Sanghoek Kim

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.

Midfield Wireless Powering for Implantable Systems

Efficient wireless power transfer across tissue is highly desirable for removing bulky energy storage components. Most existing power transfer systems are conceptually based on coils linked by slowly varying magnetic fields (less than 10 MHz). These systems have many important capabilities, but are poorly suited for tiny, millimeter-scale implants where extreme asymmetry between the source and the receiver results in weak coupling. This paper first surveys the analysis of near-field power transfer and associated strategies to optimize efficiency. It then reviews analytical models that show that significantly higher efficiencies can be obtained in the electromagnetic midfield. The performance limits of such systems are explored through optimization of the source, and a numerical example of a cardiac implant demonstrates that millimeter-sized devices are feasible.

Implantable biomedical devices: Wireless powering and communication

In recent years, there has been major progress on implantable biomedical systems that support most of the functionalities of wireless implantable devices. Nevertheless, these devices remain mostly restricted to research, in part due to limited miniaturization, power supply constraints, and lack of a reliable interface between implants and external devices. This article provides a tutorial on the design of implantable biomedical devices that addresses these limitations. Specifically, it presents analysis and techniques for wireless power transfer and efficient data transfer from both theoretical and practical standpoints. Their potential implementations are also discussed.

Wireless power transfer to a cardiac implant

We analyze wireless power transfer between a source and a weakly coupled implant on the heart. Numerical studies show that mid-field wireless powering achieves much higher power transfer efficiency than traditional inductively coupled systems. With proper system design, power sufficient to operate typical cardiac implants can be received by millimeter-sized coils.

Wireless Power Transfer to Miniature Implants: Transmitter Optimization

This paper examines transmitter optimization for wirelessly powering a small implant embedded in tissue. The wireless link between the transmitter and receiver is first modeled as a two-port network and an expression for the power transfer efficiency derived. For a given small receiver in a multilayer tissue model, the transmitter is abstracted as a sheet of magnetic current density for which the optimal distribution is analytically found. The optimal transmitter is compared to the point and uniform source across a range of frequencies. At higher frequencies, the optimal current distribution is shown to induce fields that exhibit focusing. The effects of constructive and destructive interference substantially improves the power transfer efficiency and reinforces operation in the low GHz-range. The optimal transmitter establishes an upper bound on the power transfer efficiency for a given implant and provides insight on the design of the optimal transmit antenna.

Optimizations of source distribution in wireless power transmission for implantable devices

Implantable medical devices will play an important role in modern medicine for preventive and post-surgery monitoring, drug delivery, local stimulation, and biomimetic prosthesis. To reduce the risk of wire snapping, and replacement and corrosion of embedded batteries, wireless delivery of energy to these devices is desirable. Current studies in wireless power transmission into biological tissue tend to operate below 10 MHz because of the common belief that lower operating frequency yields higher power transfer efficiency. Our previous work [1], however, showed that the optimal frequency maximizing the power transfer efficiency is in the low GHz frequency range. The corresponding wavelength inside tissue is a few centimeters which is in the same order as the dimension of the transmit coil. This suggests that we should consider using an array of transmit coils to focus the electromagnetic energy and hence boost up the received power further.

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.

Optimal transmit dimension for wireless powering of miniature implants

RF wireless interface enables remotely powered implantable devices. To maximize the received power under the safety constraint, we analytically solved the optimal current source distribution and investigated the theoretical upper-bound of the efficiency obtainable with planary vertical magnetic current sources. The optimal solution reveals that a finite dimensional source is sufficient to approach the theoretical upper-bound. At the low MHz-range, a coil of approximately 2 cm in diameter is adequate. At the low GHz-range, multiple coils each with different phases and magnitudes are required. However, the efficiency at the low GHz-range is 16-dB higher than that at the low MHz-range for typical depth of the implant.

Motion control of multiple autonomous ships to approach a target without being detected

We introduce a motion control which is a stealth strategy of a team of autonomous ships to approach a target without being detected. The target is equipped with a radar to detect a nearby ship. In the team, one ship, the leader, has a stealth ability not to be detected by a radar as it approaches the target. We steer the leader so that it moves to the target while appearing to be at the same bearing line from the target to the leader. Assuming that the target can readily observe optical flow, but only poorly sense looming, this type of motion by the leader is difficult to be detected by the target. We steer every ship, except for the leader, so that a pulse signal emitted from the radar on the target cannot reach any ship, since the signal is blocked by the leader. In this way, even in the case where multiple ships approach the target, the target cannot detect any ship. In our control law, a ship, except for the leader, does not have to access the position of the target in real time. However, a ship must access the bearing line from the target to the leader in real time. Our motion control is inspired by line-of-sight guidance control laws and is developed in discrete-time systems. We present MATLAB simulations to verify the performance of our motion control law.

Stealth path planning for a high speed torpedo-shaped autonomous underwater vehicle to approach a target ship

Abstract Let us consider a scenario where a high speed torpedo-shaped autonomous underwater vehicle (AUV) approaches a target ship. In an underwater environment, radio waves are easily dissipated. Thus, sound is the main energy transmitted from the AUV to the target ship. We plan the path of the AUV to reduce both the time required to meet the target ship and the AUV’s sound measured by the target. We consider a torpedo-shaped AUV equipped with backward propellers to move forward and fins for heading control. For this type of vehicle, there is an instantaneous maximum turn rate for its motion. In order to increase the safety of path, we plan the AUV’s path so that a sharp corner along the path is avoided. We verify the effectiveness of our stealth path planning method using MATLAB simulations.