Date Published: March 10, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Xiaoling Wei, Lan Luan, Zhengtuo Zhao, Xue Li, Hanlin Zhu, Ojas Potnis, Chong Xie.
Understanding brain functions at the circuit level requires time‐resolved simultaneous measurement of a large number of densely distributed neurons, which remains a great challenge for current neural technologies. In particular, penetrating neural electrodes allow for recording from individual neurons at high temporal resolution, but often have larger dimensions than the biological matrix, which induces significant damage to brain tissues and therefore precludes the high implant density that is necessary for mapping large neuronal populations with full coverage. Here, it is demonstrated that nanofabricated ultraflexible electrode arrays with cross‐sectional areas as small as sub‐10 µm2 can overcome this physical limitation. In a mouse model, it is shown that these electrodes record action potentials with high signal‐to‐noise ratio; their dense arrays allow spatial oversampling; and their multiprobe implantation allows for interprobe spacing at 60 µm without eliciting chronic neuronal degeneration. These results present the possibility of minimizing tissue displacement by implanted ultraflexible electrodes for scalable, high‐density electrophysiological recording that is capable of complete neuronal circuitry mapping over chronic time scales.
Implanted neural probes1, 2 such as microwires,3 tetrodes,4 and silicon‐based microelectrodes5, 6, 7 are among the most important techniques in both fundamental and clinical neuroscience. Scientifically, they remain our only option to temporally resolve the fastest electrophysiological activities of individual neurons, which provides critical information to dissect the neural circuitry.8, 9, 10, 11 Clinically, neural electrodes have been successfully used in the treatment for a number of neurological disorders12, 13 such as Parkinson’s disease,14 epilepsy,15 and obsessive compulsive disorder.12 Moreover, they allow for direct communication between brain and man‐made devices, which can enable applications such as human brain–machine interface and neuroprosthetics.16, 17 However, these conventional neural electrical probes typically have dimensions substantially larger than neurons and capillaries, which fundamentally precludes the possibility of interrogating the whole neuronal population in a functional brain region. In particular, microwire electrodes,3 tetrodes,4 and Utah array18 host only one recording site at the tip of each wire, and therefore cannot simultaneously record neural activity at multiple depths. Micro‐electromechanical system‐based silicon probes19, 20 have significantly increased the number of recording sites on one probe. However, these silicon probes typically have cross‐sectional areas around or greater than 103 µm2, which gives the volume per electrode similar to that of the microwire and tetrodes, all at two orders of magnitude larger than the average size of a neuronal soma (Table S1, Supporting Information). In addition, because these probes are strongly invasive to living brain tissue,21, 22 to maintain tissue vitality their highest implantation density is limited by only allowing at most 1–2% of the enclosed volume to be occupied by the electrode array.23 Therefore, the smallest interprobe spacing is limited to be at least several hundred micrometers for both microwire array and silicon probes.24 Although flexible neural electrodes are generally believed to induce less tissue reaction than their rigid counterparts, their highest implantation densities demonstrated to date are still at similar values.25, 26
We adopted a hybrid method involving both electron beam and optical lithography38 to fabricate NET‐e devices with high throughput (detailed fabrication procedures in Section S1 and Figure S1 in the Supporting Information). We used EBL to define the implanted section (the “thread”) where dimension constraints were stringent and used photolithography with relaxed resolution requirement for larger structures that were not implanted into brain tissue (an example of EBL and photolithography sections are shown in Figure S2 in the Supporting Information). Figure1a–f shows the overview and zoom‐in images of the as‐fabricated EBL section on NET‐e probes with different patterns of electrode arrays. NET‐e‐l (Figure 1a) hosted a linear array of electrodes with a cross‐sectional profile of 0.8 µm × 8 µm, the smallest among all reported electrodes to our knowledge (Figure S3 and Table S1, Supporting Information). NET‐e‐t (Figure 1b) was designed to function similarly as multiple tetrodes spanning across the cortical depth, hosting groups of four closely spaced electrodes every 100 µm along the thread. NET‐e‐o (Figure 1c) had a continuum of individually addressed electrodes along the thread. For improved single‐unit detection and sorting yield, both NET‐e‐t and NET‐e‐o were designed to have electrode spacing smaller than their detection range to enable spatial oversampling in action potential recording. To minimize NET‐e probe’s cross‐sectional area, we designed a multilayer architecture with no substrate similar to previous work,36 where interconnect traces and electrodes were fabricated on different layers separated by insulating layers (Figure 1g,h). To facilitate the transition from EBL to optical lithography and to avoid disconnected interconnects and insulating layers, we intentionally overlapped the sections of EBL and photolithography by at least 4 µm in all directions in all layers as shown in Figure S2 (Supporting Information).
In this study, we demonstrated the possibility of integrating neural electrode arrays within a subcellular form factor and their implantation in a high‐density, scalable manner. By applying nanofabrication techniques on unconventional substrate‐less design of neural probes, we drastically reduced the physical dimensions of neural probes. Combining these nanofabricated ultraflexible probes with minimally invasive implantation methods at subcellular surgical footprints, we developed a practical approach to overcome current physical limits in the design and implantation of intracortical neural electrodes, which paves the road for chronic, full‐coverage, neural recording and complete circuit‐level mapping of neural activity.
Shuttle Device Fabrication and Assembly: A straight segment of carbon fiber was attached to a stainless steel microneedle (prod# 13561‐10, Ted Pella, Inc.) for convenient handling. It was then cut to the designed length (2–3 mm) using focused ion beam (FIB). An anchor post was micromilled at the tip of the shuttle device using FIB to shape a well‐defined micropost (≈3 µm in diameter, 4 µm in height).
The authors declare no conflict of interest.