Effects of reservoir location on stent mechanical integrity (blind-hole reservoirs).
\r\n\t
",isbn:"978-1-83968-930-7",printIsbn:"978-1-83968-929-1",pdfIsbn:"978-1-83968-931-4",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"f159c09dab49a9bc6239b42660d8e8ec",bookSignature:"Dr. Yongxia Zhou",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/10310.jpg",keywords:"Brain Science, Brain-Computer Interface, Imaging of Neural Networks, Brain Networks, Brain Function, Molecular Imaging, Brain and Mind, Functional Imaging, Multimodal Imaging, Neuroplasticity Enhancement, Learning, Memory",numberOfDownloads:null,numberOfWosCitations:0,numberOfCrossrefCitations:null,numberOfDimensionsCitations:null,numberOfTotalCitations:null,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"September 28th 2020",dateEndSecondStepPublish:"October 26th 2020",dateEndThirdStepPublish:"December 25th 2020",dateEndFourthStepPublish:"March 15th 2021",dateEndFifthStepPublish:"May 14th 2021",remainingDaysToSecondStep:"3 months",secondStepPassed:!0,currentStepOfPublishingProcess:4,editedByType:null,kuFlag:!1,biosketch:"Yongxia Zhou had completed her Ph.D. from the University of Southern California in Biomedical imaging (2004) and had been trained and worked as a neuroimaging scientist in several prestigious institutes including Columbia University, New York University, University of Pennsylvania. Her research interest is focused on neuroimaging and neuroscience applications.",coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"259308",title:"Dr.",name:"Yongxia",middleName:null,surname:"Zhou",slug:"yongxia-zhou",fullName:"Yongxia Zhou",profilePictureURL:"https://mts.intechopen.com/storage/users/259308/images/system/259308.jpeg",biography:"Yongxia Zhou obtained a PhD from the University of Southern California in Biomedical Imaging in 2004. Her main research interest is in radiology and neuroscience applications. She had been trained and worked as a medical imaging scientist at several prestigious institutes including Columbia University, University of Pennsylvania, and the National Institutes of Health (NIH). Her research focuses on multimodal neuroimaging integration including MRI/PET and EEG/MEG instrumentation that makes the best use of multiple modalities to help interpret underlying disease mechanisms. She has authored six monograph books, and edited several books for well-known publishers including IntechOpen and Nova Science. She has published more than 100 papers and presentations in many reputed international journals and conferences, and served as reviewer and editor for several well-known associations.",institutionString:"University of Southern California",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"2",totalChapterViews:"0",totalEditedBooks:"3",institution:{name:"University of Southern California",institutionURL:null,country:{name:"United States of America"}}}],coeditorOne:null,coeditorTwo:null,coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"16",title:"Medicine",slug:"medicine"}],chapters:null,productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"247041",firstName:"Dolores",lastName:"Kuzelj",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/247041/images/7108_n.jpg",email:"dolores@intechopen.com",biography:"As an Author Service Manager my responsibilities include monitoring and facilitating all publishing activities for authors and editors. From chapter submission and review, to approval and revision, copyediting and design, until final publication, I work closely with authors and editors to ensure a simple and easy publishing process. I maintain constant and effective communication with authors, editors and reviewers, which allows for a level of personal support that enables contributors to fully commit and concentrate on the chapters they are writing, editing, or reviewing. I assist authors in the preparation of their full chapter submissions and track important deadlines and ensure they are met. I help to coordinate internal processes such as linguistic review, and monitor the technical aspects of the process. As an ASM I am also involved in the acquisition of editors. 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A metallic stent is a tiny, coiled wire-mesh tube that can be deployed into an artery and expanded using a catheter during angioplasty to open a narrowed artery. However, intimal cells can proliferate due to the artery injury during stenting, often leading to in-stent restenosis of the artery. Restenosis, the re-narrowing of the artery after the intervention, is the most common occurrence after angioplasty procedures in early days [1-3].
In the past decade, stent technology has evolved from the metallic stent to the so-called drug-eluting stent (DES). A drug-eluting stent has a metallic stent platform coated with an anti-proliferative drug (e.g., Sirolimus or Everolimus) that is known to interfere with the restenosis process. The drug is typically mixed with a polymer compound (durable or biodegradable) to precisely control its release rate and timing to the artery wall. The adoption of the drug-eluting stent has resulted in a dramatic lowering in restenosis rates from 20 to 30% for the metallic stent to the single digit now [4, 5], leading to a worldwide embrace of this new technology in healthcare. Since the introduction of the drug-eluting stent, restenosis has become less of an issue for the treatment of coronary artery diseases. The drug-eluting stent has thus become the gold standard for PCI procedures since then.
Although the drug-eluting stent has been hugely successful in lowering restenosis, the stenting technology continues to evolve in a quest for better solutions. The biodegradable vascular scaffold (BVS) presents the next frontier. Biodegradable stents stay in the blood vessel for a limited period of time, give mechanical support, and then degrade to non-toxic substances. Potential advantages of having the stent disappear from the treated site include reduced late stent thrombosis, facilitation of repeat treatments to the same site, and freedom from fracture-induced restenosis. The biodegradable stents with poly lactic acid (PLA) have good physical properties such as high strength and processability; and in a suitable disposal site it will degrade to natural products [6-8]. BVS has the potential to act as local drug delivery systems. Therefore, it is possible to design a BVS, not only offering a physical support to the vessel wall, but also presenting a pharmacological approach in the prevention of thrombus formation and intimal proliferation [9-11].
In recent years, another novel concept in smart drug delivery is the emergence of the depot stent: a metallic stent laser-drilled with micro-sized holes, or called “reservoirs,” that can be loaded with single or multiple drugs, potentially in various doses or formulations [12, 13]. The drug-polymer mix can be varied from one reservoir to the next, allowing a high flexibility of controlled release of different drugs. For example, on the outer side of the reservoirs close to the artery wall, drugs preventing neointimal proliferation can be filled, while on the inner side of the reservoirs close to the blood stream, thrombocyte inhibitors can be filled to prevent stent thrombosis (Figure 1). In addition, it is believed that this stent concept could become even more powerful in some cases such as the renal indication for potential treatment of both renal artery stenosis (RAS) and its associated kidney diseases at the same time, as renal artery stenosis is usually related to progressive hypertension, renal insufficiency, or kidney failure reciprocally. For example, drugs preventing neointimal proliferation can be applied on the outside portion of the reservoirs closer to the artery wall, whereas on the inside portion of the reservoirs, drugs for kidney diseases can be loaded and carried by the blood stream to the distal kidney organ for direct target therapy. This proposed method could potentially help to treat two problems in one attempt. Figure 2 shows that different types of drugs can be administered independently at each stage after intervention.
Depot stent concept with different types of drugs and release rates administered independently of opposite sides of the stent.
The depot stent has other advantages. Unlike drug-eluting stents, the depot stent does not need to be surface-coated. Therefore, it is free of surface coating layers, thereby reducing direct contact between the artery wall and the polymer compound. Such contact is believed to increase the potential risk of chronic inflammation or late stent thrombosis. Another advantage is the decrease of the overall stent profile due to the absence of surface coating layers. Lower stent profile allows a stent to access narrower lesion sites and offers physicians easier deliverability. Given these potential advantages, however, creating reservoirs on the stent struts inevitably weaken the stent structure and compromise its mechanical integrity, namely, its abilities to sustain various loading conditions including crimping onto a balloon catheter during manufacturing, stent expansion during deployment, radial resistance to blood vessels from collapsing inward, and long-term fatigue resistance to systolic/diastolic pressure loadings. Therefore, the objective of this paper is to investigate the impact of the micro-sized reservoirs on the overall mechanical integrity of the depot stent.
Computational modeling has emerged as a powerful tool for optimization of stent designs and can be used along with bench testing to improve stent clinical performance [14-18]. Such computational tools could provide valuable insights to various aspects of stent design tactics which may consequently reduce the potential risk of vascular injury and restenosis. It also gives extensive information under a highly-controlled environment, making it feasible to screen numerous design iterations prior to costly prototyping. Therefore, in this study, computational models were developed to assess key clinical attributes of the depot stent using finite element analysis (FEA). Based on these findings, we propose an optimal depot stent design in an effort to increase the drug capacity without significantly comprising its mechanical integrity.
Schematic of sequential release of multiple drugs for each period after intervention.
An L-605 cobalt–chromium balloon-expandable stent was used as the “standard” stent for baseline in this study. Micro-sized drug reservoirs were created on this standard stent struts in order to investigate their effects on the stent mechanical integrity. The stent was designed to form a series of nested rings interconnected with bridging connectors (CO). The design parameters such as crown radius (CR) and strut dimension were tailored to optimize the overall stent performance.
The standard stent, which has exactly the same geometry as the investigated depot stents but without drug reservoirs, was first evaluated to establish the baseline information. The FEA simulation was then conducted to investigate the effects of reservoir location on the mechanical integrity of the depot stent. Five equally spaced circular (cylindrical if considering depth) reservoirs were created on three major locations of the depot stent, namely, connectors, bar arms (BA), and crowns (Figure 3). The diameter of each circular reservoir was 50% of the strut width, whereas the reservoir depth was varied for evaluation. The spacing between two adjacent reservoirs was 0.15 mm, as defined by the length between two reservoir centers along the strut centerline. The choice of the reservoir number and size was based on the condition that the total reservoir capacity of the drug–polymer mix inside the reservoirs of the depot stent was able to fully replace the total volume of surface coating layers on a typical drug-eluting stent with an average coating thickness of 5μm [4].
Depot stent with reservoirs on (a) connectors, (b) bar arms, (c) crowns, and (d) entire stent surfaces.
Since adding more reservoirs increases the total drug capacity, investigation was also carried out on the depot stent with reservoirs uniformly spread on the entire stent to understand whether the mechanical integrity of such a depot stent is further compromised or not (Figure 3(d)). In this special case, the entire stent was covered by reservoirs with the same circular diameter and depth aforementioned. Another method of increasing the total drug capacity is by drilling through-holes instead of blind-holes into the stent struts. Therefore, FEA simulation was also conducted to investigate the effects of reservoir depth on the mechanical integrity. Further studies were conducted on the depot stent to investigate whether the reservoir shape, size, and number affect the stent mechanical integrity or not.
