894IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 4, APRIL 2009 Performances of an Implanted Cavity Slot Antenna Embedded in the Human Arm Wei Xia, Kazuyuki Saito, Member, IEEE, Masaharu Takahashi, Senior Member, IEEE, and Koichi Ito, Fellow, IEEE Abstract—Implantable devices have been investigated with great interest as communication tools. These implantable devices are embedded into the human or pet body. The vital information (such as temperature, blood pressure, cardiac beat, etc.) can be transmitted from implantable devices to the external equipment by use of a wireless communication link. Therefore, the research on the antenna for implantable devices (implanted antennas) is very important. This paper proposes an implanted H-shaped cavity slot antenna for short-range wireless communications. This type of antenna, which is designed to operate at the industrial-scientiﬁc-medical band (2.45 GHz), is investigated by using ﬁnite-difference time-domain calculation. We analyzed the performances of the proposed antenna which is embedded into the human body between the shoulder and the elbow. However, since the proposed antenna is too small to fabricate, a scale model is adopted for antenna measurements. Some characteristics of the scale model of the antenna are also calculated and measured by using the 2/3 muscle-equivalent phantom. The results show that the proposed antenna has promise for use in an implant. Index Terms—Finite-difference time domain (FDTD), implanted antenna, scale model, 2/3 muscle-equivalent phantom. I. INTRODUCTION I N THE 1990s, the role and possibility of implanted device systems were suggested for future telecommunications . Meeting expectations, implanted devices are largely researched for biotelemetry, e-healthcare, hyperthermia, etc. These devices can be used in different kinds of applications. The examples of the applications for humans are measurement of temperature, blood pressure, and continuous monitoring of cardiac beat in sickrooms or intensive care units. Also, the medical records or body information, such as allergies, can be saved in the implantable devices. Thus, these devices are useful for the decision of a diagnosis and treatment method. Moreover, identity conﬁrmation and attestation of a person are also assumed for institution management, shopping, etc. Some other applications are ﬁnding lost pets, controlling farm animals for safety and quality management, etc. It is suggested that the implantable devices transmit the recorded information by using a reading device. The personal information can be received outside the body by use of a data exchange between the implantable device and the computer network. Obviously, these implanted devices must be wireless when used to communicate with the exterior, because they are embedded into the human or animal body. Therefore, it is appreciated that the antenna of an implanted device system has a particularly important role as the part of transmitting and receiving power in the human body. Until now, Microstrip or planar-inverted F antennas (PIFA) were proposed for implantable devices, which were covered with a dielectric material in the frequency band at 402–405 MHz –. In these studies, the characteristics of the antennas, such as input impedance, radiation pattern, and speciﬁc absorption rate (SAR) around the antenna were presented. However, these antennas are not small enough to be embedded into the human body. As one method of reducing the size of the antenna, it can be used in the high frequency. Here, it is chosen to be 2.40–2.48 GHz, because there is a commercially available transceiver for this band. Some implanted antennas were investigated for 2.45-GHz applications –. Especially, in , an H-shaped cavity slot antenna was proposed and the performances of the antenna were calculated by the ﬁnite-difference time-domain (FDTD) method. However, this antenna is a little big for being embedded into the human body. Moreover, the research of this antenna was only discussed in simulation. Therefore, in this paper, we optimize this antenna which is assumed to be used at 2.4–2.485 GHz in the industrial-scientiﬁc–medical (ISM) band. The antenna can be miniaturized by setting up the H-shape slot. As a result, the dimension of the optimized antenna (2.8 mm 4.0 mm 1.6 mm) is 38.5% of the previous antenna (5.2 mm 2.8 mm 3.2 mm). The characteristics of the antenna are also calculated by the FDTD method. In addition, since the antenna is too small for fabrication, the performances of the antenna are measured by using a scale model to conﬁrm the validity of the numerical calculation. II. LINK BUDGET FOR WIRELESS COMMUNICATION In this paper, it is assumed that the proposed antenna is used