Design and Analysis of an Implantable CPW-Fed X-Monopole Antenna for 2.45-GHz ISM Band Applications
Srinivasan Ashok Kumar, ME, PhD Candidate, and Thangavelu Shanmuganantham, ME, PhD Department of Electronics Engineering, Pondicherry University, Pondicherry, India.
Abstract A novel antenna design that effectively covers the industrial, scientific, and medical (ISM) band at 2.45 GHz using an X-shaped structure with a coplanar waveguide (CPW) feed is described. The antenna has a compact size of 67.6 mm3. The proposed design is effective for ISM band biotelemetry with a wakeup controller (2.45 GHz). An experimental prototype of the compact implantable CPW-fed X-shaped monopole antenna was fabricated on a biocompatible alumina Al2O3 ceramic substrate. The optimal antenna was fabricated and tested in minced tissue from the front leg of a pig and on a human body phantom liquid. The simulated and measured bandwidths are 180 MHz and 210 MHz in the ISM band, respectively. Key words: biomedical applications, implantable antenna, industrial, scientific, and medical band, method of moments, coplanar waveguide
Introduction
R
better replacement to deal with this issue. In such a case, an implantable antenna can be placed in the patient’s body. This antenna can communicates the patient’s condition regularly to the concerned personnel at the hospital as shown in Figure 1.6 In this case, the patient saves the transport charges and time, and simultaneously the doctor serves several patients at the same time. An antenna can be designed in either the air or the dielectric properties of the body. If the antenna is designed in the air, the antenna’s best performance will be achieved when air surrounds the implant. If the implanted antenna is designed in the dielectric properties of the body, the best performance from the implant will be achieved when the implant is actually inside the body cavity.7 The medical implant communication service frequency band has been chosen to reduce the wave attenuation in the complex human environment. As a drawback, the use of the medical implant communication service band requires electrically very small antennas.8,9 Until now, the microstrip (or) planar inverted F antenna has been proposed for implantable devices, which were covered with dielectric materials in the frequency band of 402–405 MHz.10 The proposed antenna is found to be compact in size and have reasonable return loss of 10 dB to cover the industrial, scientific, and medical (ISM) band. Furthermore, the return loss property of the antenna is insensitive to the variation of the electrical properties of the human body.11 Therefore, to design an implanted antenna, it is essential to place the implant in the medium in which it will be expected to operate. In this article, an implantable antenna is proposed for ISM applications. The use of bigger implantable units restricts the transmission distance of the signal. This is due to the fact that our body fluids and skin greatly attenuate the signal. This has an adverse effect
ecent research suggests that antennas are finding their ways into human bodies and animal bodies. Implanted antennas for use in biomedical therapy and diagnostics have been designed to produce hyperthermia for treating tumors and monitor various physiological parameters. In order to design an optimal communication link, parameters of implantable antennas and radiation characteristics of the body must be concordant with four techniques: (1) miniaturization, (2) design of the antenna that effectively operates in the surrounding body environment, (3) safety issues based on specific absorption rate distributions, and (4) communication link characteristics.1 Biotelemetry provides wireless communication between inside the body to outside it, or vice versa. Higher-frequency telemetry links are being developed for medical implants. An implantable antenna for biomedical implants is an advanced rapid system. The miniaturization of an implantable antenna is limited. Moreover, power conservation is critical for long-term continuous monitoring of medical implants.2–5 Fig. 1. Hospital applications of the proposed antenna. ECG, electroSome patients find it difficult to check up on the condition of cardiogram; EEG, electroencephalogram; EMG, electromyogram; EOG, their health daily at the hospital. This proposed system may be a electro-oculogram.
246 TELEMEDICINE and e-HEALTH M A R C H 2 0 1 4
DOI: 10.1089/tmj.2013.0186
IMPLANTABLE CPW-FED X-MONOPOLE ANTENNA FOR 2.45 GHZ
Matching the mode impedance of the coplanar waveguide (CPW) to 50 O is obtained by tuning the distance between the tracks and, as well as, the width of the tracks as shown in Figure 2. The value for each couple of dielectric constants and related conductivity is reported in Table 1.
THEORETICAL MODELING
Fig. 2. Proposed antenna structure. All dimensions are in millimeters. on transmission power and the coverage. Such problems can be avoided by the use of repeater units. The radiation characteristics of tissue-implantable antennae mounted over a human body are simulated and analyzed.
