Wireless Implant Technology in Medicine
The broad impact of wireless microelectronics in cardiovascular disease and neuroscience.
Wireless biomedical implants primarily communicate via a biosensor or battery-operated device surgically placed within a defined location of the human body to measure a specific physical condition1. The device can be improved to be fully/partially biodegradable to avoid a second surgery for its retrieval after the intended period of use2. Wireless sensors within the body are able to monitor conditions of pressure and/or temperature2 as intracranial measurements when treating traumatic injury3 and monitor heart function post-surgery with implants known as CardioMEMS4,5 built as microelectromechanical systems (MEMS) (Fig 1 A,B) 4,5. Despite technical achievements of implantable medical devices that started with battery operated-pacemakers in 19586, the unique nature of an implant’s application environment i.e., the human body, has imposed several challenges for the design of low power consuming, miniaturized sensors1. The evolving field of biomedical microelectronics technology includes design and fabrication of ultra-low power miniature transistors, to redefine smart implants for widespread applications7.
In wireless communication, telemetry is the automated process by which measurements (data) collected at remote/inaccessible points are transmitted (via radiofrequency, ultrasonic or infrared systems) to receiving equipment for monitoring7. When the concept is applied to biomedical telemetry it enables communication with an implanted wireless device for cutting-edge biomedical research8. Ongoing studies of the implants in their designated clinical/translational roles allow existing deficiencies and overlooked issues to be identified and resolved to develop next-generation wireless biosensors3,4.
DESIGN AND FUNCTION -
Most biomedical implants are composed of an internal device (in vivo) and an external host (ex vivo) that controls it while collecting data provided by the internal segment1. In practice, radiofrequency-driven, resistor-inductor-capacitor (RLC) resonators are commonly used for wireless data transmission in short-range (distance of centimeters) telemetry, powered by a low-frequency inductive link, whereas long-range (> 2 m) telemetry devices are battery operated with stricter design regulations and powered by authorized frequency bands approved for Medical Implant Communication Service (MICS)1. Depending on the desired function, implants can either telemeter data externally, receive and execute commands from an external host or perform both operations7. Whilst the design considerations for biomedical telemetry devices are vast1,8, the main constraints during device design include its size, lifetime and associated power consumption/losses2,8. Next generation implants can be made partially or fully biodegradable via fabrication with biodegradable metals or polymer materials, to ensure the device is disposed on completion of its function (Fig 2)2.
CardioMEMS TO MONITOR HEART DISEASE
The close relationship between volume and pressure in chronic heart disease can be monitored with implantable devices4, which led to the hypothesis that hemodynamic guided care of heart failure (HF) in patients classified as New York Heart Association class III HF maybe a superior strategy compared to traditional clinical care tools9. The wireless sensor technology CardioMEMS does not require leads (a sensing wire) or batteries to measure pulmonary artery pressure (PAP), simplifying the implantation process9. By design, the HF sensor constitutes a coil and pressure-sensitive capacitor encased in a hermetically sealed silica capsule covered with silicone, with two nitinol loops at the ends of the capsule allowing attachment of the device to the pulmonary artery branch9 (Fig 3). A variation of the sensor can be integrated with a coronary stent to improve placement versatility in vivo (Fig 1B) and monitor function of the stent themselves in addition to monitoring heart failure5.
CardioMEMS HF sensor is a permanent implant, which typically functions as the coil and capacitor forms an electrical circuit that resonates at a specific frequency10. In the clinic, an external antenna is held against the patient’s body to power the implanted sensor, which returns a signal when energized. Since the capacitance of the sensor circuit varies with the pressure of the environment in vivo, the resonant frequency varies accordingly4,9. The shift in the resonant frequency received is sent to the electronics unit for processing into a pressure waveform, to complete cardiac pressure monitoring. Patients are also able to independently monitor their own PAP with a similar home-based setup. While CardioMEMS demonstrated statistically significant effectiveness in clinical trials for HF management as expected9 and subsequently received US FDA approval, its cost-effectiveness relative to ‘quality adjusted life-year (QALY)’ gained and the HF classification remain to be determined relative to trial effectiveness over long periods of time 11.
