Purpose: A new generation of miniaturised, implanted, cardiac sensing devices, possibly introduced via a catheter, is being developed by disruptive product designers. Conventional medical batteries are packaged within metallic cans for safety purposes; they are also typically primary (non-rechargeable) and must contain the whole energy required during the life of the device they power from first day of implantation. For these reasons, miniaturisation of conventional medical batteries is limited to a few 10s of cm3. New active implanted sensing devices are being designed that are less than 1 cm3 in volume, including some measuring cardiac-related parameters, hence requiring a new energy source. Millimetre-scale solid-state batteries, which do not need significant packaging, have been developed to uniquely enable miniaturisation of next-generation implantable cardiac sensing devices.
Methods: Solid-state batteries were fabricated by physical vapour deposition and sputtering. Key developments to increase energy density used the following methods:
- Implement photolithography as a method for patterning the battery’s sub-layers at the micron level, enabling miniature features
- Thin down the substrate, i.e. the mechanical support for the batteries, enabling high-energy density
- Stack and interconnect single cells on top of each other, to multiply the energy of the resulting battery for the same footprint
- Increase the cathode thickness in order to store more energy.
The rechargeable batteries have been developed on Ilika’s first volume manufacturing line, which opened in Southampton, Hampshire, in 2021, the first of its kind in the UK.
Results: Ilika’s first solid-state batteries were produced, down to a 15 mm2 footprint and total thickness 1 mm. The batteries consisted of six stacked cells, interconnected in parallel, yielding a total capacity of 300 uAh and nominal voltage of 3.5 V. The arial energy density of the stacked battery was measured to be approximately 12.5 uAh/mm2. Internal resistance of the full stack was measured to be about a sixth of that of each single cell forming the stack, enabling peak power of a few mA. These batteries could be recharged in as little as 8 minutes with heating of the battery less than 2°C upon fast charging. These batteries are going through their final development stage and will go through full medical certification in 2023.
Discussion: A novel technique for stacking and interconnecting solid-state cells was shown to significantly increase the energy density and decrease the internal resistance of the battery stack. This development could enable further development in implanted cardiac sensors by providing an energy source of minimal size (mm-scale footprint and µm-scale thickness), appropriate energy density for increasing functionalities, and long life avoiding the risk and cost of removal. Use of rechargeable batteries for in-the-body applications have historically suffered from patient compliance with regards to regular charging. Whilst a new conversation with the patient is required, the benefit of miniaturising non-life-critical sensors that can be recharged in less than 10 minutes, is expected to outweigh the need for regular recharging. ❑