A noninvasive planar complementary split-ring resonator (CSRR) for measuring the dielectric constants and loss tangents of material is presented in this letter. Measurements were developed to extract the complex permittivity of the material under test (MUT) using the equivalent RLC resonator circuit model. This technique enables a single-step measurement of the MUT without the need for machining or reshaping. The method requires calculating only two parameters, the resonant frequency (fr) and the magnitude (dB) response, thus achieving a substantial reduction in cost and computation time. A CSRR sensor operating in the 1.8 to 2.8 GHz band is fabricated and tested for verification. The measurement errors in the dielectric constant were less than 7.6%. An efficient technique of CSRR measuring complex permittivity is experimentally verified.

This paper presents a novel noncontact measurement technique that entails using a single-compound triple complementary split-ring resonator (SC-TCSRR) to determine the complex permittivity and thickness of a material under test (MUT). The proposed technique overcomes the problem engendered by the existence of air gaps between the sensor ground plane and the MUT. In the proposed approach, a derived governing equation of the resonance frequencies is used to estimate the thickness and complex permittivity of the MUT by calculating the resonant frequency (f r) and magnitude response in a single-step noncontact measurement process. This study theoretically analyzed and experimentally verified a simple and low-cost SC-TCSRR measurement method for assessing materials in a noncontact method. For a 0.2-mm air gap, the experiments yielded average measurement errors of 4.32% and 5.05% for the thickness and permittivity, respectively. The proposed SC-TCSRR technique provides excellent solutions for reducing the effect of air-gap conditions on permittivity, thickness, and loss tangent in noncontact measurements.

This paper presents a non-invasive microwave method based on a square-shaped complementary split-ring resonator (CSRR) to measure the thickness and permittivity of multilayer dielectric structures. The CSRR sensor is etched on the ground plane of a microstrip line. The change of resonance frequency depends on the thickness and permittivity of the multilayer dielectric sample below the ground plane. For resolution analysis, the resonance frequency shifts caused by a variation of permittivity (Δε = 0.01) and thickness (Δd=0.01 mm) in the detection layer were compared across various design dimensions. Sensor size optimization improved the resolution in permittivity and thickness measurement by 66% and 37%, respectively. Subsequently, the permittivity and thickness resolution was improved by 28% and 16%, respectively, by optimizing the separation of the etched CSRRs. The analysis results show that a CSRR sensor can be designed with excellent resolution in core layer permittivity changes and thickness resolution in multilayered dielectric structures.

This paper demonstrates a closed-loop wireless power transfer (WPT) system for deep brain stimulation-implanted biomedical applications. A novel two-layer PCB FR4 coil design is proposed as a second coil for enlarging the WPT efficiency. High-efficiency implantable WPT technology was achieved by applying an inner dual-layer printed spiral coil (PSC) and outer helical coil and this coordinated with a T-type impedance-matching network. To reduce the loading effect, which lowers Q factor in implanted devices, four-coil WPT systems are recommended compared with two- or three-coil WPT systems. A practical four-coil system was implemented for verification, for which a dual-layer printed circuit board composed of a PSC, with a load loop on the other side of FR4, was designed and implemented around the system on a chip. PSCs for biomedical implants can be optimized by designing the geometric parameters. The implant coil occupies an area measuring 5 × 5 mm, and is implemented on both an FR4 board and an implantable flexible substrate with the optimized geometric parameters. Finally, the transmission efficiencies of using FR4 and flexible printed circuit boards, at a distance of 10 mm, were 19.1% and 14.8% through the air and 11.7% and 7.7% through the tissue, respectively, thus enabling the supply of milliwatts of power to the stimulation circuits, and successfully demonstrating the stimulation tissue model with a V/I output current of up to 180 μA.

This paper presents the design and analysis of a single-compound complementary split-ring resonator (SC-CSRR) that induces two resonance frequencies for simultaneously measuring the thickness and permittivity of dual-layer dielectric structures. Two resonance frequencies were generated using two distinct embedded resonator current lengths in a single complementary split-ring resonator. These two resonance frequency responses were combined to determine the thickness and permittivity of a dual-layer dielectric sample. Methods proposed in this paper were used to analyze the equivalent permittivity relationship, and thus, determine the thickness and permittivity of the material under test. The proposed simple low-cost SC-CSRR measurement method for assessing the permittivity of materials in a compact area was experimentally analyzed and verified in experiments. The experimental results indicated that the average thickness and permittivity measurement errors were 6.26% and 4.63%, respectively, for single-layer samples, and 5.26% and 6.48%, respectively, for dual-layer samples.

This paper evaluates RF powering techniques, and corresponding propagation through tissue, to supply wireless-energy for miniature implantable devices used to monitor physical-conditions in real-time. To improve efficiencies an impulsive powering technique is used with short duty-cycle high instantaneous-power-bursts, which biases the rectifier in its nonlinear regime while maintaining low average input-powers. The RF rectifier consists of a modified two-stage voltage multiplier which produces the necessary turn-on voltage for standard low-power CMOS systems while supplying the required current levels. The rectifier, fabricated on the TI 130 nm CMOS process, measures 215 μm × 265 μm, and is integrated with an antenna to quantify wireless performance of the power transfer. In-vivo studies performed on New Zealand white rabbits demonstrate the ability of implanted CMOS RF rectifiers to produce 1 V across a 27 kΩ load at a distance of 5 cm with a transmit-power of just over 1.5 W. Using a pulsed-powering technique, the circuit generates just under 0.9 V output with an average transmit-power of 300 mW. The effects of implantation on the propagation of RF powering waves are quantified and demonstrated to be surmountable, allowing for the ability to supply a low-power wireless sensor through a miniature rectifier IC.