An Ultra-Miniaturized Circular Polarized Implantable Antenna With Gain Enhancement by using DGS and Holey Superstrate for Biomedical Applications

In this paper, a new approach is presented for achieving circular polarization (CP) characteristics and gain enhancement of an ultra-miniaturized antenna for biomedical applications. The proposed antenna operates in the frequency of the industrial, scientific, and medical (ISM) bands of 2.4 GHz. The integration of the defected ground structure (DGS), and the Holey supertrate produces a significant gain improvement with the CP characteristic at the desired frequency. As a result, the proposed antenna does not only have an ultra-compact dimension of 2.5 mm × 2.5 mm × 1.28 mm (8 mm3), but also has CP characteristic, high gain value, and an acceptable radiation efficiency of 0.25 %. The performance of the proposed antenna is tested via numerical and experimental measurement. The designed antenna is fabricated on a low loss, flexible, and biocompatible PCB material, Taconic CER-10 (εr = 10.2, σ = 0.0 035). The measurement results assembles a good impedance matching at 2.4 GHz with the bandwidth of 33 % and a maximum peak gain of -14.3 dBi. Moreover, the antenna shows a low specific absorption rate (SAR) with the value compliance the IEEE standard safety guidelines. To the best of our knowledge, the proposed CP antenna is the compactest size with high performance and great gain enhancement (approximate 3.2 dBi) compared to previously reported works. Finally, the proposed antenna with the approach and its successful integration is a potential candidate and suitable for biomedical implant applications.


I. INTRODUCTION
R ECENTLY implantable medical devices (IMDs) have many applications in everyday human life for healthcare monitoring such as implantable glucose, pressure sensor, capsule pills, pacemakers, defibrillators, retinal implants, etc [1] - [6]. The implantable devices include many contain components consisting of antenna, sensors, and battery. The antenna is the major block of the wireless linked IDMs, therefore, the design of the implantable antenna is not the same with the fundamental antennas which operate in free space [7] - [10]. The implantable antennas need to satisfy many conditions of biocompatibility, tissue coupling, communication ability, bandwidth, miniaturization [11], specific absorption rate (SAR) for the patient safety [12], [13], sufficient radiation efficiency [14]. Besides, the operating frequencies must be taken into consideration such as Medical Device Radio Communication Service (MedRadio,401-406 MHz), Industrial Scientific Medical (ISM)(433-438 MHz; 886-906 MHz; 2.4-2.48 GHz; 5.725-5.875 GHz) and Wireless Medical Telemetry Service (WMTS). In addition, the implantable antennas are placed inside the human body and communicate with external devices, it should maintain fixed orientation towards the external devices, compete the posture movements and common human activities. The circular polarization of the implantable antenna is crucial and necessary. With the CP characteristic, the implantable antenna offers better mobility reduction of multipath as well as improves the lower biterror rate and link stability for high quality communication [15]- [17]. Owning to the advances of the CP characteristic, the design of implantable antennas has been gaining huge interest in the field of circular polarization.
Various techniques are proposed for the CP characteristic achievement of the implantable antennas. Liu et.al designed a capacitively loaded CP antenna with a -3 dB axial ratio (AR) bandwidth about 1.6 % with significant large size and narrow bandwidth [18]. In [19] a loop antenna with a reactive impedance substrate (RIS) for an AR bandwidth of about 7.8 %. A CP implantable antenna with 2.49 % AR bandwidth for 2.45 GHz is designed by adding a pair of open stubs in the inner boundary of the annular ring [20]. By employing four C-shaped slots and a complementary CSRR, a CP implantable antenna operating at 2.4 GHz with miniaturized is proposed [21]. The CP characteristic of antenna can be obtained by the techniques proposed by Zhan Xia [22], L.J Xu [23], Z.J Yang [24]. Besides, miniaturization and antenna gain value are some of the most important aspects of implantable devices, and closely related to each other. The miniaturization degrades the gain of the antenna and vice versa. Many techniques are presented for miniaturization and improve the gain of the implantable antennas. In [25], an implantable antenna with a compact dimension of 3 mm × 4 mm × 0.5 mm 3 is presented. The antenna exhibits satisfactory size value but insufficient gain value of -25.9 dBi. In our previous work, a 1.2 Ω resistor is used to minimize the meander line antenna structure for a compact volume of 2.5 mm × 2.5 mm × 1.28 mm [26]. Otherwise, a high gain of -9 dBi is achieved by the inclusion of metamaterial structure, which is proposed by Das and Mitra [27]. However, in their work, the antenna has a relatively large size of 10 mm × 10 mm × 0.4 mm 3 . Other researches on implantable antennas have been presented in [28] - [30]. In general, the aforementioned researches have developments and achieved good results, however, each report has the different drawbacks, either not obtaining circular polarization, low gain value, or high profile.
In this paper, an ultra-miniaturized implantable antenna with enhanced gain and CP characteristics in the frequency band of 2.4 GHz (industrial, scientific, and medical band (ISM)) is investigated for biomedical applications. In order to do this, the source antenna with the meander line structure embedded with a resistor is utilized. The CP characteristic and bandwidth enhancement are achieved by etching defected ground structure (DGS) which has the format of a metamaterial unit cell. More importantly, a Holey superstrate with different radius of drilling holes is employed to increase the antenna gain value. To best of our knowledge, the proposed implantable antenna with CP has the smallest size of only 8 mm 3 (2.5 mm × 2.5 mm × 1.28 mm) and highest enhanced gain value (3.2 dBi) compared to the most relevant researches stated in the literature. A detailed of the comparison with recently published reports about the implantable antennas is presented in Table. 1. It is evident that regardless of its size, bandwidth, CP characteristic, and gain the proposed implantable antenna has a better performance compared to those studied. Moreover, to validate the numerical simulation results, the designed antenna is fabricated and tested with minced pork to imitate the human tissue model. In addition, for the safety concern, the SAR is investigated

