Polymers that can respond to external stimuli are of great interest in medicine, especially as controlled drug release vehicles. In this critical review, we consider the types of stimulus response used in therapeutic applications and the main classes of responsive materials developed to date. Particular emphasis is placed on the wide-ranging possibilities for the biomedical use of these polymers, ranging from drug delivery systems and cell adhesion mediators to controllers of enzyme function and gene expression (134 references).

The ability of polymers displaying lower critical solution temperatures (LCSTs) to mediate bioadsorptive processes was assessed. Three carboxyl-terminated polymers P1 − 3 with LCSTs respectively of 20, 32, and 42 °C were prepared by free-radical polymerization of N-isopropylacrylamide with and without comonomers acrylamide and N-tert-butylacrylamide. The polymers were grafted to amine-functionalized glass substrates, and their surface properties were investigated by contact angle goniometry and atomic force microscopy. Increases in water contact angle of up to 24° were observed between 10 and 37 °C for polymers with LCSTs of 20 and 32 °C, whereas no change was apparent for control amine-functional and the LCST 42 °C polymer surfaces over this temperature range. Variations in topography in water were also apparent from atomic force microscopy (AFM) studies for all the polymer grafts but not the amine surfaces over these temperatures. Adsorption of 3H-labeled bovine serum albumin and cytochrome c also increased to polymer grafts above the LCST, with the greatest change in the amount of attached protein being exhibited by polymer P1 (1.13 pmol·cm-2 cytochrome c at 10 °C, 3.95 pmol·cm-2 at 37 °C):  adsorption to control surfaces varied by less than 10% in this assay. Incubation of the graft and control substrates with a gram negative and motile bacterium (Salmonella typhimurium) and gram positive, nonmotile species (Bacillus cereus) showed the same overall pattern of attachment as the protein adsorption experiments, with polymers P1 and P2 retaining more bacteria (increases of up to 1350%) at 37 °C than at temperatures below their LCST, while amine-functional and P3 polymer surfaces showed less than 20% changes in the number of attached microorganisms. Further incubations at temperatures below polymer LCST resulted in fewer adsorbed cells at the surfaces showing the reversibility of short-term attachment to these materials. The results show that protein adsorption and short-term bacterial attachment correlate well with observed changes in surface properties as determined by contact angle goniometry and indicate that control of bioadhesion is possible by grafting suitably functionalized polymers capable of temperature-mediated hydrophilic−hydrophobic switching.

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