Materials and Methods
In this article, the CPW feeding structure is shown in Figure 1, and the characteristics of the proposed antenna are discussed in detail. For theoretical modeling, this proposed structure is divided into two symmetric parts. The symmetry of this device implies that the two fundamental quasitransverse electromagnetic modes will be propagated in the outgoing waveguide. Figure 3 shows the imaginary part of an equivalent circuit model for the CPW transition in Figure 2. The parts are described by homogeneous transmission lines for which the characteristic impedance (Z0), effective dielectric constant, and attenuation constant (in dB/cm) are determined by the quasistatic formulas based on the conformal mapping technique.13 Figure 4 shows the equivalent transmission circuit model using L and C components. Based on the model order reduction method the circuit can be simplified, and the input impedance of an implantable antenna can be found. It follows that Zin = Z1 ==fZ2 + (Z3 ==Z4 ==Z5 )==(Z6 + Z7 )g
H(s) =
ANTENNA DESIGN The geometry of the proposed implantable antenna has dimensions of 67.6 mm3 (13 mm · 8 mm · 0.65 mm), as shown in Figure 2. The shape consists of a 1-mm width of the center strip and 0.5 mm for the gap width. The X shape is chosen to add the sharp edges. During this study, the origin point of the coordinate system is located at the center of the plane, according to Figure 2. Biocompatible alumina ceramic substrate (relative permittivity [er] = 9.8, loss tangent = 0.002) has long been used in implantable antenna design, and it was selected as the dielectric material.12
(sL)2 s10 L5 C 5
+ 5s8 L4 C 4
+ 10s6 L3 C 3
+ 10s4 L2 C 2 + 5S2 LC + 1
(1)
(2)
Using the values of input impedance, the reflection coefficient, voltage standing wave ratio (VSWR), return loss, and bandwidth can be computed using the following relations: Reflection coefficient k =
VSWR =
Zin - Z0 Zin + Z0
1+k 1-k
(3)
(4)
Table 1. Phantoms Used for Testing of Implantable Antennas TISSUE
SHAPE
STATE
Skin
Rectangular
Gel
Scalp
Rectangular
Rat tissue Skin, muscle, and fat
INGREDIENTS
PERMITTIVITY
REFERENCE
Deionized water, sugar, agarose
er = 38
Italian National Research Council9
Gel
Water, salt, acrylamide, TMEDA, ammonium persulfate
er = 28
Rahmat-Samii and Kim12
Rectangular
Gel
Deionized water, DGBE, Triton X-100
er = 45.2
Lee et al.13
Rectangular
Gel
Deionized water, sugar in deionized water, salt, vegetable oil, flour in deionized water, sugar, salt
er = 38 er = 52.7 er = 5.28
Wong16
DGBE, diethylene glycol butyl ether; er, relative permittivity; TMEDA, tetramethylethylenediamine.
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Fig. 3. Imaginary part of the equivalent circuit model of the proposed antenna.
Return loss = 10 log
1 = - 20 log (k) k2
(5)
where Z0 is 50 O. Figure 5 shows a comparison between the analyzed and simulated return loss of the antenna in the planar state. The simulations are performed using the IE3D simulator from Mentor Graphics (Wilsonville, OR). Because the IE3D simulator is a threedimensional simulator, the finite size of antenna typically results in a shift of the resonance frequency to lower frequencies, so for the initial design in momentum, to cover the ISM band, the antenna is designed to resonate at 2.45 GHz. A radiation response of the proposed implantable antenna shows the frequency and phase responses at resonance frequency as displayed in Figure 6.
Fig. 4. Network model of the proposed antenna.
248 TELEMEDICINE and e-HEALTH M A R C H 2 0 1 4
Fig. 5. Return loss characteristics of the analysis and simulation part.
Finally, the numerical analysis operation is based on the flowchart shown in Figure 7. The algorithm for the design process is discussed in the following steps: Step 1. Start the numerical solution for the given proposed antenna. Step 2. Select the er, loss tangent, and thickness of the dielectric material. Step 3. Draw the equivalent transmission line model. Step 4. Solving the L and C coefficients, calculate input impedance using full wave analysis. Step 5. Calculate the S-parameter. Step 6. Check whether the S-parameter is matching or not. If matching, stop the process; otherwise go with step 4.
Fig. 6. Magnitude and phase response of the proposed implantable antenna.
IMPLANTABLE CPW-FED X-MONOPOLE ANTENNA FOR 2.45 GHZ
Fig. 7. Flowchart for the design process.
