Soft, Multifunctional MXene-Coated Fiber Microelectrodes for Biointerfacing

Flexible fiber-based microelectrodes allow safe and chronic investigation and modulation of electrically active cells and tissues. Compared to planar electrodes, they enhance targeting precision while minimizing side effects from the device-tissue mechanical mismatch. However, the current manufacturing methods face scalability, reproducibility, and handling challenges, hindering large-scale deployment. Furthermore, only a few designs can record electrical and biochemical signals necessary for understanding and interacting with complex biological systems. In this study, we present a method that utilizes the electrical conductivity and easy processability of MXenes, a diverse family of two-dimensional nanomaterials, to apply a thin layer of MXene coating continuously to commercial nylon filaments (30–300 μm in diameter) at a rapid speed (up to 15 mm/s), achieving a linear resistance below 10 Ω/cm. The MXene-coated filaments are then batch-processed into free-standing fiber microelectrodes with excellent flexibility, durability, and consistent performance even when knotted. We demonstrate the electrochemical properties of these fiber electrodes and their hydrogen peroxide (H2O2) sensing capability and showcase their applications in vivo (rodent) and ex vivo (bladder tissue). This scalable process fabricates high-performance microfiber electrodes that can be easily customized and deployed in diverse bioelectronic monitoring and stimulation studies, contributing to a deeper understanding of health and disease.


Main factor analysis
Averaging resistances per parameter revealed that MXene concentration and filament diameter exert a more substantial influence than drawing speed, as evidenced by their steeper slopes and wider range coverage of resistances, following the factor analysis methodology (Figure S4, Supporting Information).To further illustrate parameter interactions, we depicted the mean resistance of one parameter on the x-axis, with a separate line for each level of a second parameter (Figure S5, supporting information).The multiple crossings or potential crossing points between the lines indicate intricate interactions among the three parameters.The interactions can be attributed to the complexity of the fluidics in the meniscus and the rheology of MXene.

Liquid-Crystalline MXene and Rocking Curve XRD Analysis
The calculations for liquid crystalline concentration were carried out following prior research, where the theoretical MXene concentration () at the isotropic-nematic transition, calculated by  = 1.03   , demonstrated good agreement with experimental data. 6In the calculation, a Ti 3 C 2 T x density () of 5.15 g/cm 3 was adopted.The aspect ratio () was obtained by dividing the MXene width () by its thickness (), where  is the intensity-weighted mean hydrodynamic size (Z average) of flakes from DLS and  is 1 nm.Following the example, based on the average flake size of the Ti 3 C 2 T x synthesized for this study, a theoretical MXene concentration of 16.6 mg/mL at the isotropic-nematic transition was calculated.This suggests the 110 mg/mL MXene solution has liquid crystalline ordering while the 10 mg/mL solution does not.We hypothesized that the spontaneous formation of a nematic liquid crystalline state in the 110 mg/mL solution would lead to coatings with superior flake alignment, akin to the results previously observed in wet spinning and blade coating processes. 4,6o confirm the hypothesis, we employed rocking curve XRD data to investigate the flake alignment (Figure S9, supporting information).The (002) peak of the 110 mg/mL MXene coated filament was more pronounced than that of the 10 mg/mL, indicating a coating with a higher degree of alignment is achieved with the elevated liquid crystalline forming concentration (Figure S10, supporting information).Such alignment, coupled with a thicker MXene deposit resulting from increased viscosity, contributes to a reduction in electrical resistance.

Figure S10.
Rocking chair XRD of 300 µm nylon filaments coated with a) 110 mg/ml and b) 10 mg/ml MXene solution.The two (002) peaks of 110 mg/ml and 10 mg/ml are nearly at the same position.An additional layer of water was observed in 110 mg/ml MXene-coated filaments.This is likely due to insufficient drying as the dried surface layer acts as a barrier for the moisture underneath.

Figure S11
. Linear resistance of MXene-coated nylon filaments and silver-plated nylon filaments (Shieldex®) as a function of the weight percentage of active material.Details can be found in Table S3.S4.

