Highly polar poly-vinyliden (PVDF) or "piezofilm" is a polymeric material with unusual properties. With the proper manufacturing techniques this material can exhibit both piezo- and pyro-electric effects. Because of its thinness (as thin as .0003 inch), large dynamic range, and durability, PVDF is now frequently used in sensor design. Because of this remarkable product, sensors that were once though to be impossible are now being used to measure force, pressure, temperature, and acceleration.

Research into applications for thin film transducers recently led ANCO’s John Stoessel to the offices of prosthetists Jan Stokosa, of Lansing, Michigan, and Anthony Layton, of Lawton, Oklahoma, where he conducted successful field tests on the use of the material to accurately measure and record the pressure place on various areas of the limb in the fitting of prosthetic devices.

FITTING IS A COMPLEX PROCESS

The construction of a prosthetic socket is a highly complex process that involves a great deal of subjectivity and a series of castings and recastings, until a socket with a near-perfect fit is achieved. The goal is to build a socket which creates an equal pressure on all surfaces of the amputated limb - something that is currently achievable only through trial and error, according to Layton.

ACCURATE PRESSURE MEASUREMENT OFFERS MANY BENEFITS

By being able to accurately measure the pressure placed on the amputee, a prosthetist could fit an amputee with greater precision, reduce the number of test sockets used in a fitting, and extend the life of an artificial limb.

A minimum of three test sockets are presently used to fit each amputee. If that number could be reduced by just one socket, the savings to the customer would be at least $1,500.

Currently, most prostheses must be replaced every two to five years, due to changes in fit, function and appearance. Because an amputee must bear weight on an unnatural weight-bearing portion of the body; limbs change. Muscles that are no longer needed atrophy, others which are used more than ever increase in bulk, and fatty tissue progressively deteriorates.

By incorporating the pressure transducer into the prosthesis, changes in pressure over time can be monitored as the prosthesis is being worn. Corrective adjustments can then be more efficiently made to the socket thus extending the prosthesis useful life and delaying its replacement.

MISSING LINK IN CAD-CAM

"ANCO’s pressure transducer could also prove to be a vital link in the computer-aided design system currently in use by prosthetists throughout the United States and the United Kingdom," added Layton.

The computer gives prosthetists better communications, information storage and retrieval, and dimension manipulation, but not yet the ability to manufacture a socket with a better fit.

"The link that is missing is the pressure transducer to make the fit. If we had a numeric representation of the pressure exerted on the inside of the socket, we could store that information and relay it to a manufacturer. Then we’d have the whole ball of wax," Layton said.