Tactile sensing is crucial for robotic haptics. Traditional approaches using sensor arrays to directly measure multiple physical quantities face challenges in manufacturing complexity and limited sensor types. An alternative is detecting fields of tactile-relevant quantities like stress fields. Previous studies have used optical methods or the sensing material itself as a field carrier, but these methods lack accuracy or are unsuitable for complex environments. Human skin, with its layered structure and mechanoreceptors like Meissner corpuscles, efficiently transforms complex forces into an internal stress field. This inspired the development of biomimetic mechanoreceptors (BMRs), an electronic skin design mimicking the structure of human skin. BMRs consist of a deformable elastomer layer sandwiched between epidermal and dermal sensing arrays, sampling the stress field distribution. This approach enables precise 3D force measurement and object hardness distinction, eliminating the need for multiple sensors. The stress field sensing principle allows for a modular design, enabling easy fabrication, integration onto curved surfaces, and replacement of components for specific applications.

The authors review existing electronic skin technologies, highlighting the limitations of traditional multi-sensor array approaches. These limitations include complex manufacturing processes, restricted sensor types, and the need for high sensor density to achieve accurate measurements. The review then examines alternative field-sensing approaches, such as optical methods and methods utilizing the sensing material as a field carrier. These approaches, however, are criticized for their limited accuracy, applicability to complex environments, or lack of accordance with human sensory perception. The authors emphasize the superior performance of human skin's stress field sensing mechanism as inspiration for their proposed solution.

The BMRs utilize a sandwich structure: an elastomer deformation layer (Ecoflex Gel) sandwiched between epidermal and dermal sensing arrays. The sensing arrays consist of TPU/CB foam pressure-detecting units, mimicking Meissner corpuscles. These units are isolated within patterned silicone structures for stability under deformation. The fabrication process involves several steps: creating porous TPU/CB films, laminating them with Cu/PI layers onto PDMS substrates, laser etching serpentine interconnects and island-bridge structures, dispensing patterned silicone around sensing units, and finally, assembling the epidermal and dermal arrays with the Ecoflex Gel deformation layer. 3D force detection is achieved by analyzing the pressure distribution measured by the epidermal and dermal arrays. The center of gravity of the pressure distributions is calculated, and the deviation vector between the centers of gravity represents the 3D force vector, where length corresponds to magnitude and direction to angle. Hardness sensation is assessed by analyzing the pressure sum and diffusion length (a measure of pressure distribution concentration) during indentation with objects of varying hardness. Modularity is demonstrated by intentionally misaligning the arrays, and showing that the affine transformation caused by the misalignment is correctable, enabling the application of the e-skin to curved surfaces. Finite element analysis using COMSOL 5.4 was also employed to simulate the behavior of the elastomer under applied forces. The experimental setup includes a custom-built 3D force testing platform and a 16x16 readout circuit for data acquisition.

The BMRs demonstrate high accuracy in 3D force detection, achieving a polar angle resolution of 1.8° and an azimuthal angle resolution of 3.5°, a significant improvement in spatial resolution (up to 71-fold) compared to sensor spacing. The system's ability to accurately measure 3D forces even with random assembly of sensing units or when applied to curved surfaces highlights its robustness and modularity. The BMRs also successfully distinguish objects of different hardness based on pressure sum and diffusion length analysis. The analysis of the stress field provides redundant hardness-related information, ensuring fast and accurate identification. The modularity of the BMRs was demonstrated through the successful calibration and accurate measurement of 3D forces even when the sensor arrays were misaligned. The e-skin's functionality was further validated by its successful application on the curved surfaces of a robotic arm and elbow.

The findings demonstrate the effectiveness of the stress field sensing approach in creating a simple yet highly accurate and versatile multi-parameter electronic skin. The significant improvement in spatial resolution achieved by analyzing the stress field simplifies the design and reduces the number of sensors required compared to traditional methods. The ability to measure 3D forces and object hardness, combined with the modular and adaptable design, enhances the potential applications of this e-skin in robotics and other fields. The demonstrated robustness to misalignment and suitability for curved surfaces broadens its applicability and practicality.

This research successfully developed a bio-inspired electronic skin (BMRs) based on stress field sensing. The BMRs achieve high accuracy in 3D force and hardness detection, demonstrating a significant advancement in tactile sensing. Its modular design allows for flexible fabrication and integration into various applications. Future research could explore the integration of other sensing modalities (temperature, humidity) within this framework, or investigate more advanced algorithms for stress field reconstruction and data analysis.

While the study demonstrates significant advancements, limitations include the specific materials and fabrication methods used, which may need optimization for specific applications. The current design might have limitations in detecting very high forces or extremely subtle tactile stimuli. Further research is necessary to explore the long-term stability and durability of the BMRs under extended use.