Stanford University engineers have invented a wireless pressure sensor that has already been used to measure brain pressure in lab mice with brain injuries. The underlying technology has such broad potential that it could one day be used to create skin-like materials that can sense pressure, leading to prosthetic devices with the electronic equivalent of a sense of touch. The wireless sensor is made of a thin layer of rubber between two strips of copper. The copper strips act like radio antennas and the rubber serves as an insulator. The technology involves beaming radio waves at this simple antenna-and-rubber sandwich. When the device comes under pressure, the copper antennas squeeze the rubber insulator and move infinitesimally closer together. That tiny change in proximity alters the electrical characteristics of the device. Radio waves reflected by these antennas slow down in terms of frequency. When pressure is relaxed, the copper antennas move apart and the radio waves accelerate in frequency. The engineers proved that this effect was measurable, giving them a way to gauge the pressure exerted on the device by tracking the frequency of radio waves interacting with the device.