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differential feeding structure, which provides the directional radiation with an ultra-thin design. The antenna is fabricated using LTCC technology with an overall ...
The 2014 International Workshop on Antenna Technology

A Wideband Differential Directional Antenna for Head Implants Terence S.P. See1, Wei Liu1, Xianming Qing1, Zhi Ning Chen1, 2 1

RF, Antenna and Optical Department Institute for Infocomm Research, Singapore {spsee, liuw; qingxm; chenzn}@i2r.a-star.edu.sg 2

Department of Electrical and Computer Engineering National University of Singapore [email protected] Monopoles and planar inverted-F antennas (PIFA) are popular candidates in wireless implants because of the simple geometry and small form factor [12]. Such antennas are single-ended and the radiation is omni-directional. On the other hand, the output of the integrated circuit (IC) chip is normally designed to be differential. In order to ease the system integration and achieve the maximum power transfer, a differential antenna which is conjugate-matched to the chip is more preferable. Moreover, in order to reduce the SAR and to maximize the gain in the direction of the external device, an antenna with directional radiation pattern is more suitable.

Abstract—This paper presents the design of a wideband differential antenna which is suitable for head implants. The antenna consists of a ground plane beneath the patches and a differential feeding structure, which provides the directional radiation with an ultra-thin design. The antenna is fabricated using LTCC technology with an overall size of 24 × 10 × 0.95 mm3. In order to ensure that the antenna is bio-compatible inside the head, a Teflon casing is constructed to house the antenna, wherein the head is modeled as a four-layered lossy structure. The antenna implanted in the head exhibits an impedance bandwidth of about 20% with a reference impedance of 100Ÿ at the center frequency of 4 GHz, the directivity of more than 5 dBi and the boresight gain of more than -8.3 dBi.

In this paper, a wideband directional differential antenna is designed to be implanted inside the head and fabricated using low temperature co-fired ceramics (LTCC) technology. This antenna can be easily integrated in medical devices to extract the neurological data for diagnosis or implantable neural recording systems for the control of prosthetic devices [34].

Keywords—Implants; ISM band; MICS band; ultra-wideband; telemetry; ultra-wideband; differential antenna.

I.

INTRODUCTION

Antennas implanted inside the human body can be used for a variety of medical applications such as therapy and diagnosis. In addition, telecommunications in medical devices for transmitting important diagnostic information is also one of the important functions. These devices extract physiological data from the body and transmit them to an external device wirelessly. The challenges for the implanted antenna designs include form factor, efficiency, radiation pattern, impedance matching, and safety in terms of the biocompatibility as well as the specific absorption rate (SAR). All these factors are essential to establish an efficient and reliable communication link for the medical implant communications service (MICS) systems. Several frequency bands have been approved for MICS, which include from the band of 402405 MHz; industrial, scientific and medical (ISM) bands of 433.05434.79 MHz, 866868 MHz, 902928 MHz, 2.42.4835 GHz, 5.7255.875 GHz as well as ultra-wideband (UWB) of 3.110.6 GHz. The unlicensed UWB band is desirable for high speed data transmission within the implantable devices because of the wide bandwidth. Moreover, the cost and power consumption of UWB systems are low.

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II.

ANTENNA DESIGN & CONFIGURATION

The antenna design is shown in Fig. 1. It is fabricated on Ferro A6M (Hr = 5.9, tanG = 0.002) substrate that is composed of 10 layers. The thickness of each layer is 0.095 mm. The patches are printed on the top layer and each patch measures 9 mm × 8 mm. The size of the substrate and the ground plane is 24 mm × 10 mm. The differential feeding structure is positioned one layer beneath the patch. The width of the differential microstrip lines is 0.77 mm and the gap between them is 0.7 mm, which gives a differential impedance of about 100Ÿ. In order to expose the feed lines so that they can be soldered to the cable structure for measurement, a cavity of size 2.2 mm × 2 mm was constructed. The antenna casing is made of Teflon material (Hr = 2.1, tanG = 0.0002), which is bio-compatible with the tissues. The thickness of the wall of the casing is 2 mm. As the antenna performance is highly dependent on the presence of the lossy and dispersive tissues around it, it is important to take into account the effect of the tissues during the design. In this study, the head is modeled as a four-layered homogeneous structure comprising skin, tendon, cortical bone, and grey matter. As the space beneath the top of the head is

The 2014 International Workshop on Antenna Technology

more suitable to be implanted with the antenna, the tissue type and thickness of each layer is consistent with those at the top of the head. Table I tabulates the thickness and dielectric properties of each layer at 4 GHz [5]. The size of the tissues is 125 mm × 87 mm, which is based on the size of the top of the head. As shown in Fig. 2, the antenna with the Teflon casing is placed inside the skin tissue. The top of the casing is 2 mm below the surface of the skin and there is an air gap of 0.5 mm above the antenna.

III.

