",isbn:"978-1-83962-718-7",printIsbn:"978-1-83962-717-0",pdfIsbn:"978-1-83962-754-5",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!0,hash:"4df95c7f295de7f6003e635d9a309fe9",bookSignature:"Dr. Yajuan Zhu, Dr. Qinghong Luo and Dr. Yuguo Liu",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/8969.jpg",keywords:"Water Cycle, Water Use Strategy, Vegetation Dynamics, Plant Community, Precipitation, Carbon Emission, Soil Respiration, Autotrophic Respiration, Algae Crust, Wind, Temperature, Vegetation Stability",numberOfDownloads:null,numberOfWosCitations:0,numberOfCrossrefCitations:null,numberOfDimensionsCitations:null,numberOfTotalCitations:null,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"January 26th 2021",dateEndSecondStepPublish:"February 23rd 2021",dateEndThirdStepPublish:"April 24th 2021",dateEndFourthStepPublish:"July 13th 2021",dateEndFifthStepPublish:"September 11th 2021",remainingDaysToSecondStep:"2 months",secondStepPassed:!0,currentStepOfPublishingProcess:3,editedByType:null,kuFlag:!1,biosketch:"Dr. Zhu holds a Ph.D. in Ecology and is currently an Associate Research Professor at the Chinese Academy of Forestry at the Institute of Desertification Studies, she has led a number of national projects while working there.",coeditorOneBiosketch:"Dr. Luo holds a Ph.D. in Physical Geography and is currently a Research Professor at the Institute of Afforestation and Sand Control, Xinjiang Academy of Forestry. She is a holder of several technological patents in her area of research.",coeditorTwoBiosketch:"Dr. Liu holds a Ph.D. in Ecology and is currently an Assistant Professor at the Institute of Desertification Studies, Chinese Academy of Forestry. He has published several international works that have been recognized.",coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"180427",title:"Dr.",name:"Yajuan",middleName:null,surname:"Zhu",slug:"yajuan-zhu",fullName:"Yajuan Zhu",profilePictureURL:"https://mts.intechopen.com/storage/users/180427/images/system/180427.jpg",biography:"Dr. Yajuan Zhu obtained her Bachelor's degree in Agriculture from Northwest Agriculture and Forestry University in 2002 and PhD in Ecology from Chinese Academy of Sciences in 2007. She was a postdoctoral fellow working on the topic of land desertification control in the Research Institute of Forestry, Chinese Academy of Forestry, followed by her appointment as an Assistant Professor at the Institute of Desertification Studies, Chinese Academy of Forestry and currently she is an Associate Research Professor at the same institute. 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1. Introduction
The Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) (or just MOS) is widely used and presents many advantages over the bipolar transistors (BJT) in many applications.
It requires less silicon area and its fabrication process is relatively simpler. It is possible to implement most analog and digital circuits using almost exclusively MOS transistors. All these properties allow packing a large number of devices in a single integrated circuit.
Additionally, and most important, its operation requires less power, making it extremely suitable to RFID circuits.
This chapter aims to provide background on MOS transistors, from its physical operation to modeling, including RF modeling. The basic knowledge is essential to analyze and to design RFID circuits implemented using CMOS transistors. The chapter also presents noise analysis which is essential to low voltage signal, as it is the case of RFID circuits.
2. Physical CMOS operation
Fig. 1 shows the physical structure of the n-channel MOS transistor, or just nMOS transistor. The transistor is fabricated in a p-type silicon substrate. Two heavily doped n-type regions, indicated as n+, are created in the substrate and will act as the source and drain (in terms of structure, source and drain can be interchanged). A thin layer of silicon oxide (SiO2), of thickness tox (typically between 2 and 50 nm), is formed on the surface of the substrate, between the drain and the source regions. The silicon oxide is an excellent electrical isolator. Metal (or polysilicon, which is conductor) is deposited on top of the oxide layer to form the gate electrode. Metal contacts are also made in the source and drain regions, in addition to contact to the bulk, also known as the substrate or body. Therefore, the four contacts were formed: D-drain, S-source, G-gate and B-bulk.
The gate region has a length L and a width W, which are two important design parameters of the MOS transistor. Usually L is in the range of 0.1μm to 3μm while W is in the range of 0.2μm to 100μm.
There is also the p-channel MOS transistor, or just pMOS transistor, in which the dopings are reversed to the nMOS transistor.
2.1. Forming the channel
As can be observed from the Fig. 1, the substrate forms pn junctions with the drain and the source. In normal operation both junctions must be kept reverse-biased, or at least out of the
Figure 1.
Physical structure of an nMOS transistor.
forward condition all the time. Since the drain is biased at a positive voltage, it is only necessary to connect the bulk to the ground in order to keep both junctions cut off.
With no bias applied to the gate, there are two back-to-back diodes between drain and source, and consequently, there is no current. This is true since each pn junction forms a diode. In fact, the resistance between drain and source under this circumstance is in the range of 1012Ω.
When a positive voltage is applied between gate and source - vGS, holes (which are positively charged) are repelled from the surface of the substrate. As the voltage increases, the surface becomes completely depleted of charge. The voltage at which this occurs is known as threshold voltage – Vt.
If vGS is further increased, electrons (which are negative charges) accumulate near the surface, under the gate, and an n region is created, thus forming a channel between drain and source, as indicated in Fig. 2. The channel was formed by inverting the substrate surface from p type to n type. Fig. 2 also shows the depletion region that forms around the channel and the two junctions.
Figure 2.
nMOS with an induced channel.
The symbols for the nMOS transistor are given in Fig. 3, although other symbols may be found in the literature. The symbol in Fig. 3.a corresponds to the four terminal connection, and the symbol in Fig. 3.b corresponds to the three terminal connection, where source and substrate are shorted.
Figure 3.
Symbols for nMOS transistor; (a) four terminals and (b) three terminals.
2.2. Triode condition
Now, if a very small voltage vDS is applied between drain and source, as indicated in Fig. 4, there will be a current flow through the channel. The current through the channel, named drain current - iD is directly dependent on the voltage vGS and the voltage vGS. If vGS increases, the channel becomes deeper and more current can flow. If vDS is increased, based on Ohm`s Law, there will be more current, since the channel behaves as a resistance. If follows that the transistor is operating as a linear resistance whose value is controlled by vGS. The resistance is very high for vGS\n\t\t\t\t\t≤ Vt and it decreases as vGS increases.
This condition of operation is known as ohmic, linear or triode.
Figure 4.
Conduction under very small vDS.
2.3. Saturation condition
As vDS increases, the difference vDS – vDS becomes smaller at the edge between the gate and the drain diffusion, and therefore the channel becomes shallow. Therefore, the channel assumes a tapered shape, as indicated in Fig. 5. Since the channel becomes smaller at the drain end, its resistance increases, and therefore, the transistor does not operate ideally as a linearly controlled resistor.
Figure 5.
Conduction under 0 < vDS\n\t\t\t\t\t\t\t< vGS\n\t\t\t\t\t\t\t- Vt.
At the condition vDS\n\t\t\t\t\t= vGS\n\t\t\t\t\t- Vt, the channel ceases to exist at the drain side, as shown in Fig. 6. This situation is known as pinch off. At this point, further increases in vDS moves the end of the channel further away from the drain, as presented in Fig. 7. This condition of operation is referred as saturation, therefore vDS is referred as vDSSAT\n\t\t\t\t\t= vGS\n\t\t\t\t\t- Vt.
Figure 6.
Conduction under vDS\n\t\t\t\t\t\t\t= vGS\n\t\t\t\t\t\t\t- Vt..
Once the transistor enters the saturation region of operation, the drain current iD becomes independent of the vDS.
Fig. 8 summarizes the conditions of operation of an nMOS transistor. Close to vDS = 0, current iD is directly proportional to vDS, with slope proportional to vGS\n\t\t\t\t\t- Vt. As vDS approaches vDS\n\t\t\t\t\t= vGS\n\t\t\t\t\t- Vt, the curve of bends because the channel resistance increases. After the vDS\n\t\t\t\t\t= vGS\n\t\t\t\t\t- Vt, the current becomes independent of vDS.
Figure 7.
Conduction under vDS\n\t\t\t\t\t\t\t> vGS\n\t\t\t\t\t\t\t- Vt.
Figure 8.
Operation condition of an nMOS transistor.
2.4. Deriving the iD - vDS relationship
Consider the biasing depicted in Fig. 9. Since the channel potential varies from zero at the source to vDS at the drain, the local voltage difference between gate and the channel varies from vGS to vGS – vDS. Therefore, the channel density, or charge per unit length, is given as:
Qd(x)=WCox[vGS−v(x)−Vt]E1
where v(x) is the potential at x and Cox is the capacitance, per unity area, formed by the gate and the channel.
Since, by definition, current is proportional to charge times velocity, and considering the current is the same along the channel, then:
iD=−WCox[vGS−v(x)−Vt]vE2
Figure 9.
Biasing of an nMOS.
The minus signal is due to the negative charge of electrons. The velocity of carriers at low fields is the product of mobility (μ) and the electric field (E). Noting that E(x)=−dV/dxand representing the electrons mobility by μn, then expression (2) can be rewritten as:
iD=WCox[vGS−v(x)−Vt]μndV(x)dxE3
Now integrating along the channel, one obtains:
∫0LiDdx=∫0VDSWCox[vGS−v(x)−Vt]μndV(x)E4
Thus, the expression for the drain current in the triode region is:
iD=μnCoxWL[(vGS−Vt)vDS−vDS22]E5
The value of the current for the saturation operation can be obtained by replacing vDS\n\t\t\t\t\t= vGS\n\t\t\t\t\t- Vt into expression (5), as:
iD=12μnCoxWL(vGS−Vt)2E6
As described earlier, the current does not depend on vDS. It can be observed from expressions (5) and (6) that the current is proportional to the ratioW/L, which is know as the aspect ratio. The designer can alter the aspect ratio to obtain the desired i-v characteristic.
Observe that expression (6) was obtained using the value of L, as given in Fig. 9. Nevertheless, when the transistor is saturated, the channel becomes shorter, as shown in Fig. 7. A reduction in the length of the channel, known as channel length modulation, means a variation in the resistance, and therefore a variation in the current iD.
Expression (6) can be modified in order to include the variation in the channel length, represented as L-ΔL, as:
Since ΔL/L is proportional to vDS (the larger vDS the larger will be ΔL), then:
iD≅12μnCoxWL(vGS−Vt)2(1+λvDS)E9
where λ is the parameter of proportionality.
The effect of channel length modulation can be seen in the iD - vDS characteristic of a MOS transistor shown in Fig. 10. The dependence of vDS on iD in the saturation region can be seen is represent by (1+λvDS) in expression (9) and can be observed in Fig. 10.
Figure 10.
Effect of channel modulation on saturation current.
An extrapolation of iD - vDS intercepts the vDS axis at vDS = − VA, known as Early voltage. For a given process, VA is proportional to L, selected by the designer. Typically, VA is in the range of 5 V/μm to 50 V/μm.
2.5. Output resistance
Fig. 10 and expression (9) show that an increase in vDS causes an increase in iD, meaning a resistive behavior. The value of the resistance is given as:
ro=[∂iD∂vDS]−1=[λ12μnCoxWL(vGS−Vt)2]−1E10
which can be simplified to:
ro=1λiD=VAiDE11
Therefore, a MOS transistor in the saturation region is not totally independent of vDS and presents an output impedance given by (11)\n\t\t\t\t
Considering the transistor operating in the triode region, as given by expression (5), if the value of vDS is sufficiently small, vDS2can be neglected, and therefore:
iD≅μnCoxWL[(vGS−Vt)vDS]E12
This relationship represents the behavior of the MOS transistor as a linear resistance whose value is controlled by vGS, as given by:
rds=vDSiD=[μnCoxWL(vGS−Vt)]−1E13
2.6. Transconductance
The large signal behavior of a MOS transistor in the saturation region is given by expression (6). Nevertheless, for a given biasing, the designer may be interested in the small signal behavior of the transistor. For a given small variation in the vGS, around the biasing, there will be a variation in the iD current, given by the transconductance, as:
gm=∂iD∂vGS|vGS=VGSE14
which results in:
gm=μnCoxWL(vGS−Vt)E15
Observe the transconductance depends on the ratio W/L and on the value of vGS, and they can be controlled by the designer. By using expression (6), then expression can be written as:
gm=2μnCoxWLiDE16
In this case, the transconductance depends on the ratio W/L and the iD current. That expression can be written also as:
gm=2iD(vGS−Vt)E17
It clearly does not depend on ratio W/L but it depends on both vGS and iD.
2.7. Body effect
In many circuits, the substrate and the source are not at the same potential, as it is possible to stack transistors. In that case, the substrate it is at lower potential than the source, and therefore the source-substrate junction becomes reversed biased. This reverse biasing widens the depletion layer, which in turn reduces the channel depth.
The effect of the bulk-source voltage VSB can be easily represented by a change in the threshold voltage - Vt, as given by:
Vt=Vt0+γ[2ϕf+VSB−2ϕf]E18
where Vt0 is the threshold voltage for VSB\n\t\t\t\t\t= 0, ϕfis a physical parameter (usually 2ϕf= 0.6V) and γ is a fabrication-process parameter given by:
γ=2qNAεsCoxE19
where q is the electron charge (1.6 x 1019\n\t\t\t\t\tC), NA is the doping concentration of the substrate and εs is the permissivity of silicon (1.17ε0\n\t\t\t\t\t= 1.17 x 8.854 x 10-14 = 1.04 x 10-12\n\t\t\t\t\tF/cm).
Any signal between substrate and source promotes a drain current component. The substrate acts as a second gate, and in turn will present a corresponding trasnconductance, named body transconductance, given as:
gmb=∂iD∂vDS|vGS=VGSvBS=VBSE20
From expressions (6), (17) and (18), then it is possible to state that:
gmb=χgmE21
where χ is given by:
χ=∂Vt∂VSB=γ22φf+VSBE22
And it is in the range of 0.1 to 0.
2.8. Small signal model
Considering the output impedance, the transconductance and the body effect, the small signal model of a nMOS transistor is given by Fig. 11, known as hybrid-π model.
Figure 11.
Hybrid-π model.
