Antenna for Medical Implant Applications at UWB Frequency …Antenna for Medical Implant Applications at UWB Frequency Band Kamya Yekeh Yazdandoost and Ryu Miura Dependable Wireless

Antenna for Medical Implant Applications at UWB Frequency Band

Kamya Yekeh Yazdandoost and Ryu Miura

Dependable Wireless Laboratory, Wireless Network Research Institute, National Institute of Information and Communications Technology, 3-4 Hikarino-oka, Yokosuka 239-0847, Japan

Tel: +81-46-847-5435 Fax: +81-46-847-5431 [email protected], [email protected]

Abstract

The narrow bandwidth at low frequency of current available wireless capsule endoscopy do not provide clear

images of digestive organs. The Ultra Wideband (UWB) at frequency of 3.1-10.6 GHz is one of the possible frequency band to provide clear images from small intestine and stomach. This paper discusses on an UWB antenna to be placed inside stomach of a human body model.

1. Introduction

The wireless body area networks (WBANs) is going to transform health monitoring, with its enormous number of possible applications in hospital, elderly care and home. The WBN is to extensively contribute to improvement in quality, access and efficacy of health care with providing health professionals with access to timely relevant information at the point of need. The limited bandwidth at low frequencies such as Industrial, Scientific and Medical (ISM) band and Medical Implant Communication Service (MICS) band do not make available comprehensible imagery from stomach and small intestine and required lots of time to transfer data images. Therefore, available higher frequency band such as Ultra Wideband (UWB) at frequency of 3.1-10.6 GHz for communication from in-body implanted device to on-body or outside the body is one of the strong candidate for biomedical applications [1]. Antennas and propagation are key points in the design of wireless body implanted devices. The communication performance will be severely affected by body tissues when devices are operated in a human body [2]. Therefore, implanted antennas should provide enough gain and efficiency while needs to be compact and light weight. Antennas implanted in a human body must be designed with deep understanding of surrounding environment, due to different environment and as result different electrical properties from free space. The resonance characteristics of the implanted antenna and their radiation performance outside the body must be evaluated, while an antenna placed inside the body tissue [3]. The rest of this paper is as follows. The antenna design is described in section 2. Scenario and Results are provided in Section 3. Finally concluding remarks are expressed in Section 4.

2. Antenna Design The medical device need to be in proper size to be placed inside the human body, hence, an antenna should be enough small to be fitted in the medical device package. The requirements, complexities and difficulties to design an acceptable antenna for wireless body area network, make it impossible to use tradition antenna design for free space. The procedure to design an antenna with human tissue layers is presented in Figure 1. The antenna has been placed in a simplified biological tissue model consists of skin, fat, and muscle. The antenna is positioned at a certain distance from the bottom of the tissue model, which is muscle, and covered by Duroid substrate. On the top there is a Duroid substrate and the rest of the tissue model which consists of muscle, fat and skin, as shown in Figure 1. For the simulations we took the dielectric properties of skin, fat and muscle from [4, 5], with thickness of 2 mm of skin, 3 mm of fat and 25 mm and 30 mm of muscles on top and bottom respectively. The implant antenna was placed in the muscle (25 mm from top and 30 mm from bottom) and parallel to it. Also we consider the relative permittivity and thickness of adhesive material. Figure 1 shows the layers of the antenna including supporting substrates, adhesive material and body tissues which have been used for the simulation.

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Figure 1. Model for antenna simulation

The proposed antenna has dimension of 10x7.5x1.091 mm3 and made of Rogers 6010 substrate with dielectric constant of 10.2 and thickness of 0.508 mm. The antenna is covered by Duroid substrate material with relative permittivity of 2.2 and thickness of 0.254 on top and bottom in order to prevent the effect of the human body tissues on antenna performance. We have used glue for the connection of the Duroid substrate on the top and bottom. The glue has relative permittivity of 3.7 and thickness of 0.02 mm. However, it was impossible to have uniform thickness of 0.02 mm on the entire surface of glue. The configuration of antenna and its prototype is shown in Figure 2.

a) b)

Figure 2. Antenna a) Dimenssion (unit in mm) b) Prototype.

3. Scenario and Results

An antenna used for the implantable device should be able to radiate outside the body and yet the power associated with the device must comply with the radiation requirements. Most tissues have significant attenuation and, therefore, there will be losses, when signal passing through different tissue layers or organs. For capsule endoscopy application we have been performed simulation for deep tissue implanted, where an implanted antenna is placed 10 cm below body surface. Figure 3 shows a close-up of the implanted antenna in the stomach and Figure 4 shows received signal strength on the body surface on the abdominal area.

Figure 3. Implanted antenna in stomach.

Figure 4. Signal strength (V/cm) on the surface of abdominal .

The antenna reflection coefficient and total realized gain which is antenna's radiation intensity in a given direction to the total incident power to the antenna port at frequency of 7 GHz is shown in Figure 5 a) and b) respectively.

2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 11.00 12.00Frequency [GHz]

-22.50

-20.00

-17.50

-15.00

-12.50

-10.00

-7.50

-5.00

-2.50

0.00

S11[

dB]

-40.00

-30.00

-20.00

-10.00

90

60

30

0

-30

-60

-90

-120

-150

-180

150

120

a) b)

Figure 5. Antenna a) reflection coefficient b) Total realized gain pattern at 7 GHz for φ = 0o and φ = 90o .

Antenna

4. Conclusion

Transferring high resolution images from digestive organs needs wide bandwidth at high frequency, which make UWB is very attractive for this application. A simple structure and small form factor of the implanted UWB loop antenna to meet the requirement for wireless capsule endoscopic has been proposed. The environment differences and as results differences on the permittivity and conductivity of body organs with free space, makes antenna performance evaluation very difficult. Therefore numerous challenges need to be overcome. The High Frequency Structure Simulator (HFSS) has been used to carry out the antenna performance.

5. References

1. IEEE Standard for Local and Metropolitan Area Networks, IEEE 802.15.6-2012 – Part 15.6: Wireless Body Area networks, 2012. 2. K. Yekeh Yazdandoost, K. Sayrafian-Pour, "Channel Model for Body Area Network," IEEE P802.15 Working Group for Wireless Personal Area Networks, IEEE P802.15-08-0780-12-0006, November 2010. 3. J. Ryckaert, C. Desset, A. Fort, M. Badaroglu, V. De Heyn, P. Wanbacq, G. Van der Plas, S. Donnay, B. Van Poucke, and B. Gyselinckx, “Ultra-wide-band transmitter for low-power wireless body area networks: Design and evaluation,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 52, 12, 2005, pp. 2515–2525. 4. Gabriel and S. Gabriel, Compilation of the dielectric properties of body tissues at RF and microwave frequencies, AL/OE-TR-1996-0037, June 1996,http://www.brooks.af.mil/AFRL/HED/hedr/reports/dielectricReport/Report.html. 5. Italian National Research Council, Institute for Applied Physics, “Dielectric properties of body tissues,” http://niremf.ifac.cnr.it.