II. METHODOLOGY
This section describes the geometry of the proposed antenna and outlines the steps of the procedure applied to the designed antenna. The concept of the antenna to achieve CP characteristic, wide bandwidth, and gain enhancement are presented here.

A. ANTENNA STRUCTURE AND DESIGN 1) Phantom model
For the first step, the simulations are conducted inside a single layer of homogeneous skin phantom (HSP) with the dimension of 25 mm × 25 mm × 25 mm, and the human skin properties at 2.4 GHz of the dielectric permittivity ε r = 38.1 and the conductivity σ = 1.44 (S/m). The HSP is surrounded by a radiation boundary box with dimensions of 100 mm × 100 mm × 100 mm. The implantable antenna is placed at the middle of the HSP, as shown in Fig. 1. Moreover, to test the antenna performance in realistic environment, the proposed antenna is implanted in the heterogeneous of head, arm, and leg model.
Compare to reference antenna [26], the radiating patch has a meander line structure with a corner truncated (Rc = 0.29) for improving matching impedance, as shown in Fig. 2(a). The antenna source is reduced size by using of a resistor R. When the resistor R is embedded to the antenna, the capacitance will decrease and the resistor R turns into a component to produce a 1/4 λ antenna (a quarter-wavelength monopole). Besides that, Fig. 2(b) shows the ground plane of the proposed antenna with a DGS element, which supports for bandwidth extending, and CP characteristic achievement. Fig. 2(c) show the superstrate with different size hole, which is responsible for obtaining higher antenna gain value. The Taconic CER-10 having a dielectric constant ε r = 10.2 and VOLUME 4, 2016 loss tangent of σ = 0.0035 is utilized for both substrate and superstrate, respectively. The antenna is excited by using a flexible 50 Ω coaxial feed with a diameter of 0.3 mm and located at the point of fx = 1.05 mm and fy = 1.1 mm. The exploded of the antenna is presented in Fig. 2(d). Finally, all dimension have optimized to achieve the bset performance of the proposed antenna and listed in Table 2.

