Thermoresponsive artificial extracellular matrix for tissue engineering:: Hyaluronic acid bioconjugated with poly(N-isopropylacrylamide) grafts

Thermoresponsive hyaluronans (HAs) were prepared by graft polymerization of N-isopropylacrylamide (NIPAM) on HA (number-averaged molecular weight, M-n, ca. 1.5 x 10(5) and 5.0 x 10(5)) using dithiocarbamate which is a kind of iniferter (initiator, transfer agent and terminator). The degree of dithiocarbamylation (DD) as an iniferter ranged from 0.4 to 11.4% per disaccharide unit of HA. The estimated M-n of the grafted polyNIPAM (PNIPAM) ranged from approximately 5.0 x 10(3) to 8.4 x 10(4). The PNIPAM-grafted HAs (PNIPAM-HAs) were water-soluble at room temperature, while they precipitated at temperatures above approximately 34 degreesC in water. The temperature at the onset of precipitation (lower critical solution temperature: LCST) was independent of parameters of molecular architecture such as M-n of HA, degree of grafting of PNIPAM, and M-n of PNIPAM. Equilibrium transmittance of the aqueous solution above LCST decreased with an increase in both degree of grafting and M-n of PNIPAM. At physiological temperature, the PNIPAM-HA film cast from a cold solution was very wettable with water. A markedly reduced adhesion of endothelial cells to the film was observed, indicating that the PNIPAM-HA film may serve as a non-cell-adhesive matrix. Scanning electron microscopic observation appeared to differentiate supramolecular structures between rapidly freeze-dried PNIPAM-HA and nongrafted HA:PNIPAM-HA exhibited a nonuniform fibrous network, whereas the morphology of which is markedly different from that of a nongrafted HA gel exhibited a mixture of sharp needle- and platelike structures.