The human fingertip is an exquisitely sensitive instrument for perceiving objects in our environment via the sense of touch. A team of Chinese researchers has mimicked the underlying perceptual mechanism to create a bionic finger with an integrated tactile feedback system capable of poking at complex objects to map details below the surface layer, according to a recent paper published in the journal Cell Reports Physical Science .
“We were inspired by human fingers, which have the most sensitive tactile perception we know of,” said co-author Jianyi Luo of Wuyi University. “For example, when we touch our own body with our fingers, we can sense not only the texture of our skin, but also the outline of the bone beneath it. This tactile technology opens up a non-optical way of non-destructive testing of the human body and flexible electronics.”
According to the authors, previously developed artificial tactile sensors could only recognize and distinguish external shapes, surface textures and hardness. But they are unable to record subsurface information about these materials. This usually requires optical technologies, such as CT scanning, PET scanning, ultrasound tomography (which scans the outside of a material to reconstruct an image of its internal structure), or MRIs, for example. But all of these also have drawbacks. Similarly, optical profilometry is often used to measure the profile and finish of the surface, but it only works on transparent materials.
When we touch something with our fingers, the skin experiences mechanical deformation such as compression or stretching, which triggers mechanoreceptors to send out electrical impulses. These impulses travel through the central nervous system to the brain’s somatosensory cortex. The brain integrates these electrical impulses to identify the properties of the object we touch. The tactile feedback enables us to recognize a material’s shape, surface texture and stiffness or softness.
The smart bionic finger mimics this feedback system. A metal cylinder mounted on top of the finger acts as the contact tip, while carbon fiber beams act as tactile mechanoreceptors (the sensing unit). These are connected to a signal processing module. The finger “scans” the target object’s surface by periodically applying pressure, akin to a jab or jab. This compresses the carbon fibers, and how much the material compresses conveys information about its relative stiffness or softness. This information, along with where on the surface it was recorded, is then sent to a computer, which translates the data into a 3D map.
The authors put their bionic finger to the test using various complex objects. For example, they tested the finger’s ability to detect and map a rigid letter “A” just beneath a soft silicon layer (see video above), along with other abstract shapes. The fingers could even tell the difference between stiff and soft inner materials, and the soft outer silicone coating.
They also created a 3D-printed physical model of human tissue from three layers of hard polymer (for the “skeleton”) and a soft outer layer of silicone (for the “muscles”). The bionic finger scanned and successfully reproduced a 3D profile of the model tissue’s structure, including the location of a “blood vessel” located beneath the “muscle” layer.
Finally, the authors tested the bionic finger on a faulty electronic device, creating a map of its internal components. The finger could find out where the circuit was disconnected and identify a hole that was incorrectly drilled without breaking through the surrounding outer layer. “Next, we want to develop the bionic finger’s capacity for omnidirectional detection with different surface materials,” Luo said.
DOI: Cell Reports Physical Science, 2023. 10.1016/j.xcrp.2023.101257 (About DOIs).