Wrist-wearable bioelectrical impedance analyzer with miniature electrodes for daily obesity management.

Bioelectrical impedance analysis (BIA) is used to analyze human body composition by applying a small alternating current through the body and measuring the impedance. The smaller the electrode of a BIA device, the larger the impedance measurement error due to the contact resistance between the electrode and human skin. Therefore, most commercial BIA devices utilize electrodes that are large enough (i.e., 4 × 1400 mm2) to counteract the contact resistance effect. We propose a novel method of compensating for contact resistance by performing 4-point and 2-point measurements alternately such that body impedance can be accurately estimated even with considerably smaller electrodes (outer electrodes: 68 mm2; inner electrodes: 128 mm2). Additionally, we report the use of a wrist-wearable BIA device with single-finger contact measurement and clinical test results from 203 participants at Seoul St. Mary's Hospital. The correlation coefficient and standard error of estimate of percentage body fat were 0.899 and 3.76%, respectively, in comparison with dual-energy X-ray absorptiometry. This result exceeds the performance level of the commercial upper-body portable body fat analyzer (Omron HBF-306). With a measurement time of 7 s, this sensor technology is expected to provide a new possibility of a wearable bioelectrical impedance analyzer, toward obesity management.

Dynamics of a microliquid prism actuated by electrowetting.

A microliquid prism is a microchannel filled with two immiscible liquids, whose interface acts as a refractive surface. To steer a light beam that constructs optical images, the interface profile or the contact angle is modulated through electrowetting on a dielectric. Accurate, yet agile actuation of the liquid prism critically depends on the understanding of dynamics of the fluid interface. Here we fabricate liquid prisms, visualize the shape evolution of the interface, and theoretically model its dynamics. By comparing the magnitude of capillary forces to those of viscous, inertial and hydrostatic forces, we find that the meniscus motion within submillimetric channels is dominated by the capillary effect. The theoretical predictions for microscale meniscus dynamics are shown to agree well with the experimental measurements. We then discuss the formation of waves in millimetric liquid prisms, which may significantly limit fast and reliable operation of the optofluidic device.

Improvement in the breakdown properties of electrowetting using polyelectrolyte ionic solution.

We report the improvement in the breakdown properties of electrowetting using a mixture of poly(acrylic acid) (PAA) polyelectrolyte and a surfactant (Tween 80, TW80). Onset of breakdown was initially determined via visual observation and further verified by investigating impedance phase shift. Breakdown characteristics of the large-molecule ionic solution were compared with those of conventional electrolytes (Na(2)SO(4)) that produce small molecules. Experiments with various conductivities and hydrophobic coatings on a thin silicon nitride dielectric layer (∼500 Å) showed that the breakdown voltage of the PAA-TW80 system was at least two times higher than that of the Na(2)SO(4)-TW80 system. Our results demonstrate that defects in the dielectric and hydrophobic layers are less vulnerable to larger ionic molecules.