Prelude
The search for artificial replacements for failing human organs is long and filled with great successes. Today, hemodialysis is routinely used to replace kidney function, artificial hip prostheses allow millions of people to walk, and artificial lenses provide cataract sufferers with clear vision. There are many disappointments as well; despite decades of serious effort, there is still no proven artificial heart or liver.
This chapter describes the success of several artificial organs, including hemodialysis for treating kidney failure and artificial hips. It also describes the efforts to build artificial hearts, livers, and pancreases, and the challenges that remain for these artificial organs. Because all artificial organs depend on the use of materials, usually manmade materials, the chapter begins with a review of biomaterials science, and the body's response to implanted biomaterials.
Biomaterials
A biomaterial is often defined as “a nonviable material used in a medical device, intended to interact with biological systems” (1). Notice that, in this definition, biomaterials are “nonviable,” or not living: They are nonliving materials that are introduced, for some purpose, into a living body. To be most useful, these materials should be biocompatible, or not harmful to the living body in which they are placed.
Over the years, some materials have been demonstrated to be biocompatible after implantation, although the degree of compatibility can vary significantly depending on the site of implantation and the function of the material at that site. Biomaterials are now used routinely in diverse medical applications—from contact lenses to artificial hearts. Frequently, the materials used in biomedical applications are polymers (Table 15.1) or metals (Table 15.2).
The concept of biocompatibility is essential for understanding how biomaterials are used in humans. Unfortunately, biocompatibility in materials is difficult to define precisely. A useful definition, which is accepted by most experts, is “the ability of a material to perform with an appropriate host response in a specific application” (2).