To directly characterize the thermoresponsive structural changes of a poly(N-isopropylacrylamide) (PNIPAAm) graft layer at the microscopic level, the force-distance curve (f-d curve) was measured on a well-tailored end-grafted PNIPAAm surface in aqueous solution at 25 and 40 °C, using an atomic force microscope (AFM). The PNIPAAm surface was prepared by an iniferter-based photograft polymerization technique. The approach trace of the f-d curve exhibited a steric repulsion profile at 25 °C, while the range of repulsion decreased 1 /10 to 1 /20 at 40 °C, confirming the ascending-heat-induced collapse of the PNIPAAm graft layer. The change in thickness of the graft layer was complementarily measured from the scanning images of the boundary between the grafted and nongrafted regions under well-defined scanning forces. The thermoresponsive characteristics of the PNIPAAm graft layer including its interaction with proteins and the applied-load dependence of the measured graft thickness are discussed.
Thermoresponsive hyaluronans (HAs) were prepared by graft polymerization of N-isopropylacrylamide (NIPAM) on HA (number-averaged molecular weight, Ma, 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 Mn 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 degrees C in water. The temperature at the onset of precipitation (lower critical solution temperature: LCST) was independent of parameters of molecular architecture such as Mn of HA, degree of grafting of PNIPAM, and Mn of PNIPAM. Equilibrium transmittance of the aqueous solution above LCST decreased with an increase in both degree of grafting and Mn 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.
A series of poly(N-isopropylacrylamide)-grafted gelatins (PNIPAM gelatins) of three different graft densities (approx. 11, 22 and 34 graft chains per gelatin molecule) and three different molecular weights of their graft chains (molecular weight approximately 1.2 x 10(4), 5.0 x 10(4) and 1.3 x 10(5) g/mol) were prepared by multiple derivatization of dithiocarbamyl (DC) group in a gelatin molecule and subsequent iniferter (acts as an initiator, transfer-agent and terminator)-based photopolymerization of NIPAM. The weight ratio of PNIPAM graft chains to gelatin (P/G) varied from 1.4 to 49. Aqueous solutions of PNIPAM-gelatins showed thermo-responsiveness, depended on the graft density and the molecular weight of PNIPAM graft chain or P/G. Aqueous solutions (10 or 20%, w/v) of PNIPAM-gelatins with P/G of more than 5.8 were converted to gels at 37 degrees C. Focal plane images of PNIPAM-gelatin gels by confocal laser scanning microscopy revealed that the size of hydrophobically clustered aggregates increased with P/G, whereas the space of microvoids decreased with concentration. Compressive strain-stress measurements revealed that compressive strength of PNIPAM-gelatin increased with P/G. Bovine smooth muscle cells (SMCs)-entrapped gels were produced from PNIPAM-gelatin-containing cell-suspended medium solutions at 37 degrees C. The entrapped cells proliferated in the gel with P/G of more than 12. A higher cell proliferativity was obtained at low concentration (5%, w/v) and higher P/G (>18). Tissue formation composed of proliferative SMCs and cell-secreted extracellular matrices (collagen) was obtained at 14 days incubation. The inter-relationship between the molecular parameters of PNIPAM-gelatin, internal structural features and cell proliferation potential was discussed.
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