Powering miniaturized devices within the human body is a key technological challenge for next-generation bioelectronic systems. Most devices today are powered by batteries, which are bulky and have limited lifetimes. Wireless powering has long been studied as an alternative , starting from the first pacemaker implanted in 1958 to commercial medical devices today such as cochlear and retinal implants. In nearly all devices, the system is based on inductively coupled coil pairs. This technology is well suited for large devices implanted near the skin surface, but cannot efficiently power miniaturised devices implanted deep in the body because of the evanescent nature of the electromagnetic field.
Almost seven years ago, Professor Ada Poon at Stanford University pointed out that operating in a different electromagnetic regime could be advantageous for powering tiny devices . By changing the operating frequency such that the wavelength in tissue is comparable to the distance of separation, power can be transported by propagating rather than evanescent fields. In this regime, the three-dimensional field pattern created within the body can be shaped by interference. This observation was not entirely new: for decades, researchers had been designing phased arrays to focus microwaves at select regions of the body for applications in hyperthermia therapy . But the ability to focus the field depends on the spatial phase resolution — how fast the phase can change over a length scale. To achieve even moderate resolutions, phased arrays rely on many antennas independently driven with different phases. Previous systems required racks of equipment, could not be adapted for different curved surfaces, and usually had to be submerged in water in order to match the surrounding material to tissue.
Figure 1. Wireless powering of a microdevice with the phased surface.
We looked for a way to integrate high-resolution phase control into a compact and conformal electromagnetic device suitable for wireless powering. We took inspiration from recently developed optical metasurfaces, which use resonant elements much smaller than the wavelength to implement phase shifts . The device we designed here - which we call a phased surface - generates a focal spot deep in tissue when placed on the body, within which very small devices can be wirelessly powered .
Of course, the body is highly heterogeneous and time-dynamic, and careful experiments were needed to show that our system could work in such an environment. Our co-investigator Professor Hung-Fat Tse, an expert in cardiac pacing and electrophysiology, invited us to test our system in his large animal research facility at the University of Hong Kong. Packing all our equipment, we flew to his facility to conduct wireless pacing experiments in adult pig models. We were able to use the system to pace the heart with a 2 mm diameter wireless stimulator, demonstrating the extreme degree of miniaturization that could be achieved in vivo.
To make the system practical for continuous use, improvements in the long-term reliability of the system will be needed. But looking ahead, we think that the most compelling applications are those in physiological sensors, neuromodulation, or cancer therapy where the device only needs to be activated periodically for short durations. The ability of the phased surfaces to wirelessly power devices that are smaller and deeper than currently possible may pave the way for new approaches to treat disease.
Our Paper: Agrawal, D. R. et al. Conformal phased surfaces for wireless powering of bioelectronic microdevices. Nat. Biomed. Eng. 1, 0043 (2017).
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 Agrawal, D. R. et al. Conformal phased surfaces for wireless powering of bioelectronic microdevices. Nat. Biomed. Eng. 1, 0043 (2017).