On 18 April 2010, the US Food and Drug Administration (FDA) published an official document, “Non-Clinical Engineering Tests and Recommended Labeling for Intravascular Stents and Associated Delivery Systems,” to serve as a guide for the medical device industry. In addition to the routine stent dimensional tests such as dimensional verification, percent surface area, foreshortening, and recoil, this document lists several key clinical relevant functional attributes in which the FDA is interested when reviewing future new stent submissions. For the balloon-expandable stent, these key clinical attributes include radial strength, stresses–strains, and fatigue resistance. Each of these properties can serve as an indicator with respect to various aspects of the stent integrity. In this study, three key clinical attributes were used to assess the mechanical integrity of the depot stent.
One of the most important functions of the stent is to create a scaffolding structure in the artery and exert radial force against the artery wall to prevent its reclosure. Radial strength represents the ability of a stent to resist radial collapse under external pressure loadings exerted by the artery wall. It is defined as the maximum pressure at which the stent experiences irreversible deformation.
Since the in vivo deployment of a balloon-expandable stent involves large plastic deformation, the equivalent plastic stresses and strains have to be calculated throughout the stent. This stress analysis provides a risk assessment of acute device failure. The stress or strain contour plots provide the overall distribution of plastic stresses or strains and identify the most fracture-prone locations of a stent. Stent fracture may cause loss of the radial strength or perforation of the blood vessel by the fractured stent struts.
Stent fracture due to long-term pulsatile fatigue loading may result in loss of the radial strength, thrombus formation, focal restenosis, or perforation of the blood vessel by the fractured stent struts. FDA recommends using a Goodman life analysis to determine the fatigue resistance of a stent to clinically relevant pressure loadings up to 4×108 cycles. The Goodman life analysis, combined with the stress analysis aforementioned and accelerated fatigue bench testing, provides a comprehensive risk assessment of long-term stent durability. The resulting fatigue safety factor shows how safe a stent is from fatigue failure based on the Goodman life analysis.
A stent deployed in the vasculature system is subjected to various loading modes which may consequently compromise the stent mechanical integrity during its service life. In this study, finite element models were developed to evaluate the mechanical integrity and fatigue resistance of a stent to various loading conditions involved in manufacturing and deploying a stent consistent with the current practice. The entire stress–strain history of the stent in each loading step was considered to incorporate the effects of accumulated residual stress–strains throughout the procedures. It includes manufacturing (crimped onto a balloon catheter), in vivo deployment (expanded into an artery), and their corresponding recoil and pulsatile loading subjected to systolic/diastolic pressures. The FEA simulation determines the distribution of stress and strain, fatigue safety factor, and radial strength imposed by the following steps:
Step 1. Crimping a stent from 2.54 mm to 2 mm OD (crimp).
Step 2. Removing outer constraint to allow stent recoil after crimping (crimp-recoil).
Step 3. Expanding a stent to 6.0 mm ID (expansion).
Step 4. Removing inner constraint to allow stent recoil after expansion (expansion-recoil).
Step 5. Applying 180/80 mmHg systolic/diastolic pressures for stent fatigue assessment (or applying external pressure for radial strength assessment).
It should be noted that step 5 can be used to calculate either the fatigue safety factor or radial strength of a stent.
The ABAQUS/standard finite element solver (Dassault Systemes Simulia Corp., Providence, RI, USA) was used to perform the stent FEA analysis. Since a stent has repeated patterns in its axial and circumferential directions, three representative rings instead of the entire stent were modeled to save computational time (Figure 3). In order to simulate the manufacturing (crimp onto a balloon catheter) and in-vivo deployment (expansion inside an artery) steps, two rigid cylinders with diameters of 2.54 and 1.12 mm were incorporated into the stent model with one cylinder inside the stent and the other one outside the stent. Gervaso et al. and De Beule et al. demonstrated that using the displacement-control expansion of a rigid cylinder for simulation of a balloon expansion could provide reliable and accurate information regarding the stent shape, stress–strain behavior, etc., when reaching the stent nominal diameter [19, 20]. Therefore, the displacement-control simplification was used in this study, as it is computationally less expensive than simulations involving balloon-driven expansion. Our goal is to assess the impact of drug reservoirs to the “standard” stent on a relative scale, so the displacement-control expansion serves that purpose well.
The stent model was meshed with the eight-node linear brick element in incompatible mode (C3D8I) with the element size of one-sixth of the strut width and one-third of strut thickness. This specific mesh size was chosen after a mesh sensitivity study to ensure that stress–strain variation on the stent was adequately captured. The inside and outside rigid cylinders were meshed with the four-node quadrilateral surface element (SFM3D4).
The material properties of L-605 cobalt–chromium alloy along with the ABAQUS von Mises plasticity model with isotropic hardening for large deformation analysis were used. Its Young’s modulus, Poisson ratio, yield stress, ultimate stress, ultimate strain, and fatigue endurance limit are 203 GPa, 0.3, 590 MPa, 1689 MPa, 60%, and 483 MPa, respectively.
A frictionless contact was used to prevent penetration between two surfaces of the model during crimping and expansion, with the following contact pairs implemented:
The first contact pair was defined as the surface contact between the inner surface of the outer rigid cylinder and the outer surface of the stent.
The second contact pair was defined as the surface contact between the outer surface of the inner rigid cylinder and the inner surface of the stent.
The third contact pair was defined as the side contact between any two stent struts during crimping.
In step 1, stress–strain analysis was conducted to simulate the crimping of a stent onto a balloon catheter. The outer rigid cylinder was compressed in the radial direction with the displacement control, forcing the stent to collapse inward. In step 2, stent recoil after crimping due to elastic strain energy was modeled by removing the outer rigid cylinder to allow for the stent recovery. In step 3, to simulate the in-vivo deployment, the inner rigid cylinder was expanded in the radial direction with the displacement control to simulate the stent deployment to the target size. In the final step, stent recoil after expansion due to elastic strain energy was modeled by removing the inner rigid cylinder to allow for the stent recovery. Figure 4 demonstrates the configuration of the stent and rigid cylinders at each step of the modeling scheme.
Configuration of the stent/cylindrical surfaces at each stage of the loading scheme: (a) crimp, (b) crimp-recoil, (c) expansion, and (d) expansion-recoil.
Following the previous four steps, systolic/diastolic arterial blood pressures of 180/80 mmHg were applied to simulate the pulsatile fatigue loading. In order to account for the external loading exerted by the arterial wall, an arterial pressure loading corresponding to the interaction between the stent and the artery was also imposed on the stent. The Goodman life analysis was performed using the multi-axial stress state experienced during the pulsatile fatigue loading to determine the fatigue resistance of a stent after implant [14]. It states that fatigue failure will occur if the stress state satisfies the following relationship:
where
The Goodman diagram is a plot of the normalized stress amplitude
where
When making a stent, a design drawing is first sketched on the 2D plane, wrapped around a target cylinder, and coded into the 3D cylindrical coordinate using the CAD/CAM software. The coded stent geometry is then input into the laser cutting machine and the design pattern is cut onto a seamless hypotube of 1–2 mm. During the laser cutting process, the hypotube was rotated and translated in the axial direction by the motor stage while the laser source remained stationary.
In this study, a laser module consisting of a 100W Yb-doped pulsed fiber laser (Rofin-Baasel Taiwan Ltd.), a linear X-Y motor stage (Aerotech, Inc.), and a Z-direction server motor was assembled and integrated (Figure 5). The precision motor stage provides linear motion and rotation of the hypotube, whereas the Z-direction server motor controls the distance between the laser source and hypotube surface for the optimal focal position (Figure 6). The X-Y motor stage accuracy is ±2 μm and ±25 arc-second in the axial and circumstantial direction, respectively; on the other hand, the linear encoder resolution for the Z-direction server motor is up to 0.5 μm. Position synchronized output (PSO), a control algorithm that greatly enhances the efficiency and quality of the laser cutting, was used to coordinate the linear X-Y motor stage with the timing of laser firing. It can minimize the heat-affected zone (HAZ) during the laser cutting process. A3200 controller (Aerotech, Inc.) allows us to perform up to 32 axes of synchronized motion control and therefore could accomplish very sophisticated laser cutting patterns effortlessly.
The principles of laser cutting and drilling are based on fusion cutting, which involves a melting mechanism where the heated materials transformed into a molten state are expelled from the cut kerf by a high pressure assisted gas such as nitrogen or argon. Laser cutting quality is typically controlled by several input laser parameters such as focal position, average laser power, pulse repetition rate, assisted gas pressure, etc. An appropriate position of the laser focal spot significantly improves the cutting outcome. The focal position of our laser module was assessed by measuring the kerf width on the hypotube while adjusting the distance between the laser source and hypotube surface. The smallest kerf width can be achieved by precisely focusing the laser sweet spot on the hypotube surface. A proper selection of laser power is crucial to the outcome as well, as excessive laser power results in a wider kerf and a thicker recast layer. On the other hand, insufficient laser power produces dross due to incomplete melting [21, 22]. Pulse repetition rate is another important laser parameter related to surface roughness and material removal. A higher pulse repetition rate corresponds to a better cutting surface and more effective material removal. Inert gas is a favorable assisted gas since it can avoid oxidation of the materials. High pressure inert gas was used during the cutting process to enhance drag forces and achieve high cutting quality [23].
Integrated laser module with a magnified view of the linear X-Y motor stage and laser source in the right window.
Stent design pattern cut onto a seamless hypotube by laser.
The laser cutting process inevitably generates dross, heat-affected zone, and other defects at stent surface. To achieve high quality of mirror-like surface, electro-polishing was performed on laser-cut depot stent prototypes. A 100 ml glass beaker was used as a bath. The experiment setup used for electro-polishing is illustrated in Figure 7. The stent was used as an anode, whereas the cathode was made of a thin lead sheet. The electrolyte consisted of 60 wt% phosphoric acid (H3PO4), 20 wt% sulfuric acid (H2SO4), and 20 wt% distilled water. Electro-polishing process was performed with continuous stirring of electrolyte to prevent oxygen bubbles from adhering to stent surface. Furthermore, several important polishing parameters including bath temperature, current, and time were determined through experiments to find the optimal conditions for electro-polishing of depot stents. The stents were then cleaned ultrasonically using distilled water for 5 minutes and were dried by air blowing as the final step.
Experimental setup for electro-polishing of depot stent prototypes.
The “standard” stent was first evaluated to establish the baseline information for this study. The FEA simulation was then conducted to investigate the impact of reservoir location on the mechanical integrity of the depot stent (e.g., equivalent plastic strain, radial strength, and fatigue safety factor). Five equally spaced circular (cylindrical if considering depth) and blind-hole reservoirs were cut on three major locations of the depot stent, namely, connectors, bar arms, and crowns (Figure 3).
