Daeshik Kang

A soft, wearable microfluidic device for the capture, storage, and colorimetric sensing of sweat

A soft, skin-mounted microfluidic device captures microliter volumes of sweat and quantitatively measures biochemical markers by colorimetric analysis. Better health? Prepare to sweat Wearable technology is a popular way many people monitor their general health and fitness, tracking heart rate, calories, and steps. Koh et al. now take wearable technology one step further. They have developed and tested a flexible microfluidic device that adheres to human skin. This device collects and analyzes sweat during exercise. Using colorimetric biochemical assays and integrating smartphone image capture analysis, the device detected lactate, glucose, and chloride ion concentrations in sweat as well as sweat pH while stuck to the skin of individuals during a controlled cycling test. Colorimetric readouts showed comparable results to conventional analyses, and the sweat patches remained intact and functional even when used during an outdoor endurance bicycle race. The authors suggest that microfluidic devices could be used during athletic or military training and could be adapted to test other bodily fluids such as tears or saliva. Capabilities in health monitoring enabled by capture and quantitative chemical analysis of sweat could complement, or potentially obviate the need for, approaches based on sporadic assessment of blood samples. Established sweat monitoring technologies use simple fabric swatches and are limited to basic analysis in controlled laboratory or hospital settings. We present a collection of materials and device designs for soft, flexible, and stretchable microfluidic systems, including embodiments that integrate wireless communication electronics, which can intimately and robustly bond to the surface of the skin without chemical and mechanical irritation. This integration defines access points for a small set of sweat glands such that perspiration spontaneously initiates routing of sweat through a microfluidic network and set of reservoirs. Embedded chemical analyses respond in colorimetric fashion to markers such as chloride and hydronium ions, glucose, and lactate. Wireless interfaces to digital image capture hardware serve as a means for quantitation. Human studies demonstrated the functionality of this microfluidic device during fitness cycling in a controlled environment and during long-distance bicycle racing in arid, outdoor conditions. The results include quantitative values for sweat rate, total sweat loss, pH, and concentration of chloride and lactate.

Bioinspired reversible interlocker using regularly arrayed high aspect-ratio polymer fibers.

A reversible interlocker that is inspired by the wing locking device of beetles is presented. It exploits the van der Waals force-assisted binding between high-aspect-ratio polymer fibers. The two-layered interlocker is highly flexible and displays an extremely high shear locking force and easy normal lift-off.

Dramatically Enhanced Mechanosensitivity and Signal-to-Noise Ratio of Nanoscale Crack-Based Sensors: Effect of Crack Depth

The sensitivity of a nanoscale crack-based sensor is enhanced markedly by modulating the crack depth. The crack-depth-propagated sensor exhibits ≈16 000 gauge factor at 2% strain and a superior signal-to-noise ratio of ≈35, which facilitates detection of target signals for voice-pattern recognition.

Shape-Controllable Microlens Arrays via Direct Transfer of Photocurable Polymer Droplets

A simple method is presented to form an array of shape-controllable microlenses by partial photocuring of an UV-curable polymer and direct transfer. Using the transferred lens array, nanoscale metal patterns as small as 130-nm gaps are detected under an optical microscope with a distinguishable resolution.

Diffusion-weighted magnetic resonance imaging in Wernicke's encephalopathy

Objective – To report diffusion‐weighted imaging (DWI) findings and postulate the pathogenic mechanism of Wernickes encephalopathy (WE). Patient – A 47‐year‐old‐woman presented with altered consciousness, ophthalmoplegia, and ataxia. DWI revealed the abnormal signal changes in periaqueductal gray matter, mamillary bodies and bilateral medial thalami. Apparent diffusion coefficient (ADC) map revealed the high signal intensity lesions in bilateral medial thalami, suggestive of vasogenic edema. The abnormal signal intensity lesions disappeared on follow‐up imaging with clinical improvement. Conclusions – Vasogenic edema plays an important role in the pathogenesis of WE and can be reversed by proper management. DWI findings in the early stage of WE may provide useful information about the prognosis.

Ultra-sensitive Pressure sensor based on guided straight mechanical cracks.

Recently, a mechanical crack-based strain sensor with high sensitivity was proposed by producing free cracks via bending metal coated film with a known curvature. To further enhance sensitivity and controllability, a guided crack formation is needed. Herein, we demonstrate such a ultra-sensitive sensor based on the guided formation of straight mechanical cracks. The sensor has patterned holes on the surface of the device, which concentrate the stress near patterned holes leading to generate uniform cracks connecting the holes throughout the surface. We found that such a guided straight crack formation resulted in an exponential dependence of the resistance against the strain, overriding known linear or power law dependences. Consequently, the sensors are highly sensitive to pressure (with a sensitivity of over 1 × 105 at pressures of 8–9.5 kPa range) as well as strain (with a gauge factor of over 2 × 106 at strains of 0–10% range). A new theoretical model for the guided crack system has been suggested to be in a good agreement with experiments. Durability and reproducibility have been also confirmed.

