An ionic skin that could provide tactile sense and texture recognition to robots

Humans are able to touch and hold onto things in their environment with their fingertips, as well as sense the tactile properties of objects and other changes in the atmosphere. Fingertips are staggeringly delicate pieces of our skin, with interesting finger impression designs decorated on them.

Roboticists have been attempting to make progressively upgraded fake fingertips, as these could permit robots to separate the fine surfaces of items and possibly even screen for climate change in their current circumstances. While a few skin-like materials have been proposed throughout the course of recent years, the majority of these don’t successfully imitate the similar material capacities of people.

Specialists at Donghua College in China have recently presented another ionic skin that could be utilized to imitate the capability of fingertips in robots. This skin, which was first shown in a paper published in Advanced Materials, may one day enable robots to recognize various textures or materials and sense the tactile properties of objects.

“This may be too thin for future soft robotic applications. In future research, we hope to incorporate Liesegang patterns into an ultra-thin matrix to create thin-film artificial ionic skins.”

 Dr. Shengtong Sun, one of the researchers who carried out the study,

According to Dr. Shengtong Sun, one of the study’s authors, “This work was initially inspired by the fingertips and fingerprints that we use to touch and perceive the world every day.” Even though numerous “hard” biomimetic fingertips have been reported in recent years, designing artificial “soft” ionic skins with fingertip-like tactile capabilities still faces two major obstacles. One is the way to beat the strain impedance in material tension detection. Another example of high tactile sensitivity is the ability to recognize fine textures like fingertips.

Dr. Sun and his colleagues set out to overcome these two major obstacles that are typically encountered when developing ionic skins, or soft skins, made of ions. They first considered how they could imitate the structure and function of human fingertips by forming and characterizing fingerprint patterns on a soft material.

According to Dr. Sun’s explanation, “We noticed that fingertips are featured with a modulus-contrast fingerprint structure with periodic ridges and valleys.” The biochemical Turing effect is thought to be the cause of a non-equilibrium reaction-diffusion process that results in such an endogenic periodic pattern.”

The rigid ridges inside the fingertips are thought to function as mechanical microlevers that can quickly transmit tactile vibration signals to receptors in the dermis—the thick layer of tissue beneath the skin—when the fingertips are sensing the tactile properties of the objects they are touching. These ridges can maintain almost constant contact with surfaces or objects because they only slightly deform when in contact with them. Therefore, there is almost no strain distortion in the pressure sensitivity of the fingertips.

Dr. Sun stated, “We fabricated an aesthetic ionic skin also grown from a non-equilibrium Liesegang patterning process, inspired by these features of fingertips.” The ability to recognize textures and the fingertip-like strain-undisturbed pressure sensitivity of such an aesthetic ionic skin were discovered to be interesting properties.

Dr. Sun and his colleagues employed the Liesegang patterning technique, which involves precipitation reactions to create patterns in hydrogel materials, to create this aesthetic ionic skin. They mimicked the stiff ridges found in human fingertips by embedding them in an elastic hydrogel using this method.

According to Dr. Sun, “Liesegang patterning typically occurs when a precipitation reaction is coupled with the mass transport of reagents in porous gel media, resulting in periodic bands of precipitates.” By covering it with a fluoroelastomer layer, the Liesegang-designed ionic skin can go about as a profoundly touchy triboelectric material sensor. The triboelectric tactile sensor is able to sensitively detect dynamic pressure stimuli and recognize fine textures thanks to the modulus contrast of soft valleys and rigid ridges. It also shows almost unchanged pressure sensitivity even when stretched to the maximum failure strain (50%) of human skin.”

The specialists combined the designed ionic skin they made with another piezoresistive ionogel to emulate the natural multimodal material’s tactile framework significantly more, everything being equal. The resulting artificial skin-like tactile sensor showed promising results in a series of preliminary tests, sensing tactile information even when stretched.

“This work inventively presents the non-harmony response dispersion peculiarity into the plan of ionic detecting materials,” Dr. Sun said. “The resulting aesthetic ionic skin has stiff ridges embedded in a soft hydrogel matrix and features periodic Liesegang patterns. The ionic skin is given the intriguing tactile sensing properties of triboelectric-based strain-undisturbed pressure sensitivity and fine texture recognition that have only rarely been achieved in previous artificial ionic skins thanks to this modulus-contrast hybrid structure.

The hydrogel-based artificial tactile sensor developed by this group of researchers could be used in the future to create soft robotic skins or artificial fingers that can acquire dynamic and static pressure signals in real time as a robot grasps objects. Additionally, Dr. Sun’s and his colleagues’ work may serve as a model for the development of additional promising Liesegang-patterning ionic skins for robotics applications.

“The Liesegang-designed stylish ionic skin was manufactured on a hydrogel substrate, and the subsequent material has a specific thickness of around 2 mm,” Dr. Sun added. “This might not be thin enough for soft robotic applications in the future. To prepare thin-film artificial ionic skins, we will attempt to incorporate Liesegang patterns into an ultra-thin matrix in our subsequent research.

More information: Haiyan Qiao et al, Non‐equilibrium‐Growing Aesthetic Ionic Skin for Fingertip‐Like Strain‐Undisturbed Tactile Sensation and Texture Recognition, Advanced Materials (2023). DOI: 10.1002/adma.202300593

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