Nonwoven fabrics in tissue engineering

Written by: Kunwar Venkteshwar Singh

Bombay Textile Research Association,Mumbai

Technical textiles are one of thefaster growing sectors of the global textile industry. The world textileindustry is moving rapidly toward the manufacture of high value-added textilestructures and products such as medical textiles, protective textiles andsmart textiles. Textile materials used in the medical and applied healthcareand hygiene sectors are an important and growing part of the textile industry.

According to Technical Textiles andIndustrial Nonwovens: World Market Forecast to 2010 published by David RigbyAssociates(DRA), it is forecast that the world market for technicaltextiles and industrial nonwovens will increase by 3.5% per annum between 1995and 2005, and 3.8% per annum from 2005 to 2010 in volume terms, to reach 23.8mntonnes with a value of $126bn by 2010. On the basis of DRAs research, over1.5mn tons of textile materials, with a value of US$5.4bn, were consumedworldwide in the manufacture of medical and hygiene products in 2000. This ispredicted to increase in volume terms by 4.5% per annum to 2010 to reach 2.4mntons with a value of US$8.2bn. This sector probably offers the greatest scopefor the development of the most sophisticated and highest value textiles forniche applications.

Technical textiles find many differentkinds of application with medical and hygiene products in the healthcaresector. The diversity of applications encountered in medical and healthcareproducts is quite remarkable, e.g. simple bandages, biocompatible implants andtissues, antibacterial wound treatment material, prosthetics, and intelligenttextiles. Each of these categories covers a broad range of applications, andthe many end-uses with their disparate requirements create opportunities forall kinds of textile such as fibers, mono- and multi-filament yarns, woven,knitted, nonwoven, braiding and composite fabrics.

Medical textiles embrace all thosetextile materials used in health and hygiene applications in both the consumerand medical markets. As such, it comprises a group of products withconsiderable variations in terms of product performance and unit value.Because of the nature of their application, many medical products aredisposable items. Nonwovens account for a high part of the sector overall interms of tons of fiber used. Also, another feature of the medical textilemarket will be the growing proportion of composite materials used in woundmanagement products. This will mean the combination of textiles with suchmaterials as films, foam and adhesives to form structures for the treatment ofwound and healthcare products. The increased use of textiles in compositeapplications will provide major growth fiber consumption in terms of volume.

Trends in World Trade of Medical Textiles

The market for medical textiles isbeing driven by a number of factors:

Population growth rates, particularly in newly developing global regions,
Changes in demographics, including the ageing of the population in the Western European market,
Changes in living standards,
Attitude to health risks; increased awareness of the risks to health workers from health threats from blood-borne diseases and airborne pathogens,
The continuing dominance of the leading suppliers and brands (especially in the consumer market),
Ongoing enhancement in product performance,
The growing dominance of purchasing which demands increasing value for money,
The increasing share of nonwovens on the medical world market in relation to traditional textile materials.

These trends will be further fed bythe increasing development of the medical textile market and industry.

Nonwoven in medical textiles

Nonwovens possess many properties due to which they became famous in medical field, as various parameters can be controlled easily like porosity, weight of fabric, thickness; easy to sterilize nonwovens; various manufacturing technique options exist according to applications and economical manufacturing process etc. The applications of nonwovens are characterized usually by their high porosity, large fiber surfaces and the absorbance capability for other substances. The properties of non-woven 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 expanda suture, for example, can expand to fill the needle hole made in a tissueallowing for tissue in growth in applications such as arterial and patch grafts. Polyurethane-based nonwovens produce a product that resembles collagen us 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. Hence by controlling the various parameter of non-woven they can utilize effectively in different applications of medical. Regarding implants the following fields are covered by nonwovens:

Nonwovens as drug carrier or delivery system;

The use of the (semi-) permeability of nonwovens: patches for defect covering or wound dressing

Nonwovens as scaffolds for tissue engineering.

Tissue Engineering

Tissue engineering is a combination of biological and engineering disciplines in order to culture viable human tissues outside the body. Tissue engineering provides surgeons with the possibility of implanting living tissue which will eventually integrate fully with the patients own tissue. A vital part of tissue engineering is the textile scaffold which supports the tissue forming cells. A common approach in tissue engineering involves the use of porous synthetic or naturally occurring polymeric scaffolds (or matrices) on which cells are seeded. Assuming cells attach to the fibers in the scaffolds; this promotes cell division and the production of an extracellular matrix. As the new tissue forms, encapsulation of the fiber is apparent and, concurrently if the scaffolds structure is based on a resorbable polymer it degrades to leave a self-supporting biological structure.

