Calcium phosphates PHEMA hydrogels

The deposition of sparingly soluble calcium phosphates on PHEMA hydrogels, both in vitro and in vivo, can be modified by the incorporation of citric acid into the PHEMA hydrogels. In the presence of citrate anions the formation of hydroxyapatites was prevented. The calcium phosphate deposits which formed in vitro on PHEMA were mainly monocalcium phosphate monohydrate and dicalcium phosphate dihydrate. The types of deposits formed in vivo were quite different from those formed in vitro. The in vivo deposits formed on PHEMA were mostly hydroxyapatites deficient in calcium and hydroxyl ions. Citrate anions were also observed to prevent significantly the deposition in vivo of protein onto PHEMA hydrogels. 

The inhibitoiy effect of citrate anions on the calcification of PHEMA hydrogels has been evaluated in this study. Here the release of citric acid from the hydrogels was designed such that the molar ratios of calcium to citric acid were unfavorable for nucleation of calcium phosphates (30). The study constitutes the first model investigation of the role of citrate anions in prevention of calcification of PHEMA hydrogels. 

The opacity was attributed to the formation of stable complexes between calcium ions and low molecular weight biomolecules, possibly lysozymes, rather than being caused by calcium phosphate deposits. Lysozyme has been reported to be able to penetrate the intertices of PHEMA hydrogel networks (45,46). It has molecular dimensions of approximately 4.0 nm x 4.0 nm x 1.9 nm (47), which are smaller than the PHEMA hydrogel mesh size of 10 - 100 nm which was estimated from SEM micrographs (48). 

It has been demonstrated that the release of citric acid from PHEMA hydrogels hinders the formation of calcium phosphates, especially hydroxyapatites. Because of this inhibitory effect, the calcium phosphate phases formed during in vitro calcification were mainly present as non-apatite phases, possibly MCPM and DCPD. The porous morphology of the outer surface of the spherical calcium phosphate deposits could be due to the dissolution of precipitates in the presence of citric acid. The results obtained after subcutaneous implantation ofPHEMA and PHEMA containing citric acid in rats confirmed the resistance of PHEMA-citric acid to calcification. The calcium phosphate deposits which formed in vivo consisted mainly of Ca2+ and OH deficient hydroxyapatites. However, it is not yet known whether or not the differences between the calcium phosphate phases found in vivo and in vitro arise from the presence of proteins/peptides in the in vivo calcifying medium. 

A urea-mediated mineralization technique was developed to enable the formation of pHEMA-based hydrogel-calcium phosphate composites with excellent polymer-mineral interfacial adhesion strength that is desirable for bone mimics. This mineralization method was also applied to generate more sophisticated composites containing functional hydrogels that possess anionic groups mimicking the extracellular matrix proteins in bone. 

Here we introduce a novel mineralization method that leads to rapid, high-affinity integration of calcium phosphate with pHEMA-based hydrogels, a key step in the fabrication of functional bone-like composites. 

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