Daniel McCormick

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.

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.

Wireless Power Supply for ICP Devices With Hybrid Supercapacitor and Battery Storage

Hydrocephalus patients use a shunt to drain excess fluid from the brain, however, these shunts fail regularly. The most valuable shunt diagnostic tool would be a pressure sensor implanted in the brain. This paper presents a wireless power transfer system being able to support the measurement of intracranial pressure (ICP) for a lifetime operation. A rechargeable battery is complemented by a new energy storage element-an electric double-layer capacitor to form a hybrid energy storage system, and a dynamic energy management algorithm is developed to control the power delivery. The charging circuitry for the hybrid system is founded on using the inductive power transfer technology, and the energy management scheme schedules the flow to power the ICP device automatically to meet both short-and long-term operation requirements. The voltage levels of the supercapacitor and rechargeable battery are continuously monitored, and the experimental results demonstrate that it takes only 3 seconds to charge up the supercapacitor for 1-minute operation of ICP for quick data collection, while longer durations of monitoring can be supported by the battery.

Chronic measurement of left ventricular pressure in freely moving rats

Measurements of left ventricular pressure (LVP) in conscious freely moving animals are uncommon, yet could offer considerable opportunity for understanding cardiovascular disease progression and treatment. The aim of this study was to develop surgical methods and validate the measurements of a new high-fidelity, solid-state pressure-sensor telemetry device for chronically measuring LVP and dP/dt in rats. The pressure-sensor catheter tip (2-Fr) was inserted into the left ventricular chamber through the apex of the heart, and the telemeter body was implanted in the abdomen. Data were measured up to 85 days after implant. The average daytime dP/dt max was 9,444 ± 363 mmHg/s, ranging from 7,870 to 10,558 mmHg/s (n = 7). A circadian variation in dP/dt max and heart rate (HR) was observed with an average increase during the night phase in dP/dt max of 918 ± 84 mmHg/s, and in HR of 38 ± 3 bpm. The β-adrenergic-agonist isoproterenol, β1-adrenergic agonist dobutamine, Ca(2+) channel blocker verapamil, and the calcium sensitizer levosimendan were administered throughout the implant period, inducing dose-dependent time course changes and absolute changes in dP/dt max of -6,000 to +13,000 mmHg/s. The surgical methods and new technologies demonstrated long-term stability, sensitivity to circadian variation, and the ability to measure large drug-induced changes, validating this new solution for chronic measurement of LVP in conscious rats.

A high bandwidth fully implantable mouse telemetry system for chronic ECG measurement

We report on the development of a novel system that enables the wireless transmission of high-bandwidth physiological data from a freely moving mouse. The system employs inductive power transfer (IPT) to continuously power a battery-less transmitter using an array of overlapping planar coils placed under the animal. This arrangement provides a minimum of 20mW at all locations and orientations across the mouse cage by selecting a coil which will sufficiently power the transmitter. Coil selection is performed by feedback control across the 2.4 GHz wireless link. A device was constructed utilizing this novel IPT system and was used to capture high-fidelity electrocardiogram (ECG) signal sampled at 2 kHz in mice. Various attributes of the ECG signal such as QT, QRS, and PR intervals could be obtained with a high degree of accuracy. This system potentially provides lifetime continuous high bandwidth measurement of physiological signals from a fully implanted telemeter in a freely moving mouse.

MRI interactions of a fully implantable pressure monitoring device

To investigate the potential patient risk and interactions between a prototype implantable pressure monitoring device and a 3T clinical magnetic resonance imaging (MRI) machine to guide device design towards MR Conditional safety approval.

Design methodology for inductive power transfer systems targeting high power implantable devices

Developing a resonant inductively powered system for implantable devices is difficult due to the constraints on the operating coupling range, power level, and size. It is not trivial to determine the suitable resonant topology, tuning capacitors, power transfer coils, and operating frequency. Often the system parameters, such as the operating frequency, are designed using nominal values or a trial and error approach. We present a detailed numerical model which included frequency dependent parameters to accurately model the resonant system of an inductive power system, so that unexpected characteristics that might be hidden through approximations will be revealed. A procedure based on numerical simulation in the frequency domain is proposed to determine the resonant topology and tuning capacitances for achieving maximum efficiency with a set of power transfer coils across the specified operating coupling and loading conditions. A wireless power supply has been designed using this methodology, which increased the power efficiency by 4% on average over the full operating conditions.

