Seokbeom Kim

Multifunctional hydrogel nano-probes for atomic force microscopy

Since the invention of the atomic force microscope (AFM) three decades ago, there have been numerous advances in its measurement capabilities. Curiously, throughout these developments, the fundamental nature of the force-sensing probe—the key actuating element—has remained largely unchanged. It is produced by long-established microfabrication etching strategies and typically composed of silicon-based materials. Here, we report a new class of photopolymerizable hydrogel nano-probes that are produced by bottom-up fabrication with compressible replica moulding. The hydrogel probes demonstrate excellent capabilities for AFM imaging and force measurement applications while enabling programmable, multifunctional capabilities based on compositionally adjustable mechanical properties and facile encapsulation of various nanomaterials. Taken together, the simple, fast and affordable manufacturing route and multifunctional capabilities of hydrogel AFM nano-probes highlight the potential of soft matter mechanical transducers in nanotechnology applications. The fabrication scheme can also be readily utilized to prepare hydrogel cantilevers, including in parallel arrays, for nanomechanical sensor devices.

Stretching and Twisting Sensing With Liquid-Metal Strain Gauges Printed on Silicone Elastomers

This letter reports ultra-stretchable strain gauges based on a liquid metal (eutectic gallium-indium) and a platinum-catalyzed silicone elastomer (EcoFlex). A custom liquid metal printing setup was constructed and operated in the pressure controlled mode to offer high quality printing by eliminating undulation typically encountered in the flow rate control mode via a syringe pump. Printed liquid-metal strain gauges were thoroughly tested under cyclic uniaxial stretching and twisting. We achieved the stretchability of ~700%, which can cover any high strain from human motion. In case of moderate strain of 350%, our liquid-metal strain gauge could be operated more than 4500 cycles without showing any degradation.

Hollow Microtube Resonators via Silicon Self-Assembly toward Subattogram Mass Sensing Applications

Fluidic resonators with integrated microchannels (hollow resonators) are attractive for mass, density, and volume measurements of single micro/nanoparticles and cells, yet their widespread use is limited by the complexity of their fabrication. Here we report a simple and cost-effective approach for fabricating hollow microtube resonators. A prestructured silicon wafer is annealed at high temperature under a controlled atmosphere to form self-assembled buried cavities. The interiors of these cavities are oxidized to produce thin oxide tubes, following which the surrounding silicon material is selectively etched away to suspend the oxide tubes. This simple three-step process easily produces hollow microtube resonators. We report another innovation in the capping glass wafer where we integrate fluidic access channels and getter materials along with residual gas suction channels. Combined together, only five photolithographic steps and one bonding step are required to fabricate vacuum-packaged hollow microtube resonators that exhibit quality factors as high as ∼ 13,000. We take one step further to explore additionally attractive features including the ability to tune the device responsivity, changing the resonator material, and scaling down the resonator size. The resonator wall thickness of ∼ 120 nm and the channel hydraulic diameter of ∼ 60 nm are demonstrated solely by conventional microfabrication approaches. The unique characteristics of this new fabrication process facilitate the widespread use of hollow microtube resonators, their translation between diverse research fields, and the production of commercially viable devices.

Direct-current triboelectricity generation by a sliding Schottky nanocontact on MoS 2 multilayers

The direct conversion of mechanical energy into electricity by nanomaterial-based devices offers potential for green energy harvesting1–3. A conventional triboelectric nanogenerator converts frictional energy into electricity by producing alternating current (a.c.) triboelectricity. However, this approach is limited by low current density and the need for rectification2. Here, we show that continuous direct-current (d.c.) with a maximum density of 106 A m−2 can be directly generated by a sliding Schottky nanocontact without the application of an external voltage. We demonstrate this by sliding a conductive-atomic force microscope tip on a thin film of molybdenum disulfide (MoS2). Finite element simulation reveals that the anomalously high current density can be attributed to the non-equilibrium carrier transport phenomenon enhanced by the strong local electrical field (105−106 V m−2) at the conductive nanoscale tip4. We hypothesize that the charge transport may be induced by electronic excitation under friction, and the nanoscale current−voltage spectra analysis indicates that the rectifying Schottky barrier at the tip–sample interface plays a critical role in efficient d.c. energy harvesting. This concept is scalable when combined with microfabricated or contact surface modified electrodes, which makes it promising for efficient d.c. triboelectricity generation.A large triboelectric direct current can be generated via the nanoscale sliding friction of a conductive-AFM tip on a MoS2 thin film.

Improved Capacitive Pressure Sensors Based on Liquid Alloy and Silicone Elastomer

This letter reports sensitivity-improved capacitive pressure sensors fabricated using liquid alloy and highly compliant silicon elastomer. Multilayer elastomer layers prepared by replica moulding were stacked and bonded to form three isolated disk-shaped cavities whose diameters are 3 or 5 mm. Top and bottom cavities (both are 200-

Micropatterning of Liquid Metal by Dewetting

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Sensitivity-Enhanced

tall) were filled with eutectic gallium–indium to realize compliant parallel electrodes. The cavity in the middle (1-mm tall) was left empty to make it compressed with applied pressure. The overall diameter and height of fabricated pressure sensors are 6 or 7.5 mm and 4 or 6 mm, respectively. After fabricated liquid alloy capacitive pressure sensors were connected with external inductors to construct LC resonant circuits, their resonance frequencies were characterized with applied pressures ranging from 0 to 10 kPa. We found that liquid-alloy capacitive pressure sensors exhibit improved sensitivities over identically sized sensors with solid electrodes.

LC

Although gallium-based liquid metals are attracting growing interest thanks to its potential applications in deformable, flexible electronic devices, challenges in fabrication associated with the high surface tension and oxide skin remain to be overcome. We report a novel fabrication technique for liquid metal circuits using dewetting. An excessively thin liquid film spontaneously shrinks on a substrate to reduce surface free energy. In the case of a thin liquid metal film on a substrate with microgrooves, the oxide on the microgroove wall and the additional viscous resistance delay the shrinkage in the grooves, which separates the liquid volume inside the microgrooves from the external volume. Utilizing this mechanism, we successfully produced 20-

Pressure Sensor for Wireless Bladder Pressure Monitoring

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Fabrication and characterization of hydrogel mems resonators via dynamic mask lithography

-thick conductive lines of eutectic gallium indium (EGaIn) on polydimethylsiloxane. This fabrication technique is simple, fast, and cost-effective and requires no top covering layer. The resultant metal lines show potentially applicable electrical resistance to flexible and stretchable electronic devices. [2017-0082]