As implantable devices become smaller and demand “smart” functionalities like wireless communication and advanced sensor arrays, an enormous strain is placed on their existing power architectures. Reduced implant battery life is quite concerning, because more implantation surgeries—and further chance for infection due to those surgeries—become both a health risk and added expense for patients.
One strategy to keep an implant’s battery chugging along effectively is to wirelessly charge the battery. However the traditional inductive coupling power transfer strategy typically used to charge cell phones has biocompatibility and alignment issues of its own. MDT wanted to learn more about alternative ways to wirelessly charge an implant’s battery, so spoke with Colin McCarthy, senior manager of business development for the medical, industrial, and defense industries at WiTricity. Here’s what he had to say:
MDT: What is the problem with using traditional magnetic induction to wirelessly charge implantable devices?
McCarthy: One of the central challenges of using traditional magnetic induction to wirelessly charge implantable devices is that induction requires very close proximity and precise alignment, meaning that this kind of technology is unable to charge deep inside the body. In addition, when not perfectly aligned, magnetic induction power transfer can generate heat that causes pain and burns to the skin. With magnetic resonance, on the other hand, more power can be safely transferred through the skin without the heating, invasiveness and risks associated with induction solutions.
MDT: How does coil misalignment during traditional magnetic induction heat up skin?
McCarthy: With traditional magnetic induction, even a slight misalignment of coils can result in substantially decreased efficiency. Efficiency loss manifests itself as heat. Therefore, in a traditional magnetic inductive system, when coils are misaligned, the resulting efficiency loses causes heating on the coils which in turn can substantially heat the skin around the implant.
MDT: Reliability has been a concern for wireless implant charging this far; how can it be improved?
McCarthy: With magnetic induction, implantable devices need to be closely aligned with the source in order to charge. Because it can be difficult to establish and maintain perfect alignment with an implanted device, charging can be difficult and lead to frustration from patients and caregivers.
Magnetic resonance, rather, delivers positional freedom — meaning the device does not need to be precisely aligned to the source to charge effectively. As a result of this positional freedom and efficiency, the system is much easier to use and can be more reliably charged. Even if the charging source is not precisely aligned or the implant moves around, high efficiency power transfer can still be achieved.
MDT: How much power transfer is safely possible with magnetic resonance induction?
McCarthy: WiTricity has developed systems capable of safely transferring milliwatts of power for wearable devices; watts for cell phones, tablets and notebooks; and kilowatts for electric vehicles. The amount of power that can be safely transferred to a medical implant is closely tied to the device’s power requirements, form factor, battery type and device side electronics design.
MDT: How does the design approach to a power module differ for more deeply implanted devices?
McCarthy: Magnetic resonance enables smaller, simpler designs that are less burdensome due to the increased efficiency of power transfer. With efficient wireless charging, batteries can be made smaller — thereby decreasing weight and size while offering a more comfortable experience for users.
MDT: Which implantables are you finding to be the best candidates for wireless charging? Why?
McCarthy: Implantable devices that require higher levels of power transfer, need to go deep into the body and/or require more frequent charging are good candidates for WiTricity’s technology as it enables flexible positioning and increased distance for high efficiency charging, as well as the ability to charge through non-metallic materials such as plastic and glass (and of course, human tissue).
MDT: Which implantables present a particular challenge to wirelessly charge? Why?
McCarthy: With traditional inductive charging, devices that need to go deep or in hard-to-reach parts of the body are challenging to charge, as are devices that require higher levels of power transfer (above 5W) or frequent charging. However, with magnetic resonance, all of these challenges are eliminated.
MDT: What new developments/opportunities do you think we’ll see in the next 5 years?
McCarthy: Through advancements made by our partners, like Greatbatch, that are committed to giving designers the tools they need to develop smaller, safer, easier to use and more reliable solutions for physicians and patients, we can expect patients to have more convenient charging experiences. We can also expect reduced battery sizes, deeper implantation, smaller more localized devices, as well as more complex neurostimulation and therapies that may require higher amounts of power.