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Article

Fully Integrated Miniaturized Wireless Power Transfer Rectenna for Medical Applications Tested inside Biological Tissues

by
Miguel Fernandez-Munoz
1,*,
Mohamed Missous
2,
Mohammadreza Sadeghi
3,
Pablo Luis Lopez-Espi
4,
Rocio Sanchez-Montero
4,
Juan Antonio Martinez-Rojas
4 and
Efren Diez-Jimenez
1
1
Mechanical Engineering Area, Universidad de Alcalá, Alcalá de Henares 28801, Spain
2
Department of Electrical and Electronic Engineering, University of Manchester, Manchester M13 9PL, UK
3
Research and Development Department, Advanced Hall Sensors Ltd., Manchester M17 1RW, UK
4
Radiation and Sensing Group, Universidad de Alcalá, Alcalá de Henares 28801, Spain
*
Author to whom correspondence should be addressed.
Electronics 2024, 13(16), 3159; https://doi.org/10.3390/electronics13163159
Submission received: 27 June 2024 / Revised: 29 July 2024 / Accepted: 7 August 2024 / Published: 10 August 2024
(This article belongs to the Special Issue Recent Advances in RF Rectifying Technology for EM Energy Harvesting and Wireless Power Transfer)
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Abstract

This work presents the results of the characterization of two 1 × 5 mm2 miniaturized rectennas developed for medical applications. They have been designed for relatively high voltage and high-power applications, given the size of the rectennas. Both rectennas were tested in open-air conditions and surrounded by pork fat and muscle tissues, whose properties are similar to those of the human body. The resonant frequencies of the rectennas were found, and the incident electric field on the rectennas tests was increased. The first chip showed a maximum output voltage of 5.29 V and a maximum output power of 0.056 mW, at 1.446 GHz, under an incident field on the rectenna of 340 V/m, and the second chip, 4.62 V and 4.27 mW, at 1.175 GHz, under 535 V/m. The second rectenna can provide an output power greater than 5 mW. The rectennas presented in this article are beyond the state of the art, as they can deliver about three times more power and voltage than those of similar dimensions reported in the literature. Based on the test results, the efficiency of the rectennas was analyzed at different locations of the human body, considering different thicknesses of tissues with high and low water content. Finally, potential applications are described in which the rectennas could power implantable medical devices or microsurgery tools, for example, pulmonary artery pressure monitors.