If the source and the substrate are at the same potential, then the model can be simplified, as the term gmbvbs goes to zero. The simplified hybrid-π model is shown in Fig. 12.
In a pMOS transistor, a p channel is formed on an n substrate. Therefore, its operation is virtually the same as the nMOS transistor, except that all voltages and currents are opposite as in the nMOS transistor. Fig. 13 shows the symbols for the nMOS transistor, although other symbols may be found in the literature. The symbol in Fig. 13.a corresponds to the four terminal connection, and the symbol in Fig. 13.b corresponds to the three terminal connection, where source and substrate are shorted.
Figure 13.
Symbols for pMOS transistor; (a) four terminals and (b) three terminals.
3. RF CMOS model
Unfortunately, the structure and the operation of a MOS transistor present parasitic capacitances that limit its frequency of operation. The parasitic capacitances may result from the capacitor formed between the gate and the channel, between gate and source/drain, and between drain/source and substrate.
3.1. Gate capacitances
The gate, the dielectric and the channel form a capacitor. When the transistor is working in the triode region with a small voltage vDS, the channel will be of uniform depth, as shown in Fig. 4. Therefore, the gate-channel capacitance can be considered equally divided between the source and the drain, and their values are:
Cgs=Cgd=12WLCox(triode region)E23
When the transistor is working in the saturation region, the channel presents a tapered shape and it is pinched off at the drain end, as presented in Fig. 7. It can be seen that the gate to channel capacitance is almost entirely modeled at the source, since the drain does not present a channel. It can be shown that the capacitances are:
Cgs=23WLCox(saturation region)Cgd≈0E24
If the transistor is cut off, there is no capacitance between gate and channel, since there is no channel for cut off. The entire capacitance is then between the gate and the substrate, therefore:
Cgs=Cgd=0(cut off)Cgb=WLCoxE25
As can be observed from Fig. 1, the gate extends over the drain and the source areas. Therefore, there is an overlapping capacitance between the gate and the drain/source. Denoting the overlapping length by Lov, then the overlap capacitance can be seen to be:
Cgsov=Cgdov=WLovCoxE26
For modern processes, Lov is usually in the range of 5% to 10% of L.
3.2. Junction capacitances
As shown by Fig. 2 there are two reversed biased junctions formed between the substrate and source/drain. Each junction consists of two semiconductors (drain/source and the substrate) and the depletion layer, thus forming a capacitor. The source-substrate capacitance can be found to be:
Csb=Csb01+VSBV0E27
where V0 is the junction built-in voltage (0.6 V to 0.8 V), VSB is the magnitude of the reversed bias voltage and Csbo is the capacitance at zero reverse bias voltage.
By the same way, the drain-substrate capacitance is given by:
Cdb=Cdb01+VDBV0E28
3.3. The high frequency model
The small signal model of the MOS transistor given in Fig. 11 can be update to include the gate and the junction capacitances, as presented in Fig. 14. Although this model represents the transistor for high frequencies, it is very complex for manual analysis.
Figure 14.
Hybrid-π model including the parasitic capacitances.
If the source and the substrate are shorted, the model can be greatly simplified, as shown in Fig. 15.
Figure 15.
Simplified high frequency model for source and substrate shorted.
4. Unity gain frequency
An important Fig. of merit for the MOS transistor is the unit gain frequency that is defined as the frequency in which the short circuit current gain becomes unit. This definition is based in the common source configuration, as shown in Fig. 16.
Figure 16.
Circuit model used to obtain the unit gain frequency.
The current Io in the short circuit is given by:
Io=gmVgs−sCgdVgs≅gmVgsE29
The approximation is due to the fact that Cgd is very small and can be neglected. Also, from the circuit, Vgs can be expressed as:
Since the magnitude of IoIi should be 1, as per definition, and considering physical frequencies (s=jω), then:
ωT=gm(Cgs+Cgd)E32
Therefore, the unit gain frequency is:
fT=gm2π(Cgs+Cgd)E33
As can be observed, the unit gain frequency is directly proportional to gm and inversely proportional to the internal capacitances. Therefore, in terms of frequency response the transistor should have large gm and small capacitances.
4. RF CMOS noise model
The two most important types of noise in MOS devices are the 1/f noise and the thermal noise.
4.1. Thermal noise
The main source of thermal noise in a MOS transistor is due to the resistive channel in the active region, and has a value of:
id2=4kTγgmE34
where k is the Boltzmann’s constant (about 1.38 x 10-23\n\t\t\t\t\tJ/K), T is the absolute temperature in kelvins and γ is a constant that is approximately 2/3 for long channel transistors and increase to the range 1-2 for short channel devices.
The other source of thermal noise is the gate. Fluctuation in the channel potential couples capacitively into the gate terminal, which in turn translates into a noise gate current. Noise gate current can also be produced by the resistive material of the gate. This total noise gate can be ignored at low frequencies but becomes significant at high frequencies as it is the case of RF circuits. It has been shown the gate noise may be expressed as:
ig2=4kTδggE35
where δ is approximately 4/3 for long channel transistors and increase to the range 2-4 for short channel devices, and gg is given by:
gg=ω2Cgs25gmE36
Mostly of the time, instead of using a current source at the gate, it is more convenient to consider an equivalent voltage source. The equivalent voltage source of expressions (31) and(32) is given by:
vg2=4kTδrgE37
where rg is given by:
rg=15gmE38
4.2. 1/f noise
The 1/f noise, also known as flicker noise or pink noise, arises mainly due to the surface imperfections that can trap and release charges. Since MOS devices are naturally surface devices, they produce much more 1/f than bipolar devices (which are bulk devices). This noise is also generated by defects and impurities that randomly trap and release charges. The trapping times are statistically distributed in such a way that lead to a 1/f noise spectrum.
The 1/f noise can be modeled by a voltage source in series with the gate, of value:
vf2=βWLCoxfE39
For pMOS devices, β is typically about 10-28C2/m2, but it can be up to 50 times larger for nMOS devices.
As can be observed from expression (53), the 1/f noise is smaller for larger devices. This occurs because the large capacitance smoothes the fluctuation in the channel charge. Therefore, in order to achieve good 1/f performance, larger devices should be used.
The 1/f can also be modeled as a current source at the drain whose value is:
if2=βgm2WLCox2f≅βfωT2AΔfE40
where A is the area of the gate.
4.3. Noise model
The noise model of an nMOS transistor is presented in Fig. 17, where the transistor is considered noiseless. The decision of placing the noise sources as a voltage source at the gate, or as a current source at the drain is just a matter of convenience according to the circuit under analysis. As an example, the values of Fig. 17 could be:
v2=vg2=4kTδrgi2=id2+if2=4kTγgm+βgm2WLCox2fE41
5. Conclusions
The proper understanding of physical operation to modeling of CMOS transistors is essential to the analysis and design of RFID circuits. Among its advantages, the CMOS transistors demands lower power consumption than other transistors.
Noise analysis of CMOS transistors is also fundamental to analysis and design of any circuit, including RFID.
\n',keywords:null,chapterPDFUrl:"https://cdn.intechopen.com/pdfs/16516.pdf",chapterXML:"https://mts.intechopen.com/source/xml/16516.xml",downloadPdfUrl:"/chapter/pdf-download/16516",previewPdfUrl:"/chapter/pdf-preview/16516",totalDownloads:2077,totalViews:305,totalCrossrefCites:0,totalDimensionsCites:0,hasAltmetrics:0,dateSubmitted:"November 9th 2010",dateReviewed:"March 8th 2011",datePrePublished:null,datePublished:"July 20th 2011",dateFinished:null,readingETA:"0",abstract:null,reviewType:"peer-reviewed",bibtexUrl:"/chapter/bibtex/16516",risUrl:"/chapter/ris/16516",book:{slug:"current-trends-and-challenges-in-rfid"},signatures:"Tales Pimenta, Robson Moreno and Leonardo Zoccal",authors:[{id:"28692",title:"Prof.",name:"Tales",middleName:null,surname:"Pimenta",fullName:"Tales Pimenta",slug:"tales-pimenta",email:"tales@unifei.edu.br",position:null,institution:null},{id:"38289",title:"Prof.",name:"Robson",middleName:"Luiz",surname:"Moreno",fullName:"Robson Moreno",slug:"robson-moreno",email:"moreno@unifei.edu.br",position:null,institution:{name:"Federal University of Itajubá",institutionURL:null,country:{name:"Brazil"}}},{id:"77855",title:"Prof.",name:"Leonardo",middleName:null,surname:"Zoccal",fullName:"Leonardo Zoccal",slug:"leonardo-zoccal",email:"lbzoccal@unifei.edu.br",position:null,institution:{name:"Federal University of Itajubá",institutionURL:null,country:{name:"Brazil"}}}],sections:[{id:"sec_1",title:"1. Introduction",level:"1"},{id:"sec_2",title:"2. Physical CMOS operation",level:"1"},{id:"sec_2_2",title:"2.1. Forming the channel",level:"2"},{id:"sec_3_2",title:"2.2. Triode condition",level:"2"},{id:"sec_4_2",title:"2.3. Saturation condition",level:"2"},{id:"sec_5_2",title:"2.4. Deriving the iD - vDS relationship",level:"2"},{id:"sec_6_2",title:"2.5. Output resistance",level:"2"},{id:"sec_7_2",title:"2.6. Transconductance",level:"2"},{id:"sec_8_2",title:"2.7. Body effect",level:"2"},{id:"sec_9_2",title:"2.8. Small signal model",level:"2"},{id:"sec_10_2",title:"2.9. Summary",level:"2"},{id:"sec_11_2",title:"2.10. pMOS transistor",level:"2"},{id:"sec_13",title:"3. RF CMOS model",level:"1"},{id:"sec_13_2",title:"3.1. Gate capacitances",level:"2"},{id:"sec_14_2",title:"3.2. Junction capacitances",level:"2"},{id:"sec_15_2",title:"3.3. The high frequency model",level:"2"},{id:"sec_17",title:"4. Unity gain frequency",level:"1"},{id:"sec_18",title:"4. RF CMOS noise model",level:"1"},{id:"sec_18_2",title:"4.1. Thermal noise",level:"2"},{id:"sec_19_2",title:"4.2. 1/f noise",level:"2"},{id:"sec_20_2",title:"4.3. Noise model",level:"2"},{id:"sec_22",title:"5. Conclusions",level:"1"}],chapterReferences:[{id:"B1",body:'AllenP. E.HolbergD. R.\n\t\t\t\t\t2002\n\t\t\t\t\tCMOS Analog Circuit Design- 2nd Ed., Oxford University Press, 0-19511-644-5'},{id:"B2",body:'JohnsD. A.MartinK.\n\t\t\t\t\t1997\n\t\t\t\t\tAnalog Integrated Circuit Design, John Wiley & Sons, 0-47114-448-7'},{id:"B3",body:'SedraA. S.SmithK. C.\n\t\t\t\t\t2009\n\t\t\t\t\tMicroelectronic Circuit- 6th Ed., Oxford University Press, 0-19532-303-3'},{id:"B4",body:'LeeT. H.\n\t\t\t\t\t2004\n\t\t\t\t\tThe Design of CMOS Radio-Frequency Integrated Circuits- 2nd Edition, Cambridge University Press, 0-52183-539-9'},{id:"B5",body:'ColemanC.\n\t\t\t\t\t20040\n\t\t\t\t\tAn Introduction to Radio Frequency Engineering, Cambridge University Press, 0-52183-481-3'},{id:"B6",body:'GilmoreR.BesserL.\n\t\t\t\t\t2003\n\t\t\t\t\tPractical RF Circuit Design for Modern Wireless Systems- Vol. II, Artech House Publishers, 1-58053-522-4'},{id:"B7",body:'RogersJ.PlettC.\n\t\t\t\t\t20030\n\t\t\t\t\tRadio Frequency Integrated Circuit Design, Artech House Inc, 1-60783-979-2'},{id:"B8",body:'ZielA.\n\t\t\t\t\t1986\n\t\t\t\t\tNoise in Solid State Devices and Circuits, John Wiley and Sons, 0-47183-234-0'}],footnotes:[],contributors:[{corresp:"yes",contributorFullName:"Tales Cleber Pimenta",address:"",affiliation:'
Universidade Federal de Itajuba, Brazil
'},{corresp:"yes",contributorFullName:"Robson L. Moreno",address:"",affiliation:'
Universidade Federal de Itajuba, Brazil
'},{corresp:"yes",contributorFullName:"Leonardo B. Zoccal",address:"",affiliation:'
Universidade Federal de Itajuba, Brazil
