Self-Powered Viscosity and Pressure Sensing in Microfluidic Systems

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Self-Powered Viscosity and Pressure Sensing in Microfluidic Systems based on the Piezoelectric Energy Harvesting of Flowing droplets Zhao Wang, Lun Tan, Xumin Pan, Gao Liu, Yahua He, Wenchao Jin, Meng Li, Yongming Hu, and Haoshuang Gu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08541 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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ACS Applied Materials & Interfaces

Self-Powered Viscosity and Pressure Sensing in Microfluidic Systems based on the Piezoelectric Energy Harvesting of Flowing Droplets Zhao Wang, Lun Tan, Xumin Pan, Gao Liu, Yahua He, Wenchao Jin, Meng Li, Yongming Hu and Haoshuang Gu* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials — Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics & Electronic Sciences, Hubei University, Wuhan, 430062, P.R. China Keywords: Energy harvesting; Piezoelectric; Microfluidic; Self-powered Sensor; PVDF

Abstract: The rapid development of micro-scaled piezoelectric energy harvesters has provided a simple and highly efficient way for building self-powered sensor systems through harvesting the mechanical energy from the ambient environment. In this work, a self-powered microfluidic sensor that can harvest the mechanical energy of the fluid and simultaneously monitor their characteristics was fabricated by integrating the flexible piezoelectric poly(vinylidene fluoride) (PVDF) nanofibers with the well-designed microfluidic chips. Those devices could generate open-circuit high output voltage up to 1.8 V when a droplet of water is flowing past the suspended PVDF nanofibers and result in their periodical deformations. The impulsive output

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voltage signal allowed them to be utilized for droplets or bubbles counting in the microfluidic systems. Furthermore, the devices also exhibited self-powered sensing behavior due to the decreased voltage amplitude with increasing input pressure and liquid viscosity. The drop of output voltage could be attributed to the variation of flow condition and velocity of the droplets, leading to the reduced deformation of the piezoelectric PVDF layer and the decrease of the generated piezoelectric potential.

1.

Introduction Micro-total-analysis systems (µTAS) based on microfluidic devices have been widely applied

in variety of fields such as nanoparticle synthesis, nucleic acid analyses, medicine development, medical tests and environmental science, etc.1 Such so called “Lab on a Chip” technique allows people to integrate various functions including physical, chemical and biological tools inside a micro-scaled microfluidic chip, and realize the personalization of the complex analyses that usually need to be accomplished in traditional laboratories.2, 3 Among the various functions of the µTAS, the monitoring of flow properties such as the pressure, velocity and viscosity is one of the most basic and crucial aspects.4-7 Besides the direct observation method by using the microscopy and high-speed CCDs, the other conventional methods to detect those fluidic properties are always based on the detection of displacement induced by the fluids. For instance, the micro-electromechanical-system (MEMS)based piezoresistive cantilevers could be used for the detection of droplet motions and measure the viscosity of the fluid.8 Moreover, the electro-fluidic resistors could be used for measuring the variation of the fluidic pressure according to their pressure-induced electrical resistance


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changes.9 Furthermore, the fluidic velocity could be measured by the micro-particle image velocimetry system.10 However, such conventional methods are still suffering from low compatibility with the microfluidic system, the complex external circuit and equipment and large device sizes. Those drawbacks restrict the miniaturization of the whole system and their practical applications in many fields such as the implantable and in vivo biomedical monitoring devices. Recently, the piezoelectric and triboelectric nanogenerators (PENGs and TENGs), which could harvest the dynamic mechanical energy and convert them into electrical output, have exhibited great potential in building high-output and self-powered sensor systems.11-14 With the assistance of the nanogenerators, we can make use of the mechanical energy from air flows, human body movements and dynamic pressures to power the micro-scaled electronic devices, or to realize the active sensing of the object’s mechanical status. The simple device structure and self-powered operating mode makes them one of the best candidate for building the miniaturized smart sensor systems such as the portable or implantable µTAS. For example, X. Li et al. have demonstrated a self-powered triboelectric nanosensor, which could be used to monitoring the flowing liquid and solution chemistry in the microfluidic system without any powering unit or driving circuit.15 However, the output voltage of that self-powered nanosensors was lower than 500 mV, which might be due to relatively lower interaction between the fluid and the functional layer. Comparing with TENGs, the PENGs based on the piezoelectric nanowires (NWs) and nanofibers (NFs) have exhibited higher energy conversion efficiency. Moreover, the output voltage and current of PENGs exhibit good linearity with both the amplitude and the rate of the strain, which makes them more suitable for building the self-powered mechanical sensors to monitor the velocity and pressure of the fluids.16, 17 Because the output voltage is the sensing


