The integration of the biological world with information networks necessitates highly accurate and safe bio-signal interfaces. Wearable, soft platforms with dry needle electrodes have shown promise in applications like visual potential decoding. Epidermal electronics offer a potential interface for continuous physiological and biochemical parameter collection. Electromyography (EMG), measuring electrical potential in muscle cells, is an intuitive human-computer interaction (HCI) interface, particularly surface EMG. Challenges in EMG detection include the high impedance of the skin's stratum corneum and motion artifacts. Conventional Ag/AgCl wet electrodes use conductive gel to reduce impedance, but this requires skin preparation, can cause allergic reactions, and dries out over time. Dry electrodes have been proposed to address these issues, but they suffer from high impedance. Microneedle electrodes, which penetrate the stratum corneum, have shown potential to overcome this limitation, maintaining stable impedance for long-term detection. Various materials and fabrication methods have been explored, but existing silicon-based microneedles lack sufficient stretchability and robustness for long-term, dynamic use. This paper presents a novel skin-integrated, biocompatible, robust, and stretchable silicon microneedle electrode (SSME) for long-term EMG monitoring during movement, inspired by the structure of plant thorns.

Existing literature highlights the challenges and progress in developing effective EMG monitoring technologies. Studies have explored wearable soft platforms for brain-computer interfaces and epidermal electronics for various physiological monitoring applications. While conventional wet electrodes provide good signal quality in short-term measurements, they suffer from limitations such as the need for conductive gel, potential for skin irritation, and impedance instability over time. Dry electrodes aim to address these issues but often struggle with high impedance due to the stratum corneum. Microneedle electrodes offer a promising alternative, minimizing skin-electrode contact impedance. Previous research has explored various microneedle materials and designs, including those utilizing SU-8, PDMS, stainless steel, and silicon. However, many of these designs lack the necessary stretchability and robustness required for long-term, in-motion EMG monitoring. The researchers reviewed these existing approaches and identified a need for a design that is both highly biocompatible and mechanically robust enough to endure repeated stretching and flexing of the skin.

The SSME design mimics the structure of plant thorns, using silicon microneedles, serpentine interconnects encapsulated in polyimide (PI), and a polyurethane substrate. The microneedles are partially encapsulated by PI to improve adhesion and reduce stress concentration at the base. The serpentine interconnects enhance stretchability. Fabrication begins with deep reactive ion etching (DRIE) to create silicon pillars, followed by isotropic etching to form microneedles. Oxygen plasma treatment improves adhesion before PI spin-coating and curing. The serpentine interconnects are created using a semi-additive process: sputtering a Ti/Au layer, patterning with photoresist and lithography, electroplating, and removing excess metal. A second PI layer is applied, patterned, and cured. Finally, the silicon substrate is removed, and the device is transferred to a polyurethane substrate. The mechanical properties were evaluated using tensile and fatigue tests, while biocompatibility was assessed via cytotoxicity tests using human keratin-forming cells (HaCaT). Electrode-skin impedance was measured using an impedance analyzer, and EMG signals were recorded during both static and dynamic (motion) conditions. Finite element analysis (FEA) was employed to simulate the microneedle insertion process and to evaluate the structural integrity of the SSME under stress.

The fabricated SSME demonstrated a stretchability exceeding 45%, with no cracks or delamination after 1000 cycles of stretching at 20% elongation. Cytotoxicity tests showed excellent biocompatibility with over 95% cell viability after 72 hours. Impedance measurements indicated comparable impedance to wet electrodes in the 40-1000 Hz range, but lower impedance in the 40-1000 Hz range compared to wet electrodes, and demonstrated significantly better long-term stability than wet electrodes. EMG signal recordings during both static and dynamic exercises showed comparable or superior performance to wet electrodes in terms of signal amplitude and signal-to-noise ratio (SNR). Specifically, the SSME consistently recorded EMG signals with higher amplitude and maintained signal quality significantly better than wet electrodes over a six-hour period, highlighting the device's suitability for long-term, in-motion EMG monitoring.

The SSME's superior performance compared to wet electrodes can be attributed to its design features. The microneedles effectively penetrate the stratum corneum, significantly reducing electrode-skin contact impedance and enabling more stable signal acquisition. The stretchable design and serpentine interconnects accommodate skin deformation during movement, minimizing motion artifacts. The biocompatibility ensures safe long-term wear. The high signal quality and long-term stability of the SSME are promising for applications in wearable healthcare monitoring, myoelectric prostheses, and advanced human-computer interfaces. The bioinspired design strategy is shown to be effective in enhancing the performance and durability of microneedle electrodes.

This study successfully demonstrated a biocompatible and highly stretchable silicon microneedle electrode for long-term EMG monitoring, outperforming traditional wet electrodes in terms of stability and signal quality during movement. Future work should focus on increasing microneedle density for higher spatial resolution, integrating signal processing chips onto the substrate, and exploring applications in disease diagnosis and treatment.

While the SSME shows significant improvement over existing technologies, there are areas for future enhancement. The current design uses a 6x6 array of microneedles which could be further optimized. While the study demonstrated the device's performance for a specific set of movements, additional testing across a wider range of activities would be beneficial. The long-term effects of repeated microneedle insertion on the skin were not fully explored beyond 48 hours, necessitating further investigation to fully assess long-term skin health after prolonged usage. Finally, larger scale studies with diverse populations would be needed to fully confirm the generalizability of the findings.