Shielding effectiveness and attenuation %.
\r\n\tComputational fluid dynamics is composed of turbulence and modeling, turbulent heat transfer, fluid-solid interaction, chemical reactions and combustion, the finite volume method for unsteady flows, sports engineering problem and simulations - Aerodynamics, fluid dynamics, biomechanics, blood flow.
",isbn:"978-1-83968-248-3",printIsbn:"978-1-83968-247-6",pdfIsbn:"978-1-83968-321-3",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!0,hash:"1f8fd29e4b72dbfe632f47840b369b11",bookSignature:"Dr. Suvanjan Bhattacharyya",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/10695.jpg",keywords:"Free Turbulent Flow, Discretisation Methods, Aerodynamics, Phase Flow, Bluff-Body, Complex Geometries, Drag Force, Flow Separation, Laminar Diffusion Flame, Non-Premixed Combustion, Fluid Dynamics, Biomechanics",numberOfDownloads:null,numberOfWosCitations:0,numberOfCrossrefCitations:null,numberOfDimensionsCitations:null,numberOfTotalCitations:null,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"January 28th 2021",dateEndSecondStepPublish:"February 25th 2021",dateEndThirdStepPublish:"April 26th 2021",dateEndFourthStepPublish:"July 15th 2021",dateEndFifthStepPublish:"September 13th 2021",remainingDaysToSecondStep:"3 days",secondStepPassed:!0,currentStepOfPublishingProcess:3,editedByType:null,kuFlag:!1,biosketch:"Dr. Suvanjan Bhattacharyya is currently working as an Assistant Professor in the Department of Mechanical Engineering of BITS Pilani, Pilani Campus. His research interest lies in computational fluid dynamics, experimental heat transfer enhancement, solar energy, renewable energy, etc.",coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"233630",title:"Dr.",name:"Suvanjan",middleName:null,surname:"Bhattacharyya",slug:"suvanjan-bhattacharyya",fullName:"Suvanjan Bhattacharyya",profilePictureURL:"https://mts.intechopen.com/storage/users/233630/images/system/233630.png",biography:"Dr. Suvanjan Bhattacharyya is currently working as an Assistant Professor in the Department of Mechanical Engineering of BITS Pilani, Pilani Campus, India. Dr. Bhattacharyya completed his post-doctoral research at the Department of Mechanical and Aeronautical Engineering, University of Pretoria, South Africa. Dr. Bhattacharyya completed his Ph.D. in Mechanical Engineering from Jadavpur University, Kolkata, India and with the collaboration of Duesseldorf University of Applied Sciences, Germany. He received his Master’s degree from the Indian Institute of Engineering, Science and Technology, India (Formerly known as Bengal Engineering and Science University), on Heat-Power Engineering.\nHis research interest lies in computational fluid dynamics in fluid flow and heat transfer, specializing on laminar, turbulent, transition, steady, unsteady separated flows and convective heat transfer, experimental heat transfer enhancement, solar energy and renewable energy. He is the author and co-author of 107 papers in high ranked journals and prestigious conference proceedings. He has bagged the best paper award in a number of international conferences as well. 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The oscillations of the electric field and the magnetic field are perpendicular to each other and they are also perpendicular to the direction of EM waves propagation. EM waves travel with a constant velocity of 3.0 × 108 m/s in vacuum. Unlike mechanical waves (sound waves) which need a medium to travel, EM waves can travel through anything, such as air, water, a solid material or vacuum. EM radiation refers to the EM waves, propagating through space–time, carrying EM radiant energy [1]. It is a form of energy that is all around us. Human activities like using global positioning system (GPS) device to navigate precise location, heating up a food in a microwave or using X-rays detection by a doctor would be impossible without EM radiation. Figure 1 shows the EM spectrum used to describe different types of EM energy according to their frequencies (or wavelengths). The EM spectrum ranges from lower energy waves (longer wavelength), like radio waves and microwaves, to higher energy waves (shorter wavelength), like X-rays and gamma rays. As for the radiated emission which is focused on in this chapter, the frequency locates in the radio frequency spectrum (3 KHz–300 GHz).
\nA diagram of the EM spectrum showing various properties across the range of frequencies and wavelengths.
Electromagnetic interference (EMI) is a disturbance generated by conduction or external radiation that affects an electrical circuit. The interference emission sources are from the conducted emission (several KHz–30 MHz) to the radiated emission (30 MHz–12 GHz) [2]. The conducted emission is the noise which is internally generated from the poor designed electrical circuit such as electrical cables and power wires. The radiated emission that is externally generated is in the form of transmitting EM waves such as the intended EM radiation from the radio broadcasting antenna and the unintended EM radiation from the high-speed transceivers. While detecting the EMI shielding of the device, it is usually relevant to the radiated emission lonely. The conducted emission is another subject especially for the noise prevention in system level.
\nEMI is encountered by all of us in our daily life and are expected to face exponential rise in future due to the growing numbers of wireless devices and standards, including cell phones, GPS, Bluetooth, Wi-Fi and near-field communication (NFC). Great effort has been dedicated for the development of EMI shielding materials. EMI shielding can be achieved by prevention of EM waves passing through an electric system either by reflection or by absorption of the incident radiation power. In the past, metals were conveniently used in many occasions. Among them, galvanized steel and aluminum are the most widely used. Copper, nickel, pre-tin plated steel, zinc and silver are also used for some purposes. When the trend in today’s electronic devices become faster, smaller and lighter, metals are disadvantageous in weight consideration. Moreover, the EM pollution is not truly eliminated or mitigated since the EM signals are almost completely reflected at the surface of the metal protecting the environment only beyond the shield [3]. Hence, intensive research efforts have been focused on the development of EMI shielding materials that work by tunable reflection and absorption based on novel materials that possess lightness, corrosion resistance, flexibility, easy processing, etc.
\nThis chapter is divided into two sections. In the next section, we will describe the EMI shielding theory in details and the parameters that influence the shielding by reflection and absorption. After that, we introduce three categories of lightweight EMI shielding materials, namely, polymer-based composites, foams and aerogels.
\nThe EMI capability of a material is called shielding effectiveness (SE). It is defined in terms of the ratio between the incoming power (Pi) and outgoing power (Po) of an EM wave as [4]:
\nThe unit of EMI SE is given in decibels (dB). According to Eq. (1), how much attenuation is blocked at given SE is given in Table 1.
\nSE (dB) | \n20 | \n30 | \n40 | \n50 | \n60 | \n70 | \n
Attenuation % | \n99 | \n99.9 | \n99.99 | \n99.999 | \n99.9999 | \n99.9999 | \n
Shielding effectiveness and attenuation %.
According to the distance r between the radiating source and the shield material, an EM wave can be divided into near field wave and far field wave relative to the total wavelength λ of the EM wave. As shown in Figure 2, the region within the distance r > λ/2π is the far field while the distance r < λ/2π is the near field.
\nWave impedance in far field and near field [5].