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signal, the unstable output voltage may lead to the disordered operation of the self-powered sensor systems. Therefore, the generation of sustainable vibration and stable output voltage of the PENGs under volatile external environment is a crucial factor that may affect their practical application in self-powered systems. Zhang and co-authors have employed a triangle-like bluff body to modulate the air flows around a flexible PENG, and generate a stable vortex to make the PENG vibrating with the wind.18 Therefore, the output voltage of the PENG could maintain at a stable level under the same wind-velocity. Based on that, the device could be utilized for the selfpowered in situ monitoring of the wind-velocity. However, there are no reports about the integration of PENGs with the µTAS, or their energy harvesting behavior in the µTAS for fluidic sensors, although the microfluidic technology is a simple and highly efficient way for the microfluid control. The electrospun poly(vinylidene fluoride) (PVDF) NFs have exhibited outstanding piezoelectric energy harvesting performance in many literatures.19-22 Although their piezoelectric constant was relatively lower than the traditional perovskite materials such as PZT and PMN-PT, the much higher flexibility of the PVDF materials can greatly enhanced the energy harvesting efficiency of the PENGs, especially for the application with slight deformations and strains. Moreover, the low-temperature fabrication process of that polymer NFs provides better compatibility for integrating with the microfluidic chips. In this work, the electrospun NFs with high piezoelectric energy conversion performance were assembled into a T-shaped microfluidic channel, which could generate the continuous flow of droplets. The as-fabricated microfluidic device could generate stable and sustaining output voltage with the droplets flowing past the NFs. The self-powered fluid sensing properties including the pressure and viscosity sensing performance were also investigated. 2. Experimental details


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2.1.

Materials

PVDF powder (MW 534000, Sigma-Aldrich) was used for preparing the piezoelectric PVDF NFs. Absolute acetone and N, N-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Fluid samples with different viscosity are mixtures of glycerol and deionized water with different ratios. All reagents were analytical grade and used as received without further purification. 2.2

The preparation of PVDF NFs

PVDF solutions with concentration of 0.13 g/ml were prepared by adding PVDF powder to the DMF-acetone mixture of 10 mL (3/7 v/v). The mixtures were then magnetic stirred at 80°C for 3 h to obtain the homogeneous PVDF precursor solutions. The PVDF NFs were prepared by far-field electrospinning method, which is similar to our previous reported works. Firstly, the transparent PVDF precursor solution was transferred into a Hamilton 5 mL syringe. A LongerPump TJP-3A/W0109-1B syringe pump and Betran DC High Voltage power supplier were used for the electrospinning. The feed rate of the syringe pump was set in 50 µl/min and a high voltage of 15 kV/cm was applied to the syringe needle. The electrospun fibers were collected by a receiving plate, which is made of Perspex sheet with two pieces of grounded aluminum foil sticking on the surface. A 2.5 cm gap was left between the Al foils to make the NFs arranging along the same orientation. The sample collector was placed 15 cm away from the needle during the electrospinning process. Lately, the collected nanofibers were dry at 80 oC for 1 hour to remove excess solvent. 2.3

Assembling of microfluidic chips


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The microfluidic chips used in this work were based on the polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning) polymer materials. Firstly, a hard template was fabricated by the negative photolithography technique of SU8-GM1070 photoresist, followed by the reverse molding of PDMS layers to obtain the as-designed microchannel (top layer). Then a thin layer of PDMS (middle layer) was obtained through whirl coating with first-step rotating speed of 800 rpm for 10 seconds and second-step rotating speed of 4000 rpm for 40 seconds. After that, the bottom layer of the microfluidic chip with a micro pore and markers were fabricated with a similar technique. The Pt/Ti electrodes were then deposited on the top surface of Part III by sputtering method with a pre-patterned shadow mask. A thin piece of as-fabricated PVDF NFs was then transferred onto the center part of the electrodes to form the Pt/PVDF/Pt structure. Finally, all three parts of the microfluidic chips were aligned and bonded together to form the microfluidic chips. All the above-mentioned PDMS was curing in the oven at the temperature of 90°C for 1 hour. 2.4