From Table 1, it is clear that the creation of blind-hole reservoirs on either bar arms or connectors resulted in little or no change in equivalent plastic strain and radial strength. However, the degradation in the fatigue safety factor was the most significant among major clinical attributes investigated, with a 16–18% reduction compared to the “standard” stent. This indicates that the depot stent with reservoirs on the bar arms or connectors is resistant to vessel collapse or acute stent fracture but susceptible to long-term stent fatigue failure. In other words, the depot stent is more sensitive to dynamic loading than static loading.
However, the depot stent with blind-hole reservoirs on the crowns led to noticeable changes in equivalent plastic strain (+12%) and radial strength (–8%). Its fatigue safety factor declined further from the standard case of 3.05 to 2.33, a significant 24% reduction. By comparing these three reservoir locations, it is clear that cutting reservoirs on the stent crowns has the most significant impact among the three major locations investigated. This is not surprising considering the standard case that the maximum von Mises stress and maximum equivalent plastic strain always occur on the inner surface of the curved crowns, whereas the connectors and the bar arms are mostly under elastic deformation.
\n\t\t\t\tModel\n\t\t\t | \n\t\t\t\n\t\t\t\tRS\n\t\t\t\t \n\t\t\t\t(N/mm)\n\t\t\t | \n\t\t\t\n\t\t\t\tVariation\n\t\t\t\t \n\t\t\t\t(%)\n\t\t\t | \n\t\t\t\n\t\t\t\tPEEQ\n\t\t\t\t \n\t\t\t\t(%Strain)\n\t\t\t | \n\t\t\t\n\t\t\t\tVariation\n\t\t\t\t \n\t\t\t\t(%)\n\t\t\t | \n\t\t\t\n\t\t\t\tFSF\n\t\t\t | \n\t\t\t\n\t\t\t\tVariation\n\t\t\t\t \n\t\t\t\t(%)\n\t\t\t | \n\t\t
Standard | \n\t\t\t3.78 | \n\t\t\t- | \n\t\t\t40.5 | \n\t\t\t- | \n\t\t\t3.05 | \n\t\t\t- | \n\t\t
Hole- CO | \n\t\t\t3.77 | \n\t\t\t-0.26 | \n\t\t\t40.2 | \n\t\t\t-0.74 | \n\t\t\t2.56 | \n\t\t\t-16.07 | \n\t\t
Hole- BA | \n\t\t\t3.66 | \n\t\t\t-3.17 | \n\t\t\t39.7 | \n\t\t\t-1.98 | \n\t\t\t2.49 | \n\t\t\t-18.36 | \n\t\t
Hole- CR | \n\t\t\t3.49 | \n\t\t\t-7.67 | \n\t\t\t45.2 | \n\t\t\t11.6 | \n\t\t\t2.33 | \n\t\t\t-23.61 | \n\t\t
Hole- All | \n\t\t\t3.45 | \n\t\t\t-8.73 | \n\t\t\t45.1 | \n\t\t\t11.36 | \n\t\t\t2.17 | \n\t\t\t-28.85 | \n\t\t
Effects of reservoir location on stent mechanical integrity (blind-hole reservoirs).
Adding the number of reservoirs increases the total drug capacity of the depot stent. Therefore, investigation was also conducted for reservoirs evenly spread on the entire stent to understand whether the mechanical integrity of such a stent is further compromised or not (Figure 3(d)). Simulation results show that this specific depot stent followed a similar trend to the previous case (blind-hole reservoirs on the crowns only). Their equivalent plastic strain and radial strength were almost identical, whereas the fatigue safety factor continued to decline further, with a 29% reduction compared to the “standard” stent. This again demonstrates that the mechanical integrity of the depot stent is mainly dominated by the reservoirs located on the stent crowns.
Another method of increasing the total drug capacity is to cut through-holes instead of blind-holes into the stent struts. The through-hole reservoir design also allows different types of drugs with different release rates to be administered independently on opposite sides of the stent. Since this dual-side drug delivery concept is quite interesting, simulation was then carried out to investigate the effects of reservoir depth on the mechanical integrity of the depot stent. The depth of the reservoirs was increased from 50% to 75% and 100% of the strut thickness, with the last case equivalent to the through-hole scenario.
Table 2 summarizes the effects of reservoir depth on the key stent attributes for the case of depot reservoirs evenly spread on the entire stent. Figures 8 and 9 show the contour plot comparisons of the equivalent plastic strain developed during different stages of the loading process (crimping and expansion, respectively) between the standard case and the depot stent with through-hole reservoirs on the entire stent. It shows again that the maximum equivalent plastic strain occurred on the inner surface of the most critical region of the stent, the curved crowns. The maximum equivalent plastic strain was increased by 16%, and its strain distribution, as indicated by the colors, changed significantly due to the appearance of the through-hole reservoirs on the stent crowns. It was more evenly spread along the crown arc for the standard case but became non-uniform when the maximum stress–strain occurred at the 6 o’clock location of each reservoir.
\n\t\t\t\tModel\n\t\t\t | \n\t\t\t\n\t\t\t\tRS\n\t\t\t\t \n\t\t\t\t(N/mm)\n\t\t\t | \n\t\t\t\n\t\t\t\tVariation\n\t\t\t\t \n\t\t\t\t(%)\n\t\t\t | \n\t\t\t\n\t\t\t\tPEEQ\n\t\t\t\t \n\t\t\t\t(%Strain)\n\t\t\t | \n\t\t\t\n\t\t\t\tVariation\n\t\t\t\t \n\t\t\t\t(%)\n\t\t\t | \n\t\t\t\n\t\t\t\tFSF\n\t\t\t | \n\t\t\t\n\t\t\t\tVariation\n\t\t\t\t \n\t\t\t\t(%)\n\t\t\t | \n\t\t
Standard | \n\t\t\t3.78 | \n\t\t\t- | \n\t\t\t40.5 | \n\t\t\t- | \n\t\t\t3.05 | \n\t\t\t- | \n\t\t
Hole- All (50%) | \n\t\t\t3.45 | \n\t\t\t-8.73 | \n\t\t\t45.1 | \n\t\t\t11.36 | \n\t\t\t2.17 | \n\t\t\t-28.85 | \n\t\t
Hole- All (75%) | \n\t\t\t3.33 | \n\t\t\t-11.9 | \n\t\t\t46.8 | \n\t\t\t15.56 | \n\t\t\t2.06 | \n\t\t\t-32.46 | \n\t\t
Hole- All (100%) | \n\t\t\t3.08 | \n\t\t\t-18.52 | \n\t\t\t46.8 | \n\t\t\t15.56 | \n\t\t\t2.02 | \n\t\t\t-33.77 | \n\t\t
Effects of reservoir location on stent mechanical integrity (blind-hole reservoirs).
Figure 10 shows the radial strength comparison between the “standard” stent and the depot stent with through-hole reservoirs on the entire stent. The radial strength dropped by 19% in this case, and its peak value shifted toward the right by 0.1 mm when compared to the “standard” stent. This suggests that the through-hole depot stent is not as strong as its standard counterpart and could be collapsed by the arterial pressure at an earlier stage. Figure 11 shows the Goodman diagram comparison of the pulsatile fatigue loading between the “standard” stent and the depot stent with through-hole reservoirs on the entire stent. Simulation data of the “standard” stent were far below the Goodman diagram failure line, indicating that the “standard” stent is able to pass the fatigue life of 4 x 108 cycles with ease under pulsatile fatigue loading. Comparing Figure 11 (top) with Figure 11 (bottom), wherein the same stent but with through-hole reservoirs was evaluated for pulsatile fatigue loading, shows that the simulation data of the depot stent migrated toward the Goodman diagram failure line, indicating a significant drop of 34% in FSF and thus much lower fatigue resistance to systolic/diastolic blood pressures in this specific case.
Contour plot of the PEEQ of the standard stent (top) and the depot stent with through-hole reservoirs (bottom) at crimping with a magnified view of the stent crown in the right window.
Contour plot of the PEEQ of the standard stent (top) and the depot stent with through-hole reservoirs (bottom) at expansion with a magnified view of the stent crown in the right window.
Radial strength comparison of the standard stent and the depot stent with through-hole reservoirs.
Goodman diagram comparison of the standard stent (top) and the depot stent with through-hole reservoirs (bottom).
Figure 12 shows the effects of reservoir depth on the key stent attributes of the depot stent with the reservoir depth ranging from 50%, 75% to 100% of the strut thickness on the entire stent. It is shown that the radial strength decayed almost linearly with the reservoir depth. The loss in radial strength reached the maximum, a 19% reduction compared to the standard case, when the reservoirs were completely cut through. The equivalent plastic strain rose moderately with the reservoir depth but eventually reached a plateau for an approximately 15–20% gain. The fatigue safety factor remained the most critical factor among these key stent attributes. According to the chart, it fell significantly right from the beginning, even with shallow blind holes, but eventually reached a plateau and settled with an approximately 30–35% loss when compared to the standard case. Since the major stent clinical attributes suffered significant losses, it is not a good idea to pursue this specific design with through-hole reservoirs spread all over the entire stent. Given the fact that the crown is a critical region in a stent, we propose that an optimal depot stent should have through-hole reservoirs on the stent bar arms and/or connectors for the maximum drug capacity without compromising its mechanical integrity significantly. It should be noted that although the depot stent has the wonderful feature of precise and programmable drug release control, it was found that its fatigue safety factor could be compromised to certain degrees for all various forms of the depot stent.
Variation of key clinically relevant functional attributes versus drug reservoir depth.
A typical DES (3-ring model) with a 5 μm coating thickness carries a total drug-polymer volume of 0.0535 mm3 (Table 3). For the depot stent, our chosen reservoir size is 50% of the strut width and 100% completely through the strut thickness. Such a design is able to carry approximately 0.0004 mm3 of drug-polymer mix per reservoir. Therefore, by creating 135 through-hole reservoirs on the 3-ring depot stent model, its total reservoir capacity is enough to fully replace the surface coating layers of 0.0535 mm3 on a typical drug-eluting stent. This helps to completely eliminate the surface coating layers and thus reduce the overall stent profile.
The density and size of the reservoirs could be changed to increase the total drug capacity of the depot stent. For example, the total volume of the drug-polymer mix can be quadrupled to 0.221 mm3 when the through-hole reservoirs are evenly distributed on the entire stent, as shown in Figure 3(d). Our proposed depot stent in the next section, same stent as the one in Figure 3(d) without reservoirs on the crowns, also triples the total volume to 0.149 mm3.
