Biomaterials: defining the mechanical properties of natural tissues and selection of replacement materials

This article reviews the problems which are encountered in defining the mechanical properties of natural tissues, and in replacing them with synthetic materials in the human body. It describes how death, ageing, degeneration, pathology and individual variability influence the properties of natural tissues. Experimental problems arise from degradation and testing conditions; these are illustrated by the properties of the nucleus pulposus of the intervertebral disc. Replacement of natural tissues by graft materials and the products of tissue engineering is then described. Synthetic replacement materials should be biocompatible, i.e. they should not cause adverse reactions in the human body. However, polymers which hydrolyse in the body fluids may be useful for implants which are intended to have a limited life or for controlled release of drugs. Synthetic implant materials may attempt to mimic natural tissues but there may be a problem of attaching them to the surrounding tissue, Artificial ligaments provide an example of implants of this kind. Total hip replacement is used to illustrate the successful use of conventional engineering materials, Finally, safety issues are described; an implant material must be biocompatible, have the required mechanical strength, be sterile and should be incorporated into a device so that its performance in the living patient can be monitored.