High-Density Stretchable Electrode Grids for Chronic Neural Recording

Abstract

Electrical interfacing with neural tissue is key to advancing diagnosis and therapies for neurological disorders, as well as providing detailed information about neural signals. A challenge for creating long-term stable interfaces between electronics and neural tissue is the huge mechanical mismatch between the systems. So far, materials and fabrication processes have restricted the development of soft electrode grids able to combine high performance, long-term stability, and high electrode density, aspects all essential for neural interfacing. Here, this challenge is addressed by developing a soft, high-density, stretchable electrode grid based on an inert, high-performance composite material comprising gold-coated titanium dioxide nanowires embedded in a silicone matrix. The developed grid can resolve high spatiotemporal neural signals from the surface of the cortex in freely moving rats with stable neural recording quality and preserved electrode signal coherence during 3 months of implantation. Due to its flexible and stretchable nature, it is possible to minimize the size of the craniotomy required for placement, further reducing the level of invasiveness. The material and device technology presented herein have potential for a wide range of emerging biomedical applications.

Keywords: nanowires; neural electrodes; neural interfaces; soft electronics; stretchable electronics.

Figure 1

Stretchable Au-TiO₂NW-PDMS composite. a) The TiO₂NWs are coated with gold by reducing HAuCl₄ salt with HDA in the presence of a capping agent (PVP). b) The addition of HCl stabilizes the dispersion and promotes a color change from brown–grey to orange–red. c–e) SEM images of the Au-TiO₂NWs at different stages during the coating process. c) A partial gold coating is produced when 1/10 of the gold salt solution has been added. d) At the end of the reaction, a continuous gold coating has been produced. e) The initial gold coating is rough (left), but smoothens out after one week (right). f) The sheet resistance R_S of films made from freshly produced Au-TiO₂NWs improves over time (■). The conductivity of the NWs stored in the dispersion improves over time as well . g) Stretchable conductors are produced by embedding the NWs in PDMS. The sheet resistance remains below 10 Ω □⁻¹ up to 100% strain. h) The conductors show excellent stability for 1000 strain cycles at 20%, 50%, and 100% strain. Scale bars: c,d) 500 nm, e) 200 nm.

Figure 2

Soft and stretchable electrode grids. a) The electrode grid is made of Au-TiO₂NW conductors embedded in PDMS and comprises 32 electrodes with 200 μm pitch. b) The stretchability allows the SEG to establish conformal contact around curved surfaces. c) An opening in the PDMS layer is etched to form the 50 μm large electrodes. A thin layer of platinum is electroplated onto the exposed Au-TiO₂NWs to improve electrode performance. d) Optical surface profilometry reveals that the electrode openings are ≈7 μm deep. e) The SEM image of the electrode shows how the Pt-coated Au-TiO₂NWs are partially embedded in the PDMS. f) The impedance of the electrode before (—) and after (—) electroplating of platinum. g) Measured impedance at 1 kHz of the electrodes of an SEG before (◆) and after Pt coating. h) Microscopy images of the electrode at 0% and 30% strain. i) The impedance of an electrode at 0% (—) and 30% (—) strain. Scale bars: a) 1 mm, b) 500 μm, c) 50 μm, e) 20 μm and 1 μm, h) 50 μm.

Figure 3

Implantation and in vivo neural recording of soft SEGs. a) Intraoperative microphotograph of soft SEG showing the probe in a craniotomy lying on the surface of the brain. Note the hydrophobic nature of the device demonstrated by a drop of water on the ribbon area. b) Zoomed-in microscopy image showing the ability of the probe to be inserted in smaller craniotomy than the probe geometry. c) Freely moving rat with chronically implanted SEG. d) Intraoperative recording of SSEPs using the soft SEG. Top: raw LFP time traces showing SSEPs generated by hind limb electrical stimulation. Right: heat map demonstrating the anatomical localization of SSEPs to the somatosensory cortex. Left: electric impedance of the SEGs at 1 kHz, showing a uniform impedance range across devices. Scale bars: a,b) 1 mm, c) 10 mm, d) 100 ms, 500 μV.

Figure 4

In vivo chronic recording and spatial distribution of local field potential. a) Time–frequency spectrogram of LFP recorded by the soft SEG during sleep in somatosensory cortex. The spectrogram during an NREM epoch contains slow oscillations (2–4 Hz) and sleep spindles (9–16 Hz); during a REM epoch theta (4–8 Hz) and gamma (30–80 Hz) oscillations were seen. Colors represent normalized power, with warmer colors indicating higher power. b) Raw LFP trace (top) and corresponding spectrogram (bottom) from 3 month chronically implanted SEG during an NREM epoch. c) Raw LFP trace highlighting a spatially patterned gamma oscillation during an NREM epoch. d) Average coherence between recording electrodes at each frequency band in an NREM session (— 2–4 Hz), (— 4–8 Hz), (— 9–16 Hz), (— 30–80 Hz). e) Mean power of recorded signal after 1 (—), 2 (—), and 3 (—) months of implantation. f) Number of functional recording electrodes after 1, 2, and 3 months of implantation, with high-quality recording electrodes in blue and low-quality recording electrodes in light blue. Scale bar: c) 100 ms.