A core-shell structured hydrogel thin layer on surfaces by lamination of a poly(ethylene glycol)-b-poly(D,L-lactide) micelle and polyallylamine

A thin hydrogel possessing layer-by-layer structure was prepared on substrates from a stabilized reactive micelle from a poly(ethylene glycol) -poly(D,L-lactide) (PEG- PLA) bearing an acetal group at the PEG end and a methacryloyl group at the PLA end. The hydrogel layer was formed by coating the aminated surfaces with the micelle and polyallylamine (PAlAm) alternately in the presence of a reducing reagent. Each step of the alternate coating of the micelle and PAlAm was characterized with scanning probe microscopy and the zeta-potential measurement over a pH range of 2-11. When the micelle was the topmost layer, the zeta-potential exhibited a small absolute value, suggesting full masking of the electrostatic charge from the inner layer. The zeta-potential of the surface with a PAlAm top layer showed a large positive value of similar to 50 mV up to pH 8 and declined to zero, attributed exclusively to the protonation and deprotonation of PAlAm. The alternation of zeta-potential with the coatings indicates the unmixing of the micelle and PAlAm. Scanning probe microscopy revealed that the surface coated with a monolayer of micelles consisted of granules on the order of the micelle size. With an increase in the number of coatings, the surface undulation was promoted and the nodular size increased. The thickness of the layer, estimated by tapping-mode scanning of the area scratched by the strong force contact mode, was found to increase with the number of coatings, and the increase corresponded approximately to the size of the micelle for each step of the coating. Of interest, the diminution of the well scratched by the probe was observed after a short period indicative of the reorganization of the elastic network to recover entropic reduction. Because of its unique structure, the thin hydrogel layer can be applied as a controlled release matrix of hydrophobic drugs.