1. Introduction

Microantennas are key components of any communication system capable of operating inside the human body. Maximizing their performance while reducing their size is a must for any biomedical application. Wireless systems have gained great importance in the medical field [1,2] due to their applicability in wireless power transfer (WPT) configurations [3,4,5,6,7,8], implants such as biosensors or stimulators [9,10,11], actuators [12,13,14,15], and other implantable apphlications [16,17,18,19]. These wireless systems offer not only the possibility of communication but also the ability to power devices without wires, supporting the development and investigation of small implantable, ingestible, and injectable devices [20]. They also eliminate the need for batteries, which have the drawback of large size and in most cases take up most of the volume of the implant [21]. With wireless power transfer, the lifetime of the devices is not limited by the battery. In addition, the risk of infections because of non-biocompatible materials in batteries is eliminated. Biomedical wireless systems are typically near-field systems [22] to reduce the absorption of electromagnetic waves by the body. They are composed of an external power source that emits energy and a receiver that captures and transforms the energy to feed devices [23,24], which consists of microantennas and circuits to adapt and rectify the signals. Sometimes, the microantennas are also used to transmit or receive data simultaneously [25,26].
The resonant frequency of the antennas decreases in inverse proportion to the antenna size [27,28,29]. That is, the lower the frequency, the larger the size of the antenna. This effect presents one of the main difficulties in obtaining miniaturized antennas capable of operating at low frequencies. Antennas designed for in-body applications must have low resonant frequencies, as the transmission efficiency depends on the electromagnetic wave absorption by the human body, which increases with operating frequency [30]. This behavior can be explained by the electromagnetic absorption behavior of water, of which the human body is mainly composed.
Any WPT system will require a minimum amount of power to work properly. An unlimited increase in the emitted power is not a solution to this problem because it raises safety concerns for human tissue [31]. The Specific Absorption Rate (SAR) cannot exceed the limits defined by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines [32,33,34] and the Institute of Electrical and Electronics Engineers (IEEE) Standards Coordinating Committee [35]. If the emitted power is excessive, there is a harmful increase in the local temperature and SAR [36,37]. To avoid these effects, it is critical to optimize the miniaturized receiving antennas in order to reduce the necessary emitted radiation [38].
There are several studies reported in the literature about on-chip rectennas that have faced the challenge of improving the efficiency of RF power conversion. Authors of Reference [39] presented a 2.45 GHz near-field RF identification (RFID) system with passive on-chip antenna tags of only 1 × 0.5 mm2, which delivers 94.7 μW. Reference [40] is about two antennas integrated on a chip for wireless powering applications. They work at 5.8 GHz and their dimensions are 3 × 1.5 mm2. An on-chip antenna for implantable intraocular pressure monitoring is presented in [41]. It works at 5.2 GHz and measures 3.2 mm long and 1.5 mm wide. The antenna presented in [42] works at a much higher frequency, 24 GHz, and it is 3.7 × 1.2 mm2. Authors of Reference [43] describe an on-chip antenna for millimeter-sized biomedical implants. Its dimensions are 1.6 × 1.6 mm2 and it can deliver 1.2 mW. A rectenna working at 28 GHz for wireless power transfer experiments of only 1.8 × 1.8 mm2 is presented in [44]. It can deliver 8.6 μW if 25 kW is emitted at a distance of 5 m.
The design and preliminary results of a 1 × 5 mm2 miniaturized antenna with an integrated rectifier circuit (hereinafter rectenna) for WPT systems presented in this paper were first introduced in [45]. The motivation of this proposal is to demonstrate its operation in a controlled exposure environment, as well as to test its performance as a possible power source in an implantable device. For this purpose, its operation has been tested in an anechoic chamber under free space conditions and surrounded by tissue. Characterization results are shown in both cases: free space and surrounded by pork muscle and fat tissues, whose properties are similar to those of human tissues. From these results, the expected available output voltage and power that the rectennas can deliver when working at different locations of the human body were obtained. The results demonstrate that the rectenna could be a candidate for implementation in microsurgical tools or implantable devices, as it can provide high power compared to its small volume. Compared to other results in the literature (Table 1), the proposed rectenna operates at a lower frequency, obtaining higher values of output voltage and available power.
The text is structured as follows. Section 2 is dedicated to the development of the rectenna, putting in the context of this proposal the previous results of its first validation. Section 3 describes the characteristics of the measurement site, antennas, and sensors used to radiate the rectenna and test the applied electric field levels. Section 4 is dedicated to the free space tests. First, a frequency sweep is performed to detect the optimum value of the radiation frequency. For this optimal frequency value, a subsequent sweep of electric field values applied to the rectenna is performed to establish its conversion parameters. Section 5 and Section 6 analyze the possibilities of the rectenna as an implantable device. Firstly, its behavior surrounded by different types of tissue is tested and then the conditions for its possible use in different parts of the human body are discussed. Finally, Section 6 highlights the conclusions reached with this study.

2. Rectenna Development

The complete development of the design, simulation, and optimization of the patch rectenna, whose characterization in free space conditions and simulated intracorporeal conditions is presented in this work, is described in Reference [45]. The antenna was designed to be used mainly in medical applications, such as its integration into surgical microtools or implantable medical devices. Therefore, its design was carried out following an iterative optimization process seeking to reduce its size while trying to reduce losses due to electromagnetic absorption in the human body, maximizing its efficiency and reducing its resonant frequency [46].
The final design consists of a miniaturized geometry rectenna of only 1 × 5 × 0.6 mm3, with a folded antenna structure constructed from two symmetrical radiating open-ended arms built with multiple L-shaped conducting sections. The antennas were designed and fabricated on 0.625 mm semi-insulating GaAs substrate with a relative permittivity (εr) of 12.9 and a dielectric loss tangent (tanδ) of 0.006.
The rectenna is expected to have low performance due to the very small effective radiating area of the antenna. Generally, the formation of dipole geometry involves inevitable trade-offs in terms of reduced size, reasonable gain, and radiation efficiency. Additionally, the output voltage or power levels of the full rectenna can be increased to compensate for the low antenna performance. A Cockcroft–Walton voltage doubler [47] was used to overcome the low ηrad and gain of the antenna. An Asymmetric Spacer Layer Tunnel Diode (ASPAT) was used as the active rectifier due to its high non-linearity and temperature insensitivity features, as described in Reference [48]. Figure 1a shows a microscopic view of the rectenna.
Finally, two output resistance values were considered for the rectifier circuit. The first one, 500 kΩ, integrated into the Generation 2 rectenna (hereinafter G2 rectenna), was planned to maximize the voltage at the output terminals of the rectenna. The second one, 5 kΩ, integrated into the Generation 3 rectenna (hereinafter G3 rectenna), was planned to maximize the available power at the output terminals of the rectenna. Several prototypes of the G2 and G3 rectennas were manufactured in a clean room using i-line lithography and Physical Vapor Deposition (PVD) techniques. The output terminals of the manufactured rectenna prototypes were wire-bonded to a larger Printed Circuit Board (PCB) (Figure 1b,c) with standard connection ports.

