Modifying the aggregation behavior of poly (N-isopropylacrylamide) thermoreversible gel by a bile salt

One possible method of altering the critical solution temperature (CST) of poly (N-isopropylacrylamide) (PNIPAM) gel is the addition of surfactants. Because of the biological origin of bile salts, their inclusion in PNIPAM could lead to better biocompatibility when the materials are used for biomedical applications. The gelling behaviour of PNIPAM was studied at various concentrations: PNIPAM concentrations in the range of 1% to 12% (w/v) and sodium cholate (NaC) concentrations varied from 0 to 20 mM. From fluorescence, DLS, rheology and turbidity studies it was found that in the presence of NaC, the CST shifts to lower temperature. This effect of the bile salt is in contrast to the effect of conventional surfactants which are known to shift the CST to higher values, due to mutual solubilization. A study of fluorescence spectroscopic parameters like fluorescence anisotropy, spectral shift, intensity and DLS measurements suggest that a NaC-induced aggregation could be responsible for this unusual observation. © 2007 IOP Publishing Ltd.