Designing an active robotic prosthesis can be a great challenge, having in mind how difficult it can be to provide sensory feedback. Skin is the largest organ of the body and is stretchable up to 75% strain. Considering covering prosthetics with electronic skins is to consider a design that allows free movements as much as a design easy to fabricate and replicate, minimizing its per-area cost [1].

Besides, users’ acceptance their artificial limb depends on how good the artificial limb can mimic the impaired limb, but also how easy it is to use it, including its weight, cost, durability and appearance [2], [3], [4], [5]. First, a skin-like coverage that looks and feels like the real skin may improve comfort and social acceptance. Second, providing sensory feedback from the artificial limb may improve the perception of ownership of the new limb. Additionally, completing the lacking limb with artificial sensory pathways can alleviate phantom limb pain. Third, sensory feedback is essential for optimal and successful manipulation. However, it may ease the utilisation of the prosthetic limb by restoring information about proprioception and grip forces. 

To meet these requirements, future electronic skin aims to improve the following key performance parameters: sensitivity, dynamic range, response time, relaxation time, and detection limit. In the field of force and strain transductions, various mechanisms are employed, with capacitive and resistive approaches being particularly common. Capacitive sensors rely on changes in capacitance, which occur when two electrodes separated by a dielectric layer experience movement. On the other hand, resistive sensors generate an output signal through either measuring the intrinsic material piezoresistivity or detecting changes in contact resistance between a conductor and an electrode.

In the present study, the primary aim was to enhance the sensitivity of a shear force sensor through scientific means. Previous research has demonstrated a decrease in the sensitivity of shear force detection when subjected to normal forces [6], [7]. Capacitive-type sensors are preferred over piezoelectric and triboelectric sensors due to their resistance to electromagnetic interference, and ease of fabrication. While resistive sensors depend on materials properties, capacitive sensors exhibit fast response times, low power consumption, and are suitable for both static and dynamic force sensing, making them ideal for electronic skin applications. To that purpose, a highly conductive flexible conductive polymer composite is developed in order to pattern the electrodes, and a reproducible fabrication process for a porous dielectric layer is under research.

References

[1] A. Chortos, J. Liu, and Z. Bao, “Pursuing prosthetic electronic skin,” Nat Mater, vol. 15, no. 9, pp. 937–950, 2016, doi: 10.1038/nmat4671.

[2] R. Dahiya, “E-Skin: From Humanoids to Humans [Point of View],” Proceedings of the IEEE, vol. 107, no. 2, pp. 247–252, Feb. 2019, doi: 10.1109/JPROC.2018.2890729.

[3] W. Navaraj, C. Smith, and R. Dahiya, “E-skin and wearable systems for health care,” in Wearable Bioelectronics, Elsevier, 2019, pp. 133–178. doi: 10.1016/B978-0-08-102407-2.00006-0.

[4] M. Soni and R. Dahiya, “Soft Eskin: Distributed touch sensing with harmonized energy and computing,” Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 378, no. 2164, 2020, doi: 10.1098/rsta.2019.0156.

[5] J. C. Yeo, J. Yu, Z. M. Koh, Z. Wang, and C. T. Lim, “Wearable tactile sensor based on flexible microfluidics,” Lab Chip, vol. 16, no. 17, pp. 3244–3250, 2016, doi: 10.1039/C6LC00579A.

[6] C. M. Boutry et al., “A hierarchically patterned, bioinspired e-skin able to detect the direction of applied pressure for robotics.,” Sci Robot, vol. 3, no. 24, pp. 1–10, Nov. 2018, doi: 10.1126/scirobotics.aau6914.

[7] J. Park et al., “Giant tunneling piezoresistance of composite elastomers with interlocked microdome arrays for ultrasensitive and multimodal electronic skins,” ACS Nano, vol. 8, no. 5, pp. 4689–4697, 2014, doi: 10.1021/nn500441k.