Nanostructured electrodes for biocompatible CMOS integrated circuits

This paper reports on the adaptation of standard complementary metal oxide semiconductor (CMOS) integrated circuit (IC) technology for biocompatibility, enabling a low-cost solution for drug discovery pharmacology, neural interface systems, cell-based biosensors and electrophysiology. The basis for the process is the anodisation of IC aluminium electrodes to form nanoporous alumina. The porous alumina was electrochemically thinned to reduce the alumina electrode impedance. For applications where a porous electrode surface is either preferred or acceptable, we demonstrated that porosity can be manipulated at room temperature by modifying the anodising electrolyte to include up to 40% polyethylene glycol and reducing the phosphoric acid concentration from 4% (w/v) to 1%. For applications requiring a planar microelectrode surface, a noble metal was electrodeposited into the pores of the alumina film. Limited success was achieved with a pH 7 platinum and pH 5 gold cyanide bath but good results were demonstrated with a pH 0.5 gold chloride bath which produced planar biocompatible electrodes. A further reduction in impedance was produced by deposition of platinum-black, which may be a necessary additional step for demanding applications such as neuronal recording. During this work a capability for real-time electrochemical impedance spectroscopy (EIS) was developed to study anodisation, barrier oxide thinning, oxide breakdown and electrodeposition processes. To study the pore morphology, focused ion beam (FIB) was employed to produce cross-sectional cuts of the IC features which were inspected by SEM with an 'In-lens' detector. The anodisation process and the optional electrodeposition steps require only simple bench equipment operated at room temperature and is therefore a viable route for manufacturing low-cost biocompatible electrodes from standard CMOS ICs.