Broadband Loop Antenna on Soft Contact Lens for Wireless Ocular Physiological Monitoring

1 Department of Electrical Engineering, National Cheng Kung University, Tainan City 701, Taiwan 2Department of Electronic Engineering, Southern Taiwan University of Science and Technology, Tainan City 71005, Taiwan 3Department of Electronic and Computer Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan 4 Institute of Medical Science and Technology, National Sun Yat-sen University, Kaohsiung, 804, Taiwan


Introduction
Recently, monitoring various physiological parameters with biological telemetry system has become an important research topic in wireless monitoring control.To accomplish the wireless monitoring control, the antennas implanted in human body are used to establish communication links between medical sensing devices and external instruments for short-distance biotelemetry.Eye ball monitoring system that is an application of biological telemetry system has also been studied recently, such as monitoring intraocular pressure monitoring systems [1,2], wireless powered micro-LED display systems [3], and glucose-monitoring systems [4].This biotelemetry system can transfer the physiological data into signal and transmit the signal to the external receiving instrument through the antenna on the contact lens.These instruments can share valuable data through the telephone system or the Internet.Through the wireless instruments, the detected physiological data can be sent regularly to the medical institution.An antenna is a key component in the biotelemetry medical systems to transfer available data.However, the antennas used in human body are difficult to accomplish for interference and size limits.This paper presents a novel loop antenna design for WOPM applications.

Antenna Design
2.1.Antenna Geometry.Using the Ansoft simulation software high-frequency structure simulator (HFSSTM), a novel broadband antenna was designed for the biotelemetry applications (ISM band 2.4 and 5.8 GHz).The geometry of the proposed antenna shown in Figure 1(a) is fabricated on the parylene C substrate with thickness of 11 m and permittivity of 4. Figure 1(b) shows the photograph of proposed antenna with minicable.The proposed antenna is connected on a soft contact lens with conductive epoxy glue.The overall dimension of loop antenna is  × 6.5 × 6.5 mm 2 with width of the 1 mm (W1).The parameters of the antenna are listed in Table 1.

Comparison of Permittivity and Conductivity between
Human's and Pig's Eye.In experimental operation, pig's eye must be used to replace human's eye.Therefore, the parameters of their tissue must be verified and studied.Figure 2 presents the measurement environment of the minced pig's eye tissue on an Agilent 85070E dielectric probe kit and an 8753E network analyzer.Figure 3 shows the comparison results of permittivity and conductivity against frequency between the test tissue (pig's eye) and the reference tissue (human's eye) [5].The results indicate that the characteristics of pig's eye are similar to human's eye and can be used to replace human's eye.

Results and Discussion
The simulated results of the proposed antenna are obtained by high frequency structure simulator (HFSS) [6] and          small variations and close to 50 Ω impedance matching.The property of small impedance variations also creates broadband characteristic and covers 2.4 and 5.8 GHz for ISM band. Figure 6 shows the simulated S 11 for different loop sizes of the proposed antenna.From the simulated results, smaller loop excites higher resonant modes, which meet the theory of resonance.Another feature of the proposed design is that the resonant modes are tunable.Figure 7 shows the surface current distributions of the proposed antenna at 2.4 and 5.8 GHz.In Figure 7(a), two zeroes and three poles on the loop form a 1  resonance.At the higher mode (Figure 7(b)), five zeroes and four poles create a 1.5  resonance.To sum up, the proposed design excites two resonant modes to obtain broad operation for ISM band that are verified.Figures 8(a) and 8(b) present the measured antenna 2D radiation pattern.In Y-Z plane, the pattern shows near omnidirectional pattern at 2.4 GHz.At higher band at 5.8 GHz, the patterns are twist and have more zeros.Figure 9 shows the measured antenna radiation gain and efficiency.At 2.4 and 5.8 GHz, the antenna gains are stable and close to 0 dBi.Besides, the radiation efficiency is also stable and close to 1% at that band.Because the human body is a lossy dielectric material, the efficiency of the antenna is always small [7].In Figures 10(a) and 10(b), the simulated 10 g averaged SAR distributions at ISM band (2.4 and 5.8 GHz) are obtained from the proposed antenna attached on eye tissue.

Conclusions
This work presents a novel broadband loop antenna design (optimized bandwidth of 4.46 GHz) for wireless ocular physiological monitoring applications at ISM band (2.4 and 5.8 GHz).The proposed antenna has advantages of simple geometry and easy fabrication.Another feature of this design is that the resonant modes can be changed by varying the loop size.The simulated and measured results are also suitable for wireless ocular physiological monitoring applications.

Figure 1 :
Figure 1: Geometry of the proposed antenna.(a) Substrate model.(b) Photograph of a fabricated antenna with minicable.

Figure 3 : 4 Figure 4 :Figure 5 :
Figure 3: Comparing the permittivity and conductivity against frequency of the test tissue (pig's eye) and the reference tissue (human's eye).

Figure 6 :
Figure 6: Simulated S 11 for different loop sizes of the proposed antenna.

Table 1 :
Parameters of the proposed antenna.