High-Tech Fibrous Materials - American Chemical Society

Chapter 9

Textiles as Bioimplants

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 15, 2016 | http://pubs.acs.org Publication Date: April 3, 1991 | doi: 10.1021/bk-1991-0457.ch009

Roger W. Snyder Harbour Biomedical Consultants, Inc., 2716 Bent Tree Trail, League City, TX 77573

Basic textile structures and yarns have been used in numerous implanted devices. Since variables in structure and yarn are not always independent, trade-offs between ingrowth, strength and handling of medical textiles must be made. Those properties known to affect degradation and biocompatibility include base polymer, fiber shape, processing and contaminants. Primary areas of current research in the medical use of textiles such as biologic composite structures, solvent bonded nonwovens and new yarns are briefly discussed. Although textiles have been used in medical devices for many years, new uses are still being found.

The use of textiles in medicine is an ancient and varied art. Current applications range from external uses such as simple bandages, complicated trusses and stockings; to internal uses ranging from simple sutures to artificial arteries and ligaments. Any application requiring flexibility, uniaxial or biaxial strength and/or porosity can use a textile. A number of materials that can be fabricated into textiles are biostable or at least degrade in a predictable manner. In addition, the fabrication of textiles is an old art and a wide variety of techniques are available which allow the design of an appropriate structure.

Uses Textiles have long been used externally for wound dressings, pressure bandages or supports, casts, etc. The use of textiles as implants began with the use of threads as sutures. Vorhees (see, for example Callow (1)) noted that a suture inserted into the right ventricle retained fibrin and cells. Soon several investigators were trying various structures as cardiovascular patches and vascular replacements. Certain textiles soon demonstrated the ability to replace or bypass damaged arteries. These textiles functioned by serving as a scaffold for fibrin, collagen or, in some cases, cells.

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Textile vascular grafts and cardiovascular patches still represent one of the largest uses for textiles as implants. However, other uses have also developed. In the cardiovascular field, vascular grafts have been used for arterial-venous shunts, connectors for the artificial hearts currently under development and felt pledgets to prevent sutures from cutting through fragile tissue. In a related application, many of the early heavy pacemakers were inserted into textile pouches, which prevented migration and encouraged the formation of an intramuscular cavity. The discovery that textiles could be used to encourage ingrowth led to a number of other applications such as cuffs around various access devices, liners for some artificial heart models, nerve regeneration tubes and tabs to anchor other implants. Many textiles can also be heat set or shrunk into various shapes which turns out to be very useful for improving handling characteristics in some applications such as vascular grafts. The majority of textile vascular grafts are crimped. This is primarily a heat setting process. The crimps allow the length of the graft to be adjusted and keeps the cross section circular, even when the textile is wet and sticky with blood. This makes the surgeon's job easier. These heat setting and shrinking characteristics are also useful in other applications such as fitting sewing rings to various devices. Finally, textiles also have the property of being very stiff and strong uniaxially or biaxially , at least in tension, and very flexible in bending. This property is very useful in designing unidirectional structures such as artificial ligaments and tendons. Of course, the property of encouraging ingrowth needs to be overcome, especially at the midpoints of these devices, where ingrowth or scar tissue might interfere with movement. Thus these structures are normally designed with minimal porosity, except perhaps at the ends where anchoring is important. Structures There are three basic structures for textiles; nonwovens, wovens and knits. Each has properties that directly impact upon the application. Nonwovens are basically piles or mats of short fibers which can be held together in a number of ways. Felts are tangled or intertwined by passing barbed needles back and forth through the thickness. Spun bonded or melt bonded materials have fibers that are bonded together by adhesive, solvent welding, partial melting (by direct heat or ultrasonically) or by another polymer. Typically, nonwovens have been used for sewing rings , pledgets or other applications where thickness or compressibility might be required. Thin nonwoven materials have not demonstrated the strength required for load carrying applications. Woven materials consist of at least two independent yarns typically crossing each other at right angles (warp and weft), as shown in Figure 1a. These materials tend to be very strong if tightly woven and have minimal porosity. Fine yarns can be woven tightly enough to be nearly waterproof. The patterns can vary from simple over under patterns to complicated tweeds and other patterned textiles. Additional floater yarns can be added to give a napped or plush surface. Yarns can be varied to yield structures that might be partially degradable. One other class of woven structures are braids. Braids are woven structures with the yarns running



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Figure 1. (a) Simple woven structure; (b) Simple weft knit structure; (c) Typical warp knit structure.

at angles to the length of the structure. This structure is very strong in the axial direction, but difficult to stabilize against unraveling. Such structures are commonly used to reinforce tubing, such as cardiovascular catheters, or as braided sutures. Knits are complicated structures which generally consist of yarns running in one direction or the other. For example, a weft knit might be formed of a single yarn traveling back and forth across the fabric, such as illustrated in Figure 1b. A warp knit consists of yarns all generally traveling along the length of the fabric. Like the woven, additional yarns can be laid in to yield patterns or plush surfaces. An example of one such pattern is illustrated in Figure 1c. In the warp knit,


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different yarns can be used to form a partially degradable fabric. However, each yarn must be accompanied with a stable yarn to prevent the fabric from unraveling.


