Modeling of a compact, implantable, dual-band antenna for biomedical applications

Citation: Bahrouni, M.; Houzet, G.; Vuong, T.P.; Mendes, P.M.; Dinis, H.; Silva, R.; Trabelsi, H. Modeling of a Compact, Implantable, Dual‑Band Antenna for Biomedical Applications. Electronics 2023,12, 1475. https:// doi.org/10.3390/electronics12061475 Academic Editor: Giovanni Leone Received: 8 January 2023 Revised: 20 February 2023 Accepted: 20 February 2023 Published: 21 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). electronics Article Modeling of a Compact, Implantable, Dual‑Band Antenna for Biomedical Applications Majdi Bahrouni 1,2,3,*, Gregory Houzet 2,3, Tan Phu Vuong 2, Paulo M. Mendes 4,5 , Hugo Dinis 4,5, Rui Silva 4,5 and Hichem Trabelsi 6 1Microwave Electronics Research Laboratory, Department of Physics, Faculty of Sciences of Tunis, University of Tunis El Manar, Tunis 2092, Tunisia 2Institute of Microelectronics Electromagnetism and Photonics‑Microwave Laboratory and Characterization, Grenoble INP, Grenoble Alps University, 38000 Grenoble, France 3Institute of Microelectronics Electromagnetism and Photonics‑Microwave Laboratory and Characterization, UniversitéSavoie Mont Blanc, 73000 Le Bourget du Lac, France 4Center for MicroElectromechanical Systems (CMEMS‑UMinho), University of Minho, 4800‑058 Guimarães, Portugal 5LABBELS—Associate Laboratory, 4710‑057 Braga, Portugal 6National School of Engineers of Carthage, University of Carthage, Charguia II 2035, Tunisia *Correspondence: [email protected] Abstract: Different implantable antenna designs exist to establish communication with implantable devices depending on the domain of use and the implantation space. Owing to their nature and purposes, these antennas have many imposed criteria on various characteristics, such as bandwidth, multiband behavior, radiation pattern, gain, and specific absorption rate (SAR). This presents a chal‑ lenge when it comes to achieving satisfying results without a major compromise in any of these crucial parameters. Additionally, many of the existing designs do not follow a specific approach to obtain results. Measuring different parameters of such fabricated structures requires special condi‑ tions and special environments mimicking the tissues where they are supposed to be placed. For such issues, the use of biological or synthetic phantoms is widely employed to validate what is ob‑ tained in simulation, and a multitude of formulas exist for the creation of such phantoms, each with its advantages and drawbacks. In this paper, a miniature dual‑band structure derived from the first iteration of the Koch fractal structure is designed to operate 2 mm below the skin in the arm of the hu‑ man body, with the MICS (Medical Implant Communication System) and ISM (Industrial, Scientific, Medical) 2.4 GHz bands. The purposes of the design are to derive structures from commonly used shapes with certain behavior while maintaining miniaturization, and to easily design dual‑band im‑ plantable antennas. More than one band is used to diversify uses, since bands such as the MICS band are mainly dedicated to telemetry. The structure is characterized not only by its low profile compared to various structures found in the literature with dimensions of 17.2 ×14.8 ×0.254 mm3, but also its ease of design, independent shifting of resonant frequencies, and the absence of the need for a matching circuit and a shorting pin (via) for miniaturization. It exhibits satisfying performance: bandwidths of 23 MHz in the MICS band and 190 and 70 MHz in the vicinity of the ISM 2.4 GHz band, and measured gain in the latter band of −18.66 and −17 dBi in the azimuth and elevation radiation patterns, respectively. To validate the antenna’s properties in a skin‑mimicking environment, two simple phantom formulas found in the literature were explored and compared in order to identify the best option in terms of accuracy and ease of fabrication. Keywords: antenna; implantable; miniature; fractal; phantom characterization; biocompatibility 1. Introduction With the evolution of various existing technologies, implantable medical devices (IMDs) continue to evolve not only to solve the already existing challenges, but also to face newer Electronics 2023,12, 1475. https://doi.org/10.3390/electronics12061475 https://www.mdpi.com/journal/electronics Electronics 2023,12, 1475 2 of 15 ones. This all revolves around the objective of simplifying the lives of patients by improv‑ ing performance and establishing new approaches, purposes, and ways of operation, while maintaining less invasiveness and lower costs. This is performed without compromising the health of patients, mainly in terms of biocompatibility and unwanted tissue stimulation, often evaluated through the specific absorption rate (SAR). Implantable antennas, part of an IMD that needs to connect to the outside world, are therefore also changing. Miniaturization of these radiating structures is crucial to main‑ tain sizes compatible with those of downsized implantable devices, making their design processes challenging. A large variety of antenna design concepts and approaches exist to maintain this performance/miniaturization rule. The design presented in [1] has a compact structure of only 8 ×8×0.2 mm3. It was simulated in a skin‑mimicking homogeneous phantom. It has a bandwidth of 290 MHz covering the ISM 2.4 GHz band and a simulated gain value of −14 dBi at its resonant frequency. Despite its characteristics, the structure includes slots on both sides, along with a via for miniaturization, making the manufac‑ turing process complicated. All this, in addition to its monoband nature, makes its uses limited to the mentioned band. A similar structure is presented in [2]. Although smaller, at just 7 ×7×0.2 mm3, the antenna has a simulated gain of −15 dBi and a total bandwidth of 420 MHz, including the ISM 2.4 GHz band. The simulation was performed in skin, stomach, and bronchi‑mimicking environments. The antenna contains relatively complex shapes on both sides and a via, which makes it even more complex in terms of manufactur‑ ing, aside from the fact that, similarly to the previously presented structure, it remains a monoband antenna. In [3], a dual‑band meander‑line/ring shape is presented, with dimen‑ sions of 6 ×6×2.54 mm3. The antenna operates in the ISM 915 MHz and 2.4 GHz bands, in a homogeneous skin box, and has measured gain values of −13.14 and −28 dBi in the mentioned bands, respectively. Although simple in design and having no vias, the antenna has a relatively large volume of 91.44 mm3and low gain in the ISM 2.4 GHz band. In [4], an even simpler structure operating in the MICS and ISM 2.4 GHz bands is presented, with dimensions of 22.5 ×22.5 ×2.5 mm3. It was simulated in a three‑layer muscle–fat–skin tissue model, providing measured bandwidths of 141.2 and 170.4 MHz in the MICS and ISM 2.4 GHz bands, respectively. The structure is not very efficient in terms of miniaturiza‑ tion, as it has large dimensions and a volume of 1265.63 mm3and requires a shorting pin, making the fabrication process complicated. A dual‑band ISM 915 MHz/2.4 GHz antenna was proposed in [5]. It has simulated bandwidths of 200 and 450 MHz, and levels of gain reaching −26.71 dBi and −17.5 dBi in the mentioned bands, in a seven‑layer human head model. Its overall dimensions are 11 ×19 ×1.25 mm3, and it includes a via. In this paper, an approach based on fractal structures is adopted, as they are known to exhibit multiband behavior depending on the chosen shape. The purpose of the approach is to design dual‑band structures with known shapes that allow obtaining resonant fre‑ quencies in the desired bands, while maintaining a reduced size. The reason behind the adoption of the dual‑band characteristic is to diversify the use of the antenna. In the case of the presented work, the MICS band is dedicated to telemetry, whereas the ISM 2.4 GHz band can have other uses. One application of interest is wireless power transfer (WPT), as it has a multitude of advantages in the field of implantable devices. These include a reduction in the dependency on batteries, allowing smaller battery sizes for dependent implantable devices, and directly powering up implantable devices when the battery is not needed. The proposed design is based on the first iteration of Koch’s fractal structure before it is modified for miniaturization purposes, with final dimensions of 17.2 ×14.8 × 0.254 mm3. The resonant frequencies, which are independently modifiable, are obtained by tuning the specific areas where the antenna resonates. The model is designed to be placed 2 mm below the skin of a human arm, with no matching circuit, and does not re‑ quire any shorting pin for miniaturization. The bands of interest are the 400 MHz MICS and the ISM 2.4 GHz. To validate simulation results through measurements, a phantom imitating a biological tissue is required. Various methods and mixtures exist to create such phantoms, but two mixtures found in the literature were a matter of interest. One is based Electronics 2023,12, 1475 3 of 15 on sodium chloride (NaCl) and sugar [4], and the other is based on DGBE (diethylene glycol monobutyl ether) and the detergent Triton X‑100 [6]. A parametric study was per‑ formed to compare the two mixtures in terms of ease of fabrication and the accuracy of their permittivity and conductivity. 2. Antenna Design Biocompatible materials are used to make antennas implantable in any living biolog‑ ical tissue without triggering any reaction from the immune system. The antenna was therefore initially designed based on the use of alumina (εr= 9.8; tanδ= 10−3) as a sub‑ strate and superstrate [7], but due to its low availability, a similar, widely marketed non‑ biocompatible dielectric was used solely for in vitro measurements: the Rogers RO3010 (εr= 10.2; tanδ= 2.2 ×10−3) with a thickness of 127 µm. It was selected for this design as it has comparable dielectric parameters to alumina. 2.1. Geometry The proposed antenna design was initially based on the first iteration of Koch’s fractal structure, a synthetic curve, the shape of which maintains the same pattern in each of its parts, forming smaller self‑copies, similarly to natural fractal curves, i.e., tree roots. This shape is used to fill a defined space with numerous smaller self‑copies for miniaturiza‑ tion, as shown in Figure 1. This allows the designed antenna to inherit fractal structures’ compactness and the multi‑band behavior [8]. Electronics 2022, 11, x FOR PEER REVIEW 3 of 15 2.4 GHz. To validate simulation results through measurements, a phantom imitating a biological tissue is required. Various methods and mixtures exist to create such phantoms, but two mixtures found in the literature were a matter of interest. One is based on sodium chloride (NaCl) and sugar [4], and the other is based on DGBE (diethylene glycol monobutyl ether) and the detergent Triton X-100 [6]. A parametric study was performed to compare the two mixtures in terms of ease of fabrication and the accuracy of their permittivity and conductivity. 2. Antenna Design Biocompatible materials are used to make antennas implantable in any living biological tissue without triggering any reaction from the immune system. The antenna was therefore initially designed based on the use of alumina (εr = 9.8; tanδ = 10−3) as a substrate and superstrate [7], but due to its low availability, a similar, widely marketed non-biocompatible dielectric was used solely for in vitro measurements: the Rogers RO3010 (εr = 10.2; tanδ = 2.2 × 10−3) with a thickness of 127 μm. It was selected for this design as it has comparable dielectric parameters to alumina. 2.1. Geometry The proposed antenna design was initially based on the first iteration of Koch’s fractal structure, a synthetic curve, the shape of which maintains the same pattern in each of its parts, forming smaller self-copies, similarly to natural fractal curves, i.e., tree roots. This shape is used to fill a defined space with numerous smaller self-copies for miniaturization, as shown in Figure 1. This allows the designed antenna to inherit fractal structures’ compactness and the multi-band behavior [8]. (a) (b) (c) Figure 1. Koch fractal curve: (a) no iterations applied to a segment line; (b) first iteration applied; (c) second iteration applied. The design approach consists in creating and modifying a printed dipole based on the first iteration, as shown in Figure 2a. The dipole is placed between a RO3010 127 μm thick substrate and superstrate. The latter is added to avoid direct contact between the radiating element and the human body to avoid any possible unwanted electrical flow in the surrounding tissues, and to ensure biocompatibility when using alumina. In the figure, the structure resonates at 2.4 GHz prior to development and optimization. Figure 2b,c show the modification process for miniaturization while maintaining the overall length and width of the antenna. While the resulting modified structure resembles a spiral shape, multiband behavior is observed in the iterations presented in these figures. In Figure 2d, the structure is designed to reach the MICS band and includes modifications for better matching while maintaining similar behavior to its previous iterations. (a) (b) Figure 1. Koch fractal curve: (a) no iterations applied to a segment line; (b) first iteration applied; (c) second iteration applied. The design approach consists in creating and modifying a printed dipole based on the first iteration, as shown in Figure 2a. The dipole is placed between a RO3010 127 µm thick substrate and superstrate. The latter is added to avoid direct contact between the radiating element and the human body to avoid any possible unwanted electrical flow in the surrounding tissues, and to ensure biocompatibility when using alumina. In the figure, the structure resonates at 2.4 GHz prior to development and optimization. Figure 2b,c show the modification process for miniaturization while maintaining the overall length and width of the antenna. While the resulting modified structure resembles a spiral shape, multiband behavior is observed in the iterations presented in these figures. In Figure 2d, the structure is designed to reach the MICS band and includes modifications for better matching while maintaining similar behavior to its previous iterations. The design process is conducted as follows: •Tilt the structure’s elements to reduce size while maintaining comparable resonant frequencies. For this purpose, a printed fractal dipole is studied at first, as shown in Figure 2a. •The structure’s elements are extended inwards, as shown in Figure 2b,c. •The structure is parametrically studied to identify parts where it resonates in the de‑ sired bands. For better matching, a parametric study on different parts of the antenna is conducted. The lower side of the radiating element is modified and parametrically optimized, as seen in Figure 2d. •For further miniaturization, one element is removed and is replaced with a ground plane at the bottom of the substrate, cutting the structure’s width almost in half, as shown in Figure 2e. This results in a slight degradation in terms of performance. •For further miniaturization, the antenna’s sides are flattened and parametrically stud‑ ied to ensure comparable behavior to the initial design. Electronics 2023,12, 1475 4 of 15 •After all modifications are done, the structure’s matching is enhanced again by creat‑ ing and tuning two parts, “A” and “B”, as shown in Figure 3, where: ◦A: is the space resonating in the MICS band; ◦B: is the space resonating in the ISM 2.4 GHz band. Electronics 2022, 11, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/electronics Article 1 Modeling of a compact implantable dual band antenna for bio2 medical applications 3 4 (a) (b) 5 (c) (d) 6 7 (e) 8 Figure 1. Different iterations obtained throughout the optimization process: (a) Koch 1st iteration 9 resonating at 2.4 GHz (b) Modified Koch structure (c) Extended modified Koch structure for lower 10 frequency resonance (d) modified structure for better matching (e) single element structure 11 Figure 2. Different iterations obtained throughout the optimization process: (a) first Koch iteration resonating at 2.4 GHz; (b) modified Koch structure; (c) extended modified Koch structure for lower frequency resonance; (d) modified structure for better matching; (e) single‑element structure. Region B is shaped into a rectangle. Its dimensions are defined following a parametric study for best matching. In the MICS band, the enhancement is obtained by applying several extensions at the tips of region A. Region 3 is where the reflection coefficient is enhanced. Table 1shows the different antenna dimensions’ values: Overall, the proposed antenna has a volume of 64.65 mm3, and dimensions equivalent to (2.3% ×3.6% ×0.03%) λat 400 MHz and (14.01% ×12.1% ×0.2%) λat 2.4 GHz. 2.2. Simulation The antenna was simulated using the Ansys High Frequency Simulation System— HFSS—in a single layer‑mimicking environment to facilitate the measurement and valida‑ tion procedure, as shown in Figure 4. Electronics 2023,12, 1475 5 of 15 Electronics 2022, 11, x FOR PEER REVIEW 2 of 3 12 (a) 13 14 (b) 15 Figure 2. Final proposed structure: (a) Face view (b) Side view with mounted U.FL connector 16 17 18 19 20 (a) (b) Figure 3. Antenna simulation conditions (a) in a homogenous skin mimicking phantom (b) with a 21 the U.FL connector 22 23 e l=17.2 mm w=14.8 mm e=200 mm e e w l e Radiation box Antenna U.FL connector Figure 3. Final proposed structure: (a) face view; (b) side view with mounted U.FL connector. Table 1. Proposed antenna’s dimensions. Parameter Value (mm) L 17.2 W 14.8 Dielectric thickness 0.127 Sl 0.7 Sw 1.6 Sd1 1.8 Sd2 1 Lm1 9 Lm2 4.1 Lm3 5.3 Lm4 7.5 Lm5 3.9 Lm6 7.4 Lm7 1.9 Lmw 0.1 Lio 0.3 Wio 0.1 Le1 3.2 Le2 2 Wext 11.6 Lma 0.1 Lia 4.5 Wia 10.4 Electronics 2023,12, 1475 6 of 15 Electronics 2022, 11, x FOR PEER REVIEW 2 of 3 12 (a) 13 14 (b) 15 Figure 2. Final proposed structure: (a) Face view (b) Side view with mounted U.FL connector 16 17 18 19 20 (a) (b) Figure 3. Antenna simulation conditions (a) in a homogenous skin mimicking phantom (b) with a 21 the U.FL connector 22 23 e l=17.2 mm w=14.8 mm e=200 mm e e w l e Radiation box Antenna U.FL connector Figure 4. Antenna simulation conditions (a) in a homogenous skin‑mimicking phantom (b) with the U.FL connector. The simulation was carried out in two configurations. In Figure 4a, the antenna is submerged in a 200 ×200 ×200 mm3cube filled with a skin mimicking phantom, as it is intended to be placed subcutaneously in the arm 2 mm below the skin. The first simula‑ tion phases were performed in the configuration presented in Figure 4a. Due to simulator’s limitations, the antenna was excited using an internal ideal port. To consider the use of an external connector, including its dimensions, the second configuration, shown in Figure 4b, was used. It consisted of putting a 17.2 ×14.8 ×200 mm3skin mimicking phantom above the antenna to simulate placement of a commercially available U.FL connector as an exter‑ nal port in order to take into account its effects during simulation. The U.FL connector was used because of its small dimensions and to simplify connection between the antenna and the devices. Human skin is characterized by certain electrical parameters, including permittivity and conductivity. They depend on the frequencies of the signals used. In the MICS and ISM 2.4 GHz bands, these parameters are εr= 46.787, σ= 0.68807 S/m, and εr= 38.063, σ= 1.4407 S/m, respectively [9]. The simulated S11 coefficient of the antenna presented in Figure 4b configuration is shown in Figure 5a,b. The simulated results indicate that in the MICS band, the parameter S11 has a value of −32.18 dB at 402.5 MHz. The frequency band in which the S11 coefficient is less than −10 dB runs from 395 to 409 MHz (a bandwidth of 14 MHz). In the ISM 2.4 GHz band, the antenna exhibits relatively wide‑band behavior. The S11 remains below −10 dB at 2.33 to 2.51 GHz (a bandwidth of 180 MHz), and a minimum reflection coefficient equal to −19.65 dB is obtained at 2.42 GHz. A parametric study was performed in case there was a mismatch on any of these bands for any reason. Samples of the modified antennas with different dimensions are presented in Figure 6a–d, and the resulting S11 parameters are shown in Figure 7a,b for the MICS and ISM 2.4 GHz bands, respectively. Figure 7a shows that different values of the “Lm7” parameter residing in the radiating region of the MICS band contribute to the shifting of the resonant frequency, without degrading the impedance matching of the antenna. Figure 7b shows a similar behavior in the ISM band, where the “Wext” parameter helps in shifting the resonant frequency without strongly af‑ fecting the impedance matching. The shifting in terms of frequency in one band does not affect the other, allowing one to perform frequency‑independent tuning. The frequency shifting sensitivity obtained by tuning the mentioned parameters is not the same: in the MICS band, shifting the resonant frequency from 402.5 MHz to 390 MHz requires a vari‑ ation of 4.8 mm in the “Lm7” parameter, whereas in the ISM 2.4 GHz band, it takes only 0.5 mm of difference to shift the resonant frequency from 2.42 to 2.51 GHz. Region B is therefore more vulnerable to structural modifications. Electronics 2023,12, 1475 7 of 15 Electronics 2022, 11, x FOR PEER REVIEW 7 of 16 (a) (b) Figure 5. Simulated reflection coefficient of the antenna in the (a) MICS band and (b) ISM 2.4 GHz band. The simulated results indicate that in the MICS band, the parameter S11 has a value of −32.18 dB at 402.5 MHz. The frequency band in which the S11 coefficient is less than −10 dB runs from 395 to 409 MHz (a bandwidth of 14 MHz). In the ISM 2.4 GHz band, the antenna exhibits relatively wide-band behavior. The S11 remains below −10 dB at 2.33 to 2.51 GHz (a bandwidth of 180 MHz), and a minimum reflection coefficient equal to −19.65 dB is obtained at 2.42 GHz. A parametric study was performed in case there was a mismatch on any of these bands for any reason. Samples of the modified antennas with different dimensions are presented in Figure 6a–d, and the resulting S11 parameters are shown in Figure 7a,b for the MICS and ISM 2.4 GHz bands, respectively. Figure 7a shows that different values of the “Lm7” parameter residing in the radiating region of the MICS band contribute to the shifting of the resonant frequency, without degrading the impedance matching of the antenna. Figure 7b shows a similar behavior in the ISM band, where the “Wext” parameter helps in shifting the resonant frequency without strongly affecting the impedance matching. The shifting in terms of frequency in one band does not affect the other, allowing one to perform frequency-independent tuning. The frequency shifting sensitivity obtained by tuning the mentioned parameters is not the same: in the MICS band, shifting the resonant frequency from 402.5 MHz to 390 MHz requires a variation of 4.8 mm in the “Lm7” parameter, whereas in the ISM 2.4 GHz band, it takes only 0.5 mm of difference to shift the resonant frequency from 2.42 to 2.51 GHz. Region B is therefore more vulnerable to structural modifications. Lm7 = 1.9 mm Lm7 = 6.7 mm Figure 5. Simulated reflection coefficient of the antenna in the (a) MICS band and (b) ISM 2.4 GHz band. Electronics 2022, 11, x FOR PEER REVIEW 3 of 3 24 25 (a) (b) 26 27 (c) (d) Figure 4. Proposed structure with different parameter variations: (a) Lm7= 1.9 mm (b) Lm7= 6.7 mm 28 (c) Wext=11.6 mm (d) Wext= 11.1 mm 29 30 Lm7=1.9mm Lm7=6.7mm Wext=11.6mm Wext=11.1mm Figure 6. Proposed structure with different parameter variations: (a) Lm7 = 1.9 mm, (b) Lm7 = 6.7 mm, (c) Wext = 11.6 mm, (d) Wext = 11.1 mm. Electronics 2023,12, 1475 8 of 15 Electronics 2022, 11, x FOR PEER REVIEW 8 of 16 (a) (b) (c) (d) Figure 6. Proposed structure with different parameter variations: (a) Lm7= 1.9 mm, (b) Lm7= 6.7 mm, (c) Wext = 11.6 mm, (d) Wext= 11.1 mm. (a) (b) Figure 7. Reflection coefficient of the antenna with variations in the resonant frequency in the (a) MICS band and (b) ISM band. The simulated radiation patterns of the structure in both bands are shown in Figure 8a,b. Wext = 11.6 mm Wext = 11.1 mm Figure 7. Reflection coefficient of the antenna with variations in the resonant frequency in the (a) MICS band and (b) ISM band. The simulated radiation patterns of the structure in both bands are shown in Figure 8a,b. Electronics 2022, 11, x FOR PEER REVIEW 8 of 15 (c) (d) Figure 6. Proposed structure with different parameter variations: (a) Lm7 = 1.9 mm, (b) Lm7 = 6.7 mm, (c) Wext = 11.6 mm, (d) Wext = 11.1 mm. (a) (b) Figure 7. Reflection coefficient of the antenna with variations in the resonant frequency in the (a) MICS band and (b) ISM band. The simulated radiation patterns of the structure in both bands are shown in Figure 8a,b. (a) (b) Figure 8. Simulated radiation pattern in (a) the MICS band and (b) the ISM band. Wext = 11.6 mm Wext = 11.1 mm Figure 8. Simulated radiation pattern in (a) the MICS band and (b) the ISM band. The antenna exhibits quasi‑omnidirectional radiation patterns in both MICS and ISM 2.4 GHz bands. Gain values reach −42.97 and −19.82 dBi in these bands, respectively. The obtained low gain values are due to the low profile of the antenna and the lossy environ‑ ment it is placed in. Electronics 2023,12, 1475 9 of 15 3. Tissue‑Mimicking Phantom Characterization To validate the various results obtained by simulation, measurements must be carried out either in vivo or in vitro. For the latter, the environment simulating real conditions must imitate the biological tissues of interest. The antenna is designed to operate at a depth of 2 mm below the skin of the human arm. Thus, a phantom mimicking human skin was required. In the literature, a variety of mixtures exist, two of which are the sugar/NaCl and Triton X‑100/DGBE/NaCl formulas. A parametric study was performed to establish the validity and accuracy of these formulas. For the study conducted in this work, the Keysight 85070E open‑ended coaxial probe was used for characterization, and the configuration is presented in Figure 9. Electronics 2022, 11, x FOR PEER REVIEW 9 of 15 The antenna exhibits quasi-omnidirectional radiation patterns in both MICS and ISM 2.4 GHz bands. Gain values reach −42.97 and −19.82 dBi in these bands, respectively. The obtained low gain values are due to the low profile of the antenna and the lossy environment it is placed in. 3. Tissue-Mimicking Phantom Characterization To validate the various results obtained by simulation, measurements must be carried out either in vivo or in vitro. For the latter, the environment simulating real conditions must imitate the biological tissues of interest. The antenna is designed to operate at a depth of 2 mm below the skin of the human arm. Thus, a phantom mimicking human skin was required. In the literature, a variety of mixtures exist, two of which are the sugar/NaCl and Triton X-100/DGBE/NaCl formulas. A parametric study was performed to establish the validity and accuracy of these formulas. For the study conducted in this work, the Keysight 85070E open-ended coaxial probe was used for characterization, and the configuration is presented in Figure 9. Figure 9. Phantom characterization setup (Keysight E5071C VNA and Keysight 85070E dielectric probe kit). 3.1. Sugar/NaCl Mixture The composition of this formula is based on widely available components: sugar and an electrolyte, which in this case was regular salt, sodium chloride (NaCl), mixed in deionized water. The desired dielectric parameters can be achieved by adjusting these components. Permittivity is dependent on the quantity of sugar added into the mixture, whereas the electrolyte helps with adjusting the conductivity. However, as seen in [4], at higher frequencies, and particularly in the ISM 2.4 GHz band, the formula obtained shows that although the permittivity can be accurately controlled, the conductivity increases drastically without the use of NaCl, exceeding the desired values uncontrollably. For the ISM 2.4 GHz band, a trial was carried out to analyze the dielectric properties of a mixture comprised of 100 mg of sugar dissolved in 100 mL of deionized water. The obtained results are shown in Figure 10. Figure 9. Phantom characterization setup (Keysight E5071C VNA and Keysight 85070E dielectric probe kit). 3.1. Sugar/NaCl Mixture The composition of this formula is based on widely available components: sugar and an electrolyte, which in this case was regular salt, sodium chloride (NaCl), mixed in deion‑ ized water. The desired dielectric parameters can be achieved by adjusting these compo‑ nents. Permittivity is dependent on the quantity of sugar added into the mixture, whereas the electrolyte helps with adjusting the conductivity. However, as seen in [4], at higher frequencies, and particularly in the ISM 2.4 GHz band, the formula obtained shows that al‑ though the permittivity can be accurately controlled, the conductivity increases drastically without the use of NaCl, exceeding the desired values uncontrollably. For the ISM 2.4 GHz band, a trial was carried out to analyze the dielectric properties of a mixture comprised of 100 mg of sugar dissolved in 100 mL of deionized water. The obtained results are shown in Figure 10. As can be seen in Figure 10a,b, both permittivity and conductivity increase propor‑ tionally to the amount of added sugar without adding salt. With the addition of 100 mg of sugar, the dielectric parameters obtained were εr= 38.08 and σ= 2.24 S/m, though there was an overshoot in the desired conductivity, as expected. Moreover, aside from the inability to control a crucial parameter, the dissolution of sugar is slow because of its high concentra‑ tion. This makes this formula unreliable for making phantoms for high‑frequency bands. 3.2. Triton X‑100/DGBE Mixture This formula requires essentially two commercially available ingredients: Triton X‑ 100 and DGBE. These allow mixing a liquid with the desired dielectric parameters of hu‑ man tissues in the bands of interest [6]. Additional ingredients are used [10], but for the phantoms required to validate the performance of the presented antenna, only NaCl was used. Figures 11–14 show various parametric studies at the MICS and ISM 2.4 GHz bands with different amounts of ingredients added to 100 mL of deionized water.