3. Test Setup

The rectenna prototypes were tested at the Centre for High Technology and Homologation (Centro de Alta Tecnología y Homologación, CATECHOM [49]) at the Escuela Politécnica Superior of the Universidad de Alcalá. Specifically, tests were conducted in a semi-anechoic chamber, measuring 6.75 m long, 3.07 m wide, and 3.05 m high, with 100 dB shielding and a range of ferrites and absorbers from 30 MHz to 18 GHz.
The transmitter antenna used for the tests was a Broadband TEM Horn Antenna TEMH 6000. The TEMH 6000 is a calibrated antenna with a frequency range of 300 MHz to 8 GHz, and an isotropic gain of 2 to 10 dBi. The transmitter antenna was coupled to a mast with adjustable height and position. The antenna was connected to an automatic switch controlling multiple power amplifiers. The switch automatically selects the appropriate power amplifier among those available so that the highest power can be transmitted depending on the working frequency. For the tests between 0.3 and 3 GHz, a 100 W KALMUS 7100LC amplifier (Ametek, Devon-Berwyn, PA, USA) (0.3–1 GHz) and a 60 W R&S BBA150 amplifier (Rohde & Schwarz, Munich, Germany) (1–3 GHz) were used, connected to a R&S SMB100B signal generator (Rohde & Schwarz, Munich, Germany).
The rectenna prototypes, which worked as receiving antennas, were attached to another mast placed 4 cm from the horn mouth and connected to an oscilloscope placed outside the chamber to measure the output voltage. Finally, an isotropic E-field probe (model AR FL7006) was placed over the prototypes. Before the tests, it was verified that although the size of the sensor is much larger than the size of the prototypes, this does not affect the voltage measurements. The cables of the probe and the transmitter antenna are cables that are always present in the chamber and whose electromagnetic interference is calibrated and neutralized. The cable used to measure the voltage in the prototype was a coaxial cable directly connected to the prototype SMA with ferrite coupling cores attached to reduce noise. Figure 2 below shows some pictures of the setup.

4. Open Air Tests

4.1. Frequency Sweep Tests

First, a frequency sweep test was performed at constant transmit power to determine the resonant frequency of the prototypes, following the process described in Figure 3. The constant transmit power was controlled by measuring the electric field with the probe placed on the prototypes, which was set to 20 V/m. The sweep was performed from 0.3 to 3 GHz, and several prototypes were tested. The test results of the rectenna prototypes G2 and G3, measuring the voltage at the output terminals of the rectennas to determine the maximum values (corresponding to the resonant frequencies), are shown in Figure 4a,b. The output power was calculated considering the value of the output resistance of the rectifier circuit of the rectennas, with the result being 500 kΩ in the G2 rectenna and 5 kΩ in the G3 rectenna.
Firstly, it can be seen that the G2 rectennas have their main resonant frequency at 1.46 GHz while resonances also appear at 0.58 GHz and 2.78 GHz. Furthermore, it is appreciated that below 0.3 GHz there could also be another resonant frequency. However, the transmitter antenna used for the experiments does not show stable behavior below this frequency, so this range could not be analyzed. This is very interesting from the point of view of the low absorption that it would have in the human body at low frequencies, so this is an aspect that will be analyzed when the appropriate equipment is available. Although the highest performance is obtained by working at the main resonance frequency, resonant frequencies in the order of a hundred megahertz are very convenient to achieve low attenuation due to the energy absorption of organic tissues, and the higher ones could also be useful to have for low-power communication channels.
Secondly, the main and only resonant frequency in the analysis range of the G3 rectenna was located at 1.175 GHz. Although the voltage generated at the resonant frequency was of the same order of magnitude as that in the G2 rectenna, the power is about sixty times larger because of the lower output resistance.

