David Budgett

A Frequency Control Method for Regulating Wireless Power to Implantable Devices

This paper presents a method to regulate the power transferred over a wireless link by adjusting the resonant operating frequency of the primary converter. A significant advantage of this method is that effective power regulation is maintained under variations in load, coupling and circuit parameters. This is particularly important when the wireless supply is used to power implanted medical devices where substantial coupling variations between internal and external systems is expected. The operating frequency is changed dynamically by altering the effective tuning capacitance through soft switched phase control. A thorough analysis of the proposed system has been undertaken, and experimental results verify its functionality.

What sets the long-term level of renal sympathetic nerve activity: a role for angiotensin II and baroreflexes?

Abstract— Increasing evidence suggests elevated sympathetic outflow may be important in the genesis of hypertension. It is thought that peripheral angiotensin II, in addition to its pressor actions, may act centrally to increase sympathetic nerve activity (SNA). Without direct long-term recordings of SNA, testing the involvement of neural mechanisms in angiotensin II–induced increases in arterial pressure is difficult. Using a novel telemetry-based implantable amplifier, we made continuous recordings of renal SNA (RSNA) before, during, and after 1 week of angiotensin II–based hypertension in rabbits living in their home cages. Angiotensin II infusion (50 ng · kg−1 · min−1) caused a sustained increase in arterial pressure (18±3 mm Hg). There was a sustained decrease in RSNA from 18±2 normalized units (n.u.) before angiotensin II to 8±2 n.u. on day 2 and 9±2 n.u. on day 7 of the angiotensin II infusion (P <0.01) before recovering to 17±2 n.u. after ceasing angiotensin II. Analysis of the baroreflex response showed that although angiotensin II–induced hypertension led to resetting of the relationship between mean arterial pressure (MAP) and heart rate, there was no evidence of resetting of the MAP-RSNA relationship. We propose that the lack of resetting of the MAP-RSNA curve, with the resting point lying near the lower plateau, suggests the sustained decrease in RSNA during angiotensin II is baroreflex mediated. These results suggest that baroreflex control of RSNA and thus renal function is likely to play a significant role in the control of arterial pressure not only in the short term but also in the long term.

Wireless Power Supply for Implantable Biomedical Device Based on Primary Input Voltage Regulation

This paper presents a wireless power supply system for implantable biomedical devices. Magnitude of the input voltage supplied to the primary power converter is dynamically regulated according to the power demand of the device. The major advantage of such a system is that its average power loss is minimized. Unlike methods implemented at implantable secondary (pick-up) side, the magnitude regulation is undertaken at the external primary side. Thus the heating effect and physical size of the implantable secondary can be reduced. The system utilizes parallel tuning circuit to boost the voltage induced in the secondary pick-up, and does not require a tight coupling between the primary and secondary coils. As a result, the system has great tolerance to the variation in the air gap distance between the coils. The characteristics of the magnitude regulated power flow have been thoroughly analyzed, and both simulations and laboratory experiments have verified the proposed system.

Experimental Study of a TET System for Implantable Biomedical Devices

Time-varying magnetic fields can be used to transfer power across the skin to drive implantable biomedical devices without the use of percutaneous wires. However, the main challenges of a transcutanoues energy transfer (TET) system are the temperature rise caused by power loss in the implanted circuitry and the changes in positioning between the external and internal coils due to fitting and changes in posture. This study presents a TET system with a closed-loop frequency-based power regulation method to deliver the right amount of power to the load under variable coil coupling conditions. After implanting a TET system into adult sheep, the temperature rise in the internal and external coils of a TET system was measured for power delivery in the range of 5 W to 15 W. The sheep was housed in a temperature controlled (16 plusmn1degC, humidity 50plusmn10%) room, in accordance with the standard protocols implemented at the University of Auckland for sheep studies. A power-loss analysis for the overall system was performed. The system was capable of regulating power for axially aligned separations of up to 16 mm. The maximum power efficiency of the overall system was 82.1% and a maximum temperature rise of 2.7degC was observed on the implanted secondary coil.

Switching Frequency Analysis of Dynamically Detuned ICPT Power Pick-ups

Dynamic detuning methods have been used in inductive contactless power transfer (ICPT) systems for power flow control. However, the highly variable switching frequency involved in the detuning operation will contribute to electromagnetic interference (EMI) and power losses. It is difficult to determine the detuning frequency precisely due to nonlinear features of power pick-ups. Uncertainty in the operating frequency can result in difficulties in designing filters with suitable bandwidths and choosing suitable switching devices. Based on detailed analytical analysis in four segments of the detuning process, a numerical method is developed in this paper to determine the boundaries of the switching frequencies. An iterative algorithm is presented using a flow chart to illustrate the process taken in the numerical analysis. Simulation and practical experiments are conducted to verify the algorithm so as to ensure the calculated results are sufficiently accurate for designing EMI filters and choosing suitable switching devices.

