In situ quantitative visualization and characterization of chitosan electrodeposition with paired sidewall electrodesElectronic supplementary information (ESI) available: Neutralization process illustration; Nernst-Planck equation governing the migration of OH− and positively charged chitosan; calculation of ΔNe and ΔNNH3+; estimation of hydrogel density; experimental details and Videos S1, S1′, S2 and S2′. See DOI: 10.1039/c0sm00124d

We report the first in situ quantitative visualization and characterization of electro-induced chitosan hydrogel growth in an aqueous environment. This was enabled with a pair of sidewall electrodes within a transparent fluidic system, which allowed us to resolve the electrogelling mechanism and interpret the dominant causes responsible for the formation and density distribution of the deposited hydrogel. The pH and the time-dependent growth profiles of the chitosan hydrogel were directly visualized, analyzed, and characterized. The results indicate that the gelation and immobilization of chitosan onto the cathode at a pH above its p K a value (∼6.3) are due to the electrochemically generated concentration gradient of reactant OH − ions, and their subsequent neutralization of the NH 3 + groups of chitosan chains in solution near the cathode. The increased gel density around the fringes of the electrodes was demonstrated and correlated with the electrophoretic migration of chitosan cations during deposition. Simulation of the electric potential/field distribution, together with the corresponding dry film topography confirmed the non-uniform, electric field-dependent density distribution of deposited hydrogel. This report provides fundamental understanding towards the mechanism and the kinetics of the electro-induced chitosan gel formation. It also provides important guidelines for pursuing its application in bio-components integrated microsystems. The method in use exemplifies a simple, effective and non-destructive approach for in situ characterization of electro-responsive biopolymers in an aqueous environment. We report the first in situ quantitative visualization and characterization of chitosan hydrogel electrodeposition in aqueous environments using sidewall electrodes.