THE ELECTRICAL PROPERTIES OF PORCINE TISSUE In current studies, simulated human skin fluids have been developed by varying the concentrations of alcohol and salt.14 Here minced porcine tissue was adopted to offer an easy approach to mimicking the environment of ISM band applications and to verify the characteristics of the proposed antenna. In this case, the experimental test tissue was front leg tissue of a pig. The dielectric probe kit and network analyzer from Agilent Technologies (Santa Clara, CA) were used to make the dielectric measurements, which were taken of the test tissue between 300 and 3,000 MHz as shown in Figure 8. The dimensions of the test tissue were 100 mm · 100 mm · 50 mm. Figure 9 compares the permittivity and conductivity of the test tissue (porcine) with that of
Fig. 8. Dielectric measurement (porcine tissue).
Fig. 9. Electrical properties of porcine tissue: (a) permittivity at 2.45 GHz and (b) conductivity at 2.45 GHz. reference skin and muscle.15 Therefore it is suitable for verifying the ISM band implantable CPW-fed antenna design.
Results and Discussion Experimental investigations are needed for validate the simulation results of implantable CPW-fed antennas. Because it is not possible to carry out measurements inside the human body, investigations are performed by measuring laboratory Cochin University of Science and
Fig. 10. Fabricated proposed antenna.
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Fig. 12. Comparison of return of loss versus frequency.
Fig. 11. Photograph of the experimental setup.
Technology (Kerala, India)–fabricated prototypes (Fig. 10) inside tissue-equivalent media (phantoms). Because of the unavailability of biocompatible materials in some laboratories, we purchased alumina ceramic material from Alibaba Manufacturer, China. Prototype fabrication of implantable antennas meets all classical difficulties of miniature antennas. The proposed antenna is fabricated with biocompatible alumina ceramic substrate (er = 9.8, h = 0.65 mm) as shown in Figure 10. The CPW feed used to connect the antenna with the network analyzer may give rise to radiating currents on the outer part of the cable, which in turn causes deterioration of measurements (Fig. 11). Based on this occurrence, the numerical antenna model must be slightly adjusted in order to take prototype fabrication considerations into account. Numerical simulations and experimental measurements must be carried out with the exact same antenna structure in order to be able to validate the design. Canonically shaped phantoms have so far been used for testing of implantable antennas. In this case, the main challenge lies in the formulation and characterization of
tissue-emulating materials. Example phantoms and tissue recipes reported in the literature are given in Table 1.9,11,13,16 Recipes proposed mainly included ultrapure water, sugar, and salt contents. An increase in sugar content concentration has been found to drastically decrease er, while slightly increasing r. An increase in salt content concentration decreases er and significantly increases r as tabulated in Table 2.13 Adding an agarose to solidify the liquids and form multilayer gel phantoms was also examined.14 Numerical simulations and experimental measurements must be carried out with the exact same antenna structure in order to be able to validate the design. Return loss characteristics of porcine tissue, human phantom liquid, and simulated results are plotted in Figure 12. These characteristics correspond to those of human skin and muscle between 300 MHz and 3 GHz. The typical current distribution of the proposed antenna shows the maximum current in the edges as displayed in Figure 13. Finally, an implantable antenna was tested inside minced tissue from the front leg of a pig. In vivo investigations are also vital in order to investigate the effects of live tissue on the performance of implantable patch antennas, while providing valuable feedback for antenna design and analysis.
Table 2. Preparation of Human Body Phantom Liquids SKIN
FAT
MUSCLE
50%
2.9%
59.5%
NaCl
—
0.1%
0.5%
Sugar
50%
—
40%
Vegetable oil
—
30%
—
Flour
—
67%
—
Deionized water
Fig. 13. Current distribution.
