Temperature-responsive thermoresponsive polymer

The most popular examples of switchable substrates are the thermosensitive surfaces that change from hydrophobic to hydrophilic substrates upon small temperature changes. Thermoresponsive polymers, which exhibit a lower critical solution temperature (LCST) in water close to body temperature, are the mostly used responsive materials to produce surfaces with tunable cell attachment/detachment (da Silva et /., 2007). 

Among stimuli-responsive or thermoresponsive polymer microspheres, poly(N-isopropylacrylamidc) (PNIPA) was investigated most intensively. Cross-linked PNIPA is a thermosensitive hydrogel which undergoes a volume phase transition at its lower critical solution temperature (LCST) of around 34 °C. Several authors reported the synthesis of PNIPA microspheres [68-73]. 

KUN Kunugi, S., Yamazaki, Y., Takano, K., and Tanaka, N., Effects of ionic additives and ionic comonomers on the temperature and pressure responsive behavior of thermoresponsive polymers in acpieous solutions, Langmuir, 15,4056, 1999. 

Due to the relative ease of control, temperature is one of the most widely used external stimuli for the synthesis of stimulus-responsive bmshes. In this case, thermoresponsive polymer bmshes from poly(N-isopropylacrylamide) (PNIPAM) are the most intensively studied responsive bmshes that display a lower critical solution temperature (LOST) in a suitable solvent. Below the critical point, the polymer chains interact preferentially with the solvent and adopt a swollen, extended conformation. Above the critical point, the polymer chains collapse as they become more solvophobic. Jayachandran et reported the synthesis of PNIPAM homopolymer and block copolymer brushes on the surface of latex particles by aqueous ATRP. Urey demonstrated that PNIPAM brushes were sensitive to temperature and salt concentration. Zhu et synthesized Au-NPs stabilized with thiol-terminated PNIPAM via the grafting to approach. These thermosensitive Au-NPs exhibit a sharp, reversible, dear opaque transition in solution between 25 and 30 °C. Shan et al. prepared PNIPAM-coated Au-NPs using both grafting to and graft from approaches. Lv et al. prepared dual-sensitive polymer by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from trithiocarbonate groups linked to dextran and sucdnoylation of dextran after polymerization. Such dextran-based dual-sensitive polymer is employed to endow Au-NPs with stability and pH and temperature sensitivity. 

PNIPAAM, is a well-known temperature-responsive polymer [47] that has been used in the preparation of thermoresponsive membranes [48,49]. The colloidal nanopores were modified with PNIPAAM brushes (Scheme 8.4) [50], and the... 

Abstract This chapter describes polymers that undergo a temperature-induced phase transition in aqueous solution providing an important basis for smart materials. Different types of temperature-responsive polymers, including shape-memory materials, hquid crystalline materials and responsive polymer solutions are briefly introduced. Subsequently this chapter will focus on thermoresponsive polymer solutions. At first, the basic principles of the upper and lower critical temperature polymer phase transitions will be discussed, followed by an overview and discussion of important aspects of various key types of such temperature-responsive polymers. Finally, selected potential apphcations of thermoresponsive polymer solutions will be described. 

The third and most widely studied type of thermoresponsive polymers are polymers that undergo a solution liquid-liquid phase transition in response to variation of the temperature, that is, phase separation occurs from a homogeneous solution into a concentrated polymer phase and a diluted polymer phase. This phase transition is often accompanied by a transition from a clear solution to a cloudy solution, also known as the cloud point temperature (Tcp), for low concentration polymer solutions. This clouding is due to... 

Most interesting, however, are thermoresponsive polymer phase transitions in aqueous solutions since this phenomenon provides high potential for biomedical applications, such as drug dehvery and switchable synthetic cell culture surfaces (de las Heras Alarcon et al, 2005 Schmaljohann, 2006 Ward and Theoni, 2011). The remainder of this chapter will focus on such temperature-responsive polymers in aqueous solution, by discussing basic principles (Section 2.2) and key types of temperature-responsive polymers (Section 2.3) as well as selected applications (Section 2.4). 

Thermoresponsive polymers have been used in basic cell studies. For example, a PNIPAAm thiol (PNIPAAm-PEG-thiol) was synthesized and used to regulate the adhesion of cells in a microfluidic channel [66], Fibroblasts were initially cultured at 37°C to induce cell spreading and adhesion to the microfluidic surface. Then, the temperature of the microfluidic environment was reduced to 25 °C for a period of time. After 20 min, cell monolayers became detached and the morphology of individual cells was transformed to a spherical shape. Cell attachment and detachment as a function of the surface response to changing temperature is provided in Figure 6.5. 

Chemomechanical gels that function with phase transition caused by temperature changes Various pol)maers exhibit reversible phase transition in aqueous solution due to temperature variations. Representative examples include poly(vinyl methyl ether) (PVME) and poly(N-isopropylacrylamide) (PNIPAAm) [16, 17]. Common features of thermoresponsive polymers are the coexistence of hydrophilic and hydro-phobic portions in the same polymer chain. Increased hydrophobic interaction at an elevated temperature causes phase separation to take place. Gels obtained by crosslinking these polymers also show thermo-responsivity. The PNIPAAm gel shows the phase transition at 33°C in pure water. It swells at a temperature below the transition and vice versa (see Fig. 5) [18]. 

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