4.2. Power Sweep Tests

Second, a power sweep test was performed at the measured resonant frequencies to determine the rectenna maximum output power and the relationship between the received and the generated power, following the process described in Figure 5. The sweep was programmed between an incident electric field of 10 V/m to the maximum that the amplifier could generate, although the output was limited by protection since the reflected power was high.
The test results on the G2 prototype for the main resonance frequency are shown in Figure 6a. A maximum voltage of 5.29 V and a power of 0.056 mW was measured at 1.446 GHz, under an incident field on the rectenna of 340 V/m. From this incident electric field value, the voltage and power curve at the output of the rectenna stabilize. For all other frequencies, the power obtained is lower, and the curves do not stabilize when the incident electric field is increased further. A much larger electric field would be necessary to reach the same values as that of the main resonant frequency. However, the output power when working at lower resonant frequencies could be higher when working inside the body, as a consequence of the low absorption of electromagnetic energy of the tissues.
The test results on the G3 prototype for the main resonance frequency are shown in Figure 6b. The maximum voltage measured at the rectenna terminals was 4.62 V, with a maximum field of 535 V/m that could be generated with the available equipment. However, it can be seen that the rectenna is not at its operating limit, since the curves present a positive slope. The power generated is much greater in this prototype, reaching 4.27 mW, although it is estimated that it could be greater than 5 mW under an incident electric field of sufficient magnitude.
These results have been compared with those of other studies in the literature using on-chip antennas as shown in Table 1. As can be seen, G2 and G3 rectennas show a good compromise between low resonant frequency, which reduces absorption losses in intracorporeal applications; size, which facilitates their insertion into the body and their integration into microsurgery tools and implantable medical devices; and transmission efficiency, which is in the same order of magnitude as the rest of the antennas reported in the literature. Additionally, they can deliver much higher maximum power and voltage.

5. Biological Tissues Tests

The rectenna prototypes were also tested by simulating their operation in intracorporeal conditions. To conduct the simulation, they were introduced into 3 cm thick pieces of pork muscle and fat, whose properties are similar to those of human tissues. To protect the rectennas, mainly from the wire bonding, and prevent direct contact with the tissues, plastic covers were 3D printed, as shown in Figure 7a. The covers were manufactured with the minimum wall thickness allowed by the 3D printer so that the shielding was as low as possible. The covers turned out to be completely transparent to the signal, which was ideal for the tests.
The rectennas protected by the printed covers were then introduced into the pork muscle and fat tissues, as shown in Figure 7b–f, and frequency sweep tests were performed again to determine the new resonant frequencies under intracorporeal conditions. Once the resonant frequencies of the two rectenna prototypes were determined, surrounded by muscle and fat tissues, power sweeps tests were carried out at each of the determined resonant frequencies. These results were presented in graphs, in which the limits allowed by the ICNIRP were delimited considering the incident field on the tissues under continuous and pulsed signal transmission [33]. The results for the best performance cases are shown below in Figure 8 and Figure 9.
The G2 rectenna showed resonant frequencies at 590 MHz, 785 MHz, and 905 MHz when surrounded by pork muscle. The highest output values, 4.28 V and 36.64 μW, were obtained when working at 785 MHz. The output voltage and power measurements at this frequency of 785 MHz are shown in Figure 8a. These values are possible when working inside the human body only if the duty cycle of the emitted signal is controlled, so as not to exceed the limits imposed by the ICNIRP. By lowering the duty cycle of the emitted signal, it is possible to increase the maximum safe electric field incident on the skin. The electric field safety limit, indicated in the graphs as a “harmful zone”, has been defined for the duty cycle specified in each graph, although it could be increased, if necessary, by controlling the duty cycle of the signal.
The G2 rectenna also showed resonant frequencies at 360 MHz, 650 MHz, 1.28 GHz, and 2.295 GHz when surrounded by pork fat. In this case, the highest output values, 5.51 V and 60.72 μW, were obtained when working at 1.28 GHz. The measured output values at this frequency of 1.28 GHz are shown in Figure 8b. Again, these values are achievable only with a pulsed emission of the signal.
Regarding the G3 rectenna, it showed resonant frequencies at 785 MHz and 900 MHz when surrounded by muscle. The highest output values, 1.4 V and 389.2 μW, were obtained when working at 785 MHz. The output voltage and power measurements at the frequency of 1.28 GHz are shown in Figure 9a. By surrounding the G3 rectenna with fat, it was observed that the new resonant frequencies were 305 MHz, 650 MHz, 1.05 GHz, and 2.25 GHz. The highest output values, 4.22 V and 3.55 mW, were obtained when working at 1.05 GHz. The measured output values at 1.05 GHz are shown in Figure 9b. For these two cases, the emitted signal also needs to be pulsed to be able to increase the maximum safe value of the incident electric field.

