On-skin interfaces are becoming a more common form factor for wearable devices as sensors and processing units shrink in both size and cost. Such interfaces have the advantage of being in direct contact with the skin, which allows onboard sensors to collect information about the wearer and their activities. Most on-skin devices tend to be either impractical for real world use, or uncomfortable for long-term wear.
A team from Cornell University is working to make on-skin devices a reality for the masses with their fabrication process for the device they call WovenProbe. Unlike other on-skin devices, WovenProbe is woven into a fabric, bandage-like interface that is capable of operating without being tethered to an external power source or processing system.
Weaving was chosen as the fabrication method because the interlaced pattern allows for organized routing of traces for creating circuits, and for integrating distributed PCBs into the fabric. Woven fabrics also have the advantage of being resilient, comfortable for long-term wear, and being socially acceptable in appearance.
The first step in creating a new design is to sketch the electrical layout of the device on a stencil overlaid on the skin. The stencil is then used as a guide in fabricating the mesh of thin yarns and wires with a weaving loom. Next, the electronics are soldered into their places in the circuit and a thin layer of silicone is applied to insulate the PCBs. Finally, the completed interface is attached to the skin — the team used eyelash glue for this job.
Using this method, the researchers built a prototype device that attaches to the forearm, and extends onto the index finger and thumb. There are a total of four inertial measurement units (ST LSM6DSOX), two on the index finger, and two more on the thumb. A microcontroller (ST STM32L071KZ) provides onboard processing, an SD card provides storage, and a LiPo battery powers it all.
Three co-authors of the paper conducted a study in which they wore the prototype device for a six hour period to better understand the durability of the system. They found that the interface maintained good adhesion with the skin for the duration of the trial, with some minor detachments at the fingertips. No major electrical issues were discovered, however, some minor wire breakages were identified. Those areas were reinforced before moving on to the field study.
Seven participants were recruited for the field study in which they wore the device for a six hour period. During this time, they were asked to perform different hand gestures to assess functionality. Some issues were detected with both the attachment of the device to the skin, and the functioning of the electronics. The participants were interviewed at the conclusion of the study, and the device was generally described as comfortable and noninvasive.
Recognizing that needing to recharge a battery every few hours can dramatically impact the usability of an on-skin interface, the team explored wirelessly powering their device through near-field communication (NFC) coils. They suggest that a smartwatch could provide the power for the device over NFC, however, the amount of power it can supply is very limited. Only very specialized, low-power device designs could be supplied with sufficient power in this way.
The fabrication method introduced by WovenProbe has the potential to make on-skin devices more practical and more socially acceptable, however, there are still some issues with durability that need to be worked out. With further refinement, it may see the light of day outside of a research lab one day.