RESULTS

The simulation was carried out using CST Microwave Studio. The measurement was performed using a pair of semirigid co-axial cables with the outer conductor soldered together and the inner conductors connected to the pair of differential microstrip lines as shown in Fig. 3.

Fig. 3. Photo of antenna with casing and cable fixture.

By considering a virtual ground plane in between the antenna radiators, each terminal of the antenna radiators and the virtual ground plane can be considered as a “port”. The antenna may then be considered as a “two-port” network and its impedance will be related to the network parameters of the equivalent two-port network, which can be characterized by measuring the network parameters such as S-parameters [67].

Fig. 1. Antenna geometry.

The differential impedance Zd can be expressed as TABLE I. Tissue

Thickness (mm)

TISSUE PARAMETERS AT 4 GHZ Relative Permittivity

Conductivity (S/m)

Skin

5

40.85

2.70

1125

Tendon

2.5

40.24

3.17

1151

Cortical Bone

5

10.53

0.728

1850

Grey Matter

12

46.58

3.09

1036

teflon

24 0.5 (air)

2 2

28

air skin 0.95 (antenna) 2 tendon

(1)

Using the Agilent PNA N5230A and “port-extension” calibration technique, the influence of the cable fixture can be de-embedded to achieve an accurate result [7]. First, the inner and outer conductors of the cable-pair were short-circuited by soldering them to a small metal plate. Next, the antenna was connected to the cable fixture and with the tissues in place, the real and imaginary parts of the S-parameters were extracted and the differential impedance was calculated using (1).

5 2.5 5

grey matter

12

With the impedance referenced to 100Ÿ, the impedance matching is then calculated and shown in Fig. 4. The simulated bandwidth of the antenna is about 3.74.5 GHz while the measured bandwidth is about 44.8 GHz for the reflection coefficient of less than -10 dB. The slight upward shift in the resonant frequency is mainly due to the fabrication tolerance. The average thickness of the fabricated samples is about 0.935 mm, which is slightly smaller than the simulated thickness of 0.95 mm. The measured values in the x and y-directions are generally about 0.7% lower than the design value.

mm

Fig. 2. Antenna placement inside the tissues.

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2 Z 0 1  S11S 22  S12 S 21  S12  S 21 1  S 11 1  S 22  S 21S12

where Z0 is the characteristic impedance of the connected transmission lines, which is 50Ÿ for most measurement systems.

cortical bone

125

Zd

Density (kg/m3)

185

The 2014 International Workshop on Antenna Technology TABLE II. MAXIMUM SAR & MAXIMUM ALLOWABLE NET INPUT POWER

0

Frequency (GHz)

Maximum SAR (W/kg) 1 g-avg

Max. Net Input Power (mW) [ANSI C95.1-1999]

3.5

36.6

43.7

4.0

53.8

29.7

4.5

47.5

33.7

Relection coefficient, dB

-5 -10 -15

IV.

-20

Simulated Measured

-25 2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

In this paper, the design of a wideband differential patch antenna has been presented. The antenna was fabricated using LTCC technology and placed inside a casing that was made of bio-compatible material. By extracting the S-parameters, the differential impedance of the antenna was calculated. The measured results have shown that the impedance bandwidth of about 20% was achieved with the reference impedance of 100Ÿ. From the simulations, the directivity of more than 5 dBi and gain of more than -8.3 dBi were obtained when the antenna was implanted inside the head. Due to the directional radiation property of the antenna, a lower SAR was achieved. The antenna can be used in head implants for bio-telemetry, deep brain stimulation, monitoring, and neural prosthesis.

6.0

Frequency, GHz Fig. 4. Reflection coefficient of the antenna.

The radiation patterns of the antenna were computed and the directivity and gain at the boresight are shown in Fig. 5. It can be observed that the directivity of more than 5 dBi and a gain of more than -8.3 dBi across the impedance bandwidth can be achieved when the antenna is implanted inside the head.

Directivity and Gain, dBi

10

REFERENCES

5

[1]

0 -5

[2]

-10 -15

[3]

-20 Directivity Gain

-25 -30 3.0

CONCLUSIONS

3.5

4.0 4.5 Frequency, GHz

[4]

5.0 [5]

Fig. 5. Directivity and gain at boresight of antenna implanted inside the head.

[6]

The specific absorption rate (SAR), which measures the maximum amount of energy absorbed by a unit mass of tissue, is an important parameter in order to ensure the safety of the patient. The maximum 1-g averaged SAR values and the corresponding maximum allowable net input power are given in Table II for the delivered peak power of 1 W. In order to meet the ANSI C95.1-1999 regulation of 1.6 W/kg in 1 g of tissue [8], the power that is delivered to the implant antenna should be reduced accordingly. Generally, it is important that the SAR to be as low as possible. With the low SAR, it will not be necessary attenuate the net input power to the antenna by too much in order to meet the regulations since less input power will lead to a reduction in the transmission range.

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[7]

[8]

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