Browsing by Author "Kim, Keekyoung"
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- ItemOpen AccessDevelopment of Multifunctional Polymer Nanocomposites with Hybrid Structures for Fabrication of Stretchable Strain Sensing and Wearable Electronic Devices(2020-06-25) Shajari, Shaghayegh; Sudak, Sudak, Leszek Jozef; Sundararaj, Uttandaraman; Kim, Keekyoung; Dalton, Colin; Roberts, Edward P. L.; Naguib, Hani E.Stretchable and flexible electronics have been proposed and practiced as promising alternatives to traditional rigid electronics for the next generation of smart devices in the fields of biomedicine, soft robotics, and energy harvesting. Particularly, the next generation of personal portable devices for remote health assessment requires wearable and attachable smart systems. Examples of these monitoring devices are stretchable and skin-mountable strain sensors for human motion detections, sport activities monitoring, soft robotics, and entertainment technology. Several requirements such as high stretchability, flexibility, a wide working strain ranges, durability, fast response, easy signal collections are considered for wearable sensing systems. Polymer nanocomposites are well established in wearable fashion due to their light weight, flexibility, deformability and easy processing. In this thesis, unique processing techniques were employed to develop novel filler network structures in polymers to improve electrical conductivity and mechanical properties and electromechanical properties and thus strain sensing performances. One dimensional (1D) nanofillers such as carbon nanotubes (CNTs), silver nanowires (AgNWs) and stainless steel fibers (SSFs) were employed due to their effective network connections in polymers. Developing effective filler network structures in polymers even facilitated reducing electrical and strain sensing percolations. In this regard, firstly, a new double percolated network was introduced in CNT combined with fluoroelastomer FKM using an internal melt-mixing process. The two percolation networks provided wide range of low to high stretchability with high sensitivities. High strain sensing of more than 200% with high sensitivity or gauge factor (GF) of greater than 8×(10)^3 was achieved. Very long AgNWs (70-100 µm) with high aspect ratio of >500 and high conductivity of (10)^5 S.cm-1 were synthesized via a modified polyol process. Novel 3D hybrid network structures of AgNWs with CNTs in fluoroelastomer FKM including bridging and shell like structures were developed using optimized solution processing techniques such as solution mixing and layer by layer assembly (LBLA) methods. These hybrid network morphologies led to high conductivity of 2× (10)^5 S. m-1, ultra-high stretchability of up to 300% with ultra-high sensitivity at GF of 2× (10)^6. In LBLA method, only small hybrid nanofiller loadings of 0.88 wt% were employed for ultra-thin sensing elements with less than 10 µm in thickness. Compared to the recent stretchable strain sensors based on polymer nanocomposites reported in the literature, this is the best reported combination of strain sensing performances including stretchability, sensitivity and conductivity in thin film structures and using low filler concentrations. Moreover, the hierarchical hybrid network of SSFs and CNTs in polypropylene (PP) was developed via an internal melt-mixing process. This synergistic network structure in a semicrystalline polymer contributed to the simultaneous enhanced EMI shielding effectiveness (SE) of 57.4 dB and mechanical properties such as strain to failure at 3.5 vol% hybrid filler concentrations. The hybrid nanocomposite with SSFs and CNTs in PP is a good coating candidate to protect the sensor signals by 99.99% from any interference by electromagnetic (EM) waves in the X-band freq. The reliability and usability of wearable sensors made from CNT/FKM nanocomposites was verified for human motion monitoring and as flexible interconnectors for fabricating stretchable light emitting diodes (LEDs) circuits. These inexpensive and simple fabrication techniques satisfy the new demands for cost-effective and high performance flexible and wearable electronics.