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IMPLANTABLE CPW-FED X-MONOPOLE ANTENNA FOR 2.45 GHZ
The radiation characteristics of the antenna inside the liquid simulating muscle, fat, and skin tissue are determined in terms of radiation patterns and gain. To simulate, the antenna is directed toward the surface of the gel (muscle, fat, and skin), along the z-direction the distance to the surface of the gel is set to 15 mm in the xy-plane, and the antenna is placed in the center of the surface of the human model phantom. The computed radiation patterns in the E-plane and H-plane are shown in Figures 14 and 15, respectively. The patterns are computed at 2.45 GHz, at a reference distance of 1 m, and using an input power of 1 W. The maximum gain is equal to - 6 dBi for h = 0 and u = 0, and the radiation efficiency is 0.34%. These values are comparable to other results in the literature.15 Radiation efficiency is very low because the antenna is not in free space, but embedded into human tissue, simulated as a very lossy tissue medium. In the future, some patients may be in a coma. In this stage, spontaneous activities may occur, and the eyes can open in response
Fig. 15. Radiation pattern for E-field: (a) copolarization and (b) cross polarization. to external stimulus. Patients may even suddenly grimace, laugh, or cry. Although a patient in a persistent vegetative state may appear slightly normal, he or she does not speak and is unable to respond to commands. So, because very rarely change may occur in the progress of the comatose patient, we have to monitor him or her regularly. In this case an implantable antenna may be placed into the human body, and it will collect the movement signal from the human body to communicate to the responsible person at the hospital through a wireless network. Hence, this proposed implantable antenna may be suitable for regularly monitoring the physical change in the body conditions of a comatose patient.
Conclusions
Fig. 14. Radiation pattern for H-field: (a) copolarization and (b) cross polarization.
A novel flexible implantable CPW-fed X-monopole antenna for ISM band biomedical applications is presented with a compact size of 13 · 8 · 0.65 mm3. Hence it can be conveniently embedded into human body liquid. The resonant frequency of the in-body antenna is that of the ISM band (2,450 MHz), and the bandwidth is 210 MHz
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from the measurements. Because of superior permittivity and the quality factor of the ceramic substrates, implantable antennas exhibit lower return loss, good VSWR, and better impedance matching at 50 O with the CPW structure. Therefore, the proposed antenna is the suitable structure for the ISM band frequency of 2.45 GHz for applications in the field of biomedical engineering.
Disclosure Statement No competing financial interests exist.
REFERENCES 1. Kumar SA, Thangavelu S. Implanted CPW fed monopole antenna for biomedical applications. Adv Intell Syst Comput 2013;178:97–105. 2. Scarpello ML, Kurup D, Rogier H, Vande Ginste D, Axisa F, Vanfleteren J, Joseph W, Martens L, Vermeeren G. Design of an implantable slot dipole conformal flexible antenna for biomedical applications. IEEE Trans Antennas Propagation 2011;59:3556–3564. 3. Lee CM, Yo TC, Luo C-H, Tu CH, Juang TZ. Compact broadband stacked implantable antenna for biotelemetry with medical devices. Electron Lett 2007;43:660–662. 4. Huang L, Ashouei M, Yazicioglu F, et al. Ultra-low power sensor design for wireless body area networks: Challenges, potential solutions, and applications. Int J Digital Content Technol Applic 2009;3:136–146. 5. Chien T-F, Cheng C-M, Yang H-C, Jiang J-W, Luo C-H. Development of nonsuperstrate implantable low-profile CPW-fed ceramic antennas. IEEE Antennas Wireless Propagation Lett 2010;9:599–602. 6. Kumar SA, Shanmuganantham T. Implantable CPW fed dual folded dipole antenna for biomedical applications. IEEE International Conference on ICCCNT2012, IEEE-20180. Piscataway, NJ: IEEE, 2012:1–5. 7. Liu WC, S. Chen H, Wu CM. Bandwidth enhancement and size reduction of an implantable PIFA antenna for biotelemetry devices. Microwave Opt Technol Lett 2009;51:755–757. 8. IEEE standard for safety levels with respect to human exposure to radio frequency electromagnetic fields, 3 kHz to 300GHz. IEEE standard C95. Piscataway, NJ: IEEE, 1999.
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9. Institute for Applied Physics, Italian National Research Council. Dielectric properties of body tissues. Available at http://niremf.ifac.cnr.it (last accessed November 29, 2011). 10. Karacolak T, Hood AZ, Topsakal E. Design of a dual-band implantable antenna and development of skin mimicking gels for continuous glucose monitoring. IEEE Trans Microwave Theory Tech 2008;56:1001–1008. 11. Rahmat-Samii Y, Kim J. Synthesis lectures on antennas: Implanted antennas in medical wireless communications. San Rafael, CA: Morgan & Claypool Publishers, 2006. 12. www.fcc.gov/Bureaus/Engineering_Technology /Orders /2000 /fcc00211.doc (last accessed November 29, 2011). 13. Lee C-M, Yo T-C, Huang F-J, Luo C-H. Bandwidth enhancement of planar inverted-F antenna for implantable biotelemetry. Microwave Opt Technol Lett 2009;51:749–752. 14. Lee C-M, Yo T-C, Luo C-H. Compact broadband stacked implantable antenna for biotelemetry with medical devices. IEEE Annual Wireless and Microwave Technology Conference, 2006. WAMICON ’06. Piscataway, NJ: IEEE, 2006:1–4. 15. Soontornpipit P. Design of implantable antennas for communication with medical implants [MS thesis]. Logan, UT: Department of Electrical and Computer Engineering, Utah State University, 2002. 16. Wong K-L . Compact and broadband microstrip antennas. New York: Wiley, 2002.
