Design of Conformal Spiral Dual-Band Antenna for Wireless Capsule System

This paper presents a conformal spiral antenna that is miniaturized and with dual-resonant for the wireless implantable capsule system. The spiral antenna conforms to a swallowable capsule with a radius of 3 mm and a length of 26 mm without occupying the internal space of the capsule. The compact antenna adopts two spiral arms to extend the effective current path for miniaturization. Biocompatible flexible polyimide was used as the dielectric substrate and capsule shell, achieving conformal properties of the antenna as well as compatibility with human tissue. The antenna has been simulated in different environmental models. The bandwidth of the antenna can reach 39.16 % (1.82 GHz-2.76 GHz) and 12.06 % (5.36 GHz-6.06 GHz) at 2.4 GHz and 5.8 GHz. The maximum gains of −35.2 dBi and −28.1 dBi can be achieved at 2.4 GHz and 5.8 GHz, respectively. In addition, the transmission characteristics of the antenna were experimentally verified in the minced pork and pig intestine. By analyzing the communication link, the communication distance between transceivers at 2.4 GHz and 5.8 GHz can meet 14 m and 5 m. These results show that the proposed antenna is suitable for wireless implantable capsule systems.


I. INTRODUCTION
Nowadays, wireless technology is widely used in implantable medical devices as it gets rid of the body from the limitations of wired devices [1]. In recent years, implantable devices can be implanted into the human body for the auxiliary treatment of various diseases, including capsule endoscopes [2], cardiac pacemakers [3], and intracranial pressure monitoring [4].
A wireless implantable capsule system with dual-band wideband antennas is selected for transmission as capsule speculum and cardiac pacemaker, as showing in Fig. 1. The wireless implantable capsule is promising to be used in the digestive system as a capsule speculum, and the heart as a cardiac pacemaker as long as changing its internal structure. The operating band of the proposed antenna could cover 2.42-2.48 GHz and 5.725-5.850 GHz, both of which belong to the industrial, scientific, and medical (ISM) frequency bands [5].
The associate editor coordinating the review of this manuscript and approving it for publication was Debdeep Sarkar . With the development of medical technology, the demand for the transmission of real-time video is greater than for the transmission of simple pictures [6]. The transmission antenna with a wideband should be designed to meet the transmission needs of the implantable capsule system to be able to transmit video and other big data signals in real-time. In addition, to adapt to the complex implanted environment and improve the robustness of the antenna, it is required that the implanted antenna should be achieved VOLUME 9, 2021 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ a wideband [7]. Due to the requirements for high data transfer rates, the life cycle of the implanted system needs to be as long as possible [8]. To extend the battery life, the antenna can be made with dual-frequency or multi-frequency [9]. Features, so that the system has dual-mode working characteristics. Therefore, designing a dual-frequency and broadband implanted antenna has important research value and significance. In [10], a spiral patch, high-dielectric substrate, and an open-end ground slot were used to achieve the multi-frequency at 402 MHz, 1.6GHz, and 2.4 GHz, and the maximum bandwidth of 219 MHz is obtained. Although this design implements multi-frequency features, the bandwidth is relatively small and takes up some space for implantable devices. In [11], the proposed antenna is designed to operate in 915 MHz and 2.45 GHz with the bandwidth of 107.5 MHz and 560 MHz, by adding an open-ended ground slot, shorting pin, and hexagonal and T-shaped slots in the radiator. Though its bandwidth can be increased, its complex structure increases the dimensions of the device. In studies [12], [13], although dual-band or multi-band characteristics are realized, the problems of narrow bandwidth and large internal space occupied still exist. Due to the limitations of space in implantable devices, how to maintain antenna dual-band and broadband while reducing the space occupied by the antenna is the top priority. Some miniaturization techniques, such as high dielectric constant substrate [14], [15], meandered line [16], spiral line [10], opening slot [17], and adding shorting pins [18], as well as stacked antennas [19], are used to reduce the space. However, these miniaturization technologies usually bring difficulties to the antenna design and production process and still occupy the limited space. Compared with the use of miniaturization, conformal can improve miniaturization performance. The conformal structure can effectively use the surface of the capsule, to avoid competing with the electronic components inside the capsule for the valuable space of capsules [20], [21]. The conformal characteristics are achieved by using flexible materials for bending, such as in studies [22], although the proposed antennas can be conformal with flexible materials, their performance fails to achieve multi-band and wide-band. Based on the above literature considerations, how to design a conformal antenna that has wide dual-band characteristics and reduces the contact area with the internal circuit is a problem worthy of consideration. In this approach, a dual-band spiral antenna is investigated. It is conformed to a wireless implantable capsule system with the size of π× 3 2 × 26 mm 3 . We implanted the antenna in the large intestine model and heart simulation model to verify the stability. To ensure safety, the specific absorption rate (SAR) has been analyzed. The entire system is also conducted in the different aforementioned heterogeneous implanted organs. A lot of experiments have been verified our design. Minced pork and pig intestine are used as the measured materials environment to the simulated human body. A significant result has been obtained.