In far field, the EM waves can be regarded as plane waves and EMI should consider both electric field (E) and magnetic field (H) effects. It fulfills the conditions as follows,
\nwhere Z is the intrinsic impedance or what is sometimes called wave impedance. |E| and |H| are the electric and magnetic fields’ amplitudes, respectively. For air (σ = 0, μ = μ0, ε = ε0), the wave impedance (Z0) is always equal to 377 Ω and can be expressed as
\nwhere σ is the electrical conductivity, μr is the relative permeability (μ = μ0μr), μ0 is the permeability of air (4π × 10−7 H/m), ε0 is the permittivity of air (8.85 × 10−12 F/m).
\nIn near field, the wave front is curved instead of planar, so the wave front is not parallel to the surface of the shielding material. In this case, the wave impedance (|E|/|H|) is not constant and depends on the distance and the dominant field. For an electrical radiation source, the electrical field dominates. The wave impedance is higher than 377 Ω and decreases as the distance r increases. It can be expressed as [5].
\nFor a magnetic radiation source, the near field is mainly magnetic. The wave impedance is lower than 377 Ω and increases as the distance r increases, it can be expressed as [5].
\nIn this chapter, all the formulations and results are taken based on far field condition because a distance of 48 cm associated with operating at a frequency of 100 MHz is already considered as far field.
\nFigure 3a illustrates the reflection and transmission of an EM wave when it strikes on a shield material. The uniform EM wave with the electric field Ei and magnetic field Hi is normal incident to the material. When the EM wave strikes the left boundary of the material, portions of the EM wave are reflected in the opposite direction with the electric field Er and magnetic field Hr. Other portions of the EM wave are transmitted though the material with the electric field Et and magnetic field Ht. The electric field SE can be expressed as:
\n(a) Schematic illustration of EM plane wave is normal incident to a material with thickness t and (b) schematic illustration of attenuation of an incident EM wave by a shield material (thickness of shield material = t).
The magnetic field SE can be expressed as:
\nTheoretically, the SE of a material is contributed from three mechanisms including reflection, absorption and multiple-reflections., the materials with mobile charge carriers (electrons or holes) can interact with the incoming EM wave to facilitate reflection. Absorption depends on the thickness of the shield materials. It increases with the increase of the thickness of the shield materials. For significant absorption, the shield materials possess electric and/or magnetic dipoles which could then interact with the EM fields. Multiple-reflections is the third shielding mechanism, which operates via the internal reflections within the shield material. Therefore, the overall SE is the sum of all the three terms:
\nThe EMI SE of the material depends on the distance between radiation source and the shielding material. When the radiation source is far from the shielding material, the SE is called as far field SE. In the case of the short distance between radiation source and the shielding material, the SE is called as near field SE.
\nFigure 3b illustrates three EMI shielding mechanisms in a conductive shield material. When an EM wave strikes the left boundary of the homogenous conductive material, a reflected wave and a transmitted wave will be created at the left external and right external surface, respectively. As the transmitted wave propagates within the shield material, the amplitude of the wave exponentially decreases as a result from absorption, and the energy loss due to the absorption will be dissipated as heat [6]. Once the transmitted wave reaches the internal right surface of the shield (t), a portion of wave continues to transmit from the shield material and a portion will be reflected into the shield material. The portion of internal reflected wave will be re-reflected within the shield material, which represents the multiple-reflections mechanism. The skin effect would affect the effect of multiple-reflections to the overall shielding to a great extent. The depth at which the electric field drops to (1/e) of the incident strength is call the skin depth (δ), which is given as follows [7]:
\nwhere f is frequency (Hz). μ and σ are the magnetic permeability and the electrical conductivity of the shield material, respectively. If the shield is thicker than the skin depth, the multiple-reflections can be ignored. However, the effect of multiple-reflections will be significant as the shield is thinner than the skin depth.
\nAs shown in Figure 3b, in case the shield material is a good conductor, Zm ≪ Z0, then [8].
\nwhere \n
where ω = 2πf, σr = σ/σCu is the relative conductivity of the material, it is related to the electrical conductivity of the copper, the electrical conductivity of copper is σCu = 5.8 × 107 S/m. If the shield material possesses electric and/or magnetic dipoles, the attenuation of incident EM wave happens inside the shield material due to the absorption and multiple-reflections, the amplitude of the EM wave declines during wave traveling, and it can be expressed as [8].
\nwhere t is the thickness of the shield material, δ is the skip depth under the operation frequency, β is the propagation constant.
\nThe mechanism of multi-reflections is complicated. For a good conductor material, the multiple-reflection is usually insignificant because most of the incident EM waves are reflected from the external conductive surface of the shield material, and only few penetrated EM waves can be retained for multiple-reflections. The influence is more important for a material that has high permeability and low electrical conductivity. In this case, EM waves can easily penetrate through the external surface of the shield material and most penetrated EM waves are reflected from the second surface of the shield material. The influence is more important in low frequency and is reduced when the frequency gets higher because the ratio between material thickness and skin depth (t/δ) become larger as the frequency increases.
\nComposites are made from fillers and matrices with significantly different physical or chemical properties. Hence, EMI shielding mechanisms are more complicated than those for homogeneous shield materials because of the huge surface area available for reflection and multiple-reflections. The EMI SE of composites can be measured experimentally, and it also can be calculated theoretically. The effective relative permittivity εeff of composites, which is one of the most important parameters in the calculation, can be approximately calculated from the Maxwell Garnett formula as [9]:
\nwhere εe is the relative permittivity of the matrix, εi is the relative permittivity of the filler and f is the volume fraction of the filler. If the filler are electrical conductive particles, the relative permittivity εi can be expressed as [10]:
\nwhere ε′ and ε′′ are the real and imaginary part of the complex relative permittivity of the filler, respectively. σ is the electrical conductivity of the filler. As shown in Figure 3b, the transmission coefficient T can be expressed as [10]:
\nwhere T1 and T2 are the transmission coefficients at the boundary 0 and t, respectively. R1 and R2 are the reflection coefficients at the boundary 0 and t, respectively. γm is the complex propagation constant. The T1, T2, R1, and R2 can further be expressed in terms of the impedance Z0 and Zm [10]:
\nwhere Z0 and Zm are the impedance of the air and the composite material, respectively. Z0 can be expressed in Eq. (4) and Zm can further be expressed as:
\nThe propagation constant γm can be expressed as [10]:
\nSo, the SE can be calculated in terms of T,
\nWhen modern electronic devices are designed, high performance EMI shielding materials are highly demanded. In addition, lightweight is one additional important technical requirement for potential applications especially in the areas of automobile and aerospace. In the following section, we will briefly review state-of-the-art research work regarding polymer-based composite, foams and aerogels used for EMI shielding.
\nPolymer/conductive fillers composites was seen as a promising advanced EMI shielding materials since the discovery that an insulating polymer would allow the flow of current through the conductive network stablished by conductive fillers above the percolation threshold. The conductive composite materials preserve the advantages of lightness of polymers, low cost, design flexibility and ease of processing, and the incorporation of conductive fillers circumvent intrinsic nature of polymers being transparent to EM waves through interaction between EM wave and the conductive fillers. Metallic fillers, intrinsically conductive polymers and carbon based electrically conductive fillers are discussed in this section with specific examples. Polymer/magnetic particles composites will also be briefly introduced as magnetic portion is an important component in EM waves that should not be ignored. This section aims to provide a general overview on the preparation of polymer-based EMI shielding materials and the advantages and challenges faced by each category and possible strategies towards enhancing the EMI shielding performances.