in a sickroom as illustrated in Fig. 1 for medical data transmission. It is necessary to know some parameters related to a link budget for communications between the implantable antenna and the receiver. The parameters of the communication environment are as follows: the operating frequency is ﬁxed to 2.45 GHz and the input power to the implanted antenna is 25 W (from the European Research Council (ERC) limitation ). The implanted antenna is assumed to be used in short-range communication; therefore, the distance between the implanted antenna and the receiving antenna is set to be 4 m. The receiving antenna is assumed to be a monopole antenna on the ceiling. The availability . If the link of the communication is decided by exceeds required , wireless communication is possible. Manuscript received January 07, 2008; revised October 26, 2008. Current version published April 08, 2009. W. Xia and K. Ito are with the Graduate School of Engineering, Chiba University, Chiba 263-8522, Japan (e-mail: [email protected]
). K. Saito and M. Takahashi are with the Research Center for Frontier Medical Engineering, Chiba University, Chiba 263-8522, Japan. Digital Object Identiﬁer 10.1109/TAP.2009.2014579 0018-926X/$25.00 © 2009 IEEE XIA et al.: PERFORMANCES OF AN IMPLANTED CAVITY SLOT 895 TABLE II CALCULATED RESULTS OF THE LINK BUDGET Fig. 1. One example of the application for a human in a sickroom. TABLE I PARAMETERS OF THE LINK BUDGET Fig. 2. Conﬁguration of the cavity slot antenna with the coaxial feeding. III. ANTENNA CONFIGURATION AND CALCULATION MODEL A. Antenna Conﬁguration Fig. 2 shows the conﬁguration of a cavity slot antenna with the H-shape slot proposed as an implanted antenna. The dimension of the antenna is shown in Table III (original model). A cavity slot antenna has the merit of high miniaturization and mechanical robustness. The cavity slot antenna with the H-shape slot is made of a conductor sheet. In addition, as illustrated in is ﬁlled into the implanted Fig. 2, dielectric material antenna, preventing human tissues and ﬂuids from getting inside. The antenna is fed by coaxial cable at the center of the antenna in the direction and 0.95 mm from the upper conductor sheet layer in the direction. The coaxial cable is 0.86 mm in diameter and 184 mm (1.5 ) in length. The inner conductor of the coaxial cable is 0.20 mm in diameter and 186.8 mm in length. Moreover, for simplicity in the simulation of radiation characteristics, the antenna model only has the inner conductor which in length. Then, gap feeding is set between the inner conis ductor and perfect electrical at the left edge in Fig. 2. It should be noted that in this case, it is necessary to conﬁrm the performances of the antenna from the measurement. So the antenna model considers using the coaxial feeding. In practical use, by using the circuit, the antenna can be fed by a battery which is considered to be set inside the antenna or connected outside the antenna. B. Calculation Model Fig. 3 shows the numerical calculation model when the antenna is embedded into the human’s upper arm. The antenna is assumed to be implanted subcutaneously into the human model between the shoulder and the elbow. The dimension of the model is 180 mm 60 mm 60 mm (original model in Table III). In this research, muscle-equivalent phantom is employed as the human model. The electrical properties of muscle-equivalent phantom at 2.45 GHz were found the 1.16 S/m . The upper surface of to be the antenna is directed toward the surface of the skin, and the Table I shows the parameters used to calculate the link and required . The link used (1), (3), and (4) and used (2), (5), and . the required Link (1) Required (2) (3) (4) (5) The calculated results are shown in Table II. Since the proposed implanted antenna gain is more than 26.5 dBi, the link exceeds the required . 896 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 4, APRIL 2009 TABLE III GEOMETRICAL SCALE MODEL Fig. 4. Reﬂection characteristic (calculation) (original model). Fig. 5. Impedances of the antenna (calculation) (original model). Fig. 3. Numerical calculation model using the 2=3 muscle-equivalent phantom. distance from the surface of the antenna to the surface of the skin is set to 4 mm. The antenna is placed in the center of the surface of the human model. IV. CALCULATED RESULTS Fig. 4 shows the reﬂection characteristic of the antenna (original model), when the antenna is fed by coaxial feeding and gap performance feeding. From this result, it is conﬁrmed that is lower than 10 dB from 2.13 GHz to 2.80 GHz when the antenna is fed by the coaxial feeding. Moreover, the fractional bandwidth at the target frequency (2.45 GHz) is approximately performance is lower than 