The development of neural interfaces requires new electroactive and biocompatible materials, capable to apply electric fields without secondary effects, as large impedances at the interface or radical formation, which can cause damage in the tissues and the degradation of the electrode functionality. Currently, different types of electroactive materials are available for application as electrodes in the neural system: gold, platinum, glassy carbon, Pt-Ir, TiN or IrOx, among others, being the last, the one with superior performance. Properties such as high electrochemical efficiencies, good bio-stability and significant biocompatibility, have turned out IrOx into one of the most promising material for neural recording and stimulation electrodes. However, new technological breakthroughs have generated a demand of novel materials, with enhanced properties and which also minimize the drawbacks found in the actual ones, as low stability under electrochemical conditions, small values for charge capacity or the inherent rigidity of these oxides, which involves low compatibility with soft tissues. These improvements required may be achieved by hybrid materials, which join different properties from both counterparts. In this sense, IrOx-CNTs have been electrochemically prepared with enhanced properties. The chemical composition at the surface is very similar to that for IrOx, but the incorporation of carbon nanotubes makes the surface rougher, increasing the available interface area of the material. These properties, joined with the conductivity provided by the CNTs, yield very high values for charge storage capacity in electrochemical measurements. Also, the stability of the resulting coatings is improved in comparison with bare IrOx. The biocompatibility tests have shown high cellular survival and neuron functionality, similar to those values obtained for bare IrOx or borosilicate (used for reference), which validates these new materials as promising neural electrodes. IrOx hybrids with graphite and graphene also have been prepared. In both coatings, the presence of carbon particles has been demonstrated, although the confirmation of graphene sheets instead of few-layered graphene needs more experimental studies. The electrochemical properties of these IrOx-graphene and IrOx-graphite hybrids are similar than those obtained for IrOx-CNTs electrodes, with high values of charge storage capacity. However, the stability during consecutive cycling for the graphite-hybrid is poor and the coating is finally delaminated. These results are presumably due to heterogeneous structure in graphite-hybrids, in which the big carbon particles are not completely embedded in the IrOx matrix. Also, IrOx hybrids with N-doped graphene have been prepared, showing promising properties and very high values for charge storage capacity and stability, even when compared with non-doped IrOx-graphene coatings. The enhanced conductivity of these materials can be related with the presence of nitrogen, which induces the increase of the defects in the graphene sheets. The biocompatibility of these graphitic materials is under study. Polymeric tri-hibrids, IrOx-PEDOT-CNTs, have been also electrochemically synthesized. The use of a polymeric matrix is an effort to confer more flexibility to the electrode, which is desirable for soft tissue applications. However, the first results show that the polymer may encapsulate the CNTs and the IrOx particles, minimizing the electrochemical properties of these species. As a consequence, the electrochemical performance of the hybrid material is similar to those obtained for other polymers, as PEDOT-PSS. The biocompatibility tests have shown low neuronal viability in these substrates; however, co-cultures have been proposed as a novel method to improve biocompatibility in these types of materials. The materials obtained in all cases, are well adehered coatings, which leads to an easy future perpespective for their use as electrodes or cells substrates.
|Date of Award||23 May 2014|
|Supervisor||Maria Nieves Casaña Pastor (Director) & Jaume Casabo Gispert (Tutor)|