Ultrahigh-transparency and pressure-sensitive iontronic device for tactile intelligence

Tactile sensing, mimicking human mechanoreception, is a crucial area of research and development. Integrating optical transparency into tactile sensors offers the potential for combined tactile and visual intelligence, leading to applications in medical imaging, health monitoring, and human-machine interfaces. For example, a transparent tactile endoscope could provide real-time feedback during procedures, while transparent wearable electronics could enable continuous and imperceptible physiological signal detection. However, achieving both high transparency and sensitivity simultaneously remains challenging. Improving light transmittance typically involves using materials with low absorption and scattering. While selecting and modifying materials can minimize absorption and scattering loss, interfacial light reflection remains a significant obstacle, particularly with the coarse interfaces often needed for high sensitivity. High sensitivity, however, typically requires engineering interfaces with high surface areas, leading to increased roughness and hence reduced transparency. The flexible iontronic sensing mechanism (FITS) offers a potential solution, as its building materials can be intrinsically transparent, and it can possess an optically smooth sensing interface. Previous iontronic sensors, however, suffered from limited transparency due to optical loss at interfaces. This work introduces a new architecture for the TIS device that addresses these challenges.

Existing research on transparent flexible tactile sensors highlights a trade-off between optical transparency and device sensitivity. Methods to enhance sensitivity, such as creating rough interfaces (pyramidal, micro-needle, nanofibrous, or bioinspired structures), often reduce transparency. Studies focusing on improving transparency primarily modify the intrinsic optical transparency of materials, achieving high transmittance (e.g., 99.94%) for individual components but struggling to maintain high overall device transparency due to interfacial reflections. The introduction of the flexible iontronic sensing mechanism (FITS) showed promise, with early devices achieving 77% transmittance. Subsequent improvements, utilizing porous PVDF membranes filled with refractive index (RI)-matched ionic liquids, achieved 94.8% material transmittance but only 90.4% overall device transmittance, along with low sensitivity (1.2 kPa⁻¹). The state-of-the-art in touch panel technology already achieves >95% light transmittance, highlighting the potential for improvement in tactile sensing.

The proposed TIS device features a two-layer architecture. The ionode layer consists of a transparent silver nanowire (AgNw) conductive film coated with an array of microscopic hemispherically shaped transparent ionic elastomer. The counter electrode is a pristine AgNw surface. A non-ionic RI-matching liquid fills the gap between the ionode and the electrode, eliminating air-solid interfaces and minimizing reflections. The ionic gel is a photo-crosslinkable polymeric gel containing 1-ethyl-3-methylimidazolium triflate (EMIMOTF) dispersed in a polymer matrix of poly(ethylene glycol) diacrylate (PEGDA) and hydroxyethyl methacrylate (HEMA). The ratio of HEMA, PEGDA, and EMIMOTF is optimized to achieve high transparency, elasticity, and ionic conductivity. The RI-matching liquid, a mixture of silicone oil and liquid paraffin, is carefully formulated to match the RI of the ionic gel and maintain chemical stability. The sensitivity is optimized by controlling the geometrical parameters of the hemispherical array (diameter and density). Several characterization techniques were employed, including UV-Vis spectrophotometry for light transmittance measurement, LCR meter for capacitance measurements, a dynamometer for pressure application, and a piezoelectric actuator for dynamic response testing. The TIS device was integrated into three different applications: a 3D touchscreen, an endoscope, and a wearable health monitor. For the endoscope application, tissue stiffness was measured by analyzing the slope rate of the force-displacement curve and comparing it to calibration samples with known Young's moduli.

The TIS device demonstrated an overall optical transmittance of 96.9%, the highest reported value in the literature. The sensitivity reached 83.9 kPa⁻¹ in the pressure range of 0-20 kPa, three orders of magnitude higher than capacitive devices. The response and reset times were 61 ms and 50 ms, respectively, while the pressure resolution was 10 Pa. The device exhibited high repeatability, with less than 10% signal variation after 5000 cycles. The integration of the TIS device into a 3D touchscreen allowed for high-resolution pressure mapping during writing and object recognition. In an endoscope, the TIS device allowed for quantitative assessment of tissue stiffness, distinguishing between normal pancreatic tissue and pancreatic cancer tissue. The wearable TIS device successfully detected high-fidelity arterial pulse waveforms, enabling imperceptible health monitoring.

The TIS device successfully addresses the long-standing challenge of simultaneously achieving high transparency and sensitivity in flexible tactile sensors. The refractive index matching strategy eliminates interfacial reflections, maximizing transparency without compromising the sensitivity provided by the micro-hemispherical array. The high sensitivity enables the detection of subtle pressure variations, while the high transparency preserves the visual properties of the underlying system. The device's performance across diverse applications (touchscreen, endoscope, wearable sensor) demonstrates its versatility and broad applicability. The quantitative assessment of tissue stiffness using the TIS-enabled endoscope offers a promising advancement in minimally invasive diagnostics.

This study presents a novel TIS device with unprecedented levels of transparency and sensitivity, paving the way for more advanced and versatile human-machine interfaces in both medical and industrial applications. Future research could focus on expanding the range of detectable pressures, improving response times further, and exploring additional applications of this technology, such as advanced robotics and virtual reality interfaces.

While the device demonstrates excellent performance, several limitations exist. The response time, though adequate for many applications, is not as fast as some existing iontronic sensors. The fabrication process might need further optimization for large-scale production. The current study focused on specific applications; further research is needed to evaluate the device's performance in a broader range of environments and conditions.