The Future of Modified Fibers

J. Vincent Edwards1, Steven C. Goheen2, and Gisela Buschle-Diller3

1USDA-ARS, Southern Regional Research Center, 1100 Robert E. Lee Blvd., New Orleans, LA 70124, U.S.A.

2Battelle Northwest, Richland, Washington 99352, U.S.A. 3 Textile Engineering Department, Auburn University, AL 36849, U.S.A.

The future of fiber technology for medical and specialty applications depends largely on the future needs of our civilization. It has been said that "unmet needs drive the funding that sparks ideas". In this regard recent emphasis on United States homeland security has encouraged new biofiber research, resulting in the development of anti-bacterial fibers for producing clothing and filters to eliminate pathogens and enzyme-linked fibers to facilitate decontamination of nerve toxins from human skin [1]. Magnetic fibers may also have future security applications including fiber-based detectors for individual and material recognition. Interest in smart and interactive textiles is increasing with a projected average annual growth rate of 36% by 2009 [2]. More specific markets including medical textiles and enzymes will grow even more rapidly. Among the medical textiles are interactive wound dressings, implantable grafts, smart hygienic materials, and dialysis tubing. Some of the medical and specialty fibers inclusive of these types of product areas are discussed in this book. A recent review of the surface modification of fibers as therapeutic and diagnostic systems relevant to some of these new product areas has appeared and Gupta reviewed current technology for medical textile structures [3] with focus on woven medical textile materials.

The design of new fibers for use in healthcare textiles has increased rapidly over the past quarter of a century. Innovations in fiber design have led to improvements in the four major areas of medical textiles: non-implantable, implantable, extracorporeal, and hygienic products. The use of natural fibers in

J. V. Edwards et al. (eds.), Modified Fibers with Medical and Specialty Applications, 1-9. © 2006 Springer. Printed in the Netherlands.

medical applications spans to ancient times. Although wood seems an unlikely material for a medical textile, some of the earliest documented evidence of the use of natural fibers as prosthetics is from the use of wooden dentures in early civilizations [4]. Anecdotal folklore also suggests that President George Washington wore similar prosthetics; however his dentures were probably constructed of ivory [5]. It is notable that wood is still employed in splints to stabilize fractures [6]. Natural fibers are readily available and easily produced owning to their remarkable molecular structure that affords a bioactive matrix for design of more biocompatible and intelligent materials. The nano-structure of natural fibers is complex and organized in motifs that cannot be easily duplicated. Synthetic fibers typically do not have the same multilevel structure as native materials. On the other hand, specific material properties including the modulus of elasticity, tensile strength, and hardness are largely fixed parameters for a natural fiber but have been more manageable within synthetic fiber design. The molecular conformation native to natural fibers is often key to interactions with blood and organ cells, proteins, and cell receptors, which are currently being studied for a better understanding to improve medical textiles. The native conformation or periodicity of structural components in native fibers such as collagen and cellulose offers unique and beneficial properties for biomedical applications. An extension of the bioactive conformation property in fibers to rationally designed fibers that would inhibit enzymes or trigger a cell receptor is a premise of current research.

The first nine chapters of this book present work going on in the research and development of biomedical products from these four traditional areas of medical textiles.

Non-implantable textiles are applied externally. They include dressings and bandages used in wound and orthopedic care, bedpads, sheets, diapers, and protective clothing such as patient and medical personnel gowns, gloves, face masks, and related items. Non-implantable wound dressings are largely exposed to the skin and wound fluid as well as subcutaneous cells [7]. Chapters 2 and 4 both discuss recent results of work in an area of mechanism-based non-implantable fibers that address a current need to enhance wound healing by redressing the molecular imbalance of the chronic wound. Wound healing and material science are shaping new views on how dressings are being improved and expected to develop. The implications of mechanism-based dressings employing the concepts of contemporary wound bed preparation and wound healing science for future chronic wound dressings are drawn from the current state of the science. The two natural fibers collagen and cellulose play an important role in new wound dressing designs. The most common application for collagen in dermatology is tissue augmentation and wound healing [8]. An example of collagens role in non-implantable materials is evident in interactive wound dressings, which have a mechanism-based mode of action and employ either

Crystallite

Unit Cell

Cellulose Chain

Figure 1.1. A portrayal of the levels of structure of cellulose (structures are provided courtesy of Dr. Alfred D. French). The cellulose chain, which is an unbranched chain of glucose residues with B-(1-4) linkages. The second level of structure is the unit cell, which is shown here as a cross-section of cellulose chains. The unit cell is the smallest piece of a crystal that can be repeated in the x, y, and z directions to generate an entire crystal. Here, it consists of two cellobiose units. One is located at the corners of the unit cell and another at the center. Although there are chains at each corner, only one-fourth of each is inside the unit cell for a total of one corner chain. This crystallite contains 36 chains and is thought to correspond to an elementary fibril for higher plant secondary walls. Its atomic positions, like those in the unit cell, is based on the structure of cellulose that was reported in Nishiyama, Y.; Langan, P.; Chanzy, H. Crystal structure and hydrogen-bonding system in cellulose IB from synchrotron X-ray and neutron fiber diffraction. J. Am. Chem. Soc. 2002, 124, 9074-9082.

a native or electrospun form of collagen fibers to stimulate cell growth and to augment soft tissue repair.

