Page 373 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

350 BIOMATERIALS

A B
FIGURE 14.5 Surface traces of indentation cracks in alumina-glass dental composite by backscattered scanning
electron microscopy (SEM). Grain bridging sites are near the arrows in (a) and transgranular fracture sites in (b). (From
Ref. 21.)

bone cements, although this is a very slow phenomenon. Visocelastic effects such as the strain-rate
dependence of stiffness and long-term creep are proportional to the column fraction of matrix such
that highly reinforced continuous-fiber composites are less prone than short-fiber composites.

14.7 BIOLOGIC RESPONSE

In designing biomedical composites and predicting their performance, several issues must be
considered regarding the biological response. As the number of constituent materials in a composite
increases, so can the variations in the host response. Additional tests are necessary to establish that
while the individual materials may be by themselves biocompatible, their specific composition,
arrangement, and interaction are also biocompatible. This has implications for both the flexibility of
design and obtaining regulatory approval. The potential of composite design to obtain the desired set
of properties can be restricted by being conservative in the choice and number of materials used.
Even if all the materials used may be approved by the Food and Drug Administration (FDA), their
particular combination in a composite may require additional approval.
Materials can elicit a different host response in the bulk form than in the fibrous or particulate
form. For instance, UHMWPE, as in an acetabular cup of a hip prosthesis, is generally biocompatible,
whereas its fibrous form, as in a finely woven fabric, has been shown to produce a different, more
adverse reaction. Furthermore, when the discontinuous phase is particles, whiskers, platelets, or
microspheres with dimensions on a cellular scale, the inflammatory response can include their
ingestion by immune cells and transport to other parts of the body. This can be accompanied by the
release of enzymes that can adversely affect the performance of the composite, such as by altering
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the degradation kinetics of a biodegradable composite. The composite can be designed in such a
way that the fibers or particles are not exposed to the host, but this is challenging because it involves
elimination of all voids at the fiber-matrix or particle-matrix interface during processing. In addition,