Colloids and Surfaces B: Biointerfaces 70 (2009) 60–67 Contents lists available at ScienceDirect Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb The effect of sodium cholate aggregates on thermoreversible gelation of PNIPAM Anitha C. Kumar a , Himadri B. Bohidar b,∗ , Ashok K. Mishra a,∗∗ a Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India b Polymer and Biophysics Laboratory, School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India article info Article history: Received 10 October 2008 Accepted 3 December 2008 Available online 9 December 2008 Keywords: Poly(N-isopropylacrylamide) Thermoreversible gels Sodium cholate Fluorescence Lower critical solution temperature abstract Additives like salts and surfactants can alter the phase transition temperature of poly(N- isopropylacrylamide) (PNIPAM). The inclusion of a biological surfactant like sodium cholate (NaC) into PNIPAM could lead a better biocompatibility when the materials are used for biomedical applications. The phase transition behavior of PNIPAM was studied in presence of NaC. DSC study shows that the pres- ence of NaC broadens the phase transition endotherm of PNIPAM, which is also accompanied by a small shift of the critical solution temperature (CST) to lower temperature. The results were compared with the optical measurements like, turbidity, DLS, fluorescence and rheology and it was observed that optical techniques are the best suitable for finding the onset temperature of gelation. The effect of the NaC 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. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Thermoreversible hydrogels exhibit an LCST-type (lower critical solution temperature) discontinuous first order volume transition phenomenon [1,2]. Current research interest in these hydrogels arises from potential biomedical applications [3–5]. The most widely studied thermoreversible gel, poly(N-isopropylacrylamide) (PNIPAM), is especially interesting as it exhibits LCST at around 32 ◦ C in water, which is close to body temperature of homeother- mic animals [1,2]. PNIPAM is a chemical isomer of poly-leucine, having a polar peptide group in the side chain rather than in the back bone. The main driving forces for collapse of PNIPAM chain in water are the hydrophobic and hydrophilic interactions. At the phase transition temperature there is a coil-to-globule transition present in this polymer, which has been extensively studied the- oretically and experimentally [6]. The volume phase transition is reversible with temperature, which leads to some special applica- tions like membranes for molecular separation [3], controlled drug releasing devices [4] and tissue culture substrates [5]. The effect of additives such as salts and surfactants on the phase transition temperature of PNIPAM has been studied in detail [7,8]. The presence of surfactants drastically changes the behavior of ∗ Corresponding author. Tel.: +91 11 26704636. ∗∗ Corresponding author. Tel.: +91 44 22574207; fax: +91 44 22574202. E-mail addresses: bohi0700@mail.jnu.ac.in (H.B. Bohidar), mishra@iitm.ac.in (A.K. Mishra). PNIPAM, especially its solubility in water, and therefore its phase transition signatures. Generally, surfactants promote both inter and intra molecular solubility so that the phase transition temperature increases with the surfactant concentration. Wu and Zhou [8] have reported that the presence of 4.05 mM sodium dodecyl sulfate (SDS) and 4.11 mM dodecylpyridine bromide (DPB) shifts the phase tran- sition temperature of PNIPAM to ∼50 and ∼35 ◦ C respectively from the original value of around 32 ◦ C. Schild and Tirrell [9] have car- ried out extensive studies of the effects of anionic surfactants on the phase transition temperature of PNIPAM and with surfactants having different chain lengths. They reported that the solubility of PNIPAM can be decreased or enhanced depending on the alkyl chain length and concentration. Surprisingly, not much study has been carried out on the interactions of biological surfactants like bile salts with PNIPAM. The phase transition behavior of similar poly- mer systems: N-alkylacrylamide copolymers with methacrylamide derivatives of cholic acid have been studied by Avoce et al. [10] and it was found that the LCST decreased with increasing amount of NaC residue. It was shown by Benrebouh et al. [11] that the bile acid residues tend to induce the aggregation of PNIPAM copolymers with 1–5% methacrylate derivatives of cholic acid polymers. In the presence of the NaC residue, the aggregation of these copolymers started at very low concentrations. They also reported that, NaC modified PNIPAM has no significant effect on the critical micellar concentration (CMC) of pure NaC solutions. Bile salts are biological compounds synthesized in the liver, stored in gall bladder and released for lipid digestion in the gastro- intestinal tract [12]. They are surfactant molecules possessing ‘facial 0927-7765/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2008.12.004