Enhanced loading and activity retention of bioactive proteins in hydrogel delivery systems

A simple, general and effective technique is developed for increasing the loading of bioactive macromolecules into hydrogels using the principles of aqueous two-phase extraction. Model proteins, ovalbumin and alpha-amylase, were loaded into hydrated gels by soaking the gels in a buffered solution of protein containing 12 wt% PEG-10 000 and 0.22 M salt (KCl, KBr or KI). The PEG and salts were expected to enhance protein sorption according to aqueous two-phase extraction heuristics. In the absence of the solution additives, the gels absorbed little protein. But protein loading up to 270 mg ovalbumin/g polymer and 67 mg alpha-amylase/g polymer was obtained when the PEG and salt were added; loading was not significantly dependent upon salt type. Ovalbumin release from hydrated gels was diffusion-controlled. The diffusion coefficient was 1.10(-7) cm(2)/s, consistent with protein absorption into the gel rather than adsorption onto the surface. Release kinetics of both proteins from dried, glassy gels matched conventional behavior for release of absorbed drugs from glassy polymers. Finally, alpha-amylase activity was retained even after drying the loaded gel at 65 degrees C, conditions which denatured the enzyme when not absorbed in the gel. (C) 1998 Published by Elsevier Science B.V.