\n\t\t\t\tModel\n\t\t\t | \n\t\t\t\n\t\t\t\tNumber of reservoirs\n\t\t\t | \n\t\t\t\n\t\t\t\tDrug-polymer compound carried (mm3)\n\t\t\t | \n\t\t
5-μm thickness coating on stent surface | \n\t\t\t- | \n\t\t\t0.0535 | \n\t\t
Through-hole reservoir (per reservoir) | \n\t\t\t- | \n\t\t\t0.0004 | \n\t\t
Through-hole reservoirs on CO | \n\t\t\t30 | \n\t\t\t0.012 | \n\t\t
Through-hole reservoirs on BA | \n\t\t\t270 | \n\t\t\t0.108 | \n\t\t
Through-hole reservoirs on CR | \n\t\t\t270 | \n\t\t\t0.108 | \n\t\t
Through-hole reservoirs on entire stent | \n\t\t\t552 | \n\t\t\t0.221 | \n\t\t
Proposed depot stent | \n\t\t\t372 | \n\t\t\t0.149 | \n\t\t
Estimated drug capacity of a depot stent (3-ring model).
We propose an optimal depot stent, with reservoirs uniformly distributed on the entire stent except the crown region, to increase the total drug capacity without comprising its mechanical integrity significantly (Figure 13). This depot stent has eight reservoirs on the connectors and six reservoirs on the bar arms, with the spacing of approximately 0.15 mm between two reservoirs. Simulation results on the variations of major clinical attributes are listed in Tables 4 along with those on other stents for comparison. Figure 14 shows the contour plot comparison of the equivalent plastic strain developed at the expansion stage among the standard case, the depot stent with through-hole reservoirs on the entire stent, and the proposed depot stent. For the proposed depot stent, the maximum equivalent plastic strain was actually reduced by 9% and the strain distribution was spread out even more uniformly than the standard case. This was attributed to the creation of through-hole reservoirs on the bar arms, effectively shifting the stress–strains away from the crown reservoirs and re-distributing them along the crown arcs. The radial strength only dropped by 10%. The fatigue safety factor was reduced marginally by 13% when compared to the standard case, whereas the very same stent but with through-hole reservoirs on the entire stent showed a staggering 34% reduction. This significant gain in fatigue safety factor was partially due to the removal of through-hole reservoirs on the crowns and partially due to the stress–strain redistribution mentioned above.
To sum up, the total drug capacity of our proposed depot stent could be tripled, with only marginal trade-off in major clinical attributes: its radial strength and the fatigue safety factor were reduced by only 10% and 13%, respectively. Therefore, this depot stent could carry more drugs and deliver them more smartly than the modern drug-eluting stents, thereby opening up a wide variety of new treatment opportunities such as the renal disease or cancer target therapy.
Proposed depot stent with a magnified view of the stent mesh in the right window.
Contour plot of the equivalent plastic strain of the standard stent (top), the depot stent with reservoirs on the entire stent (middle), and the proposed depot stent (bottom) at expansion with a magnified view of the stent crown in the right window.
Besides the circular (cylindrical if considering depth) reservoirs aforementioned, other reservoir shapes were also investigated, for example, hexagonal and square reservoirs (Figure 15). The length on each side of the hexagonal and square reservoirs (0.0349 mm and 0.0563 mm, respectively) was determined in a way that each reservoir capacity was identical to that of a circular reservoir. The location of the reservoir center remained unchanged.
Table 4 lists the mechanical integrity of the depot stent as a function of the reservoir shape. Simulation results show that the changes in the reservoir shape caused little differences in equivalent plastic strain and radial strength, with approximately 10% across-the-board reductions when compared to the “standard” stent. In terms of the fatigue safety factor, the hexagonal reservoir had a slight 5% advantage over the circular and square reservoirs. This could be attributed to the stress re-distribution within the stent and the hexagonal reservoir makes a bigger impact than reservoirs with other shapes.
Depot stent with (a) hexagonal and (b) square reservoirs.
\n\t\t\t\tModel\n\t\t\t | \n\t\t\t\n\t\t\t\tRS\n\t\t\t\t \n\t\t\t\t(N/mm)\n\t\t\t | \n\t\t\t\n\t\t\t\tVariation\n\t\t\t\t \n\t\t\t\t(%)\n\t\t\t | \n\t\t\t\n\t\t\t\tPEEQ\n\t\t\t\t \n\t\t\t\t(%Strain)\n\t\t\t | \n\t\t\t\n\t\t\t\tVariation\n\t\t\t\t \n\t\t\t\t(%)\n\t\t\t | \n\t\t\t\n\t\t\t\tFSF\n\t\t\t | \n\t\t\t\n\t\t\t\tVariation\n\t\t\t\t \n\t\t\t\t(%)\n\t\t\t | \n\t\t
Standard | \n\t\t\t3.78 | \n\t\t\t- | \n\t\t\t40.5 | \n\t\t\t- | \n\t\t\t3.05 | \n\t\t\t- | \n\t\t
Proposed depot stent | \n\t\t\t3.41 | \n\t\t\t-9.79 | \n\t\t\t36.7 | \n\t\t\t-9.38 | \n\t\t\t2.66 | \n\t\t\t-12.79 | \n\t\t
Hexagonal reservoir | \n\t\t\t3.43 | \n\t\t\t-9.26 | \n\t\t\t36.4 | \n\t\t\t-10.12 | \n\t\t\t2.80 | \n\t\t\t-8.20 | \n\t\t
Square reservoir | \n\t\t\t3.38 | \n\t\t\t-10.58 | \n\t\t\t36.1 | \n\t\t\t-10.86 | \n\t\t\t2.65 | \n\t\t\t-13.11 | \n\t\t
Effects of reservoir shape on stent mechanical integrity.
Effects of the reservoir size and number of the depot stent were also investigated (Figure 16). The total drug loading capacity on each bar arm/connector was intended to maintain the same, whereas the size and number of the reservoirs were adjusted accordingly. Stent variation #1 had smaller but more reservoirs, with 8 reservoirs on the bar arms and 10 reservoirs on the connectors; on the other hand, stent variation #2 had larger but fewer reservoirs, with four reservoirs on the bar arms and six reservoirs on the connectors. In all cases, the total length between the farther edges of two end reservoirs remained unchanged.
Table 5 lists the mechanical integrity of the depot stent as a function of the reservoir size and number. Simulation results show that, for the same drug loading capacity, stent variation #2 with larger and fewer reservoirs yielded lower radial strength, but smaller equivalent plastic strain and thus higher fatigue safety factor. In addition, the fatigue safety factor seems to be more sensitive than the equivalent plastic strain and radial strength in this case. Its value increased from 2.36 to 2.88, a 17% jump from stent variation #1 to #2, which is consistent with the earlier observation. Therefore, stent variation #2 of larger and fewer reservoirs is a better candidate for drug delivery; its total drug capacity could be tripled with marginal trade-off in its major clinical attributes: the radial strength and fatigue safety factor were reduced by only 11% and 6%, respectively.
Model | \n\t\t\tRS (N/mm) | \n\t\t\tVariation (%) | \n\t\t\tPEEQ (%Strain) | \n\t\t\tVariation (%) | \n\t\t\tFSF | \n\t\t\tVariation (%) | \n\t\t
Standard | \n\t\t\t3.78 | \n\t\t\t- | \n\t\t\t40.5 | \n\t\t\t- | \n\t\t\t3.05 | \n\t\t\t- | \n\t\t
Proposed depot stent | \n\t\t\t3.41 | \n\t\t\t-9.79 | \n\t\t\t36.7 | \n\t\t\t-9.38 | \n\t\t\t2.66 | \n\t\t\t-12.79 | \n\t\t
Variation #1 | \n\t\t\t3.48 | \n\t\t\t-7.94 | \n\t\t\t37.6 | \n\t\t\t-7.16 | \n\t\t\t2.36 | \n\t\t\t-22.62 | \n\t\t
Variation #2 | \n\t\t\t3.38 | \n\t\t\t-10.58 | \n\t\t\t34.8 | \n\t\t\t-14.70 | \n\t\t\t2.88 | \n\t\t\t-5.57 | \n\t\t
Effects of reservoir size and number on stent mechanical integrity.
Proposed depot stent and its variation #1 and #2.
In this study, the input laser parameters used for cutting stents are listed as follows: average power 37.5 W, pulse repetition rate 80 KHz, cutting speed 5 mm/s, and Argon pressure 12 bar. Optical microscopy was used to measure the kerf width and observe the surface conditions of each stent.
Figure 17 shows the relationship of the average kerf width vs. the distance between the laser source and hypotube surface. The kerf width had the minimum value of 23.2 μm at the distance of 0.37 mm. When the distance was between 0.27 mm and 0.51 mm, the laser beam was able to penetrate through the hypotube, resulting in successful cutting within a focal-depth range of 0.24 mm. Figure 6 is the depot stent design pattern cut onto a seamless hypotube by laser prior to material removal and electro-polishing.
Average kerf width vs. distance between laser source and hypotube surface.
The polishing conditions for surface finishing of depot stent prototypes are listed in Table 6. Figure 18 shows the depot stent prototype before and after electro-polishing. These stents were able to achieve high quality of mirror-like surface finishing after polishing. Figure 19 is the prototype of our proposed depot stent of 2 mm diameter and 22 mm long for demonstration of our design concept.
\n\t\t\t\tStirring speed\n\t\t\t\t \n\t\t\t\t(RPM)\n\t\t\t | \n\t\t\t\n\t\t\t\tCurrent\n\t\t\t\t \n\t\t\t\t(A)\n\t\t\t | \n\t\t\t\n\t\t\t\tTime\n\t\t\t\t \n\t\t\t\t(sec)\n\t\t\t | \n\t\t\t\n\t\t\t\tTemperature\n\t\t\t\t \n\t\t\t\t( Co )\n\t\t\t | \n\t\t
500–550 | \n\t\t\t0.242 | \n\t\t\t105–135 | \n\t\t\t50–55 | \n\t\t
Electro-polishing conditions for surface finishing of depot stents.
Surface conditions of manufactured depot stents before (left) and after (right) electro-polishing.
Prototype of our proposed depot stent.