Characterization and Measurement

X-ray diffraction (XRD, Bruker D8 Advanced, CuKα, λ = 0.15406 nm) and Fourier Transform Infrared Spectroscopy (FTIR, Thermo Fisher Scientific, Nicolet iS10) are used to characterize the structure of electrospun NFs. The surface morphology was characterized by the field emission scanning electron microscopy (FE-SEM, JSM7100F JEOL). The properties of microfluidic chips were tested through a microfluidic chip test system which is shown in Figure S1. The system was consisted of three parts, including the gas and liquid input part composed of a Multifunctional Fluid Controller (MesBioSystem, FC-2P8-MN) and several air-tight bottles, the observation part composed of a monocular microscope formation image with CCD and a three-dimensional mobile platform, as well as the electrical test part composed of a charge


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amplifier for impedance matching (Shenzhen Cheng Tec, CT5002), a data acquisition card (National Instruments, NI USB-6210) and PC computer for data acquisition. In addition, deionized water and glycerol mixtures of different mixing ratios were prepared as different viscosities range testing liquid. The viscosities of the liquid samples were got by referring to the glycerol water solution viscosity and temperature table. 3. Results and discussions Figure 1(a) shows a photo image of the electrospun PVDF products. The white PVDF membrane exhibit good flexibility and uniform surface morphology. The SEM image shown in Figure 1(b) confirms that the membrane was consisted of well-aligned NFs with diameter of 200 ~ 800 nm. Figure 1(c) shows the XRD patterns of the as-fabricated PVDF NFs and the raw materials used in the preparation of the electrospinning precursor. The results confirm the phase transition of the PVDF materials from non-polarized α-phase to the polarized β-phase, which could be attributed to the high electrical field and mechanical stretching during the electrospinning process. Moreover, the phase transition could also be confirmed by the FT-IR results shown in Figure 1(d), in which the peaks marked with red colors belongs to the β-phase of PVDF. By calculating the relative intensity of the peaks in the FT-IR spectrum, the content of

β-phase in the final product can be estimated to be ~ 83.6%. Figure 1(e) shows a PENG fabricated by connecting the as-synthesized PVDF NFs with a pair of Pt electrodes inside a thin layer of PDMS. In order to confirm the piezoelectric property of the NFs, the PENG was fixed onto a Cu foil which was vibrating under the stimulation of a vibration exciter.22 As shown in Figure 1(f), the PENG could generate impulsive electrical signal with open-circuit voltage up to 1.2V when a periodical vibration of the device was induced by the vibration exciter. Such result could confirm the sensitive piezoelectric response of the as-synthesized PVDF NFs.


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The PVDF NFs with outstanding piezoelectric response were cut into small pieces, and then embedded into a triple-layered microfluidic chips with a separation layer of PDMS inserted to prevent the NFs and electrodes from direction contact with the microfluid. The fabrication procedure and structure of the microfluidic chip were illustrated by the schematic diagrams shown in Figure 2. As shown in Figure 2(b), a micro-pore was fabricated in the bottom layer of PDMS through microfabrication. The PVDF NFs were precisely positioned on top of the micropore to produce a suspended structure, which could enhance the deformation of the piezoelectric layer. Figure 3(a-c) shows the operating mode of the microfluidic chip while testing its energy harvesting behavior. The T-shaped microfluidic channel with two inlet ports (for air flow and water flow, respectively) and one outlet port was designed for the generation of flowing droplets/bubbles. As shown in Figure 3(d-l) and Video S1 shown in the supporting information, a stable double phase flow consisting of air bubbles and water droplets could be generated when a constant pressure of air (P1 for inlet 1) and a pulsed pressure of water (P2 for inlet 2) were applied on the input ports. Figure 4 illustrates the working mechanism of the microfluidic chips with flowing droplets/bubbles. As shown in Figure 4(a), the piezoelectric layer would be forced to bend downward and then recover to the free state when the liquid droplet was flowing past the top surface of the NFs, leading to the variation of mechanical load on them. The vibration of the piezoelectric layer finally led to the generation of piezoelectric electrical output. According to the motion of the droplet and the piezoelectric layer, the whole cycle could be divided into 4 sections: stress-free (T1), bending (T2), max deformation (T3) and recovering (T4). Figure 4(b) shows the output open-circuit voltage signal within one cycle. Firstly, no output voltage was generated until the input pressure was triggered (T1), leading to a slight vibration of the PVDF