B. CIRCULAR POLARIZATION REALIZATION
Circular polarization can be achieved by simultaneous excitation of two orthogonally polarized modes of 90 0 phase difference and with a nearly similar amplitude. To realize CP red characteristics, various methods have been proposed, such as, loading shorting pin [22], [31], introducing a pair of perturbation elements [32], [33], employing metamaterial structures [34], [35], etc. Among these, using defected ground structure (DGS) is an effective way to the obtain CP characteristic for implantable antenna [36]. Besides, embedding suitable DGS can cause meandering of the excited surface current paths, which would effectively reduces the resonance frequency and the quality factor of the antenna, and obtain wide impedance [26], contribute for size reduction [37], [38], or improve radiation property [39].
In this study, to obtain latter DGS structure for highest performance and suitability with the source antenna, the DGS is investigated and optimized. By adding the designed DGS with a format as a metamaterial unit cell in the ground plane, then a time-phase shift of 90 0 between two currents is created, circular polarization is realized. A triangle slot on the corner of the ground plane is employed to further optimize the impedance matching and axial ratio bandwidth (AR). To fully illustrate the CP operation mechanism of the proposed antenna, the presentation of the surface current at 2.4 GHz for different phase of 0 0 , 90 0 , 180 0 , 270 0 is depicted in Fig. 3. It can be observed that the direction of the predominant surface current rotates counter-clockwise, indicating the property of right-handed circular polarization (RHCP) radiation. Fig. 4(a) shows the simulated S-parameter of the antenna with and without designed DGS. It is clearly realized that the simulated result provides a bandwidth of reflection coefficient |S 11 | below -10 dB ranger from 2.11 GHz to 2.81 GHz (29.1 % compare to 16.6 % of the antenna without DGS). The axial ratio is shown in Fig. 4(b). As illustrated in the figure, the AR value of the antenna without designed DGS has a high value, however, after embedding the DSG the AR value of the antenna improves and achieves less than 3 dB with a AR bandwidth of 160 MHz (6.6 %).

C. HOLEY SUPERSTRATE DESIGN AND ENHANCEMENT OF GAIN VALUE
As well known, with the regard to bio-compatibility and proof of the miniaturization concept, the material with high permittivity is employed for both substrate and superstrate [40], [41]. The superstrate is utilized to provide insulation, avoid short circuits caused by high permittivity tissues, and stabilizes the effective permittivity fluctuations around the  antenna [42], [43]. However, in this work, we present a superstrate with different radius of drilling holes in it for as well as responses mentioned properties and use to increase the antenna gain value. Work presented in [44] described that antenna gain is increased by creating an in-phase electricfield area on the upper surface of the superstrate. To obtain the in-phase field, the effective permittivity of the superstrate has to control. The effective permittivity can be managed by changing the properties (thickness), or changing structure of the material. Assume a point source as exhibited in Fig. 5(a). The phase of the electric field some distance away from the point source is reliant on the distance In particular, when a dielectric layer, which has a thickness with a different permittivity determined by its position is located above the point source at a distance, the phase difference (∆ξ) between two points can be expressed by the following equation equation: (1) where ε 2 is the permittivity at point 2 and ε 1 is the permittivity at point 1. According to (1) the phase difference ∆ξ can be set to zero by changing ε 2 . The reflection coefficient (Γ)  at the boundary and phase delay of the superstrate are given by: From (2) and (3), the effective permittivity can be derived by: The relationship between effective permittivity and hole size can be available by using equation (4). Therefore if we obtained different effective permittivity by changing the radius and spacing of the drilling holes in the superstrate, the in-phase electric field on the upper surface superstrate can be created, which leads to increase the gain in the desired direction. The proposed superstrate structure with a size of 2.5 mm × 2.5mm × 0.64 mm consist of different holes size of r 1 = 0.52 mm, r 2 = 0.34 mm and r 3 = 0.12 mm, respectively. Fig. 5(b) shows the unit cell simulation of the hole with various radius. Two perfect electric (PEC) and magnetic (PMC) boundary conditions are used to analyze unit cell characteristics. The values of effective permittivity based on the size of hole are calculated and exhibited in Fig. 5(c).
As known, the superstrate is used to provide insulation and avoid short circuits caused by high permittivity tissues. Therefore, the simulations on the issue of the superstrate with holes which may leak body tissues into the holes is considered. Herein, the holes are fully filled with both case of the air and the skin property. However, both simulation results have no significant differences. The comparison of refection coefficients of antenna with and without Holeys supstrate is depicted in Fig. 6(a). In addition, Fig. 6(b) shows the simulated gain versus frequency of the antenna with and without using the proposed superstrate. It is observed that, the gain of antenna without the proposed superstrate structure is around -16.8 dBi, however, -13.7 dBi is obtained in the case of antenna utilizing the desiged superstrate. An approximately 3.2 dBi gain enhancement is observed. From  Fig. 6, it is evident that by loading the proposed superstrate structure, a great enhancement for the antenna gain without deteriorating CP characteristic or enlarging antenna dimension. Besides, other parametric analysis are not investigated in this study, because, these parameters variable depending on the structure of ground the plane and superstrate.

