Composite Biomaterials - Biomaterials - page 81

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Composite Biomaterials

4.1 Structure ............................................................................................. 4 -1
4.2 Bounds on Properties ....................................................................... 4 -2
4.3 Anisotropy of Composites ............................................................... 4 -3
4.4 Particulate Composites .................................................................... 4 -4
4.5 Fibrous Composites .......................................................................... 4 -6
4.6 Porous Materials ............................................................................... 4 -9
4.7 Biocompatibility .............................................................................. 4 -12
4.8 Summary .......................................................................................... 4 -13
References .................................................................................................... 4 -13

Roderic S. Lakes
University of Wisconsin,
Madison

Composite materials are solids which contain two or more distinct constituent materials or phases, on
a scale larger than the atomic. The term “composite” is usually reserved for those materials in which the
distinct phases are separated on a scale larger than the atomic and in which properties such as the elastic
modulus are significantly altered in comparison with those of a homogeneous material. Accordingly,
reinforced plastics such as fiberglass as well as natural materials such as bone are viewed as composite
materials, but alloys such as brass are not. A foam is a composite in which one phase is empty space.
Natural biological materials tend to be composites. Natural composites include bone, wood, dentin,
cartilage, and skin. Natural foams include lung, cancellous bone, and wood. Natural composites often
exhibit hierarchical structures in which particulate, porous, and fibrous structural features are seen on
different micro-scales (Katz, 1980; Lakes, 1993). In this chapter, fundamentals of composite materials
and their applications in biomaterials (Park and Lakes, 1992) are explored. Composite materials offer
a variety of advantages in comparison with homogeneous materials. These include the ability for the
scientist or engineer to exercise considerable control over material properties. There is the potential for
stiff, strong, lightweight materials as well as for highly resilient and compliant materials. In biomateri-
als, it is important that each constituent of the composite be biocompatible. Moreover, the interface
between constituents should not be degraded by the body environment. Some applications of compos-
ites in biomaterials are (1) dental filling composites, (2) reinforced methylmethacrylate bone cement and
ultra-high-molecular-weight polyethylene, and (3) orthopedic implants with porous surfaces.

4.1 Structure

The properties of composite materials depend very much on structure . Composites differ from homo-
geneous materials in that considerable control can be exerted over the larger scale structure and hence
over the desired properties. In particular, the properties of a composite material depend on the shape of
the heterogeneities, on the volume fraction occupied by them, and on the interface among the constitu-
ents. The shape of the heterogeneities in a composite material is classified as follows. The principal inclu-
sion shape categories are: (1) the particle, with no long dimension, (2) the fiber, with one long dimension,
and (3) the platelet or lamina, with two long dimensions, as shown in Figure 4.1. The inclusions may

4 -1