Tri-Component Hydrogel as Template for Nanocrystalline Hydroxyapatite Deposition Using Alternate Soaking Method for Bone Tissue Engineering Applications

Composite hydrogels containing apatite-like particles can act as scaffolds for osteoblast proliferation, with applications in bone tissue engineering. In this respect, porous biocompatible hydrogels were obtained from chitosan, oxidized pullulan, and PVA in different ratios. The stability of the hydrogels was ensured both by covalent bonds between aldehyde groups of oxidized pullulan and free amino groups of chitosan, and by physical bonds formed during freeze-thaw cycles and lyophilization. The deposition of calcium phosphates was performed by alternate soaking of the porous hydrogels into solutions with calcium and phosphate ions, assuring a basic pH required for hydroxyapatite formation. The mineralized hydrogels were characterized using FTIR spectroscopy, scanning electron microscopy, X-ray diffraction, and thermogravimetric analysis, showing that inorganic particles containing between 80 and 92% hydroxyapatite were deposited in a high amount on the pore walls of the polymeric matrix. The composition of the organic matrix influenced the crystallization of calcium phosphates and the mechanical properties of the composite hydrogels. In vitro biological tests showed that mineralized hydrogels support the proliferation of MG-63 osteoblast-like cells to a greater extent compared to pristine hydrogels.

Urease-induced calcification of segmented polymer hydrogels - a step towards artificial biomineralization

Natural organic/inorganic composites, such as nacre, bones and teeth, are perfectly designed materials with exceptional mechanical properties. Numerous approaches have been taken to synthetically prepare such composites. The presented work describes a new way of mineralizing bulk materials on a large scale following the approach of bioinduced mineralization. To this end, a series of polymer conetworks with entrapped urease were prepared. After polymerization, the entrapped urease shows high enzymatic activity. The bioactive polymer conetworks were then treated with an aqueous mixture of urea and CaCl2. The urease-induced calcification indeed allows formation of carbonate crystals exclusively within the hydrogel even at room temperature. The influence of network composition, degree of cross-linking, immobilized urease concentration and temperature of calcification were investigated. By varying these parameters, spherical, monolithic clusters, as well as bar-like nanocrystals with different aspect ratios in spherical or dendritic arrays, are formed. The grown nanocrystals improve the stiffness of the starting material by up to 700-fold, provided that the microstructure shows a dense construction without pores and strong interaction between crystals and network. The process has the potential to generate a new class of hybrid materials that would be available on the macroscopic scale for use in lightweight design and medicine.

How important is polyelectrolyte complex formation in biomimetic mineralisation? Manipulation via alcohol addition

Understanding the formation of biominerals like nacre can lead to the fabrication of more advanced biomimetic materials. Several factors are known to influence the final form of both native nacre and biomimetic synthetic variants. Two important components in calcium carbonate biominerals such as nacre are the organic scaffold and the acidic proteins. Interactions between these two components may also influence final composite characteristics. In this investigation chitosan hydrogels were prepared from acidic aqueous solution using four alcohols as cosolvents. The addition of alcohol enables direct modification of the network of the chitosan hydrogel (and thereby the nanometre and micrometre length-scale structure of the hydrogel). Both alcohol-modified chitosan and subsequently reacetylated chitin scaffolds were then mineralised with a combined soaking mineralisation method in the presence of poly(acrylic acid), the latter of which mimics the role of the acidic proteins in the native system. The effects of these structural variations of the hydrogel, induced by the presence of alcohol during fabrication, on (1) the formation of a polyelectrolyte complex between the chitosan or chitin and the poly(acrylic acid) and (2) the subsequent polymorph and morphology of calcium carbonate crystals mineralised within the hydrogel scaffold were investigated. Increasing the amount of the alcohols 1,2-propanediol or 1,3-propanediol led to increased disruption of the hydrogen bonding of the hydrogel scaffold and significant changes to, or reduced formation of, the polyelectrolyte complex formed between the scaffold carbohydrate and the poly(acrylic acid). The disruption of the polyelectrolyte complex in turn led to a loss of control over which polymorph of calcium carbonate is nucleated. These results show that the physical form of the polymer scaffold in these organic/inorganic composites, and the formation of the polyelectrolyte complex play a crucial role in determining the final composite structure and the calcium carbonate polymorphs and morphologies.