Medical Textile Structures:An Overview

Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published January 1998

TEXTILES

From their first appearance as sutures more than 4000 years ago to their present use in products ranging from gowns and wound dressings to arterial and skin grafts, fibers and fabrics have been explored as potential materials for novel applications in medicine and surgery. This continuous interest has its basis in the unique properties of fibers—which in many respects resemble biological materials—and in their ability to be converted into a wide array of desired end products.

Textile products for medical applications include such materials as woven and knitted polyester fabrics and PTFE felt and mesh. Photo: IMPRA

This article will provide a brief introduction to polymer fibers, textiles, and related structures used in medicine and will discuss the principles governing the performance of these materials. Current product concerns and developmental activities will also be reviewed.

Manufacturing Medical Fabrics Medical textile products are based on fabrics, of which there are four types: woven, knitted, braided, and nonwoven (see Figure 1). The first three of these are made from yarns, whereas the fourth can be made directly from fibers, or even from polymers. (Gore-Tex—based products or electrostatically spun materials from polyurethane are examples of products made directly from polymers.) There is, therefore, a hierarchy of structure: the performance of the final textile product is affected by the properties of polymers whose structures are modified at between two and four different levels of organization.

Figure 1. Constituent elements of medical textile products.

Of the many different types of polymers, only a few can be made into useful fibers. This is because a polymer must meet certain requirements before it can be successfully and efficiently converted into a fibrous product. Some of the most important of these requirements are:

Common fiber-forming polymers include cellulosics (linen, cotton, rayon, acetate), proteins (wool, silk), polyamides, polyester (PET), olefins, vinyls, acrylics, polytetrafluoroethylene (PTFE), polyphenylene sulfide (PPS), aramids (Kevlar, Nomex), and polyurethanes (Lycra, Pellethane, Biomer). Each of these materials is unique in chemical structure and potential properties. For example, among the polyurethanes is an elastomeric material with high elongation and elastic recovery, whose properties nearly match those of elastin tissue fibers. This material—when extruded into fiber, fibrillar, or fabric form—derives its high elongation and elasticity from alternating patterns of crystalline hard units and noncrystalline soft units.

Although several of the materials mentioned above are used in traditional textile as well as medical applications, various polymeric materials—both absorbable and nonabsorbable—have been developed specifically for use in medical products. Chemical structures of some of these materials are illustrated in Figure 2.

Figure 2. Examples of fibrous materials developed for use in medicine.

The reactivity of tissues in contact with fibrous structures varies among materials and is governed by both chemical and physical characteristics. Absorbable materials typically excite greater tissue reaction, a result of the nature of the absorption process itself. Among the available materials, some are absorbed faster (e.g., polyglycolic acid, polyglactin acid) and others more slowly (e.g., polyglyconate). Semiabsorbable materials such as cotton and silk generally cause less reaction, although the tissue response may continue for an extended time. Nonabsorbable materials (e.g., nylon, polyester, polypropylene) tend to be inert and to provoke the least reaction. To minimize tissue reaction, the use of catalysts and additives is carefully controlled in medical-grade products.

Fibers. As discussed, of the many types of polymers, only a few can be made into useful fibers that can then be converted into textile products. To make fibers, polymers are extruded by wet, dry, or melt spinning and then processed to obtain the desired texture, shape, and size. Through careful control of morphology, fibers can be manufactured with a range of mechanical properties. Tensile strength can vary from textile values (values needed for use in typical textile products such as apparel) of 2—6 g/d (gram/denier) up to industrial values (values typical of industrial products such as tire cords or belts) of 6—10 g/d. For high-performance applications, such as body armor or structural composites, novel spinning techniques can produce fibers with strengths approaching 30 g/d. Likewise, breaking extension can be varied over a broad range, from 10—40% for textile to 1—15% for industrial and 100—500% for elastomeric fibers.

Yarns. Fibers or filaments are converted into yarns by twisting or entangling processes that improve strength, abrasion resistance, and handling. Yarn properties (_y) depend on those of the fibers or filaments (_f) as well as on the angle of twist, . Modulus (E ) and strength () are reduced by the cos² factor, while the extension at break () is increased by the sec² factor:

Fabrics. Yarns are interlaced into fabrics by various mechanical processes—that is, weaving, knitting, and braiding.¹ The three prevalent fabric structures used for medical implants or sutures are woven, in which two sets of yarns are interlaced at right angles; knitted, in which loops of yarn are intermeshed; and braided, in which three or more yarns cross one another in a diagonal pattern (see Figure 3). Knitted fabrics can be either weft or warp knit, and braided products can include tubular structures, with or without a core, as well as ribbon.

Figure 3. Examples of woven (top left), knitted (top right, bottom left), and braided (bottom right) structures.

There are also numerous medical uses for nonwoven fabrics (wipes, sponges, dressings, gowns), made directly from fibers that are needle-felted, hydroentangled, or bonded through a thermal, chemical, or adhesive process. Nonwovens may also be made directly from a polymer. For example, expanded polytetrafluoroethylene (ePTFE) products such as sutures and arterial grafts and electrostatically spun polyurethane used as tubular structures are examples of medical applications of polymer-to-fabric nonwovens.²

Fabric Structures

The properties of fabrics depend on the characteristics of the constituent yarns or fibers and on the geometry of the formed structure. Whether a fabric is woven, knitted, braided, or nonwoven will affect its behavior.

