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Nonwovens & Technical Textiles
  Textile materials in implantable medical surgeries

There is vast scope in the application of textile materials for the replacement of tissue, nerve guideline, arteries, and bones, say D Somasundaram and V K Kothari, who discuss the issue of textile materials in implantable medical applications.

Surgical papyrus has the first recorded use of fibres as a medicine as long back as 4000 years. "Susanta Sambita" tells us that about 2500 year ago textile fibres were used as medicine. There are many materials used for medicinal purpose like horse hair, leather strips, cotton, animal sinew and fibrous tree bark. With time, textiles have found their way into a variety of medical applications.

In addition to protective medical apparel, textiles in fibre and fabric forms are used as implants, filters and surgical dressings. Recent decades have witnessed major developments in medical textile production, covering the fields of materials as well as technologies. Biomedical implants are used to aid or replace damaged tissues or organs. These materials are used in affecting repair to the body whether it is wound closure (sutures) or replacement surgery (vascular grafts, artificial ligaments, etc).

Although the natural way to replace a defective body part would be transplantation, this is always not possible due to many reasons. Therefore, physicians use an artificial substitute (biomaterials) such as biotextiles. A foreign or synthetic material or part used to replace a body part is referred to as prosthesis. Most people are familiar with artificial knees or hips. Recent reports show that 1 in 10 Americans has an implanted medical device.

The main attribute of a biomedical textile is that it should fulfil the purpose for which it was designed. For example, sutures may require a biodegradable textile. An artificial ligament is permanent and is able to react more with blood cells and the surrounding tissues, compared to an external bandage which is temporary and only contacts the outer skin tissue. An implantable device should be biocompatible. Biocompatibility testing evaluates the response of the host system to the medical textile. Results of this testing must be viewed along with the benefits of this device. Requirements of an implant(2)

The biological requirements of a satisfactory artificial implant may be stated as follows:

Porosity, which determines the rate at which tissue will grow and encapsulate the implant.

Fibre diameter: Small circular fibres are better encapsulated by human tissue than larger fibres with irregular cross-sections.

Non-toxicity, where fibre polymer or fabrication techniques must be non-toxic and fibres should be free of contaminants.

Biodegradability and bio-stability depending on the application; A suitable artificial surface for body cells to adhere to and grow on.

The properties of the polymer will influence the success of the implantation in terms of its biodegradability. Table 1 shows some of the areas of application, fibre used and type of fabrics for the implantable materials. Polyamide is the most reactive material losing its overall strength after only two years as a result of biodegradation. PTFE is the least reactive with polypropylene and polyester in between.

Table 1: Implantable materials

Product application Fibre type Manufacture system
Sutures
Biodegradable Collagen, polylactide, polyglycolide Monofilament, braided
Non biodegradable Polyamide, polyester, PTFE, Polypropylene, Steel Monofilament, braided
Soft tissue implants
Artificial tendon PTFE, Polyester, polyamide, silk, polyethylene Woven, braided
Artificial ligament Polyester, Carbon Braided
Artificial cartilage LDPE Nonwoven
Artificial skin Chitin Nonwoven
Eye contact lenses/ artificial cornea Polymethyl methacrylate,
Cilicone, collagen
 
Orthopedic implants
Artificial joints/bones Silicone, polyacetal, polyethylene  
Cardiovascular implants
Vascular grafts PET, PTFE Knitted, woven
Heart valves PET Knitted, woven

Source: Handbook of technical textiles Edited by A R Horrocks and S C Anand, University of Bolton, UK

Various types of implants

Sutures and ligatures

The term 'Ligature' denotes tying something such as blood vessel or a pedicle, whereas 'suture' denotes sewing by means of a needle and a thread made of suture material(2). Sutures are strategically located after a surgical operation primarily to hold the basic structural elements in their required sites and provide the necessary strength, retained over a period of two weeks upwards, depending on the specific site. Sutures are either monofilament or multifilament threads used in surgery for wound closure. The ideal suture is a monofilament with a smooth surface that can pass through the skin without being caught and can be tightened into a single knot. The polybutylene terephthalate (PBT) suture is currently the most popular because of its acceptable strength and smooth surface.

