Browsing by Author "Kim, Keekyoung"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    Open Access
    Development 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.
  • Thumbnail Image
    Item
    Open Access
    Development of Rapid, Low-cost, and Portable Device to Detect Infectious Diseases
    (2022-09-22) Lee, Yoonjung; Kim, Keekyoung; Wong, Joanna; Curiel, Laura
    With 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.
  • Thumbnail Image
    Item
    Open Access
    Effects of imperfect bonding on three-phase inclusion-crack interactions
    (2004) Kim, Keekyoung; Sudak, Les Jozev
    The 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.
  • Thumbnail Image
    Item
    Open Access
    Enhanced 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 Brent
    Three-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.
  • Thumbnail Image
    Item
    Open Access
    Rapid and Highly Controlled Generation of Multiple Emulsions via a Hybrid Microfluidic Device
    (2022-01) Azarmanesh, Milad; Mohamad, Abdulmajeed; Sanati Nezhad, Amir; Hejazi, Hossein; Kim, Keekyoung
    Multiple 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.