Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 30

Composites are composed of two or more physically distinct materials or phases (Chapter 12). Synthetic composites used as biomaterials are composed of one or more of the primary classes of materials (metals, ceramics, and polymers), and consist of a continuous phase (the matrix) and a dispersed phase (the reinforcing phase). While composites are abundant in nature, synthetic composites find only limited use as biomaterials. A well‐known example of a natural composite is bone, produced by embedded cells and composed of an inorganic (ceramic) phase of fine lath‐like particles of composition approximating that of hydroxyapatite and an organic phase composed of collagen.

Synthetic composites are often of interest when a single material cannot provide the desired combination of properties. For example, polymers have an advantage of ease of fabrication but, because of their weak mechanical properties, they are not suitable as implants to heal defects in structural bone that have to support a significant physiological stress. To better approximate the mechanical properties of bone, polymers can be reinforced with a strong material, such as a ceramic in the form of particles or fibers. Use of particles composed of hydroxyapatite or bioactive glass can also enhance the functionality of the polymer matrix, such as its bioactivity.

As ceramics are brittle, they are sometimes reinforced with another phase, typically strong ceramic particles or fibers, to improve their resistance to fracture, a property quantified by a measurable parameter called the fracture toughness. For example, zirconium oxide (ZrO₂ ) is incorporated into alumina (Al₂O₃) to form a composite, referred to as zirconia‐toughened alumina (ZTA), an improved form of which is called alumina matrix composite (AMC). Because of its better fracture resistance (higher fracture toughness) and better wear resistance, AMC has now replaced Al₂O₃ as a ceramic femoral head material in hip implants.