Voltage transients recorded for 28 µm MXene coated nylon fiber and troubleshooting of abnormal spikes
For the most accurate measurement of the voltage transients, the sampling rate of the Gamry Reference 600 potentiostat was set to the fastest possible rate of 3.33 µs.As a result, for low values of applied current, the potentiostat tended to slightly overshoot, resulting in spikes at points of steep change in signal amplitude.We can observe such overshoots in the applied biphasic current pulse for the 28 µm diameter MXene-coated nylon fibers (Figure S27a, red curve).Thus, the emergence of such overshoots at the fastest acquisition settings is responsible for the sharp peaks observed in the voltage transient (Figure 27a, blue curve).
For higher current amplitudes, the potentiostat does not generate any overshoots when applying the specified current pulses (Figure S27b-d, red curves).Therefore, no steep spikes are observed in the corresponding voltage transients (Figure S27b-d, blue curves).Reducing the speed of data acquisition might prevent an overshoot in the applied current amplitude.However, we will lose the temporal resolution necessary for accurately determining the cathodic and anodic potential excursions.

Figure S1 .
Figure S1.Ti 3 C 2 T x characterization.Measurements of a) flake size (n=3) and b) zeta potential (n=3) via dynamic light scattering (DLS).c) UV-vis of diluted Ti 3 C 2 T x solution (concentration < 0.01 mg/mL) and d) XRD pattern of freestanding Ti 3 C 2 T x film obtained from vacuum-assisted filtration, both confirming the successful synthesis of Ti 3 C 2 T x .

Figure S2 .
Figure S2.Rheology of MXene suspensions in water.a) Relationship between viscosity and shear rate for Ti 3 C 2 T x colloidal dispersions at concentrations ranging from 10 to 110 mg/mL.b) Digital images showing the consistency of Ti 3 C 2 T x dispersions at selected concentrations when scooped with a spatula.

Figure S3 .
Figure S3.Standard deviation versus average linear resistance of MXene-coated nylon filaments with their least squares regression line.

Figure S4 .
Figure S4.Main effects plots presenting the mean resistance for each parameter, thus combining the effects of the other two parameters, offering insight into the relative impact of each parameter on resistance.

Figure S5 .
Figure S5.Parameter interaction plots, with the mean resistances for the levels of one parameter presented on the x-axis and a separate line for each level of a second parameter.

Figure S6 .
Figure S6.The change in linear resistance and SD with Ti 3 C 2 T x concentrations for 10 mg/mL to 110 mg/mL for electrodes of different diameters from 100 to 300 μm.

Figure S7 .
Figure S7.SEM images and EDS maps of nylon filaments of a) 300 µm diameter coated with 110 mg/ml MXene solution, b) 300 µm diameter coated with 10 mg/ml solution, c) 100 µm diameter coated with 110 mg/mL solution.All samples were coated at the speed of 15 mm/s.The EDS images were net counts with brightness increased by 60 %, 70 %, and 60 %, respectively, to improve clarity.

Figure
Figure S9.a) MXene fiber placement and b) range of stage rotation (-4° to 4°) during rocking chair XRD measurement.

Figure S12 .
Figure S12.Bending stiffness of MXene-nylon-Parylene C fiber electrodes as a function of diameter compared to fiber electrodes of other materials in literature.Details can be found in TableS4.

Figure S14 .
Figure S14.SEM and EDS of cross-sectional images of MXene-nylon-Parylene C fiber electrodes before deployment, after the ends were cut with a fresh blade against a glass substrate.a) 300 µm diameter electrodes coated with 110 mg/ml MXene solution, b) 300 µm diameter electrodes coated with 10 mg/ml MXene solution, c) 100 µm diameter electrodes coated with 110 mg/ml MXene solution, d) 28 µm diameter electrodes coated with 110 mg/ml MXene solution.All filaments were coated at the rate of 15 mm/s.The EDS images are based on the net count of the elements and increased with 50% brightness for clarity.

Figure
Figure S17.a) CV and b) EIS spectra of 300 µm diameter electrodes (110 mg/mL MXene, 15 mm/s) compared before and after Parylene C encapsulation in 5 mM RuHex in 1 M KCl at 20 mV/s scan rate.

