Industrial Electronics

Flexible tactile sensors for wearable tech

21 September 2022
This flexible tactile sensor measures the constituents of sweat on the skin. Source: University of California San Diego

Tactile perception is vital to the fundamental perceptual ability of human skin and is one of the primary ways in which most people obtain data from the environment. During the first several weeks after birth, a baby's eyes are not developed enough to clearly perceive the mother's face. The tactile sensing ability of an infant, on the other hand, is remarkably responsive to the mother's skin. .

Tactile sensing ability is incorporated into a variety of social relationships, such as handshakes and hugs. With the speedy advancement of flexible electronics, robotic technologies, and artificial intelligence (AI) in recent years, the demand for friendly and safe communication between humans and machines has grown. A man-made tactile sensor transforms external stimuli into quantifiable or recordable signals. The majority of stimuli are generated through physical interaction, and a tactile sensor, like human skin, is anticipated to recognize strain, pressure, temperature and even moisture.

How are flexible tactile sensors different?

The development of materials for sensing, new manufacturing techniques and electrical sensing phenomena has resulted in noteworthy advancements in flexible tactile sensors. Certain flexible tactile sensors have exhibited extreme flexibility, superior sensitivity, high stretchability, ultra-conformity and cost-effectiveness, in addition to large-area manufacturing. Several flexible tactile sensors, in particular, equipped with sophisticated sensing materials and efficient electrodes, show sensing abilities that exceed those of human skin. Such distinguishing characteristics of tactile sensors assist and support uses in monitoring human activities, healthcare and AI.

Considerations when designing flexible tactile sensors

Wearable technology incorporating flexible tactile sensors with a high degree of sensitivity, stretchability, latency and stability necessitates novel material design and structural engineering approaches. It should be possible to provide high-quality materials, new manufacturing processes and revolutionary device design requirements that can fulfill a variety of needs in practical applications. Particular attention should be paid to how a device is constructed, its manufacturing costs and its power consumption.

The sensing mechanism plays an important role when constructing a flexible tactile sensor. For example, the sensing efficiency of the majority of piezoresistive tactile devices is normally characterized by the bulk material's mechanical characteristics. Due to the relatively large modulus and viscoelasticity, the sensor has a low sensitivity and a slow response time. A potential solution to this problem is to employ micro/nanopatterned structures, which can significantly alter contact resistance in response to outside stimuli. Numerous design methods dependent on bionic micro/nanostructures are known to improve the mechanical compliance, selectivity, sensitivity and latency rate of tactile sensors.

For practical applications, it is also critical to fabricate flexible tactile sensors consuming low power. The piezoelectric and triboelectric sensing mechanisms enable the development of self-powered tactile sensors. Significant advancements have been made in the exploration of related functional materials and the fabrication of self-powered tactile sensing products. The micropatterned plastic film-based triboelectric device is an intriguing example. A self-powered pressure tactile sensor can successfully induce voltage in response to an outside stimulus and identify a falling feather or water droplet. Such exceptional properties, combined with the capability to generate power upon deformation, open up a world of possibilities for self-powered micromechanical elements.

To achieve the flexible tactile sensor's low manufacturing cost, a simple device framework and effective fabrication techniques are preferred. Numerous simple-structured tactile sensors with cost-effective frameworks have been developed. A micropatterned surface can help tactile sensors perform better. However, the traditional approach to microstructure acquisition relies heavily on lithographic processes that are generally complex, costly and time-consuming. Nonetheless, to solve such problems, many interesting methods have been proposed. For example, micropatterned films have been shown to be molded using a silk textile and even a lotus leaf. Such structures exhibited extremely high sensitivity, high stability, a short response time and repeatability.

Desirable characteristics in flexible tactile sensors

To accurately mimic the complicated features of the sensing abilities of the human body, tactile sensors must be capable of discriminating between different mechanical stimuli, such as shear, torsion, stretching and bending that are typically generated during the interaction of the human and its environment, in addition to responding to exerted pressure. Besides mechanical stimuli, tactile sensors for a variety of physical stimuli are also in demand, including temperature, biological and chemical variables, and humidity.

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Tactile sensors are a new class of electronic components that enable machines to interact with their environment. Conformable flexible tactile sensors have enabled friendly interaction between machines and humans, or between machines. When designing a particular flexible tactile sensor, special attention should be paid to device construction, manufacturing costs and power consumption.

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