Researchers at Ecole Polytechnique Fédérale de Lausanne’s (EPFL) Reconfigurable Robotics Lab (RRL), headed by Jamie Paik, PhD, and Laboratory for Soft Bioelectronic Interfaces (LSBI), headed by Stéphanie Lacour, PhD, at the School of Engineering, have teamed up to develop a soft, flexible artificial skin made of silicone and electrodes. Both labs are part of the NCCR robotics program.

Technology capable of replicating our sense of touch, also known as haptic feedback, can greatly enhance human-computer and human-robot interfaces for applications such as medical rehabilitation and virtual reality.

The skin’s system of soft sensors and actuators enable the artificial skin to conform to the exact shape of a wearer’s wrist, for example, and provide haptic feedback in the form of pressure and vibration. Strain sensors continuously measure the skin’s deformation so that the haptic feedback can be adjusted in real time to produce a sense of touch that’s as realistic as possible.

The scientists’ work (“Closed-loop haptic feedback control using a self-sensing soft pneumatic actuator skin“) was recently published in Soft Robotics.

“In this article, we achieve a closed-loop control over haptic feedback, the first time for an entirely soft platform. We prototyped a novel self-sensing soft pneumatic actuator (SPA) with soft strain sensors, called SPA-skin, which withstands large multiaxial strains and is capable of high-frequency sensing and actuation. To close-loop control the haptic feedback, the platform requires a cohesively integrated system. Our system consists of a stretchable low profile (<500 μm) SPA and an ultra-compliant thin-metal film strain sensor that creates a novel bidirectional platform for tactile sensing via force-tunable vibratory feedback. With this prototype, we demonstrated control of the actuator shape in real time up to 100 Hz at output forces up to 1 N, maintained under variable mechanical loadings,” the investigators wrote.

“We further characterized the SPA-skin platform for its static and dynamic behavior over a range of actuation amplitudes and frequencies as well as developed an analytical model of this system to predict the actuator inflation state only using the embedded sensor’s resistance. Our SPA-skin is a multifunctional multilayer system that can readily be implemented as a high-speed wearable bidirectional interface for contact sensing and vibrotactile feedback.”

Artifical skin
The skin’s system of soft sensors and actuators enable the artificial skin to conform to the exact shape of a wearer’s wrist. [EPFL]
“This is the first time we have developed an entirely soft artificial skin where both sensors and actuators are integrated,” said Harshal Sonar, PhD, the study’s lead author a doctoral candidate at EPFL. “This gives us closed-loop control, which means we can accurately and reliably modulate the vibratory stimulation felt by the user. This is ideal for wearable applications, such as for testing a patient’s proprioception in medical applications.”

The artificial skin contains soft pneumatic actuators that form a membrane layer which can be inflated by pumping air into it. The actuators can be tuned to varying pressures and frequencies (up to 100 Hz, or 100 impulses per second). The skin vibrates when the membrane layer is inflated and deflated rapidly. A sensor layer sits on top of the membrane layer and contains soft electrodes made of a liquid-solid gallium mixture. These electrodes measure the skin’s deformation continuously and send the data to a microcontroller, which uses this feedback to fine-tune the sensation transmitted to the wearer in response to the wearer’s movements and changes in external factors.

The artificial skin can be stretched up to four times its original length for up to a million cycles. That makes it particularly attractive for a number of real-world applications. For now, the scientists have tested it on users’ fingers and are still making improvements to the technology.

artificial skin video snapshot
Future applications include rehabilitation and virtual and augmented reality. [EPFL]
“The next step will be to develop a fully wearable prototype for applications in rehabilitation and virtual and augmented reality,” said Sonar. “The prototype will also be tested in neuroscientific studies, where it can be used to stimulate the human body while researchers study dynamic brain activity in magnetic resonance experiments.”

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