Supplementary Figure 2: Mechanical and electrical characterization of the multifunctional fibers.

(a) Bending stiffness measured for a multifunctional fiber, silica optical fiber and a stainless steel microwire. Stiffness was measured by a dynamic mechanical analyzer (DMA, Q800, TA Instrument) in a single cantilever mode with a displacement amplitude of 50 μm. Shaded areas represent standard deviation (n=3). (b) Impedance of gCPE electrodes following multifunctional fiber implantation into the mouse brain. Impedance decreased over the course of the first 3 days of implantation and remained stable over 3 months. Number of samples n=6, * p < 0.05, ** p <0.01, ***p < 0.001 determined by one-way ANOVA. 1 day: p = 0.7972, 2 days: p = 0.0153, 3 days: p = 0.0002, 1 week: p < 0.0001, 2 weeks: p = 0.0005, 1 month: p < 0.0001, 3 months: p = 0.0004, F_{7, 40} = 8.3. Error bars represent standard deviation. (c) Comparison of commercial CPE and gCPE composite electrodes. SEM images of (i, ii) commercial CPE and (iii, iv) composite graphite-doped gCPE electrodes. Scale bars are (i, iii) 10 μm and (ii, iv) 1 μm, respectively. Composite gCPE electrodes exhibit higher porosity and surface roughness than commercial CPE, which may contribute to the decrease in impedance observed in gCPE electrodes following a soak in a salt solution. (d) Impedance of commercial CPE and gCPE composite electrodes within multifunctional fibers. The impedance was measured when a sinusoidal voltage is 10mV, and Vpp is applied at 1 kHz using a LCR meter (HP4284A, Agilent Technologies). The impedance for commercial CPE electrodes increased proportionally to their length, dominated by sheet-resistance of the material. The impedance of gCPE electrodes does not have proportional correlation with the length likely dominated by the tip geometry. Shaded areas represent standard deviation (Number of samples n=6). (e, f) Evaluation of electrical insulation within fiber probes. (e) An illustration of experimental setup for evaluation of electrical insulation. Impedance of gCPE electrodes was measured for the fibers in surface contact with phosphate buffered saline (PBS) solution and for the fibers immersed into PBS by 1-3 cm. Fiber length was fixed to 5 cm. (f) Impedances of gCPE electrodes in the multifunctional fiber measured for the scenarios in (e) at 1 kHz. No significant differences were identified for the immersed devices as compared to the devices in contact with PBS surface (Number of samples n=6, One-way ANOVA, p = 0.992, F_{3, 20} = 0.0323. Error bars represent standard deviation. (g) Evaluation of chemical stability of gCPE electrode surfaces in PBS. Cyclic voltammetry (CV) curves collected for a gCPE electrode within a multifunctional fiber at scan rates between 2-100 mV/s. No peaks corresponding to reduction or oxidation are observed within a voltage window exceeding that of extracellular potentials.