Analysis of Preload-Dependent Reversible Mechanical Interlocking Using Beetle-Inspired Wing Locking Device

We report an analysis of preload-dependent reversible interlocking between regularly arrayed, high aspect ratio (AR) polymer micro- and nanofibers. Such a reversible interlocking is inspired from the wing-locking device of a beetle where densely populated microhairs (termed microtrichia) on the cuticular surface form numerous hair-to-hair contacts to maximize lateral shear adhesion. To mimic this, we fabricate various high AR, vertical micro- and nanopillars on a flexible substrate and investigate the shear locking force with different preloads (0.1-10 N/cm(2)). A simple theoretical model is developed based on the competition between van der Waals (VdW) attraction and deflection forces of pillars, which can explain the preload-dependent maximum deflection, tilting angle, and total shear adhesion force.

Transparent ITO mechanical crack-based pressure and strain sensor

Sensors to detect motion with high precision have been extensively studied in diverse engineering research fields. Among them, transparent devices, which have strong adaptability in various fields such as display panels, have not gained much academic interest. In this study, we present a highly sensitive pressure and strain sensor based on a cracked transparent epilayer, indium-tin oxide (ITO), deposited on a transparent PET substrate. This sensor system, with which we demonstrate how to detect pressure and finger motions, exhibits ultra-sensitivity to strain (gauge factor about 4000 at 2% strain), pressure (sensitivity is about 1.91 kPa−1 at pressures from 30 to 70 kPa), and transparency (up to 89% at a wavelength of 560 nm). Also, durability has been validated over 5000 cycles. The sensor thus boasts broad applications including touchscreens and motion detectors.

Artificial Slanted Nanocilia Array as a Mechanotransducer for Controlling Cell Polarity

We present a method to induce cell directional behavior using slanted nanocilia arrays. NIH-3T3 fibroblasts demonstrated bidirectional polarization in a rectangular arrangement on vertical nanocilia arrays and exhibited a transition from a bidirectional to a unidirectional polarization pattern when the angle of the nanocilia was decreased from 90° to 30°. The slanted nanocilia guided and facilitated spreading by allowing the cells to contact the sidewalls of the nanocilia, and the directional migration of the cells opposed the direction of the slant due to the anisotropic bending stiffness of the slanted nanocilia. Although the cells recognized the underlying anisotropic geometry when the nanocilia were coated with fibronectin, collagen type I, and Matrigel, the cells lost their directionality when the nanocilia were coated with poly-d-lysine and poly-l-lysine. Furthermore, although the cells recognized geometrical anisotropy on fibronectin coatings, pharmacological perturbation of PI3K-Rac signaling hindered the directional elongation of the cells on both the slanted and vertical nanocilia. Furthermore, myosin light chain II was required for the cells to obtain polarized morphologies. These results indicated that the slanted nanocilia array provided anisotropic contact guidance cues to the interacting cells. The polarization of cells was controlled through two steps: the recognition of underlying geometrical anisotropy and the subsequent directional spreading according to the guidance cues.

Battery-free, wireless sensors for full-body pressure and temperature mapping

Battery-free, soft, skin-mounted wireless sensors enable continuous, full-body spatiotemporal mapping of pressure and temperature on human subjects. Feeling the heat under pressure Pressure ulcers, or bedsores, can develop at skin sites overlying bony areas of the body when a patient remains in one position for an extended period. These sores can be difficult to detect in their early stages. To begin to address this, Han et al. developed flexible, adherent sensors that measure skin temperature and pressure in real time. The small sensors use wireless power to communicate with external reader antennas. Data acquired from multiple sensors were used to create full-body pressure and temperature maps, which detected changes in pressure due to adjusting the angle of hospital bed incline and changes in skin temperature during sleep in human participants during proof-of-concept studies. Thin, soft, skin-like sensors capable of precise, continuous measurements of physiological health have broad potential relevance to clinical health care. Use of sensors distributed over a wide area for full-body, spatiotemporal mapping of physiological processes would be a considerable advance for this field. We introduce materials, device designs, wireless power delivery and communication strategies, and overall system architectures for skin-like, battery-free sensors of temperature and pressure that can be used across the entire body. Combined experimental and theoretical investigations of the sensor operation and the modes for wireless addressing define the key features of these systems. Studies with human subjects in clinical sleep laboratories and in adjustable hospital beds demonstrate functionality of the sensors, with potential implications for monitoring of circadian cycles and mitigating risks for pressure-induced skin ulcers.