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 CaCl 2 , 0:6x mM KH 2 PO 4 , and PAA at pH ¼ 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 (x T ), 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 x T , 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 Ca 2þ and HPO 4 2À 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 ¼ x T ; 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.KEY WORDS: Biomineralization / Hypercomplex Gel / Organic/inorganic Solid Solution / Miscibility / Phase Transition / Catilaginous Material / Skeletal Material / Skeletal materials (endoskeleton and exoskeleton) formed by biomineralization are not pure Ca 5 (PO 4 ) 3 (OH), CaCO 3 , or SiO 2 but composites of organic polymers and the respective minerals. The polymers play a principal role in biomineralization and are believed to endow skeletal materials with resistance to fractures by overcoming the fragility inherent in the mineral component. Biomineralization is controlled by both species-specific genetic information and physicochemical parameters. Once the necessary solute ions and organic polymers have been produced and assembled by the organism, the subsequent stages of biomineralization are controlled primarily by physicochemical factors such as pH and the concentrations of the polymers and ions, which affect interactions between them in aqueous solution. Although the physicochemical mechanism of biomineralization has been investigated for many years, the mechanism is still unknown, even for cases in which the monomer sequence of the polymers is clarified.In previous work, we discovered the physicochemical regularity in the polymer systems related to hydroxyapatite (HAP), CaCO 3 and SiO 2 skeletal materials, and an artificial biomineralization system has been constructed using a polymer complex hydrogel composed of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) t...

Formation of Hydroxyapatite Skeletal Materials from Hydrogel Matrices via Artificial Biomineralization

“…Hence, the artificial bone displays mechanical properties similar to those of material found in biological systems, and also shows similarities in both the physicochemical properties of the constituent polymers and in the bone formation process. 15 We have also recorded SEM images of the PVA/PAA-hnetwork before and after mineralization. The initial network film has a dense homogeneous structure in the dry state ( Figure 3 (a)).…”

“…Conversely, the skeletal phase is mimicked by the homogeneous solid of HA/polymer network. 15 The skeletal phase grows thicker at the cost of the cartilaginous phase as mineralization proceeds, until it completely occupies the volume of the sample. Since, in this way, there is no intermediate state between skeletal and cartilaginous phases, the discontinuous change from the hypercomplex gel to the solid solution will be a phase transformation process.…”

“…The gel changed to a three-dimensional dense monolithic solid composite in an appropriate salt solution. 14,15 From the observation of this artificial biomineralization system, we deduced the following biomineralization mechanism: The matrix hydrogel is a complex gel formed via hydrogen bonding between proton-donating and protonaccepting polymers. In bone formation processes, the hydrogel traps both hydrogen phosphate anions (HPO 4 2− ) and calcium cations (Ca 2+ ) through hydrogen bonding and electrostatic interaction, respectively, thus forming a hypercomplex gel.…”

Calcium Carbonate Skeletal Material Is Synthesized via Phase Transition of the Calcium Carbonate Cartilaginous Material

“…In the artificial biomineralization of HA skeletal materials, the Ca-rich, high- W , and crystalline phase is not simply a solid dispersion of the polymer network and HA, but rather a solid solution of these two (i.e., a network/HA solid solution). 11 This model can be applied to the S phase of the present system. We can determine the equilibrium state of the S phase of composite immersed in a solution unsaturated with respect to CC, by examining the chemical potentials of CC at the interface between the solid composite and solution.…”

CaCO3 mineralization in polymer composites with cellulose nanocrystals providing a chiral nematic mesomorphic structure

“…Calcium chloride dihydrate (CaCl 2 •2H 2 O) and ammonium carbonate ((NH 4 ) 2 CO 3 ) were purchased from Nacalai Tesque, Inc. and used as ion sources for mineralization. A low-molecular-weight PAA (L-PAA; average degree of polymerization (DP) = 25, Sigma-Aldrich) was used as a precipitation inhibitor in the salt solution of ion server [25,37]. Other conventional chemicals were purchased from Wako Pure Chemical Ind., Ltd. or Nacalai Tesque, Inc. and used as received.…”