Evolution in textile technology for medical use

Excerpt: Bioabsorbable implants are already used in regenerative medicine, but tissue engineering approaches using porous Bioabsorbable scaffolds, either with preseeded cells or without, are also emerging.

Abstract

The need to repair musculoskeletal tissues is increasing worldwide due to the ageing of the population and the increasing number of sports injuries. Bioabsorbable implants are already used in regenerative medicine, but tissue engineering approaches using porous Bioabsorbable scaffolds, either with preseeded cells or without, are also emerging. The term scaffold refers to highly porous, bioabsorbable implant with interconnected, suitably sized pores. Our paper discusses how woven, braided, and knitted scaffolds have been used in bone, cartilage, tendon, and ligament tissue engineering because of their suitable mechanical properties. They can form highly oriented scaffolds and support the oriented in growth of cells. Textiles provide the greatest benefits for healing tissue when these special features are utilized. If textile scaffolds are used to support healing tissue, then long- term support is most likely needed and slowly degrading fibres are chosen for the scaffolds. First, we describe the different steps for manufacturing filaments, yarns, and Bioabsorbable textiles. Then we discuss issues related to the characterization and modelling of fabrics and scaffolds. We can conclude that knitted fabrics are the most flexible, and woven structures are the most stable, while the braided ones are in between. Overall flexibility can be useful in application such as skin substitutes, and braids have been utilized in tendon replacements because of their suitable uniaxial behaviour. For bone, stronger scaffolds are needed hence woven are preferable.

Keywords: Textile scaffold, Bioabsorbable fabric, Bone, Cartilage, Tendon, Ligament.

Introduction

TISSUE Regeneration is also known as tissue engineering and regenerative medicine. Regenerative medicine combines the physical nature of product with living cells. It is an emerging revolutionary approach in modern medicine as it delivers living tissue, stimulating the body's own natural healing process by activating the body's inherent ability to repair and regenerate. Innovative regenerative medicine therapies are now available that now aim to heal or reconstruct diseased tissue and support the regeneration of diseased or injured organs. The best example of tissue engineering/ regeneration device is scaffold. Scaffold is the central components, which are used to deliver cells, drug and gene into the body.

  1. Importance of Scaffold Matrices in Cell Delivery
  2. Key Considerations which are important when determining the suitability of a Scaffold Biomaterials for scaffold fabrication
  3. Manufacturing methods of scaffolds
  4. Medical products by textile technique
  5. Future trends.

Importance of scaffold matrices in cell delivery

  • Scaffolds provide growth of cells either seeded within the porous structure of the scaffold or migrating from surrounding tissue.
  • Scaffolds matrices can be used to achieve cell delivery with high loading and efficiency to specific sites.
  • Scaffold must provide a suitable substrate for cell attachment, cell proliferation and cell migration.
  • To permit the transport of biological signaling factors, nutrients and wastes to allow for cell survival.
  • Possess relatively easy process ability and malleability into desired shapes.

Highly porous with a large surface volume ratio which provides high cell attachment.

Key considerations

Key considerations which are important when determining the suitability of a Scaffold is as follows:-

Biocompatibility

The very first criterion of any scaffold for tissue engineering is that it must be biocompatible; cells must adhere, function normally, and migrate onto the surface and eventually through the scaffold and begin to proliferate before laying down new matrix. After implantation, the scaffold or tissue engineered construct must elicit a negligible immune reaction in order to prevent it causing such a severe inflammatory response that it might reduce healing or cause rejection by the body.

Biodegradability

The objective of tissue engineering is to allow the body's own cells, over time, to eventually replace the implanted scaffold or tissue engineered construct. Scaffolds and constructs, are not intended as permanent implants. The scaffold must therefore be biodegradable so as to allow cells to produce their own extracellular matrix. The by-products of this degradation should also be non-toxic and able to exit the body without interference with other organs.

Mechanical Properties

Scaffold must be strong enough to allow surgical handling during implantation. While this is important in all tissues, it provides some challenges for cardiovascular and orthopedic applications specifically. Producing scaffolds with adequate mechanical properties is one of the great challenges in attempting to engineer bone or cartilage. For these tissues, the implanted scaffold must have sufficient mechanical integrity to function from the time of implantation to the completion of the remodeling process. The factors influencing the structural properties of the scaffolds can be divide into-a) yarn related factors- filament number, fibre type, fibres strength & twist. b) fabric related factors- yarn density, fill direction, weave & crimp. A further challenge is that healing rates vary with age; for example, in young individuals, fractures normally heal to the point of weight-bearing in about six weeks, with complete mechanical integrity not returning until approximately one year after fracture, but in the elderly the rate of repair slows down.

Scaffold Architecture

The architecture of scaffolds used for tissue engineering is of critical importance. Scaffolds should have an interconnected pore structure and high porosity to ensure cellular penetration and adequate diffusion of nutrients to cells within the construct and to the extra-cellular matrix formed by these cells. Furthermore, a porous interconnected structure is required to allow diffusion of waste products out of the scaffold, and the products of scaffold degradation should be able to exit the body without interference with other organs and surrounding tissues.

