The deformation (stretching and compression) of the stretchable sensor can cause its resistance to change. 6.2 3D view with (a) top view and (b) bottom view, and (c) side view of the simulation setup of the floating EBG.
Background
The liquid metal in the stretchable antenna flows in response to the elongation of the elastomer and changes the electrical properties of the antenna (Wang et al., 2015, Cheng and Wu, 2010). This feature in the stretchable antenna allows it to act as a strain sensor, where a change in strain causes a change in resonant frequency (So et al., 2009, Cheng and Wu, 2011).
Problem Statements
However, without proper insulation from the human body, the antenna can emit harmful radiation into the human body. Thus, the EBG, which has in-phase reflection, can be placed under the antenna to effectively isolate the human body from the antenna and improve the radiation performance.
Aim and Objectives
The antenna could respond to different environmental pressures by expanding or contracting its shape. Air cavity is also tactfully included to make the proposed antenna work well as both the antenna and air pressure sensor.
Thesis Outline
The design description, working principle and fabrication procedure will be presented in detail. The design description, working principle and manufacturing procedures of the wearable antenna will be presented in detail.
Introduction of Stretchable Electronics
Types of Stretchable Insulators
Polyurethanes (PUs) are a diverse family of polymers with only one aspect in common - the presence of the urethane group (-NHCO-O-). Due to the two blocks having different polarities and chemical characters, they can be classified into two phases: "soft" and "hard", as shown in Figure 2.2.
Types of Stretchable Conductors
The stretchability of thin films is determined by the material and geometrical properties of the thin films and substrates (Song et al., 2009). Direct deposition of solid thin metal on the elastic substrate can limit its extent due to internal mechanical and chemical incompatibility.
Fabrication Methods for Stretchable Microfluidic Devices
Imprinting liquid metal such as EGAIn with elastomeric molds is a simple patterning technique (Gozen et al., 2014). This ensures strong wetting between the PDMS channel and the liquid metal, but increases the manufacturing process complexity.
Introduction to Stretchable Strain Sensors
Resistive Sensing
The resistance of a conductor is given by 𝑅 = 𝜌𝐿/𝐴, where A is the cross-sectional area, L is the length, and 𝜌 is the electrical resistance of the conductor. As a result, the resistance of the conductor can be increased by increasing the length of the conductor or by decreasing the cross-sectional area of the conductor.
Capacitive Sensing
To monitor respiration and human movements, this capacitive voltage sensor was integrated into a chest strap and wearable arm sleeve, respectively (Atalay et al., 2017). However, it has a smaller sizing factor than a resistive strain sensor due to the decrease in the relative permittivity of the dielectric elastomer during strain (Shintake et al., 2018; Tagarielli et al., 2012).
Introduction of Patch Antenna: Background, Theory and Analysis
For the circular patch, only one degree of freedom can be manipulated, which is the radius of the patch. Most conventional microstrip patch antennas are rigid and cannot be bent, as they are often constructed by etching the copper cladding to form static conductor shapes on the rigid substrates (Alzoubi et al., 2011).
Introduction of Dielectric Resonator Antenna: Background, Theory and Analysis
Solid Dielectrics
The authors in (Leung et al., 2012) showed that the dielectric glass resonators could be used as light covers. A light-emitting diode (LED) was inserted into the air gap of a transparent glass through the ground plane, resulting in a DRA that could be used as a light cover for the home lighting system. A plastic-based superformed DRA was also introduced in (Simeoni et al., 2011, Simeoni et al., 2009), where polyvinyl chloride (PVC) was used as a dielectric material due to its low cost and easy-to-process properties.
Liquid Dielectrics
LDRAs open up a new possibility for antenna design where its physical shape can be changed dynamically, such as changing the volume of the resonator to change the resonant frequency (O'Keefe et al., 2007). LDRA can be used for reconfiguration of pattern (Chen and Wong, 2017b) and polarization (Chen and Wong, 2017a) by controlling the liquid volume in its container. A small air gap may therefore exist and this may result in large discrepancies between simulated and measured results (Junker et al., 1994a, Junker et al., 1995, Junker et al., 1994b).