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C. Nascimento and J. C. da S. Lacava",authors:[{id:"17182",title:"Dr.",name:"José Carlos",middleName:"Da Silva",surname:"Lacava",fullName:"José Carlos Lacava",slug:"jose-carlos-lacava"},{id:"21173",title:"PhD.",name:"Daniel",middleName:null,surname:"Nascimento",fullName:"Daniel Nascimento",slug:"daniel-nascimento"}]},{id:"14707",title:"Analysis of a Rectangular Microstrip Antenna on a Uniaxial Substrate",slug:"analysis-of-a-rectangular-microstrip-antenna-on-a-uniaxial-substrate",signatures:"Amel Boufrioua",authors:[{id:"18041",title:"Prof.",name:"Amel",middleName:null,surname:"Boufrioua",fullName:"Amel Boufrioua",slug:"amel-boufrioua"}]},{id:"14708",title:"Artificial Materials based Microstrip Antenna Design",slug:"artificial-materials-based-microstrip-antenna-design",signatures:"Merih Palandöken",authors:[{id:"19051",title:"Dr.Ing.",name:"Merih",middleName:null,surname:"Palandöken",fullName:"Merih Palandöken",slug:"merih-palandoken"}]},{id:"14709",title:"Particle-Swarm-Optimization-Based Selective Neural Network Ensemble and Its Application to Modeling Resonant Frequency of Microstrip Antenna",slug:"particle-swarm-optimization-based-selective-neural-network-ensemble-and-its-application-to-modeling-",signatures:"Tian Yu-Bo and Xie Zhi-Bin",authors:[{id:"21523",title:"Dr.",name:"Yu-Bo",middleName:null,surname:"Tian",fullName:"Yu-Bo Tian",slug:"yu-bo-tian"}]},{id:"14710",title:"Microstrip Antennas Conformed onto Spherical Surfaces",slug:"microstrip-antennas-conformed-onto-spherical-surfaces",signatures:"Daniel B. Ferreira and J. C. da S. Lacava",authors:[{id:"17182",title:"Dr.",name:"José Carlos",middleName:"Da Silva",surname:"Lacava",fullName:"José Carlos Lacava",slug:"jose-carlos-lacava"},{id:"21171",title:"Dr.",name:"Daniel",middleName:null,surname:"Ferreira",fullName:"Daniel Ferreira",slug:"daniel-ferreira"}]},{id:"14711",title:"Mathematical Modeling of Spherical Microstrip Antennas and Applications",slug:"mathematical-modeling-of-spherical-microstrip-antennas-and-applications",signatures:"Nikolaos L. Tsitsas and Constantinos A. Valagiannopoulos",authors:[{id:"20921",title:"Dr.",name:"Nikolaos L.",middleName:null,surname:"Tsitsas",fullName:"Nikolaos L. Tsitsas",slug:"nikolaos-l.-tsitsas"},{id:"20922",title:"PhD.",name:"Constantinos A.",middleName:null,surname:"Valagiannopoulos",fullName:"Constantinos A. Valagiannopoulos",slug:"constantinos-a.-valagiannopoulos"}]},{id:"14712",title:"Cavity-Backed Cylindrical Wraparound Antennas",slug:"cavity-backed-cylindrical-wraparound-antennas",signatures:"O. M. C. Pereira-Filho, T. B. Ventura, C. G. Rego, A. F. Tinoco-S., and J. C. da S. Lacava",authors:[{id:"17182",title:"Dr.",name:"José Carlos",middleName:"Da Silva",surname:"Lacava",fullName:"José Carlos Lacava",slug:"jose-carlos-lacava"},{id:"19227",title:"PhD.",name:"Odilon",middleName:null,surname:"Pereira Filho",fullName:"Odilon Pereira Filho",slug:"odilon-pereira-filho"},{id:"19450",title:"Mr.",name:"Thiago",middleName:null,surname:"Ventura",fullName:"Thiago Ventura",slug:"thiago-ventura"},{id:"19451",title:"PhD.",name:"Cassio",middleName:null,surname:"Rego",fullName:"Cassio Rego",slug:"cassio-rego"},{id:"19452",title:"Prof.",name:"Alexis F.",middleName:null,surname:"Tinoco Salazar",fullName:"Alexis F. Tinoco Salazar",slug:"alexis-f.-tinoco-salazar"}]},{id:"14713",title:"Analysis into Proximity-Coupled Microstrip Antenna on Dielectric Lens",slug:"analysis-into-proximity-coupled-microstrip-antenna-on-dielectric-lens",signatures:"Lawrence Mall",authors:[{id:"18397",title:"Dr.",name:"Lawrence",middleName:null,surname:"Mall",fullName:"Lawrence Mall",slug:"lawrence-mall"}]},{id:"14714",title:"Methods to Design Microstrip Antennas for Modern Applications",slug:"methods-to-design-microstrip-antennas-for-modern-applications",signatures:"K. Siakavara",authors:[{id:"18497",title:"PhD.",name:"Katherine",middleName:null,surname:"Siakavara",fullName:"Katherine Siakavara",slug:"katherine-siakavara"}]},{id:"14715",title:"Fractal-Shaped Reconfigurable Antennas",slug:"fractal-shaped-reconfigurable-antennas",signatures:"Ali Ramadan, Mohammed Al-Husseini, Karim Y. Kabalan and Ali El-Hajj",authors:[{id:"17558",title:"Dr.",name:"Mohammed",middleName:null,surname:"Al-Husseini",fullName:"Mohammed Al-Husseini",slug:"mohammed-al-husseini"},{id:"21637",title:"Mr.",name:"Ali",middleName:null,surname:"Ramadan",fullName:"Ali Ramadan",slug:"ali-ramadan"},{id:"21638",title:"Prof.",name:"Karim",middleName:null,surname:"Kabalan",fullName:"Karim Kabalan",slug:"karim-kabalan"},{id:"21639",title:"Prof.",name:"Ali",middleName:null,surname:"El-Hajj",fullName:"Ali El-Hajj",slug:"ali-el-hajj"}]},{id:"14716",title:"A Microstrip Antenna Shape Grammar",slug:"a-microstrip-antenna-shape-grammar",signatures:"Adrian Muscat and Joseph A. Zammit",authors:[{id:"19668",title:"Dr.",name:"Adrian",middleName:null,surname:"Muscat",fullName:"Adrian Muscat",slug:"adrian-muscat"},{id:"19669",title:"Dr.",name:"Joseph A.",middleName:null,surname:"Zammit",fullName:"Joseph A. Zammit",slug:"joseph-a.-zammit"}]},{id:"14717",title:"Electrically Small Microstrip Antennas Targeting Miniaturized Satellites: the CubeSat Paradigm",slug:"electrically-small-microstrip-antennas-targeting-miniaturized-satellites-the-cubesat-paradigm",signatures:"Constantine Kakoyiannis and Philip Constantinou",authors:[{id:"19252",title:"Dr.Ing.",name:"Constantine",middleName:"G.",surname:"Kakoyiannis",fullName:"Constantine Kakoyiannis",slug:"constantine-kakoyiannis"},{id:"21863",title:"Prof.",name:"Philip",middleName:null,surname:"Constantinou",fullName:"Philip Constantinou",slug:"philip-constantinou"}]},{id:"14718",title:"Circularly Polarized Microstrip Antennas with Proximity Coupled Feed for Circularly Polarized Synthetic Aperture Radar",slug:"circularly-polarized-microstrip-antennas-with-proximity-coupled-feed-for-circularly-polarized-synthe",signatures:"Merna Baharuddin and Josaphat Tetuko Sri Sumantyo",authors:[{id:"18521",title:"PhD.",name:"Merna",middleName:null,surname:"Baharuddin",fullName:"Merna Baharuddin",slug:"merna-baharuddin"},{id:"21582",title:"Prof.",name:"Josaphat",middleName:null,surname:"Tetuko Sri Sumantyo",fullName:"Josaphat Tetuko Sri Sumantyo",slug:"josaphat-tetuko-sri-sumantyo"}]},{id:"14719",title:"Circularly Polarized Slotted/Slit-Microstrip Patch Antennas",slug:"circularly-polarized-slotted-slit-microstrip-patch-antennas",signatures:"Nasimuddin, Zhi-Ning Chen and Xianming Qing",authors:[{id:"21459",title:"Dr.",name:"N",middleName:null,surname:"Nasimuddin",fullName:"N Nasimuddin",slug:"n-nasimuddin"}]},{id:"14720",title:"Microstrip Antenna Arrays",slug:"microstrip-antenna-arrays",signatures:"Albert Sabban",authors:[{id:"16889",title:"Dr.",name:"Albert",middleName:null,surname:"Sabban",fullName:"Albert Sabban",slug:"albert-sabban"}]},{id:"14721",title:"Microstrip Antennas for Indoor Wireless Dynamic Environments",slug:"microstrip-antennas-for-indoor-wireless-dynamic-environments",signatures:"Mohamed Elhefnawy and Widad Ismail",authors:[{id:"17004",title:"Dr.",name:"Widad",middleName:null,surname:"Ismail",fullName:"Widad Ismail",slug:"widad-ismail"},{id:"17005",title:"Dr.",name:"Mohamed",middleName:null,surname:"Elhefnawy",fullName:"Mohamed Elhefnawy",slug:"mohamed-elhefnawy"}]},{id:"14722",title:"DBDP SAR Microstrip Array Technology",slug:"dbdp-sar-microstrip-array-technology",signatures:"Shun-Shi Zhong",authors:[{id:"4123",title:"Prof.",name:"Shun-Shi",middleName:null,surname:"Zhong",fullName:"Shun-Shi Zhong",slug:"shun-shi-zhong"}]},{id:"14723",title:"Microwave Properties of Dielectric Materials",slug:"microwave-properties-of-dielectric-materials",signatures:"JS Mandeep and Loke Ngai Kin",authors:[{id:"21035",title:"Prof.",name:"Mandeep",middleName:null,surname:"Singh Jit",fullName:"Mandeep Singh Jit",slug:"mandeep-singh-jit"},{id:"135784",title:"Prof.",name:"Ngai Kin",middleName:null,surname:"Loke",fullName:"Ngai Kin Loke",slug:"ngai-kin-loke"}]},{id:"14724",title:"Hybrid Microstrip Antennas",slug:"hybrid-microstrip-antennas",signatures:"Alexandre Perron, Tayeb A. Denidni and Abdel R. Sebak",authors:[{id:"11473",title:"Prof.",name:"Tayeb A.",middleName:null,surname:"Denidni",fullName:"Tayeb A. Denidni",slug:"tayeb-a.-denidni"},{id:"21901",title:"Prof.",name:"Alexandre",middleName:null,surname:"Perron",fullName:"Alexandre Perron",slug:"alexandre-perron"},{id:"21902",title:"Prof.",name:"Abdel R.",middleName:null,surname:"Sebak",fullName:"Abdel R. Sebak",slug:"abdel-r.-sebak"}]},{id:"14725",title:"Integration of 60-GHz Microstrip Antennas with CMOS Chip",slug:"integration-of-60-ghz-microstrip-antennas-with-cmos-chip",signatures:"Gordana Klaric Felic and Efstratios Skafidas",authors:[{id:"18389",title:"Prof.",name:"Gordana Klaric",middleName:null,surname:"Felic",fullName:"Gordana Klaric Felic",slug:"gordana-klaric-felic"},{id:"21215",title:"PhD.",name:"Efstratios",middleName:null,surname:"Skafidas",fullName:"Efstratios Skafidas",slug:"efstratios-skafidas"}]},{id:"14726",title:"A Practical Guide to 3D Electromagnetic Software Tools",slug:"a-practical-guide-to-3d-electromagnetic-software-tools",signatures:"Guy A. E. Vandenbosch and Alexander Vasylchenko",authors:[{id:"18667",title:"PhD.",name:"Alexander",middleName:null,surname:"Vasylchenko",fullName:"Alexander Vasylchenko",slug:"alexander-vasylchenko"},{id:"20908",title:"Prof.",name:"Guy A. E.",middleName:null,surname:"Vandenbosch",fullName:"Guy A. E. Vandenbosch",slug:"guy-a.-e.-vandenbosch"}]}]}]},onlineFirst:{chapter:{type:"chapter",id:"68698",title:"Nanofibrous Scaffolds for Skin Tissue Engineering and Wound Healing Based on Nature-Derived Polymers",doi:"10.5772/intechopen.88602",slug:"nanofibrous-scaffolds-for-skin-tissue-engineering-and-wound-healing-based-on-nature-derived-polymers",body:'\n
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1. Introduction
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Nanofibrous scaffolds are one of the most promising materials for skin tissue engineering and wound dressing, because they resemble nanoarchitecture of the native extracellular matrix (for a review, see [1]). Therefore, they can serve as suitable carriers of cells for tissue engineering and also as suitable wound dressings, which are able to protect the wound from external harmful effects, mainly microbial infection, and at the same time, they can keep appropriate moisture and gas exchange at the wound site.
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Nanofibrous scaffolds for skin tissue engineering have been fabricated from a wide range of synthetic and nature-derived polymers, which can be either biostable or degradable within the human body. Biostable synthetic polymers used in nanofiber-based skin regenerative therapies include, for example, polyurethane [2], polydimethylsiloxane [3], polyethylene terephthalate [4], polyethersulfone [5], and also hydrogels such as poly(acrylic acid) (PAA, [6]), poly(methyl methacrylate) (PMMA, [7]), and poly[di(ethylene glycol) methyl ether methacrylate] (PDEGMA, [8]). Degradable synthetic polymers typically include poly(ε-caprolactone) (PCL, [9]) and its copolymers with polylactides (PLCL, [10]), polylactides (PLA, [11]) and their copolymers with polyglycolides (PLGA, [12]), and also so-called auxiliary polymers, such as poly(ethylene glycol) (PEG), poly(ethylene oxide) (PEO, [13]) or poly(vinyl alcohol) (PVA, [14]), which facilitated the electrospinning process and improved the mechanical properties and wettability of the chief polymer. However, the synthetic polymers, although they are well-chemically defined and tailorable, are often bioinert, hydrophobic and thus not promoting cell adhesion, and also not well-adhering to the wound site. Therefore, they need to be combined with other bioactive substances, particularly nature-derived polymers.
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This chapter is focused on nature-derived polymers used for fabrication of nanofibrous scaffolds for skin tissue engineering and wound healing. The advantages of most of these polymers are their better bioactivity, flexibility, wettability, and adhesion to the wound site. Similarly as synthetic polymers, also nature-derived polymers can be divided into polymers with none or limited degradability, when implanted into human tissues, and polymers well-degradable in human tissues. The first group includes glucans, such as cellulose, schizophyllan, dextran, starch, and other polysaccharides and proteins, such as pullulan, xylan, alginate, pectin, gum tragacanth, gum arabic, silk fibroin, and sericin. The second group of polymers degradable in human tissues includes collagen and its derivative gelatin, elastin, keratin, glycosaminoglycans such as hyaluronic acid, heparin and chondroitin sulfate, and also polymers not produced in the human body, namely chitosan, gellan gum, zein, and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV).
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Some of the polymers degradable in human tissues, such as collagen, gelatin, elastin, keratin, and glycosaminoglycans, contain specific cell-binding motifs in their molecules, for example, specific amino acid sequences in proteins and oligosaccharide domains in glycosaminoglycans, which are recognized by cell adhesion receptors of integrin and non-integrin families (for a review, see [15, 16]). These molecules are often used in allogeneic or xenogeneic form, thus they can be associated with pathogen transmission or immune reaction. However, some synthetic polymers, for example PLA and PCL, have been reported to induce a more pronounced inflammatory reaction than gelatin [17].
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This review chapter summarizes earlier and recent knowledge on skin tissue engineering and wound dressing applications, based on nanofibrous scaffolds made of nature-derived polymers, including our results.
