A new artificial skin that can aid in rehabilitation and in VR

Materials World magazine
29 Oct 2019

An artificial skin has been developed with the potential to improve virtual reality experiences. Idha Valeur reports.

A synthetic soft-skin has been created that supplies haptic feedback and can adapt to the wearer’s movements using a built-in self-sensing mechanism. According to the researchers, the technology has several suitable applications, including medical rehabilitation and virtual reality (VR).

Scientists from the Reconfigurable Robotics Lab (RRL) and Laboratory for Soft Bioelectronic Interfaces (LSBI) from École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, teamed up to create the skin. Their joint efforts were published in the paper, Closed-loop haptic feedback control using a self-sensing soft pneumatic actuator skin, published in Soft Robotics in September 2019.

The material is made of silicone and electrodes, which makes a skin that is both soft and flexible. These are important properties to ensure it is adaptable to the wearer and their movements. ‘We chose PDMS (DC184) due to the widely available literature on material properties, biocompatibility and also the sensor technology being previously developed with the same material,’ lead author and PhD student, Harshal Sonar, told Materials World.

‘DC184 also allowed for oxygen plasma bonding that created a covalently bonded monolithic sensor-actuator integration, making the sensors act as part of actuators themselves.’

The new material’s mechanical properties are similar to human skin, which makes the soft pneumatic actuator (SPA)-skin interface compliant. ‘SPA-skin can not only sense the applied pressure but also provide coherent vibrotactile feedback, making it an interactive and bi-directional user interface. This gives us closed-loop control, which means we can accurately and reliably modulate the vibratory stimulation felt by the user,’ Sonar said.

Tactile feedback

Sonar explained the benefits of having an artificial skin able to provide haptic feedback as, ‘providing useful information to communicate with computers or other humans’.

He said this development is the first of its kind where ‘an entirely soft sensor-actuator integrated platform is able to provide high-fidelity tactile feedback with closed-loop control’. The system’s combination of soft sensors and actuators are what makes the skin capable of accurately adjusting to a person’s wrist, for example.

Providing further details on the actuators, Sonar said, ‘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 100Hz, 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. The sensors will keep measuring how the skin deforms, enabling the haptic feedback to be adjusted in real-time. This ensures that the sense of touch is as real as it can be at present time with current technologies.

In the paper, the team showed that the skin can successfully be stretched approximately four times its length for one million cycles.

‘We obtained robust performance where it is being strained at high rates. For the commercialisation aspects, it is dependent on the end-user application for the design modifications and also, converting the technology into a product will require a development lifecycle,’ Sonar said, and added that the material can be adapted for applications on any part of the body. ‘A glove for hands, band for the forearm, or integrated patch for the upper part of the feet or back of the neck. This invites for a proper mechanical grounding and support in these situations which needs to be further investigated in detail,’ he said.

Virtual reality

Closed-loop control that enables the user to feel accurate and reliable vibrations makes the skin ideal for applications such as testing a patient’s proprioception in medical applications, according to 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,’ he said.

Another application that is somewhat unexpected is VR. ‘With a mechanically transparent, wearable interface like SPA-skin, feeling the objects that a virtual avatar is seeing or manipulating in the virtual reality becomes a reality. The applications of this can range from at-home rehabilitation, in surgical simulations for doctors, to the gaming and entertainment industry,’ Sonar said.

Sonar expects the SPA-skin to help patients through VR or augmented reality aided assistance, particularly for rehabilitation in regaining motor skills. ‘It can be used as auxiliary feedback for the people wearing exoskeletons to transfer what interaction forces the exoskeleton is feeling while handling objects. Similarly in medical research, due to lack of any metal parts, this skin makes it possible to have vibrational feedback on the body of the subject and study its effects on human brain activity inside an MRI scanner,’ he said.

The skin has recently been tested on fingers and the team is working to extend this. ‘The next step will be to develop a fully wearable prototype for applications in rehabilitation and virtual and augmented reality,’ Sonar said.