Design and development of implantable CPW fed monopole L-Slot antenna at 2.45 GHz ISM band for biomedical applications

1 Introduction

At the moment, to carry out the diagnostic and therapeutic functions in medicine, implantable devices have become more predominant Ashok Kumar & Shanmuganantham 2014. They are more favorable while being able to reach out wirelessly. To blow away the issues with rate of transmission, range of communication, sensitivities within the coils, etc., Tutku Karacolak, et al., 2010; & Chih-Yu Huang, 2011, biotelemetries empowered with antenna is earning more importance. These antennas, since had to be implanted to the human, size should be small. There shouldn’t be any trade-offs in data rate and patient safety. There are some frequency bands like MICS, ISM, etc., that have been approved for medical implant communications.

Amidst those, Medical Implant Communication Service [MICS] ranging within the frequencies of 402 MHz to 405 MHz is most prevalently used for medical implant communication Jasik 1961, while Industrial, Scientific and Medical [ISM] band ranging within 2.4 GHz to 2.48 GHz is additionally used Kiourti 2010. Size is another important touchstone for implantable antennas. A few antennas like Circular Inder Bahl 2003, Square Stacked PIFAs [Planar Inverted F Antennas] Fu-Jhuan Huang et al., 2011, Simplified MICS Half Wavelength Dipole Antennas and MICS Magnetic-Type Loop Antennas Electronic Communications Committee (ECC) 2010 have also been reported for implantation on the human skull but have hardly concentrated in this aspect. Antennas with high resonance frequency provide us with a couple of advantages allied to reduced size and high bit rate transmission, Implantable H shaped slot cavity antennas are preferred for 2.45 GHz applications Ashok Kumar, 2014; Karacolak 2008; antennas working in both ISM and MICS bands are available, but, none of them were analyzed with biocompatible materials. They lack flexibility except the cardiovascular stent that works at 2.45 GHz and was implanted in a live porcine Wong, 2002.

Previously, few analysis regarding home supervision and short range communication were made based on the data available in internet Merli & Skrivervik, 2010. This helped in designing a low power blood pressure monitoring wireless system where an adjustable head set was used to control the device Sani et al., 2010. The contemporary being the use of cloud computing, as immense data are stored using smart phones and the condition of the patient can be monitored from anywhere in the world. Another wearable architecture that uses a wearable body sensor network to support a cuffless blood pressure monitoring over a protracted period Ashok Kumar & Shanmuganantham, 2014.

In this paper, monopole L-slot implantable antenna, operating at 2.45 GHz ISM band for biomedical applications. To be superior enough while implanted, it is set in Al₂O₃ support with reflection coefficient simulation on a human tissue equivalent at 2.45 GHz. Thus the radiation characteristics were reviewed and when tested with various types of tissues, obliged well without any embarrassment.

2 Antenna design

Here we have come up with a CPW fed antenna, constituted with a radiating patch and a bottom patch that reduces the effect of human body on antenna performance. Aspects of the designed antenna for dual band applications at 688 MHz and 2.45 GHz are shown in Fig. 1.

Printed on 25×16 mm² alumina ceramic substrate of 0.5 mm thickness and ɛ_r = 9.8, the antenna is designed with a 50 Ω CPW transmission line with 1 mm signal strip width, L – shaped monopole with slots and 0.5 mm gap between strip and ground plane that is used as a feed.

Antenna’s fabrication and the way in which network analyzer was used to take measurement over the test tissue of dimension 250 mm×250 mm×80 mm at 300 – 3000 MHz are shown in Fig. 2 and Fig. 3.

Figures 3 and 4 shows S11, when antenna is simulated with Mentor Graphics IE3D simulator and while being measured with network analyzer. Being a 3D simulator, the frequency will shift from resonant frequency to lower frequencies when the dimension changes from finite size although described to reproduce at 0.688 GHz and 2.45 GHz respectively. Comparison of the return loss of the antenna while being measured and simulated is presented in Fig. 5. On being fabricated and measured, the antenna ensures that the required bandwidth with –10 dB return loss the range (660–735 MHz) and (2290 – 2490 MHz) are covered.

Testing inside phantoms are relatively easy and practical to implement. The fabricated prototype is immersed inside a tissue phantom and measured. For validation purposes, the same scenario as that of the numerical simulations has to be considered. Canonically-shaped phantoms have so far been used for testing 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 Ashok Kumar & Shanmuganantham, 2014. Testing inside animal tissue can be performed by implanting antenna inside tissue samples from donor animals, or by surgically implanting the antenna inside live model animals. In the first case, electrical properties of the test tissue can be measured using a dielectric probe kit and a network analyzer. The use of animal tissue samples provides an easy approach to mimic the frequency dependency characteristics of the electrical properties of tissues. This can prove highly advantageous when carrying out measurements for multi-band implantable antennas. In the literature, an implantable patch antenna with dual resonances at 380 and 440 MHz was tested inside test tissue obtained by grinding the front leg of a pig Merli, & Skrivervik, 2010. The electrical properties of the adapted pork were found to be between those of human skin and muscle in the ISM band.

When placed inside liquid simulating muscle fat and directed towards gel’s surface along z-direction and at 10 mm from its surface along x-y direction, the radiation characteristics are persuaded in terms of gain and radiation pattern.

With reference distance of a meter and 1 W input power at 2.45 GHz, the radiation pattern is computed in E and H planes as shown in Fig. 6 and Fig. 7. We have obtained very low radiation efficiencies of 0.11 % and 0.32 % as we have nested the antenna in a lossy human tissue medium. For θ= 0 and φ= 0, maximum gain obtained is –18.7dBi and –15.3dBi.

3 Conclusion

This CPW fed monopole implantable slot antenna of 25 mm×16 mm×0.5 mm size is proposed for biomedical applications. Biocompatibility, energy and size reduction are some characteristics that the design affirms. Energy management really increases the life of this implantable antenna since it rises up only when any notice is sent.

There are many identical designs available for implantable antennas, since the proposed model is designed with alumina ceramic substrate that has good ɛ_r, the antenna provides us with low return loss, better VSWR, impedance matching and gain [Table 1]. Hence, this antenna suits biomedical applications at 2.45 GHz.