II. DESIGN AND ANALYSIS OF ANTENNA A. THE STRUCTURE OF THE ANTENNA
The implantable antenna works inside the body; and the size must be small enough, without affecting the surrounding tissues. The conformal structure is one of the best for the antennas which takes up almost no space [23]. The antenna proposed in this work uses a conformal design of the spiral structure and the hemispherical structure of the capsule. The structure of the antenna is mainly made up of two spiral arms. One arm is composed of a rectangular patch surrounding a hemispherical function. Another is obtained by rotating the first arm 180 degrees around the center. The hemispherical parameter function is as follows In the above function, t is a variable, the range of t is 0-n × 2π, n is the number of turns of the spiral, r is the radius of the hemispherical spiral, and S is the pitch. The x t is the position coordinate function in the x-direction of the variable t, the y t is the position coordinate function in the y-direction of the variable t and the z t is the position coordinate function in the z-direction of the variable t. As illustrated in Fig. 2(a), the two spiral arms relate to two strip lines and are distributed on both sides of the annular dielectric substrate under the spiral arms. The dielectric is polyimide with 0.15 mm thickness which has a relative permittivity ε r of 3.5, loss tangent tanδ of 0.008. The antenna is installed inside the top of the capsule with a length of K and a diameter of d at both ends of the sphere. The thickness of the outer layer of the capsule is 0.15 mm thick.

B. THE SIMULATION ENVIRONMENT
The wavelength in the human body is shorter than in the free space due to the antenna is affected by the relatively high permittivity of the human tissue, which works to profitably miniaturize the physical size of the antenna [24]. A simple muscle model, a large intestine model, and a heart model were established to simulate different working environments.
Electromagnetic properties of human tissues in various parts are tabulated in Table 1 [25].

C. PARAMETER STUDY AND DISCUSSION
We implant the antenna in the single muscle simulation model which dimension is π× 50 2 × 100 mm 3 as shown in Fig. 3. According to the effective current distribution at 2.4 GHz and 5.8 GHz of the antenna in Fig. 4, we can see that different parts of the antenna excite the resonance at different frequencies. We analyze some important antenna structural parameters. Firstly, the antenna mainly depends on the spiral arm radiation, so the spiral arm around the number of turns parameter n and the winding line width L of the spiral arm have a great impact on the performance of the antenna.  It is observed that as the turns of the spiral arm changed, the effective path of the current increased, and the whole operating frequency moved with n changing from 0.85 to 1.6, as shown in Fig. 5 (a). The |S 11 | with different L is presented in Fig. 5 (b). It is observed the higher frequency point decrease and bandwidth in the low band increases as the winding line width L increases, which means that the change of L has a more obvious impact on the high frequency. According to the above results, the optimal n is 1.35, and the optimal size L is 0.5 mm.
In addition to the spiral arm, we have studied the influence of the size of the ground plane and the width of the antenna's microstrip. The |S 11 | with different heights of the ground H is shown in Fig. 6 (a). The |S 11 | with different widths of the antenna's microstrip W is shown in Fig. 6(b). The |S 11 | has not changed much when change the H and W , which means that the antenna is stable.
The parameter n affects the bandwidth at 2.4 GHz, and L affects the frequency at 5.8 GHz as shown in Fig. 5.  We studied the influence of the parameter n on the gain at 2.4 GHz and the influence of parameter L on the gain at 5.8 GHz as shown in Fig. 7. The parameter n and L affects both bandwidth and gains at 2.4 GHz and 5.8 GHz.
According to the above results, the optimal size of the antenna is tabulated in Table 2.

D. SIMULATION WITH COMPLEX HUMAN MODEL
After analyzing the parameters of the antenna, the antenna was placed in a more complex simulation environment  to simulate to further verify the stability of the antenna performance. The large intestine model and heart model were established to simulate the working environment. The large intestine model is fat, muscle, and mucosa membrane from the outermost layer to the innermost layer. The thickness of this fat, muscle, and mucosa membrane is 15 mm, 20 mm, and 30 mm, respectively. The simulation model is a cylinder with a height of 100 mm, as shown in Fig. 8. And then, we established the heart model with fat, outer mucosa membrane, muscle, and mucosa membrane from the outermost layer to the innermost layer. The thickness of this fat, second layer mucosal membrane, muscle, and internal mucosa membrane is 10 mm, 5 mm, 10 mm, and 35 mm. The model is a sphere with a radius of 60 mm, as shown in Fig. 9. Then we compared the |S 11 | in a complex and simple environment as shown in Fig. 10. According to the results, the proposed antenna could work stably in different environments due to the stability of the antenna structure and the protective effect of the capsule shell.

