Ultra-thin IDE Pulse Wave Sensor

The monitoring of vital signs is used to determine human health status. Healthcare monitoring devices are usually attached to the human skin to obtain information about the human body. However, the main inconvenience of using conventional electronic devices is the mechanical mismatch between the devices and the skin. This issue can lead to measurement errors, and patient comfort can be affected negatively when these devices are used continuously. Therefore, it is needed to develop skin-conformal electronic devices to overcome these drawbacks. This thesis explores the fabrication process of ultrathin interdigitated pulse wave sensors based on the piezoelectric effect. The aim of this research is to demonstrate that printed electronics technologies are an excellent alternative to fabricate low-cost skin-conformal sensors. First, this thesis explores the theoretical background of piezoelectricity, flexible and ultrathin piezoelectric pressure sensors, and printed electronics technologies. Then, the fabrication process is analyzed. The sensor is fabricated onto a Parylene-C substrate using the piezoelectric polymer P(VDF-TrF) and the conductive polymer PEDOT:PSS. Preliminary experiments are done to determine substrate wettability and to characterize the electrical properties of the conductive ink. A substrate surface treatment is used to modify the wetting properties of the substrate. The effect of the surface treatment exposure time is evaluated by measuring the width of printed lines. The experiment results are used to evaluate the sensor structure printing process. IDE structure is fabricated by inkjet printing, and the piezoelectric layer is screen printed on top of the electrodes. Electrical properties and piezoelectric sensitivity of the final samples are characterized. The results of this research show that the ink and substrate properties have an impact on the performance of the printed structures. The surface energy of the substrate is modified to improve its wettability. Thus, UV/O₃ surface treatment can be used to make Parylene-C hydro-philic. Furthermore, the IDE structure can be fabricated by inkjet printing technology. However, the coffeering effect is observed in narrow PEDOT:PSS inkjet printed lines (i.e. IDE fingers). This may have an impact on the conductivity of the lines due to the non-uniform distribution of the material. On the other hand, the validation of the piezoelectric sensitivity characterization suggests that the poling process has to be improved to guarantee the operation of the device as a piezoelectric sensor. The results of this research validate that ultrathin sensors can be fabricated using printed electronics technologies. The overall thickness of the sensors is below 6 µm. In conclusion, further research has to be done to activate properly the piezoelectric properties of the P(VDF-TrFE) material in this sensor configuration

Electrowetting: from basics to applications

Electrowetting has become one of the most widely used tools for manipulating tiny amounts of liquids on surfaces. Applications range from 'lab-on-a-chip' devices to adjustable lenses and new kinds of electronic displays. In the present article, we review the recent progress in this rapidly growing field including both fundamental and applied aspects. We compare the various approaches used to derive the basic electrowetting equation, which has been shown to be very reliable as long as the applied voltage is not too high. We discuss in detail the origin of the electrostatic forces that induce both contact angle reduction and the motion of entire droplets. We examine the limitations of the electrowetting equation and present a variety of recent extensions to the theory that account for distortions of the liquid surface due to local electric fields, for the finite penetration depth of electric fields into the liquid, as well as for finite conductivity effects in the presence of AC voltage. The most prominent failure of the electrowetting equation, namely the saturation of the contact angle at high voltage, is discussed in a separate section. Recent work in this direction indicates that a variety of distinct physical effects¿rather than a unique one¿are responsible for the saturation phenomenon, depending on experimental details. In the presence of suitable electrode patterns or topographic structures on the substrate surface, variations of the contact angle can give rise not only to continuous changes of the droplet shape, but also to discontinuous morphological transitions between distinct liquid morphologies. The dynamics of electrowetting are discussed briefly. Finally, we give an overview of recent work aimed at commercial applications, in particular in the fields of adjustable lenses, display technology, fibre optics, and biotechnology-related microfluidic devices

Actuation Of Droplets Using Transparent Graphene Electrodes For Tunable Lenses And Biomedical Applications