\nMetals are typical wave-reflection materials used for EMI shielding purpose owing to their abundance in mobile charge carriers that can interact with the incident EM radiation. Metallic fillers of various physical forms, such as fibers or nanoparticles, were dispersed in the polymer matrix to increase the interaction with the incident EM radiation. Injection-molding provides a direct method to disperse metallic fillers into a polymer matrix. Stainless steel fibers (SSF) introduced into polycarbonate matrix through injection molding shown that EMI SE is heavily dependent on the molding parameters which would give an optimum electrical conductivity [11]. Blended textiles of polyester fibers with SSF showed that the EMI SE is more than 50 dB in the frequencies ranging from 30 MHz to 1.5 GHz [12] (see Figure 4a). As shown in Figure 4b and c, comparison of reflectance, absorbance and transmittance, (identified as reflectivity, absorptivity and transmissibility in Figure 4) for SSF and SSF/polyester fiber fabrics as a function of frequency revealed absorption as the dominant EMI shielding mechanism. In the case of SSF/polyester with 10 wt% SSF, EMI shielding by absorption increased from 30 MHz to maximum at 500 MHz and then decreased with the increase in frequency.
\n(a) The EMI SE of the SSF/PET fabric as a function of frequency; (b) reflectivity/absorptivity/transmissibility of SSF fabric and (c) SSF/PET fabric with 10 wt% SSF as a function of frequency [12].
The challenges in achieving a good dispersion of metallic fillers and the weight increase make polymer/metallic fillers composites a less popular choice. Much attention was switched to intrinsically conductive polymers (including polyaniline, polyacetylene, and polypyrrole), carbon-based materials (including carbon fibers, carbon black, graphite, graphene, carbon nanotubes and mesoporous carbon), and magnetic materials like carbonyl iron and ferrites (including Fe3O4 and α-Fe2O3).
\nBlends of a polymer with an intrinsically conductive polymer results in a composite combining the desired properties of the two components, that is, adequate mechanical properties of the polymer matrix for mechanical support and the electrically conducting component for interaction with the EM radiation. Conducting polymers are conjugated polymers, which on doping exhibit electronic conductivity. Distinctive to metallic fillers, the electrical conductivity of conducting polymers arises from the polymer molecular structure. Alteration of parameters such as chain size, doping level, dopant type and the synthesis route directly affect the molecular structure, hence the EMI shielding properties of the material.
\nAmong the available conducting polymers, polypyrrole (PPY) and polyaniline (PANI) are the most widely used conductive fillers for EMI shielding purposes. PPY is known to possess high conductivity, easy synthesis, good environmental stability and less toxicological problem. Chemical and electrochemical polymerization of PPY on a polyethylene terephthalate (PET) fabric is given as an example for electrically conducting composite. Pyrrole was first dissolved in an aqueous solution containing 10 wt% polyvinyl alcohol (PVA) and sprayed on the PET fabric before subject to electrochemical polymerization at room temperature under a constant current density. The resultant PPY coated PET fabric was shown to exhibit EMI SE about 36 dB over a wide frequency range up to 1.5 GHz [13].
\nPANI was studied extensively for its various structures, unique doping mechanism, excellent physical and chemical properties, stability, and the readily obtainable raw materials. Lakshmi et al. [14] prepared PANI-PU composite film by adding aniline to polyurethane (PU) solution in tetrahydrofuran (THF). Doping of composites was done by adding camphor sulfonic acid to the composite solution. The EMI SE of the PU-PANI film was found to increase with thickness and the frequency specific material is ideal for shielding at 2.2 and 8.8 GHz.
\nOther intrinsically conducting polymers, such as poly(p-phenylene-vinylene) [15, 16] and poly(3-octylthiophene) [17], were also investigated for EMI shielding applications, but too much lesser extent, mainly due to the unsatisfactory performance and complex processing procedures involved.
\nIn general, the EMI shielding performance arises by the addition of conductive polymer consequently dominated by reflection mechanism due to the increase of the level of impedance mismatch with air. One obvious advantage of such polymer-polymer system is the lightweight being preserved, also there is no issue on substrate flexibility as those associated with metallic or carbon-based fillers. However, the main drawbacks of such composites include (1) poor mechanical properties of the most of the intrinsically conducting polymers require a matrix material for structural support; (2) the insoluble and infusible characteristics caused conducting polymers to exhibit poor processability and (3) high filler (conducting polymer) level is usually needed for acceptable performances.
\nSimilar to metallic fillers, carbon-based fillers come in various shapes and aspect ratios. Carbon black (CB), including graphite and CB, is the generic name given to small particle size carbon pigments which are formed in the gas phase by thermal decomposition of hydrocarbons [18]. Carbon fibers (CFs) are 1D carbon structure of diameter generally lies between 50 and 200 nm and aspect ratios around 250 and 2000, largely produced by chemical vaporization of hydrocarbon [19, 20]. Carbon nanotubes (CNTs) can be considered as rolled-up hollow cylinders of graphene sheets of very high aspect ratio due to the small diameter, constituted of a single hollow cylinder, that is, single-walled carbon nanotubes (SWCNTs) or of a collection of graphene concentric cylinders, that is, multi-walled carbon nanotubes (MWCNTs) [21, 22]. Graphene sheet (GS), an atomically thick two-dimensional structure, exhibited excellent mechanical, thermal and electrical properties [23]. Both CNTs and graphene offer substantial advantages over conventional carbon fillers and the percolation threshold can be achieved by both at very low content if properly dispersed.
\nIn general, carbon fillers with high aspect ratio are generally more effective in imparting electrical conductivities to a polymer matrix, hence it is no surprise to observe the highest SE from fillers with the highest aspect ratio, that is, SWCNTs > MWCNTs > CNFs > CB when the volume fraction of the fillers is the same. The different methods of fillers dispersion and various carbon filler surface modification methods were comprehensively reviewed in the published paper and will not be discussed in detail here [3, 24]. The EMI shielding performance of the polymer/carbon-fillers composites can also be found in Ref. [3, 7, 24, 25].
\nA binary or even ternary component consists of two or more types of the fillers provide an effective way to bypass the inherent shortcomings of a single-filler composite. The incorporation of magnetic components will supplement the attenuation properties of a carbon-based EMI shielding material.
\nPhysical blending or deposition of metallic particles within a polymer blend or structure is the most direct way of incorporation a third element, however, such method faces the problem of uniform dispersion and deposition at the bottom layers due to the higher density of metallic particles. Electroless plating of metals on carbon substrates provides a neat way of incorporating metal components uniformly into a system without excessive weight addition. Works by Kim et al. [26] and Yim et al. [27] dispersed nickel coated MWCNT through electroless plating in epoxy and high-density polyethylene, respectively. Figure 5a gives an illustration of the nickel coated MWCNTs. It is apparent that the nickel coated MWCNTs appeared rougher comparing to the pristine ones due to the presence of nickel particles as shown Figure 5b. Yim achieved 140% (at 1 GHz, Figure 5c) in enhancement of the EMI SE compared to the pristine MWCNT/polymer composites. The enhancement was attributed to the increased surface conductivity. Figure 5d shows the proposed shielding mechanism of Ni-MWCNTs/HDPE. EM wave was firstly reflected at the composite surfaces upon reaching the surface of the composite. When the penetrated EM wave meets the nickel layer on the MWCNTs, the metallic layer functioned as EM absorbable or reflective fillers. It is evident that the EMI absorbing nature of the metallic layer can be used as an effective additional shielding material despite the small amount present in the systems.