A novel low temperature transcutaneous energy transfer system suitable for high power implantable medical devices: performance and validation in sheep.

Transcutaneous energy transfer (TET) systems use magnetic fields to transfer power across the skin without direct electrical connectivity. This offers the prospect of lifetime operation and overcomes risk of infection associated with wires passing through the skin. Previous attempts at this technology have not proved suitable due to poor efficiency, large size, or tissue damage. We have developed a novel approach utilizing frequency control that allows for wide tolerance in the alignment between internal and external coils for coupling variations of 10 to 20 mm, and relatively small size (50 mm diameter, 5 mm thickness). Using a sheep experimental model, the secondary coil was implanted under the skin in six sheep, and the system was operated to deliver a stable power output to a 15 W load continuously over 4 weeks. The maximum surface temperature of the secondary coil increased by a mean value of 3.4 +/- 0.4 degrees C (+/-SEM). The highest absolute mean temperature was 38.3 degrees C. The mean temperature rise 20 mm from the secondary coil was 0.8 +/- 0.1 degrees C. The efficiency of the system exceeded 80% across a wide range of coil orientations. Histological analysis revealed no evidence of tissue necrosis or damage after four weeks of operation. We conclude that this technology is able to offer robust transfer of power to implantable devices without excess heating causing tissue damage.

A fully implantable telemetry system for the chronic monitoring of brain tissue oxygen in freely moving rats

The ability to monitor tissue oxygen concentration in a specific region of the brain in a freely moving animal could provide a new paradigm in neuroscience research. We have developed a fully implantable telemetry system for the continuous and chronic recording of brain tissue oxygen (PO(2,BR)) in conscious animals. A telemetry system with a sampling rate of 2kHz was combined with a miniaturized potentiostat to amperiometrically detect oxygen concentration with carbon paste electrodes. Wireless power was employed to recharge the telemeter battery transcutaneously for potential lifetime monitoring. Rats were implanted with the telemeter in the peritoneal cavity and electrodes stereotaxically implanted into the brain (striatum or medulla oblongata). While the animals were living in their home cages the sensitivity to changes in oxygen was validated by repeatedly altering the inspired oxygen (10%, 100%, respectively) or a pharmacological stimulus (carbonic anhydrase inhibitor: acetazolamide 50mg/kg IP). Basal level of PO(2,BR) was monitored for 3weeks and showed good overall stability and good correlation to movement such as grooming. During hypoxia, PO(2,BR) decreased significantly by -51%±2% from baseline, whereas it increased by 34%±3% during hyperoxia. Following the systemic administration of acetazolamide, PO(2,BR) increased by 38%±4%. We propose this new technology provides a robust method to measure changes in oxygen concentration in specific areas of the brain, in conscious freely moving rats. The ability to track long term changes with disease progression or drug treatment may be enabled.

Wireless power delivery system for mouse telemeter

Implantable telemetry systems provide chronic measurements of physiological parameters such as the electrocardiogram and blood pressure from research animals. Device operation is typically limited by battery life which also determines device size. We present a system for powering implanted telemetry devices for use in mice which provides lifetime monitoring. It uses coils under the home cage of the animal to generate a magnetic field capable of supplying power continuously to a telemetry system implanted inside the mouse. A guaranteed minimum power of 20mW is provided over a 100mm × 100mm area independent of mouse orientation which facilitates high bandwidth (2kHz sample rate) continuous physiological data.

Efficient Power-Transfer Capability Analysis of the TET System Using the Equivalent Small Parameter Method

Transcutaneous energy transfer (TET) enables the transfer of power across the skin without direct electrical connection. It is a mechanism for powering implantable devices for the lifetime of a patient. For maximum power transfer, it is essential that TET systems be resonant on both the primary and secondary sides, which requires considerable design effort. Consequently, a strong need exists for an efficient method to aid the design process. This paper presents an analytical technique appropriate to analyze complex TET systems. The systems steady-state solution in closed form with sufficient accuracy is obtained by employing the proposed equivalent small parameter method. It is shown that power-transfer capability can be correctly predicted without tedious iterative simulations or practical measurements. Furthermore, for TET systems utilizing a current-fed push-pull soft switching resonant converter, it is found that the maximum energy transfer does not occur when the primary and secondary resonant tanks are “tuned” to the nominal resonant frequency. An optimal turning point exists, corresponding to the systems maximum power-transfer capability when optimal tuning capacitors are applied.

Powering Implantable Telemetry Devices from Localized Magnetic Fields

This paper presents a novel method of inductively powering an implantable telemetry device over a large area. The system is based around an array of individually tuned series resonant circuits distributed across a charging pad. By varying the frequency of the driving voltage, the location of the charging field is changed. Presented are a method of controlling the resonant frequency and techniques for determining the geometry of the charging pad. Results from a nine coil system operating between 97 kHz and 209 kHz are given which can deliver 100 mW to an implanted telemeter at a height of 5 cm above the charging pad.