EXPERIMENTAL BIORESORBABLE SENSORS FOR THE BRAIN
In modern clinical medicine, standard permanent electronic hardware can be replaced with next-generation bioresorbable sensors to eliminate device extraction altogether and prevent long-term pathological tissue reactions in vivo3. An experimental bioresorbable silicon electronic sensor (Fig 1A) implanted in laboratory rats can diligently monitor intracranial pressure and temperature as intended for 3-5 days, before natural resorption via hydrolysis in the organism, to prevent a second surgery for its extraction3.
In the experimental study, pressure responses recorded in an in-lab artificial intracranial set-up (in vitro) using the short-lived biosensor were in agreement with commercial biosensors of clinical standards, followed by similar trends of intracranial pressure (ICP) and temperature measured in freely moving rats (in vivo) (Fig 4 ab). The operational principle of the logic chip containing piezoresistive and thermoresistive sensors is driven by wireless delivery of power via an external reader, to provide small currents and assess the response (where changes in resistance represent changes in pressure or temperature in the intracranial space), then recorded and transmitted to the external reader3.
The experimental field of biomedical electronics is continuously evolving with increasingly sophisticated capabilities introduced in alignment with medical practice guidelines12 to improve the applications of wireless communication. Wireless powering, a central theme in biomedical systems design, dominates the architectural and implantation decisions of microdevices, optimized via high-frequency energy transfer mechanisms13 and the integration of ‘phased surfaces’14 to enable energy penetration across heterogeneous tissues13 and power microdevices at greater depths than previously accessible14.
In the immediate future, further research on wireless implantable technology envisioned by the Neural Engineering Systems Design of the Defense Advanced Research Projects Agency (DARPA), collaboratively aims to create a ‘brain intranet’; implanting multitudes of microdevices as small as grains of salt (neurograins) to wirelessly monitor function at the level of a single neuron in the brain. Such unprecedented research efforts will assist clinical therapeutic strategies that aim to restore brain function lost to injury or disease. Attaining such unparalleled levels of precisely detailed neural activity may also inspire ongoing development of modern computing systems that strive to understand mechanisms of our brain for better performance.
Poster Image: An animation on Connecting Networks.
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- Boutry, C. M. et al. Towards biodegradable wireless implants. Philosophical Transactions of the Royal Society A 370, 2418-2432, (2012).
- Kang, S.-K. et al. Bioresorbable silicon electronic sensors for the brain. Nature 530, 71-76, (2016).
- Castro, P. F. et al. A Wireless Pressure Sensor for Monitoring Pulmonary Artery Pressure in Advanced Heart Failure: Initial Experience. The Journal of Heart and Lung Transplantation 26, 85-88, (2007).
- Chow, E. Y., Chlebowski, A. L., Chakraborty, S., Chappell, W. J. & Irazoqui, P. P. Fully wireless implantable cardiovascular pressure monitor integrated with a medical stent. IEEE Trans Biomed Eng 57, 1487-1496, (2010).
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- Cirmirakis, D., Demosthenous, A. & Nikita, K. S. in Handbook of Biomedical Telemetry 27-55 (John Wiley & Sons, Inc.) (2014).
- Adamson, P. B. et al. CHAMPION∗ Trial Rationale and Design: The Long-Term Safety and Clinical Efficacy of a Wireless Pulmonary Artery Pressure Monitoring System. Journal of Cardiac Failure 17, 3-10, (2011).
- Joy, J., Kroh, J., Ellis, M., Allen, M. & Pyle, W. (Google Patents, 2007).
- Sandhu, A. T. et al. Cost-Effectiveness of Implantable Pulmonary Artery Pressure Monitoring in Chronic Heart Failure. JACC: Heart Failure 4, 368-375, (2016).
- Poole, J. E. Present Guidelines for Device Implantation. Clinical Considerations and Clinical Challenges From Pacing, Implantable Cardiac Defibrillator, and Cardiac Resynchronization Therapy 129, 383-394, (2014).
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- Agrawal, D. R. et al. Conformal phased surfaces for wireless powering of bioelectronic microdevices. 1, 0043, (2017).