Address correspondence to: Srinivasan Ashok Kumar, ME, PhD Candidate Department of Electronics Engineering Pondicherry University Pondicherry, 605014 India E-mail: [email protected] Received: May 29, 2013 Revised: July 16, 2013 Accepted: July 16, 2013
Srinivasan Ashok Kumar, ME, PhD Candidate, and Thangavelu Shanmuganantham, ME, PhD Department of Electronics Engineering, Pondicherry University, Pondicherry, India.
Abstract A novel antenna design that effectively covers the industrial, scientific, and medical (ISM) band at 2.45 GHz using an X-shaped structure with a coplanar waveguide (CPW) feed is described. The antenna has a compact size of 67.6 mm3. The proposed design is effective for ISM band biotelemetry with a wakeup controller (2.45 GHz). An experimental prototype of the compact implantable CPW-fed X-shaped monopole antenna was fabricated on a biocompatible alumina Al2O3 ceramic substrate. The optimal antenna was fabricated and tested in minced tissue from the front leg of a pig and on a human body phantom liquid. The simulated and measured bandwidths are 180 MHz and 210 MHz in the ISM band, respectively. Key words: biomedical applications, implantable antenna, industrial, scientific, and medical band, method of moments, coplanar waveguide
Introduction
R
better replacement to deal with this issue. In such a case, an implantable antenna can be placed in the patient’s body. This antenna can communicates the patient’s condition regularly to the concerned personnel at the hospital as shown in Figure 1.6 In this case, the patient saves the transport charges and time, and simultaneously the doctor serves several patients at the same time. An antenna can be designed in either the air or the dielectric properties of the body. If the antenna is designed in the air, the antenna’s best performance will be achieved when air surrounds the implant. If the implanted antenna is designed in the dielectric properties of the body, the best performance from the implant will be achieved when the implant is actually inside the body cavity.7 The medical implant communication service frequency band has been chosen to reduce the wave attenuation in the complex human environment. As a drawback, the use of the medical implant communication service band requires electrically very small antennas.8,9 Until now, the microstrip (or) planar inverted F antenna has been proposed for implantable devices, which were covered with dielectric materials in the frequency band of 402–405 MHz.10 The proposed antenna is found to be compact in size and have reasonable return loss of 10 dB to cover the industrial, scientific, and medical (ISM) band. Furthermore, the return loss property of the antenna is insensitive to the variation of the electrical properties of the human body.11 Therefore, to design an implanted antenna, it is essential to place the implant in the medium in which it will be expected to operate. In this article, an implantable antenna is proposed for ISM applications. The use of bigger implantable units restricts the transmission distance of the signal. This is due to the fact that our body fluids and skin greatly attenuate the signal. This has an adverse effect
ecent research suggests that antennas are finding their ways into human bodies and animal bodies. Implanted antennas for use in biomedical therapy and diagnostics have been designed to produce hyperthermia for treating tumors and monitor various physiological parameters. In order to design an optimal communication link, parameters of implantable antennas and radiation characteristics of the body must be concordant with four techniques: (1) miniaturization, (2) design of the antenna that effectively operates in the surrounding body environment, (3) safety issues based on specific absorption rate distributions, and (4) communication link characteristics.1 Biotelemetry provides wireless communication between inside the body to outside it, or vice versa. Higher-frequency telemetry links are being developed for medical implants. An implantable antenna for biomedical implants is an advanced rapid system. The miniaturization of an implantable antenna is limited. Moreover, power conservation is critical for long-term continuous monitoring of medical implants.2–5 Fig. 1. Hospital applications of the proposed antenna. ECG, electroSome patients find it difficult to check up on the condition of cardiogram; EEG, electroencephalogram; EMG, electromyogram; EOG, their health daily at the hospital. This proposed system may be a electro-oculogram.
