Simple implantable wireless sensor platform to measure pressure and force.

Smart implants have the potential to enable personalized care regimens for patients. However, the integration of smart implants into daily clinical practice is limited by the size and cost of available sensing technology. Passive resonant sensors are an attractive alternative to traditional sensing technologies because they obviate the need for on-sensor signal conditioning or telemetry and are substantially simpler, smaller, less expensive, and more robust than other sensing methods. We have developed a novel simple, passive sensing platform that is adaptable to a variety of applications. Sensors consist of only two disconnected parallel Archimedean spiral coils and an intervening solid dielectric layer. When exposed to force or pressure, the resonant frequency of the circuit shifts which can be measured wirelessly. We fabricated prototype pressure sensors and force sensors and compared their performance to a lumped parameter model which predicts sensor behavior. The sensors exhibited a linear response (R2 > 0.91) to dynamic changes in pressure or force with excellent sensitivity. Experimental data were within 13.3% and 6.2% of the values predicted by the model for force and pressure respectively. Results demonstrate that the sensors can be adapted to measure various measurands through a span of sensitivities and ranges by appropriate selection of the intervening layer.

A simple sensing mechanism for wireless, passive pressure sensors.

We have developed a simple wireless pressure sensor that consists of only three electrically isolated components. Two conductive spirals are separated by a closed cell foam that deforms when exposed to changing pressures. This deformation changes the capacitance and thus the resonant frequency of the sensors. Prototype sensors were submerged and wirelessly interrogated while being exposed to physiologically relevant pressures from 10 to 130 mmHg. Sensors consistently exhibited a sensitivity of 4.35 kHz/mmHg which is sufficient for resolving physiologically relevant pressure changes in vivo. These simple sensors have the potential for in vivo pressure sensing.

Reducing the effect of parasitic capacitance on implantable passive resonant sensors.

Passive, LC resonators have the potential to serve as small, robust, low cost, implantable sensors to wirelessly monitor implants following orthopedic surgery. One significant barrier to using LC sensors is the influence on the sensor's resonance of the surrounding conductive high permittivity media in vivo. The surrounding media can detune the resonant frequency of the LC sensor resulting in a bias. To mitigate the effects of the surrounding media, we added a "capping layer" to LC sensors to isolate them from the surrounding media. Several capping materials and thicknesses were tested to determine effectiveness at reducing the sensor's interaction with the surrounding media. Results show that a 1 mm glass capping layer on the outer surfaces of the sensor was sufficient to reduce the effects of the media on sensor signal to less than 1%.