Tissue replacement from mechanical point of view

 Tissue replacement from mechanical point of view  

Transplantation of natural tissues is not always possible due to lack of availability of natural organ or tissue. In such cases, artificial tissue analogues such as blood substitutes, artificial aortic valves, or bone-like polymeric composites are used as substitutes.

However, there are a number of problems in making artificial tissues safe in clinical use. The ideal materials should:
 * 1) have physical properties and structure (eg, elasticity, smoothness of surface, and durability) as close as possible to the natural tissue to be replaced;
 * 2) be non-allergic, so they do not trigger inflammatory reactions;
 * 3) be non-toxic, so they do not release  toxic substances, causing tissue or organ damage;
 * 4) be bio-degradable, in the case of temporary tissue replacement like surgical sutures.

As in other areas of biomedical research, nature may be a good guide in design of new materials. The development of replacement materials should be based on understanding of the structure and function to be substituted.

Two examples of these properties in clinical practice are artificial aortic valve prosthesis and artificial bone replacement materials.

Elasticity and smoothness of the graft material is especially important in artificial aortic valve prostheses. Elasticity ensures proper hemodynamic behavior of the artificial valve, and smoothness of the surface prevents excessive agglutination, thus preventing clot formation. Excessive blood clotting on the surface of artificial heart valves is one of the most important problems in artificial heart valve replacement.

Enhancement of stiffness and strength is important in replacement of mineralized tissue, such as bones and teeth.

Natural bone is a composite with variable density ranging from very dense and stiff (eg, the cortical bone), to a soft, foamy structure (eg, the trabecular bone). Structurally, the trabecular bone matrix consists of type 1 collagen fibers responsible for flexibility, and it is reinforced by minerals (hydroxyapatite crystals) responsible for the stiffness.

In the past, the first bone implants used for bone fixation and total joint replacement were done with the use of metal prostheses, which have much greater stiffness than typical bone. Under pressure conditions in which there is a difference in stiffness between the bone and metal, most of the force is carried by the metal device. This can lead to osteoporosis, which compromises tissue healing.

To overcome this mechanical problem, the bone analogue concept was developed to develop a material or mixture of materials that will replicate the mechanical behavior of natural bone.

The researcher’s attention was focused on the design of a prosthesis based on a combination of two materials: flexible porous polymers, reinforced with a ceramic material. Polymers, which are light and flexible, in this setting played the role of tubular bone, while the ceramic gave mechanical reinforcement to the polymer. Bone prostheses built of such a combination of materials imitate the bioactive character of the natural bone.

Other interesting materials available for polymer reinforcement are aramid fiber (also known as Kevlar), natural bamboo fibers, and bioactive glass. Bioactive glass is a special type of glass which give mechanical reinforcement to the polymer matrix.

The greatest advantage of composite materials is that they allow tailoring its properties by changing the proportions of the polymer and reinforcement agent, as well as the size and orientations of particles. The use of such composite materials avoids the stiffness mismatch between metal implants and bone, thus preventing serious clinical complications such as osteoporosis.

Further research is necessary to develop the safest and most desirable combination of replacement materials. Carbon fibers, recently investigated in this application, offer hope for finding an even better solution—they are radiolucent, heat-resistant, very light weight, and extremely strong.

Bibliography:


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 * Evans SL, et al.: Composite technology in load-bearing orthopedic implants. Biomaterials,1998:19:1329-42