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layer and a weak voltage signal (marked by the black triangle). Once the liquid was contacted with the PVDF layer (T2), the change of mechanical load will lead to the bending of the PVDF layer, which generated the piezoelectric potential along the poling direction. The electrons in the external circuit were driven to the opposite electrode to compensate the piezoelectric potential, leading to the generation of a positive output voltage (marked by the green area).23 When the whole surface of the PVDF layer was covered by the liquid droplet (T3), the deformation got to the maximum level. As a result, the open-circuit voltage impulse increased to the highest level (~ 1.8 V). Then the output voltage decreased to zero because the electron flow stopped when the system returned to the equilibrium state (fully compensation of the piezoelectric potential by the accumulation of electrons). Thereafter, the liquid droplet left the surface of the PVDF and result in the recovering of the piezoelectric layer (T4). Due to the decrease of the piezoelectric potential during the recovering process, a reverse flow of the accumulated electrons was triggered, generating a negative output voltage impulse. After the PVDF layer recovered to the free state, the electron flow was terminated and no output voltage would be generated (T1). The generation of the small voltage peaks after T4 could be attributed to slight shaking of the PVDF layer due to the flow of unshaped water droplets after the stable ones. Figure 4(c) shows the output voltage of the device for 6 cycles. A long-term testing result for evaluating the continuous operation performance was also shown in Figure S2. The results show that the output voltage generated by the microfluidic chips remain stable after continuous working for more than 1000 s, which suggested the good stability of the device for long-term applications. Due to the switching behavior of the output voltage in response to the flow of the droplets, the device could be used for counting the bubbles or the droplets in the microchannel, which is an important function of many microfluidic chips in biological and medical applications.24, 25 Moreover, the output power


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density of the devices were also tested by connecting the devices with various external load resistance. As shown in Figure 5(a), the output voltage was increase with the increasing load resistance in the external circuit. The maximum power density of the microfluidic chips was ~ 65 nW/cm2 with load resistance of 1 MΩ. In order to optimize the output performance of the microfluidic chip, samples with different channel sizes were fabricated by fixing the width ratio between main channel (w1) and branch channel (w2) as w1/w2 = 2, and increasing w1 from 100 µm to 500 µm, as illustrated in Figure 6(a). All devices were tested with the testing at room temperature. The input pressure is 0.3 bar and the liquid (deionized water) input frequency is 1 Hz. As shown in Figure 6(b-f), all devices could generate continues voltage output to the external circuit under the driving force of water droplets. Figure 6(g) shows the change of peak-to-peak voltage of the devices with the microchannel size. The output voltage was increased with channel size from 100 to 300 µm, and then decreased when the channel size was further increased to 500 µm. The change of the output voltage could be attributed to the variation of the contact area and the motion status of the droplets. Firstly, the increased contact area between the PVDF layer and the microfluid with the increase of channel size may lead to higher strain of the piezoelectric layer and higher output voltage of the device. Secondly, the Reynolds number of the microfluid was also changed with the channel size. It is known that the Reynolds number (Re = ρvd/µ) is a non-dimensional parameter that was used to characterize the flowing condition of fluid, where ρ, v, µ and d represent the density, velocity, and viscosity of the microfluid as well as the feature size of the microfluidic channel, respectively. Moreover, the influence of the fluid viscous force is more significant with smaller Reynold number, while the influence of the fluid inertia become


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more significant with bigger Reynold number.26, 27 As a result, a larger channel size may lead to a bigger Reynolds number, and result in the change of the fluidic state from laminar flow to turbulent flow. The unstable turbulent flow could decrease the strain of the PVDF layer, leading to the decrease of output voltage of the devices. Therefore, the increase of output voltage with the channel size at relatively lower level should be attributed to the increase of contact area with stable laminar flow state. Once the channel size was higher than 300 µm, the decrease of the output voltage should be attributed to the decrease of the strain level due to the unstable turbulent. According to the Reynolds number equation, the change of viscosity and velocity of the microfluid may also lead to the variation of the microfluidic state in the channels and result in the change of the output voltage. Therefore, the devices could be applied as a self-powered sensor of the microfluid states. In order to test the viscosity sensing performance of the devices, a series of liquid samples with different viscosity was prepared by mixing DI water and glycerol with certain volume ratio. Those liquid samples were injected into the microfluidic system through input 2 for generating the droplet flows. The energy harvesting performance was tested by fixing the input pressures at 0.5 bar and channel size of 300 µm. Figure 7(a-e) shows the variation of output voltage with the viscosity of the liquid droplets. With the viscosity increasing from 1.05 to 2.94 mPa·s, the peak-to-peak value of the output voltage generated by the devices decreased from ~1.13 to 0.22 V. According to the Reynolds number equation, the increase of viscosity would lead to the decrease of Reynolds number, making all fluids in the samples remaining at stable laminar flow state. At laminar state, the volume flow rate (Q) will be decreased due to the increase of viscosity according to the Poiseuille’ law as