Figure S19 .
Figure S19.Phase angle of EIS of 100 mg/ml MXene coated electrodes of 28 µm, 100 µm and 300 µm diameter in 1X PBS.Data are plotted as means with shaded regions corresponding to SDs.

Figure S22 .
Figure S22.CV cyclability at 100 mV/s of 300 µm electrodes (110 mg/mL, 15 mm/s) under two windows: a) an extended voltage window of -1.3 V to 0.4 V and b) a narrowed voltage window of -1.1 V to -0.1 V, for 5000 cycles respectively.

Figure S23 .
Figure S23.CVs of 300 µm electrodes (110 mg/mL, 15 mm/s) at three conditions of 1) tested right after fabrication at a 35 mm immersion depth, 2) after 12 months of bench storage at the same 35 mm immersion and 3) after 12 months of bench storage at a reduced 2 mm immersion.All 3 tests were conducted in 1X PBS at a scan rate of 100 mV/s.

Figure S24 .
Figure S24.Electrochemical characterizations.a) CVs, b) impedance and c) phase angle of 110 mg/mL MXene coated electrodes of different diameters in 5 mM RuHex and 1 M KCl.

Figure S25 .
Figure S25.CV of a carbon fiber electrode in 5 mM RuHex and 1 M KCl at 20 mV/s.

Figure S26 .
Figure S26.Voltage transients measured as a response to biphasic current pulses for 110 mg/ml MXene-coated electrodes of a) 300 µm, b) 100 µm, and c) 28 µm diameter.Dotted black lines denote the safe operating window of -1.3 to 0.4 V.

Figure S27 .
Figure S27.Voltage transients recorded for 28 µm MXene-coated nylon fiber and troubleshooting of abnormal spikes.Measured voltage transients (blue) and applied current pulses (red) for a representative 28 µm diameter Ti 3 C 2 T x MXene coated nylon fiber at current amplitudes of a) 0.5 μA, b) 1.0 μA, c) 2.0 μA, and d) 5.0 μA.Arrows in a) denote the spikes in the applied current pulse and the corresponding peaks measured in the voltage transient.

Figure S28 .
Figure S28.The cathodic charge injection capacity CICc when normalized by active material area for electrodes of diameters of 300 um and 100 µm.

Figure S29 .
Figure S29.Photos of the In vivo experiment on a rat with a) 300 µm and b) 100 µm diameter electrodes.

Figure S30 .
Figure S30.Identifying the location of the stimulation artifact as recorded by the electrophysiology recording setup.

Figure S31 .
Figure S31.Evoked electromyography response under increasing stimulation amplitudes.The potential measured as a function of time for bipolar stimulation with a pair of 300 µm electrodes and EMG recorded with a 300 µm electrode.Grey plots denote the individual pulses (n=10), and the red plots denote the average of individual traces.

Figure S32 .
Figure S32.The potential measured as a function of time for bipolar stimulation with a pair of 100 µm electrodes and EMG recorded with a 100 µm electrode.Grey plots denote the individual pulses (n=10), and the red plots denote the average of individual traces.

Figure S33 .
Figure S33.a) Potential measured as a function of time for bipolar stimulation with a commercially available W electrode and EMG recorded with a commercially available electrode.Grey plots denote the individual pulses (n=10) and the red plots denote the average of individual traces.b) Measured peak-to-peak evoked EMG response as a function of increasing stimulation currents.

Figure S34 .
Figure S34.Optical images of 300 μm-diameter nylon filaments coated with a) 70 mg/mL V 2 C solution (average flake size 500 nm, zeta potential -36.4 eV) and b) a solution of 30 mg/mL graphene oxide (GO) and Ti 3 C 2 in a 1:1 weight percentage ratio.

1
Table S1 Linear resistance and filament conductivity of the dip coating parametric study.

Table S3 .
Resistance, conductivity and density comparison between MXene-coated nylon filaments and commercial silver-plated nylon filaments.

Table S4
Bending stiffness of MXene-nylon-Parylene C fiber electrodes compared to representative fiber electrodes in the literature that are made of other materials.