Design and examination of an antioxidant-containing polyphosphazene scaffold for tissue engineering

Polyphosphazenes were synthesized that contained the antioxidant, ferulic acid, and amino acid esters as co-substituents. The synthesis protocol utilized the replacement of chlorine atoms in poly(dichlorophosphazene) by ferulic acid with either glycine, alanine, valine, or phenylalanine ethyl ester. Ferulic acid protects cells from free radical damage, and its steric characteristics have the potential to generate polymeric materials with high mechanical strength for hard tissue engineering scaffolds. Incorporation of the amino acid esters allows for control over the polymer hydrolysis while releasing the antioxidant at different rates. Macromolecular substitution reactions were carried out utilizing the allyl ester of ferulic acid to prevent side reactions during halogen replacement. The allyl protecting group was then removed using mild conditions. The polymers were characterized by 1H and 31P NMR, GPC, and DSC techniques. Static water contact angles were measured to monitor the change in hydrophobicity/hydrophilicity. Before deprotection, the water contact angles were 82–88° and after deprotection the water contact angles were 56–71°. A pH-dependent hydrolysis study revealed that the polymers are hydrolytically sensitive and decomposed ∼5–25% over 8 weeks yielding a final pH between 6.0–6.8. Polymer hydrolysis resulted in the release of ferulic acid. The polymers were photocrosslinked by a [2 + 2] cycloaddition of the ferulic acid moieties induced by exposure to long-wave UV light. The crosslinking was monitored by UV-spectroscopy via the apparent decrease in the 320 nm absorbance, attributed to the cyclization of the ferulic acid moieties. Increased UV exposure time led to an increase in the level of crosslinking. After 60 s, the polymers were crosslinked ∼41–62%. Hydrolysis experiments of the crosslinked materials showed only a 0–5% weight loss after 8 weeks with a pH between 6.6–7.0.