Collagen is a key component in several different tissues, and though the fibrous form of the protein is varied it fulfills the requirements of an important structural component of both non-implantable and implantable materials. Collagen possesses multiple levels of structure (Figure 1.1), which are interesting to contemplate for its role in a variety of biocompatible materials as viewed. Collagen has a repeating amino acid sequence. Two out of three of these sequences are identical (alpha-1) left-handed helices with a pitch of 9.5 A. The third is a nearly identical (alpha-2) chain with the same left-handed pitch. These three strands of amino acids are bound together in a right-handed triple helix with a pitch of 104 A. These helices are coupled by hydrogen bonds between the HN group of glycine in one chain and O=C groups of an adjacent amino acid. Each super helix is about 1000 residues long, and these residues are staggered to form 668 A repeating units at the higher structural level, the microfibril. Microfibrils are further organized at several levels resulting in the final structure of collagen.

Other natural fibers such as elastin, silk, and wool, which are also proteina-ceous are as complex and unique as cellulose and collagen. Some researchers have examined ways to modify wool [9, 10] and silk [9, 11] to enhance their bacterial resistance. The work with these fibers has been expanded to include other natural fibers and the enhancement of anti-fungal properties [12]. Silk is also commonly used for sutures although may not be as effective as other tissue sealing methods when underivatized [13] and may some day be used to augment bone repair [14, 15]. Genetically engineered forms of elastin have been used for cartilage tissue repair [16]. Closely related research areas address the ability of natural or synthetic fibers to either resist microbe adhesion [17] or produce anti-microbial fabrics from other fibers.

Cellulose is similar in its structural complexity to collagen. However, cellulose is composed of carbohydrate residues. Differences between cotton and wood cellulose, for example, are significant at the macromolecular level, but the molecular sequences are similar. In the cotton fiber, many levels of organization have been discovered based on the arrangement of the crystalline microfibrils that are ordered in multilayer structures. Figure 1.2 demonstrates an analog of progressing from the smallest unit that is the cellulose molecule in-

Figure 1.2. A simplified illustration representing the three major levels of structure of collagen fibers: Triple helical collagen (3000 A by 16 A) molecules are packed into collagen microfibrils that are assembled into the native collagen fiber

visible to light microscopes to the cotton fiber visible to the naked eye. Chapter 4 examines blood proteins, their adsorption to cotton, and their potential role in wound healing. Much of the concern about modified fiber performance for medical applications involves the interface between the fiber and its immediate environment. In Chapter 4, Goheen et al. present an approach to understanding the interaction of the blood protein albumin with a modified cotton wound dressing fiber and an enzyme that takes up destructive residence in chronic wounds. In Chapter 6, Sun and Worley present current product-oriented work on a type of non-implantable hygienic textile with biocidal activity, in which they attach halamines to the surface of cotton and cotton/polyester fibers. This work is an important chapter in the development of regenerable anti-microbial fabrics and represents a growing effort to control microbes in hospital textiles and protective fabrics. Modified cellulose has also been used to generate microcapsules to deliver pharmaceuticals [18]. There has been recent research on the use of modified cellulose derivatives to create ultra thin coatings on biomaterials [19]. Regioselectively derivatized cellulose has also been explored for its anti-coagulant activity, which is another example of bioactive fibers from biopolymers. In Chapter 8, Negulescu et al. further discuss the bioactive polymer idea from a drug discovery paradigm and give examples from their own work of biologically active polysaccharide polymers from plants. Indeed, polysaccharide fibers offer interesting possibilities for drug discovery from both rational design and combinatorial motifs.

In Chapter 7, Bide et al. review the medical uses of polyester fibers, which along with polytetrafluoroethylene predominate the market of vascular grafts. Implantable fibers are placed in vivo for wound closure or replacement surgery. Factors in determining the biocompatibility of a textile include biodegradabil-ity, toxicity, fiber size, porosity, and tissue encapsulation. Implantable medical textile product groups that are currently being researched and developed are arterial grafts, surgical sutures, stents, and ligaments. An important area of research is concerned with improving the fabric failure of conventional grafts within the harsh hemodynamic milieu especially when coupled to stents [20]. Vascular grafts have been used for over 40 years to replace diseased or damaged arteries. Implants are also exposed to several different types of tissues, depending on the location of the implant. Much of the current interest in fiber biocompatibility with fluids and tissues reverts to the compatibility between the implant (or wound dressing) and the proteins in the immediate environment. Protein binding to implant materials has been the subject of a large body of literature over several decades. To summarize this body of literature on protein/material binding the statement "water soluble proteins tend to resist binding to highly hydrophilic surfaces " conceptualizes the primary issue. This property of protein/material binding exists because water forms a partially impenetrable layer between the protein and the surface. However, hydrophilic surfaces are not necessarily more biocompatible than hydrophobic surfaces. In this regard, it is still not entirely clear whether blood coagulation and tissue rejection can be predicted based on simple surface parameters as surface tension determinations.