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2. Nature-derived nanofibers with none or limited degradability in the human tissues
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Nature-derived nondegradable polymers or polymers with limited degradability in human tissues include polymers not occurring in the human body and synthesized by other organisms, such as plants, algae, fungi, insects, and bacteria.
\n
\nCellulose is a typical natural polymer nondegradable in human tissues. Cellulose belongs to the group of glucans, that is, polysaccharides derived from D-glucose, linked by glycosidic bond. In the cellulose molecules, these glycosidic bonds are of the β-type, thus the cellulose is a β-glucan. It is structural polysaccharide consisting of a linear chain of several hundred to over ten thousand β(1 → 4) linked D-glucose units. Cellulose is synthesized by plants, algae, fungi, some species of bacteria (Gluconacetobacter xylinus), and also by some animals, namely tunicates (Styela clava) (for a review, see [18, 19]).
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Nanofibrous cellulose can be prepared in three basic forms: bacterial cellulose, which contains cellulose nanofibrils, synthesized by bacteria, nanofibrillar cellulose prepared from plants, particularly from wood, by hydrolysis, oxidation, and mechanical disintegration, and cellulose nanofibers created by electrospinning (for a review, see [19]). For electrospinning, cellulose should be solved. Well-known solvent of cellulose is N-methylmorpholine-N-oxide (NMMO). Another possibility is N-alkylinidazolium-derivate ionic liquid and N,N-dimethylacetamide containing 8 wt% of LiCl. However, any of them did not prove to be a good solvent for needleless electrospinning. The most favorable solvent of cellulose was found to be trifluoroacetic acid (TFA). However, TFA causes severe skin burns and is toxic for aquatic organisms even in low concentrations [20]. These problems, which limit the use of cellulose for creation of electrospun scaffolds for biomedical applications, can be solved by substituting the natural cellulose by its derivatives. The mostly used derivative of cellulose is cellulose acetate (CA), mainly due to its easier solubility and biocompatibility. CA can be dissolved in several solvents, however the best ones for electrospinning proved to be acetic acid (AA), and mixtures of acetone and N,N-dimethylacetamide (DMAC). Some results of successfully spun fibers by needleless electrospinning in our experiments can be found in \nFigure 1\n, demonstrating differences in the fiber morphology. The 95% aqueous mixture of AA showed the best results in comparison with acetone/DMAC mixtures due to production of smoother fibers and lower cytotoxicity.
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Figure 1.
Scanning electron microscopy of nanofibrous layers produced by wire needleless electrospinning using different solvents, namely 12 wt% of CA in acetone/DMAC (9:1) (left) or 14 wt% of CA in 95% AA (right).
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All the mentioned forms of cellulose have been widely applied as wound dressings releasing various bioactive agents into wounds (antimicrobial, anti-inflammatory, antioxidative agents, cytokines, and growth and angiogenic factors), as transparent wound dressings for direct optical monitoring of wounds, for systemic transdermal drug delivery (analgesics, antiphlogistics, corticoids, and antihypertensives) and for construction of epidermal electronics for monitoring wound healing or physiological status of the organism. Non-degradable nanocellulose has also been used as a temporary carrier for delivery of keratinocytes, dermal fibroblasts, and mesenchymal stem cells into wounds (for a review, see [19]).
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However, for use as direct scaffolds for skin tissue engineering, cellulose should be rendered degradable in human tissues. Cellulose is degradable by cellulase enzymes (exoglucanases and endoglucanases), which hydrolyze 1,4-beta-D-glycosidic linkages. These enzymes are not synthesized in human tissues, but they can be incorporated into cellulose scaffolds in order to degrade them gradually [21, 22]. These enzymes are believed to be non-toxic for mammalian cells [23, 24]. Moreover, the final product of cellulose degradation by these enzymes is glucose, which is a natural nutrient for the cells, by contrast with the acidic by-products of the standard currently used biodegradable PLA or PLGA scaffolds [25]. Another possibility how to use cellulase enzymes in skin tissue engineering (and in tissue engineering in general) is cell sheet technology. First, cells can be grown on the top of non-degradable cellulose substrates. After reaching the cell confluence, self-standing cell sheets can be released by exposure of the cellulose substrates to cellulases. Unlike the proteolytic enzymes conventionally used for detaching cells from their growth supports, cellulases do not disintegrate the extracellular matrix (ECM) formed by cells and do not cleave extracellular parts of cell adhesion receptors binding the ECM [26]. The cell sheets can be then replanted in the wound bed.
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Another interesting approach how to render the cellulose degradable was metabolic engineering of Gluconacetobacter xylinus, which then produced modified cellulose molecules with intercalated N-acetylglucosamine (GlcNAc) residues, susceptible to degradation with lysozyme, present in the human body. After subcutaneous implantation in mice, the modified cellulose was completely degraded within 20 days [27, 28].
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Other approaches how to render the cellulose degradable, at least partially, is its oxidation and other chemical modifications of cellulose, such as its conversion into regenerated cellulose or 2,3-dialdehydecellulose. In addition, cellulose of animal origin, that is, from tunicates, degraded more quickly than plant cellulose. For example, when cellulose films from Styela clava were implanted subcutaneously into rats for 90 days, they lost almost 24% of their initial weight, while the films prepared from wood pulp cellulose lost only less than 10% (for a review, see [19]).
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\nSchizophyllan is another β-glucan used for potential wound healing application. It is an extracellular β-1,3 beta-glucan with β-1,6 branching, produced by the fungus Schizophyllum commune. In blends with PVA, it was used for electrospinning of nanofibrous scaffolds, which provided a suitable growth support for human dermal fibroblast. In experimental wound models in vivo, schizophyllan attracted macrophages, necessary for the first physiological phase of wound healing, that is, inflammation. Schizophyllan and other 1,3-β-glucans also increased collagen deposition, cellularity, formation of granulation tissue, and vascularity at the wound site [29].
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Other glucans used for fabrication of nanofibrous scaffolds for skin tissue engineering and wound healing include dextran, starch and pullulan. According to the type of their glycosidic bonds, these polysaccharides belong to α-glucans.
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\nDextran is a branched complex glucan, in which the D-glucose units are linked by α-1,6 glycosidic bonds with branches from α-1,3 linkages. Dextran is of microbial origin; it can be produced, for example, by some lactic acid bacteria from sucrose. Dextran was used as a component of nanofibrous polyurethane-based wound dressings, in which dextran promoted neovascularization of the wound site, and also served as carrier for β-estradiol, an endogenous estrogen, a potent anti-inflammatory agent, and mitogen for keratinocytes. In addition, the presence of dextran made the polyurethane dressing softer, more flexible, more wettable, and well-adherent to the wound and promoted hemostatic activity of the dressing. In vitro, the presence of dextran and β-estradiol enhanced the proliferation of 3T3-L1 fibroblasts on the scaffolds [30].
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Dextran was also used as component of a bilayer scaffold for skin tissue engineering. The upper part of the scaffolds was made of electrospun blend of poly(ε-caprolactone-co-lactide) and poloxamer (i.e., Pluronic), and the lower part was made of a hydrogel composed of dextran and gelatin without the addition of a chemical crosslinking agent. The lower dextran/gelatin hydrogel layer provided a highly swollen three-dimensional environment similar to extracellular matrix (ECM) of soft tissues. Both part of the scaffolds supported the growth of adiposetissue-derived stem cells; however, the number of these cells on the hydrogels decreased with increasing content of dextran [31].
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Dextran is degradable by dextranases, enzymes hydrolyzing (1 → 6)-alpha-D-glycosidic linkages. This enzyme is produced mainly by bacterial and fungi, but it was also detected in animal and human tissues, namely liver and spleen. Therefore, dextran is often chosen for biomedical applications, particularly drug delivery, because it is slowly degradable in human organism. Dextran molecules with Mw higher than 40 kDa are sequestered in the liver and spleen, and then hydrolyzed by endo- and exodextranases. Dextran molecules with Mw lower than 40 kDa can be eliminated through renal clearance [32]. However, dextran hydrogels implanted subcutaneously or intramuscularly into rats did not show signs of degradation 6 weeks post-implantation and were surrounded by a thin fibrous capsule and some macrophages and giant cells, which is a response typical for a number of non-degradable materials [32].
\n
\nStarch is another α-glucan, containing both α-1,4- and α-1,6 glycosidic bonds. It serves an energy storage polysaccharide in plants, and from this point of view, it is considered to be an analogue of glycogen, energy storage polysaccharide in animals. Starch consists of two types of molecules, namely linear amylose and branched amylopectin (for a review, see [33]). Electrospun starch-based nanofibrous meshes were proposed for wound healing applications. The electrospinning of starch was facilitated by addition of PVA, that is, a noncytotoxic, water-soluble, biocompatible synthetic polymer which reduced the repulsive forces produced in starch solution. The scaffolds then promoted the proliferation of mouse L929 fibroblasts [34]. Starch is degradable by amylases, that is, hydrolases that act on α-1,4-glycosidic bonds. Amylases occur in three forms, namely α-, β-, and γ-amylases. These enzymes are synthesized by microorganisms (bacteria and fungi), plants, and with exception of β-amylases, also in animals. Alpha-amylases are present in human organism, but not currently in all tissues-they are important enzymes of gastrointestinal tract and are produced by salivary glands and pancreas. Interestingly, α-amylases were also found in brain, and their lower expression there is probably associated with the pathogenesis of Alzheimer’s neurodegenerative disease [35].
\n
\nPullulan is also an α-glucan with both α-1,4- and α-1,6 glycosidic bonds. It is a linear polysaccharide consisting of maltotriose units, in which three glucose units in maltotriose are connected by an α-1,4 glycosidic bond, whereas consecutive maltotriose units are connected to each other by an α-1,6 glycosidic bond. Pullulan is produced from starch by the fungus Aureobasidium pullulans. It shows a high water-absorbing capability, adhesive properties, and the capability to form strong resilient films and fibers. It is degradable by pullulanase, a specific kind of glucanase, produced in bacteria and not present in human tissues. When pullulan hydrogels alone or in combination with dextran were implanted subcutaneously into rats, they induced inflammatory reaction and were surrounded by a fibrous capsule [36]. Nevertheless, pullulan is water-soluble and thus removable from human issues, and in combination with chitosan and tannic acid, it was used for fabrication of electrospun nanofibrous meshes promising for wound healing [37]. In combination with dextran and gelatin, pullulan was used for electrospinning of nanofibrous scaffolds promising for skin tissue engineering. These scaffolds, especially when crosslinked with trisodium trimetaphosphate, supported the adhesion and spreading of human dermal fibroblasts and formation of actin cytoskeleton in these cells [38].
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\nXylan is a plant polysaccharide belonging to the group of hemicelluloses, that is, polymers often associated with cellulose. While cellulose is made of glucose units, hemicelluloses contain many different sugar monomers. Xylans are polysaccharides made of β-1,4-linked xylose (i.e., a pentose sugar) residues with side branches of α-arabinofuranose and α-glucuronic acids, which contribute to crosslinking of cellulose microfibrils and lignin through ferulic acid residues. Xylans are considered as relatively available and cost-effective natural materials for tissue engineering. Electrospun nanofibers containing beech-derived xylan and PVA were tested as potential dermal substitutes for skin tissue regeneration. These scaffolds provided a good support for the adhesion and proliferation of human foreskin fibroblasts and for production of collagen by these cells [39]. Bagasse xylan was also a component of hydrogels endowed with shape memory, namely carboxymethyl xylan-g-poly(acrylic acid) hydrogels, applicable in tissue engineering and biosensorics, particularly for construction of electronic skin [40].
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\nAlginates, for example, sodium alginate or calcium alginate, are salts of alginic acid, a linear polysaccharide composed of (1,4)-β-D-mannuronic acid and (1,3)-α-L-guluronic acid. Alginates are produced by various species of brown algae, and also by the bacterium Pseudomonas aeruginosa, a major pathogen found in the lungs of patients with cystic fibrosis. The structure of alginates is similar to glycosaminoglycans, an important component of ECM in human tissues including skin [41]. Alginates have a great ability to keep moisture in the wound site and to adhere to skin. However, alginates are poorly spinnable, and therefore, for skin tissue engineering and wound dressing applications, they were electrospun together with other polymers, such as PVA [41, 42] or PEO [43]. Poor mechanical properties of alginates have been compensated by the combination with chitosan [44] or PCL [44, 45]. In addition, alginates themselves are not adhesive for mammalian cells, which was compensated by their combination with collagen and gelatin, containing ligands for cell adhesion receptors [41]. Alginates were modified with a cell adhesive GRGDSP oligopeptide, which acts as ligand for integrin cell adhesion receptors [43]. Sodium alginate was used for attachment of arginine to the surface of chitosan nanofibers in order to increase healing capability of this wound dressing [46]. Alginate nanofibers supported by PCL were impregnated with an extract from Spirulina, a photosynthetic cyanobacterium producing bioactive molecules with anti-oxidant and anti-inflammatory effects [45]. Electrospun sodium alginate nanofibers containing silver nanoparticles were used for fabrication of an electronic skin capable of pressure sensing and endowed with antibacterial activity [47].
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The degradability of alginate in human organism is limited. Alginate is naturally degraded by alginate lyases or alginate depolymerases, which have been isolated from marine algae, marine animals, bacteria, fungi, viruses, and other microorganisms, but are not present in the human organism. Degradability of alginate can be increased by its oxidation and at low pH. Also the hydrophilicity and water uptake capacity of alginate can help in its removal from the wound site (for a review, see [48]).
