Thermoresponsive materials temperature

Temperature variations may result in reversible changes in properties such as structural arrangement, size, solubihty, and shape. Many materials designed for biomedical or biotechnology appHcations are confined to a narrow temperature spectrum in order to be effective in a physiological environment. The following thermoresponsive materials are discussed in the next section poly(N-iso-propylacrylamide (PNIPAAm), polymer brushes, and shape-memory polymers. 

In addition, the copolymers with a CP close to body temperature have a robust phase transition that does not significantly vary with concentration (Figure 22.14, right). As such, these copolymers of EtOx and /tPropOx are thermoresponsive materials with a robust and tunable CP, making them suitable candidates for biomedical applications as alternatives to PNIPAM. 

In the next section, an overview of the main biomedical tpplications based on shape-changing materials in response to natural stimulus (temperature—thermoresponsive materials, pH— pH-responsive materials, and mechanical—mechano-electrical materials) will be presented. 

Materials that typify thermoresponsive behavior are polyethylene—poly (ethylene glycol) copolymers that are used to functionalize the surfaces of polyethylene films (smart surfaces) (20). When the copolymer is immersed in water, the poly(ethylene glycol) functionaUties at the surfaces have solvation behavior similar to poly(ethylene glycol) itself. The abiUty to design a smart surface in these cases is based on the observed behavior of inverse temperature-dependent solubiUty of poly(alkene oxide)s in water. The behavior is used to produce surface-modified polymers that reversibly change their hydrophilicity and solvation with changes in temperatures. Similar behaviors have been observed as a function of changes in pH (21—24). 

There is a host of other intriguing phenomena associated with the structure and dynamics of stars, which we only list here. The inhomogeneous monomer density distribution in Fig. 2 is responsible for temperature and/or solvency variation in analogy to polymer brushes attached on a flat solid surface [198]. In fact, multiarm star solutions display a reversible thermoresponsive vitrification (see also Sect. 5) which, in contrast to polymer solutions, occurs upon heating rather than on cooling [199]. Another effect is the organization of multiarm stars in filaments induced by weak laser light due to action of electrostrictive forces [200]. This effect was recently attributed [201] to local concentration fluctuations which provide localized-intensity dependent refractive index variations. Hence, the structure factor speciflc to the particular material plays a crucial role in the pattern formation. 

The hexagonaUy ordered mesoporous silicas with different pore sizes (10,17,30 nm) materials were tested as carrier, in smart controlled drug (Indomethacin (I)) release using the thermoresponsive poly(iV-isopropylacrylamide) (PNIPAm) hybrid nanoporous structures during stepwise temperature changes between 25°C and 40°C (Chang et al. 2004). 

Lendlein et al. demonstrated the possibilities to design thermoresponsive macroscopic self-folding objects using shape-memory polymers based on different poly(e-caprolactone) [12]. At low temperature, the materials are in their temporary shape. The films recover their permanent shape and irreversibly fold by heating, which could be accompanied by a change of transparency. 

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