An array of highly flexible electrodes with a tailored configuration locked by gelatin during implantation-initial evaluation in cortex cerebri of awake rats

Abstract

Background: A major challenge in the field of neural interfaces is to overcome the problem of poor stability of neuronal recordings, which impedes long-term studies of individual neurons in the brain. Conceivably, unstable recordings reflect relative movements between electrode and tissue. To address this challenge, we have developed a new ultra-flexible electrode array and evaluated its performance in awake non-restrained animals.

Methods: An array of eight separated gold leads (4 × 10 μm), individually flexible in 3D, were cut from a gold sheet using laser milling and insulated with Parylene C. To provide structural support during implantation into rat cortex, the electrode array was embedded in a hard gelatin based material, which dissolves after implantation. Recordings were made during 3 weeks. At termination, the animals were perfused with fixative and frozen to prevent dislocation of the implanted electrodes. A thick slice of brain tissue, with the electrode array still in situ, was made transparent using methyl salicylate to evaluate the conformation of the implanted electrode array.

Results: Median noise levels and signal/noise remained relatively stable during the 3 week observation period; 4.3-5.9 μV and 2.8-4.2, respectively. The spike amplitudes were often quite stable within recording sessions and for 15% of recordings where single-units were identified, the highest-SNR unit had an amplitude higher than 150 μV. In addition, high correlations (>0.96) between unit waveforms recorded at different time points were obtained for 58% of the electrode sites. The structure of the electrode array was well preserved 3 weeks after implantation.

Conclusions: A new implantable multichannel neural interface, comprising electrodes individually flexible in 3D that retain its architecture and functionality after implantation has been developed. Since the new neural interface design is adaptable, it offers a versatile tool to explore the function of various brain structures.

Keywords: brain machine interface; brain micro-motions; embedding material; flexible electronic implant; mechanical compliance; neural probe; neuromodulation; stable neural recordings.

Figure 1

A schematic of the 3D flexible electrode array. Note the Z-shaped proximal part of the electrode array located between the cranium and the brain surface, and the tips (10 μm) of the distal protrusions which serve as recording sites.

Figure 2

Photograph taken 3.5 h after implantation of the gelatin embedded electrode, showing that the gelatin has dissolved and the brain surface contracted around the electrode array.

Figure 3

Electrode array comprising eight thin (4 × 10 μm) gold leads insulated with Parylene C, except for an area (10 × 10 μm) at the tip of the 100 μm long protrusions. (A) An electrode array before embedding into a gelatin based matrix. (B) The same electrode array after being embedded into a gelatin matrix shaped as a needle, and in (C), same electrode array inside a section of clarified brain tissue 3 weeks post implantation. Note that the conformation of the electrode array is well preserved inside the gelatin matrix as well as after implantation in the brain.

Figure 4

Samples of 1 s long recordings extracted at 0, 29, and 52 min from one electrode channel in an awake non-restrained rat. The unit spike with the highest signal to noise ratio in each segment is shown to the right of respective sample (mean waveform ± standard deviation). Note the high level of stability of the recordings with respect to spike-waveforms (waveform correlation > 0.99 between the indicated unit waveforms of all three segments) and amplitude, as well as the overall recording qualities. The spike-times of the indicated single-unit are marked in red below the sweeps.

Figure 5

Analysis of unit responses to tactile stimulation of the hind paw in awake, non-restrained rat. (A) Peristimulus-Time-Histogram (PSTH) for an identified single-unit (superimposed 1.6 ms long spike wave-forms for the unit are shown to the right, n = 150). (B) Principal component analysis (PCA). Note that the single unit (black stars) is well isolated from the background noise (gray dots) in the principal component (PC1 and PC2) feature space.

Figure 6

Characterization of electrode performance over time. (A) Noise level estimated by Equation (1) (distributions shown as median and percentiles values) including all electrode channels. The noise increased significantly (^***p < 0.001) between weeks 1 and 2, but remained stable between weeks 2 and 3 (p > 0.05). (B) Signal to noise ratio (SNR) of single-units (distribution including all identified units shown as median and percentile values) remained stable (p > 0.05) between week 1 and 2, but increased significantly between week 2 and 3 (^**p < 0.01). This reflects the fact that high amplitude units were added during week 3. The increase in SNR and yield suggest that the overall recording conditions improved during the course of the experiment. (C) Bar diagram depicting yield in percentage (number of channels with units divided by number of channels is shown on top of each bar). It can be seen that yield increased for both good (SNR ≥ 4) and fair (SNR ≥ 2) units (C), although the increase was not strictly monotonic for fair units (peaked during week 2).

Figure 7

Characterization of relative amplitudes of single unit spikes with high waveform correlation (>0.96) during two consecutive weeks. (A) Relative difference in amplitude shown as median and percentile values. The amplitude of units showed a tendency to increase over time, but this difference was not significant (p > 0.05). (B,C) Representative recordings (100 ms long) during week 2 and 3 in two different animals. Recordings are made from the same channel and show highly correlated units (waveform correlation > 0.96). Mean waveform ± standard deviation is shown to the right of each recording. In some cases, the amplitude of the units increased from week 2 to 3 (B), whereas in some other cases, the amplitude decreased (C).