Which of the following is not considered a natural biomaterial?

In the context of biomaterials used in tissue engineering, natural biomaterials are derived from biological sources and mimic the properties of tissues found in nature.

PCL, or polycaprolactone, is a synthetic biodegradable polymer. It is not derived from natural biological sources but instead is chemically synthesized, which distinguishes it from the other options listed. Natural biomaterials like chitosan, hydroxyapatite, and calcium phosphate are obtained from natural sources or are mineral components found in biological materials.

Chitosan comes from chitin, which is found in the exoskeletons of crustaceans, hydroxyapatite is a naturally occurring mineral form of calcium phosphate found in bones and teeth, and calcium phosphate is a major component of bone that can also be derived from natural sources. These materials demonstrate properties that are more compatible with native tissues, making them favorable for various applications in tissue engineering.

Thus, the identification of PCL as the correct answer lies in its synthetic origin, which inherently categorizes it outside the realm of natural biomaterials.