Fabrication and modification of implantable micro-probes for neuro-studies

Abstract

In this work, different types of implantable probes fabricated on Si and polyimide substrates for sensing neurotransmitters, recording extracellular action potentials and stimulating the neurons have been developed. Several electrode configurations have been designed for implantation at various locations in the central nervous system. Each probe contains a multi electrode array (MEA) of 3, 5, 7 or 16 metal electrodes (Au or Pt) fabricated by electron-beam evaporation and lift-off processes.

For sensing purpose, as a proof-of-principle, dopamine (DA) and L-glutamate sensors have been implemented. The surface of electrodes was modified with nanostructures by different methods appropriate to the substrate and electrode material to enhance the performance of the sensors. Self-referencing technique and selective membrane deposition were also used to get better selectivity and limit of detection (LOD). A reference electrode was implemented in the same probe with the working electrodes, which presented an integrated, compact and robust system. A Ag/AgCl thin film reference electrode was used as the traditional method and pseudo-reference electrodes using IrO x film were also developed to solve the biocompatibility issue. Cell and animal experiments were carried out to prove the performance.

A method to modify the working electrode with nanowires using the vapor-liquid-solid (VLS) mechanism was realized with Si based probes containing an MEA of Pt, in order to increase the sensitivity of the electrochemical neurotransmitter sensors. The sensor probes were manufactured from a 300 μm thick 4-inch silicon (Si) wafer and then tailored into individual probes. The surfaces of electrodes were observed and characterized by scanning electron microscopy (SEM) and cyclic voltammetry (CV). With polyimide substrate, the probes were fabricated from a 125 μm thick Kapton sheet ( Dupont ). The flexibility of the probe helps to prevent scar forming in tissues aiming for long-term in vivo monitoring. A comparison between Au and Pt thin films was conducted by cyclic voltammetry. Several low-temperature processes were tried to modify the electrode surfaces with nanoparticles. The complete devices were made and used to demonstrate the enhancement in performance contributed by nanostructures in the enzyme-based electrochemical sensing of L-glutamate and DA. Comparison between electrodes with and without nanostructures modification was conducted showing that the modification methods were better to improve the performance of electrochemical sensors. In vivo experiments were also carried out to demonstrate the capability of the sensor with the live animals. A pain study was followed with approved protocols to prove that the sensor could distinguish different levels of excitations.

Those probes were also tested to record action potentials.

For stimulation, the above-mentioned probes could be used to electrically stimulate the neurons like the traditional approaches. Beside, a specially-configured probe was designed for optical stimulation as well. The probe was implemented to have three action potential recording electrodes and two bigger pads to mount a commercially available micro-LED for generating the stimulation light. Transgenic mice with visual cortex expressing channelrhodopsin-2 (ChR2) were used to test the performance of the integrated probes. The result was compared with that recorded by a conventional tungsten electrode. Data showed that the device was capable of stimulating as well was recording the neuron activities.