Variable focal length liquid microlenses are the next candidate for a wide variety of applications. Driving mechanism of the liquid lenses can be categorized into mechanical and electrical actuation. Among different actuation mechanisms, EWOD is the most common tool for actuation of the liquid lenses. In this dissertation, we have demonstrated versatile and low-cost miniature liquid lenses with graphene as electrodes. Tunable focal length is achieved by changing both curvature of the droplet using electrowetting on dielectric (EWOD) and applied pressure. Ionic liquid and KCl solution are utilized as lens liquid on the top of a flexible Teflon-coated PDMS/parylene membrane. Transparent and flexible, graphene allows transmission of visible light as well as large deformation of the polymer membrane to achieve requirements for different lens designs and to increase the field of view without damaging of electrodes. Another advantage of graphene compared to non-transparent electrodes is the larger lens aperture. The tunable range for the focal length is between 3 and 7 mm for a droplet with a volume of 3 μL. The visualization of bone marrow dendritic cells is demonstrated by the liquid lens system with a high resolution (more than 456 lp/mm). The Spherical aberration analysis is performed using COMSOL software to investigate the optical properties of the lens under applied voltages and pressure. We propose a prototype of compound eye with specific design of the electrodes using both tunable lenses and tunable supporting membrane. The design has many advantages including large field of view, compact size and fast response time. This work maybe applicable in the development of the next generation of cameras, endoscopes, cell phones on flexible platform. We also proposed here the design and concept of self-powered wireless sensor based on the graphene radio-frequency (RF) components, which are transparent, flexible, and monolithically integrated on biocompatible soft substrate. We show that a quad-ring circuit based on graphene transistors may simultaneously offer sensing and frequency modulation functions. This battery-free and transparent sensors based on newly discovered 2D nanomaterials may benefit versatile wireless sensing and internet-of-things applications, such as smart contact lenses/glasses and microscope slides

Electrical Textile Valves for Paper Microfluidics

This paper describes electrically-activated fluidic valves that operate based on electrowetting through textiles. The valves are fabricated from electrically conductive, insulated, hydrophobic textiles, but the concept can be extended to other porous materials. When the valve is closed, the liquid cannot pass through the hydrophobic textile. Upon application of a potential (in the range of 100–1000 V) between the textile and the liquid, the valve opens and the liquid penetrates the textile. These valves actuate in less than 1 s, require low energy (≈27 µJ per actuation), and work with a variety of aqueous solutions, including those with low surface tension and those containing bioanalytes. They are bistable in function, and are, in a sense, the electrofluidic analog of thyristors. They can be integrated into paper microfluidic devices to make circuits that are capable of controlling liquid, including autonomous fluidic timers and fluidic logic.Chemistry and Chemical Biolog

In-plane anisotropic nanocolumnar films for advanced functional applications

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Water Droplet Impact on Functional Surfaces

The impact and spreading of picolitre-sized water droplets on a substrate is of importance in many applications such as rapid cooling, delayed freezing, crop spraying, and inkjet printing. In this thesis, the effects of substrate chemistry, roughness, hardness, charge, and porosity on such droplet impact are studied. The effect of roughness was investigated through the use of superhydrophobic CF4 plasma fluorinated polybutadiene. Comparison of the maximum spreading ratio and droplet oscillation frequencies with literature models shows that both are found to be lower than theoretically predicted. Further study of the effect of multiple types of surface topography was carried out via the CF4 plasma texturing of honeycomb surfaces, leading to hierarchical surfaces with roughness on two length scales. This led to the discovery that surfaces with similar static contact angles can give rise to different droplet impact dynamics, governed by the underlying surface topography. The effect of the mechanical properties of the substrate upon picolitre droplets can be important in microfluidics. The oscillatory dynamics of picolitre droplets following impact were found to depend upon the thickness and elasticity of the substrate. Higher oscillation frequencies are measured for softer and thicker films, which are correlated to larger surface deformations around the contact line. Static buildup during inkjet printing is known to affect print quality. The role of surface charge on picolitre droplet impact onto polymer substrates is found to give rise to increased droplet impact velocities. Higher surface potentials can result in unexpected behaviour such as droplet bouncing or increased contact area diameters leading to a decrease in print resolution. Printing on porous materials is important as porosity can aid ink adhesion and durability. CF4 plasma fluorination of porous membranes can inhibit droplet spreading laterally over a surface, with little change in the imbibition behaviour in the material, leading to printing that is more highly defined. These hydrophobic membranes remain oleophilic and could also find use in oil–water separation. Similarly, a hydrophilic–oleophobic switching surface can be beneficial in a range of applications such as anti-fogging, self-cleaning, and oil– water separation. Polelectroyle–fluorosurfactant complexes were found to exhibit excellent switching, resulting in a surface that quickly becomes hydrophilic whilst remaining oleophobic