- ItemOpen AccessDevelopment of Rapid, Low-cost, and Portable Device to Detect Infectious Diseases(2022-09-22) Lee, Yoonjung; Kim, Keekyoung; Wong, Joanna; Curiel, LauraWith the spread of COVID-19, which started the global pandemic in 2019 and continues to be prevalent these days, the importance of developing effective diagnostic methods to limit the spread of infectious diseases has emerged. The standard method used to diagnose severe acute respiratory syndrome coronavirus 2 is reverse transcriptase polymerase chain reaction (RT-PCR). Still, its disadvantages include high cost, complex equipment, and long diagnostic time. This study developed two loop-mediated isothermal amplification (LAMP) based diagnostic methods (Saliva-Dry LAMP and Direct Dry-LAMP) which are rapid, sensitive, and near-patient to overcome the limitations of RT-PCR. Saliva-Dry LAMP has the advantages of the LAMP method and requires saliva samples using a customized portable all-in-one box. Direct Dry-LAMP has a more rapid detection time with a heat inactivation step instead of RNA extraction, and the customized device can be executed with batteries and the developed application. The development of these devices reduces the capital cost of instruments significantly, and both methods have shown great performances with excellent positive percent agreement and negative percent agreement compared to each reference RT-PCR. Another convection-based device combined with real-time detection was developed to perform Direct Dry-LAMP. Although this device is still in development, it underscores the growing need for a random-access platform with real-time detection. Overall, Saliva-Dry LAMP and Direct Dry-LAMP can provide rapid and accurate detection of COVID-19 with portable and low-cost devices. With more widespread use, both these methods could play a central role in efficiently limiting the spread of infectious diseases, especially in resource-limited regions.
- ItemOpen AccessEffects of imperfect bonding on three-phase inclusion-crack interactions(2004) Kim, Keekyoung; Sudak, Les JozevThe solution for the elastic three-phase circular inclusion problem plays a fundamental role in many practical and theoretical applications. In particular, it offers the fundamental solution for the generalized self-consistent method in the mechanics of composites materials. In this thesis, a general method is presented for evaluating the interaction between a pre-existing radial matrix crack and a three-phase circular inclusion. The bonding at the inclusion-interphase interface is considered to be imperfect with the assumption that the interface imperfections are constant. On the remaining boundary, that being the interphase-matrix interface, the bonding is considered to be perfect. Using complex variable techniques, we derive series representations for the corresponding stress functions inside the inclusion, in the interphase layer and in the surrounding matrix. The governing boundary value problem is then formulated in such a way that these stress distributions simultaneously satisfy the traction free condition along the crack face, the imperfect interface condition and the prescribed asymptotic loading conditions. Stress intensity factor (SIF) calculations are performed at the crack tips for different material property combinations and crack positions. The results illustrate convincingly the role of an interphase layer as well as the effects of an imperfect interface on crack behavior. Moreover, the conclusions reached in this dissertation provide a quantitative description of the interaction problem between a three-phase circular inclusion with interface imperfections and a radial matrix crack.
- ItemOpen AccessElectrokinetic Transport of Silica Nanoparticles in a Biomimetic Porous Medium(2024-04-18) Sreeram, Priyanka; Natale, Giovanniantonio; Benneker, Anne; Kim, Keekyoung; Hejazi, HosseinColloidal particles can be manipulated under an applied external field, one of which is an electric field, to transport them to their desired location. The movement of charged particles or fluids due to an applied electric field is known as electrokinetics. This has widely been used in drug delivery and electrokinetic soil remediation. This phenomenon allows for an easy way to manipulate charged particles. Previous work investigating the application of drug delivery has focused on porous mediums that is homogeneous and physiochemically different from that found in tissues. The electrokinetic and hydrodynamic transport of nanoparticles in biomimetic porous medium has not been studied extensively. In this work, we have incorporated Gelatin methacrylamide (GelMA) hydrogel in a microfluidic chip to explore the transport of silica nanoparticles through the hydrogel. To accomplish this, we studied the transport of silica nanoparticles at varying crosslinker concentration, pressure driven flow and electric field intensity. Our results indicate that by increasing the crosslinker concentration, the pore size gets smaller and the nanoparticle transport is more constrained. Increasing the flow rate increases the distance that the nanoparticles can travel while also increasing the number of aggregates formed. We also explore the transport of silica nanoparticles at varying electric field strengths. We observed that increasing the electric field strength increases the distance travelled by the nanoparticles through the hydrogel and reduces the number of aggregates formed which benefits the transport of the nanoparticle. The experimental results in this thesis open up routes for electrokinetic drug delivery through heterogeneous porous media, and allow for further investigation of the effect of electric fields and more directed drug delivery.