A. ANTENNA FABRICATION
In order to validate the results achieved from the simulation scenarios, the prototypes of the proposed antenna are fabricated, as shown in Fig. 7. The proposed structure of the biomedical implantable antenna is fabricated using PCB (printed circuit board) technology with the material of Taconic CER-10 having a dielectric constant ε r = 10.2 and VOLUME 4, 2016 loss tangent of σ = 0.0035. The Musashi MS-1 solder machine is utilized for soldering the resistor and SMA coaxial cable to the fabricated antenna prototype, then, associate with the superstrate on the top by glue. The characteristics of the prototype are experimentally measured. The reflection coefficient of the antenna is measured in a box filled with minced pork through the Protek A333 vector network analyzer (VNA) with covering frequency of 300 kHz to 3.2 GHz, as depicted in Fig. 8(a). The minced pork is used to emulate human tissue models properties. Moreover, the radiation patterns of the antenna are measured in a microwave anechoic chamber, as shown in Fig. 8(b). The antenna system is placed in far-field on transmitting antenna and the positioned on the position can be rotated freely. For measuring the radiation pattern as a function of angle, the prototype is rotated so that the transmitting antenna illuminates the prototype from different angles. The measurement shift angles are done at 10 deg, 15 deg, and 30 deg, respectively. Fig. 9 depicts the comparison of simulated and measured reflection coefficient of the proposed biomedical implantable antenna. It is clearly realized that the measured result in minced pork provides the measured bandwidth of reflection coefficient |S 11 | below -10 dB is observed from 2.05 GHz to 2.85 GHz (33 %). Fig. 10 illustrates the difference between the simulation and experimental results for the axial ratio of the proposed antenna. The measured 3 dB AR bandwidth of the antenna is observed to be 5.8 %. The simulated AR behavior is closely correlated with the measured scenario. Fig. 11 shows the results of the gain, and radiation efficiency. From the figure, the measured result provides the measurement high gain of -14.3 dBi, and the radiation efficiency of 0.25 % (caused by the implantation depth, the efficiency of the implantable antenna is regularly found less than 1 %). Compare with the simulated result, the measured results in minced pork show a slightly shifts to higher frequency (2.435 GHz) in case of operating frequency, a reduction of 0.7 dBi from -13.6 dBi to -14.3 dBi in case of realized gain. The slight difference results can be caused by the soldering issue, or by the air gap between two layer (which could be avoided only by using highly accurate manufacturing machine [45]), or by the dielectric properties of the measurements [46]. However, the antenna provides an acceptable radiation efficiency, high gain, and wide bandwidth with covered the desired frequency band of 2.4 GHz To clarify the affections of the antenna radiation characteristics implanted in different heterogeneous environments of human body, the simulations of the antenna in different scenario (skin phantom, head, arm, and leg model) are implemented. Fig. 12 demonstrates the far-field patterns in the human tissues and measured ( in minced pork) using the setup in Fig. 8 show more smooth graphs, show continuity in the results, and easy for viewing, some bad points are removed while plotting which come may be due to connections, stacking. Generally, in all cases of the simulations results and measurements exhibit a good agreement and exhibits adequately high gain for the entire bandwidth.

B. RESULTS AND DISCUSSION
To evaluate the safety of the proposed antenna, the investigation on the value of specific absorption rate (SAR) is necessary, because of the radiating electromagnetic energy,   (5).
where σ and ρ denote the electrical conductivity, and the mass density, respectively, E is the electric field intensity. Fig. 13 demonstrates the SAR distribution in the head, leg, and arm model of the human body. For evaluating the SAR value, an input power of 1 W is set. The maximum SAR of 394 W/kg with a maximum acceptable input power of 4.06 mW is realized in the leg model (highly conductive nature environment). The detailed SAR values and corresponding maximum allowable input power at the frequency of 2.4 GHz are given in Table 3. The SAR value is not a concerning issue in this study, however, the analysis SAR values of the proposed implantable antenna meet the IEEE standard safety guideline .

IV. CONCLUSION
This paper addressed the design evolution, the analysis and the experimental measurements of a ultra miniaturized implantable antenna with enhanced gain and obtained CP characteristic biomedical applications at the frequency band of 2.4 GHz. The CP characteristic and bandwidth enhancement are achieved by etching metamaterial unit cell defected ground structure (DGS). Besides, by employing a Holey superstrate with different radius of drilling holes, the antenna gain value is significantly enhanced. Consequently, the proposed implantable antenna with CP has the smallest size of only 8 mm 3 (2.5 mm × 2.5 mm × 1.28 mm), high gain with highest enhanced gain value (