246 TELEMEDICINE and e-HEALTH M A R C H 2 0 1 4
DOI: 10.1089/tmj.2013.0186
IMPLANTABLE CPW-FED X-MONOPOLE ANTENNA FOR 2.45 GHZ
Matching the mode impedance of the coplanar waveguide (CPW) to 50 O is obtained by tuning the distance between the tracks and, as well as, the width of the tracks as shown in Figure 2. The value for each couple of dielectric constants and related conductivity is reported in Table 1.
THEORETICAL MODELING
Fig. 2. Proposed antenna structure. All dimensions are in millimeters. on transmission power and the coverage. Such problems can be avoided by the use of repeater units. The radiation characteristics of tissue-implantable antennae mounted over a human body are simulated and analyzed.
Materials and Methods
In this article, the CPW feeding structure is shown in Figure 1, and the characteristics of the proposed antenna are discussed in detail. For theoretical modeling, this proposed structure is divided into two symmetric parts. The symmetry of this device implies that the two fundamental quasitransverse electromagnetic modes will be propagated in the outgoing waveguide. Figure 3 shows the imaginary part of an equivalent circuit model for the CPW transition in Figure 2. The parts are described by homogeneous transmission lines for which the characteristic impedance (Z0), effective dielectric constant, and attenuation constant (in dB/cm) are determined by the quasistatic formulas based on the conformal mapping technique.13 Figure 4 shows the equivalent transmission circuit model using L and C components. Based on the model order reduction method the circuit can be simplified, and the input impedance of an implantable antenna can be found. It follows that Zin = Z1 ==fZ2 + (Z3 ==Z4 ==Z5 )==(Z6 + Z7 )g
H(s) =
ANTENNA DESIGN The geometry of the proposed implantable antenna has dimensions of 67.6 mm3 (13 mm · 8 mm · 0.65 mm), as shown in Figure 2. The shape consists of a 1-mm width of the center strip and 0.5 mm for the gap width. The X shape is chosen to add the sharp edges. During this study, the origin point of the coordinate system is located at the center of the plane, according to Figure 2. Biocompatible alumina ceramic substrate (relative permittivity [er] = 9.8, loss tangent = 0.002) has long been used in implantable antenna design, and it was selected as the dielectric material.12
(sL)2 s10 L5 C 5
+ 5s8 L4 C 4
+ 10s6 L3 C 3
+ 10s4 L2 C 2 + 5S2 LC + 1
(1)
(2)
Using the values of input impedance, the reflection coefficient, voltage standing wave ratio (VSWR), return loss, and bandwidth can be computed using the following relations: Reflection coefficient k =
VSWR =
Zin - Z0 Zin + Z0
1+k 1-k
(3)
(4)
Table 1. Phantoms Used for Testing of Implantable Antennas TISSUE
SHAPE
STATE
Skin
Rectangular
Gel
Scalp
Rectangular
Rat tissue Skin, muscle, and fat
INGREDIENTS
PERMITTIVITY
REFERENCE
Deionized water, sugar, agarose
er = 38
Italian National Research Council9
Gel
Water, salt, acrylamide, TMEDA, ammonium persulfate
er = 28
Rahmat-Samii and Kim12
Rectangular
Gel
Deionized water, DGBE, Triton X-100
er = 45.2
Lee et al.13
Rectangular
Gel
Deionized water, sugar in deionized water, salt, vegetable oil, flour in deionized water, sugar, salt
er = 38 er = 52.7 er = 5.28
Wong16
DGBE, diethylene glycol butyl ether; er, relative permittivity; TMEDA, tetramethylethylenediamine.
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Fig. 3. Imaginary part of the equivalent circuit model of the proposed antenna.
Return loss = 10 log
1 = - 20 log (k) k2
(5)
where Z0 is 50 O. Figure 5 shows a comparison between the analyzed and simulated return loss of the antenna in the planar state. The simulations are performed using the IE3D simulator from Mentor Graphics (Wilsonville, OR). Because the IE3D simulator is a threedimensional simulator, the finite size of antenna typically results in a shift of the resonance frequency to lower frequencies, so for the initial design in momentum, to cover the ISM band, the antenna is designed to resonate at 2.45 GHz. A radiation response of the proposed implantable antenna shows the frequency and phase responses at resonance frequency as displayed in Figure 6.
Fig. 4. Network model of the proposed antenna.
248 TELEMEDICINE and e-HEALTH M A R C H 2 0 1 4
Fig. 5. Return loss characteristics of the analysis and simulation part.
