Patch Me In

Sometimes it feels like input technologies for our digital devices have just been completely figured out. We have a handful of options that are used by the vast majority of devices, and they seem to serve us quite well. So maybe we have reached peak user interface? Are touchscreens, keyboards, and mice eating the world?

Not likely. Of course these input devices are very efficient for desktop computers, laptops, smartphones, and the like, but as technology advances, entirely new types of computing platforms are being developed. And our tried and true input devices are not always well-suited for use with them. Consider wearables, for example. These on-body electronic devices have the potential to forever change everything from healthcare to communication and entertainment for the better. But you would not want to have to carry around a keyboard to interact with a device built into your shirt, now would you?

It is safe to say that just about no one would, so alternatives are clearly needed as these emerging technologies make their way into our everyday lives. A new option that is especially well-suited to wearable applications has recently been developed by a team at Meta Reality Labs Research. Their input device, named TouchpadAnyWear, is a flexible array of tactile sensors that can readily be integrated into clothing or other textiles.

TouchpadAnyWear employs a high-density capacitive sensor array (25 sensors per square cm) that is embedded into fabric to offer precise detection of touch location, size, and pressure. The system's high resolution is important for maintaining accuracy despite the challenges associated with body movement and fabric deformation. These arrays enable a broad range of inputs, including micro-gestures, multi-touch capabilities, and continuous 2D input.

The core of the sensor is built from an elastic knitted fabric, chosen for its high elasticity and ability to compress significantly under force. This fabric is embedded with poly-islands, which are silicone rubber structures patterned onto the fabric. Made from Bluesil RTV 3040, the poly-islands enhance the stiffness of the textile by an order of magnitude, improving its ability to handle motion artifacts and maintain sensitivity during dynamic activities.

Stretchable silver electrodes are fabricated using a specialized silver ink that provides excellent conductivity and strong adhesion to a thermoplastic polyurethane substrate. These electrodes are patterned using a 3-axis automated dispensing robot, which ensures precision in design and scalability in production. The electrodes maintain low resistance and consistent electrical performance, even under mechanical stretching of up to 50 percent. Layers of electrodes are positioned above and below the fabric layer.

To shield the device from electromagnetic interference and provide grounding, a layer of conductive fabric tape is used as the outermost layer. This material is both conductive and flexible, allowing safe direct skin contact while protecting the inner components from external disturbances. The conductive tape is laser-engraved into custom kirigami patterns, which enhance flexibility and conformability.

TouchpadAnyWear was evaluated in a number of user studies focused on gesture recognition and 2D continuous input. In these studies, the sensor was adhered to the participants' pants. They then performed 50 gestures each, such as Left, Right, Up, Down, and Tap, with feedback provided after each attempt. The system demonstrated a high overall gesture recognition accuracy of 93.7 percent. Furthermore, users generally found the touchpad to work well for continuous input when it was connected to a laptop, however, they noted that the amount of force they had to supply was surprisingly high.

Looking ahead, the team hopes to fully integrate TouchpadAnyWear and the associated processing components into an article of clothing. Doing so would go a long way toward the goal of offering an alternative user interface for wearable devices.