Kush Agarwal

Enabling Wireless Powering and Telemetry for Peripheral Nerve Implants

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.

Wearable AMC Backed Near-Endfire Antenna for On-Body Communications on Latex Substrate

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 0.044λ0 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 0.4λ0×0.4λ0×0.076λ0 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.

Highly efficient wireless energy harvesting system using metamaterial based compact CP antenna

This paper presents a highly efficient 2.4 GHz wireless energy harvesting system comprising of a metamaterial based circularly polarized (CP) antenna and a power management circuit. The antenna is designed at 2.4 GHz using a circular slotted truncated corner square patch radiator placed on reactive impedance surface (RIS) for antenna size miniaturization, better impedance matching and to improve the front-to-back ratio. The power management system integrates a matching circuit with a single stage Dickson charge pump and an ultra low power with high efficiency DC/DC boost converter/charger. The Dickson charge pump uses three Schottky diodes to minimize the losses at high frequency. The DC-DC converter (BQ25504) is capable of acquiring and managing low power levels. The measured axial ratio (boresight) is below 3-dB for the entire 2.40-2.48 GHz band and the 10-dB return loss band is from 2.35-2.49 GHz. The gain (boresight) of the antenna is around 4.6 dBic at 2.44 GHz. The proposed antenna shows an improvement in front-to-back ratio of around 3 dB with size reduction of approximately 22%. The power management system generates an output voltage level of 1.5 volts at -10 dBm and 4 volts at 0 dBm input RF power respectively. The overall efficiency of the proposed energy harvesting system is above 28%.

Wireless Power Delivery to Flexible Subcutaneous Implants Using Capacitive Coupling

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 sub-GHz 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.

Interaction of electromagnetic waves with humans in wearable and biomedical implant antennas

Antennas are the key components for far-field wireless transmission of data and power by electromagnetic (EM) radiation. These passive devices are the only radiating structures in biomedical communication systems, so its design needs to be carefully studied to ensure human safety. In this paper, we analyse and discuss the effect of wearable and implantable biomedical antennas on humans and vice versa. The design process of these electromagnetic radiating structures in body-worn devices and implantable medical devices (IMDs), which take into consideration the human body effects and FCC/FDA safety regulations, has been demonstrated using a wearable endfire antenna on artificial magnetic conductor metasurface intended for wireless body area network communications and an implantable rectenna for far-field wireless powering. Both proposed antenna designs perform considerably well in/on the voxel model used for the human body analysis and adhere to the FCC/FDA human safety limits of specific absorption rate (SAR) and thermal heating by EM radiation.

Wireless Power Transfer Strategies for Implantable Bioelectronics: Methodological Review

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. Patients 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.

Dual-band circularly polarized stacked microstrip antenna over RIS for GPS applications

A compact, dual band, circularly polarized (CP) multilayered stacked microstrip antenna over reactive impedance surface (RIS) is studied and presented in this paper. The CP radiation with compact antenna size is achieved by placing two asymmetric slit square patch (ASSP) and cross-shaped slotted square patch (CSSP) radiators over the RIS. Dual band is achieved by using the CSSP and ASSP stacked patches placed over RIS, fed by a coaxial probe at proper location to generate CP radiation. The measured results of the proposed antenna are 1.61% (1.235-1.255 GHz): L2 band and 1.25% (1.585-1.605 GHz): L1 band for 3-dB axial ratio bandwidth (BW), 2.00% (1.235-1.260 GHz): L2 band and 1.57% (1.580-1.605 GHz): L1 band for 10-dB impedance BW, and 2.68 dBic: L2 band and 4.46 dBic: L1 band for gain at the boresight for compact antenna overall volume of 0.26λo × 0.26λo × 0.018λo at 1.2 GHz.

Miniaturized Circularly Polarized Stacked Patch Antenna on Reactive Impedance Surface for Dual-Band ISM and WiMAX Applications

This paper proposes a compact microstrip patch antenna for operating in 2.4 GHz ISM and 3.5 GHz WiMAX bands with circularly polarized (CP) radiation. The CP radiation in dual-bands is a result of two multilayered truncated corner stacked square patches, while the reactive impedance surface (RIS) is used for antenna size miniaturization for the lower operating frequency band. Since the overall lateral antenna dimensions are controlled by the lower frequency band (higher wavelength), reducing the electrical size of the antenna for lower band results in overall smaller antenna dimensions. The measured 3-dB axial ratio bandwidths of the in-house fabricated antenna prototype are 6.1% (2.40–2.55 GHz) for the lower band and 5.7% (3.40–3.60 GHz) for the upper band, while the 10-dB bandwidths for the two bands are 8.1% (2.39–2.59 GHz) and 6.9% (3.38–3.62 GHz), respectively. The maximum gain at boresight for the lower band is 2.93 dBic at 2.5 GHz, while the gain for the upper band is 6.26 dBic at 3.52 GHz. The overall volume of the proposed antenna is 0.292λo × 0.292λo × 0.044λo, where λo is the corresponding free-space wavelength at 2.5 GHz.

Unidirectional AMC reflector backed L-band annular slot antenna

This paper presents an artificial magnetic conductor (AMC) reflector backed low-profile annular slot-antenna with unidirectional radiation operating at the L band (1-2 GHz). The proposed antenna comprises of an FR4 substrate with a square shaped annular slot on one side and a feed element on the other side. The intrinsically bidirectional radiation of the slot antenna is made unidirectional by backing it with an AMC surface. An AMC has an added advantage because it can be kept closer to the radiating structure without distorting its antenna properties compared to the traditional perfect electric conductor (PEC) reflector, thus resulting in a low antenna profile. Three different types of AMC unit cells on grounded Rogers 3010 substrate, namely square patch, jerusalem cross and ring have been compared as back reflectors for this purpose. An overall antenna volume of 0.6 λ0 × 0.6 λ0 × 0.1222 λ0 at 1.5 GHz has been achieved using an AMC backed reflector for a close separation of ~0.1 λ0 between the annular slot radiator and the AMC reflector surface.

Latex based near-endfire wearable antenna backed by AMC surface

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 ISM band. The bidirectional-endfire radiation pattern of Yagi-Uda latex antenna is changed to off-axis near-endfire radiation using an AMC reflector also printed on latex. The antenna is separated from upper AMC surface using flexible Styrofoam of thickness 0.044λo at 2.4 GHz for best compromise between keeping the antenna structure low-profile and achieving an off-axis beam tilt radiation of ~74° towards end-fire direction. 0° reflection phase AMC surface is proposed to reduce downward radiations and consequently improve the antenna tolerance to positioning on the human body, and reduce the specific absorption rate (SAR) level for 2.4 GHz frequency band gap. Antenna performance in terms of return loss, radiation efficiency, extent of frequency detuning, gain and SAR level are studied for free space as well as CST MWS tissue-equivalent voxel model for proposed antenna design. The Yagi-Uda antenna backed with AMC reflector demonstrates the measured return loss bandwidth of 40 MHz (2.43-2.47 GHz) and gain of -0.2 dBi in endfire direction with improved on-body radiation efficiency of 74.82 % and reduced peak SAR level of 1.24 W/kg for 10 g tissue for the compact overall flexible latex antenna volume of 0.4λo × 0.4λo × 0.076λo at 2.4 GHz.