\n(a) Schematic diagram of the electroless Ni-plating process; (b) SEM images of (1) pristine MWCNTs and (2) Ni-coated MWCNTs, respectively; (c) comparison of the EMI SE of MWCNTs/HDPE and Ni-MWCNTs/HDPE and (d) the proposed shielding mechanism of Ni-MWCNTs/HDPE [27].
In view of the rigid index of fuel-economy in the applications of automobile and aerospace, lightweight EMI shielding materials with the combination of reduced density and high EMI SE are much preferred. In this section, we aim to provide a general overview on the preparation of foam and aerogel materials used in EMI shielding and the advantages and challenges faced by each category and possible strategies towards enhancing their EMI shielding performances. The specific EMI SE, defined as the ratio of the EMI SE to the density (SSE) or both density and thickness (SSE/t), is a more appropriate criterion to compare the EMI shielding performance with those of other typical materials for the applications where lightweight is required.
\nConductive polymer-based composites foams offer significant reduction in weight, while the pores decrease the real part of the permittivity, accordingly reducing the reflection at the material surface. The porous structure enhances the energy absorption through wave scattering in the walls of the pores. Electrically conductive fillers, including CNFs, CNTs and graphene sheets, are commonly used to form a desirable conducting network within the inherently insulating polymer foam matrix. Yang et al. [28] first reported CNFs reinforced polystyrene (PS) composite foam as a conductive foam for EMI shielding application. The EMI SE of PS/CNFs foam containing 1 wt% CNFs was less than 1 dB, upon increasing CNFs content to 15 wt%, EMI SE increased to 19 dB. Following this work, the authors reported PS/CNTs composite foam with varying CNTs contents from 0 to 7 wt% [4]. The PS/CNTs composite foam achieved a higher EMI SE of above 10 dB compared to 3 dB for the PS/CNFs composite foam at the same filler content of 3 wt%. The difference in the results originated from the remarkable electrical and structural properties of CNTs, such as larger aspect ratio, smaller diameter, higher electrical conductivity and strength, compared to CNFs.
\nSyntactic foam, filling hollow spheres in a matrix, is a kind of lightweight composite materials. The approaches to enhance the EMI SE of syntactic foams include (i) hollow particles made of a conductive material; (ii) coating a conductive layer onto the surface of hollow particles and (iii) adding a second conductive filler in syntactic foam matrix.
\nZhang et al. [29] added a second conductive filler, (CNFs, chopped carbon fiber (CCF), and long carbon fiber (LCF)), into syntactic foams containing conductive hollow carbon microspheres (HCMs). The EMI SE values of used syntactic foams at the same filler content were compared, as shown as Table 2. The results showed that CNFs is more effective in providing EMI shielding compared to CCF and LCF due to the larger aspect ratio of CNFs.
\nFiller content (vol%) | \nCNF Aspect ratio: 500–1700 | \nCCF Aspect ratio: 6–50 | \nLCF Aspect ratio: 150–750 | \n
---|---|---|---|
0.5 | \n5.2 | \n2.2 | \n2.8 | \n
1.0 | \n11.3 | \n3.4 | \n4.4 | \n
1.5 | \n16.4 | \n3.7 | \n6.5 | \n
2.0 | \n24.9 | \n4.3 | \n7.5 | \n
Comparison of the EMI SE (dB) of CNF, CCF, and LCF reinforced syntactic foam.
Zhang et al. [30] also demonstrated the effect of functionalization of HCMs on the EMI SE of the epoxy-HCMs syntactic foam. HCMs were coated with polydopamine (PDA) via the self-polymerization of dopamine. The PDA coating promotes dispersion and served as a reducing agent to deposit silver (Ag) particles on the surface of HCMs as illustrated in Figure 6a. The average EMI SE of the epoxy-HCMs syntactic foam containing Ag-PDA-HCMs with 28.5 and 30.5 wt% of silver in the X-band achieved 49.5 and 60.2 dB, respectively as shown in Figure 6b. The SSE reached up to 46.3 dB cm3/g, demonstrating the prospect of epoxy/Ag-PDA-HCMs syntactic foam as a lightweight high-performance EMI shielding material. The corresponding EMI shielding mechanism of this syntactic foam was analyzed by comparing the values of reflectance (R), absorptance (A), and transmittance (T) in Figure 6c. The specimens were both reflective and absorptive towards EM radiation at silver content less than 17.8 wt%. The contribution of reflection (0.83) towards EMI SE surpassed that from absorption (0.16) when silver content increased to 28.5%. The dense and thick electrically conductive silver formed due to further increasing the silver content to 30.5 wt% increased the R to 0.97 and resultant in reflection as the dominant shielding mechanism.
\n(a) Schematic illustration of the procedure for preparation of PDA-HCMs and Ag-PDA-HCMs; (b) EMI SE in the frequency range from 8 to 12 GHz for syntactic foam containing pristine HCMs and Ag-PDA-HCMs with different silver contents; and (c) reflectance (R), absorbance (A), and transmittance (T) of EM radiation over syntactic foams containing Ag-PDA-HCMs with different silver content at 10 GHz [30].
Xu et al. [31] fabricated syntactic foams (“hybridized epoxy composite foams” according to authors) through impregnating expandable epoxy/MWCNT/microsphere blends into a preformed, highly porous, and 3D silver-coated melamine foam (SF) sponge. The highly conductive SF resolved the problem of the foam reduction of high filled epoxy blends and provided channels for rapid electron transport. MWCNTs were used to offset the loss of conductive pathways due to the crystal defects in the silver layer and the insulating epoxy resin. As a result, the EMI SE of 68.1 dB was achieved with only 2 wt% of MWCNTs and 3.7 wt% of silver due to the synergy of the MWCNT and SF.
\nCarbon foam is a class of three-dimensional (3D) architecture consisting of a sponge-like interconnected network of porous carbon. Carbon foams have been wildly used as candidates for realistic EMI shielding applications due to their excellent properties, such as low density, resistance to chemical corrosion, high thermal and electrical conductivity, and high temperature resistance.
\nZhang et al. [32] prepared a novel ultralight (0.15 g/cm3) carbon foam by direct carbonization of phthalonitrile (PN)-based polymer foam, as shown in Figure 7a. High EMI SE of ∼ 51.2 dB (see Figure 7b, C1000 was labeled as the carbonization of 1000°C) was contributed by the high graphitic carbonaceous species and the intrinsic nitrogen-containing structure. The carbon foams showed the best SSE of 341.1 dB cm3/g so far when mechanical property was considered. The carbon foam developed by Zhang provides an excellent low-density and high-performance EMI shielding material for use in areas where mechanical integrity is desired.