Knits are typically more porous and not as strong as wovens. This is because yarns generally must make tighter bends in a knit, decreasing their effective strength and making it difficult to form a tight fabric. On the other hand, because of these bends, knits are in general more pliable, often containing pivot points where the fabric can bend without bending and stretching the actual yarns. Materials A number of biostable materials, suitable for implant have been identified over the past 20 years. Probably the material with the most experience in terms of length of implant and number of cases is polyester, specifically polyethylene terephathalate as manufactured by DuPont in fiber form. Dacron vascular grafts have been explanted after 18 years, still functioning (2). Polytetrafluorethylene, as Teflon fibers by DuPont has shown excellent long term biostability; however with fewer implants to date than polyester. Data out to 17 years for textile vascular grafts demonstrates a lot less decrease in strength than Dacron (£ ). Polypropylene has also been used successfully In sutures for long implant times. Materials which have shown mixed results include ivalon, nylon, silk and polyurethane among others. Ivalon and nylon were used in some of the early vascular grafts. Both approached zero strength after approximately two years implant. This is an example which should be remembered by anyone designing implants. Silk sutures are seldom used for any long term application. Some polyurethane fabrications have also shown degradation in long term implant situations. Some materials have been designed to degrade. Most of these materials to date are based on the polyglycolic acid chemistry. The use of these materials in textiles other than as sutures have been limited. Several mechanisms have been demonstrated or proposed for polymer degradation. Hydrolysis was the first mechanism proposed and is probably the most prevalent. The ability of some cells and bacteria to actively degrade some polymers, probably through secreted enzymes or change in pH environment has been demonstrated (3). These degradation mechanisms are affected by a number of factors. The formulation of the polymer will make a difference. For example, textiles formulated from polyester copolymers have shown signs of degradation not seen in the original polyester homopolymer textiles (4). Processing can also make a difference. Cracks and fissures in the polymer can be caused by excessive temperatures in the presence of moisture. Finally the presence of residual surfactants might enhance surface damage.


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Filament shapes which increase surface area and cause local stress concentrations accelerate degradation. Fiber bulk will also impact the process. Large filaments or a larger number of filaments increase the bulk to be attacked and decrease the stress levels in individual fibers which will lessen stress mediated degradation. Finally, sterility and biocompatibility will also impact degradation. The presence of a few bacteria on a "sterile" product may lead to local degradation as well as a long term infection. The presence of certain cells whose function is to remove foreign bodies will also contribute to degradation (2.)- Sterilization itself is a thermal, chemical or radiation process which can damage polymers. Biocompatibility Although the mechanisms of biocompatibility are not fully understood, four factors have been demonstrated to effect how a biological entity reacts to an implanted textile. Probably the most important is porosity. It was demonstrated many years ago by Weslow (£) that an optimum porosity existed which would encourage ingrowth or encapsulation. The lower limit of porosity is normally limited by the type of structure and the upper limit is determined by the stability of the structure and the ability of the implanter. The optimum porosity is normally achieved by a fairly tight knitted structure. A tightly woven structure is too tight for optimum ingrowth. The size and shape of the filaments also effects the host acceptance. Smaller fibers with circular cross-sections are better encapsulated than larger fibers with irregular cross-sections. Third, toxic substances such as monomers, solvents or polymerization byproducts leaching out of a polymer will have a negative impact on the tissue reaction to the implant. Surface contaminations such as sizing agents, lubricants or surfactants can also negatively impact the results. As shown in a study by Sawyer(&), different methods of processing the same material may result in substances left on the surface or toxic solvents eluting from the polymer, with a negative impact on the host. Some bacteria, even though killed during sterilization, can leave toxic molecules on the surface of the polymer. These molecules elicit a toxic reaction from the host and are extremely difficult to eliminate. Finally the properties of the polymer itself and certainly the surface configuration of the polymer will effect the outcome. Among the polymers which have been used as implants, nylon seems the most reactive and PTFE the least, with polypropylene and polyester in between. A fifth area in which the data is not as clear is the compliance of the structure. Evidence suggests that compliance matching at tissue/material interfaces will minimize any stress induced or mediated growth. On the other hand, evidence published by Sauvage (Z), among others, suggests that a more rigid textile tube yields better results in certain vascular implant situations. Certainly, textiles made from polymers such as polyester or PTFE have compliances much lower than natural tissue. It may also be that movement of the textile structure causes excessive tissue growth. This is one area in which additional research is needed.


