Two new kinds of on-skin electronics allow users to build and customize them directly on the body – with potential applications in biometric sensing, medical monitoring, interactive prosthetic makeup and more.

SkinLink, developed by the Hybrid Body Lab led by Cindy Hsin-Liu Kao, assistant professor of human centered design in the College of Human Ecology, is an on-skin electronic interface that can be fabricated right on the body, providing flexibility in design depending on the intended use. And ECSkin is an electrochromic display interface that also can be fabricated in situ, and features modular design through tiles that can be arranged as desired.

Some of the potential applications for SkinLink include vital-sign and posture monitoring, proximity sensing and body art.

“Both of these projects are modular prototyping toolkits, to enable much more intricate on-skin circuitry prototyping,” Kao said. “And users are able to build the circuitry directly on the skin surface.”

Doctoral student and lab member Pin-Sung Ku is lead author of “SkinLink: On-body Construction and Prototyping of Reconfigurable Epidermal Interfaces,” which was presented in early October at UbiComp/ISWC ’24, the Association for Computing Machinery’s international joint conference on pervasive and ubiquitous computing.

The work earned a Distinguished Paper Award from the journal, Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies, where it was originally published in June 2023.

ECSkin: Tessellating Electrochromic Films for Reconfigurable On-skin Displays, which published May 15, 2024, in the same journal and for which Ku is also lead author, was also presented at the conference. Kao is senior author for both papers.

SkinLink represents a step forward from SkinKit, which the lab developed and presented at UbiComp in 2021. The earlier iteration featured slim and flexible printed circuit boards in temporary tattoo form, but the boards had predefined behaviors and larger surface areas, making customizability a challenge.

“There were several issues we didn’t address in SkinKit, such as how to allow users to program the modules the way they want to,” Ku said. “For SkinKit, everything was pre-programmed; users had to attach things in a certain order. With SkinLink, there is more customizability of the functions they like.”

With SkinKit, for example, the circuitry had to be built before applying it to the body; with SkinLink, users can put one module on the body, then another, for more customizability. 

The SkinLink toolkit consists of functional circuit modules, made of flexible printed circuit boards, and flexible, custom-designed wiring (called trace modules, or traces) that connect the sensor and actuator modules to the microcontroller board.

That’s a key difference from SkinKit: Smaller circuit boards connect via flexible wiring, allowing for greater freedom of motion without sacrificing connectivity. 

“The circuit boards are somewhat rigid,” Ku said, “but we created the traces to be elastic, stretchable and flexible, so as not to hinder body movement.”

The on-body fabrication process starts with selecting and programming the circuit board and trace modules. The microcontroller board can be repeatedly programmed during this step, to fine-tune the circuit functions. The boards can be temporarily placed on the body and customized, before final placement.

The researchers conducted A/B testing of SkinLink, comparing its usability with SkinKit in a 14-person study, and found that SkinLink enables a more flexible wiring process than SkinKit, along with unrestricted circuit arrangement and superior wearability. Compared to the SkinKit, the SkinLink interface has a much smaller footprint, better stretchability, easier fabricating and more seamless wear.

SkinKit, Kao said, offered a “low floor” – a term coined by computer scientist Ben Shneiderman, meaning that people with little or no experience can quickly get started designing and prototyping wearable tech. SkinLink takes it a couple of steps further, she said.

“Another goal is what we call ‘high ceilings and wide walls,’” she said. “High ceilings means increasing the complexity of the prototypes; wide walls refers to the vast array of things that could be designed. That’s what I think SkinLink really does.”

Kao said she sees both SkinLink and ECSkin as “enabling technologies,” with a variety of potential uses. Some of the studies with SkinLink involved artists, wearable tech researchers, and psychology researchers for physiology sensing; she also envisions applications beyond humans. 

“Being able to use this as a functional technology for physiological sensing or artistic practice is one application,” she said. “Our work has focused on skin interfaces for humans, but I think there could also be potential for designing ‘skin’ interfaces for other living beings, such as animals, and sensing for agricultural purposes, even on plants. We are interested in bringing SkinLink to broader disciplines to support on-skin prototyping.”

This work was supported by the National Science Foundation.