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\nPectin is a complex of structural polysaccharides present in the cell walls of terrestrial plants, rich in galacturonic acid. Pectin is known as gelling agent in food industry, but it is also widely used in medicine, for example, against digestive disorders, such as obstipation and diarrhea, for oral drug delivery, as a component of dietary fibers trapping cholesterol and carbohydrates, as a demulcent, that is, a mucoprotective agent, and also in wound healing preparations [49]. Pectin is known as a natural prophylactic substance against poisoning with toxic cations, and its hemostatic and curing effects are well-documented in healing ointments [50]. Pectin is degradable by enzymes produced by bacterial, fungal and plant cells, and not present in human tissues [51, 52, 53]. Thus, pectin is degradable, at least partly, only in the intestinal tract populated with bacteria. However, pectin is water-soluble and quickly dissolves in the water environment, including the tissues. Therefore, in order to increase its stability, it was combined with chitosan and TiO2 nanoparticles for wound dressing applications [50] or used for construction of composite chitosan-pectin scaffolds for skin tissue engineering. Blending chitosan with pectin markedly improved the mechanical properties of the scaffolds, such as their Young’s modulus, strain at break and ultimate tensile strength, in comparison with pure chitosan scaffolds, although the proliferation of cells (i.e., fibroblasts) was slightly slower on pectin-containing scaffolds [54, 55]. The reason is that pectin does not contain cell binding domains. The cell adhesion on pectin nanofibers was markedly enhanced by oxidizing pectin with periodate to generate aldehyde groups, and then crosslinking the nanofibers with adipic acid dihydrazide to covalently connect pectin macromolecular chains with adipic acid dihydrazone linkers. In addition, the crosslinked pectin nanofibers exhibited excellent mechanical strength and enhanced body degradability [56].
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Other polysaccharides explored for creation of nanofibrous scaffolds for skin tissue engineering and wound healing are gum tragacanth and gum arabic, both polysaccharides of plant origin, degradable by bacteria and fungi, for example, in soil [57, 58].
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\nGum tragacanth is a viscous water-soluble mixture of polysaccharides, mainly tragacanthin and bassorin. Tragacanthin dissolves to give a colloidal hydrosol. Bassorin, representing 60–70% of the gum, is insoluble and swells to a gel. Chemically, tragacanthin is a complex mixture of acidic polysaccharides containing D-galacturonic acid, D-galactose, L-fucose (6-deoyl-L-galactose), D-xylose, and L-arabinose. Bassorin is probably a methylated tragacanthin. A small amount of cellulose, starch, protein and ash are also present (https://colonygums.com/tragacanth). In order to improve electrospinning and mechanical properties of the gum tragacanth, it was combined with PVA and PCL [59]. Gum tragacanth is endowed with microbial resistance and wound healing activity, which was further enhanced by curcumin, a naturally occurring poly-phenolic compound with a broad range of favorable biological functions, including anti-cancer, anti-oxidant, anti-inflammatory, anti-infective, angiogenic, and healing properties [60].
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\nGum arabic, also known as gum acacia, is a complex and water-soluble mixture of glycoproteins and polysaccharides consisting mainly of arabinose and galactose. For skin tissue engineering, it was electrospun with PCL and also with zein, a storage plant protein [61].
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\nSilk fibroin is a water-insoluble elastic protein present in silk fibers produced by larvae of Bombyx mori and some other moth of the Saturniidae family, such as Antheraea assama, Antheraea mylitta, and Philosamia ricini [62, 63, 64]. Silk fibroin occurs in the fibers together with sericin, a water-soluble serine-rich protein, which forms a glue-like layer coating two singular filaments of fibroin.
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In biomaterial science, silk fibroin is considered to be degradable, but in mammalian organism, this degradation is long-lasting and can take more than 1 year. As a kind of biomaterial approved by the Food and Drug Administration (FDA) for medical use, silk is defined by United States Pharmacopeia as non-degradable for its negligible tensile strength loss in vivo. However, silk fibroin is susceptible to biological degradation by proteolytic enzymes such as chymotrypsin, actinase, carboxylase, proteases XIV, XXI and E, and collagenase IA. The final degradation products of silk fibroin are amino acids, which are easily absorbed in vivo (for a review, see [65]).
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The degradation behavior of fibroin scaffolds depends on the preparation method and structural characteristics, such as processing condition, pore size, and silk fibroin concentration (for a review, see [65]). For example, three-dimensional porous scaffolds prepared from silk fibroin using all-aqueous process degraded within 2–6 months after implantation into muscle pouches of rats, while the scaffolds prepared using an organic solvent, hexafluoroisopropanol (HFIP), persisted beyond 1 year. It was probably due to a lower original silk fibroin concentration, larger pore size, and a higher and more homogeneous cellular infiltration of aqueous-derived scaffolds than in HFIP-derived scaffolds [66].
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For skin tissue engineering and wound healing, silk fibroin has been combined with various synthetic and natural polymers and other bioactive substances. The polymers included, for example, PCL, [67], poly(L-lactic acid)-co-poly(ε-caprolactone) (PLACL, [68]), carboxyethyl chitosan, PVA, [69], chitin [70], cellulose-based materials modified by oxidation [71] or with lysozyme [72], collagen [73], gelatin [74], and hyaluronan [75]. The bioactive substances were, for example, growth factors, such as epidermal growth factor [64], vitamins, such as vitamin C [68], vitamin E [76], and pantothenic acid (vitamin B5; [77]), antioxidants, such as grape seed extract ([78]) or quinone-based chromenopyrazole [79], antibiotics, such as ciprofloxacin [64], tetracycline [68] or gentamycin [62], and other antimicrobial and wound healing agents, such as silver nanoparticles, dandelion leaf extract [63], Aloe vera [80], or astragaloside IV [74]. In order to enlarge the pore size in nanofibrous scaffolds for cell penetration, silk fibroin was electrospun together with so-called “sacrificial” crystals of ice [67] or NaCl [81, 82], that is, crystals which are removed after the electrospinning process. An interesting combination is silk fibroin with decellularized human amniotic membrane, which was used for developing a three-dimensional bi-layered scaffold for burn treatment. Adipose tissue-derived mesenchymal stem cells seeded on this scaffold increased expression of two main pro-angiogenesis factors, vascular endothelial growth factor, and basic fibroblast growth factor [83]. Also the transplantation of bone marrow-derived mesenchymal stem cells and epidermal stem cells into wounds using nanofibrous silk fibroin scaffolds supported re-epithelization, collagen synthesis, as well as the skin appendages regeneration [84]. Another interesting approach is to use silk fibroin produced by other species than Bombyx mori, namely by the moths Antheraea assama and Philosamia ricini. This “non-mulberry” silk fibroin possesses inherent Arg-Gly-Asp (RGD) motifs in its protein sequence, which facilitates binding of cells through their integrin adhesion receptors [64].
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\nSericin has also been applied in skin tissue engineering and wound healing, although in a lesser extent than silk fibroin. Sericin shows antioxidant, UV-protective, heat-protective, moisture-retaining, and antimicrobial properties, which have been reported to be more pronounced in non-mulberry sericin (e.g., from Antheraea mylitta) than in sericin produced by Bombyx mori. The reason is that wild moths like Antheraea mylitta are exposed to a hostile environment in nature than Bombyx mori raised in captive conditions. Similarly as non-mulberry silk fibroin, also sericin has been reported to be more supportive for cell adhesion than mulberry sericin (for a review, see [85]). Sericin enhanced the proliferation and epidermal differentiation of human mesenchymal stem cells on gelatin/hyaluronan/chondroitin sulfate nanofibrous scaffolds [86]. Similarly, sericin improved the growth of murine L929 fibroblasts and human HaCaT keratinocytes cultured on the PVA nanofibrous scaffolds [87] and also the growth of L929 fibroblasts on chitosan nanofibrous scaffolds, together with antibacterial properties of these scaffolds [88].
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3. Nature-derived nanofibers degradable in the human tissues
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Nature-derived polymers degradable in human tissues include, in particular, polymers that are synthesized in the human body and usually act as components of ECM. These polymers are proteins (collagen and its derivative gelatin, elastin, fibrinogen and fibrin, keratin) or polysaccharides in non-sulfated form (hyaluronic acid) and sulfated form (heparin-like glycosaminoglycans). In addition, some natural polymers synthesized by other organisms, such as bacteria, fungi, insects, crustaceans or plants, are degradable in human tissues, because they are susceptible to enzymes present in human tissues, such as lysozyme and esterases. These polymers include chitosan, gellan gum, zein, and PHBV.
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\nCollagen is the main structural protein in the extracellular space in a wide range of tissues in the body. Skin contains type I collagen, one of the most abundant collagens in the human body. Type I collagen is also abundant in tendons, ligaments, and vasculature, and it is a main component of the organic part of bone. Type I collagen is a fibrillar type of collagen; it is composed of amino acid chains forming triple-helices of elongated fibrils. That is why the nanofibrous collagen scaffolds closely mimic the architecture of the native ECM and are advantageous for tissue engineering. In addition, collagen has been reported to be relatively poorly immunogenic, even if used in allogeneic and xenogeneic forms, for example, recombinant human collagen or bovine and porcine collagen. However, mammalian collagen is associated with the risk of disease transmission, for example, bovine spongiform encephalopathy (for a review, see [89, 90, 91]). This risk can be reduced by the use of fish collagen, which became to be popular in tissue engineering, including skin tissue engineering and wound healing. In addition, the fish collagen enables an easier recovery of intact collagen triple helices than the mammalian collagen [92]. Fish collagen can be obtained from the skin, scales and bones of freshwater fish, such as tilapia [91, 92, 93, 94], and marine fish, such as hoki fish (Macruronus novaezelandiae) [92, 95], or Arothron stellatus, also known as “stellate puffer,” “starry puffer” or “starry toadfish” [96]. Nanofiber electrospun from tilapia skin collagen promoted the proliferation of human HaCaT keratinocytes, and stimulated epidermal differentiation through the up-regulated gene expression of involucrin, filaggrin, and type I transglutaminase in these cells. Moreover, the tilapia collagen nanofibers accelerated wound healing in vivo in rat models [91, 92, 93, 94]. Beneficial effects on wound healing were also observed in nanofibrous meshes electrospun from collagen obtained from Arothron stellatus [96] and from fish scale collagen peptides [90].
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Collagen is one of the most widely used natural proteins for creation of nanofibrous scaffolds for skin tissue engineering and wound healing. However, these scaffolds are usually mechanically weak, and therefore they need crosslinking or blending with synthetic polymers. Collagen crosslinking with conventionally used agents, particularly glutaraldehyde, is associated with the risk of the scaffold cytotoxicity. More benign crosslinkers used recently include, for example, citric acid [95] or quaternary ammonium organosilane, a multifunctional crosslinking agent, which improved the electrospinnability of collagen by reducing its surface tension, endowed the collagen nanofibers with potent antimicrobial activity and promoted the adhesion and metabolic activity of primary human dermal fibroblasts without any cytotoxicity, at least in a lower concentration of 0.1% w/w [97].
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Synthetic polymers used for combination with collagen in nanofibrous scaffolds included PLA [98], PLGA [99, 100], and particularly PCL, which was either blended with collagen [101, 102, 103, 104] or served as substrate for subsequent deposition of collagen [105]. Collagen has also been combined with natural polymers, such as silk fibroin [73] or chitosan in a form of blends [106] or in a form of bilayered scaffolds, where collagen was electrospun onto the chitosan scaffolds [107]. Collagen was also grafted on the surface of composite electrospun PVA/gelatin/alginate nanofibers [41]. Collagen-based or collagen-containing nanofibers have been loaded with a wide range of bioactive substances, such as vitamin C, vitamin D3, hydrocortisone, insulin, triiodothyronine, and epidermal growth factor [100], transforming growth factor-β1 [102], plant extracts such as Coccinia grandis leaf extract [96], or lithospermi radix extract [107], antibiotics such as gentamicin [103], or bioactive glass [93, 104]. Collagen and PCL and bioactive glass nanoparticles were applied for delivery of endothelial progenitor cells into wounds in order to promote their vascularization and healing [104].
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\nGelatin is a derivative of collagen, obtained by denaturing its triple helical structure. Specifically, it is a mixture of peptides and proteins produced by partial hydrolysis of collagen extracted from the skin, bone, and connective tissue of animals, such as cattle, pigs, chicken, and also fish. Gelatin can be defined as a complex mixtures of oligomers of the α subunits joined by covalent bonds, and intact and partially hydrolyzed α-chains of varying molecular weight (for a review, see [89, 92]). Properties of gelatin, including its spinnability, depend on the source of collagen, animal species, age, type of collagen, type of conversion of collagen to gelatin (i.e., acidic vs. basic hydrolysis), and particularly on the conformation of gelatin molecules [108].
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Similarly as collagen, also gelatin is the most promising for skin tissue engineering and wound healing applications in combination with various synthetic and natural polymers. For example, gelatin was combined with polyurethane [109], PLA [11, 17], and particularly with PCL, where it was incorporated into core-shell PCL/gelatin nanofibers as the core polymer [110] or electrospun independently of PCL using a double-nozzle technique, which resulted in creation of two types of nanofibers in the scaffolds, either mixed [111] or arranged in separate gelatin and PCL layers [112]. Gelatin was also combined with a copolymer of lactic acid and caprolactone P(LLA-CL) in the form of blends [113] or in the form of coaxial nanofibers with P(LLA-CL)/gelatin shell and albumin core containing epidermal growth factor, insulin, hydrocortisone, and retinoic acid [114]. Natural proteins combined with gelatin included dextran [31], pullulan [38], alginate [41], silk fibroin [74], and hyaluronan with chondroitin sulfate [86].
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For combination with synthetic and natural polymers, for example, with PCL [115], and chitosan and keratin [116], gelatin was also used in the form of photocrosslinkable gelatin methacrylate hydrogel (GelMA). On PCL nanofibers, GelMA showed a higher decoration level in comparison with native gelatin [116]. Self-standing nanofibrous matrices electrospun from GelMA enabled tuning of their water retention capacity, stiffness, strength, elasticity, and degradation by changing the exposure time to UV light [117].
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\nElastin is a protein found in the ECM, that maintains its elasticity [118]. It is the second main protein-based component of native skin ECM. The presence of elastin in composite electrospun nanofibrous scaffolds, containing gelatin, cellulose acetate and elastin, changed the fiber morphology from straight to ribbon-like structure, and decreased the swelling ratio and degradation rate of the scaffolds. In addition, elastin-containing scaffolds supported the attachment and proliferation of human fibroblasts [119].