\n(a) Schematic representation of the preparation of PN-based carbon foams and (b) EMI SE of carbon foams [32].
The EMI SE of carbon foams was closely related to the char yield of polymer precursors and the demanding carbonization conditions. Therefore, a new kind of filler-free lightweight EMI shielding material, is in demand, which can be prepared without the stringent processing conditions. In view of the lightweight requirement, assembling one dimensional (1D) CNTs and two-dimensional (2D) graphene sheets into three dimensional (3D) macroscopic porous structures (e.g., sponges, foams and aerogels) emerged as an efficient approach.
\nLu et al. [33] synthesized a flexible CNTs sponge with a density of 10.0 mg/cm3 via chemical vapor deposition (CVD) process, composed of self-assembled and interconnected CNT skeletons. The freestanding CNTs sponge showed the high EMI SE and SSE of 54.8 dB and 5480 dB cm3/g in X-band, respectively. After composited with polydimethylsiloxane (PDMS) by directly infiltrating method, the CNT/PDMS composites still exhibited excellent EMI SE (46.3 dB) at the thickness of 2.0 mm, while the CNT loading content was less than 1.0 wt%.
\nSurface modification is employed to increase the EMI shielding ability of graphene foams. Zhang et al. [34] prepared surfaced modified 3D graphene foams via self-polymerization of dopamine with a subsequent foaming process, as shown in Figure 8a. The polydopamine (PDA) served as a nitrogen doping source and an enhancement tool to achieve higher extent of reduction of the graphene through providing wider pathways and larger accessible surface areas. The enhanced reduction of graphene sheets and the polarization effects introduced by PDA decoration compensated the negative effect of the barrier posed by PDA. As a result, the resultant EMI SE showed 15% improvement compared to PDA-free graphene foam as shown in Figure 8b. Wu et al. [35] also fabricated an ultralight, high performance EMI shielding graphene foam (GF)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) composites by drop coating of PEDOT:PSS on the freestanding cellular-structured GFs, as illustrated in Figure 8c. The GF/PEDOT:PSS composites possess an enhanced electrical conductivity from 11.8 to 43.2 S/cm after the incorporation of PEDOT:PSS. The modified grapheme foam with a density of 18.2 × 10−3 g/cm3 provide a remarkable EMI SE of 91.9 dB (identified as SET in Figure 8d).
\n(a) Schematic representation of the preparation of PDA-GO and PDA-rGO; (b) EMI SE of rGO foam and PDA-rGO foam [34]; (c) schematic procedure of the preparation of GF/PEDOT:PSS composites; (d) EMI SE of GF/PEDOT:PSS composites as a function frequency [35].
Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component used in gel are replaced by air. In recent years, the great potential of graphene aerogel (GAs) in EMI shielding applications has been confirmed by several researchers. Song et al. [36] reported that the EMI SE of GA-carbon textile hybrid with a thickness of 2 mm was 27 dB. The 3D scaffold GA greatly enhances the conductive network while maintaining the advantage of light carbon textile. Singh et al. [37] studied the EMI SE of pure GA, which was 20 dB, with a density ∼75 mg /cm3 and a thickness of 2 mm. They discussed the EMI shielding mechanism by correlating the EM wave interaction with the 3D porous structure. Zeng et al. [38] fabricated an ultralight and highly elastic rGO/lignin-derived carbon (LDC) composite aerogel with aligned microspores and cell walls by directional freeze-drying and carbonization method. The EMI SE of rGO/LDC composite aerogels with a thickness of 2 mm could reach up to 49.2 and 21.3 dB under ultralow densities of 8.0 and 2.0 mg/cm3, respectively.
\nThe graphitization of GAs facilitates to improve its electrical conductivity, thus improving the EMI SE. Liu et al. [39] reported an effective method of manufacturing an integrated graphene aerogel (IGA) using a complete bridge between rGO sheets and polyimide macromolecules via graphitization at 2800°C, as shown in Figure 9a. The rGO sheets were efficiently reduced to graphene during graphitization, while the polyimide component was graphitized to turbostratic carbon to connect the graphene sheets, resulting in a high EMI SE of ∼83 dB in X-band at a low density of 18 mg/cm3, as shown in Figure 9b. The EMI shielding mechanism analysis for the porous IGA revealed that most of the incident EM wave was dissipated through absorption, thus forming an absorption-dominant EMI shielding mechanism.
\n(a) Schematic illustration for fabricating IGA and (b) effect of annealing temperature on EMI shielding performance of IGAs [39].
Different reduction process of graphene oxide (GO), including chemical reduction and thermal reduction would affect the EMI shielding performance of GAs. Bi et al. [40, 41] carried out a comprehensive study of EMI shielding mechanisms of GAs solely consisted of graphene sheets to determine the main parameters of high EMI SE. As shown in Figure 10a, two types of ultralight (4.5–5.5 mg/cm3) 3D GAs were prepared by chemical reduction and thermal reduction of GO aerogels. The EMI SE reached 27.6 and 40.2 dB for chemically reduced graphene aerogel (GAC) and thermally reduced graphene aerogel (GAT), respectively. The distinct graphene surface resulted from different processing pathway led to different EM wave response upon striking the graphene/air interface. Nitrogen-doping and side polar groups induced strong polarization effects in GAC. Higher extent of reduction of the grapheme sheets in GAT left a smaller amount of side polar groups and formed more sp2 graphitic lattice, both favored π-π stacking between the adjacent graphene sheets. The enhanced polarization effects and the increased electrical conductivity of GAT contributed to better EMI shielding performance. Bi further investigated the effect of porosity on EMI shielding mechanisms compressing the aerogel (GA9) into thin film (GA9F), as shown in Figure 10b. The highly connected conducting network resulted in a significant increase in the electrical conductivity of GA9F, while the EMI SE remained unchanged at constant rGO content. The observation was contradictory to the previous outcomes that higher electrical conductivity or better-connected network contributed to higher EMI SE. Hence, the fact can be believed that the EMI SE is highly dependent on the effective amounts of materials response to the EM waves. Despite the similar intrinsic properties of rGO, the amount of absorption of EM waves in GA9 was much higher than that in GA9F when the EM waves penetrated through the porous structure. The cavities within the highly porous GA absorbed the EM waves through multiple internal reflections and eventually depleted the energy. Hence, the tightly connected conducting network within GA9F changed the EMI shielding mechanism from absorption to reflection.
\n(a) Schematic representation of the preparation process of GAC and GAT [42] and (b) R & A of GA9 and GA9F [41].