Sutures are characterised as biodegradable or non-biodegradable. Biodegradable sutures are used mainly for internal wound closures. Non-biodegradable sutures are used to close exposed wounds and are removed when the wound is sufficiently healed. These may again be of a natural or synthetic variety. The selection of the suture will depend on physical and chemical characteristics and the biological culture of the tissue in which it is placed.

The absorbable natural sutures are Catgut prepared from the intestine of government inspected sheep. The great advantage of catgut is that it can be used even in the presence of infection where a non-absorbable suture cannot be used. The disadvantages of catgut are loss of tensile strength, doubted purity and cost. Collagen was invented to overcome the disadvantages of catgut. The flexor tendons of beefs are converted into dispersed fibrils, which are extruded and reconstituted to form collagen sutures. The absorbable synthetic sutures are Polyglycolic acid (dexon), which has the advantages of tensile strength, very little tissue reactivity and knots well. The disadvantage however is that its tensile strength falls in 15 days. Co-polymer is a suture, which overcomes the disadvantage of polyglycolic acid in that its tensile strength does not fall before 4-6 weeks.

The non-absorbable natural sutures are silk, waste silk, cotton, linen whereas the synthetic ones are polyamide, polyester, polyethylene, and polypropylene. Vicryl plus from Ethicon, is claimed to be the world's first and only suture incorporating an antibacterial agent. It is designed to reduce bacterial colonisation on the suture.

Vascular implants

The number of implants used in the surgical treatment of vascular disease has been steadily increasing over the last 20 years, so it is now considered a common procedure(4). Such tubes replace or bypass part of a blood vessel-principally arteries-and operate in a similar fashion to natural blood vessels.

However, while advances in vascular disease surgery have been made, synthetic implants are still susceptible to thrombosis or clotting, occlusions and infection caused by protein and cell adsorption and coagulation activation. Serious post-surgical problems occur in some 10% of surgical patients, which includes approximately a 2% rate of vascular graft infections. As a result, some grafts have to be replaced only a few months after being implanted.

New test device: Vascular graft is an artificial vein or artery used to replace segments of the natural cardiovascular system that are blocked or weakened(3). Grafts are implanted to bypass the blockages and restore the circulation. These are replaced in surgery to replace damaged thick arteries or veins from 6 mm, 8 mm or 1 cm diameter. Straight or branched grafts are possible by using either the weft or warp knitting technology. Knitted vascular grafts have a porous structure, which allow the graft to be encapsulated with new tissue. The disadvantage is that this can cause hemorrhage (blood leakage) through the interstices directly after the implantation. In an attempt to reduce this risk, knitted grafts with internal and external velour surfaces are used. Another method is to seal or percolate the graft with the patient's blood after implantation. Porous Teflon exhibits good bio-compatibility and anticoagulant activity. However, thin blood vessels, made from Teflon tubes lead to problems. The tube consists of an inner layer of collagen, the tube itself providing strength. Research is targeted to produce artificial blood vessels of less than 3 mm diameter. The main requirements are blood compatibility, porous structure, re-absorbable, easy for tissue growth and avoid clotting.

Soft tissue implants

  • Attempts have been made to replace or augment most of the soft tissues in the body(5).
    *Connective tissues: skin, ligament, tendon, cartilage
    *Vascular tissue: blood vessels, heart valves
    *Organs: heart, pancreas, kidney
    *Other: eye, ear, breast

  • Most soft tissue implants are constructed from synthetic polymers.
    *Possible to choose and control the physical and mechanical properties
    *Flexibility in manufacturing

  • "Soft tissue implants" can also be designed for soft tissue repair.

A major problem with implanted materials is their compatibility with host tissues(6). Researchers at Clemson University have recently patented methods for texturising soft tissue implants with micron-scale surface texture to optimise their compatibility with human body tissue. This method optimises anchorage and histocompatibility of implants to tissue without causing inflammatory tissue to form at the implantation site. The surface layer defines a three-dimensional pattern with an exterior surface defining a plurality of spaces and a plurality of solid surface portions.