27.3%. By using the gap feeding, 10 dB from 2.10 GHz to 2.61 GHz, and the fractional bandwidth is approximately 20.8%. Fig. 5 illustrates the impedances. From the imaginary parts of the impedance, it is conﬁrmed that the resonant frequency of the antenna with coaxial feeding is higher than that of the antenna with gap excitation. The difference is caused by the effects of the coaxial cable. component (or Fig. 6 indicates radiation patterns for the component cross-pol component) in the -plane and for the (or co-pol component) in the -plane. It should be noted that in the simulations, in the -plane, the maximum gain for the component is 95.5 dBi, and in the -plane, the maximum gain for the component is 106.0 dBi. The gains are quite component in the -plane small. So in this research, the component in the -plane are discussed. and 0 The direction of maximum radiation in the -plane is ( direction). The maximum gain is approximately 24.2 dBi. 38 in the On the other hand, the maximum radiation is -plane, and the maximum gain is approximately 22.3 dBi. It is conﬁrmed that the directions of maximum radiation in both planes are approximately in the direction (the direction to outside the body). According to the link budget, it is possible to use wireless communication when the gain of the antenna is more than 26.5 dBi. In Fig. 5, it can be stated that wireless communication is possible within the range of approximately 90 centered at 0 in the -plane, or 180 centered at 0 in the -plane. In addition, the radiation efﬁciency is 0.39%. This value is very low, because the antenna is embedded into the human model. Moreover, since the antenna is embedded into the human body, it is necessary to evaluate the SAR to examine whether there is an inﬂuence on the human body. From the calculated results, when the input power of the antenna is assumed to be 25 W, the maximum 1-g average SAR value is W/kg, which is much lower than the maximum 1-g average SAR standard value of ANSI (1.60 W/kg) . V. EXPERIMENTAL RESULTS In order to conﬁrm the validity of the numerical calculations, the antenna measurement is needed. However, the antenna is too small for the fabrication. Therefore, a scale model is proposed for the antenna fabrication and the measurement. In order to simplify the measurement, the scale model of the antenna is 2.5 times larger than the original antenna which is considerably XIA et al.: PERFORMANCES OF AN IMPLANTED CAVITY SLOT 897 TABLE IV COMPOSITION OF THE PHANTOM Fig. 8. Impedances of the antenna (calculation) (scale model). Fig. 6. Radiation patterns of the calculation (original model). (a) xz -plane (E ). (b) yz -plane (E ). Fig. 9. Measured reﬂection (scale model). Fig. 7. Photograph of the fabricated antenna (scale model). easier to fabricate. The parameters of the scale model of the antenna and human model are summarized in Table III and . By using the 2.5 times scale model, the resonant frequency of the antenna is expected to change from 2.45 GHz to 980 MHz. In addition, the coaxial cable used in the scale model is 0.86 mm in diameter and 460.0 mm (about 1.5 ) in length at 980 MHz. Fig. 7 is a photograph of the fabricated antenna. The performusclemances of the antenna were measured by using the equivalent phantom. Table IV shows the composition of the muscle-equivalent phantom at 980 MHz which is used in measurements . The electrical properties of the fabricated 0.43 S/m. The target value of the phantom are 0.46 S/m. So the difference electric constants is errors for the dielectric constant and conductivity between target and measured values are 3.4% and 6.7%, respectively. There is a very small inﬂuence on the antenna performances with these small differences in the electrical properties values. Fig. 8 illustrates the calculated impedances of the antenna in the scale model, when the antenna is fed by coaxial feeding and gap feeding. It is shown that the impedance bandwidth is approximately the same with that of the original model (Fig. 5). Fig. 9 shows the calculated and measured reﬂections of the scale model. The results show that the resonant frequency of the measurement corresponded approximately with that of the numerical calculation. Fig. 10 indicates radiation patterns in the - and -planes by numerical calculation and the measurement. It is shown that calculated radiation patterns are almost the same by using the original model and the scale model. The maximum gain of the -plane and measurement is 24.3 dBi, 21.8 dBi, in the -plane, respectively. The radiation directivity and maximum gain are approximately the same for the measurement and calculation from 0 to 90 . From these results, it is conﬁrmed that the 898 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 4, APRIL 2009 In future work, the performances of the proposed antenna should be analyzed when the distance from the surface of the antenna to the surface of the skin has changed. Then, the method of the measurement is also considered by using the original antenna model. REFERENCES  B. M. Steinhaus, R. E. Smith, and P. Crosby, “The role of telecommunications in future implantable device systems,” in Proc. IEEE Conf. Medicine and Biology, Nov. 1994, vol. 2, pp. 1013–1014.  