- ItemOpen AccessEnhanced Longitudinal Analysis of Bone Strength Estimated by 3D Bone Imaging and the Finite Element Method(2020-10-06) Plett, Ryan Michael; Boyd, Steven Kyle; Duncan, Neil A.; Manske, Sarah Lynn; Kim, Keekyoung; Edwards, William BrentThree-dimensional (3D) imaging with high-resolution peripheral quantitative computed tomography (HR-pQCT) and micro-finite element (FE) analysis provides important insight into bone health. Longitudinal analyses of bone morphology maximize precision by using 2D slice-matching registration (SM) or 3D rigid-body registration (3DR) to account for repositioning error between scans, however, the compatibility of these techniques with FE for longitudinal bone strength estimates is limited. This work developed and validated a FE approach for longitudinal HR-pQCT studies using 3DR to maximize reproducibility by fully accounting for misalignment between images. Using a standard imaging protocol, ex vivo (N=10) and in vivo (N=40) distal radius HR-pQCT images were acquired to estimate the efficacy of 3DR to reduce longitudinal variability due to repositioning error and assess the sensitivity of this method to detect true changes in bone strength. In our proposed approach, the full common bone volume defined by 3DR for serial scans was used for FE. Standard FE parameters were estimated by no registration (NR), SM, and 3DR. Ex vivo reproducibility was estimated by the least significant change (LSC) in each parameter with a ground truth of zero change in longitudinal estimates. In vivo reproducibility was estimated by the standard deviation of the rate of change (σ) with an ideal value that was minimized to define true changes in longitudinal estimates. Group-wise comparisons of ex vivo and in vivo reproducibility found that FE reproducibility was improved by both SM (CVRMS<0.80%) and 3DR (CVRMS<0.62%) compared to NR (CVRMS~2%), and 3DR was advantageous as repositioning error increased. Although 3D registration did not negate motion artifacts, it played an important role in detecting and reducing variability in FE estimates for longitudinal study designs. Therefore, 3D registration is ideally suited for estimating in vivo effects of interventions in longitudinal studies of bone strength.
- ItemOpen AccessMetal-Organic Framework Reinforced Highly Stretchable and Durable Conductive Hydrogel-Based Triboelectric Nanogenerator for Biomotion Sensing and Wearable Human-Machine Interfaces(Wiley, 2023-07-17) Rahman, Muhammad Toyabur; Rahman, Md Sazzadur; Kumar, Hitendra; Kim, Keekyoung; Kim, SeonghwanFlexible triboelectric nanogenerators (TENGs) with multifunctional sensing capabilities offer an elegant solution to address the growing energy supply challenges for wearable smart electronics. Herein, a highly stretchable and durable electrode for wearable TENG is developed using ZIF-8 as a reinforcing nanofiller in a hydrogel with LiCl electrolyte. ZIF-8 nanocrystals improve the hydrogel's mechanical properties by forming hydrogen bonds with copolymer chains, resulting in 2.7 times greater stretchability than pure hydrogel. The hydrogel electrode is encapsulated by microstructured silicone layers that act as triboelectric materials and prevent water loss from the hydrogel. Optimized ZIF-8-based hydrogel electrodes enhance the output performance of TENG through the dynamic balance of electric double layers (EDLs) during contact electrification. Thus, the as-fabricated TENG delivers an excellent power density of 3.47 Wm–2, which is 3.2 times higher than pure hydrogel-based TENG. The developed TENG can scavenge biomechanical energy even at subzero temperatures to power small electronics and serve as excellent self-powered pressure sensors for human-machine interfaces (HMIs). The nanocomposite hydrogel-based TENG can also function as a wearable biomotion sensor, detecting body movements with high sensitivity. This study demonstrates the significant potential of utilizing ZIF-8 reinforced hydrogel as an electrode for wearable TENGs in energy harvesting and sensor technology.