Introduction of Electromagnetic Bandgap for Wearable Antenna
Copper-based EBG
The use of the conventional non-flexible FR-4 substrate was only to study the performance of the antenna near the human hand. Because it was constructed from a conventional non-flexible FR-4 substrate, it was not suitable for wearable applications. To improve flexibility, a thin flexible substrate such as RO3010 can be used to make a flexible EBG, which makes it fit better on human skin, as shown in Figure 2.16 (Hadarig et al., 2013).
Fabric-based EBG
The integration of fabric EBG with antenna also shows 50% improvement of the bandwidth and 30% reduction in antenna size, compared to the antenna without EBG. The impact of bending and crumpling on the fabric antenna with fabric EBG was found to be very minor (Shakhirul et al., 2014, Hu et al., 2015). On the other hand, another study on the material-based portable antenna shows that the antenna bandwidth and impedance can change with the moisture levels (Shakhirul et al., 2014).
Elastomer-based EBG
The stretchability can be further improved by replacing the conductive textile with AgNWs (Jiang et al., 2017).
Conclusion
Introduction
When the spiral pattern is deformed, the electrical resistance of EGaIn increases due to the reduction of the cross-sectional area. When the sensor is extended, it elongates in the direction of extension and contracts transversely, simultaneously, according to the Poisson effect of the material. The ratio between the change in length, height and width of the sensor is defined by the load, 𝜖 = ∆𝐿/L, so that ∆𝑤 = −𝜈𝜖𝑤 and ∆ℎ = −𝜈𝜖ℎ.
Fabrication of Microfluidic Sensor
Next, the 3D printed microchannel mold, which was made of acrylonitrile butadiene styrene (ABS) material, was placed on cured Ecoflex 00-50, as shown in Figure 3.2 (a), followed by casting a second layer (1.5 mm ) Ecoflex 00-50 to close the microchannel mold, leaving only two tips exposed (shown in Figure 3.2 (b)). Once the entire Ecoflex 00-50 was cured, it was removed from the petri dish (Figure 3.2 (c)) and immersed in an acetone solution at room temperature for 24 hours to soften and melt the microchannel made of ABS until it formed hollow microchannel. designed as shown in Figure 3.2 (d). According to Figure 3.3 (a), the two sensors are then connected with a pair of 1 kΩ reference resistors (A larger.
Experimental Results and Discussion
The sensor output was measured using the NI ELVIS Board II+ (National Instrument, USA) at a sampling rate of 3.5 Hz. The hysteresis is mainly due to the properties of the polymer material (Park et al., 2012). A gait cycle is measured from one heel strike to the next heel strike of the same foot.
Summary
Introduction
The design of the proposed antenna is shown in Figure 4.1 where it is intended to operate at the resonance frequency of 5.8 GHz to miniaturize the antenna size and meet the Industrial, Scientific and Medical (ISM) standards. Due to the charging effect of the air cavity, the effective permittivity decreases, and this causes the resonance frequency to increase. Finally, the length of the feed probe (l2) can be adjusted to improve the impedance matching of the patch antenna.
Device Fabrication
Initially, we considered adding liquid metal to the cavity of the top layer before sealing it with a cover cap. However, during the experiment we found that the liquid metal curled up in the absence of oxide due to its high surface tension (~624 mN/m) (Dickey, 2017, Zrnic and Swatik, 1969). Since PDMS is highly permeable to oxygen, the liquid metal in the cavity quickly forms an oxide layer and causes the liquid metal to adhere to the surfaces of the PDMS walls and stabilize the shape of the liquid metal.
