Research And Stretchable Sensors Liquid Development Of With Wireless Capability Based

Low, Jen Hahn (2021) Research And Stretchable Sensors Liquid Development Of With Wireless Capability Based. PhD thesis, UTAR.

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Abstract

Sensors are important devices to detect physical parameters and convert them into electrical signals for computers to analyze. However, most of the conventional sensors are non-flexible and do not have wireless capability. The non-flexible of conventional sensors makes it difficult to be attached on nonplanar surfaces. Also, an additional antenna is required to transmit the sensor’s data wirelessly, which increases the overall design footprint. To solve the aforementioned constraints, this thesis has proposed the use of elastomers and liquids for the construction of the soft sensors with wireless capabilities. The sequence of developing the soft sensors with wireless capabilities is presented in 4 parts. In the first part, a stretchable microfluidic sensor is proposed for placing on non-planar surfaces such as human skin. The deformation (stretching and compressing) of the stretchable sensor can cause its resistance to change. The use of highly stretchable elastomer and liquid in the sensor gives good mechanical durability under 100 continuous cycles, and low hysteresis of 5.01% and 4.24% for normal pressure and axial strain mechanical loading, respectively. Due to its high stretchability and good compliance to human skin, it was placed on the shoe insole to detect foot pressure during walking and running. Experimental data shows that this resistive sensor can detect the pressure from human foot by capturing accurately the weightages of the stance and swing phases for a complete gait cycle. Since the sensor does not have wireless capability, an external antenna (Bluetooth) had to be connected to transfer data wirelessly. To integrate wireless functionality into the sensor, in the second and third parts, new types of sensor designs were proposed by modifying the patch antenna and dielectric resonator antenna (DRA), respectively. They were designed by using elastomer and liquid for improving their stretchability. Also, an air cavity is incorporated into the design for air pressure sensing. Numerical simulations were first performed to optimize the antenna designs, then followed by experiments to validate the simulation results. The results are in good agreement with each other. Then, air pressure tests were conducted for these antennas in high and low ambient pressure conditions to study their sensing performance, and the shift in resonant frequencies was observed. It has been observed that the resonant frequency changes linearly with the air pressure. The liquid patch antenna and liquid dielectric resonator antenna (LDRA) show a sensitivity of 176 MHz/bar and 270 MHz/bar, respectively. This proves that the proposed multi-functional antennas can work well as both microwave radiator and air pressure sensor. Here, the design concepts and working principles of the proposed wireless sensors have been successfully proven, and the next step is to implement them as wearable sensors with some useful wireless functionality. One of the most challenging issues of designing such wearable devices is that the radiated electromagnetic (EM) waves from the antenna may be harmful to human body. To alleviate this problem, a periodic-patterned ground iv plane, called electromagnetic bandgap (EBG), is studied for isolating the effects of a wearable antenna. Here, the fourth part, liquid metal is infused into the elastomer platform for designing the proposed liquid EBG (LEBG). A new design, which is the slot antenna, is proposed to be integrated with the LEBG. Experiments show that, after adding the LEBG beneath the slot antenna, the antenna can produce a stable impedance bandwidth when being placed on human body. Higher radiation gain (5.80 dBi) is also seen in the boresight direction, and better specific absorption rate (SAR) performance is achieved with a reduction of about 80% after adding the LEBG. As both the LEBG and antenna are stretchable, they can be stretched up to 30% strain, and the resonant frequency shows linear shifting (3.71 MHz/strain(%)). Finally, a wearable sensor with wireless capability has been successfully demonstrated in the final part of this thesis. The aim of the thesis is achieved where the development of multiple multifunctional antennas that can function as both the radiating element and the sensor are realized. This is different from the commercial electronic components which have only a single function. It can reduce the total footprint of the design. For future works, the antennas can be incorporated with Radio-frequency Identification (RFID) so that the resonant frequencies can be obtained using a mobile RFID reader. Furthermore, the thickness of the sensors can be reduced by using new fabrication techniques.

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