A low-cost fiber-optic strain gage system for biological applications

A new low-cost strain measurement system has been developed for the mechanical testing of biological soft tissues. The technique creates four spots of light on a tissue sample surface by piercing the tissue sample with two pairs of small light-conducting optical fibers (one pair for each axis of a biaxial stretch), terminated by high intensity infrared emitters. A large-area photodiode, located below the tissue sample, detects the light emitted from the two pairs of light-spots. Analog and digital circuitry analyze the current signal from the photodiode to determine the position of a light-spot in real time. Each infrared emitter is sequentially cycled "on" at a rate of 3 kHz and the resulting photodiode current signal, after being converted to a voltage signal, is held by an integrated circuit sample and hold amplifier. Analog differencing of pairs of light-spot voltage signals provides a final output proportional to the separation between coaxial light-spots.