6. In-Body Operation Analysis

The power that would be obtained at the end terminals of the receiving antenna at different locations of the human body was analyzed to determine the feasibility of using these antennas in real intracorporeal operating conditions. First, the attenuation of the signal at different working frequencies and different high- (muscles and skin) and low- (fat and bones) water-content tissue thicknesses were calculated from the data of [50]. In Reference [50], the penetration depth (the depth at which the energy transmission efficiency is 13.5%) in the human body for high- and low-water-content tissues at different frequencies is provided.
Then, the output power expected to be delivered by the G3 rectenna, which achieves greater power at the output terminals than the ones of the previous generations, was calculated from the results of the power sweep tests, considering the incident electric field in the rectenna. The incident electric field in the rectenna was determined by multiplying the maximum safe incident electric field for pulsed operations in local exposure by the attenuation previously calculated. The duty cycle of the emitted signal was adjusted to shift the safe limit of the incident electric field and increase the instantaneous power available at the output terminals of the rectenna in each case. The results are shown in Table 2.
The available power at the output terminals makes the rectenna a great candidate to be used in a multitude of implants and medical tools since it achieves high power in a very small volume. Furthermore, it should be remembered that, in the tests, the operative maximum output power of the rectenna, which is estimated to be greater than 5 mW under optimal conditions, was not reached.
Finally, a search was carried out to determine if the rectenna could be used in medical device applications, either in devices currently in use or in devices that could be adapted for wireless power transmission technology. After reviewing potential applications, the pulmonary artery pressure monitor was selected as the most suitable application for this specific rectenna based upon alignment with the rectenna performance specifications and the presence of a predicate commercial device, indicating an existing viable market.
Heart failure (HF) is among the most prevalent cardiovascular conditions, affecting an estimated 1–2% of adults [51]. This rate increases to over 10% among individuals older than 70 years. Each year, HF is responsible for more than one million hospital admissions in the US and Europe and affects an estimated 64 million people worldwide. Wirelessly powered, implantable pulmonary artery blood pressure (PABP) monitors have been shown to be a cost-effective means of allowing remote measurement and monitoring of sections of the HF patient population, improving quality of life and reducing rehospitalizations.

7. Conclusions

This work presents the results of the characterization of two 1 × 5 mm2 miniaturized rectennas developed for medical applications. The first was designed for enhanced output voltage applications, and the second, for enhanced output power applications. Both rectennas were tested in open-air conditions and surrounded by pork fat and muscle tissues, whose properties are similar to those of the human body. The first chip showed a maximum voltage of 5.29 V and a maximum power of 0.056 mW, at 1.446 GHz, under an incident field on the rectenna of 340 V/m, and the second chip, 4.62 V and 4.27 mW, at 1.175 GHz, under 535 V/m. However, the second rectenna did not reach its limit, and it is estimated that the output power could be greater than 5 mW.
The motivation for this work was to evaluate the performance of rectennas as a possible power source in implantable devices. The rectennas presented in this article are beyond the state of the art, as they can deliver about three times more power and voltage than those of similar dimensions found in the literature, achieving higher voltage and power than other devices with reduced volume. The proposed rectennas support the design of microsurgical tools and implantable devices with smaller overall dimensions, which do not require a physical connection as they can be wirelessly powered. Therefore, the use of rectennas in medical devices would enable interventions with microsurgical tools; moreover, the insertion of rectenna-powered implantable devices would be much less invasive for patients, opening the possibility of reaching areas of the human body with narrower cavities. Future work will explore the inclusion of the developed rectennas in medical devices.

