Abstract
In recent years, stretchable electronics have garnered considerable attention due to their potential applications in diverse areas such as wearable devices, biomedical engineering, soft robotics, and human-machine interfaces. The development of stretchable materials that can accommodate large deformations while maintaining their electronic properties is crucial for realizing these applications. Although substantial progress has been made in designing conductive polymer composites for stretchable circuits, the inherent incompatibility between conductive fillers and adjacent polymer networks often leads to premature electrical failure in stretchable conductors at strain levels lower than their mechanical ultimate strain. Particularly, the modulus mismatch between flexible conductor traces and rigid electronic components induces additional stresses at the interface during stretching, resulting in interface failure. Therefore, the development of stretchable conductive composites that can effectively integrate with rigid components while maintaining reliable conductivity over a wide range of strains is a pressing need in the field.Gallium-based liquid metals (LM), exhibiting metallic conductivity while maintaining fluidity at room temperature, stand out as a compelling conductive filler material for fabricating stretchable conductive materials. Recent advancements have witnessed the emergence of LM-polymer composites with remarkable stretchability and conductivity. However, the leak of interfacial interactions between the LM fillers and polymer substrates often results in undesired leakage of the pure LM fillers during repeated stretching or deformation. These will compromise the mechanical properties and electrical stability of the stretchable conductors, hindering their practical implementation. Supramolecular polymers have emerged as a compelling platform for the integration of conductive fillers, such as silver nanowires and carbon nanotubes, enabling the development of multifunctional conductive composites strategically engineered for applications in stretchable electronics and wearable devices. The advantageous mechanical and interfacial properties of supramolecular polymers, which arise from the versatile design of non-covalent bonds and the resulting dynamic networks, offer a promising approach to overcoming the aforementioned challenges.
In this research, we designed and fabricated a series of LM-based supramolecular polymeric composites exhibiting exceptional mechanical toughness, high electrical conductivity, and robust interfacial connections with electronic components. Leveraging these characteristics, we demonstrated its application in three-dimensional (3D) conformable electronics. Additionally, we investigated the capability of the composites to serve as electrical interconnects for wearable electronics, including stretchable circuits, near-field communication (NFC) wrists, and bio-information sensors by optimizing the mechanical properties. Specifically, there are three parts in this thesis.
In the first part, we employed a strategic molecular design approach to develop a series of typical linear polymer and small-molecule modulators functionalized with terminal 2-amino-4-hydroxy-6-methylpyrimidine (UPy) motifs. The LM-polymer composites were subsequently prepared through the co-assembly process involving the supramolecular polymer and small-molecule modulators, which facilitated the stabilization of the LM microparticles (LMP) within the resulting composites. Leveraging the synergistic combination of the high mechanical toughness and electrical conductivity inherent to the polymer matrix, the resulting LM-polymer composites have emerged as a promising solution for stretchable electrodes and pressure sensors across a variety of strain conditions.
In the second part, we systematically studied the heterogeneously structured interface in LM-polymer composites. The interface comprises a continuous conductive LM network that contributes to the overall electrical conductivity and surrounding polymer compartments that provide interfacial bonding via dynamic bonds. The reversible and dynamic nature of the interfacial interactions within the composite, both at the surface and interior, enabled the LM-polymer composite to function as a self-solder, the integration and interconnection with commercial silicon-based electronic components through a typical thermal processing method. The stable interconnect can withstand the substantial shear stresses generated during the thermoforming process, enabling the fabrication of integrated 3D electronics with exceptional stability and robustness without the requirement of encapsulation, thereby simplifying the process of component substitution and improving the recyclability of the integrated circuits.
In the third part, through further optimization, the LM-polymer composite materials have demonstrated exceptional mechanical and electrical properties. Not only do these composites exhibit excellent electrical stability during deformation, but they can also reliably interconnect with a variety of substrates compatible with printed circuit board manufacturing processes in a simplified "Press-N-Go" way. The robust and stable interconnects established between the LM-polymer composite materials and electronic components, external devices, as well as textile substrates, have collectively showcased the significant potential of the multifunctional interconnect for applications in the emergent field of wearable electronics.
Overall, we present a comprehensive investigation into the development of stretchable conductive materials by utilizing gallium-based LM and supramolecular polymers. The insights gained from this research offer valuable guidance for the design and fabrication of stretchable electronics, opening up new avenues for the practical realization of wearable devices, soft robotics, and human-machine interfaces.
| Date of Award | 29 Jul 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Xi YAO (Supervisor) |