band Microstrip Antenna for Body Wearable Wireless Devices at UWB ...

169 INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY, VOL.11, NO.3, MAY 2016

Performance Investigations and SAR Analysis of a Dualband Microstrip Antenna for Body Wearable Wireless Devices at UWB Channel Frequencies Varshini Karthik 1 and T. Rama Rao 2 1 2

Department of Biomedical Engineering, SRM University, Chennai - 603203, India RAMS Lab, Department of Telecommunication Engineering, SRM University, India E-mail: [email protected], [email protected]

Abstract— A microstrip based dual-band monopole antenna for body Wearable Wireless Devices (WWD) is proposed utilizing channels in the Ultra Wide Band (UWB) spectrum. The designed dual-band antenna targets lower band (4.7 - 5.3 GHz) and upper band (7.3 - 8.5 GHz) of the unlicensed UWB spectrum to meet low, high data rates. The antenna design considers the difference in the interaction of body tissues with the electromagnetic (EM) fields, when compared to free space. The performance of the antenna on different wearable substrates, the robustness of the antenna to work with varying skin thickness and on different sites on the body along with the thermal effects on the skin are analyzed. A simplified, human tissue layer model that is frequency dependent in its properties is used for simulations and computations. Simulated and experimental results were compared and found to be in agreement favouring the use of the antenna for wearable devices for various WBAN applications. Index Terms— Dual band, Microstrip monopole antenna, Specific Absorption Rate, Thermal effects, Ultra wideband, Wearable Wireless Devices.

I.

INTRODUCTION

A Wireless body area network (WBAN) is a special purpose sensor network designed to work autonomously to connect various Wearable Wireless Devices (WWD) located inside or outside a human body [1]. The aim of Wearable technology is to create convenient, portable access to information in real time. Wearable devices communicate by means of wearable antennas. With the use of these WWD, WBAN finds extensive applications in health care,

fitness, assisted-living and smart entertainment. Ultra Wideband (UWB) technology is presented as one of the possible physical layer solutions. The UWB specifications offer a large scope for WBAN implementations in terms of high performance, robustness, simplicity, and ultralow power operation. It also provides safe power levels to be used on human body complementing other longer-range radio technologies such as Wi-Fi and WiMAX. One major challenge for antenna design in WBANs is the variation in the antenna topology centred on the shape of the human body, which specifies the need for flexible and textile antennas. One other challenge is the electromagnetic interaction between the human body and the antenna. The human body is inhomogeneous with high loss and permittivity, which affects the properties of an antenna. Additionally, the surrounding environment, other parameters of a user like weight loss/gain, posture and changes in the skin with age also need to be considered for antenna design in WBANs. The shape, size, material limitations need to be taken into account. Moreover, the location of an antenna on the body has major control on the size and shape of the antenna being used, therefore constraining the designer. Skin tissue, muscle and fat change characteristics with respect to frequency. The power usage has to be kept at a minimum level to enhance the battery life. The heating effects on the body due to the electric field should also be taken into account in WBAN antenna design. A survey paper [2] enumerates the parameters that influence the design of in body and on body

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antennas. The field of wearable and implantable antennas is a multidisciplinary one that combines electromagnetics, electronics, material science and bioelectronics. A review paper [3] portrays research on wearable, body mounted antennas for various applications at various frequencies over a decade. In recent years, the effects of planar fabric substrate antenna when placed near to the human body, antenna for health care systems, tripod kettle antenna, dual polarized printed antenna and several antennas when placed near to human body is studied in [4-8] respectively. In this work, a dual-band microstrip based monopole antenna that works efficiently at dual bands within the unlicensed UWB frequencies in free space and on human body is proposed for WBANs utilizing WWD. The dual bands chosen enable the antenna to be used for low and high data rate applications and realistic environments when the full UWB cannot be utilized [9-10]. The performance of the designed antenna for different wearable substrates, varying skin thickness and heating effects are studied and analysed utilizing 3D EM tools of ANSYS HFSS [11] and Sim4Life by SPEAG [12]. Sim4Life is used for on-body simulations for the designed antenna at various body sites utilizing its high-fidelity computable human phantoms with advanced tissue models and HFSS (based on finite element method) used for antenna design and fabrication.

