Fiber-Integrated Force Sensor using 3D Printed Spring-Composed Fabry-Perot Cavities with a High Precision Down to Tens of Piconewton.

Developing microscale sensors capable of force measurements down to the scale of piconewtons is of fundamental importance for a wide range of applications. To date, advanced instrumentations such as atomic force microscopes and other specifically developed micro/nano-electromechanical systems face challenges such as high cost, complex detection systems and poor electromagnetic compatibility. Here, it presents the unprecedented design and 3D printing of general fiber-integrated force sensors using spring-composed Fabry-Perot cavities. It calibrates these microscale devices employing varied-diameter µ $\umu$ m-scale silica particles as standard weights. The force sensitivity and resolution reach values of (0.436 ± 0.007) nmnN-1 and (40.0 ± 0.7) pN, respectively, which are the best resolutions among all fiber-based nanomechanical probes so far. It also measured the non-linear airflow force distributions produced from a nozzle with an orifice of 2 µ $\umu$ m, which matches well with the full-sized simulations. With further customization of their geometries and materials, it anticipates the easy-to-use force probe can well extend to many other applications such as air/fluidic turbulences sensing, micro-manipulations, and biological sensing.

A Generalized Polymer Precursor Ink Design for 3D Printing of Functional Metal Oxides.

Three-dimensional-structured metal oxides have myriad applications for optoelectronic devices. Comparing to conventional lithography-based manufacturing methods which face significant challenges for 3D device architectures, additive manufacturing approaches such as direct ink writing offer convenient, on-demand manufacturing of 3D oxides with high resolutions down to sub-micrometer scales. However, the lack of a universal ink design strategy greatly limits the choices of printable oxides. Here, a universal, facile synthetic strategy is developed for direct ink writable polymer precursor inks based on metal-polymer coordination effect. Specifically, polyethyleneimine functionalized by ethylenediaminetetraacetic acid is employed as the polymer matrix for adsorbing targeted metal ions. Next, glucose is introduced as a crosslinker for endowing the polymer precursor inks with a thermosetting property required for 3D printing via the Maillard reaction. For demonstrations, binary (i.e., ZnO, CuO, In2O3, Ga2O3, TiO2, and Y2O3) and ternary metal oxides (i.e., BaTiO3 and SrTiO3) are printed into 3D architectures with sub-micrometer resolution by extruding the inks through ultrafine nozzles. Upon thermal crosslinking and pyrolysis, the 3D microarchitectures with woodpile geometries exhibit strong light-matter coupling in the mid-infrared region. The design strategy for printable inks opens a new pathway toward 3D-printed optoelectronic devices based on functional oxides.

Granular Ionogel Particle Inks for 3D Printed Tough and Stretchable Ionotronics.

Ionogels have garnered great attention as promising soft conducting materials for the fabrication of flexible energy storage devices, soft actuators, and ionotronics. However, the leakage of the ionic liquids, weak mechanical strength, and poor manufacturability have greatly limited their reliability and applications. Here, we propose a new ionogel synthesis strategy by utilizing granular zwitterionic microparticles to stabilize ionic liquids. The ionic liquids swell the microparticles and physically crosslink microparticles via either electronic interaction or hydrogen bonding. Further introducing a photocurable acrylic monomer enables the fabrication of double-network (DN) ionogels with high stretchability (>600%) and ultrahigh toughness (fracture energy > 10 kJ/m2). The synthesized ionogels exhibit a wide working temperature of -60 to 90 °C. By tuning the crosslinking density of microparticles and physical crosslinking strength of ionogels, we synthesize DN ionogel inks and print them into three-dimensional (3D) motifs. Several ionogel-based ionotronics are 3D printed as demonstrations, including strain gauges, humidity sensors, and ionic skins made of capacitive touch sensor arrays. Via covalently linking ionogels with silicone elastomers, we integrate the ionogel sensors onto pneumatic soft actuators and demonstrate their capacities in sensing large deformation. As our last demonstration, multimaterial direct ink writing is harnessed to fabricate highly stretchable and durable alternating-current electroluminescent devices with arbitrary structures. Our printable granular ionogel ink represents a versatile platform for the future manufacturing of ionotronics.

Colloidal oxide nanoparticle inks for micrometer-resolution additive manufacturing of three-dimensional gas sensors.

Micrometer-resolution 3D printing of functional oxides is of growing importance for the fabrication of micro-electromechanical systems (MEMSs) with customized 3D geometries. Compared to conventional microfabrication methods, additive manufacturing presents new opportunities for the low-cost, energy-saving, high-precision, and rapid manufacturing of electronics with complex 3D architectures. Despite these promises, methods for printable oxide inks are often hampered by challenges in achieving the printing resolution required by today's MEMS electronics and integration capabilities with various other electronic components. Here, a novel, facile ink design strategy is presented to overcome these challenges. Specifically, we first prepare a high-solid loading (∼78 wt%) colloidal suspension that contains polyethyleneimine (PEI)-coated stannic dioxide (SnO2) nanoparticles, followed by PEI desorption that is induced by nitric acid (HNO3) titration to optimize the rheological properties of the printable inks. Our achieved ∼3-5 µm printing resolution is at least an order of magnitude higher than those of other printed oxide studies employing nanoparticle ink-based printing methods demonstrated previously. Finally, various SnO2 structures were directly printed on a MEMS-based microelectrode for acetylene detection application. The gas sensitivity measurements reveal that the device performance is strongly dependent on the printed SnO2 structures. Specifically, the 3D structured SnO2 gas sensor exhibits the highest response of ∼ 29.9 to 100 ppm acetylene with the fastest total response time of ∼ 65.8 s. This work presents a general ink formulation and printing strategy for functional oxides, which further provides a pathway for the additive manufacturing of oxide-based MEMSs.