Tech Briefs

Measurement of Stresses and Strains in Muscles and Tendons

Small, electrically and chemically neutral sensors would be implanted surgically.

Miniature fiber-optic-coupled sensors based on optically excited, self-resonant microbeams are being developed for measuring stresses and strains within muscle fascicles and tendons. These sensors could be used in medical and biological research on humans and other animals, or to obtain data for the design of lifelike robots.

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Each sensor has typical dimensions of about 1 by 1 by 0.1 mm. These dimensions are suitable for surgical implantation in muscle and tendon tissues; these dimensions are also comparable to diameters of cores of multimode optical fibers, making the sensors amenable to fiber-optic coupling.

The figure depicts the main transducer portion of a sensor of this type and its mode of operation. The sensor contains a microbeam that has been micromachined as an integral part of a polysilicon housing. The microbeam is suspended in a vacuum cavity in the housing, over an embedded photodiode. The microbeam vibrates at a resonance frequency of the order of a megahertz, with a resonance quality factor (Q) of about 105.

The sensor exploits a unique gain mechanism that involves the exchange of optical, electrical, and mechanical energy during each cycle of oscillation: When the sensor is irradiated with infrared light via an optical fiber, the electric charge photogenerated in the diode electrostatically deflects the beam, setting the beam into vibration at its resonance frequency. Because the microbeam-and-cavity structure constitutes a Fabry-Perot interferometer, vibration of the beam modulates the transmitted light, thereby modulating the photovoltage and thereby, further, sustaining the oscillation.

Vibration of the beam also modulates the infrared light reflected back along the optical fiber. The reflected light is converted to a quasi-digital pulse stream by a photodetector, and the pulse-repetition frequency (equal to the frequency of vibration) is measured to obtain an indication of the physical quantity of interest (stress or strain).

For measuring stress or strain, the microbeam and housing must be configured such that the resonance frequency varies with longitudinal stress applied to the housing. The stress could be applied to the sensor via fibers or ribbons attached to a tendon. If the sensor is to be used to measure strain in a tendon, then both fibers or ribbons must be nonextensible and are attached to the tendon. If muscular tension is to be measured, then both fibers or ribbons must be nonextensible and attached to the loose ends of a severed tendon. If muscle extension/retraction is to be measured, then one fiber or ribbon must be nonextensible while the other is extensible by a known amount.

This work was done by James Weiss and Frank Hartley of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Test and Measurement category.

NPO-20464

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