Marta Vandrovcová, Julia K. Keppler, Michelle Feuereisen, Patrick Ricquier, Andreas Schieber, Andre G. Skirtach, Timothy E.L. Douglas
Mineralization of hydrogel biomaterials with calcium phosphate (CaP) is considered desirable to improve their suitability as materials for bone regeneration. One biomimetic strategy to achieve hydrogel mineralization involves incorporation of the enzyme responsible for mineralization of bone tissue, alkaline phosphatase (ALP), inside hydrogels, followed by incubation containing calcium ions and an organophosphate substrate for ALP, e.g. glycerophosphate (GP). ALP causes release of phosphate from GP, leading to elevated phosphate concentrations, which promote CaP precipitation. Another strategy is the incorporation of calcium-binding organic biomolecules inside hydrogels. This raises intrahydrogel calcium concentration, promoting CaP precipitation. Polyphenols are organic biomolecules consisting of phenolic units. Certain polyphenols have been reported to show affinity for calcium ions. A further advantage of using polyphenols is their antimicrobial activity, which can combat infection. In this study, the ability of different gallotannins (a class of polyphenols) to promote the ALP-mediated mineralizaton hydrogels of gellan gum (GG) was compared. ALP-loaded hydrogels were enriched with Pentagalloyl glucose (PGG), tannic acid preparations of differing molecular weights (1450 kD, 1060 kD and 850 kD) (all Ajinomoto OmniChem n.v., Wetteren, Belgium), and a gallotannin-rich extract from Mango kernel (Mangifera indica L.). All gallotannins and the mango extract promoted mineral formation. Mineralization was promoted most strongly by PGG, the mango extract and the high molecular weight tannic acid. All gallotannins and the mango extract interacted with ALP, as demonstrated by fluorescence quenching analysis. In addition, ALP aggregation was observed after addition of some of the gallotannons, especially for PGG (~800 nm diameter), as determined by dynamic light scattering. Further work will concentrate on the antimicrobial activity of the polyphenol-enriched mineralized hydrogels, as well as their cytocompatibility.