A series of thermoresponsive gating membranes, with a wide range of grafting yields, were prepared by grafting poly (N-isopropylacrylamide) (PNIPAM) onto porous poly (vinylidene fluoride) (PVDF) membrane substrates with a plasma-induced pore-filling polymerization method. The effect of grafting yield on the gating characteristics of thermoresponsive gating membranes was investigated systematically. The results showed that the grafting yield heavily affected both the water flux responsiveness coefficient and the thermoresponsivity of the membrane pore size. When the grafting yield was smaller than 2.81%, both the flux responsiveness coefficient and the thermoresponsivity of the membrane pore size increased with an increase in the grafting yield; however, when the grafting yield was higher than 6.38%, both the flux responsiveness coefficient and the thermoresponsivity of the membrane pore size were always equal to 1; i. e., no gating characteristics existed anymore. Diffusional permeation experiments showed that two distinct types of temperature responses were observed, depending on the grafting yield. The diffusional coefficient of a solute across membranes with low grafting yields increased with temperature, while that across membranes with high grafting yields decreased with temperature. To get a desired or satisfactory thermoresponsive gating performance, the membranes should be designed and prepared with a proper grafting yield.

Both thermoresponsive flat membranes and core-shell microcapsule membranes, with (N-isopropylacrylamide) (PNIPAM) a porous membrane substrate and grafted poly N-isopropylacrylamide PNIPAM gates, were successfully prepared using a plasma-graft pore-filling polymerization method. PNIPAM was prepared to be grafted homogeneously onto the porous membrane sub-strates, in the direction of both the membrane thickness and surface. Regardless of the solute molecular size, temperature had an opposite effection diffusion coefficients of the solute across the PNIPAM-grafted membranes with low graft yields as opposed to those with high graft yields. The PE-g-PNIPAM membranes change from positive thermo-response to negative thermoresponse types with increasing pore-filling ratios at around 30%. Phenomenological models were developed for predicting the diffusion coefficient of the solute across PNIPAM-grafted membranes at temperatures, both above and be-low the lower critical solution temperature LCST. Predicted diffusional coefficients of solutes across both the PNIPAM-grafted flat and PNIPAM-grafted microcapsule membranes fit the experimental values. To obtain an ideal result for the diffusional thermoresponsive controlled release through PNIPAM-grafted membranes, the substrates strong enough to prevent any conformation changes are more suitable for preparing thermoresponsive membranes than weak ones.

Membranes with porous substrates and functional gates that are responsive to environmental stimuli are attracting increasing interest from various fields. Their permeation properties can be controlled or adjusted by the gates according to the external chemical and/or physical environment, and they may find various applications e.g. in controlled drug delivery, bioseparation, chemical separation, water treatment, tissue engineering, and as chemical sensors. The functional gates for stimuli-responsive membranes serve as chemical valves, and have been reported to act in response to changes in environmental temperature, [1-5] pH, [5-10] ionic strength, [11] glucose concentration, [12, 13] electric field, [14] light, [15] redox properties, [16] or different molecules. [17-19] There are many cases in which environmental temperature fluctuations occur naturally, and in which the temperature stimuli can be easily designed and artificially controlled; therefore much attention has recently been focused on thermoresponsive membranes. [2-1] Up to now, almost all of the thermoresponsive gating membranes have featured positively thermoresponsive characteristics, that is, the membrane permeability increases with increasing environmental temperature, because all of the thermoresponsive functional gates were constructed from poly (Nisopropylacrylamide) (PNIPAM). [1-5] In these cases, the membrane pores change from a "closed" to an "open" state when the environmental temperature increases from below the lower critical solution temperature (LCST) of PNIPAM to above the LCST, as a result of the swelling/shrinking conformational change of the polymer. In certain applications, however, an inverse mode of the thermoresponsive gating behavior of the membranes is preferred. Here we report a novel family of thermoresponsive gating membranes with negatively thermoresponsive gating characteristics, that is, "opening" of the membrane pores is induced by a decrease rather than an increase in temperature.

A glucose-sensitive microcapsule with a porous membrane and with linear-grafted polyacrylic acid (PAAC) chains and covalently bound glucose oxidase (GOD) enzymes in the membrane pores acting as functional gates was successfully prepared. Polyamide microcapsules with a porous membrane were prepared by interfacial polymerization, PAAC chains were grafted into the pores of the microcapsule membrane by plasma-graft pore-filling polymerization, and GOD enzymes were immobilized onto the PAAC-grafted microcapsules by a carbodiimide method. The release rates of model drug solutes from the fabricated microcapsules were significantly sensitive to the existence of glucose in the environmental solution. In solution, the release rate of either sodium chloride or VB12 molecules from the microcapsules was low but increased dramatically in the presence of 0.2mol/L glucose. The prepared PAAC-grafted and GOD-immobilized microcapsules showed a reversible glucose-sensitive release characteristic. The proposed microcapsules provide a new mode for injection-type self-regulated drug delivery systems having the capability of adapting the release rate of drugs such as insulin in response to changes in glucose concentration, which is highly attractive for diabetes therapy.

Wehave successfully prepared monodispersed thermoresponsive core-shell hydrogel microspheres with a mean diameter of 200-400nm with poly (N-isopropylacrylamide-co-styrene) [P (NIPAM-co-St)] cores and poly (N-isopropylacrylamide) (PNIPAM) shells. The submicrometer-sized monodispersed P (NIPAM-co-St) core seeds were prepared by using a surfactant-free emulsion polymerization method, and the PNIPAM shell layers were fabricated onto the core seeds by using a seed polymerization method. The particle size, morphology and monodispersity, and thermoresponsive characteristics of the prepared microspheres were experimentally studied. In the preparation of P (NIPAM-co-St) seeds, with increasing the initiator dosage, the mean diameters and the dispersal coefficients were almost at the same levels at first; however, when the initiator dosage increased further to a critical amount, the mean diameters decreased drastically and the monodispersity became worse significantly. With increasing the stirring rate, the particle diameter decreased, and when the stirring rate was larger than 600 rpm, the monodispersity became worse obviously. With increasing the phase ratio, the mean diameter became larger simply, and the monodispersity became worse first and then became better again. With increasing the reaction time, the particle sizes nearly did not change, while the monodispersity gradually became better slightly. For the core-shell microspheres, with increasing the NIPAM dosage in the preparation of the PNIPAM shell layers, the mean diameters became larger simply, the monodispersity became better, and the thermoresponsive swelling ratio of the hydrodynamic diameters increased.