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\nFibrin is a provisional ECM protein, which accumulates in wounds after injury to initiate hemostasis and healing. Fibrin is formed via the polymerization of fibrinogen monomers in the presence of thrombin, and this process can be simulated in vitro [120]. Fibrin forms a fine nanofibrous mesh, which is mechanically weak and needs to be deposited on some supportive structure, for example, synthetic nanofibrous meshes made of poly(L-lactide) (PLLA) [121]. In our experiments, fibrin was deposited on PLLA in the form of two types of coating, depending on the mode of fibrin preparation. Fibrin either covered the individual fibers in the membrane (F1 nanocoating), or covered the individual fibers and also formed a fine homogeneous nanofibrous mesh on the surface of the membrane (F2 nanocoating), depending on the mode of fibrin preparation. The fibroblasts on the F1 nanocoating remained in their typical spindle-like shape, while the cells on the F2 nanocoating were polygonal with a higher proliferation rate [122]. F2 nanocoating was then used for development of a bilayer skin construct. First, a nanofibrous PLLA mesh was coated with fibrin and seeded with human dermal fibroblasts. After reaching confluence, the fibroblasts were covered with a collagen hydrogel and were allowed to migrate into this hydrogel and to proliferate inside. After sufficient colonization of the hydrogel with fibroblasts and formation of a structure resembling the skin dermis, human epidermal keratinocytes were seeded on the top of the collagen hydrogel (\nFigure 2\n) [123].
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Figure 2.
Developing a bilayer construct of keratinocytes and fibroblasts on a PLLA nanofibrous membrane with fibrin and collagen hydrogel. Left: schematic design; right: real construct.
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Also fibrinogen was used for modification of synthetic polymeric nanofibers in order to enhance the cell adhesion and growth. Nanofibrous scaffolds electrospun from blends of PCL and fibrinogen improved the adhesion, proliferation, and epidermal differentiation of adipose tissue-derived stem cells (ADSCs) in comparison with pure PCL scaffolds. Composite PCL/fibrinogen scaffolds seeded with ADSCs also markedly improved healing of full-thickness excisional wounds created in rats in comparison with acellular dermal matrix or acellular dermal matrix with ADSCs [124].
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\nKeratin is a fibrous structural protein, present in skin appendages, such as hair, wool, feather, nails, horns, claws, hooves, and in the outer (cornified) layer of epidermis [125, 126]. Keratin protects epithelial cells from damage and stress and is insoluble in water and organic solvents.
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In most studies dealing with keratin-containing nanofibers, keratin was combined with other natural or synthetic polymers in order to improve the spinnability of keratin, or to improve the bioactivity of the co-electrospun polymer. For example, in a study by Cruz-Maya et al. [127], blending keratin with PCL improved the stability of the electrospinning process, promoted the formation of nanofibers without defects, such as beads and ribbons, typically observed in the fabrication of keratin nanofibers. At the same time, keratin markedly increases the fiber hydrophilicity compared with pure PCL, which improved the adhesion and proliferation of human mesenchymal stem cells [127]. Similarly, co-electrospinning of keratose (i.e., oxidative keratin) with PVA resulted in nanofibers with uniform fibrous structure, suitable hydrophilicity and mechanical properties [125]. Properties of electrospun keratin nanofibers were also improved by incorporation of hydrotalcites, intended for delivery of diclofenac. These nanofibers displayed a reduced swelling ratio and a slower degradation profile compared to keratin-based non-woven nanofibrous mats containing free diclofenac [126]. Keratin was also a component of core-shell nanofibers, prepared by coaxial electrospinning of chitosan, PCL and keratin with Aloe vera extracts encapsulated inside the polymer nanofibers. This construct increased the adhesion and growth of L929 fibroblasts and was intended for wound healing applications [128]. Importantly, keratin was a component of bilayer scaffolds for skin tissue engineering, composed of human hair keratin/chitosan nanofiber mat and gelatin methacrylate (GelMA) hydrogel. Human dermal fibroblasts were encapsulated and grown in the hydrogel matrix, while human HaCaT keratinocytes formed a layer on the top of the scaffolds, mimicking dermis and epidermis of skin tissue [116]. Another bilayer scaffolds was constructed using polyurethane wound dressing as an outer layer, and electrospun gelatin/keratin nanofibrous mat as an inner layer [109].
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\nHyaluronic acid, also called hyaluronan, is an anionic, non-sulfated linear glycosaminoglycan. It is distributed widely throughout connective, neural, and epithelial tissues, including skin, where it is a major component of ECM. Therefore, hyaluronic acid has been widely used for skin tissue engineering and wound healing, and it is approved for clinical application [33].
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Hyaluronic acid stimulated infiltration of nanofibrous scaffold composed of hyaluronan, silk fibroin and PCL [75], and can help to promote cell proliferation [129]. Electrospinning of pure hyaluronic acid is not simple because of solubility characteristics of this polymer. Hyaluronic acid is well-soluble in water but less-soluble in most organic solvents, which can be solved by mixtures of solvents as water/ethanol or water/dimethylformamide [130]. Increasing of evaporation and decreasing of solution surface tension by the solvent mixing helps to electrospinning process. Another possibility is electrospinning of hyaluronic acid together with a suitable water-soluble polymer such as PVA [131] or PEO. The solution of pure hyaluronic acid [132] or with relatively small amount of carrier PEO was successfully spun into nanofibrous material by air-assisted electrospinning technology, that is, electroblowing [133]. For creation of nanofibrous scaffolds, hyaluronic acid was also used in combination with PCL [134], PLA [135] or gelatin, chondroitin sulfate and sericin [86].
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\nSulfated glycosaminoglycans have been relatively rarely used as components of nanofibers for skin tissue engineering and wound healing in comparison with hyaluronic acid. This group of polysaccharides include heparin, heparan sulfate, chondroitin sulfate, dermatan sulfate, and keratan sulfate (for a review, see [33]). From these polysaccharides, only heparin, heparan sulfate, and chondroitin sulfate were used as components of nanofibers for skin regenerative therapies. For example, heparin coatings on PLLA nanofibers increased the infiltration of the scaffolds with endothelial cells in vitro, and enhanced epidermal skin cell migration across the wound in a full-thickness dermal wound model in rats in vivo [136]. In a recent study by Yergoz et al. [137], a heparin-like nanofibrous hydrogel promoted regeneration of full thickness burn injury in mice. Chondroitin sulfate was used as a component of electrospun gelatin/PVA/chondroitin sulfate nanofibrous scaffolds, which supported the proliferation of human dermal fibroblasts [138], of electrospun nanofibrous composite scaffolds made of cationic gelatin/hyaluronan/chondroitin sulfate loaded with sericin, which promoted the differentiation of human mesenchymal stem cells toward epithelial lineage [86], and of electrospun gelatin/chondroitin sulfate nanofibrous scaffolds, which accelerated healing of full-thickness skin excision wounds in rats [139].
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\nChitosan is a linear polysaccharide composed of randomly distributed β-(1 → 4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). It can be prepared by alkali treatment from chitin, a poly-N-acetyl-D-glucosamine polysaccharide, the major structural component of the exoskeleton of crustaceans (crab, shrimp), and of the cell wall of fungi and yeast [140]. Chitosan is known as biocompatible, antimicrobial, and biodegradable. In the human organism, it can be degraded by lysozyme, a hydrolytic enzyme present in various secretions such as saliva, tears, mucus, and human milk, and also in cytoplasmic granules of macrophages and polymorphonuclear neutrophils. Chitosan breakdown by lysozymes happens via the removal of glycosidic bonds between polysaccharide units in the polymer. Glucosamine and saccharide are the products of this process, which can be metabolized or stored as proteoglycans in the body.
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However, electrospinning of chitosan is difficult due to its polycationic characters. Due to the presence of amine groups in the chitosan molecule, acidic aqueous solutions are the best solvents for this polymer. The best candidates for solvent system seem to be mixture of acetic acid (AA) and formic acid (FA) or trifluoroacetic acid (TFA); however, TFA is highly toxic. Electrospinning of pure chitosan has very low productivity because it requires very concentrated polymeric solutions [141]. Therefore, for creation of nanofibrous scaffolds for skin tissue engineering, chitosan has been mixed with other natural or synthetic polymers, such as collagen [142], gelatin [143], keratin [116], cellulose [144], pectin [54, 55], silk fibroin [69], PHBV [145], PCL [142], PLA [146], PLGA [147], PEO, [148], and also with PVA, which was used in our studies (\nFigure 3\n). Chitosan has also been mixed with various nanoparticles, such as halloysite nanotubes [149], graphene oxide [150] or nanodiamonds [144]. The reason of all these mixtures was to improve the stability, spinnability, wettability, mechanical properties, and biofunctionality of chitosan-containing scaffolds for skin tissue engineering. Combination of chitosan with various polymers also enabled creation of bilayer scaffolds for reconstruction of two main skin layers, that is, epidermis containing keratinocytes and dermis containing fibroblasts [116, 142]. In order to enhance the antimicrobial and wound healing activity of chitosan, this polymer was electrospun together with extract from Henna leaves [151]. In addition, chitosan nanoparticles have been incorporated in nanofibrous scaffolds as carriers for controlled drug delivery, for example, delivery of growth factors, cytokines and angiogenic factors, such as platelet-derived growth factor [152], granulocyte colony-stimulating factor [153] or angiogenin [147]. Nanofibrous scaffolds promising for skin tissue engineering and wound healing were also prepared directly from chitin, which was electrospun either alone with further modifications with fibronectin, laminin and particularly with type I collagen [154], or in combination with silk fibroin [70].
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Figure 3.
Scanning electron microscopy of nanofibrous layers produced by needle electrospinning from PVA/chitosan solution.
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\nGellan gum is a water-soluble anionic polysaccharide produced by the bacterium Sphingomonas elodea (formerly Pseudomonas elodea). The repeating unit of the polymer is a tetrasaccharide, which consists of two residues of D-glucose, one of residue of L-rhamnose and one residue of D-glucuronic acid. For skin tissue engineering and wound healing, gellan gum was electrospun with PVA in order to decrease its viscosity and repulsive forces between the polyanions along the polymer chains and to increase the stability, uniformity, and structural consistency of the nanofibers in aqueous environment. The nanofibrous scaffolds were further stabilized by crosslinking with various physical, chemical, and ionic methods, such as by heat, UV irradiation, methanol, glutaraldehyde, and by calcium chloride [155]. These scaffolds supported the adhesion and growth of human dermal 3T3-L1 fibroblasts [155, 156] and human HaCaT keratinocytes [157] and provided a better support for these cells than conventionally proposed gellan-based hydrogels and dry films. In addition, these scaffolds were endowed with antimicrobial activity by incorporation with amoxicillin, and accelerated healing of full-thickness skin excision wound in rats in comparison with non-treated wounds [157]. Similarly as chitosan, gellan gum has been reported to be degradable by lysozyme [158]. Three-dimensional printed gellan gum scaffolds also showed degradation in vitro in phosphate-buffered saline (PBS) or in simulated body fluid, and the degradation rate could be modulated by changing the ratio of surface area per mass of the scaffolds [159].
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\nZein is the major storage protein of corn, composed of amino acids such as leucine, glutamic acid, alanine and proline, and showing good biocompatibility, flexibility, microbial resistance, and antioxidant activity [61]. Zein has been shown to degradable in vitro in PBS and also in vivo when implanted subcutaneously in rats in the form of rod-like implants [160]. However, similarly as in many other natural polymers, the application of pure zein nanofibers is limited because of poor mechanical properties of these fibers. Therefore, for skin tissue engineering and wound dressing applications, zein has been mixed with various synthetic and nature-derived polymers, such as polyurethane [161], PLA [162], PCL, hyaluronic acid, chitosan [163], and polydopamine [164], and impregnated with TiO2 nanoparticles [164] or Ag nanoparticles [161] in order to enhance the antimicrobial activity of the scaffolds.
\n
\nPoly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), is a polymer produced naturally by bacteria as a storage compound under growth-limiting conditions. It is a thermoplastic linear aliphatic polyester of polyhydroxyalkanoate type. PHBV is approved by the FDA for medical use. PHBV is biodegradable by bacterial enzymes, but it is also susceptible to hydrolytic degradation in water environment, although this degradation is relatively slow. When degradation of porous PHBV scaffolds for tissue engineering was simulated in vitro in PBS at 37°C, it lasted several months [165]. In the human body, however, the degradation of PHBV can be accelerated by nonspecific esterase and lysozyme enzymes, both present in cells of the immune system (for a review, see [165]). For biomedical application, PHBV is often used as an alternative to synthetic polymers, but it has several drawbacks, such as relatively high cost, brittleness, relatively difficult processing, and also hydrophobicity, which can hamper the cell adhesion and growth. However, PHBV is piezoelectric, which can stimulate the adhesion, growth, and phenotypic maturation of cells. Pure electrospun PHBV meshes supported the adhesion, growth, and epidermal differentiation of bone marrow mesenchymal stem cells, which was induced by an appropriate composition of cell culture media, containing epidermal growth factor, insulin, 3,3′,5-triiodo-L-thyronine (T3), hydrocortisone, and 1α, 25-dihydroxyvitamin (D3), and manifested by expression of genes for keratin, filaggrin, and involucrin, that is, an early, intermediate and late marker of keratinocyte differentiation, respectively [166]. In order to increase the attractiveness of electrospun PHBV nanofibers for the cell adhesion and growth, they were coated with collagen [167] blended with collagen [168], blended with chitosan [145] or blended with keratin [169]. Collagen-coated PHBV nanofibers alone or seeded with unrestricted somatic stem cells, isolated from umbilical cord, accelerated closure of excision wounds in rats in vivo compared to unmodified PHBV nanofibers [167]. Similar wound healing effect was also obtained with PHBV nanofibers blended with keratin [169]. Mechanical properties of PHBV nanofibers were improved by addition of graphene oxide nanoparticles in the electrospinning solution, which also endowed these fibers with and antimicrobial activity [167].