The depot stent with micro-sized drug reservoirs is a novel concept for smart drug delivery and provides a promising future for highly controlled release of different medications with programmable spatial/temporal control. However, creating such drug reservoirs on the stent struts inevitably weaken the stent scaffolding and compromise its mechanical integrity. The impact of these drug reservoirs on major clinical attributes of the depot stent was systematically investigated. Several conclusions were drawn from this study:
The reservoirs on either bar arms or connectors had little effects in equivalent plastic strain and radial strength when compared to the “standard” drug-eluting stent. However, the fatigue safety factor was reduced more significantly, suggesting that the depot stent is resistant to acute stent fracture or vessel collapse but susceptible to long-term stent fatigue failure.
Creating reservoirs on the crown region of the depot stent has the most significant impact among all major locations.
The degradation in mechanical integrity is more sensitive to reservoir location than reservoir depth.
The hexagonal reservoirs resulted in a marginal increase in fatigue resistance when compared to the circular and square reservoirs.
For the same drug loading capacity, larger and fewer reservoirs resulted in a noticeable increase in the fatigue resistance over smaller and more reservoirs.
Our proposed depot stent was proven to be a feasible design. Its total drug capacity could be tripled with acceptable/marginal trade-off in major clinical attributes: the radial strength and the fatigue safety factor of our proposed depot stent were reduced by only 10% and 13%, respectively.
A prototype of our proposed depot stent (2 mm diameter and 22 mm long) was manufactured for the feasibility demonstration of our design concept.
This study can serve as a guideline to help future depot stent designs to achieve the best combination of stent mechanical integrity and smart drug delivery.
This research was supported by the Ministry of Science and Technology in Taiwan through Grants MOST-103-2622-E-002-010-CC1 and NSC-102-2221-E-002-130-MY3. The authors are grateful for the support and help.
Polyesters constitute an important group of polymers widely used in many branches of economy, such as electrical, electronic, building, clothing and packaging industries, and medicine. These polymers are characterized by excellent electro-insulating properties, mechanical strength, thermal resistance, and susceptibility to multiple processing. A common feature of polyesters is the presence of ester group in the main chain structure. This group imparts a polar character to polyesters. Aromatic groups in the repeatable units of polyester chain impart to the polymers an increased physical resistance, such as thermal (increased melting point and glass transition temperature) and mechanical resistance (increased mechanical parameters such as elasticity and strength). The content of aromatic structures in polyesters is different and has been used for the internal classification of these polymers into aliphatic, aliphatic-aromatic, and aromatic polyesters. Aliphatic polyesters contain no aromatic structures, for example, poly(ethylene adipate). This group includes biodegradable polyesters, such as poly(lactic acid) (PLA) (Figure 1B), polyesters of butyric acid (PHB), and poly(butylene succinates) (PBS) widely used in medicine and technology. Aliphatic-aromatic polyesters contain both the aliphatic and aromatic structures (e.g., poly(ethylene terephthalate), polycarbonates) (Figure 1C, D). These polyesters show very good thermo-mechanical, impact, tribological, and optical properties, as well as resistance to atmospheric and functional conditions, which qualify them for mass production and versatile use. Finally, polyesters with “purely” aromatic structures—polyarylates (poly(4-hydroxybenzoate), poly(bishfenol-A-terephthalate), liquid crystal polyester) that show excellent thermal and dimensional stability, impact resistance, fire resistance, and nonlinear optical properties (NLO) [1]. Aromatic polyesters are used in the production of membranes, films coatings for electronic and electrical industries, optical waveguides and devices doubling the frequency of electromagnetic waves [1].
Structures of chemical groups in polyesters.
The chapter presents the behavior analysis of the following polyesters in DC electric field: aliphatic-aromatic polyesters with the example of poly(ethylene terephthalate) (PET) and polycarbonate (PC) and aliphatic polyester with the example of poly(lactic acid).
The processes of nonstationary current flow in polyesters and possible mechanisms that generate the nonstationary states are presented. Using the example of PET film tests, the ionic character of current conduction is shown. It is the effect of air humidity in the environment where a product is used that is connected with the ionic character of current conductance. There observed a well-known effect of deteriorating the PET film electro-insulating properties with increasing the content of water vapor in air [2]. In this study, it has been shown that this effect additionally depends on the form of product, and it is particularly intensified in fibrous structures, where the conduction is increased by about 1000 times. The effect of the physicochemical state and coarseness of surface on the level of static electricity charge is shown. The processes of static electricity of polyesters, electricity conditioning, and hypotheses explaining these processes with a particular consideration of the static electricity of polyester-metal contact are discussed. The polyesters are important precursors for making electrets. The various techniques of electrets making and the issues of the electric charge relaxation that are of great importance for the electrets stability are presented. Information about the development trends of polyester and new fields of application are also included.
Polyesters are real dielectrics with a low electric conductance, polarizability in electric field, and a strong susceptibility to static electricity. Electric properties are determined by the presence of connected charges and trace quantities of free electric charges that are generated by defects, impurities, technological additives, or injected from electrodes or environment (e.g., low-temperature plasma). During the interaction of DC field, free charges undergo migration and connected charges are polarized. Migration and polarization processes occur in parallel, shaping a characteristic, nonstationary image of the polymer electric conductance.
The current that flows in a real polymeric dielectric under DC field is transient in nature and called absorption current. The density of absorption current decreases in time to reach a steady-state value called conductance current. The transient (absorption) current “j(t)” is presented as a sum of the steady-state conduction current “jc,” the transient current (displacement current) “jp(t),” and diffusion current “jd“:
In many polymers, the decreasing transient polarization current is in a direct relation with the function of polarization decay P(t). The polarization decay occurs according to the dependence:
where: t—time and τp—macroscopic relaxation time of electrical polarization. The decrease in current in dielectric polymer can be approximated by Curie function:
where: A—coefficient of proportionality dependent on temperature, n—power coefficient dependent on relaxation processes, n < 1 for times t > τ, while n > 1 for t < τ. With the use of the above approximation to transient conduction processes in PET, the following values of power coefficient are obtained: n = 0.75 [3], n = 0.33 [4], and n = 0.79 [5]. The transient character of current indicates a complicated behavior of electric charges in dielectric polymers. The values of conductance current are lower by one to several orders compared to the initial absorption current and the time of reaching a steady-state value is from several minutes to dozen hours depending on polymer. To metrologically characterize a polymer, it is indispensable to determine the isochronal absorption current (i.e., current at constant times).
Test results indicate that the transient character of current depends on the factors connected with the object tested (chemical structure and the presence of polar groups, physical microstructure, thermal and electric history of a sample) and test conditions (field intensity, temperature and relative humidity of the medium, and electrode material and the presence of residual charge in the material tested).
Das-Gupta in his studies [7, 8, 9] has presented several mechanisms explaining the transient current flow in polymers. His intention was to indicate a method of identifying the mechanism type. The identification consists in performing a series of tests for the given polymer: relationship between electric field and the isochronal current transient, temperature dependence, effect of electrode material, thickness dependence, and time dependence of transient current. Then, the test results are analyzed with respect to the probability of the given mechanism. Das-Gupta characterizes the features of five mechanisms potentially generating transient current flows:
Electrode polarization: isochronal current proportional to field, a strong effect of the electrode material by blocking the flow, the process is thermally activated, power coefficient: initially n = 0.5 followed by n > 1.
Orientation of dipoles uniformly arranged through the polymer volume: isochronal current proportional to field, the process is thermally activated, electrode materials and thickness sample independent of isochronal current at DC field, power coefficient 0 ≤ n ≤ 2.
Charge injection forming trapped space charge: isochronal current is controlled by injection method (electronic, corona discharge, and glow discharge methods), the thickness sample independent of isochronal current at DC field, dependence electrode material related to injection method, thermally activated process related to injection method, power coefficient 0 ≤ n ≤ 1.
Charge tunneling from electrodes to empty traps: (isochronal current proportional to field, thickness sample inversely proportional dependent of isochronal current at DC field, the process is thermally independent, strongly dependence electrode material, power coefficient 0 ≤ n ≤ 2
Hopping of charge carriers from one localized state to another: isochronal current proportional to field, the process is thermally activated, thickness sample and electrode materials independent of isochronal current at DC field, power coefficient 0 ≤ n ≤ 2.
The starting point of recognizing the conductance process is the determination of charge carrier nature (electrons, ions). The assessment of carrier types is carried out by direct testing through comparing the charge transferred with the mass of substance released on electrodes in contact with the polymer (mass spectrometry and neutron activation analysis). Indirect methods include: tests of voltage-current and current-temperature characteristics [5, 10, 11], dielectric tests [12], tests of photo-electric and electro-chemical effects as well as tests of the dependence of polymer conductance on pressure [13]. The image of the pure polymer conductance significantly differs from that of polymer containing ionogen compounds (e.g., stabilizers, catalyst residues, impurities, other additives, and products of chemical decomposition). Compounds of this type (e.g., water) easily dissociate in volume or on the polymer surface imparting an ionic character to conductance.
In studies carried out on a pure PET, their authors present divided views: Amborski [10], Saito [13] declare themselves in favor of the ionic conductivity mechanism in PET. Based on testing the electro-chemical effect with the use of thermally stimulated currents (TSC), Sawa [14] concludes that electronic conduction occurs at temperatures to Tg and ionic conductance at temperatures above Tg. The concept of ionic conductance in polymers is based on the assumption of the presence of molecules capable of dissociating in the polymer structure and various structural effects that determine the available internal volume of polymer and are responsible for the diffusion processes and ion transport in the polymer volume. The migration of free ions consists in specific, hopping change in ion positions [15] Charge motion is given by the carrier hopping over the potential barrier. It takes place from one to the other position, in which the ion achieves the minimal value of potential energy. The mobility of free ions is limited by the value of potential barrier WB, which should be defeated by the ion during migration. The electric field reduces the barrier by the value “Δ“:
where: E is DC field intensity, e—electron charge, and a—hopping dislocation path of ion. Based on the calculations of the probability of change in ion position in the polymer during migration and the rate of ion dislocation from one to the other position [15], the current density of conduction under the conditions of the DC action can be given as follows:
where: z—number of elementary charges, e—electron charge, n—density of dissociated molecules, a—path of the charge hopping dislocation, k—Boltzmann constant, WB—barrier of the charge potential energy, E—DC field intensity, T—temperature, G and H—constants in the generalized form of Eq. (5).
Poly(ethylene terephthalate) is a commonly used polymer in electrical engineering and electronics, where its electro-insulating properties are successfully used. In this study, commercial PET films, Estrofol type, containing indispensable additives, such as catalysts and stabilizers: manganese acetate (0.035% by wt.), calcium acetate (0.052% by wt.), antimony trioxide (0.040% by wt.), and phosphorus compounds (0.035% by wt.) were tested. Three variants of PET film with different supermolecular structures were tested: oriented films technologically drawn in a single-axis direction in a ratio of 2×, 3.5×, and 4×.
