Wireless far-field remote powering has the potential to revolutionize the world of medical devices. This technology addresses a growing need in the medical device field, and allows for incredible size reductions, which is beneficial in a variety of areas including neural, ocular, and cardiovascular applications. As a result of recent advances in semiconductor, packaging, and bio-interface technology, state-of-the-art millimeter-sized wireless implantable devices exits or are under development, for clinical applications such as optogenetic stimulator (OGS) for epilepsy (5 mm×10 mm), cardiovascular monitoring (3 mm×6 mm), glaucoma intraocular pressure (IOP) monitoring for mouse eye (0.7 mm×1.3 mm) and human eye (3 mm×6 mm), and implantable electromyogram (EMG) electrode (1.5 mm×10 mm) for targeted muscle reinnervation (TMR) control of prosthetic limbs.
In practice, because the quality of RF powering is very sensitive to the position of the receiver antenna within the RF electromagnetic field, the energy storage component on a chip plays a crucial role in supplying the local power for continuous and reliable operations such as wireless data transmission. Thus, there is a strong need for the development of compact energy storage solutions with appropriate size to meet strict clinical dimension constraints while still providing sufficiently high energy for useful operation.
Unfortunately, current research on integrating miniature energy storage components on a chip is still inadequate. So far, batteries are the primary choice for implantable medical devices. However, as the size of these devices decreases to millimeter scales and beyond, currently available batteries far exceed the available volume for a medical implant. Worse still, despite having a high energy density, batteries suffer from fundamental problems caused by the thermodynamic chemical reactions they are based on: slow charge rate (several hours), limited life time (hundreds to thousands of charge/discharge cycles) and safety concerns associated with using toxic metal materials. The issue of battery replacement after one-to-two-year usage is not only expensive but also involves a potentially high-risk surgery for the patient. Alternatively, clinical size constraints might be met with commercial high-dielectric ceramic surface-mount capacitors with the largest available capacitance. In this case, many capacitors are needed to build power storage units for high energy demands. However, the overall capacitance is still limited, and the 300-μm thickness of commercial capacitors and their rigid mechanical properties make implantable devices unfeasible for many anatomical placements (e.g., the anterior chamber of eye and the nerve interface of muscle). Hence, a tremendous challenge remains in the development of an efficient power supply for wireless miniature implantable devices with high energy density, long life time and mechanical flexibility.
Recently, electrochemical capacitors, also known as supercapacitors, have attracted a great deal of attention. A supercapacitor consists of two electrodes sandwiching a separator immersed in an electrolyte, storing energy based on two mechanisms: electrical double layer capacitance (EDLC, accumulation of ions at the interface between a highly porous electrode and an electrolyte) and pseudo-capacitance (fast and reversible redox reactions at the surface or near-surface of the electrode, providing much higher specific capacitance than EDLC). Compared with conventional physical electrolytic or ceramic capacitors, supercapacitors have a much higher capacitance value in relatively small volumes due to the use of an electrode material with extremely high specific surface area.
Supercapacitors achieve these advantages, however, at the expense of breakdown voltage. Nevertheless stacking devices can alleviate this issue. Stacked supercapacitors can still be smaller and have a significantly greater energy density (e.g., >1 mF/mm2) than conventional capacitors. In addition, supercapacitors do not require a slow charge/discharge process to satisfy the slow thermodynamic chemical reactions in batteries, so they can handle currents that are larger by several orders of magnitude with an efficiency exceeding 90%, and can provide a long life time of up to half a million charge/discharge cycles. The extremely long cycling life of supercapacitors potentially boosts a more-than-10-year operational lifetime before the energy capacity is reduced to 80%, which is a great advantage for implantable medical device applications.
As a consequence, supercapacitors bridge the gap between conventional capacitors and batteries. With the development of low-power circuit technology, researchers have shown great interest in extending the application of supercapacitors to micro-power systems. However, current research efforts on miniature supercapacitors have largely concentrated on the scientific level and on electrode material characterization.
There is a need, therefore, for supercapacitor design as well as implementation methods and arrangements that are suitable for use in implantable medical applications.