Development of A Hydrophobicity Controlled Microfluidic Dispenser

Ph.DDOCTOR OF PHILOSOPH

Concepts for Designing Tailored Thin Film Surfaces with Potential Biological Applications

The tailoring of surfaces with nanostructured thin films, where interfacial properties can be controlled at the nanoscale, offers multiple possibilities for biological applications. The design of thin films with appropriate properties to induce desired biological responses at cell level requires the convergence of research from physics, chemistry, material science, biology, and medicine. Here, we will discuss the main surface properties that determine the behavior of isolated cells, cell colonies, and tissues interacting with a material. Surface roughness, morphological features, stiffness, wettability, chemical nature, and protein-surface interaction characteristics, as well as spatiotemporal heterogeneities, are expected to contribute to the desired biological performance of a material. A brief review in relation to thin films for biological applications will be presented. We will focus on examples in which basic rather simple processes play a key role in determining the triggering of a particular biological cell phenotype

Nature inspired surface/interface engineering towards advanced device applications

Nature inspired surface/interface with multi-faceted functions possess promises in the frontier engineering applications in flexible electronics, energy harvesting, autonomous systems, bio-mimicking tissues, micro-fluidics, etc. Understanding the relationship between nature’s architecture and underlying science could bring enabling solutions to overcome the engineering challenges. A nature inspired surface with smart resilient features provides intrinsic complexity and their multiplicity under different stimuli, i.e. chemical, physical, electronic, mechanical and (in some cases) biological properties. By mimicking/harvesting a variety of surface and interfacial features from nature, the final composition will display an integrative design to provide further explorations in deciphering the hidden physics towards advanced device applications in real world. Specifically, we bring a few engineering examples with chemical/physical approaches to construct artificial nano/micro-structured surface, yield various functional surface for different application scenarios. • A porous layer has been realised to provide controllable generation of microarchitecture to exhibit an anti-corrosion behaviour under UV exposure with multifaceted characteristics such as profound solar absorptivity, thermal emissivity. By further treating the surface with silane, a hybrid layer has been established with superhydrophobic and anti-icing features which shares innate interests in thermal transport/aero-space engineering. • The structural conformation/ elastic instabilities of the surface are exploited to devise an extreme switchable configuration to develop a morphing strategy for switchable lipophilic/oleophobic properties. The geometrical shift of soft structure is instructed to create a steady transition of surface topology rendering a unique switchable transition that are widely inspired in sub-sea/offshore engineering for oil and water separation. • We also develop a highly-replenishable thermal energy harvesting technology via a dynamical elasto-bouncing process of polymeric hydrogel to translate the thermal energy into useful elasto-kinetic energy, then further converted into electrical energy via a simple piezo-material based system, which paves way for a future portable and conformable energy harvesting tool in the regions of extreme geo-thermal residencies and industries. • Using a one drop filling technique along with interfacial pinning points between hydrophilic and hydrophobic, a unique microfluidic approach is presented to create heterogenous structures. By exploiting the communication between swelling mismatch of different functional groups, driven via in-plane and through thickness heterogeity, a highly complex 3D soft reconfiguration is achieved which is activated by stimulation inputs. • The theoretical understandings are exploited in the above applied engineering scenarios, such as elastic mechanics, morphing structure, surface/interface interactions and kinetics of of the polymer systems experienced on a hot surface, which offers further insights into the elastic recoiling evolution and tunability of the system for effective energy translation efficiency. We hope above approaches shed more lights on the nature inspired structure in device engineering, thus, advance the knowledge in the frontier science