Dual-Resonant Implantable Circular Patch Antenna for Biotelemetry Communication

A compact broadband implantable circular patch antenna is designed and experimentally demonstrated for Medical Implant Communications Service (MICS) band (402–405MHz). Compared with other similar implantable antennas, the proposed antenna incorporates three advantages for biotelemetry communication. First, it can realize a broad impedance bandwidth by exhibiting dual resonances. Second, it can obtain a compact structure by introducing two arc-shaped slots, a rectangular slot and a circular slot on metal radiating patch. Finally, it can display a friendly shape by using a circular structure.The proposed antenna occupies a volume of about 431.5mm (10.4 × 1.27πmm), which is a compromise between miniaturization and bandwidth. The measured −10 dB impedance bandwidth is 55MHz (385–440MHz). Furthermore, the radiation performance and human body safety consideration of the antenna are examined and characterized.


Introduction
Recently, there is a growing scientific interest in implantable biotelemetry communication for monitoring physiological parameters and treating human diseases.An implantable antenna is a critical component as physiological signals are transmitted wirelessly between implantable medical devices and exterior equipment [1][2][3][4].MICS band and ISM (industrial, scientific, and medical) band (2400-2480 MHz) are two common frequency bands for biotelemetry communication.In this study, we chose the MICS band to design an implantable antenna.
Designing an implantable antenna inside human body faces many challenges, such as miniaturization, bandwidth, biocompatibility, patient safety, and good radiation performance.Nowadays, printed planar inverted-F antennas (PIFAs) have been used to design implantable antennas by many researchers due to their own advantages such as small size and omnidirectional radiation pattern [5][6][7][8][9][10].Miniaturization is a key requirement owing to some strict constraints on implantable antenna size.In [5], a compact spiral microstrip implantable antenna was proposed.In [6], miniaturization of the implantable antenna was realized by etching a meandered slot on the top metal patch with one end open.In [7], a miniaturized implantable antenna was designed by using meandering and shorting strategy.On the other hand, the implantable antenna needs a wide impedance bandwidth to avoid frequency shift due to intersubject variations of the electrical property of human body tissues and inaccuracies of fabrication and test.To broaden the antenna impedance bandwidth, some methods have been used.A pi-shape with double L-strips PIFA was proposed for implantable biotelemetry in [8], but this antenna had a relatively large volume.An implantable broadband low SAR antenna was proposed by combining a sigma-shaped monopole radiator and a novel C-shaped, coupled ground [9].Reference [10] proposed a conformal broadband dipole antenna, whose bandwidth can reach 36.1%.As shown above, the aforementioned antennas are all of rectangular shape, and they have sharp angles and edges.Compared with the rectangular antenna, a circular antenna has more friendly shape [11], and it can avoid injury to human tissues when it is directly exposed to the human body [12].In [11], a miniaturized stacked circular implanted PIFA was designed 2 International Journal of Antennas and Propagation by cutting some linear slots on two-layer radiating patches.However, the stacked PIFA usually has a high profile and a complex structure.In this paper, a novel circular antenna with a single-layer radiating patch is proposed for biotelemetry communication.By properly embedding two arc-shaped slots, a rectangular slot and a circular slot on a circular radiating patch, the designed antenna can realize a compact, friendly, and dualresonant broadband structure.The detailed characteristics and experimental results of this antenna are presented and discussed.

