Design of Novel CPW Antenna for Optimized Wireless Power Transfer in Biomedical Systems
https://doi-001.org/1025/17640809875964
Amine Harhouz,1, Djelloul Aissaoui 2 and Tayeb A. Denidni 3
1,2 Telecommunications and Smart Systems Laboratory Faculty of Science and Technology Ziane Achour University of Djelfa Djelfa 17000, Algeria
3 Centre-Energie Matériaux et Telécommunications, Institut National de la Recherche Scientifique (INRS), Montreal, Quebec H5A 1K6 CANADA
Received: 23/10/2025 ; Accepted: 13/11/2025
Abstract – Wireless power transfer (WPT) has emerged as a cutting-edge approach to supplying energy to implantable medical technologies, eliminating the requirement for physical connectors, thereby minimizing potential health hazards and increasing patient mobility. This research introduces an innovative WPT antenna configuration that relies on magnetic resonant coupling and is carefully engineered for biomedical integration. In this configuration, a transmitting antenna, measuring approximately λ/4 × λ/4 (32 × 32 mm at 2.45 GHz), is positioned in free space at nearly λ/3 (40 mm) away from a receiving implant antenna. A metallic reflector is strategically placed behind the transmitting unit, producing a considerable boost in gain and radiation directionality, which in turn enhances the efficiency of energy delivery to the implanted receiver. The receiving antenna itself is extremely compact, with dimensions of about λ/18 × λ/19 (6.83 × 6.5 mm), and it is embedded beneath three biological layers—skin, fat, and muscle—to realistically replicate the implantation conditions. The performance evaluation of the system, both with and without the metallic reflector, is carried out and analyzed in terms of impedance matching and coupling efficiency. Experimental findings reveal a marked improvement in wireless energy transfer efficiency as a result of enhanced radiation directivity, thereby validating the effectiveness of the proposed antenna system for practical biomedical implementations.
Keywords: Wireless power transfer, Coplanar waveguide, transmit antenna, received antenna in body.
1.INTRODUCTION
In contemporary research, wireless power transfer (WPT) systems have been recognized as a rapidly developing field, primarily due to their ability to provide advanced alternatives for charging batteries in specialized contexts, most notably within the biomedical domain. Implantable medical equipment has become indispensable in the realms of monitoring, diagnosing, and therapeutic interventions [1], [2]. With the proliferation of such devices, the issue of maintaining a dependable and sustainable energy supply has become increasingly urgent. WPT represents a non-invasive and efficient mechanism capable of continuously supplying energy to these implants [3]. Nowadays, scholars are dedicating significant efforts to resolving several crucial aspects surrounding WPT. For this purpose, several approaches have been developed. First technique focuses on enhancing system efficiency and reliability through techniques such as near-field inductive coupling and magnetic resonance coupling [6]–[13]. The second method aims to increasing the transmitted power capacity and the overall efficiency of WPT system[6]–[7], [9]–[10], [11]–[15]. The third approach focuses on designing miniaturized antennas that are compact enough for integration into the human body while maintaining robust performance [5], [12]. Finally, the last one aims to extending transmission distances to achieve greater adaptability for portable medical technologies [8], [15]–[17]. Typically, inductive coupling or resonant magnetic coupling remains the most widely employed technique in WPT biomedical systems [5], [6], [14], [18]–[20]. In [5], for example, an energy transfer efficiency of 46.7% has been realized at 226 MHz over a 30 mm range using a conformal, strongly coupled magnetic resonator. However, the large dimensions of the resonant structures render them unsuitable for implantation. In another investigation [17], a biocompatible and flexible circularly polarized (CP) antenna has been introduced as the receiving element. To enhance efficiency, a polarization-conversion metamaterial (MTM) has been incorporated, and the design has been validated through SAR assessment and tests in tissue-equivalent phantoms, verifying both safety and effectiveness. Similarly, the authors of [18] have developed a biomedical WPT system enhanced with a mu-negative (MNG) metasurface, which not only improves the power transfer but also allows detailed exploration of coil and metasurface misalignments during body motion. Experimental validations in this case also shows a substantial performance boost due to the metasurface.
In light of these advancements, the present study puts forward a specifically engineered transmit–receive antenna system tailored for biomedical WPT. The transmitting antenna adopts a coplanar waveguide (CPW) configuration and integrates a metallic reflector at the rear; a feature intended to increase the gain and reinforce the forward-directed radiation. In this work, the receive antenna is designed using a cavity grid slot configuration to achieve a compact size and maintain strong reception efficiency. Both antennas are tuned to operate in the ISM band, ensuring a wide bandwidth and an elevated gain performance. The proposed setup allows the energy transmission distance to be expanded without compromising safety or adversely affecting biological tissues—skin, fat, and muscle—that surround the implanted unit.