\n
\n
\n
4. Conclusions
\n
Nanofibrous scaffolds made of nature-derived polymers hold a great promise for skin tissue engineering and wound healing. These scaffolds are created from biological matrices, and from this point of view, they resemble the extracellular matrix more closely than synthetic polymers. Some of these polymers, such as collagen, gelatin, elastin, keratin, nonsulfated and sulfated glycosaminoglycans, and also nonmulberry silk fibroin, contain motifs that are recognized and bound by cell adhesion receptors. Therefore, nature-derived polymers can increase the bioactivity of synthetic polymers, when combined with them in nanofibrous scaffolds. Conversely, synthetic polymers can improve the electrospinnability and mechanical properties of the natural polymers. Similarly as synthetic polymers, nature-derived polymers can be more or less degradable in human tissues. Degradable polymers include collagen, gelatin, elastin, keratin, glycosaminoglycans, but also chitosan, gellan gum and PHBV, that is, polymers produced by other than mammalian organisms. Polymers produced by other organisms, such as bacteria, fungi, algae, plants or insects, are usually nondegradable in human tissues, or their degradability is limited due to lack of appropriate enzymes. These polymers include glucans, such as cellulose or dextran, and other polysaccharides and proteins, such as pullulan, alginate, pectin, and silk fibroin. Well-degradable polymers are recommended as direct scaffolds for tissue engineering, while less-degradable polymers are suitable for “intelligent” wound dressing for drug delivery and cell delivery.
\n
\n
Acknowledgments
\n
This review article was supported by the Grant Agency of the Czech Republic (grants No. 17-02448S and 17-00885S).
\n
\n',keywords:"skin replacements, wound dressings, nanofibers, electrospinning, epidermis, dermis, keratinocytes, fibroblasts, stem cells, vascularization, cell delivery, drug delivery, regenerative medicine",chapterPDFUrl:"https://cdn.intechopen.com/pdfs/68698.pdf",chapterXML:"https://mts.intechopen.com/source/xml/68698.xml",downloadPdfUrl:"/chapter/pdf-download/68698",previewPdfUrl:"/chapter/pdf-preview/68698",totalDownloads:604,totalViews:0,totalCrossrefCites:0,dateSubmitted:"May 5th 2019",dateReviewed:"July 15th 2019",datePrePublished:"August 21st 2019",datePublished:"May 6th 2020",dateFinished:"August 21st 2019",readingETA:"0",abstract:"Nanofibrous scaffolds belong to the most suitable materials for tissue engineering, because they mimic the fibrous component of the natural extracellular matrix. This chapter is focused on the application of nanofibers in skin tissue engineering and wound healing, because the skin is the largest and vitally important organ in the human body. Nanofibrous meshes can serve as substrates for adhesion, growth and differentiation of skin and stem cells, and also as an antimicrobial and moisture-retaining barrier. These meshes have been prepared from a wide range of synthetic and nature-derived polymers. This chapter is focused on the use of nature-derived polymers. These polymers have good or limited degradability in the human tissues, which depends on their origin and on the presence of appropriate enzymes in the human tissues. Non-degradable and less-degradable polymers are usually produced in bacteria, fungi, algae, plants or insects, and include, for example, cellulose, dextran, pullulan, alginate, pectin and silk fibroin. Well-degradable polymers are usually components of the extracellular matrix in the human body or at least in other vertebrates, and include collagen, elastin, keratin and hyaluronic acid, although some polymers produced by non-vertebrate organisms, such as chitosan or poly(3-hydroxybutyrate-co-3-hydroxyvalerate), are also degradable in the human body.",reviewType:"peer-reviewed",bibtexUrl:"/chapter/bibtex/68698",risUrl:"/chapter/ris/68698",signatures:"Lucie Bacakova, Julia Pajorova, Marketa Zikmundova, Elena Filova, Petr Mikes, Vera Jencova, Eva Kuzelova Kostakova and Alla Sinica",book:{id:"9048",title:"Current and Future Aspects of Nanomedicine",subtitle:null,fullTitle:"Current and Future Aspects of Nanomedicine",slug:"current-and-future-aspects-of-nanomedicine",publishedDate:"May 6th 2020",bookSignature:"Islam Ahmed Hamed Khalil",coverURL:"https://cdn.intechopen.com/books/images_new/9048.jpg",licenceType:"CC BY 3.0",editedByType:"Edited by",isbn:"978-1-78985-870-9",printIsbn:"978-1-78985-869-3",pdfIsbn:"978-1-78984-438-2",editors:[{id:"226598",title:"Dr.",name:"Islam",middleName:null,surname:"Khalil",slug:"islam-khalil",fullName:"Islam Khalil"}],productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"}},authors:[{id:"179175",title:"Dr.",name:"Lucie",middleName:null,surname:"Bacakova",fullName:"Lucie Bacakova",slug:"lucie-bacakova",email:"lucie.bacakova@fgu.cas.cz",position:null,institution:{name:"Palacký University, Olomouc",institutionURL:null,country:{name:"Czech Republic"}}},{id:"188643",title:"MSc.",name:"Julia",middleName:null,surname:"Pajorova",fullName:"Julia Pajorova",slug:"julia-pajorova",email:"Julia.Pajorova@fgu.cas.cz",position:null,institution:null},{id:"246891",title:"Dr.",name:"Elena",middleName:null,surname:"Filova",fullName:"Elena Filova",slug:"elena-filova",email:"Elena.Filova@fgu.cas.cz",position:null,institution:null},{id:"309511",title:"Dr.",name:"Marketa",middleName:null,surname:"Zikmundova",fullName:"Marketa Zikmundova",slug:"marketa-zikmundova",email:"Marketa.Zikmundova@fgu.cas.cz",position:null,institution:null},{id:"309513",title:"Dr.",name:"Petr",middleName:null,surname:"Mikes",fullName:"Petr Mikes",slug:"petr-mikes",email:"petr.mikes@tul.cz",position:null,institution:{name:"Technical University of Liberec",institutionURL:null,country:{name:"Czech Republic"}}},{id:"309514",title:"Dr.",name:"Vera",middleName:null,surname:"Jencova",fullName:"Vera Jencova",slug:"vera-jencova",email:"vera.jencova@tul.cz",position:null,institution:null},{id:"309515",title:"Dr.",name:"Eva",middleName:null,surname:"Kuzelova Kostakova",fullName:"Eva Kuzelova Kostakova",slug:"eva-kuzelova-kostakova",email:"Eva.Kostakova@seznam.cz",position:null,institution:null},{id:"309516",title:"Dr.",name:"Alla",middleName:null,surname:"Sinica",fullName:"Alla Sinica",slug:"alla-sinica",email:"Alla.Sinica@vscht.cz",position:null,institution:null}],sections:[{id:"sec_1",title:"1. 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Antibacterial performance and in vivo diabetic wound healing of curcumin loaded gum tragacanth/poly(ε-caprolactone) electrospun nanofibers. Materials Science & Engineering. C, Materials for Biological Applications. 2016;69:1183-1191. DOI: 10.1016/j.msec.2016.08.032\n'},{id:"B61",body:'\nPedram Rad Z, Mokhtari J, Abbasi M. Calendula officinalis extract/PCL/Zein/gum arabic nanofibrous bio-composite scaffolds via suspension, two-nozzle and multilayer electrospinning for skin tissue engineering. International Journal of Biological Macromolecules. 2019;135:530-543. DOI: 10.1016/j.ijbiomac.2019.05.204\n'},{id:"B62",body:'\nSrivastava CM, Purwar R. Fabrication of robust Antheraea assama fibroin nanofibrous mat using ionic liquid for skin tissue engineering. Materials Science & Engineering. C, Materials for Biological Applications. 2016;68:276-290. DOI: 10.1016/j.msec.2016.05.020\n'},{id:"B63",body:'\nSrivastava CM, Purwar R, Gupta AP. Enhanced potential of biomimetic, silver nanoparticles functionalized Antheraea mylitta (tasar) silk fibroin nanofibrous mats for skin tissue engineering. International Journal of Biological Macromolecules. 2019;130:437-453. DOI: 10.1016/j.ijbiomac.2018.12.255\n'},{id:"B64",body:'\nChouhan D, Chakraborty B, Nandi SK, Mandal BB. Role of non-mulberry silk fibroin in deposition and regulation of extracellular matrix towards accelerated wound healing. Acta Biomaterialia. 2017;48:157-174. DOI: 10.1016/j.actbio.2016.10.019\n'},{id:"B65",body:'\nCao Y, Wang B. Biodegradation of silk biomaterials. International Journal of Molecular Sciences. 2009;10(4):1514-1524. DOI: 10.3390/ijms10041514\n'},{id:"B66",body:'\nWang Y, Rudym DD, Walsh A, Abrahamsen L, Kim HJ, Kim HS, et al. In vivo degradation of three-dimensional silk fibroin scaffolds. Biomaterials. 2008;29(24-25):3415-3428. 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Biomacromolecules. 2008;9(4):1106-1116. DOI: 10.1021/bm700875a\n'},{id:"B74",body:'\nShan YH, Peng LH, Liu X, Chen X, Xiong J, Gao JQ. Silk fibroin/gelatin electrospun nanofibrous dressing functionalized with astragaloside IV induces healing and anti-scar effects on burn wound. International Journal of Pharmaceutics. 2015;479(2):291-301. DOI: 10.1016/j.ijpharm.2014.12.067\n'},{id:"B75",body:'\nLi L, Qian Y, Jiang C, Lv Y, Liu W, Zhong L, et al. The use of hyaluronan to regulate protein adsorption and cell infiltration in nanofibrous scaffolds. Biomaterials. 2012;33(12):3428-3445. DOI: 10.1016/j.biomaterials.2012.01.038\n'},{id:"B76",body:'\nSheng X, Fan L, He C, Zhang K, Mo X, Wang H. Vitamin E-loaded silk fibroin nanofibrous mats fabricated by green process for skin care application. International Journal of Biological Macromolecules. 2013;56:49-56. DOI: 10.1016/j.ijbiomac.2013.01.029\n'},{id:"B77",body:'\nFan L, Cai Z, Zhang K, Han F, Li J, He C, et al. Green electrospun pantothenic acid/silk fibroin composite nanofibers: Fabrication, characterization and biological activity. Colloids and Surfaces. B, Biointerfaces. 2014;117:14-20. DOI: 10.1016/j.colsurfb.2013.12.030\n'},{id:"B78",body:'\nLin S, Chen M, Jiang H, Fan L, Sun B, Yu F, et al. Green electrospun grape seed extract-loaded silk fibroin nanofibrous mats with excellent cytocompatibility and antioxidant effect. Colloids and Surfaces. B, Biointerfaces. 2016;139:156-163. DOI: 10.1016/j.colsurfb.2015.12.001\n'},{id:"B79",body:'\nKandhasamy S, Arthi N, Arun RP, Verma RS. Synthesis and fabrication of novel quinone-based chromenopyrazole antioxidant-laden silk fibroin nanofibers scaffold for tissue engineering applications. Materials Science & Engineering. C, Materials for Biological Applications. 2019;102:773-787. DOI: 10.1016/j.msec.2019.04.076\n'},{id:"B80",body:'\nSuganya S, Venugopal J, Ramakrishna S, Lakshmi BS, Dev VR. 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Antimicrobial quaternary ammonium organosilane cross-linked nanofibrous collagen scaffolds for tissue engineering. International Journal of Nanomedicine. 2018;13:4473-4492. DOI: 10.2147/IJN.S159770\n'},{id:"B98",body:'\nRavichandran R, Venugopal JR, Sundarrajan S, Mukherjee S, Sridhar R, Ramakrishna S. Composite poly-L-lactic acid/poly-(α,β)-DL-aspartic acid/collagen nanofibrous scaffolds for dermal tissue regeneration. Materials Science & Engineering. C, Materials for Biological Applications. 2012;32(6):1443-1451. DOI: 10.1016/j.msec.2012.04.024\n'},{id:"B99",body:'\nAlamein MA, Stephens S, Liu Q , Skabo S, Warnke PH. Mass production of nanofibrous extracellular matrix with controlled 3D morphology for large-scale soft tissue regeneration. Tissue Engineering. Part C, Methods. 2013;19(6):458-472. DOI: 10.1089/ten.TEC.2012.0417\n'},{id:"B100",body:'\nPeh P, Lim NS, Blocki A, Chee SM, Park HC, Liao S, et al. 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Lithospermi radix extract-containing bilayer nanofiber scaffold for promoting wound healing in a rat model. Materials Science & Engineering. C, Materials for Biological Applications. 2019;96:850-858. DOI: 10.1016/j.msec.2018.11.053\n'},{id:"B108",body:'\nAldana AA, Abraham GA. Current advances in electrospun gelatin-based scaffolds for tissue engineering applications. International Journal of Pharmaceutics. 2017;523(2):441-453. DOI: 10.1016/j.ijpharm.2016.09.044\n'},{id:"B109",body:'\nYao CH, Lee CY, Huang CH, Chen YS, Chen KY. Novel bilayer wound dressing based on electrospun gelatin/keratin nanofibrous mats for skin wound repair. Materials Science & Engineering. C, Materials for Biological Applications. 2017;79:533-540. DOI: 10.1016/j.msec.2017.05.076\n'},{id:"B110",body:'\nAdeli-Sardou M, Yaghoobi MM, Torkzadeh-Mahani M, Dodel M. Controlled release of lawsone from polycaprolactone/gelatin electrospun nano fibers for skin tissue regeneration. 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DOI: 10.1016/j.carbpol.2018.11.085\n'},{id:"B132",body:'\nUm IC, Fang D, Hsiao BS, Okamoto A, Chu B. Electro-spinning and electro-blowing of hyaluronic acid. Biomacromolecules. 2004;5(4):1428-1436. DOI: 10.1021/bm034539b\n'},{id:"B133",body:'\nPokorny M, Rassushin V, Wolfova L, Velebny V. Increased production of nanofibrous materials by electroblowing from blends of hyaluronic acid and polyethylene oxide. Polymer Engineering and Science. 2016;56:932-938. DOI: 10.1002/pen.24322\n'},{id:"B134",body:'\nQian Y, Li L, Jiang C, Xu W, Lv Y, Zhong L, et al. The effect of hyaluronan on the motility of skin dermal fibroblasts in nanofibrous scaffolds. International Journal of Biological Macromolecules. 2015;79:133-143. DOI: 10.1016/j.ijbiomac.2015.04.059\n'},{id:"B135",body:'\nStodolak-Zych E, Rozmus K, Dzierzkowska E, Zych Ł, Rapacz-Kmita A, Gargas M, et al. The membrane with polylactide and hyaluronic fibers for skin substitute. Acta of Bioengineering and Biomechanics. 