Wovens. Fabrics that are woven are usually dimensionally very stable but less extensible and porous than the other structures. One disadvantage of wovens is their tendency to unravel at the edges when cut squarely or obliquely for implantation. However, the stitching technique known as a Leno weave—in which two warp threads twist around a weft—can substantially alleviate this fraying or unraveling (see Figure 4).³

Figure 4. Fabric and vascular grafts made using a Leno weave.

Knitted Fabrics. Compared with woven fabrics, weft-knitted structures are highly extensible, but they are also dimensionally unstable unless additional yarns are used to interlock the loops and reduce the extension while increasing elastic recovery. Warp-knitted structures are extremely versatile, and can be engineered with a variety of mechanical properties matching those of woven fabrics. The major advantage of knitted materials is their flexibility and inherent ability to resist unraveling when cut. A potential limitation of knitted fabrics is their high porosity, which—unlike that of woven fabrics—cannot be reduced below a certain value determined by the construction (see Figure 5). As a result, applications requiring very low porosity usually incorporate woven materials.

Figure 5. Woven (left) and knitted (center and right) fabics used for vascular grafts, showing differences in porosity.

Braided Structures. Typically employed in cords and sutures, braided structures can be designed using several different patterns, either with or without a core. Because the yarns criss-cross each other, braided materials are usually porous and may imbibe fluids within the interstitial spaces between yarns or filaments. To reduce their capillarity, braided materials are often treated with a biodegradable (polylactic acid) or nonbiodegradable (Teflon) coating. Such coatings also serve to reduce chatter or noise during body movement, improve hand or feel, and help position suture knots that must be transported by pressure from a surgeon's finger from outside the body to the wound itself.

Nonwovens. The properties of nonwoven fabrics are determined by those of the constituent polymer or fiber and by the bonding process. For instance, expanded PTFE products can be formed to meet varying porosity requirements. Because of the expanded nature of their microstructure, these materials compress easily and then expand—a suture, for example, can expand to fill the needle hole made in a tissue—allowing for tissue ingrowth in applications such as arterial and patch grafts. Polyurethane-based nonwovens produce a product that resembles collagenous material in both structure and mechanical properties, particularly compliance (extension per unit pressure or stress). The porosity of both PTFE- and polyurethane-derived nonwovens can be effectively manipulated through control of the manufacturing processes.

Textile Performance Principles

Textile materials for medical applications typically have specific performance requirements relating to strength, stiffness, abrasion resistance, and mechanical patency.

Strength. Among the many factors affecting a fabric's strength (fiber type, molecular orientation, crystallinity) is the variability in properties—especially elongation—of its constituent elements. Usually, the greater the variability in elongation at break, the lesser the strength. Products requiring high strength (e.g., artificial ligaments) must incorporate elements whose properties range within a narrow limit.

Stiffness. Bending stiffness—which governs the handling, comfort, and conformability of a fabric—is a critical parameter in a number of medical applications. A low value is usually desirable. For example, a suture with low bending stiffness requires fewer throws to tie a secure knot and has higher knot strength. The most important factors affecting bending stiffness are the shape of the fiber and the modulus, linear density, and specific gravity of the material. Generally, the higher the denier or the modulus or the lower the specific gravity, the higher the bending stiffness. For example, polyester has a higher modulus than that of nylon, and will result in a stiffer material. Polypropylene, with a lower density than nylon, should have a higher stiffness, assuming all other factors are equal. In addition, a trilobal or tubular structure produces a stiffer product than does a solid circular structure of the same area or linear density.

Monofilament materials are much stiffer than multifilament. With all other factors constant, the bending stiffness of a monofilament product such as a suture of denier T will be roughly n times greater than a multifilament structure with n filaments of denier T/n each. The use of multifilament yarns and/or fine-denier fibers in the yarn produces a more flexible and supple end product. Knot efficiency—the ratio of the tensile strength of knotted to unknotted thread—is affected by elongation at break and bending stiffness. Most often, the greater the elongation, or the lower the stiffness, the greater the knot efficiency.

Abrasion Resistance. Whenever fibers, yarns, or fabrics rub against themselves or other structures, abrasion resistance assumes an important role. A high value is usually desirable, especially in applications such as artificial ligaments or tendons. The abrasion resistance of a yarn is influenced by several factors:

Therefore, one can conclude that microdenier fibers, low-twist yarns, rough surfaces, and highly oriented materials generally exhibit low abrasion resistance. However, coating a bundle of fibers with a low-friction polymer can enhance its resistance to abrasion.

Mechanical Patency. Implanted products that must bear loads over the long term and maintain their dimensional integrity require a high degree of mechanical patency—that is, the ability to resist permanent change in physical size, shape, structure, and properties. The factors that contribute to mechanical patency include:

Thus, material selection is extremely critical for products—such as ligament prostheses—that must continue to bear loads. The material specified must be able to resist the elongation or growth that may occur as a result of stress relaxation during each cycle of operation in the body. If no such material is available, then biological tissues will need to be integrated into the assemblage to provide partial support of the load and contribute to the product's long-term patency.

