Bio-based heparin-functionalized hydrogels for controlled VEGF release I: EDC-based crosslinking
Purpose Presently formation of fibrotic encapsulation around implants  as well as insufficient oxygen and nutrient supply in tissue engineered grafts due to poor vascularization are fundamental limitations . Controlled release of pro-angiogenic growth factors is reported to be a promising approach to stimulate vascularization and thereby biointegration. In this study we investigate heparin-functionalized biopolymer based hydrogels as vascular endothelial growth factor (VEGF) storage and release systems which can also be applied as coatings. In this study we compare carbodiimid crosslinked hydrogels based on gelatine type A, gelatine type B or albumin in terms of loading and release behavior. Methods Heparin (1 % w/w) containing hydrogel precursor solutions based on gelatin (10 % w/w) or albumin (10 % w/w) were formulated. Chemical crosslinking was mediated by EDC (N-(3-Dimethylamino-propyl)-N'-ethylcarbodiimide hydrochloride). Hydrogels were characterized concerning gel yield and degree of swelling by gravimetric analysis. VEGF165 was loaded onto the dry gels at 0,1 mg VEGF per mg dry weight of gels. VEGF165 release was quantified via ELISA (enzyme linked immunosorbent assay). Thin film coatings of hydrogels were applied onto polyethylentherepthalat (PET) membrane surfaces by doctor blading and inkjet printing of solutions with low biopolymer concentration (1 % w/w protein, 0,1 % heparin). Immobilization of the hydrogel coating onto the polymer membrane is investigated applying e.g. NaOH with subsequent EDC activation of the PET surface. Alcian blue staining was applied to visualize the coatings and to analyze the stability at the polymer surface. Results We present results on physico-chemical gel characterization. We reveal impacts of the preparation method and the gel composition on gel yield and swelling behavior. We present the distinct characteristics of the biobased hydrogels systems based on gelatin types A and B, and albumin in terms of loading capacities, loading kinetics and release kinetics of VEGF as well as the influence of heparin. We consider the characteristic netto charges of the proteins as a reason for the observed effects. Finally we show results on preparation of thin hydrogel coatings and immobilization of such coatings on track-etched PET membranes. Conclusion Crosslinkend gelatin and albumin with and without heparin provide biobased hydrogel systems with distinct release behaviours. Therefore they might be candidates for controlled drug delivery.