Hypercomplex Gel Changes to Organic/Inorganic Solid Solution by Phase Transition in an Artificial Biomineralization System

Abstract

In an artificial biomineralization system, a polymer complex consisting of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) was utilized as a mineralization field. The polymer complex was immersed in salt solution containing x mM CaCl2, 0.6x mM KH2PO4, and PAA at pH=7.4. After equilibrium was reached, the weight uptake in the polymer complex was determined in the dry state. When x was less than the transition concentration (xT), the polymer complex was transformed into a hydrogel, and the dry weight uptake gradually increased with increasing x. On the other hand, at x greater than xT, the polymer complex was transformed into an organic/inorganic solid composite, and the dry weight uptake was almost independent of x. The hydrogel formed was a hypercomplex gel composed of Ca2+ and HPO42− ions and the polymer complex held together by hydrogen bonds and electrostatic interactions. The organic/inorganic solid composite appeared to be a solid solution of the polymer complex and hydroxyapatite, as indicated by experiments on the saturation state of the salt solutions. Our results indicate that the hypercomplex gel changed discontinuously to the organic/inorganic solid solution at x=xT; this discontinuity represents a phase transition of hypercomplex gel to organic/inorganic solid solution that is an open system whose border is permeable to both energy and mass. The hypercomplex gel and the organic/inorganic solid solution can serve as models for the fundamental structures of cartilaginous and skeletal material respectively.