Stress Sensors for Biomedical Applications Student Researcher: Nicholas Stellpflug Faculty Advisors: Chiu Tai Law, Rani El-Hajjar

Introduction Develop a design for an effective sensor to monitor stress on joint implants. Synthesized using a manetostrictive composite material (MCM) of Terfenol- D (Tb0.3Dy0.7Fe2), epoxy resin, and ferromagnetic material. Uses the Villari effect (inverse magnetostriction) Under mechanical stress, magnetic domains in the sensor will line up and expand.

Previous Design Terfenol-D/epoxy MCM. Used to detect changes in stress using a toroidal coil as a probe. MCM sensor was attached to an aluminum substrate under stress, and the air gap of the coil was placed around the sensor. As stress increases, inductance of the probe raises proportionally. Permeability of the sensor is modified via the Villari effect.

Theory Test the concept of one layer of MCM on top of a layer of epoxy with a localized iron region, which will concentrate magnetic flux. Magnetic flux will traverse the MCM twice in the magnetic circuit. Same toroidal coil will be used, but it can be placed anywhere on top of the iron region. Measurements will be more consistent and less dependent on the probe position.

Procedure A piece of iron will be cut to fit the middle region of the mold. Epoxy is poured into the mold. When the epoxy is semi-cured, the iron piece is put in place When the bottom layer is almost fully cured, the Terfenol-D/epoxy mixture is poured into the mold to form the upper layer.

Methodology Critical dimensions: Thickness of the Terfenol-D layer and the iron bar. Length of the iron region. Possible variations of the setup: Volume fraction of Terfenol-D particles in epoxy. Alignment of Terfenol-D particles with the external field during the fabrication process.

Expected Results The probe will be able to indicate changes of the stress on the sensor by detecting variations in the inductance via the novel sensor with high sensitivity. We will learn more about the geometrical dependence of the sensor as well as the effects of Terfenol-D concentration.

Future Direction Modifying the sensor geometry to fit the results of the experiment. Use different magnetostrictive materials, such as Galfenol.

References Al-Hajjeh, Ashraf et al., "Characteristics of a Magnetostrictive Composite Stress Sensor," IEEE Magnetics Letters 7 (2016): 6502804. Engdahl, G., ed., Handbook Of Giant Magnetostrictive Materials, 1st ed., New York, N.Y.: Academic Press, 1999.