Al electrodes with a protective ring were evaporated on the purified film surface. The kinetics of absorption/desorption (depolarization) currents were examined for 20 min with the use of the measurement system described in paper [16]. Figure 2 shows the kinetics of the volume absorption current in PET film (Table 1, draw ratio 4.0×) measured for 10 min. After this time, DC field was turned out and with earthed electrodes, the depolarization current (desorption current) was recorded for 10 min.
Decay of the transient (A) and depolarization (D) currents Iv = f(t) in a PET film (R = 4.0×) under a DC field (Al electrodes, t = 23°C, RH = 25%).
Characteristics of PTE films.
* determined by the densitometric technique.
Current-voltage characteristics were determined on the basis of the absorption current corrected by the desorption component according to the procedure of Badiana [17]. The characteristics j = f(E) shown in Figure 3 indicate linear behavior of the films and a shift into nonlinear state at an intensity of 3.5 × 107 V/m. Based on the nonlinear range of field intensity that in the case of samples B and C is developed within a high range of the intensity of field E, the analysis of the ionic conductance of the films was carried out.
Characteristics of PET films. Films denotation according to Table 1.
The experimental current-voltage characteristics of samples B and C were approximated with the Eq. (5). The approximation consisted in directing the function: j = G.sinh H.E to a linear form, that is, to develop it into Taylor series according to the procedure given in monograph [18], taking into account only the linear elements of this solution. In calculations based on the minimization procedure, the following parameter values were obtained:
for PET (R = 3.5×) G = 7.20 × 10−13 m/V, H = 2.4 × 10−8 m2/A, a = 1.24 nm
for PET (R = 4.0×) G = 7.41 × 10−13 m/V, H = 2.4 × 10−8 m2/A, a = 1.24 nm
a—path of the hopping shift of charge between successive equilibrium positions.
Parameter “H” characterizes the migration of the charge carriers in the process of conduction. With an assumption of one carrier type (e.g., proton, z = 1), the path of protons in the PET tested amounts to 1.24 nm and is the same for the film stretched 3.5 and 4.0 times. The value calculated is probable if we consider the distances between PET macromolecules. The investigations of PET films by the method of wide-angle diffraction [19] prove that the phenyl ring planes of macromolecules in the crystalline areas are oriented in parallel to the film plane. The distance between the phenyl ring planes perpendicular to the film plane (i.e., toward field E)—thus toward the motion of charge carriers according to the calculation [19], it amounts to 0.32 nm. If the migration of charge carriers occurs in the noncrystalline areas of polymer, where the intermolecular distances are higher than 0.32 nm, the value of the charge hopping shift is real. Figure 4 shows the comparison of theoretical and experimental j = f (E) characteristics of PET films.
Comparison of theoretical and experimental density current - DC field intensity characteristics of PET films.
The conformity of current-voltage courses allows one to think that in the PET films tested, the conduction of charges is of ionic character. The resistivity of PET films calculated for the proportional range of characteristics I = f(U) is for R = 2.0× lg ρV = 15.400 ± 0.157, for R = 3.5× lg ρV = 15.500 ± 0.128, and for Rx = 4.0 lg ρV = 15.277 ± 0.177. It seems that the structural differences in the films tested do not influence the DC conductance in a statistically significant way.
Urbaniak-Domagala has presented in her study [16] the tests results of DC conductance in polylactide film. The tests were carried out with the use of Al and Au rigid electrodes within the range of DC field from 8.3 to 33.3 MV/m. In DC field, polylactide behaves as PET, showing a low conductance, a transient in nature current flow, dependence of current density on the electrode material j PET-Al-PET > j PET-Au-PET and a low energy of the conduction activation energy (0.4–0.7) eV that increases in the area of glass transition (0.9–1.1) eV.
There are many examples of end products (films woven and knitted fabrics, nonwovens) designed for protective applications, such as electro-insulating barriers and protective clothing that are used under variable environmental conditions. Environmental factors bring about polymer aging [2]. Qualitative and quantitative tests of the polyester product aging have shown a significant effect of water vapor in air as an ionogenic factor that is permanently present during operational use, while the content of water vapor is variable as the relative humidity in the medium. The interaction of water vapor is dependent not only on the polymer absorption properties, but also on the product structure. This effect is particularly distinct in textile fabrics, where the fabric structure has a hierarchic character determined by complex systems of fibers with micrometric thickness, forming a yarn and the yarn threads form a higher structure such as thread repeat. The textile fabric structure intensifies absorption processes due to capillary processes that do not occur in monolithic products such as films or foils.
The importance of the problem of the air humidity effect in environment on the processes of current flow in a product in DC field was presented using an example of woven fabric made of 100% PET fibers. In order to do that, the isochronal current absorption and current depolarization in the woven fabric for 10 min were examined using an electrostatic field intensity of E = 1 kV/mm. The fabric was first purified and its surface weight was 100.0 g/m2 and thickness of 0.2 mm. They used rigid electrodes with a diameter 50 mm and a constant unit load according to EN 1149-2:1999 + Ap1:2001, in a standard screened measurement system [16]. Before the tests, the woven fabrics were preheated to constant weight “ms” considered as dry mass. Then, isothermal environment conditions were established (t = 23°C) under which the air relative humidity was changed from 25 to 88% followed by return to the initial humidity 25%. For each variant of RH in the range of 25–88%, the samples were air conditioned for 24 h and then tested under these conditions. The tests were carried out in an air conditioning chamber Feutron. In parallel to the absorption and depolarization currents, the mass of moist fabric “mw” was determined and then the absolute humidity of fabrics “W” was calculated from the dependence:
Figure 5 presents the effect of relative humidity (RH) of air in the isothermal medium on the volume resistivity “ρV “of PET woven fabric in the process of sorption followed by desorption.
Effect of relative humidity (RH) of air in an isothermal medium on the volume resistivity “ρV” of PET woven fabric: (A) absolute humidity of woven fabric “W” = f(RH), (B)ρV = f(RH), (C) log ρV = f(W).
Figure 5A shows that with increasing RH from 25 to 88%, the volume resistivity of PET woven fabric is decreased by about 1000 times. Changes in resistivity are nonlinear and conditioned by the modifying interaction of the molecules of water absorbed from air and physico-chemically and physically added to the fabric as shown by the sigmoidal character of the sorption and desorption isotherms (Figure 5B). Moreover, it is observed that the humidity and resistivity “ρV“ depend on the way of reaching the physical equilibrium of sample by wetting or drying, which results in the hysteresis of humidity of fabrics and hysteresis of volume resistivity. The occurrence of hysteresis parameters proves how important is the definition of the fabric pre-conditioning and its acclimatization for tests.
The absolute humidity of woven fabric is an important factor influencing the processes of charge conductance and depolarization. For the PET woven fabric tested in the electrostatic field E = 1 kV/mm, the volume resistivity “ρV“ of the fabric as a function of humidity “W” (Figure 5C) satisfy the generalized dependence:
where: W—absolute humidity of fiber, a = − 10.559, b = 16.150 (Figure 5C), which is consistent with the results of Morton and Hearle [20]. Based on the above dependence, one can estimate the volume resistivity of PET woven fabrics in the state of humidity W = 0% at a level of log ρV = 16,150 with correlation coefficient R2 = 0.98.
Polyesters form a group of polymers with a high susceptibility to static electricity and a long lifetime of charges generated on surface and in volume. This feature results from a low number of free charges and a low electric conductance. On account of a high Debye radius, a charge can be formed in both the polymer top layer and volume [21]. The mechanism of static electricity of polymers is complex since the electric loading state attained constitutes a resultant effect of three partial processes: charge generation, charge storage on polymer surface, and in its volume and charge decay by relaxation and transport. Each of these partial processes proceeds differently in the case of so-called contact electricity, triboelectric, induction, injection in corona and glow discharges, γ
The next question to be solved is the assessment of the carrier nature responsible for electricity. Many authors believe that electricity depends on the injection of electrons from/to the polymer surface [26, 27]. The mechanism of contact electricity with the polymer-metal system is considered when we use the notions of band structure of a solid body (insulator or semiconductor) to describe the polymer energetic structure. For polymers, the occurrence of a wide band of forbidden positions and a specified work of electron exit are assumed. A characteristic feature is the occurrence of discrete donor and acceptor energetic states in the band of forbidden energies. These states are localized at the edges of conduction and valence bands. The succession of the presence of discrete states is the broadening of band edges. According to Fuhrmann [28], Fabish [25], and Mizutani [29], these states can trap the charge carriers and take part in contact electricity.
Energetic states localized in the polymer top layer are energetically different as a result from the presence of impurities, free radicals, absorbed molecules, products of polymer oxidation, ends and branches of macromolecules, and structural defects. The nature of localized energetic states is also modified by the polymer physical microstructure, mainly by the content of crystalline phase, shape, and perfectness of crystallite and interphase structures: crystallite-amorphous zone. The use of the notion of trapping states is convenient for the description of the processes of charge displacement and accumulation in polymeric dielectrics. The filling of traps with various charges leads to a permanent presence of charges in polymer.
An open issue is the investigation method of the distribution of the charge generated in polymers. The knowledge of charge distribution is indispensable for the identification of electric conductance processes, strength of polymeric dielectrics, and their aging as well as the mechanisms of polarization in polymeric electrets. The classic methods of investigating the distribution of charges and electric relaxation of polymers include the methods thermally stimulated currents (TSC) and thermally stimulated depolarization currents (TSDC) [30]. The drawback of TSC and TSDC methods is their destructive character in relation to the sample tested. Apart from the thermally stimulated method, they developed acoustic methods suitable for a direct use in testing polymeric insulations under voltage, measurements of the distributions of electric fields [31, 32], and tracing the development of polarization in polymers under high field voltage. Three types of acoustic methods using different techniques of acoustic wave generation are now under improvement: laser induced pressure pulse (LIPP) [32], pressure wave propagation (PWP) [33], and electrically stimulated acoustic wave (ESAW) [34, 35, 36]. The analysis of signals from polymers affected by impulsing allows one to diagnose the space distribution of charge in polymers. According to the opinion of Motyl [37], the use of two measurement methods, for example, PWP and ESAW increases the reliability of results and extent the range of diagnosis. In PWP method, the deformation wave causes local changes in free and polarization charges in time, which is a source of the signal observed. In ESAW method, the applied voltage impulse to sample generates a signal from the whole charge and the homogeneously distributed dipoles are not detected.
