![]() ![]() Additionally, the ink composition can be tuned to reduce the sintering temperature to achieve high conductivity by changing the nanoparticle size and nanoparticle capping materials to promote sintering at lower temperatures. ![]() Several methods have been investigated to sinter metallic nanoparticles without damaging the substrate materials including pulsed laser, microwave, chemical, flash sintering, or plasma sintering. ![]() However, directly printed conductors on flexible and stretchable substrates are usually sintered at lower temperatures to preserve the stretchable (polymer) substrate, resulting in lower conductivities and stiffer material properties. Thermal sintering is the most common method to achieve conductivity, with higher temperatures and longer exposure times generally yielding films with higher conductivity. Printed colloidal inks must be sintered to achieve conductivity. Additional ink formulations include dispersed metallic nanowires, carbon nanotubes, or particle free metal–organic decomposition (MOD) inks. Printing methods such as aerosol jet, inkjet, screen printing, and extrusion printing typically use metallic inks composed of colloidal nanoparticles to create functional devices. Printing methods offer an attractive alternative to expensive lithographic techniques typically used for the fabrication of stretchable devices due to the low cost of operation, rapid prototyping, and an ability to scale fabrication to a roll-to-roll process. In addition to epidermal devices, prototype stretchable electronics have been developed to wirelessly monitor aneurysms in arteries. Epidermal electronic devices have been investigated widely, with recent advancements including a variety of sensors capable of wireless data and power transfer while conforming and stretching with sensitive skin. Stretchable electronics are increasingly necessary for use in fields such as health monitoring devices, soft robotics, and human machine interfaces due to their elastic modulus match with biological tissue. Finally, stretchable interconnects fully-encapsulated in PDMS polymer, with 3D pillar architectures for external connectivity are demonstrated, thus also solving an important “last-mile” problem in the packaging of stretchable electronics. An experimental step-wise variation of the thermal/atmospheric process conditions supports this hypothesis and shows that the presence of air during a low temperature drying step reduces the capillary stress to produce crack-free interconnects with high conductivities (up to 56% of bulk metal) while having an excellent compatibility with the underlying polymer materials. Additionally, the presence of oxygen promotes the removal of organic surfactants and binders in the nanoparticle ink which increases nanoparticle agglomeration, grain growth, and subsequently conductivity. Capillary forces between nanoparticles developed through rapid solvent evaporation in the colloidal ink is hypothesized to initiate cracking during drying. In this paper, the mechanisms of cracking in nanoparticle-based 3D printed and sintered stretchable interconnects are identified and architecture and processing strategies are demonstrated to achieve crack-free interconnects fully embedded in thin (<100 μm in thickness) stretchable polydimethylsiloxane (PDMS) with external connectivity. However, previous strategies to sinter metallic nanoparticles while preserving the soft polymer substrate are rife with problems such as cracking and low conductivity of the metallic features. Nanoparticle 3D printing and sintering is a promising method to achieve freeform interconnects on compliant substrates for applications such as soft robotics and wearable healthcare devices. ![]()
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