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

This article reviews the problems which are encountered in defining the mec hanical 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 t issues. Experimental problems arise from degradation and testing conditions ; these are illustrated by the properties of the nucleus pulposus of the in tervertebral disc. Replacement of natural tissues by graft materials and th e products of tissue engineering is then described. Synthetic replacement m aterials should be biocompatible, i.e. they should not cause adverse reacti ons 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 fo r controlled release of drugs. Synthetic implant materials may attempt to m imic natural tissues but there may be a problem of attaching them to the su rrounding tissue, Artificial ligaments provide an example of implants of th is kind. Total hip replacement is used to illustrate the successful use of conventional engineering materials, Finally, safety issues are described; a n implant material must be biocompatible, have the required mechanical stre ngth, be sterile and should be incorporated into a device so that its perfo rmance in the living patient can be monitored.