have become common, and satisfactory performance is experienced for decades. A PMMA material system similar to that employed in the fabrication of denture bases is used to bind metal hip replacements to the femur. Ultrahigh-molecular-weight polyethylene is used as the hip cup material. Even the metal alloys of the bone replacement are gradually beginning to be replaced by fiber-reinforced composites. Knee replacements have also become much more common and successful in recent years. This general area has made considerable progress based on increased understanding of bone growth processes that aid bonding to the prostheses. This is a huge field that is advancing rapidly as new and superior materials are introduced.

In the eye, materials are not brought into contact with blood. Contact lenses are external to the body, but the materials are maintained in intimate contact with tissue. Glass was used for many years, giving way to PMMA beginning in the 1940s. PMMA is a hard, glassy polymer that is compatible with the surface of the cornea. Soft lenses made of poly(2-hydroxy ethyl methacrylate) or simply poly-HEMA hydrogel have become popular more recently, in part because of their high oxygen permeability. Poly-HEMA will take up water to a high degree and become soft and flexible. Soft contact lenses contain about 70 percent water. The polymers currently in use are copolymers of vinyl pyrrolidone with poly-HEMA or PMMA.

Replacement lenses provided following cataract surgery are made of similar polymers. The clouded lens is removed and replaced by a hard lens (PMMA) or a soft hydrogel lens. The hydrogel lens may be inserted through a smaller incision, but it has a smaller refractive index than that of PMMA, requiring a greater thickness. An alternative procedure involving injection of a prepolymer liquid into the lens capsule and polymerization in place has been studied. The introduction of new polymer materials continues to make cataract surgery and recovery of sight safer, less distressing, and more effective.

Diagnostics

Polymers are used in diagnostics either as reagents or as enhancers. Polymeric materials can enhance performance of test materials. They are used as solid supports to bind the material being tested specifically for isolation and detection. In other uses, they serve a "reporter" role. The bioreagents are generally incorporated into the system through direct attachment via either copolymerization or cross-linking. The resulting aggregate has multiple copies of the reactive signal and thus can influence accuracy, testing time, and automation. The extent of incorporation will affect diffusivity and exchange rates of solutes, nonspecific binding, and overall binding capacity. The development of automated clinical analyzer (ACA) systems, for example, relied on the heat sealing and good optical properties of the ionomeric polymer called Surlyn®. Hence, the availability of relatively inexpensive polymers will positively influence the development



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