Hydrogels, switchable

Thermoresponsive polymers based on oligo(ethylene glycol) acrylates or methacrylates can be easily prepared by atom transfer radical polymerization under straightforward experimental conditions (i.e. in bulk or in ethanol solution and in the presence of commercially available catalysts). Thus, these stimuli-responsive macromolecules can be exploited for preparing a wide range of smart advanced materials such as thermoreversible hydrogels, thermoresponsive block-copolymer micelles and switchable surfaces. Hence, some of the results... 

Bastings MMC, Koudstaal S, Kieltyka RE, Nakano Y, Pape ACH, Feyen DAM, van Slochteren FJ, Doevendans PA, Sluijter JPG, Meijer EW, Chamuleau SAJ, Dankers PYW (2014) A fast pH-switchable and self-healing supramolecular hydrogel carrier for guided, local catheter injection in the infarcted myocardium. Adv Healthcare Mater 3 70-78. doi 10.1002/adhm.201300076... 

Liedl, T. Dietz, H. Yurke, B. Simmel, F. Controlled trapping and release of quantum dots in a DNA-switchable hydrogel. Small 2007, 3, 1688-1693. 

Zhao, X., Ding, X., Deng, Z., et al. (2005) Thermo switchable electronic properties of a gold nanoparticle/hydrogel composite, MacrorrwL Rapid Commun., 26, 1784-7. 

Micic, M., Zheng, Y., Moy, V., et al. (2002) Comparative studies of surface topography and mechanical properties of a new, photo-switchable PEG-based hydrogel. Colloid Surf. B-Biointerfaces, 27, 147-58. 

In summary, electrokinetic measurements combined with an advanced theory for soft surfaces is an effective tool for the comprehensive characterisafion of switchable hydrogel coatings. The accessible physicochemical parameters (distribution of polymer segment density, charge density, hydrodynamic softness, interphasial dilfuseness) complanent the results as obtained by techniques like ellipsometry or indenter measur ents. 

Moschou EA, Madou MJ, Bachas LG, Daimert S (2006) Voltage-switchable artificial muscles actuating at near neutral pEL Sens Actuators B 115 379 Murdan S (2003) Electro-responsive dmg delivery fiom hydrogels. J Control Release 92 1 Naficy S, Brown HR, Razal JM, Spinks GM, Whitten PG (2011) Progress toward robust polymer hydrogels. Aust J Chem 64 1007... 

Fig.l Thermal switchable microfluidic chip, (a) Parafim plugs deflne a L-shaped chamber, which can be fllled with the monomer solution (b) and polymerized by UV exposure (c). (d) Thermal treatment removes the paraffin plugs after the polymerization, (e) Detailed view on the parafim plugged channel structure. (1Q Final channel structure with a swollen hydrogel valve (Image adopted from Lin et al. (2014) with permission)... 

Schmuck and coworkers [43] recentiy reported that by attaching the self-complementary guanidiniocarbonyl pyrrole carboxylate zwitterionic moieties to the amino groups of polyethyleneimine (PEI), the polymer can be converted to a pH switchable hydrogel. After functionalization (18, Scheme 6.6), PEI was transformed from a viscous liquid to a solid. Using the characteristic UV absorption of the GCP group, the approximate loading of the zwitterionic units onto the polymer was calculated to be 40 %. This was further confirmed by GPC analysis of the molecular mass of the functionalized polymer. The functionalized PEI polymer 18 was readily soluble in water but produced more viscous samples than that observed with PEI. 

Anirudhan TS, Manasa Mohan A (2014). Novel pH switchable gelatin based hydrogel for the controlled delivery of the anti cancer drug 5-fluorouraciL RSCAdv,4,12109-12118. 

In a study, using the inclusion complex of trans azobenzene and cyclodextrin as a photo-switchable cross-linker, a dextran based photo-responsive hydrogel system has been constructed and employed for a light controlled protein release system [136]. 

The topics are classified phenomenologically (Fig. 3) (1) electrochemically addressable polymers in solution, (11) electrochemically induced micellization and demicellization (including vesicles and capsules with switchable porosity), (III) electrochemically addressable hydrogels and microgels, and (IV) thin films for electrode modification (without emphasizing classical electrochemical polymerization). An overview of polymerizations under electrochemical control is given elsewhere [18]. 

Chemical oxidants also allowed switchable hydrogel formation by reversible competitive hosting of low molecular mass ferrocene units within cyclodextrins. Here, cyclodextrin complexes the otherwise-associating alkyl side chains, which reduces the viscosity of this well-formulated mixture. However, the alkyl groups start to associate under gel formation when reduced ferrocenecarboxyUc acid is added. Then, ferrocene interacts preferentially with the cyclodextrin. This state can be reversed upon chemical oxidation due to the unfavorable inclusion complexation of ferrocenium units with cyclodextrin [254]. Based on similar mechanisms, a redox-dependent shape memory polymer has been generated [340] and also the adhesion of different gels can be switched [341]. 

Figure 1. Comparison between normal diffusion-controlling membrane and an on/off switchable hydrogel membrane.

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