2. Transmit Antenna Architecture with PEC Reflector and Electromagnetic Behavior of Perfect Electric Conductors This part explains the design framework of the transmitting antenna, which is constructed using a coplanar waveguide (CPW) feed line. In this setup, the signal conductor and the two ground planes share the same metallization layer on the dielectric substrate, simplifying fabrication while ensuring stable performance at high frequencies. To further boost the antenna’s radiation properties, a reflector fabricated from a Perfect Electric Conductor (PEC) is positioned behind the transmitting element. In microwave engineering, many practical applications involve boundaries made of highly conductive materials such as metals, which are often modeled as Perfect Electric Conductors (PEC) by assuming that their electrical conductivity approaches infinity (σ→∞) [19]. Under this condition, all electric and magnetic field components vanish inside the conductor, because an ideal PEC does not permit the existence of internal electromagnetic fields. This phenomenon is described through the concept of skin depth, which defines the depth to which electromagnetic energy can penetrate into a conductor. For materials with finite conductivity (σ < ∞), the skin depth remains very small; however, as conductivity approaches infinity, the skin depth converges to nearly zero. This indicates that electromagnetic waves cannot propagate inside a perfect conductor. Consequently, the boundary where a PEC is located is often treated as an electric wall, a surface where the tangential electric field components vanish entirely (E⃗t = 0) [20]. This boundary condition effectively acts as a short-circuit interface for the electric field and plays a pivotal role in the reflection phenomenon. At this interface, induced surface currents (J⃗s) re-radiate an electromagnetic wave that follows the classical law of reflection, redirecting energy forward. This mechanism is deliberately exploited in antenna engineering to enhance radiation strength and forward gain, as depicted in Figure 1 [21]
5.3 RESULTS AND DISCUSSION
One of the key parameters in the antenna design is the reflection coefficient, often denoted as the transmit propagation, which quantifies the portion of the incident electromagnetic wave reflected back due to impedance mismatches at the antenna input. It provides a measure of how well the antenna is matched to the feed line: the lower the reflection coefficient, the better the matching and the greater the efficiency. In this work, the transmit antenna is placed in free space with a metallic reflector, while the receiving antenna is implanted in a human body model, enabling wireless integration between the two. Results show that the reflector is essential in maintaining proper alignment between the transmitting and receiving antennas. Without the reflector, the resonant frequency of the transmit antenna’s reflection coefficient (S11) shifts from 2.45 GHz to 2.52 GHz, while the receiving antenna (S22) remains at 2.45 GHz, creating a mismatch. Additionally, the reflector enhances the transmit antenna’s directivity and gain, thereby improving power transfer efficiency. For the combined system, the transmission coefficient (S21) reaches −39 dB at 2.45 GHz, as shown in Figure 17.
The spacing between the transmit and receive antennas is another critical parameter influencing WPT performance. As distance increases, the |S21| value decreases due to near-field propagation effects, attenuation within body tissues, and dielectric losses in the surrounding medium. As depicted in Figure 18, at a distance of 40 mm, a transmission coefficient of −39 dB is obtained, which provides relatively high power transfer efficiency (PTE).
6.CONCLUSION
This study has presented an improved transmit antenna design incorporating a metallic reflector to enhance radiation characteristics. Results have confirmed that adding the reflector significantly boosts forward radiation gain and directivity while suppressing back radiation. Then, the upgraded antenna has been applied in a wireless power transfer (WPT) system by pairing it with an implantable receiving antenna inside the human body. Simulation outcomes have demonstrated that this integration enhances overall system efficiency by mitigating radiation losses. The proposed antennas design configuration exhibits strong performance in terms of radiation efficiency and bandwidth, making it a promising candidate for applications requiring reliable and efficient power delivery.
[1] R. Bashirullah, “Wireless implants,” IEEE Microw. Mag., pp. 14–23, Dec.2010.
[2] J. Walk, J. Weber, C. Soell, R. Weigel, G. Fischer, and T. Ussmueller, “Remote powered medical implants for telemonitoring,” Proc. IEEE,vol. 102, no. 11, pp. 1811–1832, Nov. 2014.
[3] X. Wei and J. Liu, “Power sources and electrical recharging strategies for implantable medical devices,” Frontiers Energy Power Eng. China, vol. 2, no. 1, pp. 1–13, Mar. 2008.
[4] A. Kurs, A. Karalis, R. Moffatt et al, “Wireless power transfer via strongly coupled magnetic resonance,” Science, vol.317, pp.83-86, Jul.6, 2007.
