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Title:High-resolution direct ink writing of soft conductive materials
Author(s):Wang, Chen
Director of Research:Nuzzo, Ralph G
Doctoral Committee Chair(s):Nuzzo, Ralph G
Doctoral Committee Member(s):Braun, Paul V; Chen, Qian; Gewirth, Andrew A; Wang, Hua
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
3D printing
conductive polymers
Abstract:Conductive polymers are important materials that attract increasing attention in diverse fields due to their tunability and easy preparation. The various applications range from tissue-engineering scaffold in the biomedical field to intelligent electronics and energy storage devices. The ability to create 3D complex structures using conductive polymers could render new possibilities to develop miniaturized tissue scaffold, personalized bioelectronics, or electrode materials that have more active sites. Traditional methods, such as drop casting or inkjet printing, rely primarily on printing 2D substrates. The gap between processability in two-dimensional and three-dimensional demands is required for new techniques such as 3D printing. In addition, 3D printing offers high freedom in geometry design and materials adjustment. Moreover, 3D printing is more cost and environmentally friendly compared to lithography-based techniques. However, the use of 3D printing to fabricate conductive polymers is still in its infancy of development, and the choice of printed materials is still limited. Among these 3D printable conductive inks, most inks rely on adding nanosized conductive polymer fillers (carbon materials such as carbon nanotubes, conjugated conductive polymer particles as polypyrrole, or polyaniline) in an insulating printable matrix. These inks have several limitations. The most vital limitation is nanoparticle aggregation at printing tips, which significantly hampered high-resolution printing (<100 μm). High-resolution 3D printing of conductive polymers offers the possibility to discriminate a single cell’s behavior rather than signals from cell clusters. Higher resolution can also increase the effective surface area. In addition to printing resolution limitations, the addition of particles also has limitations in the inhomogeneous distribution of particles within the polymer matrix. The inhomogeneous distribution significantly affects the microscopic conductivity where conductivity varies from high aggregation area to low aggregation area. Despite the demand for high-resolution 3D printable inks, the limited choice of soft materials suitable for 3D printing has dramatically thwarted further application of conductive polymers in different fields. In my thesis, three types of high-resolution 3D printable inks are developed. Central to the design of high-resolution printing is the separation of the printing process from the polymerization process, so printing through nozzles as small as 1μm is possible. The basic design idea is to mix conductive monomer (pyrrole and aniline) into a miscible ink system with ideal rheological properties for direct ink writing extrusion. After printing, the samples are treated with chemical reagents to induce in situ polymerization. The first design is based on a poly 2-hydroxyethyl methacrylate (poly HEMA) entangled network with pyrrole as the conductive monomer. The solvent used is a mixture of water and ethanol with a specifically designed ratio. This ratio allows that poly HEMA chains remain entangled while hydrophobic pyrrole is miscible with hydrophilic poly HEMA. High-resolution 3D printing is demonstrated by smooth extrusion through a one μm nozzle. The after-polymerization printed filaments have superior biocompatibility as even to promote cell attachment and proliferation. Furthermore, the high-resolution filaments offer the possibility to fabricate an electrophysiological platform to record single neuron activities. The as-prepared platform can differentiate action potential and stimulated potential with a >4.0 signal-to-noise ratio. The second design is conductive polymer-elastomer based upon oil-in-water emulsion to overcome the limitation in the first design that requires a saturated water environment. Pyrrole monomer is homogenized with silicone (polydimethylsiloxane) and forms tightly packed micelles in water, stabilized by sodium dodecyl sulfate. The in-situ polymerization of pyrrole is triggered when oxidant ions diffuse through the continuous water phase. This emulsion-based ink has better shape retention than poly HEMA-based hydrogel ink and can work in the open air after fully treated. In addition, the ink is used to prepare multidirectional sensors that can detect compressing, bending, and stretching. The third design is a porous conductive hydrogel system that has no insulating matrix after complete treatment. The system is built upon aniline and phytic acid that can form a self-supportive porous hydrogel system. The initial printing is made possible by incorporating a thermal-sensitive poloxamer (Pluronic F127), which can be removed after the self-supportive polyaniline network forms. In contrast to a polyHEMA-based system, the polyaniline network has a high surface area and porosity while maintaining conductive. The porous nature provides shorter diffusion paths for ions and electrons to reach active sites and opens the possibility to be used as electrode materials.
Issue Date:2021-06-21
Rights Information:Copyright 2021 Chen Wang
Date Available in IDEALS:2022-01-12
Date Deposited:2021-08

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