II. MODELLING HUMAN BODY TISSUES A. Effect of Electromagnetic field on Human Tissues The human body consists of different types of material, each differing in the way they interact with the EM fields. The geometry of the human body and its different parts are very complex [13].The electrical properties of the biological tissues are the result of the electromagnetic radiations interacting with constituents of the body at the cellular and molecular level [14]. Understanding the interaction and the electrical properties of body tissues is the primary need in the design work of antennas for body wearable

devices. The biological effects of antennas working in the UWB frequency ranges are proportional to the rate of energy absorption given in terms of Specific Absorption Rate (SAR) and the ability to heat human tissues. Both these effects can be hazardous if exposure is sufficiently intense or prolonged. The electrical permittivity and conductivity are the important properties that determine the electric field distribution in the body and the power dissipated in it [15]. These properties change with frequency. Tissues having the highest water content have the highest relative permittivity (e.g. skin and muscle), decreasing with increasing frequency. The tissue’s water content results in the specific permittivity value, which affects the wavelength inside tissues [16]. B. Layered Model of Human Tissue The antenna is designed for on-body applications where the influence of the outermost body tissues is considered. There are different simplified tissue models like homogeneous models, three layered body model with different cross sections as flat, rectangular and elliptical used in literature. There are also detailed 3D body models that use a lot of computational resource and time. A three layered rectangular biological tissue model made up of skin, subcutaneous fat and muscle is chosen as it represents most of the body regions [17]. The designed antenna is proposed to be placed over the stacked tissues with an average thickness of 2 mm, 3.5mm and 10mm for skin, fat and muscle layer respectively as in Fig.1. The electrical properties of the tissues at lower and upper resonating frequencies of the antenna designed are based on the works of Italian National Research Council, which is available online [18] and is given in Table 1.

Fig.1. Layered model of human tissue with antenna

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Skin, due to its in homogeneous structure has inhomogeneous dielectric properties. Muscles have intermediate dielectric properties when compared to fat which has poor dielectric properties. Table 1: The electrical properties of the tissues at the dual bands Frequency (GHz)

Tissue

εr

σ (S/m)

Tan θ (Loss Tangent)

4.9

Skin

35.8

2.98

0.31

Fat

5.03

0.24

0.17

Muscle

49.7

3.93

0.29

Skin

33.4

5.61

0.38

Fat

4.77

0.43

0.21

Muscle

45.8

7.52

0.38

7.8

input at the port is 0.9 mW. A taper microstrip line with 4.4 mm at the broad end and 1.6 mm at narrow end is used to offer good impedance matching. Two horizontal slots each measuring 6mm in length and 0.5mm in width are used on the right and left sides of the radiating patch. This ensures that the antenna has dual band operation at the required bands. The simulated return loss plots for different length of the slots are shown in Fig.4.The length of the slot was optimized as 6 mm to ensure operation at the required two bands wherein a lesser value has the effect of right shift of resonating frequencies with poor values of S11(dB). Performance metrics of the antenna considered are gain, radiation efficiency and SAR. Measurements of return loss were made using RS-ZVL Vector Network Analyzer of frequency range 9 KHz-13.6 GHz as shown in Fig.5a. The measured results were found to be in good agreement with the simulations.

III. ANTENNA SIMULATIONS AND MEASUREMENTS A compact design of the antenna is a critical requirement as the antenna is for use on the human body. The tissue environment make the antenna design requirements differ from that of a conventional antenna that is designed for free space. The simulation model and the prototype of the proposed antenna is shown in Fig.2 and 3 respectively. The antenna was designed using the model design equations [19].The antenna has dimension of 35 х 32 х1.57mm3 and made of Rogers 5880 substrate with relative permittivity of 2.2 and loss tangent of 9х10-3. The choice of the substrate is in view of the fact that it offers low electric loss and low moisture absorption. A partial ground plane with a slot and staircase rectangular patch design enhances the bandwidth of the antenna. The step size of the staircase pattern is 1mm followed by 1.5mm from both the top and bottom edges. The antenna is designed and the geometries are optimized so that the antenna works at dual bands of UWB spectrum, resonating at 4.9 GHz and 7.8 GHz on the human body. The radiating patch is fed by a 50Ω microstrip line. The power

Fig.2.