Generally, EMI shielding is defined as the prevention of the propagation of EM waves from one region to another by using shield materials. With the development of electronic industry, weight reduction is an additional technical requirement besides the good EMI shielding performance. Metal as a traditional EMI shielding material has been replacing with lighter materials, such as polymer-based composites, foams and aerogels. This chapter reviewed various types of lightweight materials with their EMI SEs corresponding to their EMI shielding mechanisms. To verify the benefits of using lightweight materials for EMI shielding applications, a comprehensive comparison was performed as shown in Figure 11. All the data in Figure 11 were collected from the reference papers listed in this chapter. Although the data are not involved all the published results, they are representative to the library of lightweight EMI shielding materials. The reported EMI SEs of polymer-based composites containing conductive fillers varied in the range of 20–60 dB corresponding to the densities higher than 0.8 g/cm3. Polymer-based foams reinforced with additional conductive fillers and carbon foams outperform polymer-based composites in terms of EMI SE. They possessed comparable EMI SE of 20–80 dB with the lower density (<0.8 g/cm3). Aerogels with ultralow densities (<100 mg/cm3) exhibited high EMI SEs in the same range of polymer- and carbon-based foams, indicating they can be used as an ideal potential lightweight EMI shielding materials though the mechanical properties of aerogels still remain a big issue.
\nComparison of EMI SEs of lightweight materials as a function of density of materials.
Liying Zhang would like to acknowledge the support by the initial research funds for young teachers of Donghua University. Shuguang Bi would like to acknowledge the financial support of Wuhan Engineering Center for Ecological Dyeing & Finishing and Functional Textiles, Key Laboratory of Textile Fiber & Product (Wuhan Textile University), Ministry of Education, Hubei Biomass Fibers and Eco-dyeing & Finishing Key Laboratory. Zhang and Bi would also thank the funding support by State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University (KF1827). Ming Liu would like to acknowledge the support from School of Materials Science and Engineering at Nanyang Technological University for this work.
\nNo conflict of interest.
In the past four decades, over 42–56% of major lower extremity amputations in the United States and Western European countries have been due to diabetes mellitus (DM) [1, 2, 3, 4]. The relative risk of major leg amputations for diabetes ranges from 5.1 to 31.5 times in comparison with that of nondiabetic populations [5, 6]. Extensive efforts have been made to improve the treatment of diabetes in regard to glycemic control and the prevention of diabetic complications, and foot ulcer treatments have improved for diabetic patients [7, 8]. Before 2004, trauma accounted for most amputations in the majority of hospitals, followed by malignancies [9]. However, the most common cause of amputation at present is diabetes mellitus [10, 11].
\nAmputation is the most appropriate therapy for an ischemic or infected limb, but the level at which to amputate is often difficult to determine. Patients who undergo only toe or trans-metatarsal amputation can walk on their own feet; however, those with major amputation require an artificial leg or a cane, which impairs their activities [12, 13]. The aim of this chapter is to describe factors that lead to amputation of a diabetic foot and propose a management strategy to prevent major amputation.
\nA retrospective descriptive study including 152 diabetic patients among 233 patients with leg ulcers who were treated in our medical center was carried out between January 2008 and December 2017. All patients had been diagnosed with type II diabetes. Diabetic foot ulcers represent more than 65 percent of all leg ulcers.
\nTo clarify the clinical characteristics of the diabetic foot, a comparison of foot ulcer patients with and without diabetes mellitus is conducted first, risk factors leading to amputation in cases of diabetic foot ulcer and “major” amputation in cases of diabetic foot are discussed, and a recommended strategy to avoid major leg amputation is presented.
\nStatistical analysis was performed using the Wilcoxon signed-rank test and chi-square test. The value of p < 0.05 was determined as significant.
\nThe ethical committee of our medical center approved this study.
\nProfiles of foot ulcer patients with and without diabetes mellitus are shown in Table 1. Of the 233 patients with a foot ulcer, 63% (147) were men, and 37% (86) were women. Of course, levels of HbA1C and blood sugar in the diabetic foot group were significantly higher than those in the nondiabetic foot group, and men were more likely to develop leg ulcers in the diabetic patient group. There were no significant differences in CRP, WBC, serum albumin, or hemoglobin between the groups.
\nProfile of foot ulcer patients with and without diabetes mellitus.
The severity of leg ulcers at discovery in patients with and without diabetes mellitus is shown in Table 2. In the groups, the ulcer stage based on the Wagner classification showed similar tendencies. About 80% of the diabetic foot group developed infection, being a significantly higher rate than in the nondiabetic foot. Methicillin-resistant Staphylococcus aureus (MRSA), methicillin-susceptible Staphylococcus aureus (MSSA), and Streptococcus were ranked high and accounted for over three-quarters of infections in both groups (Figure 1).
\nSeverity of leg ulcers at discovery in patients with and without diabetes mellitus.
Infection of leg ulcers at discovery in patients with and without diabetes mellitus (MRSA, methicillin-resistant Staphylococcus aureus; MSSA, methicillin-susceptible Staphylococcus aureus).
Because patients with diabetes are likely to develop severe infection, more than 50% of foot ulcer patients with diabetes required immediate debridement surgery, being a significantly higher rate than in the nondiabetic foot group (25%) (Figure 2).
\nThe frequency of foot ulcer patients with and without diabetes, who required immediate debridement surgery.
The frequencies of peripheral artery disease in foot ulcer patients with and without diabetes were 38.2 and 34.6%, respectively. There were no significant differences between the groups.
\nThe frequencies of hemodialysis in patients with and without diabetes were 7.2 and 6.2%, respectively. There were no significant differences between the groups.
\nThe frequencies of amputation in foot ulcer patients with and without diabetes were 53.9 and 34.6%, respectively. More than half of the patients with diabetes underwent amputation surgery, being a significantly higher rate than that in the nondiabetic foot group (Figure 3).
\nThe frequency of amputation in foot ulcer patients with and without diabetes.
We evaluated 85 amputated legs in 152 diabetic foot patients. Sixty-eight percent (104) of the patients were men, and 32% (48) were women. Profiles of diabetic patients with/without leg amputation are shown in Table 3.
\nProfiles of diabetic patients with and without leg amputation.
Men were more likely to require amputation. CRP and WBC were significantly higher, and serum albumin was significantly lower in the major amputation group, suggesting that severe infection and malnutrition are risk factors for major leg amputation in diabetic foot patients.
\nSixty-nine (82%) of 85 amputees and 36 (57.6%) of 67 non-amputees with diabetes developed infection, showing a significant difference between the groups. More than half of amputated and only 17.9% of non-amputated patients with diabetes were complicated by peripheral artery disease, showing a significant difference between the groups (Figure 4). Furthermore, the frequency of hemodialysis in amputated patients (11.8%) was also significantly higher than that in non-amputated patients (1.5%) (Figure 5).
\nThe frequency of amputation in diabetic foot ulcer patients with and without peripheral artery disease.
The frequency of amputation in diabetic foot ulcer patients with and without hemodialysis.
Of the 85 amputees with diabetes, 44 patients underwent minor amputation, and 38 received major amputation. Seventy-one percent (58) were men and 29% (24) were women. Profiles of diabetic patients with/without leg amputation are shown in Table 4. Men were more likely to require major amputation. CRP and WBC were significantly higher, and serum albumin was significantly lower in the major amputation group, suggesting that severe infection and malnutrition are risk factors for major leg amputation in diabetic foot patients.
\nProfiles of diabetic patients who underwent major and minor leg amputation.
