As an amputee walks on a prosthetic leg during the day, the natural fluid in the leg shifts and the muscles shrink slightly, which poses a problem for the fit of the prosthesis. Sandia National Laboratories researcher Jason Wheeler is working to develop a sensor to tell how a limb changes, along with a system that automatically accommodates those changes. The interface, or socket, between a prosthesis and a limb is custom-made, starting with a cast of the area. The socket follows that contour, and a clinician adjusts it for the best fit. The Sandia researchers invented a three-axis pressure sensor that is incorporated into a liner that slips into the socket of a prosthesis. The sensor can be distributed to measure normal pressure, like pushing a finger on the thigh, and shear pressure in two directions on the skin, like sliding a finger down and across the thigh. Shear forces are important because they cause such problems as rubbing, blisters and abrasions. Other designers have placed pressure sensors in sockets, but those measured only normal pressure.