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

A thin hydrogel possessing layer-by-layer structure was prepared on substra tes 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 methacr yloyl group at the PLA end. The hydrogel layer was formed by coating the am inated surfaces with the micelle and polyallylamine (PAlAm) alternately in the presence of a reducing reagent. Each step of the alternate coating of t he micelle and PAlAm was characterized with scanning probe microscopy and t he 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, su ggesting full masking of the electrostatic charge from the inner layer. The zeta-potential of the surface with a PAlAm top layer showed a large positi ve value of similar to 50 mV up to pH 8 and declined to zero, attributed ex clusively 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 m onolayer of micelles consisted of granules on the order of the micelle size . With an increase in the number of coatings, the surface undulation was pr omoted and the nodular size increased. The thickness of the layer, estimate d by tapping-mode scanning of the area scratched by the strong force contac t 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 wa s observed after a short period indicative of the reorganization of the ela stic 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.