[5] H. Hu, and S. V. Georgakopoulos. “Wireless powering of biomedical implants byconformal strongly coupled magnetic resonators.” IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, Vancouver, pp. 1207-1208, 2015.
[6] M. Zargham and P. G. Gulak, “Maximum achievable efficiency in near-field coupled power-transfer systems,” IEEE Trans. Biomed. Circuits Syst., vol. 6, no. 3, pp. 228–245, Jun. 2012.
[7] J. Choi and C. Seo,“High-efficiency wireless energy transmission using magnetic resonance based on metamaterial with relative permeability equal to -1,” Progress In Electromagnetics Research, vol. 106,33-47,2010.
[8] A. P. Sample, D. T. Meyer, J. R. Smith, “Analysis, experimental results, and range adaptation of magnetically coupled resonators for wireless power transfer”, IEEE Transactions on Industrial Electronics, vol. 58, no. 2, pp. 544-554, 2011.
[9] C. S Wang, G. Covic, O. H Stielau, “Power transfer capability and bifurcation phenomena of loosely coupled inductive power transfer systems,” IEEE Transactions on Industrial Electronics, vol. 51, no.1, pp. 148-157, 2004.
[10] B. L. Cannon, J. F. Hoburg, D. D. Stancil, “Magnetic resonant coupling as a potential means for wireless power transfer to multiple small receivers,” IEEE Trans. Power Electron., vol. 24, no. 7, pp. 1819–1825, Jul. 2009.
[11] A. K Ramrakhyani, S. Mirabbasi, C. Mu, “Design and optimization of resonance-based efficient wireless power delivery systems for biomedical implants,” IEEE Transactions on Biomedical Circuits & Systems, vol. 5, no. 1, pp.48-63, 2011.
[12] U. M. Jow, M. Ghovanloo, “Design and optimization of printed spiral coils for efficient transcutaneous inductive power transmission,” IEEE Transactions on Biomedical Circuits & Systems, vol. 1, no. 3, pp.193, 2007.
[13] N. L. Zhen, R. A. Chinga, R. Tseng, “Design and test of a high-power high-efficiency loosely coupled planar wireless power transfer system,” IEEE Transactions on Industrial Electronics, vol. 56, no. 5, pp. 1801-1812, 2009.
[14] S. Stoecklin and A. Yousaf and L. Reindl, “Efficient wireless powering of biomedical sensor for multichannel brain implants,” IEEE Trans. Instrum. Meas., vol. 65, no. 4, pp.754–764, April. 2016.
[15] Y. C. Fan, L. Li, S. X. Yu, et al. “Experiment study of efficient wireless power transfer system integrating with high sub-wavelength metamaterials,” Progress In Electromagnetics Research, vol. 141,
769-784, 2013.
[16] A. Rajagopalan, A. K. RamRakhyani, D. Schurig, and G. Lazzi, “Improving power transfer efficiency of a short-range telemetry system using compact metamaterials,” IEEE Trans. Microw. Theory Techn., vol. 62, no. 4, pp. 947–955, Apr. 2014
[17] T. Shaw and P. Samanta and D Mitra , “Efficient Wireless Power Transfer System for Implantable Medical Devices Using Circular Polarized Antennas,” IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 69, NO. 7, JULY 2021.
[18] Long Li and Haixia Liu and Huiying Zhang , “Efficient Wireless Power Transfer System Integrating With Metasurface for Biological Applications,”IEEE Transactions on Industrial Electronics , Volu: 65, Issue: 4, April 2018)
[19] K. T. McDonald, “Electromagnetic Fields inside a Perfect Conductor,” Joseph Henry Laboratories, Princeton University, Princeton, NJ, USA, Dec. 24, 2015
[20] W. C. Chew, Lectures on Electromagnetic Field Theory, Purdue University, Fall 2019, updated Dec. 4, 2019
[22] H. Haddar, B. Joly, and H.-M. Nguyen, “Construction and analysis of approximate models for electromagnetic waves scattering by imperfectly conducting scatterers,” arXiv preprint arXiv:0709.3736, 2007
[23] B. J. DeLong, A. Kiourti, and J. L. Volakis, “A radiating near-field patch rectenna for wireless power transfer to medical implants at 2.4 GHz,” IEEE J. Electromagn., RF Microw. Med. Biol., vol. 2, no. 1, pp. 64–69, Mar. 2018.
[24] C. A. Balanis, Antenna Theory: Analysis and Design, 4th ed. Hoboken, NJ, USA: John Wiley & Sons, 2016.
[25] M. Song, P. Belov, and P. Kapitanova, “Wireless power transfer inspired by the modern trends in electromagnetics,” Appl. Phys. Rev., vol. 4, no. 2, Jun. 2017, Art. no. 021102.