2D Colloidal Micropatterning for Fabrication of Pressure-Responsive Smart Films

Abstract

Organized colloids characterized by crystal structures have received special attention in photonic crystals, photovoltaic devices, and colorimetric sensors. In the case of 2D colloidal crystals, applications have extended to antifounling or adhesive coatings, displays and sensors as an effective surface treatment. 2D colloidal crystals are represented by self-assembled monolayers with hexagonal close packing or non-close packing, however, more complex patterning has been required for the various purpose of studies. In the template-assisted colloidal patterning methods, the particles are rapidly guided to pre-designed pattern by rubbing on template. It is more stable than self-assembled pattern and allows wide applicability. In this study, 2D micropatterning with functional colloids was developed to cut-and-paste pressure sensors, chemiresistive glucose sensors, and pressure-mediated adhesive patches. We first introduce a colloidal pressure sensing platform that can convert the external pressure into electrical signal by patterning the conductive elastomer microparticles between electrode pairs. The pressure can be quantified by precisely measuring current change caused when the contact area between the particles and the electrodes expands by external pressure. As the unit of pixelated pressure sensor, uniform polyurethane microparticles were prepared by using microfluidics techniques for elastic cores, and subsequently layered with carbon nanotubes to give conductivity. The particles were easily arranged in holes-patterned flexible film and developed as cut-and-paste pressure sensor platform. We have demonstrated that this new type of pressure sensing platform has high sensitivity, repeatability and applicability to flexible or stretchable e-skin sensors. We also reported a chemiresistive glucose sensor platform obtained by 2D micropatterning of glucose-conjugative, silver nanowires-deposited conductive microparticles. In the presence of glucose, silver nanowires were adhered to each other by conjugation of glucose and mercaptophenylboronic acid on both sides increasing conductivity of particles. To maximize the adhesion effect, we applied constant pressure to particles and induced the separation of silver nanowires. As the particles are deformed, separated silver nanowires were observed due to increased curvature. We observed that current change of arranged particles exactly correlated with the glucose concentration. Finally, we experimentally demonstrated that the particle-based chemiresistive sensors could detect glucose in a wide range of concentration from 0.56 uM to 56mM which includes that of blood and body fluids. Finally, we analyzed the adhesion mechanism of ivy plants and proposed pressure-mediated adhesive patch inspired from the ivy. Adhesive disc of ivy achieves initial adhesion by secretion of mucilages containing catecholic compounds and slowly solidified by crosslinking the adhered layer with inorganic species such as Fe and Ca. Inspired from the chemical and structural features of ivy disc, we prepared adhesive patch employing alginate containing microgels, DOPA-C7 nanoassemblies, and cylindrical holes-patterned film. Since the soft microgels are protruding from the film, adhesivity is activated by a little pressure and amplified by complex crosslinking of components. The mimetic patch could attach to various surfaces including metal, inorganic, and polymeric substrates. Furthermore, subsequent solidification was simulated by mineralization of Ca2+ ions which enhanced fracture stress and modulus of adhesion layer. This strategy will be further applicable for bioadhesives with diverse functionalities such as self-healing, environment-responsiveness.