On the Non-idealities of a Capacitive Link for Wireless Power Transfer to Biomedical Implants

Design and Optimization of Capacitive Links for Wireless Power Transfer to Biomedical Implants

This paper provides a comprehensive overview of capacitive wireless power transfer (C-WPT) links for biomedical implants, and proposes an algorithmic approach to optimize their design for a theoretically feasible desired power transmission efficiency (PTE). Two C-WPT links, one involving external inductors for parasitic capacitance cancellation, and another without external inductors are presented. An accurate electrical model has been presented for both cases considering the finite conductivity of the body tissue and fringe fields emanated from the metallic plates. Ex-vivo experiments were conducted with beef tissue to demonstrate the viability of the model and the optimization algorithm. The analytical and simulation results show good agreement with the measurement (with real tissue) for both types of links across a wide range of operating frequency, including one with the highest reported frequency (∼14.6 MHz) among tuned links.

Wireless power transfer system for deep-implanted biomedical devices

In this paper, a dual-band implantable rectenna is proposed for recharging and operating biomedical implantable devices at 0.915 and 2.45 GHz. The rectenna system consists of a compact dual-band antenna based on a meandered-resonator as well as efficient dual-band rectifier circuit. Both components (antenna and rectifier) are integrated inside a capsule device to simulate and experimentally validate the rectenna. The antenna occupies lower volume ([Formula: see text] [Formula: see text]), where compactness is achieved using meandered geometry and a slotted ground plane. It maintains quasi-omnidirectional radiation patterns and peak realized gains of -22.1 dBi (915 MHz) and -19.6 dBi (2.45 GHz); thus, its capability is enhanced to harvest the ambient energy from multiple directions. Moreover, a dual-band rectifier is designed using a dual-branch matching network (an L-matching network and open-circuited stub in each branch) with a radio frequency (RF) to direct current (DC) conversion efficiency of 79.9% for the input power of 1 dBm (lower band: 0.915 GHz) and 72.8% for the input power of 3 dBm (upper band: 2.45 GHz). To validate the concept of the rectenna, the implantable antenna and rectifier are fabricated and attached together inside a capsule device, with the measured results verifying the simulated responses. The proposed rectenna efficiently rectifies two RF signals and effectively superimposes on a single load, thus, providing a distinct advantage compared to single-band rectennas. To the best of the authors' knowledge, this is the first-ever implantable rectenna to perform dual-band RF signal rectification.

Floating EMG sensors and stimulators wirelessly powered and operated by volume conduction for networked neuroprosthetics

BACKGROUND

Implantable neuroprostheses consisting of a central electronic unit wired to electrodes benefit thousands of patients worldwide. However, they present limitations that restrict their use. Those limitations, which are more adverse in motor neuroprostheses, mostly arise from their bulkiness and the need to perform complex surgical implantation procedures. Alternatively, it has been proposed the development of distributed networks of intramuscular wireless microsensors and microstimulators that communicate with external systems for analyzing neuromuscular activity and performing stimulation or controlling external devices. This paradigm requires the development of miniaturized implants that can be wirelessly powered and operated by an external system. To accomplish this, we propose a wireless power transfer (WPT) and communications approach based on volume conduction of innocuous high frequency (HF) current bursts. The currents are applied through external textile electrodes and are collected by the wireless devices through two electrodes for powering and bidirectional digital communications. As these devices do not require bulky components for obtaining power, they may have a flexible threadlike conformation, facilitating deep implantation by injection.

METHODS

We report the design and evaluation of advanced prototypes based on the above approach. The system consists of an external unit, floating semi-implantable devices for sensing and stimulation, and a bidirectional communications protocol. The devices are intended for their future use in acute human trials to demonstrate the distributed paradigm. The technology is assayed in vitro using an agar phantom, and in vivo in hindlimbs of anesthetized rabbits.

RESULTS

The semi-implantable devices were able to power and bidirectionally communicate with the external unit. Using 13 commands modulated in innocuous 3 MHz HF current bursts, the external unit configured the sensing and stimulation parameters, and controlled their execution. Raw EMG was successfully acquired by the wireless devices at 1 ksps.

CONCLUSIONS

The demonstrated approach overcomes key limitations of existing neuroprostheses, paving the way to the development of distributed flexible threadlike sensors and stimulators. To the best of our knowledge, these devices are the first based on WPT by volume conduction that can work as EMG sensors and as electrical stimulators in a network of wireless devices.

A Current-Switching Technique for Intra-Body Communication With Miniaturized Electrodes

Medical implants are required to be minimized in size to alleviate surgical pains. Battery and antenna are often the main bottlenecks in system miniaturization. Wireless power transfer (WPT) is a possible way to minimize or eliminate the battery. Medical implants with WPT often use backscattering for data communication due to its low power consumption and low hardware cost. However, the conventional backscattering approach with WPT requires a large implanted antenna to ensure a relatively high efficiency and enough signal-to-noise ratio (SNR) for demodulation. In this work, we propose a current-switching technique for intra-body communication to achieve a high SNR and data rate with a pair of small implanted electrodes. Instead of the conventional electric-field based WPT and communication, a current loop is configured in the body tissue for WPT, where a new passive-communication scheme is implemented at the same time. A prototype is implemented to validate the proposed technique, in which the implanted electrodes are designed to be as small as 200 μm × 200 μm, located 13 mm deep in the tissue. The system achieves a communication rate of 10 Mbps with a bit error rate (BER) of 8.4 ×10^-4 over the 406 MHz MedRadio band, while the signal-to-blocker ratio and SNR are measured to be -35.7 dB and 12.4dB, respectively.