Bioelectronic systems enabled by wireless electromagnetic power transfer

Abstract

Wireless biomedical electronic implants are rapidly being developed to treat a variety of medical conditions. Current technologies include the pacemaker to treat arrhythmias, the cochlear implant to overcome hearing impairment and the deep brain stimulator to treat Parkinson’s disease. Researchers are aiming to create implants that are miniaturised, battery-free, and minimally invasive. This is to ensure that devices are simpler to implant, to avoid surgical battery replacement and to minimise the risk of infection. To meet these demands, future biomedical electronic implants need to be miniaturised and capable of wireless power and data transfer. This thesis explores and extends the capabilities of three different wireless power transfer technologies for biomedical electronic implants: inductive, capacitive and radiative power transfer. This thesis adopts a systems approach to extend the capabilities of wireless power transfer systems.

Wireless inductive power transfer has received thorough attention in the literature and involves the use of time-varying magnetic fields to transmit power through biological tissue. Typically, inductive power transfer involves a single transmitter and single receiver. This thesis demonstrates many receiving devices can be operated from a single transmitter - without adding complicated electronics to each receiving device. Moreover, by tuning the receiving coil on each device carefully the transmitter can power individual devices, or multiple devices simultaneously, extending the capabilities of inductive power transfer systems. Optogenetics, a technique used to transfect cells to make them light sensitive, is used to provide biological validation of the multichannel inductive receiving topology. Human embryonic kidney cells are transfected to be sensitive to blue light and then a twin channel inductive receiver with a blue and yellow light is modulated to demonstrate a cell response and no cell response respectively.

Inductive coupling is not always the most suitable power transfer scheme and wireless capacitive coupling is presented as an alternative. This is where time-varying electric fields transmit power through biological tissue via conductive plates. Stenting, a surgical procedure used to prevent blood vessels from closing, is used to validate the efficacy of capacitive coupling in a biological context. Stents are thin metal tubes resembling chicken wire made from nitinol - a conductive nickel titanium alloy. There is significant motivation to include intelligent sensors in stents as they are simple to implant via angiographic catheter. However, stents preclude the use of batteries as they cannot be removed after surgery so wireless power and data transfer is essential.

The optimal frequency to use to transmit power to a stent via capacitive coupling is derived from first principles. Then, a miniaturised circuit board, capable of wireless power and data transmission is fabricated and placed between two stents. The wireless power and data transfer capabilities of the device are validated in-vitro in excised muscle tissue and in-vivo in a live ovine model. The results demonstrate that capacitive power and data transfer is viable for stent-based biomedical implants.

An emerging area of study is wireless radiative power transfer through biological tissue. Such a technique is promising for powering miniaturised, deep tissue implants. Due to the dispersive nature of biological tissue, finite element analysis is essential to understanding how wireless radiative power transfer can power biomedical electronic implants efficiently. This thesis builds on efficient radiative power transfer schemes by proposing a new implant and antenna geometry.

Long and thin implants show promise as they have the potential to be delivered by catheter or injection - reducing surgical risk and overhead. This thesis demonstrates a technique that uses near-field radiative power transfer to efficiently power a 20 mm long implant that is sub-millimetre in diameter. To power the device, optimised wide dipole transmitting antennas are simulated, designed, fabricated, tested and measured for various implant depths. Biological validation is provided by stimulating retinal ganglion cells wirelessly with the miniaturised device designed to power a small light.

In summary, the work presented in this thesis demonstrates that by extending wireless powering schemes from the well known inductive coil to include capacitive and radiative power transfer, implants can be miniaturized and inserted in places in the body that might have not seemed previously possible. Therefore, wireless biomedical electronics implants are likely to become miniaturised, battery-free and ubiquitous.

Whilst these techniques may offer significant economic and health benefits, there are also complicated ethical questions to consider. With the promise of pervasive, safe, minimally invasive and battery free biomedical electronic implants, humans will have the choice to enhance their abilities. Naturally, the question of what it means to be human will emerge.