Temperature-modulated platelet and lymphocyte interactions with poly(N-isopropylacrylamide)-grafted surfaces.

Temperature-responsive semitelechelic poly(N-isopropylacrylamide) (PIPAAm) bearing a carboxyl end group has been chemically immobilized on aminated polystyrene particle surfaces via condensation reaction. PIPAAm-grafted particles were uniformly suspended in aqueous media at lower temperatures. With increasing temperature, PIPAAm-grafted particles aggregated and precipitated. Such reversible changes in particle colloidal behaviour was correlated to temperature-modulated hydrophilic/hydrophobic changes of particle surfaces modified by PIPAAm hydration/dehydration with temperature changes. Interactions between platelets and PIPAAm-grafted surfaces were studied by monitoring cytoplasmic free Ca2+ concentration ([Ca2+]i) changes in platelets using intracellularly-trapped Ca2+ indicator dye, Fura 2, at various temperatures. Although changes in [Ca2+]i in platelets in contact with PIPAAm-grafted particles were not observed below the critical temperature of PIPAAm, significant changes in [Ca2+]i in platelets were induced by contact with particles above this critical temperature. Furthermore, temperature-modulated cell adsorption/desorption control by PIPAAm-grafted particles was investigated using a particle aggregation assay in the presence of lymphocytes. Below the critical temperature of PIPAAm, mixed suspensions were completely homogeneous due to minimal interaction between lymphocytes and hydrated particles. In contrast, aggregated precipitates were observed by increasing the suspension temperature above the critical temperature of PIPAAm resulting from strong hydrophobic interactions between particles with lymphocytes. These precipitates are reversibly resuspended in cold buffer. The feasibility of cell activation/inactivation or cell attachment/detachment control by temperature-modulated surface changes is attractive for suspension cell culture and drug delivery at targeted sites in vivo.