PhotoFix: unraveling the mystery

The use of biological materials in prosthetic heart valves is widespread. Other articles in this issue address many key aspects of these materials. Fixation treatments, biological characteristics, mechanical performance and dysfunction, and the promise of tissue engineering are all discussed. The rest of this issue serves as an introduction to the many aspects of the use of biological materials, whether they are derived from animal sources or are bioassembled on prepared scaffolds. In this article, a particular fixation treatment, dye-mediated photooxidation (PhotoFix), is discussed. This process yields a material that is biostable, biocompatible, relatively noncalcific, and flexible. It is prepared using a relatively simple method, and the apparent chemical and physical changes in the tissue do not appear to be great. However, the resultant material exhibits dramatically altered biological properties. The mystery of why this simple process produces such dramatic changes is explored here. Despite the title of this article, like many scientific endeavors, our understanding thus far has lead to additional questions rather than providing definitive, terminal answers. Thus, "unraveling" the mystery has led to answers as well as some loose ends.