Hennecke et al. [24] have proposed an analog model of polymer electricity consisting in contacting with metals in the interrupting way repeated until the charge is established. The qualitative interpretation is based on the concept of the band polymer model, where the occurrence of the energetic levels of forbidden bands and heights of barrier between these levels in polymer were taken into account. The static electricity of polymers with the repetition of contacts has been also analyzed by Fuhrmann [28], Fabish and Duke [25], Lowell [26] and Mizutani et al. [29]. Fabish and Duke have observed a reversible change in the sign of charge transferred on polymer from metals after successive contacts. These authors explain this phenomenon with the presence of charges in the form of molecular ions in the polymer top layer. These ions form local energetic states.
Mizutani et al. [29] have determined the properties of the metal-polymer contact by means of the photo-injection of electrons from metal (Cu, Al) to PET, indicating the existence of energetic states on the surface of PET. These researchers also assessed the density of surface states amounting to 1.7 × 1014 (cm−3 eV−1) and confirmed a strong dependence of the contact energetic barrier on atmospheric conditions, including the presence of adsorbed oxygen molecules on the PET surface. Oxygen molecules, on account of a high affinity of electrons, can form surface states and act as electron traps. According to the opinion of Brennan et al. [38], during single and multiple contacts of polymer with counter-surface a charge is created whose range is limited to a depth of maximum 3 nm.
Lowell [26, 27] proposes a mechanism of charge transfer from metal to polymer (PET and PTFE). In the case of a single contact of polymer and metals (Al, Pt, and Hg), Lowell has found that the charge density does not depend on the type of metal. He puts forward a thesis that during a single contact, the tunneling of electrons occur from metal to traps in the polymer top layer, but the system does not reach thermodynamic equilibrium. The difference in electrostatic potentials created in the charge layer is inadequate to increase the energy of trap levels up to the Fermi energy of the metal. Only with a multiple contact, the charge is transferred to a higher depth. Then, it is possible to reach the equilibrium state. Lowell predicts the achievement of equilibrium charge density that will be linearly dependent on the metal work-function.
In this study, the susceptibility of PET film to static electricity in contact with metals (Al and Au) was investigated. Two pairs of cylindrical electrodes made of brass with a diameter of 50 mm and a height of 30 mm were used. Au and Al metals were evaporated onto the polished cylinder surface in the glow discharge aided with Argon. The coatings obtained were characterized by a high stability of the joint with the substrate, which made it possible to purify the surfaces before the tests of contact electricity. The PET films purified and devoid from charges were located between earthed electrodes in the following systems: Au-PET-Au and Al-PET-Al for 60s and then the electrodes were taken off in a frictionless way. The measurements of the sample surface charge were carried out in a contactless way by means of the probe of field intensity meter in the system described in paper [16]. During repeated contacts, the surface charge increased up to the saturation state that was obtained with the number of contacts of at least 100. The contact electricity was performed for PET film samples described in Table 1. The test results obtained are listed in Table 2.
Results of testing the surface properties of untreated and air plasma-treated PET films (power 100 W, flow 100sccm, pressure 20mTr).
In this study, we also attempted to answer the question how the activation of PET surface influences its susceptibility to electricity. To that end, the films were subjected to an activating plasma treatment. The process was realized in a low-temperature plasma, in glow discharge RF under reduced pressure air with the use reactor system described in paper [39]. The test results were supplemented with the examinations of the sample volume and surface resistivity in DC field with the use of rigid Al electrodes.
In Table 2 are listed the test results of volume and surface resistivity in contact with Al electrode, surface density of the charge were generated in contact with Au and Al metals. The resistivity was calculated on the basis of absorption and depolarization currents according to proceeding Badian [17]. The polarization component of the PET film surface free energy before and after the plasma treatment is also shown.
Based on the tests of the surface and volume resistance of untreated films (Table 2), one can think that the structural changes expressed by the crystallinity degree ranging from 7.4 to 19.8% and the Herman’s coefficient of orientation from 0.12 to 0.58 do not influence, in a statistically significant way, the DC conduction in the top layer and volume of the films. Also, no significant effect of changes in physical structure on the level of charge generated in contact with Al and Au metals was found. On the other hand, a significant effect of the metal type on the charge value is observed. The density of charge generated on the PET film surfaces in contact with aluminum is higher than that in contact with gold. Let us assume Lewell hypothesis [26] that the value of static charge depends on the difference in the exit works of materials in contact. According to Davies [40], the values of the exit work of the material tested are arranged in the following sequence PET > Au > Al. This relation of exit works justifies the negative sign of charge generated on the PET surface in contact with metals, as well as the higher charge density in contact with aluminum compared to that in contact with gold.
The processes of static electricity of polymers are typical surface processes. The physicochemical and physical states of surface are of importance for the course of contact electricity processes. In this study, the physicochemical and physical properties of the PET film top layer were modified by air plasma treatment. The physicochemical properties of PET films were assessed by measuring the contact angle with water and methylene iodide and the free energy calculated according to procedure of Owens and Wendt [41]. In Table 2 are listed the values of the polar component of the surface energy of films treated with plasma. The dispersion component was unchanged and therefore it was omitted. The plasma treatment was carried out for 1–30 min. Already from 1 minute to 3 minutes, the plasma treatment strongly increases the polar component of the top layer of the PET films. The physicochemical changes result in a negligible decrease in the value of surface and volume resistivity of the films. The PET film surfaces modified with plasma in contact with Al and Au metals still show a negative charge, but the surface density of charge in PET films is significantly decreased. The generation of charges is the more weakened, the longer the surface was treated with plasma. These changes can be related to the surface physical state. The plasma interaction causes significant physical changes in the film surface microtopography. Figure 6 shows the electron microscopic images of the PET film surfaces treated with plasma for 1, 3, and 30 min.
Electron microscopy images of PET film surface, (A) untreated; treated with air plasma (power 100 W, flow 100 sccm, pressure 20 mTr): (B) 1 min, (C) 3 min, (D) 30 min. The arrows show the direction of films orientation.
The effect of etching the polymer in its top layer that intensifies with increasing the plasma treatment time is observed. The surface is curved to form groove cavities located crosswise to the direction of stretching the film. The height of microroughness of surface subjected to plasma exposure for 30 min. With the strongest effect of etching can be assessed to about 0.04 μm. The character of surface carving is conditioned by the physical microstructure of PET. Based on the hypothesis of the polymer semi-crystalline structure, they are distinguished into two principal phases: crystalline and amorphous phases. The crystalline phase is characterized by a high energy of molecular cohesion that conditions its particular resistance to destructive factors, such as heat and the forces of interactions with plasma molecules. Considerably less resistant is the amorphous phase, in which the interaction between molecules are statistically incidental, and this phase is etched in the first place. In the image of the etched top layer, the protrusions seem to be formed by the crystalline phase of PET periodically occurring with long period in macro-fibrils. The etching effect generates the coarseness of surface that intensifies with prolonging the plasma action. The surface coarseness determines the real surface of film contact with metals. The coarser the surface, the lower is the charge that was found in measurements.
Electrets are materials in which a permanent electric charge occurs. The charge in electrets is stable for a longer time and its stability depends on the type of materials and exploitation conditions. It can last for years, while the material is an active source of electric field. The electrets are widely used in dust and gas filters, micro-machines, xerography, electro-acoustic transducers. Many electro-insulating polymers are used as electret precursors, including dipole (PET, PMMA and PS) and nondipole polymers (PTFE, PP and PE). The use of precursors in the form of films allows one to miniaturize the devices based on electrets. The charging state of electret material is obtained in various ways [22, 32, 46] using triboelectric effect (triboelectrets), DC electric field (thermo-electrets), UV and VIS radiations (photo-electrets), β and γ radiation (radio-electrets), glow and corona discharges (corona electrets).
Polyesters are materials with potential use as electret precursors owing to their susceptibility to dipole polarization and good mechanical parameters. Thermoelectrets are made by polymer polarization in DC electric field under conditions above the glass transition temperature and then after charging, they are refrigerated to freeze the state of oriented dipoles. The surface charge formed is called heterocharge if its sign is opposite to the sign of electrode potential in contact and is connected with ordering the dipole polarization or the separation of already existing free charges. The homocharge has a sign consistent with the electrode potential sign in contact during the electret formation and shows the incorporation of charge carriers from electrode. During the formation of thermo-electret in the external electric field, the relaxation polarization component influences the process of dipole polarization.
Two types of the relaxation mechanisms were found: α- and β-relaxation. The former one occurs at temperatures above the glass transition temperature of PET or at a low field frequency, while β-relaxation appears at lower temperatures below the glass transition temperature of PET or at higher field frequencies. According to the present views, α-relaxation results from the co-operative motion of the kinetic units of macromolecule chain with discrete changes in the energy of orientation positions in the electric field. Groups ▬O▬CH2▬CH2▬O▬ and p-phenyl bonds are the kinetic units in PET. According to Saito et al. [13], the active kinetic units of polymer contain more than one mer macromolecule. Peruccini et al. [42] present the opinion that α-relaxation is an isotropic process and occurs in amorphous areas.
The process of β-relaxation occurs under conditions of a limited mobility of the main chain of macromolecule as a result of the rotation movements of single groups of atoms ▬(CO▬O)▬ [6, 13, 43] with a stable dipole moment around the axis consistent with the axis of polymer orientation. Because of a considerable intermolecular interaction of the group of atoms, the process of β-relaxation shows an anisotropic nature [6, 13, 44, 45] and occurs in both amorphous and crystalline areas. Time changes leading to the stabilization of electret caused by relaxation processes and also by the effect of environmental conditions are known as aging [46, 47]. The study [16] describes the formation of thermo-electret from the PLA film precursor by isothermal polarization and demonstrates a stabilized density of surface charge and the dependence of charge level on the polarization intensity of filed Epolar. and charging time. In the case of field intensity Epolar. < 20 MV/m, the heterocharge is observed that is stabilized in the processes of dipole relaxation and volume charge within 30 days. The use of higher field intensities Epolar. > 20 MV/m results in the transition of homocharge into heterocharge, which can be caused by the process of an additional injection of charge from electrodes or a change in the direction of dipole moment of molecular dipoles [48]. The prolongation of the PLA isothermal charging results in an increased charge density on the surface, which is consistent with the theory of isothermal dipole polarization, according to which the charge density exponentially depends on the charging time and relaxation time.