Biomaterials for scaffold fabrication

A number of different categories of biomaterials are commonly used as scaffold for cell delivery like

Natural Polymers

Natural polymers include alginate, proteins, collagens, gelatin, fibrins, cellulose, starch, pectin, elastin, fibroin, chitosan (chitin), polyhydroxyalkanoates, etc. They can be used as biomaterials for cell/drug/gene delivery purposes. Advantages of natural polymer are their biocompatibility, commercial availability, easy processing and they can more closely mimic the natural extracellular matrix of tissues. But the limitations are short supply, expensive, batch-to-batch variation, and are susceptible to cross-contamination. (Table 1)

Synthetic Polymers

Synthetic polymers are largely divided into two categories: Biodegradable and Non-biodegradable. Biodegradable polymers are polyglycolide, polylactide and its copolymer, polycaprolactone& polyurethanes etc. Non-biodegradable polymers include polyvinyl alcohol and polyhydroxyethmethacrylate. Advantage of this scaffold is easily controlled physiochemical properties, no immunogenicity, processed with various techniques and consistently supplied in large quantities. (Table 2)

Manufacturing methods of scaffolds

Textile types each have unique properties that depend on the binding of the yarns in the structure. Different fabrics are used to achieve different physical and mechanical properties, such as thickness, strength and flexibility. Textile properties are highly dependent on the yarn geometry and the shifting capacity of the yarn in the structure, as well as the number and area of the yarns interlacing points in the structure. In general, knitted fabrics are the most flexible, and woven structures are the most stable, while the braided ones are in between. Overall flexibility can be useful in application such as skin substitutes, and braids have been utilized in tendon replacements because of their suitable uniaxial behavior. For bone, stronger scaffolds are needed. So the scaffolds are constructed by-

1) Weaving

2) Knitting

3) Braided

Weaving

The process of interlacing two yarns so that they cross each other at right angles to produce a woven fabric. A woven fabric is comprised of warp yarns in the longitudinal direction and weft yarns in the crosswise direction. The warp and weft yarns may be the same fiber and same size or they may be different in fiber type and size.

Key characteristics of woven fabrics include-

  • Dimensional stability
  • Flexible
  • Low elongation
  • Controlled porosity
  • High tensile strength in both longitudinal and crosswise directions
  • High burst strength
  • High suture retention strength

Characteristics of woven structure scaffold

  • Low profile design
  • Low delivery forces required
  • High degree of coagulation
  • Controlled permeability
  • Strength
  • Compressibility
  • Insulation against friction and tissue wear
  • Abrasion resistance

Knitting

Knitting is a method of constructing fabric by interlocking a series of loops of one or more yarns. Warp knitting is a type of knitting in which the yarns generally run lengthwise in the fabric. The yarns are prepared as warps on beams with one or more yarns for each needle.

Key characteristics of knit fabrics include-

  • Dimensional flexibility
  • Controlled elongation
  • Controlled porosity
  • Resistance to un-raveling

Characteristics of knitted structure scaffold-

  • High degree of tissue in-growth
  • Controlled permeability
  • Controlled degree of elongation
  • Compressibility

Braiding

A braid is a complex structure or pattern formed by intertwining three or more strands of flexible material such as textile fibers or wire. A braid is usually long and narrow, with each component strand functionally equivalent in zigzagging forward through the overlapping mass of the others.

Key characteristics of braided fabrics include-

  • Dimensional flexibility
  • High strength
  • High elongation
  • Controlled porosity
  • Dense braids with low porosity

Characteristics of braided structure scaffold-

  • High degree of tissue in-growth
  • Controlled permeability
  • High degree of elongation
  • Compressibility

Medical products by textile technique

Mesh Graft

Body organs can be repaired using mesh graft. The grafts are made from a fabric of a net which contains an open textures and evenly spaced holes. The utilization of a mesh grafts in a humans is based on the fact that, during the absorption period, a neomembrane is formed in the site where the mesh has been implanted.

Required properties of the mesh for hernia repair-

The fabrics used for implant must meet certain biological properties and medicinal functional properties. The biological properties include innocuity, sterilizing, bio-compatibility, bio-absorbency or bio inertness. As repair material, the medical functional properties of the mesh used for hernia repair include-

  • Having enough strength, being able to resist the stress from the inner abdomen before the new healthy tissue comes into being;
  • Having an appropriate pore size, rational pore distribution and a high pore ratio;
  • In order to decrease the chance of infection and sinus tract formation, the size of the three dimensions between the constituent filaments must exceed 10 µm;
  • Maintain good dimensional stability after the material is implanted into the body, in other words, it cannot shrink or become deformed;
  • Having good stability in structure, can easily be cut into any shape needed and is easy to suture onto tissue;
  • Having soft handle and easy to mould, increases the feasibility of operation, decreases discomfort to the patient, and increases the success of the operation.