Pulse-Width Modulation of Optogenetic Photo-Stimulation Intensity for Application to Full-Implantable Light Sources

Optogenetics allows control of neuronal activity with unprecedented spatiotemporal precision, and has enabled both significant advances in neuroscience and promising clinical prospects for some neurological, cardiac, and sensory disorders. The ability to chronically stimulate light-sensitive excitable cells is crucial for developing useful research tools and viable long-term treatment strategies. Popular optogenetic stimulation devices often rely on bench-top light-sources tethered via an optical fibre to the research animal, or significant componentry protruding externally from animal. These approaches are prone to infection, vulnerable to damage and restrict the experimental approaches that can be conducted. An ideal optogenetic stimulator would be contained entirely within the animal and provide precisely controlled optical output. However, existing prototypes of fully implantable devices rely on amplitude tuning of wireless power, which can vary strongly with environmental conditions. Here we show that pulse-width modulation (PWM) of the intensity of a light-emitting diode (LED) can enable control of photo-stimulation intensity equivalent to direct amplitude modulation. This result has significant implications for fully implantable light delivery tools, as PWM can be implemented with simple and miniaturized circuit architectures. We have modified a telemeter device previously developed by our group to include a small form-factor LED capable of generating sufficient optical power with manageable electrical power requirements and minimal heat generation. We have tested key device components in an in vitro mouse brain slice preparation and shown that pulse-width-modulation is an alternative method to modulate photo-stimulation intensity using a miniature circuit and providing easy control.

Implantable Multi-Modal Sensor to Improve Outcomes in Hydrocephalus Management

Hydrocephalus is the single most common pediatric neurosurgical problem worldwide. Current treatment of this life-threatening disorder involves diverting excess fluid from the ventricles of the brain via a prosthetic shunt. While many hydrocephalus sufferers rely heavily on their ventriculo-distal shunt to maintain a healthy intracranial pressure, shunts carry a high risk of failure. Current methods of assessing shunt patency are performed within the hospital, and many patients and their families feel bound to remaining within close proximity of a hospital in order to receive timely medical intervention in the event of a shunt failure. There is a need for a system which can detect shunt malfunction, simply and reliably. We present a novel method of obtaining flow measurements from a piezoresistive pressure transducer. This builds on an earlier development of obtaining simultaneous temperature and pressure measurement from the single ultra-miniature solid-state transducer. The flow measurement system is capable of measurements in the range 0-35 ml/h, typical of the fluid flow rates through a hydrocephalus shunt. Within the flow range 0-14 ml/hour the resolution is 2 ml/hour. For flow rates greater than 16 ml/hour the resolution is 5 ml/hour. Employing a thermal flow sensing technique, the maximum heating of the local fluid is 0.65 ± 0.02 °C. The flow signal is independent of ambient temperature. The sensor would be implanted in the shunt to allow the detection of the flow rate of fluid through it, enabling the clinician to measure the patency of a shunt in real time.

A New Tool for the Neurointensive Care Unit: Simultaneous Measurement of Temperature From a Catheter-Tip Pressure Sensor

Traumatic brain injury is a leading cause of death and permanent disability throughout the world. Over the past 15 years, basic science has made important advances in the understanding of this condition. These advances have not, however, translated into treatment of clinically proven benefit. It has been suggested that individualized care for brain injured patients should include improved monitoring of brain function. For clinical gains to be made, clinicians and researchers require appropriate diagnostic and monitoring tools. In this paper, we describe the novel method of acquiring temperature measurement from a pressure sensor. The method allows concurrent measurements of pressure and temperature. The temperature measurement has a sensitivity of 85.08 mV/°C, across the measurement range 20 °C-45 °C. The time constant of the temperature sensor is 610 ± 55 ms. The mean cross-sensitivity of the temperature signal to changes in pressure is 0.74 m°C/mmHg within the typical physiological pressure range (0-160 mmHg). A method to compensate for this pressure-related error is described. We have evaluated the accuracy of the temperature measurement and the long-term stability of 13 sensors over a period of 28 days. The mean difference between temperature measurements made from the sensors and those made from the reference sensor was <;0.2 °C.