Electronic skin (e-skin) is a significant flexible technology that can accurately replicate the various functions of human skin. It has remarkable potential for prosthetic, robot, and medical detection and diagnosis devices.
The developmental history of e-skin. Image Credit: Opto-Electron Advances
A crucial functional component for the e-skin to respond to externally rich and diverse stimuli such as temperature, pressure, strain, bending, vibration, and slippage is a flexible touch sensor that gives it an excellent sensing ability.
Based on the piezoresistive effect, the flexible tactile sensor in this paradigm has low energy consumption, is inexpensive, has high sensitivity, and broad detection limit. Thus, flexible piezoresistive sensors are finding a wide range of applications and are becoming one of the hottest research topics because of reliable and scale-up manufacturing.
The principle of force-induced resistance change forms the foundation of piezoresistive sensor design. Over the past ten years, increased research has been aimed at improving the performance of flexible piezoresistive sensors.
Systematic studies have been conducted for greater sensitivity, detection limit, uniformity, consistency, responsiveness, and recovery time. For these reasons, flexible piezoresistive sensors are designed and produced using cutting-edge material manufacturing techniques like electrospinning, lithography, laser processing, freeze-drying, and 3D printing.
The flexible piezoresistive sensor used in e-skin often must meet several requirements, including biocompatibility, flexibility when stretched, and wireless signal transmission. For instance, combining polymer with conductive material can achieve the high sensing performance of a piezoresistive sensor, which has both flexibility and expandability.
While this is going on, advancements and improvements in the field of flexible piezoresistive sensor research are being made in areas such as automatic alarm, visualization, and even in vivo implantation and sign monitoring.
These quick advancements enable flexible piezoresistive sensors to perform tasks beyond human skin, such as sound perception and micro-tactile operation of objects, assisting prosthetics on the other hand, and robots in achieving tactile function.
The researchers offer a thorough analysis of the flexible piezoresistive sensor’s design philosophy, sensing architecture, and development of application.
The current review began by introducing the fundamentals of piezoresistive response based on various materials to offer a theoretical foundation for creating a sound flexible piezoresistive sensor. Metals, conductive carbon compounds, semiconductors, conductive polymers, and insulating polymers are among the materials that can be utilized to make piezoresistive sensors.
While the piezoresistance of semiconductor materials typically relies on modified carrier mobility under external factors, the piezoresistive effect of metallic materials is primarily based on a change in geometric size.
The two materials are often quite hard and complicated to manufacture, making them less common for flexible piezoresistive sensors. Conductive composites have also received a lot of research. Percolation theory, tunneling mechanism, and the alteration of contact area and contact point are some of their fundamental concepts.
The review then thoroughly examined the vital part of the flexible piezoresistive sensor, the sensing structure. These structures, which can be created as micro-protrusion, crack, fiber, pore, and composite structures, provide innovation for bettering the key functionalities of the flexible piezoresistive sensor. The link between fabrication, structure, and sensing performance was thoroughly covered in this paradigm.
For instance, even though proper structural design can significantly increase a sensor’s sensitivity, flexible piezoresistive sensors frequently fail to strike a balance between high sensitivity and a broad detection range because of the limitations imposed by the sensing mechanism and materials used.
Although this typically comes with complex fabrication processes, flexible piezoresistive sensors can achieve excellent sensitivity and a wide linear response range with composite structure design.
As discussed in this article, the rapidly evolving 3D printing technology can be advantageous in producing a piezoresistive structure with complicated design, enabling quick fabrication with high precision and efficiency to support future advances in flexible piezoresistive sensors.
The article's summary covered the use of flexible piezoresistive sensors in e-skin for prosthesis and robot development, voice recognition, health monitoring, and other purposes. The flexible piezoresistive sensor can detect large-scale human movement and small-force vital signs in health monitoring (such as pulse and heartbeat).
The adaptable, highly sensitive piezoresistive sensor can respond to the various characteristic peaks of speech and enable volume and timbre detection. Adjustable piezoresistive sensors can also be used in medical and surgical robots to prevent needless injury.
Eventually, the authors summarized the main directions for future research on flexible piezoresistive sensors covered in this article and talked about potential applications and challenges, like maintaining high sensitivity while avoiding interference noise, pursuing stable and long-term performance, and realizing portable wireless signal transmission and collection.
Solving these issues and difficulties will undoubtedly lead to ongoing, creative improvements in flexible piezoresistive sensors.
Journal Reference:
Zhong, F., et al. (2022) Piezoresistive design for electronic skin: from fundamental to emerging applications. Opto-Electron Advances. doi.org/10.29026/oea.2022.210029.