A simplified silicon microneedle design combines wafer-scale fabrication with nanoscale platinum engineering to address a key barrier to practical dry EEG electrodes.
Paper: Platinum-nanostructured silicon microneedles with scalable fabrication for EEG biosensing. AI-generated conceptual image created using ChatGPT/OpenAI
A new study published in the journal Microsystems & Nanoengineering presents a scalable approach for fabricating platinum-nanostructured silicon microneedle electrodes for potential wearable electroencephalography (EEG) monitoring. The researchers developed a simplified microneedle fabrication process and used platinum electrodeposition to create a high-surface-area nanostructured coating. The fabricated electrodes reduced impedance and improved EEG signal quality compared with non-nanostructured platinum-coated microneedles in proof-of-concept experiments, offering a promising platform for next-generation wearable electrophysiological monitoring.
The Need for Scalable Dry Electroencephalography Electrodes
Conventional wet electrodes use conductive gels to lower the impedance between the electrode and skin. Although this approach provides reliable signal acquisition, the gel can dry over time and may cause skin irritation. Wet electrodes also require lengthy preparation and cleaning. These limitations make them less suitable for wearable, long-term, and rapidly deployable monitoring systems.
Dry electrodes eliminate the need for conductive gels, but they face another challenge. The stratum corneum, the outermost layer of the skin, has high electrical impedance. This barrier can weaken electrical coupling and reduce the quality of weak biopotential signals. Microneedles offer a way to overcome this limitation by making shallow contact with tissue beneath the stratum corneum.
The study aimed to develop a silicon microneedle electrode that combines scalable fabrication with improved electrical performance for electroencephalography recording. It addressed two key challenges: simplifying silicon microneedle fabrication while reducing electrode impedance for weak biopotential recording.
Guiding Microneedle Design and Fabrication
The researchers selected silicon for its mechanical strength, precise manufacturability, and compatibility with established microfabrication technologies. To simplify fabrication, they combined mechanical dicing with anisotropic potassium hydroxide etching, followed by platinum electrodeposition to create a high-surface-area nanostructure.
The team first used finite element simulations to guide the selection of microneedle dimensions. A 3 × 3 silicon microneedle array was modeled under a total compressive force of 5 N, with the skin represented by elastic epidermal and dermal layers. The simulations examined the effects of microneedle height and array pitch on stress, deformation, and buckling stability. Heights of 500–700 µm and pitches of at least 1000 µm provided favorable mechanical performance. The simulations assessed mechanical stability and load transfer rather than explicitly modeling tissue rupture or penetration.
The researchers then fabricated the selected structures on a 4-inch silicon wafer using a 1.7 mm dicing grid. Mechanical dicing defined the microneedle geometry, after which 30% potassium hydroxide at 80 °C sharpened the structures. After nine to ten hours, the process produced microneedles approximately 700 µm high. Finally, the researchers deposited titanium and platinum layers before electrodepositing a nanostructured platinum layer. Cyclic voltammetry was used to measure the electroactive surface area before human-skin testing.
Nanostructured Platinum Enhances Electrode Performance
Platinum nanostructuring substantially increased the electrochemically active surface area of the microneedle electrodes. Non-nanostructured platinum-coated microneedles had an electrochemically active surface area of 0.17 ± 0.05 cm². After electrodeposition, this value increased to 2.52 ± 0.09 cm², representing a 15-fold increase. The roughness factor also increased from 0.34 ± 0.09 to 5.16 ± 0.18. Scanning electron microscopy (SEM) revealed densely packed, cauliflower-like platinum nanostructures across the microneedle surfaces and the surrounding base region.
The nanostructured electrodes also showed lower electrode-skin impedance. At 20 Hz, nanostructured platinum reduced the impedance of the microneedle electrodes from 151.0 kΩ to 95.5 kΩ. Although commercial silver/silver chloride wet electrodes achieved a much lower impedance of 10.8 kΩ, the reduction achieved through nanostructuring highlights the potential of surface modification to improve the electrical performance of dry microneedle electrodes without the need for conductive gels.
The researchers then evaluated brain signal acquisition using visual and auditory steady-state responses. During visual stimulation at 12.5 Hz, the microneedle electrodes recorded a clear response at that frequency. Across 15 recordings from a single subject, the nanostructured electrodes achieved a median signal-to-noise ratio of 18.33 dB, compared with 17.39 dB for non-nanostructured platinum-coated microneedles and 21.84 dB for silver/silver chloride electrodes.
For the auditory steady-state response measurements, the nanostructured platinum electrodes also produced higher signal-to-noise ratios than the non-nanostructured platinum-coated microneedles. At 40 Hz stimulation, they achieved a median signal-to-noise ratio of 17.26 dB, compared with 14.38 dB for the non-nanostructured microneedles. Silver/silver chloride electrodes recorded the highest median value of 23.10 dB. Nine repeated measurements were performed per electrode type. These proof-of-concept results suggest that platinum nanostructuring can enhance signal quality in dry microneedle electrodes across different brain-signal recording conditions.
Toward Scalable Wearable Electrophysiological Monitoring
The study demonstrates a practical route designed to support wafer-scale fabrication of dry electrodes for wearable electrophysiological monitoring. This approach reduces reliance on complex lithography and deep reactive ion etching, offering a potential pathway toward more efficient electrode production.
The nanostructured platinum coating improves electrode performance by increasing the electrochemically active surface area and is associated with lower electrode-skin impedance. These findings indicate that nanoscale surface engineering can improve the performance of dry electrodes while retaining their practical advantages. Although long-term monitoring was not evaluated in this study, the platform could support portable, long-term brain monitoring, particularly in settings where rapid electrode placement and reduced preparation are important. The researchers identify potential applications in electrocardiography, electromyography, and bioimpedance sensing.
Future studies should examine the durability of repeated insertions, long-term electrode stability, and dimensional variability. Although the arrays were inserted multiple times without observable structural damage, ex vivo and in vivo testing could provide additional validation of their mechanical performance. Integration with flexible substrates may improve skin conformity and reduce motion-related artifacts. Overall, the findings highlight the platform's potential as a promising candidate for next-generation wearable electrophysiological monitoring.
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