Among the various biomaterials available for tissue engineering and therapeutic applications, microbial polyhydroxyalkanoates (PHAs) offer the most diverse range of thermal and mechanical properties. Of particular interest are the PHAs that contain 4-hydroxybutyrate such as poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB) and poly(4-hydroxybutyrate) [P(4HB)]. These polyesters can only be synthesized by a few types of bacteria, among which Comamonas acidovorans has the most efficient metabolic pathways to channel 4HB monomers. The resulting polyesters are bioabsorbable and are being developed as a new biomaterial for medical applications. By controlling the molar ratio of the monomers, it is possible to produce materials that are as tough and elastic as rubber.