Nanoscale eluting coatings based on alginate/chitosan hydrogels

The localized availability of bioactive biomolecules directly at the implant/tissue interface presents a promising strategy for improved wound healing and thus biointegration. Bioactive molecules that cannot be incorporated into the bulk material of a device may be delivered from a compatible surface coating, while the reservoir capacity of thin surface coatings is limited, they offer localized delivery over the first few critical hours or days of wound healing. In this study an alginate/chitosan hydrogel has been utilized as the basis for nanoscale eluting coatings to provide a hydrophilic yet water insoluble surface delivery system. The release characteristics of these hydrogel coatings were measured by employing the model molecules-fluorescein isothiocyanate dextran [FD; molecular weights (MWs) 4, 70, and 2000 kDa], fluorescein isothiocyanate albumin, and rhodamine. Scanning electron microscopy and atomic force microscopy were used to study the morphology of the hydrogel coatings on model substrates, and ellipsometry was employed for measuring the coating thickness. On silicon wafers, the coatings were of good uniformity and conformal, with a thickness of approximately 120 nm and a rms roughness of 3.0 nm. A model porous substrate, paper, which afforded deep pore penetration of the hydrogel, was used to mimic hydroxyapatite. The release of FD was observed to be dependent on the MW, the release medium, charge, and surface roughness. Sustained release was recorded for FD 70 and FD 2000 with yields of about 90% and 75%, respectively, into simulated body fluid within 26 days. Concurrent elution of different molecules from one hydrogel coating was demonstrated. The observed elution profiles were fitted to release kinetics such as the Korsmeyer-Peppas model or first order release.