Diabetic foot ulcers sometimes lead to minor or major amputation, with a high impact on patients’ life and its quality [14]. Our results suggest that risk factors for leg amputation in diabetic foot patients include male, complication of severe infection, complication of peripheral artery disease, complication of hemodialysis, and malnutrition.
\nThe importance of nutritional support in patients with wounds has been examined. Malnourished patients showed not only a higher frequency of impaired wound healing but also an increased risk of postoperative cardiopulmonary and septic complications [15, 16]. Malnutrition cannot be improved in a short time after developing foot ulcers. Thus, patients requiring surgical treatment should also receive supplemental nourishment in the perioperative period [17]. Luo et al. suggested that the geriatric nutritional risk index was a reliable and effective predictive marker of patients’ amputation-free survival, and it could identify patients early with a high risk of amputation [18]. Appropriate blood sugar control and nutritional support are required for diabetic patients to prevent leg amputation. Malnutrition usually occurs in critical limb ischemia patients as well, because of a lack of appetite and sleeplessness due to chronic pain. These patients with peripheral artery disease also require pain control and nutritional support services [18].
\nThe number of patients requiring hemodialysis has been growing because obesity-related renal diseases such as diabetes mellitus are increasing [19, 20]. Diabetic patients with renal failure had high risks of foot ulceration and lower limb complications [21]. Regarding cutaneous infection, Bencini et al. reported that the incidence of fungal infection in patients undergoing hemodialysis was 67% [22]. Because chronic renal failure patients exhibit impaired cellular immunity due to a decreased T-lymphocyte cell count, this could explain the increased prevalence of fungal infections [23]. Thus, difficulty healing wounds is a frequent problem in patients on hemodialysis [24]. Amputations of limbs are sometimes performed for these complex ulcers, because when patients receiving hemodialysis develop aggressive life-threatening infections such as sepsis, immediate surgical debridement is required in order to salvage the blood access line and save lives [25]. Fujioka reported that 13 of 17 wounds required immediate surgery, including amputation and debridement in patients with DM, while only 1 of 13 required immediate surgery in patients without DM [26].
\nPoor management of foot ulcers in patients receiving hemodialysis leads to prolonged ulceration, gangrene, amputation, depression, and death [27].
\nMarn et al. investigated the association between the implementation of a routine foot check program in diabetic incident hemodialysis patients and concluded that monthly foot checks are associated with a reduction of major lower limb amputations [28]. All patients on hemodialysis should be considered as being at high risk of developing foot complications and undergo foot checks frequently. If infection is suspected, antibiotics should be administered through the dialysis line immediately during dialysis.
\nDiabetic foot infection is a common diabetic complication, which results in lower limb amputation if not treated properly. Patients with diabetes are likely to develop infections, because of the alteration of immune defense mechanisms such as a change in the neutrophil function, suppression of the antioxidant system, and modified humoral activity due to the hyperglycemic environment [29].
\nOnce a diabetic foot develops infection, it progresses rapidly and requires the removal of all necrotizing tissue involving the bone, tendons, and skin (Figure 6).
\nA view of progressing diabetic infection in the big toe, which aggravated rapidly and required the removal of toes and metatarsal bones within 3 weeks.
If the toe infection progresses and spreads widely, the patient may have to undergo major amputation (Figures 7a and b). Thus, early and appropriate debridement to reduce infection is important.
\n(a) A view of necrotizing fasciitis in the left forearm at the first examination, which progressed rapidly to the upper arm, and the patient developed septic shock in 2 days. (b) Amputation of the infected hand at the upper arm was immediately performed to control the aggressive infection.
Soft tissue infections in diabetic patients require multidisciplinary treatment including rapid surgical intervention, antibiotic treatment, and hyperbaric oxygen therapy to restrict the growth of pathogens [30, 31, 32]. Antibiotic therapy should be instituted immediately. The initial antibiotic should act on aerobic Gram-positive and Gram-negative bacteria but also on anaerobic bacteria. Systemic antibiotics have been demonstrated in many trials to be effective in treating acute diabetic foot infections. Tchero et al. performed a systematic review to assess the clinical efficacy of antibiotic regimens in the treatment of diabetic foot infections and concluded that piperacillin/tazobactam should be recommended for severe infections and the adjuvant use of topical agents with systemic antibiotics improved the outcomes compared with systemic antibiotics alone [33]. Mustăţea et al. suggested that an initial combination of third-generation cephalosporin, quinolone, and metronidazole was initially administered. After germ identification, antibiotic therapy was administered according to the antibiogram [29]. Cellulitis, which shows inflammation and infection of the skin and subcutaneous tissue, can be treated with systemic Gram-positive bactericidal antibiotics only. However, if deep tissue infection, especially osteomyelitis, is suspected, removal of the infected bone and soft tissue, followed by 2–4 weeks of antibiotics, is required [30].
\nRegarding surgical intervention, early and appropriate debridement to reduce infection is recommended to achieve infection control (Figure 8).
\nViews of debridement for necrotizing fasciitis in the diabetic patient’s right sole. All necrotizing, contaminated tissue was removed immediately.
If the infection invades deeper to the tendon, the lesions can often be extended and spread upward rapidly along the tendon tract, which can lead to systematic sepsis and require immediate limb amputation (Figure 9a and b). As the infection developing in the diabetic patients’ limbs progresses rapidly, physicians must decide on whether to carry out debridement before the infected lesion spreads upward.
\n(a) A view of necrotizing fasciitis in the right big toe, which spreads upward rapidly. (b) Intraoperative view showing the contaminated lesion extending along the extensor tendon tract.
Case presentations
\nCase 1. A 51-year-old man developed diabetic foot gangrene with osteomyelitis of the fifth toe, which had progressed for 2 weeks (Figure 10a). The patient underwent fourth and fifth toe amputation immediately, and cleansing to reduce infection was performed for 2 weeks (Figure 10b). As abundant granulation tissue developed on the wound surface, he underwent free skin grafting (Figure 10c). The wound had completely resurfaced by 1 month after skin grafting, and the patient could walk without a cane (Figure 10d).
\n(a) Case 1. A view of diabetic foot gangrene with osteomyelitis of the fifth toe. (b) After fourth and fifth toe amputation, cleansing was performed for 2 weeks. (c) Intraoperative view showing free skin grafting on the wound. (d) A view of the foot 1 month after surgery showing favorable coverage of the wound.
Peripheral artery disease (PAD) is observed in up to 50% of patients with a diabetic foot ulcer, and the presence of PAD is an important consideration in their management [34]. PAD affects the distal vessels and results in occlusion, which is one of the major causes of ulcer development and an increased risk of amputation. The treatment for these patients often requires challenging distal revascularization surgery or angioplasty to prevent limb amputation [35]. Revascularization is commonly performed in patients with critical limb ischemia and a diabetic foot ulcer, and the ulcer-healing rate after revascularization ranges from 46 to 91% [36]. Hinchliffe et al. reviewed the effectiveness of revascularization of the ulcerated foot in patients with diabetes and PAD 1 year after surgery and reported that limb salvage rates showed a median of 85% following open surgery, and more than 60% of ulcers had healed following revascularization. They concluded that revascularization improved rates of limb salvage compared with the results of conservatively treated patients [34].
