The effects of anionic electrolytes and human serum albumin on the LCST of poly(N-isopropylacrylamide)-based temperature-responsive copolymers.

Poly(N-isopropylacrylamide) (PNIPAm) is one of the most widely studied temperature-responsive polymers among those that have been applied to biomaterials science and technology. Here, we investigated the importance of interactions between PNIPAm-based copolymers and biological factors. The effects of a series of major anionic electrolytes in biological environments and of human serum albumin (HSA) on the lower critical solution temperature (LCST) of homo-PNIPAm and PNIPAm copolymers were studied, using either a hydrophobic monomer or a cationic monomer. We synthesized P(NIPAm-co-BMA3%) with butyl methacrylate (BMA) as a hydrophobic monomer and P(NIPAm-co-DMAPAm2%) with N,N-dimethylaminopropyl acrylamide (DMAPAm) as a cationic monomer. The LCST of PNIPAm and P(NIPAm-co-DMAPAm2%) decreased with increasing salt concentrations, and the effects of anions on each polymer corresponded to the Hofmeister series. The LCST of P(NIPAm-co-DMAPAm2%) was greatly affected by anionic electrolytes compared with those of homo-PNIPAm and P(NIPAm-co-BMA3%). While the LCST of homo-PNIPAm was not affected by HSA, the LCST of P(NIPAm-co-DMAPAm2%) decreased non-linearly with increasing HSA concentrations. These effects were due to the electrostatic interactions between the positively charged polymer chains and the negatively charged HSA, as well as the stabilization of polymer aggregations with HSA. Under physiological buffer conditions, the LCST of P(NIPAm-co-DMAPAm2%) was not significantly affected by the HSA concentration. These results indicated that depending on the types of copolymers used for biological applications, it is necessary to take into account the effect of biological media while designing polymers.

Temperature-Responsive Fluorescence Polymer Probes with Accurate Thermally Controlled Cellular Uptakes.

Poly(N-isopropylacrylamide) (PNIPAAm)-based temperature-responsive fluorescence polymer probes were developed using radical polymerization, with 3-mercaptopropionic acid as the chain-transfer agent, followed by activation of terminal carboxyl groups with N-hydroxysuccinimide and reaction with 5-aminofluorescein (FL). The lower critical solution temperatures (LCSTs) of the resulting fluorescent polymer probes differed depending on the copolymer composition, and had a sharp phase-transition (hydrophilic/hydrophobic) boundary at the LCST. The cellular uptakes of the fluorescent polymer probes were effectively suppressed below the LCST, and increased greatly above the LCST. In particular, the cellular uptake of a copolymer with N,N-dimethylaminopropylacrylamide, P(NIPAAm-co-DMAPAAm2%)-FL (LCST: 37.4 °C), can be controlled within only 1 °C near body temperature, which is suitable for biological applications. These results indicated that the cellular uptakes of thermoresponsive polymers could be accurately controlled by the temperature, and such polymers have potential applications in discriminating between normal and pathological cells, and in intracellular drug delivery systems with local hyperthermia.