No abstract

Poly(ethylene oxide) (PEO) chains are introduced as graft chains maintaining freely mobile ends in thermo-responsive cross-linked poly(N-isopropylacrylamide) (PIPAAm) hydrogels by copolymerization of IPAAm with α-acryloyl-ω-methoxy-PEO. The deswelling response on raising the temperature of this gel above the gel phase transition temperature (T P) takes place within 10 min, whereas a conventionally cross-linked PIPAAm gel of the same dimensions requires 1 month for deswelling. This difference is due to the formation of water release channels within the skin layer by the hydrophilic PEO graft chains. The rapid deswelling of the grafted gel is compared with the deswelling changes of random copolymer gels composed of IPAAm and hydrophilic acrylic acid (AAc), which also accelerates gel deswelling. Deswelling is fastest in copolymers containing 1.3 wt % AAc and in grafted gels containing 13 wt % PEO. These results were interpreted as reflecting the gel structure.

Amino semitelechelic poly(Ar-isopropylacrylamide)s (PIPAAm) with three different molecular weights were synthesized by telomerization of IPAAm monomer with 2-aminoethanethiol as a chain transfer agent, changing the molar ratio of monomer to chain transfer agent. Macromonomers of thermosensitive PIPAAm were synthesized by condensation reaction of amino semitelechelic PIPAAm with A'-acryloxysuccinimide. The molecular weights of macromonomers determined by titration of the terminal amino groups were 2900, 4000, and 9000, respectively. The comb-type grafted PIPAAm hydrogels having different lengths of graft chains were synthesized by radical copolymerization of IPAAm monomer with PIPAAm macromonomer in the presence of AT,AT'-methylenebisacrylamide as a cross-linker. An important aspect of the graft-type gels is the construction of a molecular architecture different from PIPAAm normal type of gel even though the composition is same. Higher equilibrium swellings at lower temperatures were observed in graft-type gels in contrast to the normal-type gel, and longer graft chains resulted in higher equilibrium swelling due to the freely mobile grafted chains. Both the normal-type and the graft-type gels exhibited reversible swelling-deswelling changes in aqueous milieu in response to an alteration of temperature. The deswelling kinetics at 40 °C changed from equilibrium swelling states at 10 °C, however, exhibited remarkable differences, and rapid responses were observed for grafttype gels. The rapid dehydration of graft chains during gel shrinking was confirmed by analysis of DSC measurements. These dehydrated graft chains strongly aggregated with hydrophobic intermolecular forces, inducing the rapid deswelling of the gels. The attractive forces operating between dehydrated chains were larger in the gel having longer grafted chains, resulting in faster deswelling. A deswelling mechanism distinct from polymer network collective diffusion was demonstrated with these comb-type grafted hydrogels having freely mobile grafted chains in the network.

No abstract

No abstract

In this study, specific interactions between immobilized RGDS (Arg-Gly-Asp-Ser) cell adhesion peptides and cell integrin receptors located on cell membranes are controlled in vitro using stimuli-responsive polymer surface chemistry. Temperature-responsive poly(N-isopropylacrylamide-co-2-carboxyisopropylacrylamide) (P(IPAAm-co-CIPAAm)) copolymer grafted onto tissue culture grade polystyrene (TCPS) dishes permits RGDS immobilization. These surfaces facilitate the spreading of human umbilical vein endothelial cells (HUVECs) without serum depending on RGDS surface content at 37 degrees C (above the lower critical solution temperature, LCST, of the copolymer). Moreover, cells spread on RGDS-immobilized surfaces at 37 degrees C detach spontaneously by lowering culture temperature below the LCST as hydrated grafted copolymer chains dissociate immobilized RGDS from cell integrins. These cell lifting behaviors upon hydration are similar to results using soluble RGDS in culture as a competitive substitution for immobilized ligands. Binding of cell integrins to immobilized RGDS on cell culture substrates can be reversed spontaneously using mild environmental stimulation, such as temperature, without enzymatic or chemical treatment. These findings are important for control of specific interactions between proteins and cells, and subsequent "on-off" regulation of their function. Furthermore, the method allows serum-free cell culture and trypsin-free cell harvest, essentially removing mammalian-sourced components from the culture process.

Poly(N-isopropylacrylamide) (PIPAAm) exhibits a reversible, temperature-dependent soluble/insoluble transition at its critical temperature in aqueous media. When PIPAAm molecules are covalently attached to a solid surface, the graft configuration greatly affects the thermoresponsive wettability changes of PIPAAm-modified surfaces. Three types of temperature-responsive surfaces were prepared using PIPAAm grafts of different molecular architectures: PIPAAm terminally grafted surfaces, PIPAAm looped chain grafted surfaces using a copolymer of IPAAm and N-acryloxysuccinimide, and PIPAAm terminally grafted onto immobilized PIPAAm loops. These surfaces were prepared by changing the graft architecture as well as the density of PIPAAm chains to investigate temperature-responsive wettability changes. All surfaces showed temperature-responsive hydrophilic/hydrophobic surface property alterations demonstrated by observed large and discontinuous wettability changes. On both surfaces bearing terminally grafted PIPAAm, surface wettability changed dramatically over the range 32-35 °C, a temperature corresponding to the phase-transition temperature for PIPAAm in aqueous media. This implies that terminally grafted PIPAAm chains retain a highly mobile nature and respond rapidly to temperature changes. The loop-grafted surface showed relatively large wettability changes but had a slightly lower transition temperature (∼27 °C). This reduced transition temperature is likely due to restricted conformational transitions for this multipoint grafted PIPAAm. Combination of both loops and terminally grafted chains showed the largest surface free energy changes among three surfaces. We conclude that PIPAAm graft architecture strongly influences surface wettability responses to temperature changes due to differences in the dynamic motion of the grafted polymer chains.

Cross-linked poly(N-isopropylacrylamide) (PIPAAm) hydrogel-modified surfaces were prepared to investigate the effects of a three-dimensionally cross-linked structure of PIPAAm layers on both wettability changes and hydrophobic interactions with hydrophobic solutes in response to temperature changes. The temperature-responsive surface was prepared by polymerization of IPAAm in the presence of cross-linker on the substrates on which an azo polymerization initiator was covalently bonded. The PIPAAm hydrogel-modified surface showed temperatureresponsive hydrophilic/hydrophobic surface property alterations as demonstrated by a large and discontinuous wettability changes in a range of 27-32 °C, a slightly lower temperature range than the phase transition temperature for soluble PIPAAm in aqueous media. This implies that the dynamic motion in response to temperature for PIPAAm segments in the modified hydrogel is restricted due to the cross-linked structure. The effect of the threedimensional PIPAAm structure on the separation of hydrophobic steroids was investigated by high-performance liquid chromatography with an aqueous mobile phase. The retention times for steroids with different hydrophobicities were increased as the temperature was raised. Cortisone and prednisolone, those showing close hydrophobicity, were successfully separated at an elevated temperature above 25 °C owing to the amplified hydrophobic interaction of prednisolone compared to that of cortisone with the hydrophobic gel surfaces. The separation of relatively hydrophobic steroids was achieved even at lower temperature. The expanded network of the highly hydrated gel layer allowed the penetration of steroid molecules within the hydrogel layer which resulted in the changes in peak width. The cross-linked structure of PIPAAm hydrogels on substrates strongly influences both surface wettability changes and interaction with hydrophobic steroids in response to temperature due to the restricted dynamic motion of PIPAAm segments in the gel.