Neural probe arrays with high density electrode sites and low electrode impedance

Abstract

Implantable neural probes play an important role in understanding the neural activities. They can be used to investigate neural network and its functionality for explaining how our brain operates, learns, thinks, and initiates body actions. They could also be applied to treat neural diseases. We have designed and fabricated silicon (Si) based neural probes with various electrode site layouts for extracellular recording on targeted neural cells. Gold (Au) electrode sites were extended along different directions for achieving various spatial distributions. The highest electrode site density was 32 sites/10,000 μm2. The high density electrode sites had small dimensions of 2 μm diameter (dia.). They were used to collect neural signals from multiple locations of a neuron since typical neural cell body is 10 to 80 μm. These neural probes could provide high spatial resolution interface for monitoring neural signals within a neuron and multiple neurons. Polyimide (PI) neural probes were also developed since PI has lower Young’s modulus than Si which made the PI-based probes more flexible and biocompatible to soft brain tissue. Sensing quality of electrode sites with 5, 15, and 30 μm dia. was studied through in vitro and in vivo recordings. Through in vitro experiments, electrodes were used to record sinusoidal signal of 40 mV peak-to-peak voltage (Vpp). The measured signals were 15.8 to 3.1 mVpp when the corresponding electrode impedance varied from 1.2 to 8.5 MΩ. For in vivo recordings, neural probes with electrode sites of 15 and 30 μm dia. were implanted to the anterior cingulate cortex (ACC) region for acute recording of spontaneous and electrical evoked local field potential (LFP) of neural signals. Compared to 15 μm dia. electrodes, larger electrodes of 30 μm dia. were able to record 3.5 dB greater power spectral density for 60 s spontaneous LFP (≤ 500 Hz). For electrical evoked LFP, electrode sites of 15 and 30 μm dia. recorded signal of -0.02 and -0.11 mV, respectively, under 1000 μA stimulation current applied at the medial thalamus (MT) region. These results showed that larger electrode sites with lower impedance enhanced signal recording sensitivity. Although larger electrode site improved signal recording ability, neural probe size should be minimized to avoid tissue damage. In this project, tetrafluoromethane (CF4) plasma was used to increase the effective surface area of Au electrodes sites. For 15 μm dia. electrode size, the surface roughness could be increased from 1.7 to 22 nm after plasma treatment, and the electrode impedance decreased by 98%. Averaged background noise power in the range of 1 to 1000 Hz was decreased to -106 dB after the 30 μm dia. electrodes were plasma-modified, lower than the -86 dB without plasma treatment. Neural probes with plasma-modified electrode sites of 15 and 30 μm dia. were implanted to the ACC region for acute recording of spontaneous and electrical evoked LFP of neural signals. During in vivo recording, spontaneous LFP recorded by the plasma-modified electrodes of 30 μm dia. was 2 times higher compared to electrodes without treatment. For stimulation current of 400 μA, electrically evoked LFP recorded by the plasma-modified electrodes was 7 times higher than those without plasma exposure. As a result, plasma-modified electrodes improved neural probe recording quality and made it more sensitive to record spontaneous and evoked LFP in the ACC region.

Date of Award	15 Jul 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	S. Pang (Supervisor)