Dimensions of the dual –band antenna

Fig.3. Prototype of the dual –band antenna

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Fig.4. Return loss plot for various simulated length of the slot (P3) on the radiating patch of the antenna

results are tabulated in Table 2. It is seen from the table that the upper band performs well in terms of gain, efficiency when compared to lower band. The human tissues serve to dielectrically load the antenna hence worsening the communication link when the antennas are body worn. The bodily effect is important as it influences the Figure of Merit of the wearable antenna given in terms of body–induced gain, body-worn efficiency, body effect on impedance and the detuning effect by the body.

A. Antenna simulations on rectangular phantom model and measurements on human body Placement of the antenna in direct contact with the body decreases the gain and radiation efficiency with due effects on the impedance matching due to the lossy human tissue that provides dielectric loading. However, the designed antenna is kept on the human body with cotton padding [20] of 5 mm between them. The bio and electromagnetic compatibility nature of the chosen fabric serves advantageous in two ways. One, in enhancing the performance of the antenna when used on human body and two, makes the design realizable for real time use when compared to the few mm of air gap realized in previous designs [21]. Measurements of return loss when antenna placed on-body were made using RS-ZVL Vector Network Analyzer of frequency range 9 KHz-13.6 GHz as shown in Fig.5b. The measured results of return loss for antenna in free space and when placed on the body are shown in Fig.6.and they are in good agreement. The antenna has quasi omnidirectional radiation pattern as seen in Fig.7 a, b that helps lesser penetration into the body and is circularly polarized lowering the losses [22]. Among the three layers of tissue simulated, the fat layer has low water content, and hence lower dielectric constant (εr) and conductivity (σ) than the skin and muscle. This is important while considering the interactions of EM waves (thus also antennas) with body [23]. Due to the wave impedance mismatch between low (fat) and high (skin, muscle) water content tissues, significant reflections occur for far field exposure. This effect, can lead to the increased specific absorption rate (SAR) [17]. The performance of the dual bands is studied and the

Fig. 5a. Measurement of return loss when the antenna is placed in free space Table 2: Performance of the antenna at dual bands Performance Metrics/Dual Band

Total Gain (dB)

Radiation efficiency (%)

Free Space

Onbody

Free Space

Onbody

3.8

5.1

97.3

43.5

4.8

7.0

98.8

56.2

Band I

Band II

During measurements in free space and on body, a detuning in the resonating frequency of 40 MHz from 5.14 to 5.18 GHz in lower band and a shift of 90 MHz from 7.79 to 7.70 GHz in upper band are experienced. Further a comparison of gain of the proposed antenna with other antennas from literature, operating at UWB frequencies, placed on the body is given in Table 3. It is observed that the proposed antenna gives better gain values.

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(a)

.

Fig. 5b. Measurement of return loss when the antenna is placed on the human body

(b) Fig. 6. Measured return loss plot for free space and on body placement of the antenna

B. Performance study of the antenna on wearable and cost effective substrates The antenna is analyzed for its performance on five different substrates. The substrates chosen are Rogers Duroid 5880, Polyethylene, Teslin(paper), Cardboard and Fabric cotton. They are the most common substrates for patch antenna design. Duroid is a very common and most widely used substrate. Polyethylene is a common substrate in RFID tags. Teslin and Cardboard are novel to the design of such antennas but are very cost effective. Fabric cotton is often used in textile antennas. The thickness of all the substrates is taken as 1.6 mm. The electrical properties of these materials at GHz are collected from literature and tabulated in Table 4.

Fig.7. Radiation pattern of the antenna obtained through simulations showing gain (dB) for the dual bands (a) 4.9 GHz (b) 7.8 GHz when the proposed antenna is placed on body

The performance of the proposed antenna with these five substrates at the dual bands is analyzed using graphs in Fig.8 and Fig.9. It is observed that polyethylene can substitute Duroid substrate in terms of gain, efficiency and SAR values. Cardboard and Fabric cotton are found to be second options when compared to Polyethylene. This may be because of the comparatively low electrical permittivity of the substrates. Teslin showed lesser performance when compared to the other substrates. Cotton, a fabric commonly used for textiles showed almost equal values of SAR, within allowed limits, as the others when used as a substrate for the proposed antenna.

