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1  Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan


This paper reports a novel microfluidic viscometer with an embedded pressure sensor constructed using electrofluidic circuits, which are built by filling ionic liquid into microfluidic channels. The developed sensor-integrated microfluidic viscometer is made of polydimethylsiloxane (PDMS) with transparent electrofluidic circuits, which makes it feasible to real-time monitor samples under tests.  In addition, the entire device is fully disposable to prevent cross contamination between samples, which is desired for biomedical applications.  The electrofluidic circuit pressure sensor also provides great sensing linearity, long-term and thermal stability for reliable measurements. Various microfluidic viscometer designs have been developed in previous studies. However, several drawbacks, including: limitations for Newtonian fluids, limited shear rate ranges, temperature sensitivity, incapability for real-time monitoring, and complicated setup [1-5], make the devices impractical for routine measurements.  In this paper, we develop a fully disposable and optically transparent PDMS microfluidic viscometer for biomedical applications. The device can be used for Newtonian and non-Newtonian fluid measurement under different temperatures and shear rates.  The microfluidic viscometer consists of a glass substrate, a PDMS-made bottom microfluidic layer and top electrofluidic channel layer. A PDMS membrane is sandwiched between the two layers (Figure 1). In the bottom microfluidic layer, the hydrostatic pressure at the upstream is built up during the sample injection due to viscous force. By measuring the hydrostatic pressure at the upstream with known geometry of the channel, sample viscosity can be estimated. To measure the hydrostatic pressure, an integrated electrofluidic pressure sensor is designed in the top layer that provides great long-term and temperature stability [6]. A pressure transduction hole is fabricated near the inlet of the microfluidic channel, which is exploited to transfer hydrostatic pressure from bottom layer to top layer. Once the hydrostatic pressure deforms the membrane through the pressure transduction hole, electrical resistance of an electrofluidic resistor aligned to the hole will be changed due to its cross-sectional area variation.  To precisely measure the resistance change, Wheatstone bridge circuit architecture is exploited. The Wheatstone bridge circuit also provides the sensor great sensing linearity and temperature stability. Consequently, the hydrostatic pressure can be estimated by measuring the electrical voltage across the circuit, and the viscosity can be calculated based on fluid mechanics theories.  Four types of fluids are analyzed in the experiments. Water and glycerol solutions of different concentrations are exploited to demonstrate the viscosity measurement of Newtonian fluids, and the results are shown in Figure 3. To demonstrate shear thinning property of non-Newtonian fluids, 1000 ppm xanthan gum solution is tested (Figure 4). In order to further demonstrate usage of the device for real biological samples, human whole blood with EDTA is tested under different temperatures (Figure 5).  The results demonstrate the advantages of small volume (< 1 ml), disposability, and different temperature operation capability of the developed microfluidic viscometer for practical biomedical applications. We have successfully developed a fully disposable and optically transparent microfluidic viscometer based on ionic liquid electrofluidic circuit. The device can be applied widely in applications where long-term observations and various temperature operations are required.

Keywords: Microfluidic viscometer, Electrofluidic circuit, Wheatstone bridge, Pressure sensor.