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Design of Textiles For Implants


In general implants fall into two categories, synthetic and biological. Biological implants will generally consist of certain organs or tissues treated to increase biocompatibility, host acceptance and decrease antigenicity and perhaps increase fatigue life. These implants may contain synthetic materials as stents, sewing rings, or containment devices. In general, the synthetic components will be less biocompatible but more fatigue resistant and easier to reproduce. The design of textiles for any implant application is complicated by the number of choices in textile structures and the number of variables present. A number of these variables are not independent and effect the results in opposite directions, as shown in Figure 2. As an example, those fabrics with superior strength tend to have lower porosity and therefore are less likely to be well encapsulated. Even for a given type of structure, as the structure is tightened, the strength tends to increase, but porosity decreases. Also, in general as porosity increases, the flexibility or handling of the fabric improves. Thus, it is necessary to trade off strength for handling and ingrowth. As a second example, although PTFE yarn appears to be the most biostable material available, PTFE yarn filaments are relatively large in diameter and textiles fabricated from that yarn generally have higher porosity. Also the yarn feels "different". Thus one might trade off decreased risk of long term failure for lower porosity and better handling achieved with another yarn such as polyester; which, if carefully manufactured, could have sufficient longevity for many applications. Therefore, like any other good design, it is first of all important to determine what the implant is suppose to accomplish and then decide what priorities exist in terms of some of the other features. The list of parameters to be considered is extensive. An implant requiring high strength might require a woven or braided structure, particularly if ingrowth must be minimized, such as in an artificial tendon or ligament. However, an implant requiring ingrowth and subjected to relatively low loads might better use a knit, particularly if flexibility and handling are important, such as in a small diameter vascular graft, where the surgeon's skill may be a major contribution to success. Future Textile Design The primary area of research in the use of current textiles is composite structures, primarily biological composites. This is an attempt to combine the biocompatibility of biological polymers with the strength and consistency of synthetic polymers. This is currently being accomplished in two ways. The first way is with a coating such as collagen or albumin to seal the pores of the textile. This coating can be adjusted to degrade slowly or to be nearly biostable. Currently, this technology is being used in vascular grafts, but also has applications in other areas such as


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WARP KNIT

X z LU oc

CO

o oc o o.


& Q LU CO < LU OC

• WEFTKNIT

POROSITY

Q LU CO < LU CC

o z

o z

IMPROVED HANDLING

Figure 2. Schematic of variations in porosity, strength and handling characteristics. artificial ligaments and nerve regeneration tubes. Ligaments have been designed and tested using synthetic materials to coat a carbon filament substrate. A second type of composite being used, again primarily in vascular grafts, is a seeded material. Autologous cells are removed and cultured until they multiply to a useful level. They are then mixed with a suitable substrate, such as fibrin, and used to coat the textile. Until the implant matures, these cells improve the biocompatibility of the textile. This technology has also been applied to artificial skin. Another area of research related to textiles is the use of solvent bonded nonwovens as small diameter grafts. These materials are formed by spraying polyurethanes directly onto a rotating mandrel. The overlapping filaments bond while curing on the mandrel, presenting a material such as seen here. Most of the work in this area has been done with polyurethanes or polyurethane/silicone combinations. Degradation remains a problem with these designs. Pore size must be controlled to prevent penetration of macrophage cells, among others. Finally, new yarns being developed for the commercial sector, such as finer filament polyesters and new polymers will eventually find their way into medical applications. The use of these new materials will be slow to materialize, due to the amount of testing necessary to prove biocompatibility and biostability. However, benefits such as stronger or more flexible textiles will provide the incentive to undertake the expensive testing required. Conclusions Although textiles have been used in medical devices for many years, new uses are still being found. Any application requiring a combination of strength and flexibility and perhaps long term ingrowth is a candidate for a textile. Textiles can


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serve as scaffolds for natural tissue replacement and, if necessary remain as long term support structures. Certain textiles, used as vascular replacements have demonstrated a longevity approaching twenty years, in an environment that has demonstrated the ability to destroy some polymers.


Literature Cited 1. Callow, A.D. In Biological and Synthetic Vascular Prostheses; Stanley, J.C.; et al, Eds.; Grune and Stratton: New York, 1982; pp 11-26. 2. Botzko, K.; Snyder, R; Larkin, J.; Edwards, W.S. ASTM Spec. Tech.Pub.,STP 684 1979; pp76-88. 3. King, M.W.; Guidoin, R.; Blais,P.; Garton, A.; Gunasekera; K.R. ASTM Spec. Tech. Pub., STP 859; 1983; pp294-307. 4. Williams, D.F. ASTM Spec. Tech. Pub., STP 684 1979; pp61-75. 5. Wesolowski, S.A.; Fries, C.C.; Karlson, K.E.; DeBakey, M. E. ; Sawyer, P. N. Surgery (St Louis) 1961, 50, pp91-96 and 105-106. 6. Sawyer, P. N. ;Stanczewski, B. ; Hoskin, G. P. ; Sophie, Z. ; Stillman, R. M. J. Biomed. Mater. Res. 1979, 13, pp 937-956. 7. L. R. Sauvage, L.R. In Vascular Grafting: Clinical Applications and Techniques; Wright,C.B.; et al (Eds.); John Wright - PSG, Inc: Boston, 1983; pp 168186. RECEIVED July 5,

1990


High-Tech Fibrous Materials - American Chemical Society

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