Most surgical specialties have been revolutionized by the introduction of implantable devices. These advances have been founded, in large part, on biomaterials science and engineering. One of the critical determinants of the performance of the device relates to its compatibility with the structure and function of the tissue or organ (a structure comprised of two or more tissues) in which it is implanted. Moreover, the tissue response to the implant should not impede the required function of the device. This article will deal with the response to biomaterials implanted into solid tissues. Biocompatibility issues related to blood-contacting applications will be outside the scope of this discussion.
Implantation of the device requires the production of a surgical wound, and in this respect the tissue response to the implant may be looked upon as the modification of the wound healing response by the very presence of the implant. In vascularized tissues, the creation of a surgical wound elicits an inflammatory process that can be considered part of the natural course of healing. The end result of the healing process is tissue similar to that naturally occurring at the site (the process of ''regeneration'') or scar tissue (the process of ''repair''), which in many tissues and organs comprises fibrous tissue. Infectious organisms (viz., bacteria) when present serve as a persistent injurious agent that prolongs and can further incite the inflammatory process, not only jeopardizing the performance of the implant, but threatening the life of the individual. The principles of biocompatibility including the mechanisms underlying inflammatory and infectious processes apply regardless of the type of material of fabrication of the implant. There are, however, features of the biomaterials that can affect certain aspects of these processes. It can therefore be instructive to also consider issues of biocompatibility in the context of the various classes of materials: metals, ceramics, and polymers.
CLASSES OF MATERIALS
The term ''biomaterials'' generally refers to synthetic materials and treated natural materials that are employed for the fabrication of implantable devices that are to replace or augment tissue or organ function. An understanding of the chemical makeup of biomaterials can provide insights into their biocompatibility for selected applications. Generally, biomaterials may be considered ''inert'' or ''reactive'' with the biological milieu. In the latter case, the reactivity could relate to the release of moieties or the adsorption of biological molecules. Inert materials may also release small amounts of ions and molecules or nonspecifically bind biomolecules. The feature that distinguishes inert from reactive biomaterials is the degree to which the interactions of the implant with the biological environment affects the tissue response and device performance. Those materials designed to effect specific tissue responses through their reactivity may also be referred to as ''bioactive''. This article provides a brief description of materials used for the fabrication of implantable devices relative to issues related to biocompatibility. A more comprehensive discussion of biomaterials can be found elsewhere in this encyclopedia.
In metals, closely packed arrays of positively charged atoms are held together in a loosely associated ''cloud'' of free electrons. The essential features of the metallic bond are that it is nondirectional and the electrons are freely mobile. The metals most often used for the fabrication of implantable devices are stainless steel, cobalt?chromium alloys, and titanium and titanium alloy. The specific members of these families used as biomaterials are usually identified by a designation provided by the American Society for Testing and Materials (ASTM). Metallic materials have certain properties that make them ideal for load bearing applications; in particular, they can maintain very high strength under the aggressive aqueous environment in the body.
The biocompatibility of the implantable metallic materials is related to their corrosion resistance, in that they can generally be considered as inert. While they release detectable levels of metal ions (1,2), these ions have not yet been found to significantly affect tissue or organ function or cause pathological changes. One conundrum related to biomaterials is that while the ions released from certain metallic devices are known to be carcinogenic when administered to certain animals models (3,4) and when encountered by humans in certain circumstances (1), there have not yet been definitive studies relating the incidence of various cancers to the ions released by metallic implants (see later section).
The following sections provide a brief description of three metal systems most frequently used for the fabrication of implantable devices.