In the method of electret formation by irradiation with UV, VIS or ionizing β and γ, in the precursor electrons are excited from the basic state and the deep traps to the conduction band. The precursor in that time is in electric field, where a directed transport of charge carriers and trapping in new positions take place. Once the field and irradiation are turned off, the precursor shows a stable heterocharge. Zllangr et al. [23] analyzed the state of energetic traps the PET radio-electret made during the exposure to γ irradiation. The authors quantitatively characterized the average depth of trap level, density and distribution of traps by the technique of thermally stimulated currents (TSC). As precursor they used PET from various stages of production: an amorphous film, oriented in two directions and film crystallized in the process of heating. The processes of molecular orientation, increasing the content of crystalline phase and irradiation caused an increase in the depth of localized states from 1.36 to 2.15 eV linearly with increasing the molecular orientation, from 1.20 to 1.40 eV with increasing the content of the PET crystalline phase and from 1.30 to 1.70 eV with increasing the dose of γ radiation. In the interpretation of authors [23], the processes of polymer crystallization, orientation of macromolecules and ionizing radiation cause new structural defects: on the crystallites-noncrystalline border, along the border of oriented and nonoriented areas as well as cracked polymer macromolecules due to radiation, confirmed by a decrease in the molecular weight. An increase in defects generates new types of localized energetic levels, an increase in their number, decomposition, and depth. In the study [23] three models of traps are proposed, in relation to three methods of the PET treatment on the basis of quantitative data of TSC.
Currently, a great importance is ascribed to electrets in which a permanent charge is incorporated by the injection from external sources by means of corona or glow discharge [22, 49]. A charge is incorporated into polymer located in the zone of the electrode of crow discharge. Blade electrodes with a high potential generate in space electrons and various types of ions [50, 51] that recombine on the polymer surface, causing chemical changes and the residue diffuses into the polymer top layer and is trapped. During corona discharge, double bonds C〓C and carbonyl groups are formed that can constitute additional traps for current carriers. Charges are accumulated near the polymer surface. The relative depth of charge localization related to the film thickness is assessed to 5%. Charging at increased temperature increases the stability of electret, which is exploited in the formation of pneumo-thermal nonwovens. The process effects are closely dependent on the gas composition and air humidity, if it proceeds under atmospheric conditions [52]. The mechanism of a permanent charge in material proceeds as in glow discharge. A decrease in the process pressure makes it possible to perform the synthesis of polymer and the simultaneous charging of the polymer that shows the features of electret [53].
Polyesters (PET) have found their use as antistatic and electro-conductive materials. An increase in electric conductance of polyesters has been obtained to provide an effective dissipation of electrostatic charges and reduce electrostatic discharges. Owing to that, the use of polyester materials/fabrics has become safe, particularly in areas endangered with explosion and for workers using protective clothing. A high level of polyester conductance can be obtained by making polymeric composites. Composites are made according to two conceptions. The first one consists in combining at least two components, where the polymeric matrix is formed by polyester in which a conducting phase is scattered. The conducting phase can consist of: metal particle, carbon black, carbon fibers or nano-tubes, graphene, metallized fibers, conducting salts, and organic conductors, for example, conjugated polymers. The type of the scattered phase determines the electric conduction of composite. The scattered phase should be used in a quantity not lower than the percolation threshold. The value of percolation threshold depends on the coefficient of the shape of conducting particles, their dimensions and arrangement in the matrix [54, 55, 56]. Particularly beneficial is the use of nano-phase scattered in PET and PLA [57, 58, 59, 60, 61, 62, 63, 64]. The lower the value of percolation threshold, the more beneficial are the mechanic properties of composite.
The other conception of making composites consists in coating the polyester surface (substrate) with the conducting phase. The examples of conducting phase include: metals condensed on polyester surfaces by the PVD technique [65], conjugated synthetic polymers such as polyaniline and polypyrrole [62, 63, 64] deposited by the “in situ” technique or CVD, carbon nano-tube, and graphenes deposited by the printing technique [61, 66, 67, 68, 69, 70].
Table 3 presents the results of our own studies obtained for conducting composites such as PET-PPy, PLA-PPy, PC-PPy with polypyrrole coating (PPy) deposited by the CVD method. Surface resistance measurements were carried out according to standard PN-EN ISO 3915. The specific resistance related to the fabric square, expressed as, R [Ω/cm2], was the quantitative parameter of resistance properties. The polypyrrole coating imparts antistatic properties to polyesters. The combination of PPy with PET substrates is stable and resistant to the mechanical wear conditions.
Test results of different kinds of antistatic polyester films coated with polypyrrole (test conditions t = 23°C, RH = 25%).
Antistatic polyester composites find many applications as conducting elements of electronic circuits, sensors, semi-transparent, and elastic electrodes in electronic-organic elements, opto-electronic and e-textiles, active layers, and transporting charge carriers in organic electroluminescent diodes and elements of organic solar cells.
The presentation of electric properties of polyesters including conventional and biodegradable polymers indicates that these polymers are continually an attractive group of materials. In polyesters, many problems concerning their behavior in electric DC field are still open as a subject of fundamental research. This type of thermoplastic polymers is continually subjected to various chemical and physical transformations, owing to which polyesters offer newer and newer solutions for the market and economy needs.
The manuscript was financed from funds assigned for 14-148-1-2118 statuary activity by Lodz University of Technology, Department of Material and Commodity Sciences and Textile Metrology, Poland.
Edited by Jan Oxholm Gordeladze, ISBN 978-953-51-3020-8, Print ISBN 978-953-51-3019-2, 336 pages,
\nPublisher: IntechOpen
\nChapters published March 22, 2017 under CC BY 3.0 license
\nDOI: 10.5772/61430
\nEdited Volume
This book serves as a comprehensive survey of the impact of vitamin K2 on cellular functions and organ systems, indicating that vitamin K2 plays an important role in the differentiation/preservation of various cell phenotypes and as a stimulator and/or mediator of interorgan cross talk. Vitamin K2 binds to the transcription factor SXR/PXR, thus acting like a hormone (very much in the same manner as vitamin A and vitamin D). Therefore, vitamin K2 affects a multitude of organ systems, and it is reckoned to be one positive factor in bringing about "longevity" to the human body, e.g., supporting the functions/health of different organ systems, as well as correcting the functioning or even "curing" ailments striking several organs in our body.
\\n\\nChapter 1 Introductory Chapter: Vitamin K2 by Jan Oxholm Gordeladze
\\n\\nChapter 2 Vitamin K, SXR, and GGCX by Kotaro Azuma and Satoshi Inoue
\\n\\nChapter 3 Vitamin K2 Rich Food Products by Muhammad Yasin, Masood Sadiq Butt and Aurang Zeb
\\n\\nChapter 4 Menaquinones, Bacteria, and Foods: Vitamin K2 in the Diet by Barbara Walther and Magali Chollet
\\n\\nChapter 5 The Impact of Vitamin K2 on Energy Metabolism by Mona Møller, Serena Tonstad, Tone Bathen and Jan Oxholm Gordeladze
\\n\\nChapter 6 Vitamin K2 and Bone Health by Niels Erik Frandsen and Jan Oxholm Gordeladze
\\n\\nChapter 7 Vitamin K2 and its Impact on Tooth Epigenetics by Jan Oxholm Gordeladze, Maria A. Landin, Gaute Floer Johnsen, Håvard Jostein Haugen and Harald Osmundsen
\\n\\nChapter 8 Anti-Inflammatory Actions of Vitamin K by Stephen J. Hodges, Andrew A. Pitsillides, Lars M. Ytrebø and Robin Soper
\\n\\nChapter 9 Vitamin K2: Implications for Cardiovascular Health in the Context of Plant-Based Diets, with Applications for Prostate Health by Michael S. Donaldson
\\n\\nChapter 11 Vitamin K2 Facilitating Inter-Organ Cross-Talk by Jan O. Gordeladze, Håvard J. Haugen, Gaute Floer Johnsen and Mona Møller
\\n\\nChapter 13 Medicinal Chemistry of Vitamin K Derivatives and Metabolites by Shinya Fujii and Hiroyuki Kagechika
\\n"}]'},components:[{type:"htmlEditorComponent",content:'This book serves as a comprehensive survey of the impact of vitamin K2 on cellular functions and organ systems, indicating that vitamin K2 plays an important role in the differentiation/preservation of various cell phenotypes and as a stimulator and/or mediator of interorgan cross talk. Vitamin K2 binds to the transcription factor SXR/PXR, thus acting like a hormone (very much in the same manner as vitamin A and vitamin D). Therefore, vitamin K2 affects a multitude of organ systems, and it is reckoned to be one positive factor in bringing about "longevity" to the human body, e.g., supporting the functions/health of different organ systems, as well as correcting the functioning or even "curing" ailments striking several organs in our body.
\n\nChapter 1 Introductory Chapter: Vitamin K2 by Jan Oxholm Gordeladze
\n\nChapter 2 Vitamin K, SXR, and GGCX by Kotaro Azuma and Satoshi Inoue
\n\nChapter 3 Vitamin K2 Rich Food Products by Muhammad Yasin, Masood Sadiq Butt and Aurang Zeb
\n\nChapter 4 Menaquinones, Bacteria, and Foods: Vitamin K2 in the Diet by Barbara Walther and Magali Chollet
\n\nChapter 5 The Impact of Vitamin K2 on Energy Metabolism by Mona Møller, Serena Tonstad, Tone Bathen and Jan Oxholm Gordeladze
\n\nChapter 6 Vitamin K2 and Bone Health by Niels Erik Frandsen and Jan Oxholm Gordeladze
\n\nChapter 7 Vitamin K2 and its Impact on Tooth Epigenetics by Jan Oxholm Gordeladze, Maria A. Landin, Gaute Floer Johnsen, Håvard Jostein Haugen and Harald Osmundsen
\n\nChapter 8 Anti-Inflammatory Actions of Vitamin K by Stephen J. Hodges, Andrew A. Pitsillides, Lars M. Ytrebø and Robin Soper
\n\nChapter 9 Vitamin K2: Implications for Cardiovascular Health in the Context of Plant-Based Diets, with Applications for Prostate Health by Michael S. Donaldson
\n\nChapter 11 Vitamin K2 Facilitating Inter-Organ Cross-Talk by Jan O. Gordeladze, Håvard J. Haugen, Gaute Floer Johnsen and Mona Møller
\n\nChapter 13 Medicinal Chemistry of Vitamin K Derivatives and Metabolites by Shinya Fujii and Hiroyuki Kagechika
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