- ItemEmbargoPiezoelectric Systems for Force Sensing and Tunable Dynamics(2024-04-03) Rezvani, Sina; Park, Simon; Lee, Jihyun; Kim, Keekyoung; Xue, DeyiThe measurement and suppression of vibrations are crucial for modern-day devices and structures. Piezoelectric materials stand out as exceptional for vibration measurement and control due to their rapid response time. Among these materials, lead zirconate titanate (PZT) is particularly favoured for its high piezoelectric coefficients and stiffness. However, the manufacturing process of PZT can be optimized to reduce both time and energy consumption. Designing and implementing measurement systems incorporating PZT elements contributes to cost reduction and increased accuracy. Moreover, piezoelectric materials not only serve for vibration measurement but can also function as elements for active, and semi-active vibration control. Notably, they possess the capability to perform both sensing and actuation functions simultaneously, a technique known as self-sensing actuation. In this study, the hybrid microwave sintering of PZT and PZT/CNT samples is explored, investigating the effect of incorporating carbon nanotubes (CNTs) on microwave absorption. Furthermore, the effect of adding CNTs and adjusting microwave power on microstructures, and mechanical and electrical properties are investigated. A comparative analysis with conventional sintering is also conducted. A thermal modelling based on FEM is introduced to simulate the temperature variations throughout the sintering process. A vise-based force sensor system is introduced for simultaneous measurement of cutting and clamping forces in milling operations. By integrating PZT piezoelectric sensors and strain gauges, the system achieves accurate force measurement in terms of DC and AC components. Experiments are conducted to validate the effectiveness of the setup. For the purpose of vibration suppression and tuning dynamics, a friction damper based on the discrete layer jamming concept is developed, utilizing a piezoelectric actuator to change the normal force. The study conducts an in-depth exploration of the behaviour of piezoelectric friction dampers under various loading conditions. The investigation covers factors such as load stiffness, preload settings, and the damper’s response in free and forced vibrations, including flexural and axial cyclic loads. The potential applications of the damper are also highlighted.
- ItemOpen AccessRapid and Highly Controlled Generation of Multiple Emulsions via a Hybrid Microfluidic Device(2022-01) Azarmanesh, Milad; Mohamad, Abdulmajeed; Sanati Nezhad, Amir; Hejazi, Hossein; Kim, KeekyoungMultiple Emulsions (MEs) consist of droplets containing one or more microdroplets. Microfluidic approaches have been used to create monodisperse MEs in both a rapid and controlled manner. To generate monodisperse ME constructs, the design relies on the interaction between immiscible fluids in subsequent droplet formation steps. The microfluidic chip used to create the MEs consists of three liquid phases flowing through two subsequent compartments, each with a T-junction and a cross-junction. Creating high shear stress at the cross-junctions induces instability of liquid flow at the first junction, which splits the first immiscible phase into micrometer droplets surrounded by the second phase. The resulting structure is then supported by the third phase at the T-junction to generate and transport MEs. In this work, the ME formation within microfluidic chips is experimentally investigated and numerically simulated. Several critical parameters are examined concerning their effects on the physical properties of MEs. Dimensionless modelling is exploited to enable the change of one parameter at a time while assessing the system's sensitivity to that parameter. Following the optimization of ME formation, highly controlled and high-throughput MEs are formed within the microfluidic chips. It is shown that the consecutive MEs are monodisperse in size, allowing for the generation of controlled MEs for various applications, including bacterial and cell culture. Furthermore, polydimethylsiloxane (PDMS) microchannels are fabricated using conventional soft lithography techniques and coated with Tetraethyl Orthosilicate (TEOS) and Ethylamine (EA) to form a nanometer silicon oxide layer inside the microchannels. TEOS coated PDMS chips provide several advantages over bare PDMS chips. The first benefit of the TEOS coating is the prevention of fluid absorption by the PDMS bulk, which is a major challenge for long-term culture of cells in PDMS microchannels. Using the TEOS coating, bacteria successfully grow for several hours inside nanoliter-sized droplets in a controlled microenvironment, wherein the TEOS coating prevents droplets from being absorbed by the PDMS. Further engineering of this coating makes it a switchable coating, where its hydrophilic properties change to hydrophobic upon exposure to the ambient air. This feature provides selective hydrophobicity and hydrophilicity conditions and allows for the formation of diverse double and multiple emulsions. The hydrophilic property of the coating is further used for cell culture. As a proof-of-concept, MCF7 breast cancer cells and endothelium cells are cultured on the TEOS-coated PDMS chips, enabling further high-throughput drug screening on MEs containing multiple cell types and drugs.