In many situations, it's important to know how much force one object exerts on another. We designed a small, thin sticker to measure and deliver this type of data without the need for wires or batteries. Scientists at the University of California, San Diego are developing the device, called the ForceSticker. Among other uses, it could be used to monitor the weight of stacked packages in warehouses, ensure artificial knees don't damage adjacent cartilage, and even provide robots with a sense of touch.

ForceSticker attached to an interface between two touching objects/David Baillot/UC San Diego Jacobs School of Engineering

At the heart of the ForceSticker is a capacitor, about the size of a grain of rice. It consists of a flexible polymer sheet sandwiched between two conductive copper strips. When force is applied to the sticker, the polymer compresses, bringing the two copper strips closer to each other. In this way, the amount of charge in the capacitor increases.

The other major component of the ForceSticker is the RFID (radio frequency identification) tag, which is temporarily powered by a radio signal emitted by a handheld RFID reader. The tag uses an integrated antenna to transmit a modified signal back to the reader - the signal now contains information about the current charge level of the link capacitor. Custom software then converts this data into measurements of external forces.

Researchers successfully test ForceSticker in knee joint modelDavid Baillot/UC San Diego Jacobs School of Engineering

By changing the softness and hardness of the polymer used in the capacitor, it is possible to create a ForceSticker with a very high sensitivity, suitable for measuring smaller forces, or a less sensitive one, suitable for measuring larger forces. In the current tests, highly sensitive stickers were used to monitor the force on the model's knee joint, while less sensitive stickers were used to monitor the weight of various large objects placed in the box.

In both cases, ForceStickers remained effective after more than 10,000 force applications. The stickers are also relatively cheap to make, costing about $2 each, a number that should drop significantly if production is scaled up. Additionally, users will eventually be able to use their smartphones in place of dedicated RFID readers.

Chief Scientist Professor Dinesh Bharadia said: "Humans have an innate ability to sense force. Providing this kind of force-sensing capability to electronic devices and medical implants could be a game-changer for many industries."

A paper on this research will be presented at the UbiComp2023 conference in Mexico.