New wearable device stimulates skin

The device adheres to the skin to deliver realistic and immersive sensor experiences. Although it lends itself well to gaming and virtual reality (VR), the researchers anticipate that there may be applications in healthcare, to help those who are visually impaired in ‘feeling’ their environment, or provide feedback to people with prosthetic limbs.

Building on previous work

The device marks the latest advance in wearable technology from Northwestern bioelectrics pioneer John A. Rogers, the Louis A. Simpson and Kimberly Querrey Professor of Materials Science and Engineering, Biomedical Engineering and Neurological Surgery, with appointments in Northwestern’s McCormick School of Engineering and Northwestern University Feinberg School of Medicine. He also directs the Querrey Simpson Institute for Bioelectronics.

The new study, sharing information on the researchers’ work, builds on existing work published in 2019 in which John A. Rogers’ team introduced ‘epidermal VR,’ a skin-interfaced system that communicates touch through an array of miniature, vibrating actuators across large areas of the skin, with fast wireless control.

“Our new miniaturised actuators for the skin are far more capable than the simple ‘buzzers’ that we used as demonstration vehicles in our original 2019 paper,” said Rogers. “Specifically, these tiny devices can deliver controlled forces across a range of frequencies, providing constant force without continuous application of power. An additional version allows the same actuators to provide a gentle twisting motion at the surface of the skin to complement the ability to deliver vertical force, adding realism to the sensations.”

Rogers co-led the work with Northwestern’s Yonggang Huang, the Jan and Marcia Achenbach Professorship in Mechanical Engineering at McCormick; Hanqing Jiang of Westlake University in China; and Zhaoqian Xie of Dalian University of Technology in China. Jiang’s team built the small modifying structures needed to enable twisting motions.

How the device works

The new device is made up of a hexagonal array of 19 small magnetic actuators encapsulated within a thin, flexible, silicone-mesh material. Each actuator is capable of delivering different sensations, which include pressure, vibration and twisting. By using Bluetooth technology in a smartphone, the device can receive data about a person’s surroundings for translation into tactile feedback - where one sensation, like vision, is substituted for another, like touch.

The device is powered by a small battery but saves energy using a ‘bistable’ design, which refers to the fact that it can stay in two stable positions without requiring constant energy input. When the actuators press down, it stores energy in the skin and in the device’s internal structure. When they push back up, the device uses a small amount of energy to release the stored energy.

In other words, the device only uses energy when the actuators change position. Using this energy efficient design means the device can operate for longer periods of time on a single battery charge.

“Instead of fighting against the skin, the idea was ultimately to actually use the energy that's stored in skin mechanically as elastic energy and recover that during the operation of the device,” explained Matthew Flavin, the paper’s first author. “Just like stretching a rubber band, compressing the elastic skin stores energy. We can then reapply that energy while we're delivering sensory feedback, and that was ultimately the basis for how we created this really energy-efficient system.”

At the time of the research, Flavin was a postdoctoral researcher in Rogers’ lab. Now, he is an assistant professor of electrical and computer engineering at the Georgia Institute of Technology.

To find the optimal design, Huang’s team conducted systematic computational modelling in which they examined how the device interacts with skin through electromagnetic actuation.

“It is essential that this bi-stability design can be universally applied to all types of human skin. Computational modelling enables precise optimisation, ensuring this capability,” Huang added. “Achieving this solely through trial-and-error experiments would be impossible. Shupeng Li, a co-first author and postdoc in my group, led this modelling effort.”

Device testing

To test the device, the researchers blindfolded healthy subjects to test their abilities to avoid objects in their path, change foot placement to avoid injury and alter their posture to improve balance.

In one experiment, a subject had to navigate a path through obstructing objects. As the subject approached an object, the device delivered feedback in the form of light pressure in its upper right corner. As the person moved nearer to the object, the feedback became more intense, moving closer to the centre of the device.

With only a short period of training, subjects using the device were able to change behaviour in real time - demonstrating the viability of the device. By substituting visual information with mechanical, the device “would operate very similarly to how a white cane would, but it's integrating more information than someone would be able to get with a more common aid,” Flavin explained.

“As one of several application examples, we show that this system can support a basic version of ‘vision’ in the form of haptic patterns delivered to the surface of the skin based on data collected using the 3D imaging function (LiDAR) available on smartphones,” Rogers concluded. “This sort of ‘sensory substitution’ provides a primitive, but functionally meaningful, sense of one’s surroundings without reliance on eyesight — a capability useful for individuals with vision impairments."