A new publication from Opto-Electronic Advances; DOI 10.29026/oea.2022.210029 considers piezoresistive design for electronic skin.
Electronic skin (e-skin) represents one major flexible device which can mimic the diverse functions of human skin effectively, and holds promising prospect for applications in prosthetic, robot, and medical detection and diagnosis. Flexible tactile sensor that enables e-skin with an effective sensing capacity, is an important functional part for the e-skin to respond to the outside rich and diverse stimuli such as temperature, pressure, strain, bending, vibration and slippage. In this paradigm, the flexible tactile sensor designed based on piezoresistive effect has advantages such as low energy consumption, low cost, high sensitivity and wide detection range. As such, flexible piezoresistive sensor is becoming one of the research hotspots nowadays and finds a wide range of application with stable and scale-up manufacture.
Design of piezoresistive sensor is based on a principle of force-induced resistance change. Over the past decade, there are increasing research focused on the performance improvement of flexible piezoresistive sensor, and carried out systematical investigations for better sensitivity, detection range, linearity, stability, response as well as recovery time. For these purposes, advanced material fabrication technologies, such as electrospinning, lithography, laser processing, freeze-drying and 3D printing, are widely used in design and manufacture of flexible piezoresistive sensors. Generally, the flexible piezoresistive sensor used for e-skin requires multiple characteristics such as biocompatibility, flexible stretching and wireless signal transmission. For example, a combination of polymer with conductive material can realize the high sensing performance of piezoresistive sensor, which has both flexibility and extensibility. Meanwhile, the research on flexible piezoresistive sensor obtains breakthroughs and innovations in applications such as automatic alarm, visualization, and even for in vivo implantation and sign monitoring. These rapid developments make flexible piezoresistive sensor not only help prosthetics or robots to achieve tactile function, but also to obtain functions beyond human skin such as acoustic perception and micro-tactile operation of objects.
The authors of this article provide a systematic review on the design principle, sensing structure and application progress of flexible piezoresistive sensor. Firstly, this review introduced the principles of piezoresistive response based on different materials, which aimed to provide theoretical basis for designing a functional flexible piezoresistive sensor. The materials that can be used for piezoresistive sensor manufacture include metal materials, conductive carbon materials, semiconductors, conductive polymers and insulating polymers. The piezoresistive effect of metallic materials are mainly based on the change of geometric size, while the piezoresistance of semiconductor materials usually depends on altered carrier mobility under external forces. The two materials generally have great rigidity and require complex manufacturing process, thereby being less reported relatively for flexible piezoresistive sensors. At the same time, conductive composites have attracted extensive research. The basic principles of them involve percolation theory, tunneling mechanism, and the change of contact area and contact point.
Subsequently, the article systematically reviewed the key component of flexible piezoresistive sensor – sensing structure. Such structures can be designed as micro-protrusion, crack, fiber, pore and composite structures, which provided innovation for improved key performances of the flexible piezoresistive sensor. In this paradigm, the relationship between fabrication, structure and sensing performance was discussed detailly. For example, although the sensitivity of a sensor can be improved greatly by proper structural design, the flexible piezoresistive sensor is often failed to have a balance between a high sensitivity and a wide sensing range, due to the limitations by sensing mechanism and used materials. With composite structure design, the flexible piezoresistive sensor can achieve both high sensitivity and wide linear response range, although this usually accompanies with complex fabrication procedures. As reviewed in this article, the rapidly developed 3D printing technology can have advantages in the manufacture of a piezoresistive structure with complex design, which enables fast fabrication with a high precision and high efficiency to promote further developments of flexible piezoresistive sensor in future.
This article further summarized the application of flexible piezoresistive sensors in e-skin, such as for health detection, speech recognition, prosthetic and robot development. In the aspect of health detection, the flexible piezoresistive sensor can achieve a stable detection for both large-scale human movement and small force vital signs (such as pulse and heartbeat). The flexible piezoresistive sensor with a high sensitivity can respond to different characteristic peaks of speech and realize recognition of volume and timbre. In addition, flexible piezoresistive sensors can also be used in nursing and surgical robots, to help reduce unnecessary damage.
Finally, the authors summarized the main research directions on flexible piezoresistive sensors as reviewed in this article and discussed the future application prospects and challenges including the continuous pursuit of a high sensitivity while avoiding interfered noise, pursuit of a stable and long-term performance, and realization of portable wireless signal transmission and collection. Solutions to these problems and challenges definitely will give continuous and innovative developments on flexible piezoresistive sensor.