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π∆pD 4 Q= 128µ L

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(1)

where △p is the pressure difference at both ends of microchannel, D is the size of microchannel, L is the length of microchannel, µ is the viscosity of fluid in microchannel. The decrease of volume flow rate will lead to the decrease of velocity (v) of the fluid in the microchannel.28 According to our simulation results shown in Figure S3 and S4, the decrease of velocity of the fluid will decrease the stress applied on the piezoelectric PVDF layer, thus lower down the output voltage of the devices. Similar results were also found when we increase the input pressure of both the liquid and air and keeping other factors unchanged. As shown in Figure 8, the peak-to-peak value of the output voltage with input pressure of 0.3 bar is up to 1.89 V, which gradually decreased to 0.56 V as the input pressure increased to 1.0 bar. Figure 9 shows the magnified view of the output voltage curves with different input pressures. As shown in Figure 9(a), the output voltage curve under input pressure of 0.3 bar shows sharp peak of voltage signals, which indicate the stable laminar flow of the microfluids. As the input pressure increased, more relatively weak voltage peaks appeared, which indicated the unstable flow of the microfluid (Figure 9(b-g)). As shown in Figure 9(h), the voltage peak corresponding to the flow of droplets cannot be clearly distinguished from the voltage signals, which suggested that the microfluid has been changed to the unstable turbulent flow. Such phenomenon could also be explained by the Poiseuille’ law that the increase of input pressure (∆P) may lead to the increase of the volume flow rate (Q) and the velocity of the microfluid (v), thus increase the Reynolds number.28 As a result, the flow state may be changed from stable laminar flow to unstable turbulent flow, which will decrease the strain of the PVDF layer and lead to the decrease the output voltage of the devices. Video S2 in


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the supporting information shows the flow state of the bubble/droplets with input pressure of 0.8 bar. Comparing with the stable bubble flow shown in Video S1, the microfluids shown in Video S2 exhibited obvious unstable turbulent flow state, where water and air were flowing past the PVDF NFs with disordered states. Such flow state may lead to the decreased strain of the PVDF NFs and unstable output level of the devices. Both the viscosity and pressure-dependent voltage amplitude of the microfluidic devices suggested that those devices could be functionalized as a self-powered fluidic sensor without any electrical power input. Unlike most reported selfpowered sensor systems, the driving force of these devices comes from the microfluidic chip itself but not the extra input energies such as the external pressure applied on the piezoelectric layers. 4. Conclusion

In this work, PVDF nanofibers were prepared by electrospinning method. The as-synthesized PVDF nanofibers with polarized β-phase structure and ultra-long aspect ratio formed a layer of flexible piezoelectric membrane. After being assembled into flexible nanogenerators, the PVDF membrane exhibited outstanding flexible piezoelectric energy harvesting performance with output voltage up to 1.2 V. Based on the piezoelectric PVDF nanofibers, a microfluidic device for droplet generation and fluidic energy harvesting were designed and fabricated by PDMS polymers. When the water droplets flow along the microchannel, the changing of air and water on top of the PVDF layer will lead to the periodical vibrating of them and generate impulsive open-circuit output voltage with amplitude up to 1.8 V. The output voltage was closely related to the channel size of the microfluidic devices due to the variation of fluid state. Moreover, the output voltage exhibited negative correlation with the viscosity and input pressure of the


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microfluid, which indicated their great potential as self-powered microfluidic sensor for in-situ monitoring of the viscosity and pressure.


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Figure 1. The characterization result of the electrospun PVDF NFs. (a) A photo image of the electrospun products; (b) A local SEM image of the PVDF NFs; (c) The XRD patterns of the nanofibers and raw materials; (d) The FT-IR spectrum of the PVDF NFs; (e) A photo image of the nanogenerator based on the PVDF nanofibers; (f) The open-circuit voltage generated by the nanogenerator under periodical vibration.


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Figure 2. The schematic fabrication procedure and structural diagram of the microfluidic chips. (a) Photolithography process; (b) Schematic diagram of the microfluidic chips.