P. Soontornpipit, “Design of implantable antennas for communication with medical implants,” M. S. thesis, Dept. Elect. Comput. Eng., Utah State Univ., Logan, UT, 2002.  J. Kim and Y. 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Lazzi, “Impedance matching and implementation of planar space-ﬁlling dipoles as intraocular implanted antennas in a retinal prosthesis,” IEEE Trans. Antennas Propag, vol. 53, no. 8, pp. 2365–2373, Aug. 2005.  W. Sun, G. Haubrich, and G. Dublin, “Implantable medical device microstrip telemetry antenna,” U.S. Patent 5 861 091, Jan. 19, 1999.  M. D. Amundson, J. A. Von Arx, W. J. Linder, P. Rawat, and W. R. Mass, “Circumferential antenna for an implantable medical device,” U.S. Patent 6 456 256, Sep. 24, 2002.  J. A. Von Arx, W. R. Mass, S. T. Mazar, and M. D. Amundson, “Antenna for an Implantable Pacemaker,” U.S. Patent 6 708 065, Mar. 16, 2004.  H. Usui, M. Takahashi, and K. Ito, “Radiation characteristics of an implanted cavity slot antenna into the human body,” in Proc. IEEE Antennas and Propagation Soc. Int. Symp., Albuquerque, NM, Jul. 2006, pp. 1095–1098.  T. Karacolak, A. Z. Hood, and E. Topsakal, “Design of a dual-band implantable antenna and development of skin mimicking gels for continuous glucose monitoring,” IEEE Trans. Microw. Theory Tech., vol. 56, no. 4, pp. 1001–1008, Apr. 2008.  G. Collin, A. Chami, C. Luxey, P. Le Thuc, and R. Staraj, “Human implanted spiral antenna for a 2.45 GHz wireless temperature and pressure SAW sensor system,” presented at the IEEE Int. Symp. Antennas and Propagation, San Diego, CA, Jul. 2008.  S. Ohmori, H. Wakana, and S. Kawase, Mobile Satellite Communications. London, U.K.: Artech House, 1998, pp. 59–73.  C. Gabriel and S. Gabriel, Compilation of the Dielectric Properties of Body Tissues at RF and Microwave Frequencies. Louisville, KY: Armstrong Lab. [Online]. Available: http://www.brooks.af.mil/AFRL/ HED/hedr/reports/dielectric/home.html.  IEEE Standard for Safety Levels With Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz, IEEE Std. C95. 1-1999, 1999.  C. A. Balanis, Antenna Theory. New York: Harper & Row, 1982, pp. 733–735.  Y. Okano, K. Ito, I. Ida, and M. Takahashi, “The SAR evaluation method by a combination of thermographic experiments and biological tissue-equivalent phantoms,” IEEE Trans. Microw. Theory Tech., vol. 48, no. 11, pt. 2, pp. 2094–2103, Nov. 2000. Fig. 10. Radiation patterns in the xz - and yz -planes by numerical calculation and measurement. (a) xz -plane (E ). (b) yz -plane (E ). direction of the main lobes is toward the -direction (the direction to the outside of the body) for calculation and measurement. As can be seen from the calculated results and the measured results, the performances of the proposed antenna are conﬁrmed. As an implanted antenna, it has wide bandwidth, small volume, and a simple conﬁguration. Moreover, with the use of the H-shape slot, the effective length of the antenna can be changed more easily than other slots. Therefore, by setting up the H-shape slot, its volume is greatly reduced compared with  (about 38.5% of the previous antenna). VI. CONCLUSION In this paper, the cavity slot antenna with the H-shape slot is proposed for the implanted antenna. From the link budget, the proposed implanted antenna gain should be more than 26.5 dBi. Numerical results show that the proposed antenna resonates well at 2.45 GHz. The peak gain of simulation is 22.3 dBi. In addition, the SAR values around the antenna are evaluated in the simulation. The maximum 1-g average SAR is W/kg. This value satisﬁes the standard values of ANSI. Finally, in order to simplify the measurement, the antenna is fabricated by using the scale model which is 2.5 times larger than the original dimensions of the antenna. The antenna characteristics are measured in the constructed 2/3 muscle-equivalent phantom. The results show that the fabricated antenna works at the 980-MHz band as expected. The antenna characteristics for the calculation and the measurement correspond. The results show that the proposed antenna has promising use in an implant. XIA et al.: PERFORMANCES OF AN IMPLANTED CAVITY SLOT 899 Wei Xia was born in Heilongjiang, China, in September 1981. He received the B.E. degree in electric information