Polymer Composite with Carbon Nanofibers. Aligned during Thermal Drawing as a. Microelectrode for Chronic Neural Interfaces

Transcription

1 Polymer Composite with Carbon Nanofibers Aligned during Thermal Drawing as a Microelectrode for Chronic Neural Interfaces Yuanyuan Guo 1,2*, Shan Jiang 2, Benjamin J.B. Grena 3, Ian F. Kimbrough 4, Emily G. Thompson 4, Yoel Fink 3, Harald Sontheimer 4, Tatsuo Yoshinobu 1 and Xiaoting Jia 2* 1Biomedical Engineering, Tohoku University, Sendai, Miyagi , Japan; 2 Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA USA; 3 Department of Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, USA; 4 Virginia Tech Carilion Research Institute, Roanoke, VA USA Corresponding Authors: Yuanyuan Guo, yuanyuan.guo.a4@tohoku.ac.jp Department of Biomedical Engineering, Tohoku University, Sendai, Japan Xiaoting Jia, xjia@vt.edu Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA USA 1

2 Table of Contents: Supplementary Figure 1 Tuning the extensional stress of CNF composite fiber by varying draw speed and temperature during the drawing process. Supplementary Figure 2 Comparison of thermally drawn CNF composite and CPE electrodes prepared by liquid nitrogen fracture and imaged via SEM. Supplementary Figure 3 Images of the exposed inner area of the CNF composite via SEM during the course of FIB milling process. Supplementary Figure 4 Milling of a CNF composite electrode drawn at low stress (80 g/mm 2 ) via FIB. Supplementary Figure 5 Resistivity of the CNF composites drawn at different stresses. Supplementary Figure 6 Optical image of the drawn fiber sections with CNF composite and CPE electrodes, respectively. Supplementary Figure 7 Electrical impedances of the thermally drawn CNF composite and CPE. Supplementary Figure 8 Electrochemical impedance measurement of the thermally drawn CNF composite and CPE. Supplementary Figure 9 Two spike clusters indicating isolated two single unit action potentials and the principal component analysis at 15 weeks after the implantation. Supplementary Figure 10 Simultaneous electrophysiological recordings obtained at the surgical point by a single neural probe assembled with CPE electrode and CNF composite electrode with similar size. 2

3 Supplementary Figure 1 Tuning the extensional stress of CNF composite fiber by varying draw speed and temperature during the drawing process. (a) Adjustment of the drawing temperature during the process. (b) Adjustment of the drawing speed during the thermal drawing. (c) Stress experienced by the CNF composite fiber during the thermal drawing. (d) Size of the fiber measured during the drawing process. Fiber size is determined by the ratio of the draw speed in (b) to the feed speed (0.6 mm/min). 3

4 (a) (b) (c) (d) Supplementary Figure 2 Comparison of thermally drawn CNF composite and CPE electrodes prepared by liquid nitrogen fracture and imaged via SEM. (a, c) SEM images of CNF composite and (b, d) CPE. CPE has carbon black particles uniformly embedded into the polyethylene matrix, while CNF composite has CNFs with varied diameters and lengths that connect the individual carbon blacks along the longitudinal direction via its in situ unidirectional alignment during the drawing, which contributes to the significant decrease in the DC resistivity. (a) (b) Supplementary Figure 3 Images of the exposed inner area of the CNF composite via SEM during the course of FIB milling process. Scale bar is 5 µm. (a) and (b) show the aligned CNFs along the longitudinal direction indicated by the red dash line. (Scale bar: 5 μm) 4

5 Supplementary Figure 4 Milling of a CNF composite electrode drawn at low stress (80 g/mm 2 ) via FIB. (a) SEM showing the milling depth of the CNF composite in its transverse direction. (b-d) SEM of the exposed area during the milling at the irradiation angle of 52 with multiple aligned CNFs indicated by the red dashed line. (Scale bar: 5 μm) (e) The polar plot with angle of the exposed CNF with respect to the longitudinal direction and the radius defined as the distance of the exposed CNF to the surface. The green area is the angle zone smaller than 10 and most of the exposed CNFs falls into this area showing substantial alignment. 5

6 Supplementary Figure 5 Resistivity of the CNF composites drawn at different stresses. It indicates the higher degree of the CNF alignment occurs with higher drawing stresses. CNF composite CPE COC PC PC Supplementary Figure 6 Optical image of the drawn fiber sections with CNF composite and CPE electrodes, respectively. Both sections have similar cross-sectional size, CNF composite has size of 25.7 μm 16.6 μm and CPE has size of 37.2 μm 14.6 μm. Fibers with similar cross-sectional sizes were used for the resistivity measurement, electrochemical impedance measurement and electrical impedance measurement to compare their electrical performance. 6

7 Supplementary Figure 7 Electrical impedances of thermally drawn CNF composite and CPE. (a) Bode magnitude plot and (b) Bode phase plot of CNF composite and CPE samples. CNF composite and CPE samples were prepared with length of 1 cm and similar crosssectional areas as in Supplementary Figure 4, and copper wires were then connected to both terminals of the fiber samples with silver paint as electrical interfaces. A sinusoidal voltage of magnitude 10 mv and frequencies from 10 Hz to 10 khz was applied to the sample and its resulting current was measured (Interface 1000E, Gamry Instruments), which determined its ac electrical impedance. CPE has a preeminent ac impedance across the measured frequencies indicating the thermally drawn carbon blacks within the polyethylene matrix act as capacitors whereas the CNF composite has impedance of only about 300 kω and mainly exhibits resistance dominance which may attribute to the shorting of the carbon blacks capacitors by the aligned CNFs. Number of samples n=3. All shaded areas in the figure represent standard deviation. 7

8 potential (µv) PC2 Supplementary Figure 8 Electrochemical impedance measurement of thermally drawn CNF composite and CPE. (a) Bode magnitude plot and (b) Bode phase plot of the CNF composite and the CPE samples. Two-electrode configuration was adopted to mimic the electrophysiological experiments with samples as working electrode and stainless steel wire as counter and reference electrode. Samples of length 2 cm were prepared and their crosssectional areas are shown in Supplementary Figure 4. CNF composite has significantly smaller impedance across the measured frequencies compared to the CPE and its impedance is caused mainly by its tip surface electrochemistry, while CPE is mainly dominated by the capacitance behavior that is attributed to the bulk material from the sample. Number of samples n=3. All shaded areas in the figure represent standard deviation. (a) 40 (b) time (ms) Supplementary Figure 9 Two isolated single unit action potentials and the principal component analysis at 15 weeks after the implantation. It confirms the separation between the clusters by calculating the L-ratio of and the isolation distance of in the PC1- PC2 plane PC1 8

9 Supplementary Figure 10 Simultaneous electrophysiological recordings obtained at surgical point by a single neural probe assembled with CPE electrode and CNF composite electrode with similar size. This device was implanted into the hippocampal formation of the mouse and the CPE was not able to capture any spiking activities as shown in (a), while the CNF composite can record multi-unit activities shown in (b) with the detailed view of individual spikes in (c). 9