PH-responsive materials

Poly[(4-carboxylatophenoxy)(methoxyethoxyethoxy)phosphazene] copolymers of variable compositions were synthesized by Allcock [645] in 1996. These polymers were found to be soluble in alkaline solutions. When crosslinked (by y-rays or by addition of CaCl2 to the polymer solution) the resulting hydrogels were found able to contract or expand as a function of the pH of the solution and their utilization as pH-responsive materials for drug delivery systems could be envisaged. 

Potentiometric hydrogen ion concentration (pH) is an important stimulus, which can be addressed through pH-responsive materials. Upon pH variation, ionizable polymers... 

For carbon nanotubes, organosoluble PS-SWCNT nanocomposites [292] have been prepared that could be efficiently transformed to highly hydrosoluble, pH-responsive materials via direct sulfonation of the grafted PS chains [296]. In another approach, MWCNT-C H was clicked with azide-functionalized poly(glycerol methacrylate) and the final hairy MWCNTs were used for the in situ synthesis and immobilization of monodisperse 3 nm-sized palladium nanoparticles. The final heterostructure had a high catalytic activity (77%) for the C-C coupling Suzuki reaction between 4-bromobenzene and phenyl boronic acid [293],... 

Dimitrov et al, 2007 Schild, 1992). Furthermore, switchable comonomers can be introduced resulting in the formation of multistimuli-responsive copolymers. Examples of such multiresponsive polymers include pH-responsive materials based on the incorporation of pH-switchable side chains, such as tertiary amines or carboxylic adds, and UV responsive materials based on the incorporation of UV-switchable side groups, such as azobenzene or spiropyran moieties (Dimitrov et al, 2007 Roy et al, 2010,2013). 

Unlike several of the other stimuh considered in this chapter, pH is an internal stimulus and thus requires alteration of the chemical environment. As a result, the use of pH triggers is limited, particularly for basic ceU studies that are confined to a narrow pH range. The complexity of reversibility is another drawback that arises with the use of pH-responsive materials, as the solution must be removed or extracted before reuse and this may also alter the physical environment. 

Nonetheless, pH-responsive materials have shown promise in fields such as drug dehvery and microprocessing [40],... 

Since electrodeposition of sol-gel films is driven by the local pH change on the electrode surfe.ce, it allows codeposition with other pH-responsive materials such as chitosan [61,62,67,70] and cationic epoxy resin [71]. The works... 

Sol-gel-based hybrid or composite films can also be electrodeposited. The coelectrodeposition of sol-gel with other materials is based on three different mechanisms according to the properties of the material. Electrochemically active materials, such as metal ions and monomers of conductive polymers, are electrodeposited with sol-gel via reduction or oxidation forming metal or conductive polymers. pH-responsive materials, such as chitosan or multiple sol-gel... 

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. 

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