Wireless electronic ‘stickers’ developed to gauge forces between objects

Developed at the University of California San Diego, the so-called ForceSticker consists of a capacitor that is just a few millimetres thick, and a radiofrequency identification (RFID) sticker.

The capacitor is made of a soft polymer sheet sandwiched between two conductive copper strips. When an external force is applied, the polymer compresses, drawing the copper strips closer together, thereby increasing the electric charge in the capacitor.

An RFID tag is then temporarily powered up by a radio signal emitted from a handheld RFID reader. That tag utilises an integrated antenna to relay a modified version of the signal back to the reader, with information on the capacitor's charge level. Custom software then converts that data into a measurement of applied force.

The force stickers run without batteries and fit in tight spaces that make them versatile for applications that include giving robots a sense of touch, improving biomedical devices, monitoring the safety of industrial equipment, and improving the accuracy and efficiency of inventory management in warehouses.

Researchers have tested the sticker’s role in monitoring knee implants to measure the forces that implants exert on the joint. Having the ability to sense changes in these forces can be useful for monitoring an implant's fit, as well as wear and tear.

Dinesh Bharadia, professor of electrical and computer engineering at the UC San Diego Jacobs School of Engineering, said: "Humans, by nature, possess an inherent ability to sense force. This allows us to interact seamlessly with our surroundings and enables clinicians to perform delicate surgical procedures. Providing this force-sensing ability to electronic devices and medical implants could be a game-changer for many industries."

The capacitor can be customised for various force ranges: by replacing the polymer layer with a softer or stiffer one, the capacitor can be tailored to measure smaller or larger forces, respectively.

In tests, the force stickers were able to withstand more than 10,000 force applications and remained consistently accurate. Fabricated at low cost, each sticker costs less than $2, the researchers noted, with the prediction that the price will drop if production is scaled up.

The technology was presented this week at the UbiComp 2023 conference, in Cancun, Mexico. The research was originally published in the Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies in March.