Micromachining for high density neural electrode arrays: a process development for dry etching polydimethylsiloxane and it’s in vitro electro-chemical behaviour and biological response

The performance of implantable stimulating electrodes to restore sensory perceptioncan be enhanced by technical advances in material processing to develop stable neuralinterfaces. High density neural electrode arrays have been recognised as a method tostimulate a higher amount of discrete neuronal populations resulting in a greaterresolution of encoded information. This thesis addressed the issues related to theperformance of high density stimulating electrode arrays fabricated by a combination oflaser technology and surface micromachining.A reactive ion etching process was developed using sulphur hexafluoride (SF6) andoxygen (O2) plasma to pattern microstructures in medical grade polydimethylsiloxane(PDMS), to allow the transfer of current between the Pt conductor and neural tissue.The process was optimised by investigating the effects of varying the processparameters on the etched outcome. The surface topography and etch performance wereanalysed by employing surface profilometry and scanning electron micrographs (SEM).The chemical modification of the PDMS structure after SF6 and O2 plasma wasinvestigated through x-ray photoelectron spectroscopy (XPS). The surface roughness ofthe underlying platinum surface was measured using atomic force microscopy (AFM).The optimum process parameters were established based on a high etch rate, cleansurfaces and anisotropic sidewall profiles.In vitro cytotoxicity tests consisting of direct contact and extract tests (according to ISO10993-5), were used to predict the cytotoxicity of the surface processed by SF6 and O2plasma. The results showed that the morphological and compositional modifications ofPDMS after the SF6 and O2 plasma did not affect the viability, morphology andproliferation of mouse fibroblasts.The in-vitro electrochemical performance was analysed using cyclic voltammetry andpulse tests. The main findings were that the charge storage densities increasedproportionally with the size of the electrode. The charge injection capacitance for theplasma etched Pt surfaces was less than the value obtained for the laser etchedelectrodes, which was attributed to a smoother surface. The effects associated with asmaller real surface area were not found to adversely affect the electrochemical stabilityof Pt in saline.This preliminary investigation into the process of dry etching shows positive results as amethod for micro-processing PDMS for high density electrodes