Author Contributions

Conceptualization, E.D.-J., M.M. and R.S.-M.; methodology, E.D.-J. and M.M.; software, M.F.-M. and M.S.; validation, P.L.L.-E. and J.A.M.-R.; formal analysis, M.F.-M. and R.S.-M.; data curation, M.F.-M. and M.S.; writing—original draft preparation, M.F.-M. and E.D.-J.; writing—review and editing, P.L.L.-E., R.S.-M. and J.A.M.-R.; supervision, E.D.-J. and M.M.; project administration, E.D.-J. and M.M.; funding acquisition, E.D.-J. and P.L.L.-E. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 857654–UWIPOM2. This work was also funded by Universidad de Alcalá within the framework of the Predoctoral Contracts for the Training of Research Staff (2021).

Institutional Review Board Statement

Ethics approval is not required for this type of study. This study was conducted following the local legislation: https://www.global-regulation.com/translation/spain/1490937/royal-decree-53-2013%252c-1-february%252c-by-which-establish-the-basic-standards-for-the-protection-of-animals-used-in-experimentation-and-other-scientifi.html.

Data Availability Statement

Data are contained within the article. The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors want to thank David Sanguino López, Diego Pérez de Diego, Francisco Javier Dongil Moreno, and Pablo Sanz Miguel from CATECHOM for their help during the tests.

Conflicts of Interest

Authors Mohamed Missous and Mohammadreza Sadeghi were employed by the company Advanced Hall Sensors Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

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Electronics 13 03159 g001
Figure 1. (a) Microscopic view of the rectenna; (b) integration of the 1 × 5 mm2 folded rectenna on a larger PCB; and (c) detail of microwelding by wire bonding.
Figure 1. (a) Microscopic view of the rectenna; (b) integration of the 1 × 5 mm2 folded rectenna on a larger PCB; and (c) detail of microwelding by wire bonding.
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Figure 2. Test setup for the characterization of the rectennas in the semi-anechoic chamber. (1) Mast where the rectenna prototypes were attached, (2) mast where the transmitter antenna was attached, (3) switch and power amplifiers, (4) calibrated transmitting antenna, (5) electric field sensor, and (6) rectenna prototypes under test.
Figure 2. Test setup for the characterization of the rectennas in the semi-anechoic chamber. (1) Mast where the rectenna prototypes were attached, (2) mast where the transmitter antenna was attached, (3) switch and power amplifiers, (4) calibrated transmitting antenna, (5) electric field sensor, and (6) rectenna prototypes under test.
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Figure 3. Frequency sweep test procedure.
Figure 3. Frequency sweep test procedure.
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Figure 4. (a) Frequency sweep test of the G2 rectenna; (b) frequency sweep test of the G3 rectenna.
Figure 4. (a) Frequency sweep test of the G2 rectenna; (b) frequency sweep test of the G3 rectenna.
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Figure 5. Power sweep test procedure.
Figure 5. Power sweep test procedure.
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Figure 6. (a) Power sweep test at the resonant frequency of 1.46 GHz of the G2 rectenna; (b) power sweep test at the resonant frequency of 1.175 GHz of the G3 rectenna.
Figure 6. (a) Power sweep test at the resonant frequency of 1.46 GHz of the G2 rectenna; (b) power sweep test at the resonant frequency of 1.175 GHz of the G3 rectenna.
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Figure 7. (a) Cover design; (b) rectenna placed inside the 3D cover and wrapped with thin plastic sheet; (c) rectenna inserted in pork muscle; (d) rectenna inserted in pork fat; (e) test setup for the tests of the rectennas inside pork muscle; and (f) test setup for the tests of the rectennas inside pork fat.
Figure 7. (a) Cover design; (b) rectenna placed inside the 3D cover and wrapped with thin plastic sheet; (c) rectenna inserted in pork muscle; (d) rectenna inserted in pork fat; (e) test setup for the tests of the rectennas inside pork muscle; and (f) test setup for the tests of the rectennas inside pork fat.
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Figure 8. (a) Power sweep test at 785 MHz of the G2 rectenna surrounded by pork muscle tissues; (b) power sweep test at 1.28 GHz of the G2 rectenna surrounded by pork fat tissues.
Figure 8. (a) Power sweep test at 785 MHz of the G2 rectenna surrounded by pork muscle tissues; (b) power sweep test at 1.28 GHz of the G2 rectenna surrounded by pork fat tissues.
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Figure 9. (a) Power sweep test at 785 MHz of the G3 rectenna surrounded by pork muscle tissues; (b) power sweep test at 1.05 GHz of the G3 rectenna surrounded by pork fat tissues.
Figure 9. (a) Power sweep test at 785 MHz of the G3 rectenna surrounded by pork muscle tissues; (b) power sweep test at 1.05 GHz of the G3 rectenna surrounded by pork fat tissues.
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Table 1. Output voltage and power comparison with rectennas reported in the literature using on-chip antennas in open-air tests.
Table 1. Output voltage and power comparison with rectennas reported in the literature using on-chip antennas in open-air tests.
ReferenceFreq. (GHz)Area (mm2)Distance (cm) aPt (W) bVDC (V) cPDC (μW) dPTE (dB) e
[39]2.451 × 0.50.050.250.694.7−34.2
[40]5.83 × 1.57.541.80.6−68.3
[41]5.23.2 × 1.53.551.15100−47
[42]243.7 × 1.228100.91.5−68.2
[43]2.751.6 × 1.6211.11200−29.5
[44]281.8 × 1.850025,0000.198.6−94.7
G2 rectenna1.461 × 54312.355.2956−67.46
G3 rectenna1.1751 × 54256.554.624270−47.79
a Separation distance between transmitter and receiving antennas. b Transmitting antenna power. c Receiving rectenna output voltage. d Receiving rectenna output power. e Power transfer efficiency.
Table 2. Calculated power relationship with respect to the incident electric field in the G3 rectenna, at different locations of the body, considering the energy absorption of the human body.
Table 2. Calculated power relationship with respect to the incident electric field in the G3 rectenna, at different locations of the body, considering the energy absorption of the human body.
LocationTissue Thickness (cm)Transmission
Efficiency through Tissues (%)		Signal Duty
Cycle (%)		Electric Field Incident in the Skin (V/m)Electric Field
Incident in the Rectenna (V/m)		Rectenna
Output
Voltage (V)		Rectenna
Output Power (mW)
HCW aLCW b
Under the skin0.5071.59728714.28511.414.424.08
Stomach367.90854000316.312.732.52
Cerebral cortex (pia mater)2222.003102000440.053.803.51
Heart ventricle/pulmonary artery349.44454000377.763.263.01
a High-water-content tissues. b Low-water-content tissues.
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MDPI and ACS Style