IJMOT-2016-1-883 © 2016 IAMOT(dB) of the proposed antenna Fig. 6.Gain


Table 3: Comparison of gain of the proposed antenna and other antennas placed on body at UWB frequencies Antenna

Maximum Gain (dB)

Loop antenna [24]

6.00

Dipole antenna [24]

5.00

Fractal antenna [25]

3.10

Planar antenna [26]

3.90

Proposed antenna

7.12

Table 4: Electrical properties of the substrates at GHz frequencies Substrate

εr

Tan θ (Loss Tangent)

Rogers RT/ Duroid

2.20

0.0009

Polyethylene

2.25

0.0100

Teslin

2.18

0.1450

Cardboard

1.76

0.0150

Fabric cotton

1.60

0.0400

the scenario of varying skin thickness and placement of the antenna at random sites on the body. Thickness of skin decreases with age especially due to the decrease in epidermal thickness. In that view, the thickness of the skin layer used in the model is varied from 1-2mm, depicting a scenario of young to aged skin. The return loss plot for the same is seen in Fig.10. It is observed that the dual band operation is maintained for all skin thickness with the variation of a right shift towards higher resonating frequencies for the second band with a decrease in skin thickness. The dual band antenna is placed at ten different anatomical sites on the body. This is to analyze the performance of the antenna on any site on the body where the thickness of the three layers of tissue namely skin, fat and muscle vary with the placement site. The body sites considered are forehead, neck, biceps, triceps, chest, wrist, anterior abdomen, anterior thigh, posterior thigh, calf of legs as shown in Fig.11 and named in Table.5. They are the most common and comfortable sites on body for monitoring vital bio-signals. The anatomical sites are chosen considering a scenario of sports training or general health monitoring where the sensors and transceivers can be placed for the specific application. They are best sites for picking up physiological parameters like body temperature, pulse rate, blood pressure, respiration rate, ECG, heart rate, EMG, glucose level, motion and perspiration. The thickness of skin and subcutaneous fat layer for these body sites measured using ultrasonography are referred form literature .The performance of the antenna for various tissue layer combinations considering the muscle thickness as 10 mm is given in Table 5. IV. SAR AND ON-BODY THERMAL EFFECTS

Fig. 8 Return loss plot using simulations showing dual band operation for different substrates

C. Robustness study of the antenna on the body The antenna is analyzed for its robust performance on the human body by considering

Specific Absorption Rate (SAR) is a good dosimetric quantity that measures the rate of energy absorption by human body when exposed to radio frequency EM field. SAR is calculated as:

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(a) Fig. 11. Anatomical sites of the body (anterior and posterior) considered for simulations.

(1) (b)

(c) Fig. 9 (a, b, c). Performance metrics of the antenna on different substrates

Where, E is the RMS value of induced field in (V/m), σ is the conductivity of tissue in (S/m), ρm is the mass density of tissue in kg/m3. A high level of SAR, in any tissue, above standard limits, can prove dangerous for human use. The effect can be measured as an increase in tissue temperature related to the time of exposure of tissues to the EM field. It has been observed from literature that an increase of tissue temperature equal to or greater than 1K would ascertain danger. The temperature rise of tissues takes a linear relationship with the time of exposure for short term exposure of a few seconds to minutes [27]. This is because of little significant contribution made by conductive or convective heat distribution to the temperature rise. The temperature rise in tissues (K) for short time exposure is obtained from the following equation by knowing the heat capacity (c) of the tissue (J/kg/K), average SAR value for 1 gram of tissue and the time of exposure ( in seconds [27]. (2)

Fig. 10. Comparison of return loss plot for varying skin thickness of the phantom model

The short time temperature rise for a time period of 1000 seconds when the designed antenna is radiating on the human body is calculated for the three tissues and tabulated in Table 7.

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Table 5: Performance of the dual band at different body sites Performance Metrics

Total Gain (dB)

Radiation Efficiency (%)

Average SAR on skin (W/Kg)

