Polyphosphazene polymers for tissue engineering: an analysis of material synthesis, characterization and applications

Tissue engineering often utilizes biodegradable polymers in the form of porous scaffolds for regenerating de novo tissues. There is an ever-increasing need for biodegradable polymers as temporary substrates for facilitating tissue regeneration. Compared to the widely used polyesters, polyorthoesters, poly(alpha-amino acids), and poly(anhydrides), biodegradable polyphosphazenes form a unique class of polymers that has vast potential for tissue engineering applications. Polyphosphazenes are linear high molecular weight polymers with an inorganic backbone consisting of alternating phosphorous and nitrogen atoms with two organic side groups attached to each phosphorous atom. The synthetic flexibility of polyphosphazenes has enabled the development of a wide range of polymers with a variety of physical, chemical and biological properties. These biodegradable polyphosphazenes undergo hydrolytic degradation yielding non-toxic and neutral pH degradation products due to the buffering capacity of phosphates and ammonia that are produced simultaneously during polyphosphazene degradation. This review focuses on synthesis of biodegradable polyphosphazenes, their degradation characteristics, biocompatibility, and their applications as tissue regeneration and controlled delivery matrices with a particular emphasis on systems based on polyphosphazenes alone, polyphosphazene blends and polyphosphazene composites.