A wireless power link with circular polarization is studied in this letter for far-field wireless power transmission. Firstly, a miniaturized circularly polarized (CP) implantable antenna is designed at 915 ISM-Band. The proposed antenna features a good miniaturization with the dimensions of 11$\times$11$\times$1.27 mm$^3$ by employing the stub loading and capacitive coupling among the stubs. It has the smallest size and maintains good radiation performance, compared with previous CP implantable antennas. The simulated impedance bandwidth covers from 889 to 924 MHz for |S11| less than -10 dB and the AR bandwidth (AR<3dB) is from 901 to 912MHz. The results show that the antenna is suitable for far-field wireless power transmission. And the proposed wireless power link has more converted DC power, compared with previous work.

A wideband miniaturized circularly polarized implantable antenna operating at 915 MHz Industrial, Scientific, and Medical (ISM) band is presented and experimentally verified in this letter. Compared with previous work, the patch antenna with the dimensions of π × (4.7)2 × 1.27 mm ³ features a better size reduction. The miniaturization of the proposed antenna is well implemented by cutting several kinds of slots to extend the current path. Moreover, a shorting pin is introduced to lower the resonant frequency and prompt circular polarization (CP) purity. Specific absorption rate is also calculated to consider whether it can meet the safety standards. The simulated results in one-layer cubic skin phantom demonstrates that the impedance bandwidth (BW) of 12.2% and the 3 dB axial-ratio BW of 19.7% can be obtained. Finally, the antenna is measured in the skin-mimicking solution, and the measured BW achieves about 17.5%, which generally agrees with the simulated.

A conformal differentially-fed antenna at 915MHz Industrial, Scientific and Medical (ISM) (902-928 MHz) band for monitoring in-body core temperature is presented in this paper. In virtue of its characteristics of ultra-thin and flexible substrate, the peculiarity that proposed antenna can be wrapped around the capsule inner wall demonstrates the satisfactory performance of occupying little space. The volume of the conformal patch-antenna only measures 30mm3 by notching meandering slots to achieve miniaturization. The simulated impedance bandwidth (|S 11|<-10dB) covers from 861 to 942MHz. The integrated capsule system, inserted in a cubic homogeneous muscle phantom for initial parametric studies and optimization, occupies the dimensions of 22mm (length) × 11mm (diameter). Meanwhile, the performance on the sensitivity to surrounding environment and specific absorption rate (SAR) distribution are studied. In the measurements, the conformal antenna is rolled around a 3D-printed capsule inside minced pork. The pork temperature data are monitored in time and transferred wirelessly. To confirm the system reliability, hardware circuits are designed with differential concept and a data logger is also realized to record in-time temperature data. Finally, the feasibility of system function demonstrates the suitability of the conformal differentially-fed antenna in biomedical applications.

Wireless power transfer (WPT) plays critical roles in powering deep-tissue implants, which also contributes to several emerging advances for biomedical engineering. To enable a high-power density region in implants, this paper presents a method, termed the self-phasing technology, to focus electromagnetic fields from various paths at a deep-tissue spot. By performing the phase-conjugated operation on the incident signal and then retransfer back to the source, coherent RF power can be achieved without learning the precise or even dynamic locations of sources and concerning inhomogeneous medium perturbations. An external slot antenna array placed above skin surface 4mm is considered as a transmitter and an implanted rectenna consisting of a magnetic resonant coil and a RF-to-DC rectifier circuit is treated as a receiver. The conversion efficiency of the rectifier circuit is optimized within the received power range and the measured efficiency of 50% can be achieved at 0dBm. To visualize the transceiver effects of the integrated system under safety thresholds, a light-emitting diode is soldered at the terminal of the rectenna and measurements show that smooth drive can be achieved. Certain brightness of LED can demonstrate that the self-phasing technology can support wireless power transfer for biomedical applications.

A switchable ON-/OFF-body antenna operating at the same frequency of 2.45 GHz industrial, scientific, and medical (ISM) band for wireless body area network (WBAN) applications is presented. P-i-n diodes are chosen to act as the switches between ON-body and OFF-body modes with the forward or reverse direct current (dc) voltage. To achieve the same operating frequency at ON-body and OFF-body modes, suitable slots are cut on the radiator to have size reduction at OFF-body mode, and it will have less effect at ON-body mode. Bias circuit is integrated in the antenna. Total size of the antenna is π ×24 ² ×3.2 mm ³ . Specific absorption rate (SAR) value is also analyzed in this communication. Then, the antenna is fabricated to validate the design concept. Measured radiation pattern of ON-body mode is omni-direction with vertical polarization and OFF-body mode is with broadside, respectively. Communication links of ON-body mode, which assessed by |S ₂₁ | and the performance at OFF-body mode, are also studied in this communication.

A compact circularly polarized antenna operating at 915 MHz industrial, scientific, and medical (ISM) band for biomedical implantable applications is presented and experimentally measured. The proposed antenna can be miniaturized to a large extent with the compact size of 15 × 15 × 1.27 mm³ by means of loading patches to a ring-shaped microstrip patch antenna. An impedance bandwidth of 10.6% (865–962 MHz) for reflection coefficient less than −10 dB can be obtained. Meanwhile, the simulated 3 dB axial-ratio (AR) bandwidth reaches 14 MHz. Finally, the optimized design was fabricated and tested, and the measured results agree well with simulated results.