The design of “smart polymer materials” that undergo signal-triggered, reversible gel-solid transition, attracts substantial research efforts in the last decade. Chemical,100 pH,101, electrical,102 photonic103 and thermal104 stimuli were used to induce the gel-solid transitions. Different applications of signal-triggered polymer films were suggested, including the phase transition processes for sensing,105 mechanical movements of micro-objects,106 stimuli-controlled conductivity,107 permeability through the films,108 and switchable photoelectrochemistry.109

Sol-gel transitions in the solution can be stimulated by the controlled crosslinking and swelling of solubilized molecular components. The controlled reversible switching of sol-gel transitions in solutions is, however, more difficult. Photoisomerizable molecules revealed reversible light-induced sol-gel transitions,110 yet the most promising materials (especially for the present project) are biomolecules that undergo pH-induced sol-gel transitions. The reversible pH-induced gelation of pectin111 or gastric mucin112 is well established. For pectin (a polysaccharide) widely used in the food, cosmetics and pharmaceutical industries, a pH decrease to ca. pH = 4.5 results in gelation, due to the neutralization of the carboxylic acid functionalities, and the removal of the polyelectrolyte effect.

Accordingly, the solid-gel transition of polymer films linked to electrodes or the sol-gel transitions of pectin containing-solutions or the micelle-to-liposome transitions will be reversibly triggered by electrochemical signals. These processes will address the compartmentalization and containment issue that is one of the fundamental elements of the ECCell.

Polymer films that are linked to electrodes will be applied as matrices for the electronically stimulated solid-hydrogel transitions and will be based on the poly-N-isopropylacrylamide (poly NIPA). This is a thermosensitive polymer that undergoes a gel-to-solid transition at ca. 30° C. The transition temperature is affected by environmental co-effects such as the co-existence of acrylic acid moieties, and pH that controls the H-bonds in the polymer matrices. Accordingly, poly NIPA/acrylic acid copolymers will be electropolymerized on Au electrodes. Redox-active units, such as quinone, ferrocene or nile blue or even Ag+ ions will be tethered to the acrylic acid units. The temperatures for gel-to-solid transitions will be determined as a function of the redox state of the tethered units by the application of impedance spectroscopy or surface plasmon resonance (SPR) spectroscopy. These studies will identify redox-functionalized poly NIPA/acrylic acid polymers that reveal electroswitchable solid-gel transitions at a defined temperature. The study will be initially performed on macroscopic electrodes, and then miniaturized on addressable microelectrodes in microfluidic systems.

The sol-gel as well as micelle-to-liposome transitions will be stimulated by electrically triggered pH changes. Towards these goals, Au electrodes will be modified with monolayers or multilayers of pH-dependent redox labels. Monolayers consisting of quinones , nile green or polymerized films consisting of toluidine blue will be used to trigger the pH changes in the solution. Using a typical monolayer coverage of 10–10 moles . cm–2 and an electrolyte volume of 1 microliter we estimate ca. 4 orders of magnitude of pH changes. Accordingly, pectin solutions or amphiphilic DNA-block copolymers will be included in miniaturized electrochemical cells, and the sol-gel or micelle-to-liposome transitions by light scattering experiments. These studies will identify the potential regions at which the pH changes occur. The pH changes in the solutions will be followed by pH indicators or miniaturized pH electrodes. By the deposition of different redox-active monolayers/thin films on the electrodes in microfluidic systems, addressable pH changes in confined channels will be achieved, and thus, regulated sol-gel or micelle-to-liposome transitions in the respective channels will be driven.