**4. The interrogating reader**

66 Modern Telemetry

where *d*C/C is the fractional change of capacitance and ε is the strain. Using a linear fit of the differential capacitance data graphed in Fig 5, the gauge factor was ploted and calculated to be 252 for 0 to 1000 με. This value is extremly high in comparison to the current literature (Arshak 1994, 2000; Proctor & Strong 1992). By comparison, piezoresistive gauges typically provide a gauge factor less than 200 (Fraden, 1995) even at the cost of high

Fig. 5. Comparison of the calculation and experimental results of a strain sensor glued to a

The transducer circuit is an inductively coupled, load modulated design similar in concept to Radio Frequency IDentification tags used for inventory and security. The 125 kHz magnetic field sourced by the interrogating reader induces a voltage in the LC tank of the implant. The LC tank (1 cm diameter, 600 turns) is resonant at 125 kHz. The AC voltage is then rectified, filtered, and regulated using a low quiescent power regulator. A supply of 3VDC, 28 μA is required to power the oscillator circuit described above and a frequency divider circuit composed of flip-flops. The oscillator frequency is divided to less than 1/20 the carrier frequency so that detection is simplified. The frequency divider also buffers the oscillator signal so that it can drive the gate of a MOSFET placed across the LC tank. The MOSFET acts a load that modulates the 125 kHz tank output at the divider output

As it is functioned as an oscillator in Fig. 6(a), a capacitance to frequency (c-f) converter is used to convert the strain signal to a signal that can be transfered wirelessly. The c-f circuit is comprised of a pair of CMOS inverters (Lancaster, 1997) and can be seen in detail in Fig. 6(b). The oscillator produces a periodical voltage due to the charging and discharging of the RC across a threshold of an inverter input. The frequency of the RC oscillator is expressed as F=2.3RtC, where C is the variable capacitor, or a capacitive strain sensor, and Rt is the matched resistance of the oscillator. The resistor RI is to unload the RC network from clamping effect of protection diodes in the inverter. It will also result in a nearly square duty cycle and make the frequency less dependent of power supply variations. RI is normally set about 10 times as higher as Rt to minimize the effect of the protection diodes. The oscillator oscillates empirically at 20 kHz with 20 pF capacitor and 100 kOhm resistor.

frequency. A diagram that shows the functionality can be seen in Fig. 6(a).

temperature sensitivity.

spinal fusion rod.

**3. The transducer circuit** 

The interogating reader operating on 12 VDC, 175 mA provides the 125 kHz magnetic field for the implant, as illustrate in Fig. 7. The reader antenna is 24 cm in diameter and is tuned to 125 kHz. An EM Microelectronic (Marin, Switzerland), EM4095 IC contains an on-chip oscillator, antenna driver, and a demodulation circuit. The output of the demodulator is measured using an Agilent 53131A counter and logged with a computer based data acquisition system.

Fig. 7. The block diagram of the power reader.

#### **4.1 Detection region**

In the region of detection, see Fig. 8, the implant receives enough power to operate from the magnetic field sourced by the reader. There is no degradation of strain sensing performance

Inductively Coupled Telemetry In Spinal Fusion Application Using Capacitive Strain Sensors 69

The strain measurement system was tested using a corpectomy model designed as a simplified mechanical analog of a spine section and then will be tested using a human spine. Figure 10 shows a free body diagram of the forces and bending moments applied to the spinal rods through the pedicle screws due to loading of the hard plastic blocks. Note that when the spinal fusion rods are fastened to the fixtures, often the initial strain is introduced and recalibrated. On the first test, the corpectomy apparatus was assembled inside a clear acrylic water tank without water on the MTS machine for application of the load; the sensor system was oriented facing out to be detected. The MTS machine's dynamic motion only changes 3 mm in the detection distance between the interrogating reader and the strain sensor. The read range was limited to 10 cm due to the reader design used and the sensor coil design constraints. A more optimal reader would increase the range. For other applications where a larger coil diameter is acceptable the range would be increased as well.

Fig. 10. The corpectomy model: front view, top and side view, bottom.

The data acquisition is obtained with LabView software, an interfacing program designed to transfer and record live data between instruments and computer by National Instrument. The strain information is recorded by the commecial metal foil strain gauge through a strain

if sufficient DC power is available from the regulator. However at the far end of the region, planar and axial alignment becomes very important. With distance from the reader, inductive coupling is reduced thereby reducing the AC voltage across the LC tank and thus the modulated signal amplitude. The data signal also relies on the same low coupling between the implant and the reader. It is necessary to have a sensitive reader to detect the implant at the far end of the region.

Fig. 8. The detection region is within the cone shape.
