Optical images of passively deposited Pt-nanograss on a polyimide probe before (a) and after (b) mechanical cleaning. The probe in (c) shows two neighboring electrode sites with and without actively deposited Pt-nanograss on the upper and lower site, respectively.A lawn-like coating of tiny grass-like platinum wires could improve electronic devices used to communicate with the brain, according to scientists from the University of Freiburg in Germany.
Sending electrical signals directly to the brain could reduce pain by stimulating the spinal cord, treat the neurological symptoms of Parkinson’s disease, diagnose epilepsy, or control paralyzed or artificial limbs. Such communications with the brain rely on micro-sized electrodes to relay external electrical signals into neural tissue. Electrodes must be small enough to communicate with single or small number of neurons, but that gives rise to high impedance and high levels of noise.
Different ways of improving the performance of small electrodes are being explored, but the team from Freiburg has come up with a simple solution that significantly reduces impedance and, therefore, noise [Boehler et al., Biomaterials 67 (2015) 346]. The novel approach devised by Christian Boehler and his colleagues coats existing neural electrodes with a layer of grass-like nanostructured Pt. The fabrication process for the low impedance/high charge injection coating is straightforward and widely applicable.
Pt nanograss was fabricated on flexible polyimide-based probes with Pt electrodes using either an active electrochemical or a passive wet-chemical process. In both cases the electrode is first placed in platinic acid. Then either a current is applied to the electrode for a few minutes or it is left in solution for 48 hours to allow a chemical reduction reaction to take place. Excess nanograss on the probe can be removed easily by wiping or ultrasonication.
“The simple process can be applied to potentially any kind of existing electrode,” says Boehler. “It does not require special equipment like a cleanroom environment and can be realized with little effort, time, and cost.”
Nanograss-coated electrodes have significantly larger surface areas than smooth electrodes. For a similarly sized device, the nanostructured coating induces a substantial reduction in impedance of more than a factor of 60 compared with smooth, unmodified electrodes, as well as high charge injection capacity. Alternatively, the same impedance and charge injection properties can be maintained on much smaller devices just 1% of their original size.
“The coating provides an impedance that is considerably lower than commonly known materials in the field and has high potential to improve stimulation and recording properties of neural electrodes,” says Boehler.
Pt is already well established as an electrode material for biological applications because of its stability and biocompatibility. The researchers have now started in vivo testing of the modified Pt electrodes and hope to be able to simplify the fabrication process further.