Structural design of the mesh-

The three-open atlas structure, according to the properties required to knit the mesh for hernia repair. Woven fabric is made by interlacing warps and wefts. Its stability is determined by factors such as tightness of the fabric, the friction force between the warps & wefts, and the entanglement of fuzzy fibres. As a woven fabric made of monofilaments does not show the advantages concerned with the factors mentioned above, it can be imagined that the structural stability of such a woven fabric is considerably poorer. On contrary, the warp knitting mesh is made by the interlacement of one or some systems of yarns. The knitted fabric made from monofilaments has good dimensional stability after heat setting, and the stitches are not easy to drop, so it is suitable for the structural requirements of hernia-repairing mesh.

Vascular Grafts

The first artificial vascular graft was produced from polyamide fibre in1956. PTFE fibres soon replace polyamide and then PET was introduced. The implants are made from a variety of synthetic materials the main fibre includes PET, PTEF, PP, PAN. However, PET and PTFE are the most common vascular prostheses currently available. The major requirements of a good vascular graft includes-

  • Bio compatibility
  • Non-fraying
  • Flexibility
  • Durability
  • Stability to sterilization
  • Resistance to bacteria or viruses
  • Non-thrombogenecity

Structural design of the vascular graft

A hollow tubular graft comprising a single outer woven complete velour fabric from warp yarns and fill yarn, said warp yarns supporting a plurality of first fill yarn portions, said first fill yarn portions are positioned outside of warp yarns to from only outer circumferential loops exclusive of inner circumferential loop with the loops each extending outside of 4 to 8 warp yarns and the adjacent second fill yarn portions woven so they are offset from one another at least one warp yarn so that adjacent loops along the length of the tubular graft are out of alignment, said loops being substantially transverse to the longitudinal axis of the tube. Grafts which are presently in use are of a woven or knitted construction. Knitted construction grafts tend to be of high porosity and thus bleeding often accompanies their use. While woven grafts are in wide use, there is still a need for a new and improved woven graft that because of its weave construction is of low porosity, is smooth on the interior of the graft to prevent obstruction thereof by various material carried by the blood in the graft, and provides a staggered design outer looped surface extending around the circumference thereof to allow for the body tissue which grows thereabout after implantation to firmly support it in the body. The graft is also preferably crimped so that it will not kink easily during implantation.

Future Trends

In the future tissue engineering research will likely involve the development of composite scaffolds and more complex scaffold structures. Also, novel biomaterials and material combinations, either as composites or hybrid structures, will be favoured because, using single-component options, it has been challenging to find optimal solutions. Composite scaffolds with textile reinforcement and cell growth stimuli hydro gel sponge are widely studied, and the field's next challenge is to construct textile materials so that they can better serve as biologic substrates. Current material candidates for these substrates include collagen fibres. Stronger and thinner biomaterials and smaller constructions are under investigation as well. The challenges for the future are the miniaturized sizes of the structures and the scaffolds. This extremely small size scale, together with the thin fibres, poses a serious obstacle in terms of developing the machinery needed for production, and this may lead to a completely new process for designing and building the equipment for manufacturing textiles and scaffolds for tissue engineering purposes.

Conclusions

  • Scaffolds provide adequate signals to the cells, to induce and maintain them in their desired differentiation stage and for their survival and growth. Additionally, the incorporation of drugs into to scaffolds may be used to prevent infection after surgery.
  • In summary, tissue engineering is one of the most exciting interdisciplinary and multidisciplinary research areas and is growing exponentially over time. Scaffold materials and fabrication technologies play a crucial role in tissue engineering. - All these techniques for scaffold fabrication are sensitive to the various processing parameters. Innovations in the material design and fabrication processes are raising the possibility of production of implants with good performance.
  • As we have discussed in our review paper, we can conclude that knitted fabrics are the most flexible, and woven structures are the most stable, while the braided ones are in between. Overall flexibility can be useful in application such as skin substitutes, and braids have been utilized in tendon replacements because of their suitable uniaxial behaviour. For bone, stronger scaffoldsare needed hence woven are preferable.

References

Ø Kellomaki,Minna, Laine,kaisa, Ella, ville, Bioabsorbable fabrics for musculoskeletal scaffolds, chapter 4. Ø Brien,Fergal. J. O', Biomaterials and scaffold for tissue engineering. Department of Anatomy, Royal College of Surgeons in Ireland, 123. St. Stephen's Green, Dublin 2, Ireland. Ø Gaoming, Jiang, Xuhong, Miao, Dajun, Lee. Southern Yangtze., Process of warp knitting mesh for hernia repair & its mechanical properties. University, (1800) Lihu Road, Wuxi 214122, China. Ø Garg, Tarun, Bilandi, Ajay Kapoor, Bhawna, (2011) Scaffold: tissue engineering and regenerative medicine, International Research Journal of Pharmacy.2 (12), 37-42. Ø Singh Charanpreet, Wong Cynthia S and Wong Xungai, Medical Textiles as Vascular Implants and Their Success to Mimic Natural Arteries. Journal of Functional Biomaterials ISSN 2079-4983, 6, 500-517.

Author Details

Dr. M Dhinakaran, P.Hangeswaran and Hannah.J

Department of Textile Technology, Kumaraguru College of Technology, Coimbatore-641 049, India

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