An age-old question for traditional implantable medical devices: What is the maximum distance to charge a battery safely, efficiently and quickly?
Rules of physics dictate that when exposed to an alternating magnetic field, Eddy current loops are induced within a conducting material due to Faraday's law of induction. A titanium can is a conductor, so the question boils down to: How much power can be transferred across a metallic (Titanium) case for the maximum efficiency and at what distance?
The answer is: It is a compromise between minimum achievable titanium wall-thickness that is sturdy enough to protect the electronics, maximum tolerable distance - typically 1 inch - to charge a battery with enough power - typically for many hours-, and to prevent tissue damage by minimizing the energy that is being transferred into heat in the enclosure material.
The good news is that with RF-transparent and non-electrical-conducting materials, none of these limitations apply. For example, an implant enclosure can be constructed using ceramic (e.g. Zirconia). Today, brazing technologies allow materials like titanium and Zirconia to be hermetically joined for applications that require an RF transparent and non-electrically-conducting package. And this opens tremendous opportunities for better implantable devices:
Using a Zirconia-based enclosure allows antennae of near-field (e.g. back-scatter) and far-field (401-406 MHz-band MedRadio) gear to be placed inside the enclosure without limited by the tight space inside a silicone header. An RF transparent enclosure will allow higher power-level wireless power-transmitters (within the safety limits of tissue exposure) and would allow wireless-power-feeding safely and effectively, reducing and even eliminating the need for batteries (primary cell or rechargeable), thereby enabling further device miniaturization.
Technology is advancing rapidly. It is time to break the mold and consider materials other than Titanium for safer, smaller, more effective medical devices, for an expanded range of indications.
Serdar Kiykioglu is the co-founder of LSNeuro.com, a medical technology company that developed the world's first indication-agnostic pulse-generator platform.