Enhancing Flexible Neural Probe Performance via Platinum Deposition: Impedance Stability under Various Conditions and In Vivo Neural Signal Monitoring

Abstract

Monitoring neural activity in the central nervous system often utilizes silicon-based microelectromechanical system (MEMS) probes. Despite their effectiveness in monitoring, these probes have a fragility issue, limiting their application across various fields. This study introduces flexible printed circuit board (FPCB) neural probes characterized by robust mechanical and electrical properties. The probes demonstrate low impedance after platinum coating, making them suitable for multiunit recordings in awake animals. This capability allows for the simultaneous monitoring of a large population of neurons in the brain, including cluster data. Additionally, these probes exhibit no fractures, mechanical failures, or electrical issues during repeated-bending tests, both during handling and monitoring. Despite the possibility of using this neural probe for signal measurement in awake animals, simply applying a platinum coating may encounter difficulties in chronic tests and other applications. Furthermore, this suggests that FPCB probes can be advanced by any method and serve as an appropriate type of tailorable neural probes for monitoring neural systems in awake animals.

Keywords: awake animals; flexible printed circuit board (FPCB); low impedance; monitoring; platinum coating.

Figure 1

(a) Schematic image of N32-1-b probe. (b) Schematic image of electro-deposition method of platinum. (c) Real image of electro-deposition method of platinum.

Figure 2

(a) Mechanical test device; (b) schematic image of mechanical test setting. (c) Real image of side view (left) and top view (right) of the initial state of the bending test. (d) Real image of side view (left) and top view (right) of the bending state of the bending test.

Figure 3

(a) SEM image of the electrode of N32-1-b probe (left). EDS mapping data for the same image (right). Orange for copper, and yellow for gold. (b) SEM image of the electrode of N32-1-b probe (left) after electrodeposition of platinum. EDS mapping data for the same image (right). Orange for copper, yellow for gold, and light blue for platinum. (c) Average impedance data of 32 channels from reference N32-1-b electrode (d) Average impedance data of 32 channels from platinum-coated N32-1-b electrode (e) Impedance distribution at 1 kHZ of reference N32-1-b electrode (left) and impedance distribution at 1 kHZ of platinum-coated N32-1-b electrode (right). ‘****’ indicates that the observed differences between the groups are highly statistically significant, with a p-value of less than 0.0001.

Figure 4

(a) Box plot of impedance data before mechanical test (0 times for red) and after mechanical test (250 times for green, 500 times for blue). The sample-by-sample variability and ANOVA test results are shown at the bottom. (b) Box plot of impedance data before thermal test (room temperature for red) and after thermal test (150 °C for green, 175 °C for blue). The sample-by-sample variability and ANOVA test result are shown at the bottom. (c) Box plot of impedance data before chemical test (neutral for red) and after chemical test (alkaline for green, acidic for blue). The sample-by-sample variability and ANOVA test result are shown at the bottom. ‘ns’ indicates that the observed differences between the groups are not statistically significant, with a p-value over than 0.05. ‘*’ indicates that the observed differences between the groups are statistically significant, with a p-value of less than 0.05. ‘****’ indicates that the observed differences between the groups are highly statistically significant, with a p-value of less than 0.0001.

Figure 5

Simultaneous single-unit recording from multiple neurons using the neural probe. (a) Schematic of neural recording using the probe (left) and example recording site (right). Orange, DiI Fluorescence in the VPm thalamus. (b) Representative clusters of putative action potentials recorded with the probe. (c) Autocorrelation of a unit spike times. (d) Representative spike waveforms of two units recorded from four adjacent electrode sites. (e) Raster plot of neural spikes from total of 19 units. (f) Schematic of firing recording in the VPm thalamus upon stimulation before and after whisker cutting (top); example of motor movements (middle) and spikes recorded in VPm (bottom). (g) Representative raster plots (top) and PSTH (bottom) of a responsive neuron before (left) and after (right) whisker cutting. Gray line—duration of motor movement. Note that whisker cutting abolished VPm spikes upon stimulation, indicating that the signals are indeed neuronal firings.

Figure 6

Single-unit recording in awake, head-fixed animal. (a) Probe implantation for awake, head-fixed recording. (b) Schematic illustration of head-fixed recording. (c) Representative activity clusters of 2 electrodes in an awake mouse after implantation. (d) Representative autocorrelogram of unit 1. (e) Representative spike waveforms of a unit recorded from four adjacent electrode sites at day 1 (top) and day 14 (bottom). (f) Representative plot of neural spikes from total of 14 units. The red line is the units 1 day after implantation, and the blue line is the units 14 days after implantation.