Biomedical Composites

A common debate about the definition of composite materials among composite engineers and materials scientists continues to this day. More recently, biomedical engineers have used the term composite prolifically for newly developed biomaterials, and it might be argued that not every usage of the term composite for a biomaterial would satisfy the traditional composite engineer, who is used to thinking in terms of fibers, matrices, and laminates. That said, defining composites a certain way in this chapter is not meant to preclude its use outside this definition. The difficulty lies, on the one hand, in the depth of the material to which the definition refers. Practically everything is a composite material in some sense, except for pure elements. For example, a common piece of metal is a composite (polycrystal) of many grains (or single crystals). Thus alloys, ceramics, steels, etc., would be considered composites if the definition refers to the microstructure. However, if it is the macrostructure that concerns us, then we get the traditional treatment of composites as a materials system of different macroconstituents. On the other hand, there is also a question in this definition regarding how these macroconstituents are brought together and for what purpose. For instance, thin coatings on a material do not make it a typical composite, and the same could be said about adding resin-extending fillers to plastics, although the constituents exist at the macrostructure. Furthermore, a structure that is assembled with components made of different materials does not qualify it to be a composite. Thus a pacemaker lead that has a metallic core and a polymeric sheath would not be considered a composite in the strict sense, whereas a catheter tube polymer reinforced with embedded braided metal wires would. In addition, foams and porous coatings on materials will not be considered composites in this discussion. The following is an operational definition for the purpose of this chapter: