Implantable Antenna Research Articles

Dual-band Implantable Antenna Loaded with Patch Slots for Wireless Biotelemetry Systems

Dual-band Implantable Antenna Loaded with Patch Slots for Wireless Biotelemetry Systems

Electromagnetic metamaterials for biomedical applications: short review and trends

This mini-review examines the most prominent features and usages of metamaterials, such as metamaterial-based and metamaterial-inspired RF components used for biomedical applications. Emphasis is given to applications on sensing and imaging systems, wearable and implantable antennas for telemetry, and metamaterials used as flexible absorbers for protection against extreme electromagnetic (EM) radiation. A short discussion and trends on the metamaterial composition, implementation, and phantom preparation are presented. This review seeks to compile the state-of-the-art biomedical systems that utilize metamaterial concepts for enhancing their performance in some form or another. The goal is to highlight the diverse applications of metamaterials and demonstrate how different metamaterial techniques impact EM biomedical applications from RF to THz frequency range. Insights and open problems are discussed, illuminating the prototyping process.

Reduced-Size Antenna for Leadless Cardiac Pacemaker

The advancement in technology led to the development of leadless cardiac pacemaker (LCP) systems that resolved the issues that arise due to the use of leads in the conventional pacemakers (CP). However, the compact size of the LCP system also requires a miniaturized antenna that can fit inside the limited space of the LCP. Therefore, this study was conducted to propose and design a reduced-size antenna that operates at 2.4 GHz. An implantable antenna with a footprint of 4.00 5.00 0.50 mm was designed using CST Microwave Studio (student version). An LCP system was integrated with the antenna and simulated inside the heart tissue model to evaluate the performance of the antenna. Based on the S11 parameter, the antenna was further optimized, and the coaxial-feed cable position was offset until the impedance matching was achieved for the resonant frequency to be at 2.4 GHz. The antenna exhibited good performance with a S11 of less than -10 dB at 2.4 GHz. In addition, the proposed antenna satisfies the requirements of LCP with limited space. Therefore, the proposed antenna is envisaged to be implemented for LCP systems.

An implantable ISM band antenna for biomedical application

Abstract This article presents a compact implantable antenna for biomedical use in the ISM band (2.4–2.48 GHz). The antenna is designed with Roger RT/duroid 6010.2LM high permittivity substrate of thickness 0.25 mm, and feed with a 50 Ω co-axial feed. A hybrid-loop resonator slot structure and C & I-shaped slots in the radiating patch has enabled the antenna to work in the ISM band. Via less ground makes this design simple, easier to fabricate, and a suitable candidate for implantable application. The proposed antenna occupies a small volume of 0.0984 λ 0 × 0.082 λ 0 × 0.00468 λ 0 and retained 152 MHz bandwidth (2.4077–2.5599 GHz with FBW 7.64 %) with a maximum gain of −22 dBi where λ 0 is guided wavelength. The projected antenna has good performance in the multi-layer tissue model, male human head voxel model, frequency-dependent skin model, and in muscle phantom by maintaining a minimum −20 dB return-loss in the ISM band. The proposed antenna’s Device Integration using a frequency-dependent skin phantom was performed and the results are well-suited for biomedical applications in the ISM band. The bare antenna was used to study the Specific Absorption Rate (SAR) in the human scalp. Where for 1 g and 10 g tissues the investigated results are 626.05 W/kg and 84.4 W/kg for an input power of 1 W, however the input power is limited to maximum 2.55 mW (for 1 g tissue) and 23.56 mW (for 10 g tissue) to comply with IEEE SAR guidelines. The qualities of the manufactured antenna are verified in a skin-imitating gel and animal tissue that exhibits good agreement with the simulation outcomes.

Compact size antenna for skin implantable medical devices

Abstract This research work presents an implantable antenna that operates at 5.8 GHz. By using a radiator with a loop-based design, the antenna can be made smaller. Radiator is made up of three connected rectangular loops. On the substrate’s back side, an I-shaped ground plane is used. As substrate and superstrate, polydimethylsiloxane (PDMS) with dimensions of 7 mm × 5 mm × 0.3 mm is used. The conducting sections are made using copper foil that is 30 µm thick. The suggested antenna is examined by the implantable medical device using realistic human scalp phantom models and a homogenous skin box. Simulated study revealed that it operates around 5.8 GHz with a bandwidth from 5.69 to 5.92 GHz. The specific absorption rate was 0.28 and 0.26 W/kg for skin box and human scalp phantoms, respectively, at 1 mW input power across 1 g volume tissue.

A Review of Recent Innovations in Remote Health Monitoring.

The development of remote health monitoring systems has focused on enhancing healthcare services' efficiency and quality, particularly in chronic disease management and elderly care. These systems employ a range of sensors and wearable devices to track patients' health status and offer real-time feedback to healthcare providers. This facilitates prompt interventions and reduces hospitalization rates. The aim of this study is to explore the latest developments in the realm of remote health monitoring systems. In this paper, we explore a wide range of domains, spanning antenna designs, small implantable antennas, on-body wearable solutions, and adaptable detection and imaging systems. Our research also delves into the methodological approaches used in monitoring systems, including the analysis of channel characteristics, advancements in wireless capsule endoscopy, and insightful investigations into sensing and imaging techniques. These advancements hold the potential to improve the accuracy and efficiency of monitoring, ultimately contributing to enhanced health outcomes for patients.

Designing a Compact Dual-Band, Dual-Polarized Antenna for Biotelemetry Communications Using the Characteristic Mode Method.

A compact dual-band, dual-polarized, implantable antenna was designed using the characteristic mode method for operation in the wireless medical telemetry service (WMTS) band (1.4 GHz) and the industrial, scientific, and medical (ISM) band (2.45 GHz). By utilizing slotting techniques and materials with high dielectric constants, the antenna volume was minimized to 47.4 mm3, and circular polarization was gained in the WMTS band by using the characteristic mode method. A three-layer physical model of human tissues was constructed in a simulation, and a biogel and minced pork were used in the measurements. The measured peak gains are -20.85 dBi (1.4 GHz) and -22.15 dBi (2.45 GHz). The measured effective axis ratio bandwidth in the WMTS band is 170 MHz (1.33-1.50 GHz, 12.0%), and the impedance bandwidth in the ISM band is 390 MHz (2.21-2.60 GHz, 16.2%). At 1.4 GHz and 2.45 GHz, the largest 1 g average SAR values are 376 W/Kg and 318 W/Kg, which comply with IEEE C95.1-1999. Moreover, when the communication chain affinity exceeds 20 dB for 1.4 GHz and 2.4 GHz, the transceiver range reaches 8.2 m and 9.7 m. This antenna can be used for implantable wireless telemetry systems.

Metamaterial integrated highly efficient wireless power transfer system for implantable medical devices

In this article, a novel and highly efficient metamaterial (MTM) integrated approach is presented for the development of a wireless power transfer system (WPT) specifically tailored for implantable medical devices. The system is designed by using a biocompatible implantable ring slot antenna constructed on a flexible dielectric, functioning as a receiving (Rx) antenna. Additionally, a conventional patch antenna is modelled for the use in free space, serving as an external transmitting (Tx) element. By exploiting the unique characteristics of MTMs, the power transfer efficiency (PTE) of the introduced system is effectively improved. To ensure the system’s performance stability in real-time application scenario, the effects of misalignment (lateral and angular) and the flexibility of the implantable antenna are meticulously performed. The prototypes of the Tx, Rx, and MTM structures are fabricated, and comprehensive measurements are performed in human-equivalent tissue models, such as skin-mimicking gel and minced pork. Both the simulated and measured results established the proposed methodology, showcasing marked improvements in PTE due to the use of MTMs. The efficiency of the pioneering MTM-integrated WPT system increases from 0.19% to 3.07% in skin-mimicking gel and from 0.24% to 3.26% in minced pork, respectively. This study highlights the potential of MTM-integrated WPT systems in advancing the field of implantable medical devices and expanding their practical applicability.

A Triple-Band Broadband Miniature Antenna Design for Implantable Continuous Glucose Monitoring

This paper presents a miniaturized triband implantable planar antenna that covers the industrial, scientific, and medical (ISM) band and the Wireless Medical Telemetry Service (WMTS) band which can be used in a continuous blood glucose monitoring device. The antenna consists of a slotted radiating patch and a ground plane with a cross slot to miniaturize its size. The overall antenna size is only 51.35 mm3 (7.6 × 7.6 × 0.889 mm3) due to the reduced size by the slotting technique. Compared to other implantable antennas with similar performance, the proposed antenna is smaller in size, simple in construction, and easy to be fabricated. By cutting a cross slot in GND, impedance matching is improved, especially at the two higher frequency bands. We have fabricated the prototype and carried out measurements. The results show its gains are −32.2 dBi, −18.8 dBi, and −19.1 dBi at 0.9 GHz, 1.4 GHz, and 2.45 GHz, respectively. Its radiation patterns have good symmetry in the two lower operating frequency bands. In addition, the maximum SAR values comply with the IEEE standard safety guidelines (IEEE C95.1-2019).

A Miniaturized Tri-Band Implantable Antenna for ISM/WMTS/Lower UWB/Wi-Fi Frequencies.

This study aims to design a compact antenna structure suitable for implantable devices, with a broad frequency range covering various bands such as the Industrial Scientific and Medical band (868-868.6 MHz, 902-928 MHz, 5.725-5.875 GHz), the Wireless Medical Telemetry Service (WMTS) band, a subset of the unlicensed 3.5-4.5 GHz ultra-wideband (UWB) that is free of interference, and various Wi-Fi spectra (3.6 GHz, 4.9 GHz, 5 GHz, 5.9 GHz, 6 GHz). The antenna supports both low and high frequencies for efficient data transfer and is compatible with various communication technologies. The antenna features an asynchronous-meandered radiator, a parasitic patch, and an open-ended square ring-shaped ground plane. The antenna is deployed deep inside the muscle layer of a rectangular phantom below the skin and fat layer at a depth of 7 mm for numerical simulation. Furthermore, the antenna is deployed in a cylindrical phantom and bent to check the suitability for different organs. A prototype of the antenna is created, and its reflection coefficient and radiation patterns are measured in fresh pork tissue. The proposed antenna is considered a suitable candidate for implantable technology compared to other designs reported in the literature. It can be observed that the proposed antenna in this study has the smallest volume (75 mm3) and widest bandwidth (181.8% for 0.86 GHz, 9.58% for 1.43 GHz, and 285.7% for the UWB subset and Wi-Fi). It also has the highest gain (-26 dBi for ISM, -14 dBi for WMTS, and -14.2 dBi for UWB subset and Wi-Fi) compared to other antennas in the literature. In addition, the SAR values for the proposed antenna are well below the safety limits prescribed by IEEE Std C95.1-1999, with SAR values of 0.409 W/Kg for 0.8 GHz, 0.534 W/Kg for 1.43 GHz, 0.529 W/Kg for 3.5 GHz, and 0.665 W/Kg for 5.5 GHz when the applied input power is 10 mW. Overall, the proposed antenna in this study demonstrates superior performance compared to existing tri-band implantable antennas in terms of size, bandwidth, gain, and SAR values.

Miniaturized dual wideband MIMO antenna for implantable biomedical applications

AbstractA novel miniaturized multiple input multiple output (MIMO)‐based implantable antenna is designed to operate with dual bands, which are wide enough to cover three industrial, scientific, and medical (ISM) bands of 402–405 MHz, 902–928 MHz, and 2.4–2.48 GHz. The proposed antenna has an overall dimension of π × × 0.635 mm3 and uses an RT3010 substrate of thickness 0.635 mm. Each element of MIMO has a spiral structure made of meandered line to realize resonance at 915 MHz, while an arc is etched in each element for obtaining a band of 403 MHz. Semicircular slots are etched in each quadrant of the ground plane for better impedance matching at the higher band of 2.4 GHz. In a −10 dB fractional bandwidth of 95%, 45.36% is attained for the realized bands. Simulation in a skin phantom provides better impedance matching and mutual coupling of less than −15 dB in the three operating bands.

Triple‐band circularly polarized conformal antenna for vehicular communication applications

AbstractVehicular communication presents a compact wideband implantable antenna for the best wireless transmission of information. The key role of the antenna is handled by the thickness of the substrate present in that antenna. Also, the shape of the antenna causes the performance characteristics. Now, vehicle‐to‐vehicle communication is the interesting task for designers to create successful and organized vehicular communication systems. This article presents a triple‐band circularly polarized conformal antenna for vehicular communication. An antenna is introduced as a triple‐band circular polarized conformal antenna and preserved a suitable mount on the vehicle surface for greater gain to provide well‐maintained operational performance characteristics. For getting circular polarization, circular cuttings are provided in the design of the proposed antenna. The radiation gives a fixed direction by providing 0.3 mm thickness. Here, use a circularly polarized antenna to rectify the signal absorption due to the contact material used in the antenna. A polyamide material is used as a substrate to get a probable successful link and improve the flexibility of the antenna. The triple‐band circularly polarized antenna is designed to operate in vehicular communication bands such as 2.4 GHz Wi‐Fi, 3.5 GHz WiMAX, and 5.9 GHz dedicated short range communication. The antenna is designed to resonate in a triple band to improve the antenna parameters like frequency response characteristics, gain, return loss, voltage standing wave ratio, left hand circularly polarization, right hand circularly polarization, axial ratio, fidelity factor, directivity, radiated power, and input impedance. Under the operating frequency bands of 2.4, 3.4, and 5.9 GHz, the efficiency of the proposed antenna is 98.48%, 97.31%, and 97.90%, respectively. The triple‐band circularly polarized conformal antenna is implemented in the high‐frequency structure simulator platform. The proposed tri‐band antenna is applicable in vehicular communication systems and healthcare for efficient wireless transmission.

Miniaturized meander‐line dual‐band implantable antenna for biotelemetry applications

AbstractA dual‐band bio‐implantable compact antenna with a meander‐line structure is presented. The proposed meander‐line antenna resonates at the industrial, scientific, and medical (2.4 GHz) and wireless medical telemetry (1.4 GHz) bands. The meander‐line structure is selected as a radiating patch given its versatile and effective design. With a dimension of only , the designed antenna is compact. Considering a skin phantom, the proposed antenna was designed, optimized, and simulated. The Rogers RT/duroid 6010 substrate material with high dielectric constant was used to fabricate the meander‐line dual‐band implantable antenna, which was validated experimentally. The superstrate was made of the same material. Experiments were conducted on skin‐mimicking gel. The designed meander‐line antenna has a high peak gain of −21 dBi at 2.4 GHz, and its maximum specific absorption rate is compliant with IEEE safety standards.

A Compact Dual-Band Implantable Antenna for Wireless Biotelemetry in Arteriovenous Grafts

Arteriovenous grafts (AVGs) are indispensable life-saving implants for chronic kidney disease (CKD) patients undergoing hemodialysis. However, AVGs will often fail due to postoperative complications such as cellular accumulation termed restenosis, blood clots, and infections, which are dominant causes of morbidity and mortality. A new generation of hemodialysis implants equipped with biosensors and multi-band antennas for wireless power and telemetry systems that can detect specific pathological parameters and report AVGs’ patency would be transformative for CKD. Our study proposes a compact dual-band implantable antenna for hemodialysis monitoring applications. It operates in 1.4 GHz and 2.45 GHz for wireless power transfer and biotelemetry purposes. The miniaturized antenna with a current size of 5 × 5 × 0.635 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> exhibits wide bandwidth (300 MHz at 1.4 GHz band and 380 MHz at 2.45 GHz band), along with good impedance matching at two resonance frequencies. In addition, simulations are performed separately in a three-layer homogenous phantom and a realistic human body model. Measurements of the proposed antenna are evaluated in minced pork. The measured results of the fabricated antenna prototype are closely harmonized with the simulation ones, and the effect of different proportions of fat tissue in pork mince was analyzed, to verify the sensitivity of the antenna to the contacting medium. The specific absorption rate (SAR) and link budget calculation are also analyzed. Finally, the wireless biotelemetry function of the proposed antenna is realized and visualized by adopting a pair of nRF24L01 wireless transceivers, and sustainable and stable wireless data transmission characteristics are shown at a high data rate of 2 Mbps with up to 20 cm transmission distance.

Compact and Broadband Circularly Polarized Implantable Antenna for Wireless Implantable Medical Devices

This letter presents a compact and broadband circularly polarized (CP) implantable antenna for wireless implantable medical devices (WIMDs) in the 2.4 GHz Industrial, Scientific, and Medical (ISM) band. The proposed antenna consists of a main radiator, a stacked parasitic structure, and a full ground, with its entire dimension of 9.8 × 9.8 × 0.889 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . For size reduction, a modified square ring with a mitered cross inside is adopted as the main radiator. By splitting one arm of the mitered cross, the compactness can be further improved while exciting the potential CP radiation. The stacked parasitic structure not only ensures a satisfactory 3-dB axial ratio bandwidth of 28.7%, but also helps to achieve a -10-dB impedance bandwidth of 30%, with a peak realized gain of -24.7 dBi. The antenna was integrated into two types of dummy WIMDs and analyzed in the human phantoms (scalp and heart) to evaluate its device-level performance and robustness. A fabricated prototype was measured in the minced pork, and the measured results agree with the simulated ones.

A Compact Modified Two-Arm Rectangular Spiral Implantable Antenna Design for ISM Band Biosensing Applications

This paper presents a new microstrip implantable antenna (MIA) design based on the two-arm rectangular spiral (TARS) element for ISM band (Industrial, Scientific, and Medical 2.4–2.48 GHz) biotelemetric sensing applications. In the antenna design, the radiating element consists of a two-arm rectangular spiral placed on a ground-supported dielectric layer with a permittivity of ϵr = 10.2 and a metallic line surrounding this spiral. Considering the practical implementation, in the proposed TARS-MIA, a superstrate of the same material is used to prevent contact between the tissue and the metallic radiator element. The TARS-MIA has a compact size of 10 × 10 × 2.56 mm3 and is excited by a 50 Ω coaxial feed line. The impedance bandwidth of the TARS-MIA is from 2.39 to 2.51 GHz considering a 50 Ω system, and has a directional radiation pattern with directivity of 3.18 dBi. Numerical analysis of the proposed microstrip antenna design is carried out in an environment with dielectric properties of rat skin (Cole–Cole model ϵf (ω), ρ = 1050 kg/m3) via CST Microwave Studio. The proposed TARS-MIA is fabricated using Rogers 3210 laminate with dielectric permittivity of ϵr = 10.2. The in vitro input reflection coefficient measurements are realized in a rat skin-mimicking liquid reported in the literature. It is observed that the in vitro measurement and simulation results are compatible, except for some inconsistencies due to manufacturing and material tolerances. The novelty of this paper is that the proposed antenna has a unique two-armed square spiral geometry along with a compact size. Moreover, an important contribution of the paper is the consideration of the radiation performance of the proposed antenna design in a realistic homogeneous 3D rat model. Ultimately, the proposed TARS-MIA may be a good alternative for ISM-band biosensing operations with its miniature size and acceptable radiation performance compared to its counterparts.

A Lamb wave magnetoelectric antenna design for implantable devices

A 400 MHz magnetoelectric (ME) Lamb wave antenna design to function in the medical implant communication service band is proposed. The antenna employs a heterostructure of piezoelectric and magnetostrictive membranes to acoustically excite standing shear bulk wave and radiate as a magnetic dipole. Multiphysics finite element analysis simulations are performed for transmission and reception modes. In these simulations, three aspects are investigated: piezoelectricity, micromagnetic precession, and magnetic dipole radiation. An experimental demonstration of the antenna is also conducted and shows mechanical resonance with a Q-factor of 500 and ME coupling. These results indicate that the design can be operated in zero-order antisymmetric (A0) mode as a tunable oscillator or sensor. This ME approach provides a solution to the miniaturization problem of traditional current-based implantable antennas.

A Compact-Size Multiple-Band Planar Inverted L-C Implantable Antenna Used for Biomedical Applications.

