Wireless power transfer to deep-tissue microimplants

Abstract

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.biomedical electronics | microstimulator P rogress in semiconductor technology has led to electronic devices that can augment or replace physiological functions; their ability to be implanted for direct interaction with organ systems relies on overall miniaturization of the device for simplified delivery (e.g., via catheter or hypodermic needle) and access to interstitial spaces. Advances over the past few decades enable most components in a biomedical device, including electrodes, oscillators, memory, and wireless communication systems, to be integrated on tiny silicon chips. However, the energy required for electronic function remains substantial and the consumption density has not been matched by existing powering technologies (1). As a result, the vast bulk of most implantable electronic devices consists of energy storage or harvesting components.Although considerable progress has been made in energy storage technologies, batteries remain a major obstacle to miniaturization (2, 3) because their lifetimes are limited and highly constrained by the available volume, requiring periodic surgical replacement once the unit is depleted. Energy-harvesting strategies have been developed to eliminate batteries or to extend their function. Previous demonstrations include thermoelectric (4), piezoelectric (5-7), biopotential (8), or glucose (9, 10) power extraction. However, these methods are anatomically specific and, in their existing forms, yield power densities too low (<0.1 μW/mm 2 ) for a microimplant. Alternatively, energy can be transferred from an external source. I...

Toward Bioelectronic Medicine—Neuromodulation of Small Peripheral Nerves Using Flexible Neural Clip

“…Reaching and stimulating these nerves requires excess wiring as well as powering scheme. To avoid implanting a power source, we performed wireless remote neuromodulation of pelvic nerves using the FNC integrated with a tiny coil ( Figure 6 a) for the midfield powering scheme which allows the transfer of mW levels of power to FNC in deep tissue (>5 cm) 18. Figure 6b shows the assembled active FNC implanted on a pelvic nerve.…”

“…These challenges include (i) for the autonomic nervous system, complex innervation of the organs or muscles, rendering precise control of specific functions challenging; (ii) quick and mechanically secure implantation which is an important consideration in the presence of physiological motion such as respiratory and cardiovascular movements; and (iii) considerable compliance and flexibility from the neural interfaces since nerves are highly compliant and associated with moving organs. Additionally, the integration of NIT with wireless powering by either ultrasound17 or electromagnetic powers18 is a promising direction for future bioelectronic medicine.…”

ENERGY TRANSFER FOR IMPLANTABLE ELECTRONICS IN THE ELECTROMAGNETIC MIDFIELD (Invited Paper)

“…Experiments in porcine tissue in the cardiac [ Figure 3 transferred power. A microelectronic stimulator, about 2 mm in diameter and 3 mm in length, weighing 70 mg, was powered using this method and used to pace the heart of a rabbit [31]. These results show that energy transfer can be made largely insensitive to fine tissue structure, provided that some phaseadjustment mechanism is available at the source.…”

“…A metal plate patterned with circular slots was shown to reproduce the main features of the optimal current sheet [ Figure 3(a)]. The structure is excited by four radio-frequency ports [31] with different relative phases, which can be adjusted to control the position of the focal point [ Figure 3(b)]. A tiny microelectronic device, containing a coil about 2 mm in diameter, was used to measure power transfer.…”

Non-Coil, Optimal Sources for Wireless Powering of Sub-Millimeter Implantable Devices

“…The maximum efficiency was shown to be a function of the dielectric property of the tissue, distance between the source and receiver (z f ), and orientation (θ) of the receiver coil. The optimal source with its efficiency close to the global bound was physically realized and measured in [9].…”