Neural implants have emerged over the last decade as highly effective solutions for the treatment of dysfunctions and disorders of the nervous system. These implants establish a direct, often bidirectional, interface to the nervous system, both sensing neural signals and providing therapeutic treatments. As a result of the technological progress and successful clinical demonstrations, completely implantable solutions have become a reality and are now commercially available for the treatment of various functional disorders. Central to this development is the wireless power transfer (WPT) that has enabled implantable medical devices (IMDs) to function for extended durations in mobile subjects. In this review, we present the theory, link design, and challenges, along with their probable solutions for the traditional near-field resonant inductively coupled WPT, capacitively coupled short-ranged WPT, and more recently developed ultrasonic, mid-field, and far-field coupled WPT technologies for implantable applications. A comparison of various power transfer methods based on their power budgets and WPT range follows. Power requirements of specific implants like cochlear, retinal, cortical, and peripheral are also considered and currently available IMD solutions are discussed. Patient's safety concerns with respect to electrical, biological, physical, electromagnetic interference, and cyber security from an implanted neurotech device are also explored in this review. Finally, we discuss and anticipate future developments that will enhance the capabilities of current-day wirelessly powered implants and make them more efficient and integrable with other electronic components in IMDs.

Implantable devices need sustainable wireless powering for safe long-term operations. In this paper, we present a near-field capacitive coupling (NCC)-based wireless powering scheme to transfer power to implants efficiently. By modeling the power link, we identify that the optimal operating frequency of the NCC scheme for subcutaneous power delivery is in the subGHz frequency range. The proposed scheme has desirable features, such as flexible and conformal power receiver realizations, and complies well with IEEE C95.1 specific absorption rate safety standards. The NCC link was designed and tested in a nonhuman primate cadaver, and the experimental results showed that it could safely deliver up to 100 mW of power to an implant with a peak operating efficiency of over 50%. A bending deformation study of the transmitter-receiver patches was also performed to demonstrate the reliability of the NCC powering scheme, in realistic postimplantation scenarios. Our studies validate the NCC method as a safe wireless powering scheme, which can be used as an alternative to the near-field resonant inductive coupling method, for chronic use in subcutaneous implants.

A near-endfire, artificial magnetic conductor (AMC) backed wearable antenna is proposed in this paper for wireless body area networks operating in the 2.4-GHz industrial, scientific and medical (ISM) radio band. The latex substrate permittivity has accurately been characterized for realizing a flexible planar Yagi-Uda antenna printed on it using a large-area screenprinting process. The bidirectional-endfire radiation of Yagi- Uda antenna is changed to an off-axis near-endfire radiation using an AMC reflector also printed on latex. The antenna is separated from the upper AMC surface using flexible Styrofoam of thickness 0.044λ ₀ at 2.4 GHz for the best compromise between keeping the antenna structure low profile and achieving an off-axis beam-tilt radiation of ~74° toward the endfire direction. The 0° reflection phase single-layered AMC and double-layered AMC (D-AMC) surfaces are proposed to reduce the body-absorbed radiation and, consequently, minimize the peak specific absorption rate (SAR) level for 2.4-GHz frequency band. Antenna performance in terms of return loss, radiation efficiency, extent of frequency detuning, gain, and SAR level is studied for free space as well as the CST MWS tissue-equivalent voxel model for all the proposed antenna designs. Antenna deformation bending study when placed on the human body is also performed in this paper. The antenna design is first optimized and fabricated on printed circuit board to verify the concept and then designed over the latex for actual human on-body all-flexible configuration. The Yagi-Uda antenna backed with D-AMC reflector demonstrates the measured return loss bandwidth of 45 MHz (2.425-2.47 GHz) and the gain of 0.12 dBi in the endfire direction with an improved on-body (chest) radiation efficiency of 78.97% and a reduced peak SAR level of 0.714 W/kg (average over 10-g tissue) for the compact overall flexible latex antenna volume of 0.4λ ₀ ×0.4λ ₀ ×0.076λ ₀ at 2.4 GHz. To the best of our knowledge, this is the first latex-based endfire antenna for on-body 2.4-GHz wireless communications backed with an AMC periodic metamaterial surface.

A wideband, circularly polarized (CP), artificial magnetic conductor (AMC) reflector backed octagonal-shaped aperture (OSA) antenna is proposed for unidirectional radiation. The proposed antenna consists of an OSA fed by microstrip along with L-shaped stub and an AMC reflector. Bidirectional radiation of OSA antenna is changed to unidirectional radiation using an AMC reflector. The antenna height measured from the upper surface of the AMC reflector to OSA radiator is chosen to be small to realize a low-profile antenna: at the lowest analysis frequency of 4.5 GHz. Three different surfaces are studied and compared as back reflectors for OSA: perfect electric conductor (PEC), single layered AMC surface, and double layered AMC surface. The OSA with double-layered AMC surface demonstrates the largest measured 3-dB axial ratio bandwidth of 33.2% (5.20-7.19 GHz) with impedance bandwidth of 36.2% (5.04-7.21 GHz) for VSWR of 2. Almost constant gain of around 7 dBic is achieved over the band for the overall antenna volume of 0.72λ_o × 0.60λ_o × 0.19λ_o at the center frequency of 6.0 GHz.

Compact, asymmetric\symmetric-slotted\slit-microstrip patch antennas on reactive impedance surface (RIS) are proposed and studied for circularly polarized (CP) radiation. The antennas consist of a slotted-slit-microstrip patch on a RIS substrate. The CP radiation with compact size is achieved by asymmetric\symmetric-slot-slit cut along the orthogonal\diagonal directions of the patch radiator. The asymmetric\symmetric-slotted\slit microstrip patches on the RIS structure are used for further miniaturization of the antenna with improvement in CP radiation. The measured results of the compact asymmetric-cross slotted square patch antenna are 1.6% (2.51-2.55 GHz) for 3-dB axial ratio bandwidth, 5.2% (2.47-2.60 GHz) for 10-dB return loss bandwidth, and 3.41 dBic for gain over 3-dB axial ratio bandwidth. The overall antenna volume is 0.292λ_o × 0.292λ_o × 0.0308λ_o on a low cost FR4 substrate at 2.5 GHz.

Wireless power delivery and telemetry have enabled completely implantable neural devices. Current day implants are controlled, monitored, and powered wirelessly, eliminating the need for batteries and prolonging the lifetime. A brief overview of wireless platforms for such implantable devices is presented in this paper alongside an in-depth discussion of wireless platform for peripheral nerve implants covering design requirements, link design, and safety. Initial acute studies on the performance of the wireless power and data links in rodents are also presented.