Effects of Surface Cathode Properties on Biopolymer Structuring via Electrochemical Deposition

Electrochemical deposition is a versatile bottom up-approach for the realization of advanced surfaces in biomedical applications. Deposition of chitosan via cathodic polarization has been recently proposed as a simple, fast, and effective technique also for the preparation of cellular solids [1], by their peeling-off from the deposition substrate. Chitosan ((1,4)-linked 2-amino-2- deoxy-d-glucan) is a natural cationic polymer that has been widely applied in drug delivery and tissue engineering for its proven biocompatibility, biodegradability, and antibacterial properties. Its electrodeposition, based on transport phenomena and reduction reactions at the diffusion layer, offers a control of the process parameters through simple adjustment of processing time and applied potential. The morphology of the obtained structures, being in the range of interest for tissue engineering scaffolds, can be easily tuned by an appropriate selection of anionic species and their concentration. Here we show the effects of the selection of anionic species and the surface cathode properties on the morphology of deposited coatings in order to modulate the obtained porosity. Electrocoagulation processes strongly depend on local reduction at the cathode, in turn affected by the acid/base equilibrium in solution: we hence examined (i) different water-based acidic bath and (ii) surfaces with different overvoltages for the hydrogen evolution reaction. Different coating morphologies, in terms of pore size and interconnection, were obtained by modifying electrochemical bath composition and controlling the overvoltage for hydrogen evolution by selecting the proper substrate material and surface preparation. Gas evolution at the cathode surface determined the growth of pores in the deposit coatings and, depending on the process parameters, close vs. open porosities were observed. A mass increase of deposited chitosan by increasing pH was generally observed, even if this trend is not monotone by increasing the pH. An increase of film porosity has been, in fact, noticed by increasing the mass rate deposition and hydrogen evolution and entrapment in the deposited gels. The overall deposition process seems affected by different electrolytes mainly as a consequence of their acid/base equilibria. Porosity can also be tuned according to the surface properties of selected metallic substrates, based on the characteristic exchange current for hydrogen evolution reaction. An accurate control of this process, hence allows to modulate open vs. closed pore structures. The possibility of tuning surface morphology of this class of films paves the way for the design of a novel class of 3D structures supporting tissue engineering approach and advanced drug delivery.

[1] L. Altomare, L. Draghi, R. Chiesa, L. De Nardo, Materials Letters 78 (2012) 18.