Biocompatibility is defined as having properties that make amaterial or a device compatible with the human body. For example, an indwelling catheter that does not trigger blood clotting or another thrombogenic reaction as a result of its contact with the bloodstream is considered to be biocompatible. A biomaterial is any substance that has been engineered to interact with biological systems for a medical purpose - either a therapeutic or a diagnostic one. As a science, biomaterials is about fifty years old. The study of biomaterials is called biomaterials science or biomaterials engineering. For example, an indwelling catheter that does not trigger blood clotting or another thrombogenic reaction as a result of its contact with the bloodstream is considered to be biocompatible. Cell culture assays are used to assess the biocompatibility of a material or extract through the use of isolated cells in vitro. These techniques are useful in evaluating the toxicity or irritancy potential of materials and chemicals. They provide an excellent way to screen materials prior to in vivo tests.