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Wireless Implantable Sensors with No Electrical Connections Enable the Next Generation of Smart Orthopaedic Implants

Technology
Through a simple design, an L-C circuit can function as a passive, standalone force sensor. Physically, two coils of electrical conductors in close proximity to each other form an inductor. Similarly, two flat parallel conductive plates separated by a thin layer of dielectric form a capacitor. If the distance between the two plates of the capacitor changes, the capacitance is modulated and the resonant frequency changes accordingly. Applying a force to the capacitor will cause the dielectric between the plates to deform, which reduces the gap between the plates and in turn modulates capacitance and alters the resonant frequency. In this way, a simple L-C circuit can function as a force sensor.

Through a novel design, this can be achieved using a single component system with no electrical connections. In the single component design, one element acts as both inductor and capacitor. This is achieved using two flat parallel coils that are separated by only a thin layer of dielectric. When the flat coil inductors are in close proximity to each other, they also function as the capacitive plates.

What is unique is that there need not be any electrical connections between the two coils. Even with no electrical connections, the two coils comprise an L-C circuit. They have a characteristic resonant frequency that is detectable using a grid dip oscillator. The resonant frequency is a function of the inductance and capacitance. Capacitance can be modulated by loading the sensor and compressing the dielectric between the coils. In this way, the two coils comprise a complete force sensor with no electrical connections.

The flat coil inductors can be made using various gauge insulated wire of any conductor by winding the wire into coils until the desired diameter is achieved. (See Exhibit 1.) The wire is wound through epoxy, which bonds each loop of wire to the previous. This prevents the coil from unwinding once wrapped. The resting resonant frequency is a function of wire gauge, coil diameter and the gap between coils. The gap between the coils is equal to the thickness of the dielectric which separates them. The thickness of the dielectric can be customized to achieve the desired resting resonant frequency. In general, the thinner the dielectric, the lower the frequency. Dielectrics can be spin-coated, vapor deposited or manually applied to achieve thicknesses of 50 µm or less. There are several candidate dielectrics which include epoxy, silastic, PDMS or any other insulating elastomer or polymer which is hydrophilic and biocompatible. Frequencies in the 30 MHz to 100 MHz range appear to maximize read range (distance from external antenna to sensor) while allowing small physical size of the sensor. In the U.S., there are allowable industrial, scientific and medical (ISM) frequency bands at approximately 27 MHz and at approximately 40 MHz making these desirable target frequencies.  

Micro fabrication techniques including deposition and etching can be used to make these sensors in bulk which reduces fabrication costs. Currently, the cost of “laboratory grade” sensors is less than $1 using manual fabrication techniques.

Exhibit 1: Sensors appear to be small coils and can be made from various gauge wires (34, 38, 40) and diameter (6.5, 7.5 and 10 mm). The scale shown is millimeters.
Sensors appear to be small coils and can be made from various gauge wires (34, 38, 40) and diameter (6.5, 7.5 and 10 mm). The scale shown is millimeters

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