Thermo-sensitive gels

The second example used visible light absorption that increased the temperature locally within the thermosensitive gel [39]. The gel consisted of a covalently cross-linked copolymer network of N-isopropylacrylamide and chloro-phyllin, a combination of a thermo-sensitive gel and a chromophore. In the absence of light, the gel volume changed sharply but continuously as the temperature was varied. Upon illumination the transition temperature was lowered, and beyond a certain irradiation threshold the volume transition became discontinuous. The phase transition was presumably induced by local heating of polymer chains due to the absorption and subsequent thermal dissipation of light energy by the chromophore. The details will be discussed in a later section. 

Comparing both transport processes, it is evident, that thermo-sensitive gels can respond faster to the environmental change than chemo-sensitive gels. 

I. Colombo, M. Grassi, M. Fermeglia, R. Lapasin and S. Pricl, Modeling phase transitions and sorption desorption kinetics in thermo-sensitive gels for controlled drug delivery systems, Fluid Phase Equil., 1996, 116, 148-161. 

Kim S, Nlshlmoto SK et al (2010) A chltosan/beta-glycerophosphate thermo-sensitive gel for the delivery of ellagic acid for the treatment of brain cancer. Biomaterials 31 4157-66... 

Thermo-Sensitive Gels Biodegradable Hydrogels from Enantiomeric Copolymers of Poly(lactide) and Poly(ethylene glycol)... 

Yoshida, R., Sakai, K., Okano, T., and Sakurai, Y. Modulating the phase transition temperature and thermo-sensitivity in iV-isopropylacrylamide copolymer gels, J. Biomater. Set Polymer Ed., 1994, 6, 585-598. 

Poly(vinyl methyl ether), PVME, is a thermo-sensitive polymer. The aqueous solution has a Lower Critical Solution Temperature (LCST) of 37 °C. Therefore, PVME is soluble in water below its LCST, but insoluble above its LCST. When an aqueous solution of PVME is irradiated with y-rays the solution becomes PVME hydrogel [18, 19]. The gel shows thermo-sensitivity similar to the solution, and swells below 37 °C and shrinks above this temperature. It is important to form a fine porous gel structure to obtain quick response gels. There are two methods for the purpose. One is a method using micro-phase separation by heating. The other is a method using micro-phase separation by blending of polymer solutions. 

Arndt KF, Schmidt T, Reichelt R (2001) Thermo-sensitive poly(methyl vinyl ether) micro-gel formed by high energy radiation. Polymer 42(16) 6785-6791... 

Synthesis of gel particles in the pm-range (micro-gels) (Pelton 2000) using different techniques, e.g., thermo-sensitive micro-gels based on NIPAAm by inverse suspension polymerization (Bajpai et al. 2007) or inverse emulsion polymerization (Hirotsu et al. 1987). 

Microvalves are the simplest hydrogel-based components. The gel actuator is directly placed within a valve chamber (Fig. 11a). The thermo-sensitive PNIPAAm is swollen at room temperature and closes the valve. For opening the attached resistive heater has to be activated. Exceeding the volume phase transition temperature of approximately 34 °C the gel shrinks and opens the valve seat. 

It is well known that membranes of various kinds of phospholipids undergo phase transitions, such as a gel-to-liquid crystalline transition and a lamellar-to-hexagonal transition. These phase transitions have been used for the design of stimuli-sensitive liposomes. The, temperature sensitization of liposomes has been attempted using thermo-sensitive polymers that exhibit a lower critical solution temperature (CST). Attachment of these polymers to liposomes could give temperature-sensitive functionalities to the liposomes. ... 

Demonstration of a thermo-sensitive polymer below and above its LCST. Below LCST, the polymer is hydrophilic and remains in solution form. As temperature is increased to above its LCST, the polymer becomes insoluble and collapses to form a gel. 

Thermo-sensitive micro- or nano-gel particles can be obtained if the block copolymer formation is carried out in the presence of MBAm as a cross-linker. 

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