ABSTRACT
Some studies have been carried out to control the solute permeability of a microcapsule membrane depending on temperature. [54-57] One of their challenges was grafting poly(NIPAAm) on the surface of pores located in nonthermosensitive microcapsule membranes.[54] By the modification of the membrane pores with poly(NIPAAm), microcapsules have obtained a thermosensitive membrane permeability of solute. Less than 33°C solute permeability is lower than that above 33°C. One of other challenges was grafting poly(NIPAAm) on the surface of polypeptide microcapsules.[55] Microcapsule membranes containing nanosized poly(NIPAAm) gels also showed thermosensitivity.[56] The microcapsules above reported have a superiority that the membranes keep a mechanical strength even under 33°C, while poly(N-NIPAAm) hydrogel is in a swollen state and mechanically weak at the temperature. This is because almost all the structure of the micro capsule membrane is composed of nonthermosensitive materials. The thermosensitivity of the microcapsules is, however, not high, because only the pore surface or the surface of the membrane or some parts of the membrane has a thin poly(NIPAAm) hydrogel layer. As introduced in the last section, the thermosensitive property of poly(NIPAAm) hydrogel microspheres is size dependent and the smaller gels show the higher sensitivity. The microcapsule membrane prepared by an interfacial polymerization method is known to be very thin. Also, it has been reported that microcapsules show highly stimulisensitive properties by changing their shapes. That is, poly(L-lysine-terephthaloyl) microcapsules change their size, membrane permeability,[58,59] and membrane thickness[60] depending on the pH and ionic strength of the suspending media. Also, Makino et al.[61] have reported a microcapsule self-regulating delivery system for insulin. The system released insulin only when the glucose concentration outside the microcapsules became higher than a certain value. Such a highly stimuli sensitive property of microcapsules seems to be related to the thin membrane of the microcapsules. From these, it is suggested that a microcapsule having a thinner membrane is required to obtain more highly thermosensitive drug release. Poly(Llysineisopropylamide-terephthalic acid) microcapsules containing water have shown highly thermosensitive properties. The microcapsules were obtained by an interfacial polymerization at a water/oil interface between Llysineisopropylamide (abbreviated to LIPA, hereafter) and terephthaloyl dichloride.[57] The microcapsule
Fig. 28 Changes in poly(LIPA-terephthalic acid) microcapsule size depending on temperature.
