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, like blood substitutes, artificial aortic valves or bone-like polymeric composites are being used as a substitute.

However there is a number of problems which play a prominent role in making artificial tissues safe in clinical setting. The materials should:


 * 1) 1 have physical properties and structure as close as possible to the natural tissue which is to be replaced, like elasticity, smoothness of surface and endurance
 * 2) 2 be non-allergic, so they do not trigger inflammatory reactions.
 * 3) 3 be non-toxic, so they do not release toxic substances causing tissue or organ damage
 * 4) 4 be bio-degradable in case of temporary tissue replacement like surgical sutures

As in other areas of biomedical research, nature is seen as a guide to design new materials. The development of materials for any replacement application should be based on understanding of the structure and function to be substituted.

The two of the examples of those properties in clinical practice are: artificial aortic valve prosthesis and artificial bone replacement materials. On one end, elasticity and smoothness of the graft material is especially important in case of artificial aortic valve prosthesis. Elasticity ensures proper hemodynamic behaviour of the artificial valve. Smoothness of the surface, prevents excessive agglutination, thus prevents cloth formation. Excessive blood clothing on the surface of artificial heart valve is on of the most important problem associated with artificial heart valve replacement procedure.

On the opposite end enhancement of stiffness and strength is on target as it is in case of mineralized tissue replacement, such as bones and tooth.

Natural bones is a composite with variable density ranging from very dense and stiff ( the cortical bone ) to a soft and foamed structure ( trabecular bone ). Structurally the bone matrix consist of type 1 collagen fibers which is responsible for flexibility, reinforced by minerals, responsible for the stiffness.

Historically first bone implants used for bone fixation and total joint replacement have been accomplished with the use of metal prosthesis that exhibit a much higher stiffness in comparison to the typical bone. Under pressure or loading conditions the difference in stiffness between the bone and metal most of the force is carried by the metal device. This tends to promote osteoporosis which compromise tissue healing.

In order to overcome this mechanical problem, the bone analogue concept has been developed. The objective of this concept was to develop a material or mixture of materials which will replicate natural bone mechanical behavior.

The researches attention was focused on the design of prosthesis based on combination of two materials : flexible porous polymers reinforced with ceramic material. Polymers ( light and flexible) in this setting played a role of tubular bone. Ceramic assured mechanical reinforcement of the polymer. Bone prosthesis build of such combination of materials mimics bioactive character of the natural bone.

Another interesting material available for polymer reinforcement is aramid fiber (know as Kevlar), natural bamboo fibers or bioactive glass. Bioactive glass is a special type of glass, which has affinity with mineral bone enabling to obtain mechanical reinforcement and bioactivity of polymer matrix.

The greatest advantage of composite materials is that they offer the possibility of tailoring its properties by playing with the proportions of polymer and reinforcement agent, dimension of particles and its orientation. This way it will be possible to avoid the stiffness mismatch between metal implants and the bone, thus to avoid serious clinical complications such as osteoporosis.

Further research is necessary to develop most desirable and safe combination of replacement materials. Recently investigated carbon fibers which are radiolucent, heat-resistant, very light and extremely strong offer a hope for finding better yet solution.

Bibliography:


 * Black J., Hastings GW: Handbook of bimaterials properties. London; Chapman and Hall, 1998
 * Currey JD: Biocomposite - micromechanics of biological hard tissues. Curr Op in Solid State and Mat Sci, 1996:1:440-5
 * Seely RR, Stephens TD, Tate P. Anatomy and Physiology, Mosby 1995
 * Scwyzer HK, et al: Bone loss using computed topography. in Pattern SM, Biomechanics, 1984 p.383-8
 * Bonfield W: Composites for bone replacement. J Biomed Eng:1988:10:522-6
 * Evans SL, et al.: Composite technology in load-bearing orthopedic implants. Biomaterials,1998:19:1329-42