Finally, the numerical analysis operation is based on the flowchart shown in Figure 7. The algorithm for the design process is discussed in the following steps: Step 1. Start the numerical solution for the given proposed antenna. Step 2. Select the er, loss tangent, and thickness of the dielectric material. Step 3. Draw the equivalent transmission line model. Step 4. Solving the L and C coefficients, calculate input impedance using full wave analysis. Step 5. Calculate the S-parameter. Step 6. Check whether the S-parameter is matching or not. If matching, stop the process; otherwise go with step 4.
Fig. 6. Magnitude and phase response of the proposed implantable antenna.
IMPLANTABLE CPW-FED X-MONOPOLE ANTENNA FOR 2.45 GHZ
Fig. 7. Flowchart for the design process.
THE ELECTRICAL PROPERTIES OF PORCINE TISSUE In current studies, simulated human skin fluids have been developed by varying the concentrations of alcohol and salt.14 Here minced porcine tissue was adopted to offer an easy approach to mimicking the environment of ISM band applications and to verify the characteristics of the proposed antenna. In this case, the experimental test tissue was front leg tissue of a pig. The dielectric probe kit and network analyzer from Agilent Technologies (Santa Clara, CA) were used to make the dielectric measurements, which were taken of the test tissue between 300 and 3,000 MHz as shown in Figure 8. The dimensions of the test tissue were 100 mm · 100 mm · 50 mm. Figure 9 compares the permittivity and conductivity of the test tissue (porcine) with that of
Fig. 8. Dielectric measurement (porcine tissue).
Fig. 9. Electrical properties of porcine tissue: (a) permittivity at 2.45 GHz and (b) conductivity at 2.45 GHz. reference skin and muscle.15 Therefore it is suitable for verifying the ISM band implantable CPW-fed antenna design.
Results and Discussion Experimental investigations are needed for validate the simulation results of implantable CPW-fed antennas. Because it is not possible to carry out measurements inside the human body, investigations are performed by measuring laboratory Cochin University of Science and
Fig. 10. Fabricated proposed antenna.
ª M A R Y A N N L I E B E R T , I N C . VOL. 20
NO. 3 MARCH 2014
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ASHOK KUMAR AND SHANMUGANANTHAM
Fig. 12. Comparison of return of loss versus frequency.
Fig. 11. Photograph of the experimental setup.
Technology (Kerala, India)–fabricated prototypes (Fig. 10) inside tissue-equivalent media (phantoms). Because of the unavailability of biocompatible materials in some laboratories, we purchased alumina ceramic material from Alibaba Manufacturer, China. Prototype fabrication of implantable antennas meets all classical difficulties of miniature antennas. The proposed antenna is fabricated with biocompatible alumina ceramic substrate (er = 9.8, h = 0.65 mm) as shown in Figure 10. The CPW feed used to connect the antenna with the network analyzer may give rise to radiating currents on the outer part of the cable, which in turn causes deterioration of measurements (Fig. 11). Based on this occurrence, the numerical antenna model must be slightly adjusted in order to take prototype fabrication considerations into account. Numerical simulations and experimental measurements must be carried out with the exact same antenna structure in order to be able to validate the design. Canonically shaped phantoms have so far been used for testing of implantable antennas. In this case, the main challenge lies in the formulation and characterization of
tissue-emulating materials. Example phantoms and tissue recipes reported in the literature are given in Table 1.9,11,13,16 Recipes proposed mainly included ultrapure water, sugar, and salt contents. An increase in sugar content concentration has been found to drastically decrease er, while slightly increasing r. An increase in salt content concentration decreases er and significantly increases r as tabulated in Table 2.13 Adding an agarose to solidify the liquids and form multilayer gel phantoms was also examined.14 Numerical simulations and experimental measurements must be carried out with the exact same antenna structure in order to be able to validate the design. Return loss characteristics of porcine tissue, human phantom liquid, and simulated results are plotted in Figure 12. These characteristics correspond to those of human skin and muscle between 300 MHz and 3 GHz. The typical current distribution of the proposed antenna shows the maximum current in the edges as displayed in Figure 13. Finally, an implantable antenna was tested inside minced tissue from the front leg of a pig. In vivo investigations are also vital in order to investigate the effects of live tissue on the performance of implantable patch antennas, while providing valuable feedback for antenna design and analysis.