2018;20(4):91-99. DOI: 10.5277/ABB-01199-2018-02\n'},{id:"B136",body:'\nKurpinski KT, Stephenson JT, Janairo RR, Lee H, Li S. The effect of fiber alignment and heparin coating on cell infiltration into nanofibrous PLLA scaffolds. Biomaterials. 2010;31(13):3536-3542. DOI: 10.1016/j.biomaterials.2010.01.062\n'},{id:"B137",body:'\nYergoz F, Hastar N, Cimenci CE, Ozkan AD, Tekinay T, Guler MO, et al. Heparin mimetic peptide nanofiber gel promotes regeneration of full thickness burn injury. Biomaterials. 2017;134:117-127. DOI: 10.1016/j.biomaterials.2017.04.040\n'},{id:"B138",body:'\nSadeghi A, Zandi M, Pezeshki-Modaress M, Rajabi S. Tough, hybrid chondroitin sulfate nanofibers as a promising scaffold for skin tissue engineering. International Journal of Biological Macromolecules. 2019;132:63-75. DOI: 10.1016/j.ijbiomac.2019.03.208. 27\n'},{id:"B139",body:'\nPezeshki-Modaress M, Mirzadeh H, Zandi M, Rajabi-Zeleti S, Sodeifi N, Aghdami N, et al. Gelatin/chondroitin sulfate nanofibrous scaffolds for stimulation of wound healing: In-vitro and in-vivo study. Journal of Biomedical Materials Research. Part A. 2017;105(7):2020-2034. DOI: 10.1002/jbm.a.35890\n'},{id:"B140",body:'\nAzuma K, Ifuku S, Osaki T, Okamoto Y, Minami S. Preparation and biomedical applications of chitin and chitosan nanofibers. Journal of Biomedical Nanotechnology. 2014;10(10):2891-2920. DOI: 10.1166/jbn.2014.1882\n'},{id:"B141",body:'\nKai D, Liow SS, Loh XJ. Biodegradable polymers for electrospinning: towards biomedical applications. Materials Science and Engineering C: Materials for Biological Applications. 2014;45:659-670. DOI: 10.1016/j.msec.2014.04.051\n'},{id:"B142",body:'\nPal P, Dadhich P, Srivas PK, Das B, Maulik D, Dhara S. Bilayered nanofibrous 3D hierarchy as skin rudiment by emulsion electrospinning for burn wound management. Biomaterials Science. 2017;5(9):1786-1799. DOI: 10.1039/c7bm00174f\n'},{id:"B143",body:'\nGomes S, Rodrigues G, Martins G, Henriques C, Silva JC. Evaluation of nanofibrous scaffolds obtained from blends of chitosan, gelatin and polycaprolactone for skin tissue engineering. International Journal of Biological Macromolecules. 2017;102:1174-1185. DOI: 10.1016/j.ijbiomac.2017.05.004\n'},{id:"B144",body:'\nMahdavi M, Mahmoudi N, Rezaie Anaran F, Simchi A. Electrospinning of nanodiamond-modified polysaccharide nanofibers with physico-mechanical properties close to natural skins. Marine Drugs. 2016;14(7). pii: E128. DOI: 10.3390/md14070128\n'},{id:"B145",body:'\nVeleirinho B, Coelho DS, Dias PF, Maraschin M, Ribeiro-do-Valle RM, Lopes-da-Silva JA. Nanofibrous poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/chitosan scaffolds for skin regeneration. International Journal of Biological Macromolecules. 2012;51(4):343-350. DOI: 10.1016/j.ijbiomac.2012.05.023\n'},{id:"B146",body:'\nShalumon KT, Sathish D, Nair SV, Chennazhi KP, Tamura H, Jayakumar R. Fabrication of aligned poly(lactic acid)-chitosan nanofibers by novel parallel blade collector method for skin tissue engineering. Journal of Biomedical Nanotechnology. 2012;8(3):405-416. DOI: 10.1166/jbn.2012.1395\n'},{id:"B147",body:'\nMo Y, Guo R, Zhang Y, Xue W, Cheng B, Zhang Y. Controlled dual delivery of angiogenin and curcumin by electrospun nanofibers for skin regeneration. Tissue Engineering. Part A. 2017;23(13-14):597-608. DOI: 10.1089/ten.tea.2016.0268\n'},{id:"B148",body:'\nMengistu Lemma S, Bossard F, Rinaudo M. Preparation of pure and stable chitosan nanofibers by electrospinning in the presence of poly(ethylene oxide). International Journal of Molecular Sciences. 2016;17(11):E1790. DOI: 10.3390/ijms17111790\n'},{id:"B149",body:'\nKoosha M, Raoufi M, Moravvej H. One-pot reactive electrospinning of chitosan/PVA hydrogel nanofibers reinforced by halloysite nanotubes with enhanced fibroblast cell attachment for skin tissue regeneration. Colloids and Surfaces. B, Biointerfaces. 2019;179:270-279. DOI: 10.1016/j.colsurfb.2019.03.054\n'},{id:"B150",body:'\nMahmoudi N, Eslahi N, Mehdipour A, Mohammadi M, Akbari M, Samadikuchaksaraei A, et al. Temporary skin grafts based on hybrid graphene oxide-natural biopolymer nanofibers as effective wound healing substitutes: Pre-clinical and pathological studies in animal models. Journal of Materials Science. Materials in Medicine. 2017;28(5):73. DOI: 10.1007/s10856-017-5874-y\n'},{id:"B151",body:'\nYousefi I, Pakravan M, Rahimi H, Bahador A, Farshadzadeh Z, Haririan I. An investigation of electrospun henna leaves extract-loaded chitosan based nanofibrous mats for skin tissue engineering. Materials Science & Engineering. C, Materials for Biological Applications. 2017;75:433-444. DOI: 10.1016/j.msec.2017.02.076\n'},{id:"B152",body:'\nPiran M, Vakilian S, Piran M, Mohammadi-Sangcheshmeh A, Hosseinzadeh S, Ardeshirylajimi A. In vitro fibroblast migration by sustained release of PDGF-BB loaded in chitosan nanoparticles incorporated in electrospun nanofibers for wound dressing applications. Artif Cells Nanomed Biotechnol. 2018;46(supp 1):511-520. DOI: 10.1080/21691401.2018.1430698\n'},{id:"B153",body:'\nTanha S, Rafiee-Tehrani M, Abdollahi M, Vakilian S, Esmaili Z, Naraghi ZS, et al. G-CSF loaded nanofiber/nanoparticle composite coated with collagen promotes wound healing in vivo. Journal of Biomedical Materials Research. Part A. 2017;105(10):2830-2842. DOI: 10.1002/jbm.a.36135\n'},{id:"B154",body:'\nNoh HK, Lee SW, Kim JM, Oh JE, Kim KH, Chung CP, et al. Electrospinning of chitin nanofibers: Degradation behavior and cellular response to normal human keratinocytes and fibroblasts. Biomaterials. 2006;27(21):3934-3944. DOI: 10.1016/j.biomaterials.2006.03.016\n'},{id:"B155",body:'\nVashisth P, Pruthi V. Synthesis and characterization of crosslinked gellan/PVA nanofibers for tissue engineering application. Materials Science & Engineering. C, Materials for Biological Applications. 2016;67:304-312. DOI: 10.1016/j.msec.2016.05.049\n'},{id:"B156",body:'\nVashisth P, Nikhil K, Roy P, Pruthi PA, Singh RP, Pruthi V. A novel gellan-PVA nanofibrous scaffold for skin tissue regeneration: Fabrication and characterization. Carbohydrate Polymers. 2016;136:851-859. DOI: 10.1016/j.carbpol.2015.09.113\n'},{id:"B157",body:'\nVashisth P, Srivastava AK, Nagar H, Raghuwanshi N, Sharan S, Nikhil K, et al. Drug functionalized microbial polysaccharide based nanofibers as transdermal substitute. Nanomedicine. 2016;12(5):1375-1385. DOI: 10.1016/j.nano.2016.01.019\n'},{id:"B158",body:'\nBonifacio MA, Cometa S, Cochis A, Gentile P, Ferreira AM, Azzimonti B, et al. Data on Manuka Honey/Gellan gum composite hydrogels for cartilage repair. Data in Brief. 2018;20:831-839. DOI: 10.1016/j.dib.2018.08.155\n'},{id:"B159",body:'\nYu I, Kaonis S, Roland CR. A study on degradation behavior of 3D printed gellan gum scaffolds. Procedia CIRP. 2017;65:78-83. DOI: 10.1016/j.procir.2017.04.020\n'},{id:"B160",body:'\nLin T, Lu C, Zhu L, Lu T. The biodegradation of zein in vitro and in vivo and its application in implants. AAPS PharmSciTech. 2011;12(1):172-176. DOI: 10.1208/s12249-010-9565-y\n'},{id:"B161",body:'\nMaharjan B, Joshi MK, Tiwari AP, Park CH, Kim CS. In-situ synthesis of AgNPs in the natural/synthetic hybrid nanofibrous scaffolds: Fabrication, characterization and antimicrobial activities. Journal of the Mechanical Behavior of Biomedical Materials. 2017;65:66-76. DOI: 10.1016/j.jmbbm.2016.07.034\n'},{id:"B162",body:'\nZhang M, Li X, Li S, Liu Y, Hao L. Electrospun poly(l-lactide)/zein nanofiber mats loaded with Rana chensinensis skin peptides for wound dressing. Journal of Materials Science. Materials in Medicine. 2016;27(9):136. DOI: 10.1007/s10856-016-5749-7\n'},{id:"B163",body:'\nFigueira DR, Miguel SP, de Sá KD, Correia IJ. Production and characterization of polycaprolactone- hyaluronic acid/chitosan- zein electrospun bilayer nanofibrous membrane for tissue regeneration. International Journal of Biological Macromolecules. 2016;93(Pt A):1100-1110. DOI: 10.1016/j.ijbiomac.2016.09.080\n'},{id:"B164",body:'\nBabitha S, Korrapati PS. Biodegradable zein-polydopamine polymeric scaffold impregnated with TiO2 nanoparticles for skin tissue engineering. Biomedical Materials. 2017;12(5):055008. DOI: 10.1088/1748-605X/aa7d5a\n'},{id:"B165",body:'\nSultana N, Khan TH. In vitro degradation of PHBV scaffolds and nHA/PHBV composite scaffolds containing hydroxyapatite nanoparticles for bone tissue engineering. Journal of Nanomaterials. 2012;2012:190950. DOI: 10.1155/2012/190950\n'},{id:"B166",body:'\nSundaramurthi D, Krishnan UM, Sethuraman S. Epidermal differentiation of stem cells on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) nanofibers. Annals of Biomedical Engineering. 2014;42(12):2589-2599. DOI: 10.1007/s10439-014-1124-3\n'},{id:"B167",body:'\nKeshel SH, Biazar E, Rezaei Tavirani M, Rahmati Roodsari M, Ronaghi A, Ebrahimi M, et al. The healing effect of unrestricted somatic stem cells loaded in collagen-modified nanofibrous PHBV scaffold on full-thickness skin defects. Artif Cells Nanomed Biotechnol. 2014;42(3):210-216. DOI: 10.3109/21691401.2013.800080\n'},{id:"B168",body:'\nZine R, Sinha M. Nanofibrous poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/collagen/graphene oxide scaffolds for wound coverage. Materials Science & Engineering. C, Materials for Biological Applications. 2017;80:129-134. DOI: 10.1016/j.msec.2017.05.138\n'},{id:"B169",body:'\nYuan J, Geng J, Xing Z, Shim KJ, Han I, Kim J, et al. Novel wound dressing based on nanofibrous PHBV-keratin mats. Journal of Tissue Engineering and Regenerative Medicine. 2015;9(9):1027-1035. DOI: 10.1002/term.1653\n'}],footnotes:[],contributors:[{corresp:"yes",contributorFullName:"Lucie Bacakova",address:"lucie.bacakova@fgu.cas.cz",affiliation:'
Department of Biomaterials and Tissue Engineering, Institute of Physiology of the Czech Academy of Sciences, Czech Republic
BIOCEV, 1st Faculty of Medicine, Charles University, Czech Republic
Department of Analytical Chemistry, University of Chemistry and Technology Prague, Czech Republic
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Recent studies have revealed three-dimensional architecture of both human and ciliate telomerase at about 25 Å resolution, using single particle electron microscopy (EM). Telomerase supplements the tandem array of simple-sequence repeats at chromosome ends to compensate for the DNA erosion inherent in genome replication which makes it to be distinct among polymerases. Telomeres are found at the end of eukaryotic linear chromosomes and proteins that bind to them and help to protect DNA from being recognized as double-strand breaks thus preventing end-to-end fusions. The activity of telomerase is tightly regulated at multiple levels of cellular development, from transcriptional regulation of the telomerase components to holoenzyme biogenesis and recruitment to the telomere site for activation and processing. Commonly used methods in telomere biology are telomere restriction fragment (TRF), telomere repeat amplification protocol (TRAP) and telomere dysfunction induced foci (TIF) analysis. This chapter summarizes our current knowledge on the mechanisms of telomerase recruitment and activation using insights from studies in mammals and budding and fission yeasts. 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AFFILIATION
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Authors are responsible for ensuring all addresses and emails provided are correct. Under affiliation(s) all Authors should indicate where the research was conducted. Please note that no changes to the affiliation(s) can be made after the chapter has been published.
Substantially contribute to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work
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Participate in drafting or revising the work
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Approve the final version of the work to be published.
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All contributors who meet these criteria are listed as Authors. Their exact contributions should be described in the manuscript at the time of submission.
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Conversely, all contributors who do not meet these criteria should be listed in the Acknowledgments section of the manuscript, along with a short description of their specific contributions.
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CHANGES IN AUTHORSHIP
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If it is felt necessary to make changes to the list of Authors after a manuscript has been submitted or published, it is the responsibility of the Author concerned to provide a valid reason to amend the published list. Additionally, all listed Authors must verify and approve the proposed changes in order for any amendments to be made.
\n\n
AFFILIATION
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Authors are responsible for ensuring all addresses and emails provided are correct. Under affiliation(s) all Authors should indicate where the research was conducted. Please note that no changes to the affiliation(s) can be made after the chapter has been published.
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Policy last updated: 2017-05-29
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