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Figure 3. Structural diagram and photo image of the microfluidic chips. (a) The schematic diagram of the triple-layered structure of the microfluidic chip with PVDF NFs embedded. (b) A photo image of the functional area of the chip. (c) The experimental setup for measuring the energy harvesting behavior. (d-l) The local image of the microfluidic chip with flowing droplets/bubbles. The red borders represent the edge between the air bubbles and water droplets.


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Figure 4. The output voltage curve and working mechanism of the microfluidic chips. (a) The schematic diagram and top-view image of the motion of PVDF NFs with flowing droplets/bubbles. The blue areas represent the liquid in the channels (b) The output voltage signal and input pressures curve. (c) The output voltage signal within one period and the magnified waveform.


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Figure 5. The output voltage of the microfluidic chips driven by flowing droplets with different load resistance. (a) The transient voltage signal; (b) The resistance-dependent output voltage and power density.


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Figure 6. The output voltage of the microfluidic chips with different microchannel sizes. (a) The pattern and dimension of the microchannel. (b-f) The output open-circuit voltage generated by the devices. (g) The relation between the peak-to-peak value of output voltage and the microchannel size.


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Figure 7. The output voltage of the microfluidic chip with different viscosity of the liquid droplets. (a) 1.01 mPa·s; (b) 1.37 mPa·s; (c) 1.75 mPa·s; (d) 2.32 mPa·s; (e) 2.94 mPa·s. (f) The relationship of the output voltage and the viscosity.


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Figure 8. The output voltage of the microfluidic chip with different input pressure. (a) 0.3 bar; (b) 0.4 bar; (c) 0.5 bar; (d) 0.6 bar; (e) 0.7 bar (f) 0.8 bar (g) 0.9 bar (h) 1.0 bar. (i) The relationship of the output voltage and the input fluid pressure.


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Figure 9. The magnified view of the output voltage curves under different input pressures. (a) 0.3 bar; (b) 0.4 bar; (c) 0.5 bar; (d) 0.6 bar; (e) 0.7 bar; (f) 0.8 bar; (g) 0.9 bar; (h) 1.0 bar.

AUTHOR INFORMATION Corresponding Author Prof. Haoshuang Gu* E-mail: [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was financially supported by the National Science Foundation of China (Grant Nos. 11474088 and 11504099) and the Science and Technology Department of Hubei Province (Grant No. 2016AAA002, 2016CFA081). SUPPORTING INFORMATION The experimental setup for testing the energy harvesting performance of the microfluidic chips, the long-term performance, photo image of microfluidics and simulation results were included in the supporting information. REFERENCES 1. Whitesides, G. M., The Origins and the Future of Microfluidics. Nature 2006, 442, 368-373. 2. Ma, J.; Lee, S. M.-Y.; Yi, C.; Li, C.-W., Controllable Synthesis of Functional Nanoparticles by Microfluidic Platforms for Biomedical Applications - A Review. Lab Chip 2017, 17, 209-226. 3. Karle, M.; Vashist, S. K.; Zengerle, R.; von Stetten, F., Microfluidic Solutions Enabling Continuous Processing and Monitoring of Biological Samples: A Review. Anal. Chim. Acta 2016, 929, 1-22. 4. Kim, D. R.; Lee, C. H.; Zheng, X., Probing Flow Velocity with Silicon Nanowire Sensors. Nano Lett. 2009, 9, 1984-1988. 5. Li, H.; Luo, C. X.; Ji, H.; Ouyang, Q.; Chen, Y., Micro-Pressure Sensor Made of Conductive PDMS for Microfluidic Applications. Microelectron. Eng. 2010, 87, 1266-1269. 6. Nahar, M.; Nikapitiya, J.; You, S.; Moon, H., Droplet Velocity in an Electrowetting on Dielectric Digital Microfluidic Device. Micromachines 2016, 7, 71. 7. Rasmussen, A.; Mavriplis, C.; Zaghloul, M. E.; Mikulchenko, O.; Mayaram, K., Simulation and Optimization of A Microfluidic Flow Sensor. Sensor. Actuat. A 2001, 88, 121-132. 8. Nguyen, T.-V.; Nguyen, M.-D.; Takahashi, H.; Matsumoto, K.; Shimoyama, I., Viscosity


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BRIEFS (Word Style “BH_Briefs”). The flexible piezoelectric PVDF nanofibers integrated with microfluidic systems could harvest the mechanical energy of microfluidics and realize the selfpowered monitoring of fluidic pressure and viscosity under the driving of the liquid/air flows.


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Self-Powered Viscosity and Pressure Sensing in Microfluidic Systems

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