Porous Scaffolds for tissue Engineering

Any scaffold material for tissue engineering must have the right composition, biocompatibility, and 3-dimenssional architecture. The following criteria are considered necessary for designing the ideal scaffold structure.

1) The surface should allow cell adhesion, promote cell growth and permit the retention of differentiated cell functions.

2) It should be biocompatible and not produce cytotoxic degradation products.

3) It should be bioresorbable so that eventually it is eliminated from the body.

4) The porosity should be sufficient to provide space for cell adhesion, extracellular matrix generation, minimize diffusional constraints during cell culture.

5) The pore structure should be open and uniform enough to permit homogeneous cell distribution throughout the entire scaffolds.

6) The material should possess sufficient physical integrity and mechanical stability to enable the construct to be handled and relocated.

7) The material should be processable into a 3-dimessional structure and sterilizable

Textile scaffolds may be embroidered, knitted, woven, nonwoven, braided or of composite construction depending on the intended application. The scaffold acts as a three-dimensional support structure that resists compression or other modes of deformation as the tissue is created, as well as providing a delivery system guiding cells proliferation. Commonly, scaffolds have pore diameter above 10 microns to allow penetration of cells into the structure and subsequent vascularization. Many nonwovens provide this basic architectural requirement, and allow the pore dimension, porosity and fiber orientation to be adjusted within wide limits during the manufacturing process. Further, it is claimed that an advantage of nonwovens, compared with other material used as scaffolds, is that their fibrous structures have similarities with the collagenous matrices found in humans.

Needlepunching is an appropriate route for providing 3 -dimensional scaffolds for tissue engineering, because of the following advantages:

a) A very wide range of felt thicknesses and densities can be achieved;

b) A very wide range of fiber types can be processed;

c) Small-scale operations can be established; and can be obtained.

Needled fabrics made from conventional non-biodegradable materials have the potential to be used in two major areas of tissue engineering- they may be seeded with stromal tissues cells actively synthesizing the extracellular matrix and developed as:

So-called smart wound dressings for the treatment of chronic non-healing ulcers:
Dermal equivalents for the construction of living skin equivalents for use as in vitro modal system.

Scaffold Structural Design Parameters

For a scaffold to function effectively it must possess the optimum structural parameters, conducive to the cellular activities leading to neo-tissue formation; these include cell penetration and migration into the scaffold, cell attachment onto the scaffold substrate, cell spreading and proliferation and cell orientation. Such scaffold design parameters are now described with reference to these cellular activities.

Selection of polymer for tissue engineering

Collagen is the most abundant structural protein in the body and is known to exhibit minimal inflammatory and antigenic responses. It has high strength and flexibility and is already approved by regulatory authorities for medical applications related to wound dressings and artificial skin. As a fiber forming polymer it is particularly suited to tissue engineering applications since it contains cell adhesion domain that is arginine-glycine-asparitic acid, which elicit specific cellular interactions that assist in retaining the cell phenotype and activity, particularly for fibroblasts and chondrocytes. Other naturally occurring polymers and biopolymers include silk proteins, alginates, polysaccharides isolated from seaweed, and chitosan, derived from shell fish protein. These have been spun into fibrous meshes and nonwoven structures that have shown promise as tissue engineered scaffolds for seeding of cells. Polymer such as poly-α-hydroxy acids (polyesters) of glycolic acid, lactic acid and copolymers and ethylene oxide block copolymers are widely used in tissue engineering because of their inherent ability to resorb (degrade) by hydrolytic or enzymatic mechanism and return the polymer to natural metabolites and lactic acid which readily excreted from the body.

Conclusion

Textiles are very important in all aspects of medicine and surgery and the range and extent of applications to which these materials are used is a reflection of their enormous versatility. Products used for medical or surgery applications may at first sight seem either very simple or complex items. In reality, however in-depth research is required to engineer a textile for even a simplest cleaning wipe in order to meet stringent professional specifications. Advances in nonwovens have resulted in a new breed of medical textiles. Advanced composite materials containing combination of fibers and fabrics have been developed for applications where biocompatibility and strength are required. It is predicted that the nonwoven materials will continue to have greater impact in this sector owing to the large member of characteristics and performance criteria required from these materials.

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