Unlike previous implants with relatively large pore entry diameters, an implant device using this technology promotes cellular anchorage of the device at the implantation site through the growth of mature collagen, causing a minimal yet desirable encapsulation of the embedded portion of the device. The Clemson surface texture technique provides a significant improvement in the histocompatibility of implants since it results in the development of a highly desirable, thin, mature, and stable connective tissue capsule around the implant material. This type of surface eliminates the undesirable chronic inflammatory response observed in virtually all implant materials that typically results in the deformation of the implantation site and often damages to the implant.

Biomedical materials are used in applications such as soft tissue compatible artificial prostheses, artificial skin patches, artificial tendon and artificial corneas(3). Important properties that affect cell attachment and tissue growth are chemical structure, electric charge, hydrophilicity, hydrophobicity, roughness of the surface, micro heterogeneity and material flexibility. Soft tissue compatible biological polymers are collagen, silk protein, cellulose, chitin and chitosan. Soft tissue artificial materials include silicone rubber, polyurethane, hydro gels and carbon fibre. Silicone rubber is a cross linked polymer of poly (dimethyl siloxane). It has been used in artificial breasts, ears, dental works and noses.

Hernia repair

Meshes find use in hernia repair and abdominal wall replacement, where mechanical strength and fixation are very important(3). Fibres can be woven or knitted into a mesh with each side designed with a specific porosity and texture to optimise its long term function. Polypropylene mesh is an example of fabrics used in hernia repair. Polypropylene is resistant to infection and is anti allergenic. Gore-Tex soft tissue patch, which is used in hernia repair, is made of expanded PTFE.

Hard tissue implants

Hard tissue compatible materials must have excellent mechanical properties compatible to hard tissue(3). Typical characteristics of polymers related to hard tissue replacements are good processability, chemical stability and biocompatibility. Applications include artificial bone, bone cement and artificial joints. Orthopaedic implants are used to replace bones and joints, and fixation plates are used to stabilise fractured bones. Textile structural composites are replacing metal implants for this purpose. A nonwoven fibrous mat made of graphite and Teflon is also used around the implant to promote tissue growth.

Nerve guidance channel


A developing area of research is the development of nerve guidance channels that are used to bridge the damaged nerve endings and facilitate the passage of molecules secreted by the nerve and bar fibrous tissue from infiltrating the area thus preventing repair(3). An innovation is the use of electrically conducting polymers such as polypropylene to promote nerve regeneration by allowing a locally applied electrical stimulus. It is a blossoming field of textile research, since the nerve guidance channel may be a single continuous hollow tube, or it may be a hollow tube comprised of fibres.

Biomaterials in ophthalmology

Natural and synthetic hydro gels physically resemble the eye tissue and hence have been used in ophthalmology as soft corneal lenses. Soft contact lenses are made of transparent hydrogel with high oxygen permeability(3). Hard contact lenses are made of poly (methyl methacrylate) and cellulose acetate butyrate. Flexible contact lenses are made from silicone rubber.

Dental biomaterials

Major requirements of dental polymers include translucence or transparency, stability, good resilience and abrasion resistance, insolubility in oral fluids, non-toxicity, relatively high softening point and easy fabrication and repair(3). The most widely used polymer for dental use is poly (methyl methacrylate) (PMMA) and its derivatives. Other materials for denture base polymers are polysulfone and polyether polysulfone.

References

  • Dr J Hayavadana, H L Vijaya Kumar: Potential of Intelligent Textiles in Biomedical Field, Osmania Univ, India, Asian Textile Journal, Feb 2004.

  • Handbook of technical textiles, Edited by A R Horrocks, and S C Anand, University of Bolten, UK, WoodHead publishing limited, UK, published year 2000.

  • http://www.expresstextile.com/20050228/hiperformance01.shtml.

  • http://www.eureka.be/showcasePDF?prjId=2866.

  • http://ttb.eng.wayne.edu/~grimm/BME5370/ Lect20Out.html.

  • http://www.clemson.edu/research/ottSite/techs/patent/20.htm.

The authors are with the Department of Textile Technology, Indian Institute of Technology, New Delhi 110 016.

published July , 2007
 
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