In this paper, a compact-size multiple-band planar inverted L-C implantable antenna is proposed. The compact antenna has a size of 20 mm × 12 mm × 2.2 mm and consists of planar inverted C-shaped and L-shaped radiating patches. The designed antenna is employed on the RO3010 substrate (εr = 10.2, tanδ = 0.0023, and thickness = 2 mm). An alumina layer with a thickness of 0.177 mm (εr = 9.4 and tanδ = 0.006) is used as the superstrate. The designed antenna operates at triple-frequency bands with a return loss of -46 dB at 402.5 MHz, -33.55 dB at 2.45 GHz, and -41.4 dB at 2.95 GHz, and provides a size reduction of 51% compared with the conventional dual-band planar inverted F-L implant antenna designed in our previous study. In addition, the SAR values are within the safety limits with a maximum allowable input power (8.43 mW (1 g) and 47.5 mW (10 g) at 402.5 MHz; 12.85 mW (1 g) and 47.8 mW (10 g) at 2.45 GHz; and 11 mW (1 g) and 50.5 mW (10 g) at 2.95 GHz). The proposed antenna operates at low power levels and supports an energy-efficient solution. The simulated gain values are -29.7 dB, -3.1 dB, and -7.3 dB, respectively. The suggested antenna is fabricated and the return loss is measured. Our findings are then compared with the simulated results.

Single-fed broadband CPW-fed circularly polarized implantable antenna for sensing medical applications.

Biomedical telemetry relies heavily on implantable antennas. Due to this, we have designed and tested a compact, a circularly polarized, a low-profile biomedical implantable antenna that operate in the 2.45 GHz ISM band. In order to keep the antenna compact, modified co-planar waveguide (CPW) technology is used. Slotted rectangular patch with one 45-degree angle slot and truncated little patch on the left end of the ground plane generate a frequency-range antenna with circular polarization. Using a 0.25-millimeter-thick Roger Duroid-RT5880 substrate with a thickness of εr = 2.2, tanδ = 0.0009 provides flexibility. The volume of the antenna is 21 mm x 13.5 mm x 0.254 mm (0.25λg × 0.16λg × 0.003λg). The antenna covers 2.35-2.55 GHz (200 MHz) in free space and 1.63-1.17 GHz (1.17 GHz) in epidermal tissue. With skin tissue that has more bandwidth, the (x and y)-axis bends of the antenna are also simulated via the simulation. Bended antenna simulations and measurements show excellent agreement. At 2.45 GHz, the skin-like gel had -10dB impedance and 3dB axial ratio (AR) bandwidths of 47.7 and 53.8%, respectively. The ultimate result is that the SAR values are 0.78 W/kg in skin over 1 g of bulk tissue, as determined by simulations. The suggested SAR values are lower than the FCC's maximum allowable limit (FCC). This antenna is small enough to be implanted in the body, making it perfect for biomedical applications.

Miniaturized Four-Port MIMO Implantable Antenna for High-Data-Rate Wireless-Capsule-Endoscopy Applications

This study proposes a miniaturized four-port multiple-input multiple-output (MIMO) implantable antenna for wireless capsule endoscopy (WCE). The proposed MIMO implantable antenna operates in the 2.45 GHz industrial, scientific, and medical (ISM) band, has a compact size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.2\,\, \times 5.2\,\, \times 0.127~\text {mm}^{3}$ </tex-math></inline-formula> , and comprises four meandered resonators sharing a common slotless ground plane with an acceptable bandwidth (14.2%) and low mutual coupling. These features help the antenna achieve high data rates and countering multipath distortions, as confirmed using MIMO channel parameters. The calculated link budget analysis of the proposed antenna suggests that seamless high-rate (78-Mbps) information transmission can be efficiently achieved over a distance of up to 10 m (when placed in the large intestine). The proposed MIMO implantable antenna was optimized by placing it inside a capsule comprising sensors, batteries, and electronic components and was subsequently fabricated. The simulated ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula> -parameters, gain, and radiation patterns) and measured (in the minced pork) results were in close agreement with a measured bandwidth of 350 MHz and a peak measured gain of −20.1 dBi. To ensure patient safety, the specific absorption rate (SAR) was evaluated for all the four ports in different environments (stomach and small and large intestines), and satisfactory results were obtained. MIMO performance parameters, such as the envelope correlation coefficient (ECC), diversity gain (DG), and channel capacity (CC), were also evaluated and acceptable values were noted. To the best of our knowledge, this is the most compact four-element implantable MIMO antenna designed so far for biomedical applications.