In this paper we introduce a microfluidic device ultimately to be applied as a wearable sweat sensor. We show proof-of-principle of the microfluidic functions of the device, namely fluid collection and continuous fluid flow pumping. A filter-paper based layer, that eventually will form the interface between the device and the skin, is used to collect the fluid (e.g., sweat) and enter this into the microfluidic device. A controllable evaporation driven pump is used to drive a continuous fluid flow through a microfluidic channel and over a sensing area. The key element of the pump is a micro-porous membrane mounted at the channel outlet, such that a pore array with a regular hexagonal arrangement is realized through which the fluid evaporates, which drives the flow within the channel. The system is completely fabricated on flexible polyethylene terephthalate (PET) foils, which can be the backbone material for flexible electronics applications, such that it is compatible with volume production approaches like Roll-to-Roll technology. The evaporation rate can be controlled by varying the outlet geometry and the temperature. The generated flows are analyzed experimentally using Particle Tracking Velocimetry (PTV). Typical results show that with 1 to 61 pores (diameter = 250 μm, pitch = 500 μm) flow rates of 7.3 × 10-3 to 1.2 × 10-1 μL/min are achieved. When the surface temperature is increased by 9.4 °C, the flow rate is increased by 130 %. The results are theoretically analyzed using an evaporation model that includes an evaporation correction factor. The theoretical and experimental results are in good agreement.Electronic supplementary materialThe online version of this article (doi:10.1007/s10544-015-9948-7) contains supplementary material, which is available to authorized users.
In this work, we present a digital microflow meter operating in the range 30–250 nl min−1 for water. The principle is based on determining the evaporation rate of the liquid via reading the number of wetted pore array structures in a microfluidic system, through which continuous evaporation takes place. A proof-of-principle device of the digital flow meter was designed, fabricated, and tested. The device was built on foil-based technology. In the proof-of-principle experiments, good agreement was found between set flow rates and the evaporation rates estimated from reading the number of wetted pore structures. The measurement range of the digital flow meter can be tuned and extended in a straightforward manner by changing the pore structure of the device.
The surface area of liquid crystal (LC) devices in different applications, such as displays, signage devices and smart windows is becoming larger and larger. The electrical driving of such devices does not pose many problems when working on centimetre sized devices. Different types of driving issues arise when up-scaling such devices to areas of several square meter, due to the fact that the thin-film transparent conductors used in these device exhibit a finite sheet conductivity. One of the resulting issues is the non-uniformity of the electric field across the area of the device and the resulting non-uniform optical behaviour. In this work we present a simulation model that is able to predict the optical and electrical behaviour of large-sized liquid crystal devices with different applied voltage signals, including the power consumption. The simulation results are compared with optical and electrical measurements on a liquid crystal device of 1.5 m by 1.2 m.
Wearable sensors are positioned close to, on, or even inside the human body and measure vital functions such as heart rate, temperature, or even biochemical parameters. These parameters give essential information on the health and well-being of humans, and therefore wearable sensors will find applications in health monitoring, well-being, and sports.Sweat is an interesting and convenient body fluid for wearable sensor applications. The amount of sweat and its composition can be used to detect, for example, dehydration or cystic fibrosis. To enable the continuous and non-invasive monitoring of this body fluid, we have developed a wearable sweat sensor using principles that were inspired by biology. Water transportation in plants is successfully mimicked in a flexible microfluidic system: we realized a system in which (1) liquid can be collected from the skin by an absorbing structure; (2) liquid is transported through a microchannel structure by capillarity; and (3) evaporation through a porous structure at the device outlet drives a continuous and prolonged flow through the channel (by evaporative pumping) [1]. We integrated a pH sensor chip in the device.Our proof-of-concept experiments show that our prototype can be successfully used for continuous sensing [2]. It offers a base platform to integrate heterogonous sensing systems in a flexible and possibly low-cost way not only for sweat sensing but also for other applications such as continuous water quality monitoring or other bio-sensing applications where continuous flow over a sensor is required.
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