Experiment Results and Discussion
When the ambient pressure decreases (<0 bar), the air pressure in the antenna air cavity increases. Changes in pressure and volume of an air cavity can be described using Boyle's law. By changing the pressure, the volume (deformation) of the air cavity also changes.
Introduction
Dielectric Characterization
An open-ended coaxial probe (Lee et al., 2013) was used to measure the dielectric constant and loss tangent. It can be seen that water has the highest dielectric constant of 72.25 at 2.4 GHz; however, it cannot be used to design an antenna due to its high loss tangent of ~0.13. In addition, the freezing point of acetonitrile is -45ºC, where this characteristic offers the possibility for LDRA to work in cold climate areas without the need to add antifreeze, which is needed for the water antenna (Xing et al., 2015) .
Antenna Configuration and Working Principle
Since the central portion of the dielectric resonator is removed, the actual resonance frequency of an annular DRA is usually higher than that of the cylindrical DRA. Without including the Ecoflex, the simulated frequency of the liquid part is found to be 1.868 GHz. Finally, the impedance matching of the LDRA can be further improved by adjusting the length of the probe (L = 8 mm).
Fabrication Process
The reflection coefficients and input impedances of the LDRA are shown in Figure 5.5 (a) and (b), respectively, which show reasonable agreement. All displacements are shown to the same scale at all points on the surfaces. It is clear that the existence of the air cavity makes the pressure sensitivity of the structure much higher.
Summary
For high ambient pressure (> 0 bar), on the other hand, the internal air pressure is lower than the surrounding ambient pressure, causing the entire structure to be compressed. Without having an air cavity inside the LDRA, as can be seen in Figure 5.9 (c) and (d), the antenna structures are unable to deform when the external pressure is varied. The antenna can now act as both the radiating element and the air pressure sensor, reducing the need for more commercial electronic components.
Introduction
After the simulation is finished, the top surface of the LEBG unit cell is set as the reference plane. In Figure 6.5, the reflection coefficients of the moving slot antenna with the support array LEBG with the number of 3 × 2, 3 × 3 and 4 × 3 are studied, where the frequency is found to fluctuate in the range of 3.3 to 3.4 GHz. This is the optimal design point because the resonant frequency of the slot antenna has the closest operating frequency with the LEBG 0º reflection phase of the 3×2 array.
Fabrication Processes
Filling and curing of Ecoflex for (a) base layer and (b) cover of antenna. c) Bonding the two Ecoflex layers together before injecting EGaIn into the cavity to form a liquid gap antenna. Filling and curing of Ecoflex for (d) base layer, (e) intermediate layer and (f) cover on LEBG. g) Bonding the three Ecoflex layers together before injecting EGaIn into the voids to form a LEBG. Similar technique was used in the manufacturing processes of LEBG as shown in Figure 6.7 (d)-(g).
Experiment Results and Discussion
The operating frequency band of the antenna moved up when the LEBG array was added. Referring to the same figures, the center of the antenna has the most radiation. In the experiment, a 3D printed plastic holder was used to clamp the two ends of the LEBG.
Summary
It has been demonstrated that the insertion of the LEBG structure under the slot antenna can improve the antenna gain from 2.24 to 5.80 dBi by significantly reducing the back lobes. Due to the existence of LEBG, the impedance of the antenna is very stable when applied to the different parts of the human body. The antenna is also shown to radiate well with an average of 5.08 dBi even at high load conditions.
Conclusions
When the ambient pressure changes, the internal air pressure of the antenna will change accordingly, causing the antenna to expand or contract. The deformation of the antenna structure varies under different ambient pressure conditions (-0.4 bar to 0.4 bar). Placing the LEBG under a new antenna design (slot antenna) improved the antenna gain.
Future Works
Dual-function radiating glass for antennas and light shields—part II: Dielectric dual-band glass resonator antennas. Fdtd simulation of the radiation characteristics of half-volume and mode dielectric resonator antennas. In situ x-ray reflectance study of the oxidation kinetics of liquid gallium and liquid alloy.
