Silicon/SU8 multi-electrode micro-needle for in vivo neurochemical monitoring

• Miniaturized silicon/SU8 implantable micro-needle with three platinum sensing electrodes.
• SU8 microwells for precise enzyme deposition.
• Independent and linear responses of the multi-electrode micro-needle sensor.
• Continuous on-line in vivo electrochemical recordings in the rat brain.
• Low cross-talk simultaneous and real time monitoring of glucose and lactate in vivo with single penetration and minimal brain damage.

Simultaneous monitoring of glucose and lactate is an important challenge for understanding brain energetics in physiological or pathological states. We demonstrate here a versatile method based on a minimally invasive single implantation in the rat brain. A silicon/SU8-polymer multi-sensing needle-shaped biosensor, was fabricated and tested. The multi-electrode array design comprises three platinum planar microelectrodes with a surface area of 40×200 µm2 and a spacing of 200 µm, which were micromachined on a single 3 mm long micro-needle having a 100×50 µm2 cross-section for reduced tissue damage during implantation. Platinum micro-electrodes were aligned at the bottom of micro-wells obtained by photolithography on a SU8 photoresist layer. After clean room processing, each micro-electrode was functionalized inside the micro-wells by means of a micro-dispensing device, either with glucose oxidase or with lactate oxidase, which were cross-linked on the platinum electrodes. The third electrode covered with Bovine Serum Albumin (BSA) was used for the control of non-specific currents. The thick SU8 photoresist layer has revealed excellent electrical insulation of the micro-electrodes and between interconnection lines, and ensured a precise localization and packaging of the sensing enzymes on platinum micro-electrodes. During in vitro calibration with concentrations of analytes in the mM range, the micro-wells patterned in the SU8 photoresist proved to be highly effective in eliminating cross-talk signals, caused by H2O2 diffusion from closely spaced micro-electrodes. Moreover, our biosensor was successfully assayed in the rat cortex for simultaneous monitoring of both glucose and lactate during insulin and glucose administration.