Antenna Design
Figure 1 shows the configuration of the presented implantable antenna, which consists of two layers of Rogers 6010 substrate.Dielectric constant and loss tangent of the substrate are, respectively, 10.2 and 0.0023, and the thickness of each layer is 0.635 mm.The superstrate layer is usually used to achieve biocompatibility and robustness.Two arc-shaped slots, a rectangular slot and a circular slot, are cut on the circular radiating patch to realize dual resonances and extend the effective length of the current path.In order to form a PIFA structure, one can make the designed antenna resonate at a relatively low frequency, with the first short via being located at one end of the inner arc-shaped strip.Furthermore, we place the second short via at one end of the outer arc-shaped strip to construct a shorted split ring, which is electromagnetically coupling with the inner PIFA structure.Thus, the proposed antenna is considered to be a combination of the inner PIFA structure and the outer shorted split ring.Figure 2 shows the one-layer skin simulation model, and the proposed antenna is placed in the skin model, with a distance of 3 mm from the upper surface of the model and a distance of 40 mm from the other surfaces of the model.This skin model used in this work is based on [13], and the electrical properties of the skin at 402 MHz are   = 46.7 and  = 0.69 S/m.The antenna was fed by a 50-Ohm coaxial cable.The optimum dimensions of the designed antenna in Figure 1 were obtained by using electromagnetic simulation software HFSS and were listed in Table 1.In this work, broad impedance bandwidth is achieved by the combination of two close resonant frequencies at 387 MHz and 413 MHz.To better understand the operation principle of this antenna, the current distributions of the metal radiating layer at two resonant frequencies are shown in Figure 3.At 387 MHz, current flows along the same direction from the outer split ring to the inner PIFA structure, which indicates that all metal strips contribute to the low resonant frequency.At 413 MHz, the current path of the outer split ring is opposite to the inner PIFA structure, which indicates that either part of them resonates at this frequency.Thus, the antenna can obtain a broad impedance bandwidth by combining dual resonances.
In order to further explain the operation mechanism of the antenna, we considered a special case.As shown in Figure 4, we removed the outer shorted split ring and kept the inner PIFA structure of the radiating metal layer, with the same substrate and superstrate of the original one in Figure 1 and the same skin model in Figure 2 being used for comparison.As shown in Figure 5, the resonant frequency of this case is close to 413 MHz.Therefore, we can conclude that the inner PIFA structure contributes to the resonant frequency of 413 MHz.Furthermore, compared with the original antenna, the inner PIFA structure can greatly reduce antenna's size by appropriately reducing the substrate's size, but it would exhibit a narrower bandwidth.Therefore, the proposed antenna size is a compromise between miniaturization and bandwidth.
For analyzing the effect of the antenna parameters on return loss, the antenna with different short positions  1 was simulated and compared in Figure 6.We can conclude from Figure 6 that the smaller the angle  1 , the lower the low resonant frequency about 387 MHz.However, the short position  1 has little effect on the resonant frequency of 413 MHz by the simulation.The similar results could be obtained by changing the length of the outer split ring, and the longer the split ring is, the lower the low resonant frequency can be gained.Therefore, we can also conclude that the outer shorted split ring structure contributes to the low resonant frequency of 387 MHz.

Measurement and Discussion
The fabricated antenna including its superstrate is shown in Figure 7.As shown in Figure 8, we used the chopped pork phantom with the dimension of 100 × 100 × 50 mm 3 as measurement setup, which is similar to the simulation    model in Figure 2. According to [14], permittivity and conductivity of the chopped pork are similar with human skin in MICS band. Figure 9 shows a comparison of measured and simulated return loss of the proposed antenna.As can be seen from the graph, the measured return loss agrees well with the simulation result.The measured result indicates that broad  Figure 10 shows the simulated far-field gain radiation pattern of the presented antenna at 402 MHz.The pattern is almost omnidirectional, and its peak gain is −32.9 dBi.
In addition, we evaluated the 1 g averaged specific absorption rate (SAR) for the human body safety concern [15].Provided that 1 W power is delivered to the presented antenna, the maximum SAR value of 319.8 W/kg at the 402 MHz can be obtained.Therefore, the allowed transmitter power should be decreased to 5 mw for satisfying the 1 g SAR regulation.
The size, bandwidth, and peak gain of the proposed antenna are compared with the previous implantable planar antennas with the sing-layer metal patch and the full ground plane at MICS band as shown in Table 2.

Conclusions
By embedding two arc-shaped slots, a rectangular slot and a circular slot on a circular radiating metal patch, a novel dual-resonant implantable antenna at MICS band has been presented.Compared with the previous similar implantable antennas, the proposed antenna incorporates three advantages: broad bandwidth, compact structure, and  friendly shape.Good agreement is obtained between the simulation and measurement for the return loss.Evaluation of the radiation performance and the safety consideration shows that the proposed antenna is suitable for biotelemetry communication.

Figure 1 :
Figure 1: The geometry of the proposed implantable antenna.

Figure 2 :
Figure 2: Side view of simulation model.

Figure 4 :Figure 5 :
Figure 4: Geometry of the proposed antenna without shorted split ring.

FrequencyFigure 6 :
Figure 6: Comparison of | 11 | of the proposed antenna with different short positions of the split ring.

Figure 7 :
Figure 7: Photograph of the fabricated implantable antenna.

Figure 8 :
Figure 8: Photograph of the measurement setup.

Figure 9 :Figure 10 :
Figure 9: Comparison of the simulated and measured | 11 | of the proposed antenna.

Table 2 :
Characteristic comparisons of the similar implantable antennas.