Alexander J. Yeh

Wireless power transfer to deep-tissue microimplants

Significance Advances in miniaturization paved the way for tiny medical devices that circumvent conventional surgical implantation, but no suitable method for powering them deep in the body has been demonstrated. Existing methods for energy storage, harvesting, or transfer require large components that do not scale to millimeter dimensions. We report a wireless powering method that overcomes this challenge by inducing spatially focused and adaptive electromagnetic energy transport via propagating modes in tissue. We use the method to realize a tiny electrostimulator that is orders of magnitude smaller than conventional pacemakers. The demonstrated performance characteristics far exceed requirements for advanced electronic function and should enable new generations of miniaturized electronic implants. The ability to implant electronic systems in the human body has led to many medical advances. Progress in semiconductor technology paved the way for devices at the scale of a millimeter or less (“microimplants”), but the miniaturization of the power source remains challenging. Although wireless powering has been demonstrated, energy transfer beyond superficial depths in tissue has so far been limited by large coils (at least a centimeter in diameter) unsuitable for a microimplant. Here, we show that this limitation can be overcome by a method, termed midfield powering, to create a high-energy density region deep in tissue inside of which the power-harvesting structure can be made extremely small. Unlike conventional near-field (inductively coupled) coils, for which coupling is limited by exponential field decay, a patterned metal plate is used to induce spatially confined and adaptive energy transport through propagating modes in tissue. We use this method to power a microimplant (2 mm, 70 mg) capable of closed-chest wireless control of the heart that is orders of magnitude smaller than conventional pacemakers. With exposure levels below human safety thresholds, milliwatt levels of power can be transferred to a deep-tissue (>5 cm) microimplant for both complex electronic function and physiological stimulation. The approach developed here should enable new generations of implantable systems that can be integrated into the body at minimal cost and risk.

Wirelessly powered, fully internal optogenetics for brain, spinal and peripheral circuits in mice

To enable sophisticated optogenetic manipulation of neural circuits throughout the nervous system with limited disruption of animal behavior, light-delivery systems beyond fiber optic tethering and large, head-mounted wireless receivers are desirable. We report the development of an easy-to-construct, implantable wireless optogenetic device. Our smallest version (20 mg, 10 mm3) is two orders of magnitude smaller than previously reported wireless optogenetic systems, allowing the entire device to be implanted subcutaneously. With a radio-frequency (RF) power source and controller, this implant produces sufficient light power for optogenetic stimulation with minimal tissue heating (<1 °C). We show how three adaptations of the implant allow for untethered optogenetic control throughout the nervous system (brain, spinal cord and peripheral nerve endings) of behaving mice. This technology opens the door for optogenetic experiments in which animals are able to behave naturally with optogenetic manipulation of both central and peripheral targets.

Wirelessly powering miniature implants for optogenetic stimulation

Conventional methods for in vivo optogenetic stimulation require optical fibers or mounted prosthesis. We present an approach for wirelessly powering implantable stimulators using electromagnetic midfield. By exploiting the properties of the midfield, we demonstrate the ability to generate high intensity light pulses in a freely moving animal.

PLANAR IMMERSION LENS WITH METASURFACES

The solid immersion lens is a powerful optical tool that allows light entering material from air or vacuum to focus to a spot much smaller than the free-space wavelength. Conventionally, however, they rely on semispherical topographies and are non-planar and bulky, which limits their integration in many applications. Recently, there has been considerable interest in using planar structures, referred to as metasurfaces, to construct flat optical components for manipulating light in unusual ways. Here, we propose and demonstrate the concept of a planar immersion lens based on metasurfaces. The resulting planar device, when placed near an interface between air and dielectric material, can focus electromagnetic radiation incident from air to a spot in material smaller than the free-space wavelength. As an experimental demonstration, we fabricate an ultrathin and flexible microwave lens and further show that it achieves wireless energy transfer in material mimicking biological tissue.

A Small Dual-Band Asymmetric Dipole Antenna for 13.56 MHz Power and 2.45 GHz Data Transmission

Despite the disparity between typical small antenna designs for wireless powering and far-field radiation, this letter proposes a single asymmetric dipole antenna for both applications on a printed circuit board (PCB) of dimension 10 × 24 mm 2. In this design, current cancellation is applied as a technique to enhance far-field radiation efficiency while maintaining inductive wireless power transfer performance. The measured radiation efficiency at 2.45 GHz is 57%, and the power transfer efficiency at 13.56 MHz over a distance of 20 mm is 2.15 dB. The results demonstrate the feasibility of a single dual-band antenna operating at two distinct frequencies while attaining reasonable efficiencies.

Optical probe for input-impedance measurement of in vivo power-receiving microstructure

An optical method is proposed to measure the input impedance of a microstructure in vivo operating at microwave frequencies. It makes use of the non-linearity of an AC-DC conversion circuit and encodes the received power level to the pulse rate of an LED which can be accurately measured by a photodiode outside the tissue medium. An accurate measurement of the characteristics of the power-receiving structures in a wireless powering system greatly enhance the power transfer efficiency.

Exceeding Nernst limit (59mV/pH): CMOS-based pH sensor for autonomous applications

A highly sensitive field-effect sensor immune to environmental potential fluctuation is proposed. The sensor circuit consists of two sensors each with a charge sensing field effect transistor (FET) and an extended sensing gate (SG). By enlarging the sensing gate of an extended gate ISFET, a remarkable sensitivity of 130mV/pH is achieved, exceeding the conventional Nernst limit of 59mV/pH. The proposed differential sensing circuit consists of a pair of matching n-channel and p-channel ion sensitive sensors connected in parallel and biased at a matched transconductance bias point. Potential fluctuations in the electrolyte appear as common mode signal to the differential pair and are cancelled by the matched transistors. This novel differential measurement technique eliminates the need for a true reference electrode such as the bulky Ag/AgCl reference electrode and enables the use of the sensor for autonomous and implantable applications.

Wireless Powering for Miniature Implantable Systems

The miniaturization of electronics has paved way for implantable devices at the scale of a millimeter or less. Progress in energy storage technologies, however, has been slower and the miniaturization of the power source remains unsolved. Wireless powering provides a potential solution in which electromagnetic energy is transferred from an external source. In this chapter, we analyze powering in the weakly coupled regime and discuss a specific example for a cardiac implant. For a weakly coupled device, we show that optimal powering occurs in the mid-field where power transfer occurs though a combination of inductive and radiative modes in tissue, in contrast to conventional inductive coupling.