Band/ Anatomicalsite N1-Forehead

I

II

I

II

I

II

2.05

5.88

25.1

45.1

0.36

0.26

N2-Neck

2.28

5.62

23.4

44.6

0.37

0.23

N3-Biceps

5.11

7.03

43.5

56.2

0.24

0.15

N4-Triceps

5.45

7.12

44.8

54.5

0.24

0.15

N5-Chest

4.92

6.36

41.8

51.3

0.24

0.16

N6-Wrist

4.14

4.05

40.6

37.7

0.13

0.12

N7-Abdomen

4.70

5.10

45.2

44.7

0.16

0.15

N8-Anterior thigh

5.48

6.63

45.3

53.1

0.23

0.18

N9-Posterior thigh

5.93

6.43

49.0

51.2

0.20

0.15

N10-Calf

5.42

6.84

45.1

54.3

0.24

0.14

In order to obtain the relation between temperature rise and time, in the case of long time exposures, the Pennes Bio Heat equation considering the role of blood in thermoregulation of the body has been used [27]. Simplifying the Bio heat differential equation yields equation (3) that gives the maximum temperature rise over a long period of time in the order of minutes [28]. In the equation, S is average SAR for 1 gram of tissue in W/kg, ρ is the mass density of tissue in kg/m3, K is thermal conductivity of the tissue in W/m/K, and w is blood perfusion rate in ml/g/min, cb is the heat capacity of blood in J/kg/K, λ is the wavelength of the EM wave in m. The average SAR values for the tissues considered are given in Table. 6. The maximum SAR values among them are used to calculate the maximum temperature rise for the particular tissue and the temperature rise are tabulated in Table 7 using (2) and (3). It is observed that, the highest temperature increase is found in skin, with 0.11 K for short term and 0.23 K for long term exposure.

(3) Where,

;

;

The temperature increase was found to be indeed less of about fraction of a kelvin. The value of blood parameters considered are blood perfusion rate (w)and heat capacity of blood (cb) whose values are taken as 0.5 ml/g/min and 3617 J/kg/K respectively. V. DISCUSSIONS AND CONCLUSION The performance of the proposed dual-band antenna for WBANs was studied in free space and on body. It was observed that the antenna’s return loss for both simulations and measurements, satisfies S11< -10 dB, at 4.9 and 7.8 GHz, in free space and on body. The performance of the proposed antenna on other wearable and cost effective substrates was analysed. It is perceived from Fig.8 and Fig.9 that Polyethylene can substitute Duroid substrate

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177 INTERNATIONAL JOURNAL OF MICROWAVE AND OPTICAL TECHNOLOGY, VOL.11, NO.3, MAY 2016 Table 6: Parameter values of body tissues used for temperature calculations Tissue/ Param eters

Mass density (kg/m3)

Thermal conducti vity (W/m/K)

Heat capa city (J/kg /K)

Average SAR (W/kg)

Skin

1109

0.37

3391

Band I 0.370

Band II 0.260

Fat

911

0.21

2348

0.043

0.028

Muscle

1090

0.49

3421

0.096

0.064

Table 7: Maximum thermal effect at various tissues Max Average

Temperature Increase

Temperature Increase

SAR value

Short term

Long term

(W/kg)

(K)

(K)

Skin

0.370

0.11

0.23

Fat

0.043

0.02

0.19

Muscle

0.096

0.03

0.22

Tissue

as their performance is similar in terms of gain, efficiency and SAR values. The antenna performed well at the dual band for all the considered substrates. Further, the dual band antenna proposed was tested for its robustness in performance considering varying skin thickness with age and deployment at different body sites. It was observed from Table 5 that the site of placement of antenna on the body has an effect on the performance of the antenna. Antennas on sites with average skin and fat thickness (triceps, thighs) performed better than ones with skin thickness less than 1.5 mm (forehead, neck), a thin skin and fat combination (wrist) and a thick skin and fat combination (abdomen).

leading to better reflections away from the body hence improving antenna performance. However the performance of the antenna at all the considered body sites was reasonably good in terms of gain and radiation efficiency. It was analysed from Table 7 that both short term and long term exposure of the body to the EM field was found to produce a temperature increase about a fraction of a kelvin which is indeed a small value. The thermal effect was found to be greater on skin which is the outermost tissue. The SAR values were found to be far less than the maximum allowable SAR limits (2W/Kg) for 10gm averaged human tissues exposed to EM radiations given by International Council on Non-Ionizing radiation protection [29] and 1gm averaged SAR value of 1.6W/Kg given by IEEE/ICES C95.1-2005 [30]. Hence, the effects of the human body on the performance of the proposed antenna and the effects of the performing antenna on the human body in terms of thermal effects were analysed. Both are considered important when an antenna is proposed to be placed in close proximity to the human body as one used in wearable devices for WBAN applications. These observations indicate that the proposed antenna module can be used as a candidate for WWDs for WBAN applications which will proliferate very widely in near future as a promising way of monitoring bio signals for health care, sports and defense applications, where the antenna performance is greatly affected by the dielectric properties of body tissues. REFERENCES [1]

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