III. RADIATION PERFORMANCE EVALUATION A. RADIATION PERFORMANCE AT DIFFERENT ORIENTATIONS
When the antenna enters the human digestive tract, its position and orientation will be changed. We study the radiation pattern in different orientations 0, 45, and 90 degree-directed as shown in Fig. 11. The radiation patterns at 2.4 GHz are shown in Fig. 12

B. THE SAR OF SYSTEM
The safety problem of the implantable antenna should be considered, where the specific absorption rate (SAR) is usually regarded as the evaluation standard for the implantable antenna, that the SAR levels averaged over 1-g of human tissue should be less than 1.6 W/kg [25].   The simulation models are the large intestine and heart simulated models as shown in Fig. 8 and Fig. 9. The maximum averaged SAR in 1-g of the large intestine and the heart at 2.4 GHz and 5.8 GHz are listed in Table 3 when the input power is 1W. According to the results, we can calculate the maximum input power of the antenna in the large intestine simulation model and heart simulated model at 2.4 GHz and 5.8 GHz when the SAR of the antenna is less than 1.6 W/kg. The SAR and the max allowed input power are listed in Table 3.  The comparison with the proposed antennas in the previous work is displayed in Table 4. Compared with other dual-band or multi-band antennas, the proposed antenna achieved wide dual-band and conformality.  Table 2, measured in minced pork and pig intestine. The antenna is composed of a radiating patch wrapped around the capsule shell to verify the performance of the designed antenna structure. Fig. 17 is the simulated and measured |S 11 | of the antenna in minced pork and pig intestine, and the result shows the bandwidth of the antenna could cover 2.42-2.48GHz and  5.725-5.850 GHz in different environments. Fig. 18 compares the simulated radiation patterns in the large intestine model and heart model and measured radiation patterns. Fig. 18(a) and Fig. 18(b) show the comparison of the simulation results and actual measurement of radiation pattern when the antenna at 2.4 GHz and 5.8 GHz. According to the compare results, the antenna has good radiation characteristics in the simulation and measurement environment.

V. LINK BUDGET ANALYSIS
Since different tissues and organs in the human body have their electrical characteristics, their coal-consuming characteristics will absorb electromagnetic waves. Energy, internal and external objects will reflect, diffract, scatter, and absorb electromagnetic fields to varying degrees, so the communication channel between the implanted antenna and the external device will be more complicated. To prove that  the designed antenna can work normally and measure the effective communication distance of the implanted antenna in actual work, it is necessary to analyze the communication link. To simplify the calculation and make a preliminary assessment of the communication performance of the implantable antenna, we simply set the external channel environment as a free-space propagation model, with dB as the unit, the formula of path loss L f is as follow [26] L f = 20 log 10 (4πd/λ) where d is the distance between the transmitting and receiving antennas, and λ is the free-space working wavelength. Link margin (LM) with the change of communication distance is used to measure the communication performance of the implanted antenna. The expressions related to the link margin are as follows [26] LM = CNR link − CNR required (5) CNR required = E b /N 0 + 10 log 10 B r − G c + G d (7) N 0 = 10 log 10 (k) + 10 log 10 (T i ) The CNR link refers to the ratio of the signal power received by the external antenna at a certain distance and the noise power density when the implanted antenna is transmitted at a certain power. The CNR required refers to the carrier-to-noise ratio required by the receiving end to meet the requirements of a certain communication rate and bit error rate and is related to the sensitivity of the receiver. Here we adopt the BPSK modulation method, the bit error rate is required to be less than 1 × 10 −5 , and the bit rate B r is 1 Mb/s. Currently, the input power of the antenna working at 2.4 GHz and 5.8 GHz frequency is 7.60 dBm and 2.60 dBm, and the external receiving antenna adopts a circularly polarized antenna with a gain of 2.15 dBi. The above other values are listed in Table 5. The above formula can calculate the change of the communication link margin with the distance, as shown in Fig. 19. The transceiver distance reaches 14 m and 5 m when the communication link margin reached more than 20 dB at 2.4 GHz and 5.8 GHz.

VI. CONCLUSION
A spherical spiral structure and the conformal characteristics of the capsule shell are used to design a miniaturized implantable antenna that can be used as a capsule speculum and cardiac pacemaker. The Dual-band feature is implemented at 2.4 GHz and 5.8 GHz. In the simple muscle model established, the influence of the structure of the proposed antenna on the antenna is studied. The actual working environment of the antenna is simulated by building the large intestine model and heart model. The situation of different angular angles of the antenna in the process of the digestive tract system is also considered for simulation. Through the IEEE C95.1-1999 guidelines, the study of SAR ensures the safety of the proposed antenna to patients. Then compared with the previously proposed dual-band implanted antenna, the proposed antenna is distinct from the previously proposed antenna. We made a comparison of measured results in minced pork and pig intestines with the simulation to verify the feasibility of the antenna designed. Finally, by analyzing the communication link, the transceiver distance can reach 14 m and 5 m when the communication link margin reached more than 20 dB at 2.4 GHz and 5.8 GHz. KUO  He has published six articles in refereed journals and conference proceedings. He held four Chinese patents issued in 2019. His current research interests include antennas and computational electromagnetics. He has published coauthored eight papers in refereed journals and conference proceedings. His research interests include antenna, microwave circuit, computational electromagnetics, and especially in the FDTD method. He is currently working as an Associate Professor with the School of Electronics and Information Engineering, Hebei University of Technology. His research interests include microwave radiofrequency technology, flexible electronic devices, and electromagnetic compatibility.