Linlin Hou

Adhesive RFID Sensor Patch for Monitoring of Sweat Electrolytes

Wearable digital health devices are dominantly found in rigid form factors such as bracelets and pucks. An adhesive radio-frequency identification (RFID) sensor bandage (patch) is reported, which can be made completely intimate with human skin, a distinct advantage for chronological monitoring of biomarkers in sweat. In this demonstration, a commercial RFID chip is adapted with minimum components to allow potentiometric sensing of solutes in sweat, and surface temperature, as read by an Android smartphone app with 96% accuracy at 50 mM Na+ (in vitro tests). All circuitry is solder-reflow integrated on a standard Cu/polyimide flexible-electronic layer including an antenna, but while also allowing electroplating for simple integration of exotic metals for sensing electrodes. Optional paper microfluidics wick sweat from a sweat porous adhesive allowing flow to the sensor, or the sensor can be directly contacted to the skin. The wearability of the patch has been demonstrated for up to seven days, and includes a protective textile which provides a feel and appearance similar to a standard Band-Aid. Applications include hydration monitoring, but the basic capability is extendable to other mM ionic solutes in sweat (Cl-, K+, Mg2+, NH4+, and Zn2+). The design and fabrication of the patch are provided in full detail, as the basic components could be useful in the design of other wearable sensors.

Electrowetting manipulation of any optical film

Electrowetting manipulation of any optical film is reported. A square channel was constructed with four sidewall electrodes, coated with a hydrophobic dielectric, and filled with saline and oil. In a first experiment a dielectric mirror film was suspended between the oil/saline meniscus. Electrowetting at each sidewall produced a saline contact angle change of 35°<θ<170°. This change in contact angle tilted the mirror and ±105° of laser beam deflection was achieved. A second experiment utilized a Mylar film imprinted with a diffraction grating (625lines∕mm). Electrowetting tilting of the grating was shown to alter the diffraction of the laser beam.

Fabrication and Demonstration of Electrowetting Liquid Lens Arrays

Reported is the fabrication and demonstration of an array of >12 000 switchable liquid microlenses, each ~300 mum in diameter and switchable through plano-concave to plano-convex. Electrowetting is used to modulate the contact angle of an aqueous/oil liquid system over a range of 100deg, resulting in a switchable dioptric range of -360 m-1 to 230 m-1. Compared to previous reports of single 2-6 mm electrowetting lenses, the fabrication process reported herein reduces the individual lenslet size by ~10times. To dose liquids into large arrays of these small liquid lenslets, a scalable self-assembled dosing process was developed. The completed liquid lens array has a fill factor of 50% which can be extended to >80%.

Recent Progress in Arrayed Electrowetting Optics

Electrowetting devices can now be formed in arrays covering thousands of square centimeters of glass. New research is pointing the way toward exciting applications for laser radar, 3D displays, adaptive camouflage, electronic paper, retroreflector communication and lab-on-a-chip.

A full description of a scalable microfabrication process for arrayed electrowetting microprisms

Most electrowetting and liquid crystal optical devices are created by standard planar microfabrication. Arrayed electrowetting microprisms are a newer approach that offers unique performance, but which requires a challenging non-planar microfabrication process. This paper reviews a full description of a scalable fabrication process for an ~1500 element array of ~150 µm size electrowetting microprisms. The description includes creation of high aspect-ratio sidewalls, using a conventional i-line mask aligner to vertically pattern electrodes, conformal hydrophobic dielectric deposition, self-assembled and volume-controlled liquid dosing and module sealing. Also presented is a theoretical model which explores the resolution limits for vertically patterned electrodes. In addition to creating a first-generation fabrication process for arrayed electrowetting microprisms, this work may be further useful to investigators seeking methods of forming 3D arrayed electro-optic, electro-chemical or electro-mechanical devices.

Preparation and analysis of 1-chloronaphthalene for highly refractive electrowetting optics.

High-index oils are critical to the performance of optical electrowetting devices, such as lenses, prisms, and retroreflectors. Herein, the preparation and electrowetting analysis of 1-chloronaphthalene are reported. When 1-chloronaphthalene is mixed with small amounts of an alkane, the following properties can be achieved: refractive index > 1.60, viscosity < 5 cP for rapid switching, operation at <10 V, and +/-45 degrees of electrowetting modulation around the condition for a flat meniscus (90 degrees ).

Flat electrowetting optics and displays

Flat electrowetting optics currently include pixel arrays for displays and prism arrays for beam steering. Electrowetting display pixels utilize a colored oil layer that provides high efficiency control of light transmission or light reflection. Electrowetting microprisms tilt the angle of the meniscus between liquids with different refractive index and thereby cause refraction of a light beam passing through the meniscus. Both of these technologies are projected to provide an order of magnitude increase in raw performance compared to liquid-crystal and other technologies. For example, transmissive electrowetting displays are expected to achieve >80% transmission, which far exceeds the ~8% transmission of a commercial liquid crystal display. Electrowetting microprisms have a clear roadmap leading to greater than +/- 45° of continuous beam steering, which surpasses the few degrees of beam steering achieved with electro-optic phased arrays. However, before widespread commercial application can be achieved, a variety of other challenges, such as low-voltage operation, must be solved. Many of these challenges are engineering problems, not fundamental scientific discoveries, and significant technological progress is expected for flat electrowetting optics.

Experimental Validation of >1 kHz Electrowetting Modulation

Presented here is an experimental measurement of the switching speed one could expect for electrowetting optics. The switching capabilities of various sized elements are studied through basic electrowetting droplet modulation. The relative size of the droplets can be directly related to the dimensions of pixels or prisms used in arrayed electrowetting optics. Experimental results reported herein confirm that scaling down the liquid volume can potentially provide switching speeds of >1 kHz for elements that are <100 mum in size. The factors that determine switching speed are contact line velocity and damping time for the liquid meniscus oscillation.

System-level design of an RFID sweat electrolyte sensor patch.

Wearable digital health devices are dominantly found in rigid form factors such as bracelets and pucks. An adhesive RFID sensor bandage (patch) is reported, which can be made completely intimate with human skin, a distinct advantage for chronological monitoring of biomarkers in sweat. In this demonstration, a commercial RFID chip is adapted with minimum components to allow potentiometric sensing of mM ionic solutes in sweat, and surface temperature, as read by an Android smart-phone app (in-vitro tests).

Electrowetting Micro-prisms and Micro-mirrors

Electrowetting beam steering using prism refraction and mirror reflection is presented. Electrowetting at the sidewalls of a square liquid channel is able to tilt prism or mirror apex angles over +/- 55deg.