In Chapter 3, Faucheux et al. examine cell behavior and some key cellular mechanisms of proliferation and programed cell death in the presence of serum on a Cuprophan-modified surface. Extracorporeal fibers are those used in mechanical organs such as hemodialysers, artificial livers, and mechanical lungs. Historically regenerated cellulose fibers in the form of cellophane have been utilized to retain waste products from blood. Cuprophan, a cellu-losic membrane, has been the material of choice due to the selective removal of urea and creatinine while retaining nutritive molecules such as vitamin B12 in the bloodstream. Other medical applications of modified cellulose include hemodialysis membranes (vitamin E modified cellulose [21]) and cellulose di-acetate membranes [22]. A more thorough understanding of how the surface properties of extracorporeal fibers which are in contact with blood effect cells in the presence of blood proteins will improve our understanding of improved fiber design and modification.

In Chapter 9, Garcia Paez and Jorge-Herrero introduce work on the uses and preparation of biological adhesives, which is vital to tissue engineering. Tissue engineering is a discipline of biotechnology that creates biological scaffolds for the stimulation of cell growth, differentiation, viability, and the development of functional human tissue. Some of the first commercial tissue engineering products, which focused on skin replacement, will be covered in this chapter. However, technologies are under development to address the pathology of virtually every tissue and organ system. A promising area of tissue engineering is the growing research on fibrin sealants and tissue adhesives for surgical use, acceleration of wound healing, and regeneration of damaged tissue.

Tissue engineering also employs both natural and synthetic polymers elec-trospun into fibers. These electrospun fibers include collagen, elastin, gelatin, fibrinogen, polyglycolic acid, polylactic acid, polycapronic acid, and others. It has been said that this is the decade of nanoengineered materials, and in the area of medical science product potential it is virtually limitless. In Chapter 5, Buschle-Diller et al. highlight some of the principles of electrospun nanofibers and biomedical fibers of interest.

Chapters 10-12 present emerging concepts on enzyme applications to both natural and synthetic fibers. The inclusion of these three chapters on specialty applications alongside chapters for medical fibers is timely with the current interest of applying biotechnology to fibers. At a molecular level, there are close similarities between the biological modification of a fiber with an enzyme and the biological activity of a modified fiber through inhibition or promotion of enzyme activity. At this chemical/biological interface of subject areas, interest often becomes interdisciplinary and new ideas may be spawned. It is also very evident that the scientific community is now turning to enzymes in an effort to make our world more renewable and sustainable. Although enzymes have been used in textile processing for many years, it is only in the last 20 years that growing interest has been given to using a variety of enzymes for textile and fiber applications. Thus, in Chapter 10, Tzanov and Cavaco-Paulo reveal new approaches to modifying cellulose fibers with enzymes applied to the two long-studied problems of fabric crease-resistance and flame retardant finishing. The approach of surface modifying a synthetic fiber is taken up by Fischer-Colbrie et al. in Chapter 11 in the context of hydrolytic and oxidative enzymes, and their application to the many fiber surfaces that are structural components of the modern world. Finally, Kenealy in Chapter 12 extend the coverage to enzymatic modification of fibers in textile and forest products. In the closing two chapters of the book, we have come full circle from wooden dentures in ancient civilizations to the treatment of lignocellulose-containing wood and paper with cold plasmas (Chapter 13) and magnetic susceptibility properties (Chapter 14), respectively. These two chapters also turn our attention further to new technologies and green chemistries that open up promising ways of modifying lignocellulosic fibers.

Some imaginative questions that one might pose as these chapters are being read are, how will fiber technology evolve? We already have numerous military and civilian benefits from fiber development. We have clothes that selectively repel liquid water while allowing the penetration of water vapor. Will biotechnology help us design fibers or polymers to withstand intense radiation while maintaining their integrity? Will we discover that the nanostructure of natural fibers is ideal for implant biocompatibility, thereby opening the door for more successful developments of synthetic replacement organs? How interactive can we expect textile fibers of the future to be? Will we learn from natural fibers how to design synthetic fibers for better control of surface and bulk properties? We leave it to the reader to pose further imaginative questions regarding the future of modified fibers.

The technologies mentioned here are rapidly developing, but it is the editors' belief that the chapters included in this book offer current information that will form a part of the basis of future discoveries in modified fiber technology.

References

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