Evaluation of apatite-forming ability and mechanical property of pectin hydrogels

Apatite-polymer hybrids are expected as novel bone substitutes exhibiting bone-bonding ability, i.e. bioactivity, and mechanical performances analogous to those of natural bone. To fabricate such hybrids, biomimetic process has been paid much attention where bone-like apatite is deposited on organic polymers that have specific functional groups in simulated body fluid (SBF) or more concentrated solutions. Previous studies showed that carboxyl groups (-COOH) have a catalytic effect for heterogeneous apatite nucleation in SBF. In this study, we chose pectin as an organic polymer. Pectin is a natural polysaccharide containing carboxyl groups. Hydrogels were prepared from various pectins including pectic acid, apple-derived pectin and citrus-derived pectin by treatment with CaCl2 solution. Ability of apatite formation in SBF and mechanical properties of the gels were examined. The apatite-forming ability increased in the order (pectic acid) < (apple-derived pectin) < (citrus-derived pectin). It was suggested that the apatite-forming ability of the pectin gels is governed by not only the amount of carboxyl groups but also changes in Ca2+ concentration and pH in surrounding solutions. Young's modulus of the pectin gels was similar to natural bone, although tensile strength is a little lower.