engineering from Wuhan Polytechnic University, Wuhan, China, in 2004, and is currently working pursuing the M.E. degree at Chiba University, Chiba, Japan. His main interests are the development and design of the antenna for implantable devices. Kazuyuki Saito (S’99–M’01) was born in Nagano, Japan, in May 1973. He received the B.E., M.E., and D.E. degrees in electronic engineering from Chiba University, Chiba, Japan, in 1996, 1998, and 2001, respectively. Currently, he is an Assistant Professor with the Research Center for Frontier Medical Engineering, Chiba University. His main interest is in the area of medical applications of microwaves, including microwave hyperthermia. Prof. Saito received the Institute of Electrical, Information, and Communication Engineers (IEICE) Japan AP-S Freshman Award; the Award for the Young Scientist of URSI General Assembly; the IEEE AP-S Japan Chapter Young Engineer Award; the Young Researchers’ Award of IEICE; and the International Symposium on Antennas and Propagation (ISAP) Paper Award in 1997, 1999, 2000, 2004, and 2005, respectively. He is a member of the IEICE Japan, the Institute of Image Information and Television Engineers of Japan (ITE), and the Japanese Society for Thermal Medicine. Masaharu Takahashi (M’95–SM’02) was born in Chiba, Japan, on December, 1965. He received the B.E. degree in electrical engineering from Tohoku University, Miyagi, Japan, in 1989, and the M.E. and D.E. degrees in electrical engineering from the Tokyo Institute of Technology, Tokyo, Japan, in 1991 and 1994, respectively. He was a Research Associate from 1994 to 1996; an Assistant Professor from 1996 to 2000 with Musashi Institute of Technology, Tokyo; and an Associate Professor with the Tokyo University of Agriculture and Technology, Tokyo, from 2000 to 2004. Currently, he is an Associate Professor at the Research Center for Frontier Medical Engineering, Chiba University, Chiba. His main interests are electrically small antennas, planar array antennas, and electromagnetic compatibility. Prof. Takahashi received the IEEE Antennas and Propagation Society (IEEE AP-S) Tokyo Chapter Young Engineer Award in 1994. He is a member of the Institute of Electrical, Information, and Communication Engineers (IEICE), Japan. Koichi Ito (M’81–SM’02–F’05) received the B.S. and M.S. degrees in electrical engineering from Chiba University, Chiba, Japan, in 1974 and 1976, respectively, and the D.E. degree in electrical engineering from the Tokyo Institute of Technology, Tokyo, Japan, in 1985. From 1976 to 1979, he was a Research Associate at the Tokyo Institute of Technology. From 1979 to 1989, he was a Research Associate at Chiba University. From 1989 to 1997, he was an Associate Professor in the Department of Electrical and Electronics Engineering at Chiba University, and is currently a Professor at the Graduate School of Engineering, Chiba University. He has been appointed as one of the Deputy Vice-Presidents for Research, Chiba University, since 2005. In 1989, 1994, and 1998, he visited the University of Rennes I, Rennes, France, as an Invited Professor. Since 2004, he has been appointed as an Adjunct Professor to the Institute of Technology Bandung (ITB), Indonesia. His main research interests include the analysis and design of printed antennas and small antennas for mobile communications; research on evaluation of the interaction between electromagnetic ﬁelds and the human body by use of numerical and experimental phantoms; microwave antennas for medical applications, such as cancer treatment; and antennas for body-centric wireless communications. Dr. Ito is a Fellow of the Institute of Electronics, Information, and Communication Engineers (IEICE) of Japan, a member of the American Association for the Advancement of Science, the Institute of Image Information and Television Engineers of Japan (ITE), and the Japanese Society for Thermal Medicine (formerly the Japanese Society of Hyperthermic Oncology). He served as Chair of the Technical Group on Radio and Optical Transmissions, ITE, from 1997 to 2001 and Chair of the Technical Group on Human Phantoms for Electromagnetics, IEICE, from 1998 to 2006. He also served as Chair of the IEEE AP-S Japan Chapter from 2001 to 2002 and TPC Co-Chair of the 2006 IEEE International Workshop on Antenna Technology (iWAT2006). Currently, he is General Chair of the iWAT2008, which was held in Chiba, Japan, in 2008 and Vice-Chair of the 2008 International Symposium on Antennas and Propagation (ISAP2008) held in Taiwan in 2008. He is an Associate Editor for the IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION and is a Distinguished Lecturer and an AdCom member for the IEEE Antennas and Propagation Society since 2007.