Current Medical Textile Research

The literature of the past decade, including patents, provides a broad overview of current research activities as well as of some of the problems and concerns related to implantable medical textiles. Among the more intensively studied product groups are surgical sutures, vascular grafts, and artificial ligaments and tendons.

Surgical Sutures. For surgical sutures, the predominant areas of concern are strength, capillarity, sliding and positioning of knots, knot security, and handling characteristics. The recent focus of suture research has been on improving the structure of the braids (two recently proposed products are spiral- and lattice-braided materials),⁴ reducing the difference in the elongational properties between the core and the sheath yarns, using finer-denier filaments in the sheath yarns, and improving knot security and performance by exposing a two-throw square knot to laser-beam energy. In a recently conducted experiment, it was shown that exposing a two-throw square knot tied in a 3-0 Mersilene suture to energy from a CO₂ laser beam for a brief period of time not only made the knot fully secure but also led to an increase in knot strength of appoximately 16% (see Figure 6).⁵

Figure 6. The effect on knot-breaking strength of exposing a two-throw square knot tied in 3-0 Mersilene to energy from a CO₂ laser beam.

Vascular Grafts.Regarding vascular grafts, the lack of healing, compliance, and suture-line patency continue to be concerns, especially in small-caliber (< 6 mm diam) grafts.⁶ Three important efforts that highlight global developmental activities in this area are:

Experiments with endothelial cell seeding⁷ and with coated grafts containing albumin,⁸ gelatin,⁹ or collagen¹⁰ are also ongoing.

Ligaments and Tendons. Finally, in the area of artificial ligaments and tendons, desirable properties include high strength, high elasticity, low abrasion, low creep, and low stiffness. Current research endeavors are examining the use of ultra-high-strength fibers (e.g., Spectra from AlliedSignal), threads containing layers of both absorbable inelastic and nonabsorbable elastic fibers, and coatings with biocompatible polymers to reduce abrasion and restrict escape of abraded particles from within the structure.¹¹

Conclusion

Textile materials continue to serve an important function in the development of a range of medical and surgical products. The introduction of new materials, the improvement in production techniques and fiber properties, and the use of more accurate and comprehensive testing have all had significant influence on advancing fibers and fabrics for medical applications. As more is understood about medical textiles, there is every reason to believe that a host of valuable and innovative products will emerge.

References

1. Hatch KL, Textile Science, New York, West Publishing Co., pp 318—370, 1993.

2. Soldani G, Panol G, Sasken HF, et al., "Small-Diameter Polyurethane-Polydimethylsiloxane Vascular Prostheses Made by a Spraying, Phase-Inversion Process," J Mat Sci, Mat in Med, 3:106—113, 1992.

3. Kapadia I, and Ibrahim IM, Woven vascular grafts, U.S. Pat. 4,816,028, 1989.

4. Brennan KW, Skinner M, and Weaver G, Braided surgical sutures, U.S. Pat. 4,959,069, 1990.

5. Gupta BS, Milam BL, and Patty RR, "Use of Carbon Dioxide Lasers in Improving Knot Security in Polyester Sutures," J App Biomat, 1:121—125, 1990.

6. Gupta BS, and Kasyanov VA, "Biomechanics of the Human Common Carotid Artery and Design of Novel Hybrid Textile Compliant Vascular Grafts," J Biomed Mat Res, 34:341—349, 1997.

7. Williams SK, Carter T, Park PK, et al., "Formation of a Multilayer Cellular Lining on a Polyurethane Vascular Graft Following Endothelial Cell Seeding," J Biomed Mat, 26(1):103—117, 1992.

8. Merhi Y, Roy R, Guidoin R, et al., "Cellular Reactions to Polyester Arterial Prostheses Impregnated with Cross-Linked Albumin: In Vivo Studies in Mice," Biomat, 10(1): 56—58, 1989.

9 Bordenave L, Caix J, Basse-Cathalinat B, et al., "Experimental Evaluation of a Gelatin-Coated Polyester Graft Used as an Arterial Substitute," Biomat, 10(3): 235—242, 1989.

10. Guidoin R, Marceau D, Couture J, et al., "Collagen Coatings as Biological Sealants for Textile Arterial Prostheses," Biomat, 10(3): 156—165, 1989.

11. Frey O, Dittes P, and Koch R, Prosthetic implant, U.S. Pat. 5,176,708, 1993.

Bhupender S. Gupta, PhD, received his undergraduate degree in textile technology from the Punjab University, India, and his doctorate in textile physics from the University of Manchester Institute of Science and Technology (Manchester, UK). He is currently professor of textile materials science in the department of textile engineering, chemistry, and science, College of Textiles, at North Carolina State University (Raleigh, NC). His research interests include the physical and mechanical properties of fibers, yarns, and fabrics; structural mechanics of assemblies; absorbent nonwoven materials; and biomedical textiles.

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