\nCase presentations
\nCase 2. A 67-year-old man developed a diabetic foot ulcer of the right heel, which had progressed for 2 months (Figure 11a). His posterior tibial artery was not palpable. Enhanced computed tomography (CT) showed that circulation of his right lower leg was poor, with an ankle brachial pressure index (ABI) of only 0.53, which suggested that his leg ulcer might not heel spontaneously. We fashioned femoral-popliteal artery (FP) bypass to increase distal blood flow, and ABI improved to 0.83(Figure 11b). As the patient’s foot received sufficient flow, he could safely undergo resurfacing surgery using a reversed sural flap successfully and could walk 3 months after surgery (Figure 11c–f).
\n(a) Case 2. A view of a diabetic foot ulcer of the right heel. (b) Enhanced computed tomography scan image showing the poor circulation of the patient’s right lower leg due to obstruction of the right femoral artery (circles). After fashioning the femoral-popliteal artery bypass, increased distal blood flow was seen (small arrows). (c) Intraoperative view showing the debrided heel ulcer and design of the reversed sural flap. (d) Intraoperative view of heel reconstruction showing the transferred reversed sural flap. (e) A view of the reconstructed heel 3 months after surgery revealed favorable coverage of the wound. (f) The patient could walk 3 months after surgery.
Case 3. A 60-year-old man developed a diabetic foot ulcer and osteomyelitis of the calcaneus (Figure 12a). Following the removal of a sequester, he underwent FP bypass angioplasty, and ABI improved from 0.67 to 1.01 (Figure 12b). The bone-exposing wound was resurfaced using a free superficial circumflex iliac perforator (SCIP) flap (Figure 12c–e). One year after the surgery, good circulation had been achieved without infection or ulcer relapse (Figure 12f).
\n(a) Case 3. A view of a diabetic foot ulcer and osteomyelitis of the calcaneus. (b) Enhanced computed tomography scan image showing poor circulation of the patient’s right lower leg due to obstruction of right femoral artery (circle). After fashioning the femoral-popliteal artery bypass, increased distal blood flow was seen. (c) Intraoperative view showing the design of a free superficial circumflex iliac perforator flap. (d) Intraoperative view of the elevated SCIP flap. The arrow indicates the perforator of superficial circumflex iliac vessels. (e) Intraoperative view of the harvested SCIP flap. (f) A view of the reconstructed foot 1 year after surgery showing favorable coverage of the wound.
Standard stump plasty requires shortening of the remaining fine and vivid bone end to resurface the bone-exposing amputation stump (Figure 13a and b).
\n(a) A view of diabetic gangrene extending the first and second metatarsal bones. After removal of the necrotic bone, the navicular was exposed. (b) Intraoperative view of Chopart amputation followed by resurfacing with a local flap of the sole.
On the other hand, free flap transfer enables surgeons to maintain the bone length, which is a potential advantage, especially when amputation is performed at the trans-metatarsal lesion (Figure 14a–c).
\n(a) A view of a diabetic foot ulcer with osteomyelitis of the first and second metatarsal bones. (b) Intraoperative view of the harvested anterolateral thigh (ALT) flap. (c) A view of the reconstructed foot using a free ALT flap 1 year after surgery, showing favorable coverage, and the patient could walk without a cane.
This is because Chopart or transtibial amputation results in more debilitating functional outcomes than transmetatarsal amputation. Furthermore, transmetatarsal amputation preserves maximal foot length, allowing patients to achieve a better quality of life [37, 38].
\nRegarding the flap choice, the ideal flap is thought to be a good vascularized skin paddle with the same thickness and width as the wound and requiring a single-stage operation [39]. Perforator flaps are defined as flaps consisting of skin and/or subcutaneous fat, with a blood supply from isolated perforating vessels of a stem artery [40]. The development of perforator flaps has increased the number of potential donor sites because a flap can be supplied by any musculocutaneous perforator, and donor-site morbidity can be reduced [41, 42]. Furthermore, the advantage of this skin flap is that it is less invasive, so that the operation can be performed under local anesthesia if the wound is small.
\nCase presentation
\nCase 4. A 32-year-old man developed a diabetic foot ulcer on the step (Figure 15a). Following debridement, he underwent resurfacing surgery using a free superficial circumflex Iliac artery perforator flap (Figure 12b and c). As free SCIP flap transfer is less invasive, the operation can be performed under local anesthesia (Figure 15d). One year after the surgery, good circulation had been achieved without infection or ulcer relapse (Figure 15e).
\n(a) Case 4. A view of a diabetic foot ulcer of the step. (b) Intraoperative view showing the design of a free superficial circumflex iliac perforator flap. (c) Intraoperative view showing the design of a free SCIP flap. (d) Intraoperative view showing that an SCIP flap transfer is less invasive, so the patient was awake and talking with the surgeon. (e) A view of the reconstructed foot 2 months after surgery revealed favorable wound coverage.
The SCIP flap is recommended because it minimizes sacrifice at the donor site, causing no damage to the main vessels or muscles beneath the flap. The only disadvantage is that the pedicle vessel is sometimes short when a suitable recipient vessel cannot be found near the wound [43]. Identifying an acceptable recipient vessel around the contaminated area is not always easy. Chronic inflammation in recipient vessels caused by infection and fibrosis may be one of the factors leading to thrombosis of the anastomosed vessel [44]. So, it is important to select a flap with a long pedicle, as the suitable recipient vessel may be distant from the wound. The anterolateral thigh (ATL) flap is often chosen because it is supplied by the descending branch of the lateral femoral circumflex artery, which has an external diameter of more than 2 mm at the proximal end with a pedicle of more than 8 cm in length [45, 46]. This flap is also a perforator flap, so that a larger cutaneous or fasciocutaneous flap can be harvested from the thigh while avoiding the sacrificing of underlying muscle and large vessels [47, 48].
\nCase presentation
\nCase 5. A 66-year-old man developed a diabetic foot ulcer with osteomyelitis of the left fourth and fifth toes (Figure 16a). He had already undergone right below the knee amputation due to diabetic gangrene. Thus, he desired to preserve his left leg to walk. Following debridement, he underwent resurfacing surgery using a free ALT flap (Figure 16b and c). Two months after the surgery, good resurfacing had been achieved, and he could walk with an artificial right leg (Figure 16d).
\n(a) Case 5. A view of a diabetic foot ulcer. The fourth and fifth toes were amputated due to osteomyelitis. (b) Intraoperative view showing the elevation of an anterolateral thigh (ALT) flap. (c) Intraoperative view showing resurfacing of the bone-exposing wound with an ALT flap. (d) A view of the reconstructed foot 2 months after surgery revealed that favorable resurfacing had been achieved and he could walk without a cane.
I conclude that the risk factors of leg amputation due to a diabetic foot are complications of severe infection and PAD, so diabetic ulcer management should include the immediate removal of necrotic tissue and control of infection. The only way to prevent major amputation of a diabetic ischemic foot is angioplasty of the occluded lower extremity arteries, and reconstruction of the amputation stump using free flap transfers to preserve the foot length is a good option for preserving the walking function.
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