Highly stretchable and customizable microneedle electrode arrays for intramuscular electromyography

Abstract

Stretchable three-dimensional (3D) penetrating microelectrode arrays have potential utility in various fields, including neuroscience, tissue engineering, and wearable bioelectronics. These 3D microelectrode arrays can penetrate and conform to dynamically deforming tissues, thereby facilitating targeted sensing and stimulation of interior regions in a minimally invasive manner. However, fabricating custom stretchable 3D microelectrode arrays presents material integration and patterning challenges. In this study, we present the design, fabrication, and applications of stretchable microneedle electrode arrays (SMNEAs) for sensing local intramuscular electromyography signals ex vivo. We use a unique hybrid fabrication scheme based on laser micromachining, microfabrication, and transfer printing to enable scalable fabrication of individually addressable SMNEA with high device stretchability (60 to 90%). The electrode geometries and recording regions, impedance, array layout, and length distribution are highly customizable. We demonstrate the use of SMNEAs as bioelectronic interfaces in recording intramuscular electromyography from various muscle groups in the buccal mass of Aplysia.

Fig. 1.. SMNEAs.

(A) Schematic illustration of an SMNEA device before and after uniaxial stretching. The SMNEA device consists of polymer microneedles with conductive and insulation coatings, electrical interconnects in serpentine layout, and a stretchable silicone substrate. (B and C) Optical images of an SMNEA device on a glass substrate at (B) low and (C) high magnification, showing the microneedle electrodes with serpentine interconnects. (D) Side-view optical image of an SMNEA with varying microneedle lengths, ranging from approximately 800 to 1500 μm. (E) Angled-view optical image of a 6 × 6 array of microneedle electrodes with varying lengths on a stretchable silicone substrate. (F) Optical image of a 6 × 6 array of microneedle electrodes under stretching and twisting. Scale bars, 5 mm (B), 1 mm (C and D), and 3 mm (E and F).

Fig. 2.. Schematic illustration of steps for fabricating SMNEAs.

Fig. 3.. Control of the electrode recording region and electrical impedance.

(A) Schematic illustration of steps for fabricating a microneedle electrode with the conductive tip exposed. (B) Side-view microscopic images of gel-assisted etching of a copper hard mask at the tip of a microneedle. (C) SEM image of an Au-coated microneedle tip after selectively etching the parylene coating at the tip. (D) Statistics of exposed tip lengths from two groups of microneedles with target exposed tip lengths of 80 and 140 μm, respectively. (E) SEM image of a microneedle tip after electrochemical deposition of PtB at the tip. (F) Electrode impedance spectra and average electrode impedance at 1-kHz scanning frequency before and after electrochemical deposition of PtB at the microneedle tip in 0.1 M phosphate-buffered saline (PBS). Error bars correspond to the calculated standard deviation from 12 electrode measurements. (G) Impedance of the electrode with PtB tip at 1 kHz under cyclic insertion into agarose gel. Scale bars, 200 μm (B) (50 μm in inset), 10 μm (C), and 30 μm (E) (1 μm in inset).

Fig. 4.. Stretchability of SMNEAs.

(A) Optical images and the corresponding FEA results showing the maximum principal strain distribution in the PI of an SMNEA device in relaxed state, under stretching, and under a combination of stretching and twisting. Insets show the locations of the maximum values of the maximum principal strain distributions. (B) Side-view optical images of an SMNEA device under uniaxial stretching up to 100%. (C) Impedance of the electrodes in the SMNEA device as a function of the tensile strain applied. (D) Comparison of the SMNEA in this work with previously reported flexible or stretchable microneedle electrode arrays in the microneedle modulus and the device stretchability. Scale bars, 5 mm [A and B (left)] and 2 mm [B (right)].

Fig. 5.. Ex vivo recordings of intramuscular and surface EMG from the buccal mass of Aplysia using SMNEA and planar MEA devices.

(A) Schematic illustration of a retraction movement cycle in the buccal mass. (B and C) Optical image of a buccal mass with an SMNEA device and the corresponding dimensional changes during a retraction movement cycle. R represents the diameter of the buccal mass at the anterior, and L represents the length from the mouth to the esophagus. (D) Confocal microscopic images showing the microneedle electrodes inserted into the I1/I3 and I2 muscle groups of the buccal mass. (E) Optical image with identification of each microneedle electrode inserted into the buccal mass. (F and G) Intramuscular EMG and surface EMG signals recorded by (F) the SMNEA and (G) the planar MEA, respectively. MN, microneedle electrodes; P, planar electrodes. (H and I) Power spectra of the intramuscular EMG and surface EMG signals averaged across all recording channels from (H) the SMNEA and (I) the planar MEA. Scale bars, 5 mm (B and E) and 2 mm (D).