Apatite mineralization behavior on polyglutamic acid hydrogels in aqueous condition: effects of molecular weight

Apatite-polymer composites mimicking specific structure of natural bone are promised as bone substitutes with moderate flexibility able to be fabricated into desired shapes as well as bone-bonding bioactivity. In order to precipitate the apatite on polymer surfaces, aqueous processing using solution supersaturated to the apatite has been attracting as much attention. Polyglutamic acid (PGA) is a promised candidate of the polymer, since it has high apatite-forming ability owing to abundant carboxyl groups able to trigger the heterogeneous apatite nucleation. Although combination of PGA with different molecular weight is expected to provide design of organic-inorganic composites with moderate bioresorbability, precise relationship between the molecular weight of the PGA and its apatite-forming ability has been remained unclear. In the present study, PGA hydrogels with different molecular weight were prepared by covalent cross-linking using ethylenediamine. Difference in apatite formation in simulated body fluid (SBF) was interpreted in terms of their chemical structure. It was found that hydrogels prepared from PGA with higher molecular weight showed tendency to have higher apatite-forming ability. It was attributed to high content of the carboxyl group remaining on the hydrogel due to low degree of the cross-linking.