Table 2. Preparation of Human Body Phantom Liquids SKIN
FAT
MUSCLE
50%
2.9%
59.5%
NaCl
—
0.1%
0.5%
Sugar
50%
—
40%
Vegetable oil
—
30%
—
Flour
—
67%
—
Deionized water
Fig. 13. Current distribution.
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IMPLANTABLE CPW-FED X-MONOPOLE ANTENNA FOR 2.45 GHZ
The radiation characteristics of the antenna inside the liquid simulating muscle, fat, and skin tissue are determined in terms of radiation patterns and gain. To simulate, the antenna is directed toward the surface of the gel (muscle, fat, and skin), along the z-direction the distance to the surface of the gel is set to 15 mm in the xy-plane, and the antenna is placed in the center of the surface of the human model phantom. The computed radiation patterns in the E-plane and H-plane are shown in Figures 14 and 15, respectively. The patterns are computed at 2.45 GHz, at a reference distance of 1 m, and using an input power of 1 W. The maximum gain is equal to - 6 dBi for h = 0 and u = 0, and the radiation efficiency is 0.34%. These values are comparable to other results in the literature.15 Radiation efficiency is very low because the antenna is not in free space, but embedded into human tissue, simulated as a very lossy tissue medium. In the future, some patients may be in a coma. In this stage, spontaneous activities may occur, and the eyes can open in response
Fig. 15. Radiation pattern for E-field: (a) copolarization and (b) cross polarization. to external stimulus. Patients may even suddenly grimace, laugh, or cry. Although a patient in a persistent vegetative state may appear slightly normal, he or she does not speak and is unable to respond to commands. So, because very rarely change may occur in the progress of the comatose patient, we have to monitor him or her regularly. In this case an implantable antenna may be placed into the human body, and it will collect the movement signal from the human body to communicate to the responsible person at the hospital through a wireless network. Hence, this proposed implantable antenna may be suitable for regularly monitoring the physical change in the body conditions of a comatose patient.
Conclusions
Fig. 14. Radiation pattern for H-field: (a) copolarization and (b) cross polarization.
A novel flexible implantable CPW-fed X-monopole antenna for ISM band biomedical applications is presented with a compact size of 13 · 8 · 0.65 mm3. Hence it can be conveniently embedded into human body liquid. The resonant frequency of the in-body antenna is that of the ISM band (2,450 MHz), and the bandwidth is 210 MHz
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from the measurements. Because of superior permittivity and the quality factor of the ceramic substrates, implantable antennas exhibit lower return loss, good VSWR, and better impedance matching at 50 O with the CPW structure. Therefore, the proposed antenna is the suitable structure for the ISM band frequency of 2.45 GHz for applications in the field of biomedical engineering.
Disclosure Statement No competing financial interests exist.
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9. Institute for Applied Physics, Italian National Research Council. Dielectric properties of body tissues. Available at http://niremf.ifac.cnr.it (last accessed November 29, 2011). 10. Karacolak T, Hood AZ, Topsakal E. Design of a dual-band implantable antenna and development of skin mimicking gels for continuous glucose monitoring. IEEE Trans Microwave Theory Tech 2008;56:1001–1008. 11. Rahmat-Samii Y, Kim J. Synthesis lectures on antennas: Implanted antennas in medical wireless communications. San Rafael, CA: Morgan & Claypool Publishers, 2006. 12. www.fcc.gov/Bureaus/Engineering_Technology /Orders /2000 /fcc00211.doc (last accessed November 29, 2011). 13. Lee C-M, Yo T-C, Huang F-J, Luo C-H. Bandwidth enhancement of planar inverted-F antenna for implantable biotelemetry. Microwave Opt Technol Lett 2009;51:749–752. 14. Lee C-M, Yo T-C, Luo C-H. Compact broadband stacked implantable antenna for biotelemetry with medical devices. IEEE Annual Wireless and Microwave Technology Conference, 2006. WAMICON ’06. Piscataway, NJ: IEEE, 2006:1–4. 15. Soontornpipit P. Design of implantable antennas for communication with medical implants [MS thesis]. Logan, UT: Department of Electrical and Computer Engineering, Utah State University, 2002. 16. Wong K-L . Compact and broadband microstrip antennas. New York: Wiley, 2002.
Address correspondence to: Srinivasan Ashok Kumar, ME, PhD Candidate Department of Electronics Engineering Pondicherry University Pondicherry, 605014 India E-mail: [email protected] Received: May 29, 2013 Revised: July 16, 2013 Accepted: July 16, 2013