Fernandez-Munoz, M.; Missous, M.; Sadeghi, M.; Lopez-Espi, P.L.; Sanchez-Montero, R.; Martinez-Rojas, J.A.; Diez-Jimenez, E. Fully Integrated Miniaturized Wireless Power Transfer Rectenna for Medical Applications Tested inside Biological Tissues. Electronics 2024, 13, 3159. https://doi.org/10.3390/electronics13163159

AMA Style

Fernandez-Munoz M, Missous M, Sadeghi M, Lopez-Espi PL, Sanchez-Montero R, Martinez-Rojas JA, Diez-Jimenez E. Fully Integrated Miniaturized Wireless Power Transfer Rectenna for Medical Applications Tested inside Biological Tissues. Electronics. 2024; 13(16):3159. https://doi.org/10.3390/electronics13163159

Chicago/Turabian Style

Fernandez-Munoz, Miguel, Mohamed Missous, Mohammadreza Sadeghi, Pablo Luis Lopez-Espi, Rocio Sanchez-Montero, Juan Antonio Martinez-Rojas, and Efren Diez-Jimenez. 2024. "Fully Integrated Miniaturized Wireless Power Transfer Rectenna for Medical Applications Tested inside Biological Tissues" Electronics 13, no. 16: 3159. https://doi.org/10.3390/electronics13163159

APA Style

Fernandez-Munoz, M., Missous, M., Sadeghi, M., Lopez-Espi, P. L., Sanchez-Montero, R., Martinez-Rojas, J. A., & Diez-Jimenez, E. (2024). Fully Integrated Miniaturized Wireless Power Transfer Rectenna for Medical Applications Tested inside Biological Tissues. Electronics, 13(16), 3159. https://doi.org/10.3390/electronics13163159

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