Biomaterials: Facts and Fiction

Abstract

Worldwide, 30–50 million men and women benefit from implanted devices. Therefore, the clinical experiences with synthetic materials in intimate contact with human cells and tissues exceeds by three orders of magnitude the number of laboratory animals sacrificed in the elusive quest for “biocompatibility.” Furthermore, the much longer duration of observation characteristic of human implants, compared to animal studies, adds two orders of magnitude to the superiority of clinical and anatomopathological experience over laboratory animal studies. This glaring disparity must be kept in mind in the assessment of materials through in vitro and small animal studies, and the regulatory agencies’ pronouncements based on such evidence. Another sobering fact is that over 95% of the materials utilized for implants are standard commercial substances originally developed for industrial purposes. Those products which have been found to be appropriate for a specific medical device are labeled “biomaterials” (or more modestly, materials for medical use) on the grounds of established specifications and quality control, supported by continuing feedback from the clinical experience. The major obstacle to the advent of custom-designed biomaterials is that the medical device market is so small (not in numbers of implants and impact on health care budgets, but in the quantity of material used per implant) that large-scale production and amortization of industrial production expenditures is well nigh impossible for truly novel substances. In litigious countries such as the USA, suppliers are pulling out of the market because of the excessive cost of defending against legal action when materials allegedly fail in the body environment. Against this background, biomaterials science is bravely searching for new solutions to old problems. The ill-defined property of biocompatibility is slowly making way for the notion of bio acceptance as we expand our knowledge of the cellular aspects of tissue-material interactions. Bioacceptance can seek two diametrically opposed end points: biointegration, meaning that the material and the surrounding living structures form a continuum with stable, low-grade interactions; and biopassivation, meaning that the material is hardly recognized by surrounding body fluids or tissues and that its stealth characteristics can persist for clinically meaningful periods of use.