Thermoresponsive properties of polyacrylamides in physiological solutions

Polymer solutions with a lower critical solution temperature (LCST) undergo reversible phase separation when heated above their cloud point temperature (TCP or CPT). As such, they have been proposed for a wide range of biomedical applications, from injectable drug depots to switchable coatings for cell adhesion. However, in systematic studies, the TCP of these thermoresponsive polymers has been mostly measured in non-physiological solutions, thereby hindering the development of their medicinal applications. Here, we analysed the thermoresponsive properties of four acrylamide-based polymers with LCST, namely poly[(N-2,2-difluoroethyl)acrylamide] (pDFEA), poly[(N-isopropyl)acrylamide] (pNIPAM), poly[(N,N-diethyl)acrylamide] (pDEA), and poly[(N-acryloyl)pyrrolidine] (pAP). As shown by turbidimetry, their TCP in phosphate saline buffer (PBS) and foetal bovine serum (FBS) were consistently lower than those reported in the literature, typically assessed in pure water, even when using the same setup. In addition, these physiological solutions affected the variation of TCP as a function of polymer concentration (1.25 to 10.0 mg mL−1) and molar mass (20 to 50 kg mol−1). As shown by isothermal calorimetry, interactions between proteins in FBS and polymer aggregates were predominantly exothermic, which indicates that protein–polymer complexes are formed through enthalpically driven processes. In conclusion, the TCP of thermoresponsive polymers strongly depends on solvent composition and therefore should be measured under physiological conditions for future medicinal applications.