Diffusion-Controllable Biomineralization Conducted In Situ in Hydrogels Based on Reversibly Cross-Linked Hyperbranched Polyglycidol.

Abstract

We present biocompatible hydrogel systems suitable for biomineralization processes based on hyperbranched polyglycidol cross-linked with acrylamide copolymer bearing carbonyl-coordinated boronic acid. At neutral pH, diol functional groups of HbPGL react with boronic acid of polyacrylamide to generate 3D network in water by the formation of boronic ester cross-links. The dynamic associative/dissociative characteristics of the cross-links makes the network reversible. The presented hydrogels display self-healing properties and are injectable, facilitating gap filing of bone tissue. The 1H HR MAS DOSY NMR studies reveal that acrylamide copolymer plays the role of the network framework, whereas HbPGL macromolecules, due to their compact structure, move between reactive sites of the copolymer. The influence of the copolymer macromolecules entanglements and overall polymer concentrations on macromolecules mobility and stress relaxation processes is investigated. The process of hydrogel biomineralization results from hydrolysis of 1-naphthyl phosphate calcium salt catalyzed by encapsulation in hydrogel alkaline phosphatase. The environment of the hydrogel is entirely neutral toward the enzyme. However, the activity of alkaline phosphatase encapsulated within the hydrogel structure is diffusion-limited. In this article, based on the detailed characteristics of three model hydrogel systems, we demonstrate the influence of the hydrogel permeability on the encapsulated enzyme activity and calcium phosphate formation rate. The 1H HR MAS DOSY NMR is used to monitor diffusion low-molecular weight compound within hydrogels, whereas 31P HR MAS NMR facilitates monitoring of the progress of biomineralization in situ within hydrogels. The results show a direct correlation between low molecular diffusivity in hydrogels and network dynamics. We demonstrate that the morphology of in situ-generated calcium phosphate within three model HbPGL/poly(AM-ran-APBA) hydrogels of different low molecular permeability varies substantially from sparsely deployed large, well-defined crystals to an even distribution within the polymers polycrystalline continuous network.