Elsevier

Sensors and Actuators A: Physical

Volume 210, 1 April 2014, Pages 147-156
Sensors and Actuators A: Physical

The implementation of a thermal bubble actuated microfluidic chip with microvalve, micropump and micromixer

https://doi.org/10.1016/j.sna.2014.02.015Get rights and content

Highlights

  • A thermal bubble actuated microfluidic chip has been implemented with SOI wafer.

  • The sizes of thermal bubbles can be controlled steadily by applying the magnitude of direct current.

  • A better pumping efficiency can be obtained by dynamic thermal bubble actuation.

  • The mixing test was successfully implemented by using thermal bubble to create turbulent flow.

Abstract

This paper presents the implementation of a thermal bubble actuated microfluidic chip with microvalve, micropump and micromixer, based on a simple process with SOI wafer. Only two photolithography processes were required to provide an effective means of manufacturing the vertical bulk microheater and high-aspect-ratio microchannel for microfluidic applications. The static and dynamic electro-thermal coupling behaviors of the proposed resistive silicon-based microheater were evaluated by finite element analysis to provide an applicable design. The feasibility of each actuation element has also been verified by experiments. Experimental results show that the sizes of thermal bubbles, at flow rates less than 4.5 μl/s, can be controlled steadily by applying the magnitude of direct current that meets the requirement of a microvalve to modulate flow rate. When applying an alternating current with high frequency to the microheater, thermal bubbles could grow cyclically and collapse rapidly, so the liquid stream could be regulated by the repeated volume change of thermal bubbles. A maximum volume flow rate of 4.5 μl/s was obtained, under the driving voltage with a frequency of 60 Hz and 30% duty ratio. The mixing test of the multi-layer fluidics with laminar flow also was successfully implemented by using the volume of thermal bubble to create turbulent flow in the fluids. With no moving parts, the proposed microfluidic chip is well designed with high performance and reliability.

Introduction

Microfluidic chips have been widely used in biomedical applications such as detecting chemical reactions and biological features. Those miniaturized system integrates microchannels, microvalves, micropumps, and micromixers to reduce traditional feature sizes and improve complex apparatus to a chip; also including screening, separating, and transmitting functions such as a micro total analysis system (μ-TAS) or a lab-on-chip (LOC) [1], [2], [3]. They require few samples to accomplish bio-analysis and effectively shorten the cycle time [4], [5], [6], [7]. Novel designs and applications of integrated microfluidic chips have been developed by many investigators. For example, Neus et al. proposed microfluidic paper-based analytical devices (μPADs) that use printing technology to directly fabricate microchannels onto paper and use the porous structure and capillary tension phenomenon of the paper itself to achieve the objective of driving the fluid for blood separation, recirculation process, and microfluidic switches [8]. Yoo et al. proposed a high performance microfluidic system with microvalves and a micropump integrated on the same substrate using the identical fabrication process for applications in biomedical sensors [9]. Zhang et al. used micropumps, microvalves, and micromixers for an application in polymerase chain reaction (PCR) detection [10]. These works have successfully demonstrated that the integrated microfluidic chip can provide good fluid transmission ability in a miniaturized chip.

With the improvements in fabrication technology, several biomedical chips with microscaled fluidic elements have been developed such as microvalves, micropumps, and micromixers, using microeletromechnical system (MEMS) technology. Most microvalves for the control of fluid flow can be classified into two groups: active microvalves with movable components driven by thermal expansion, piezoelectric, electrostatic, and electromagnetic forces; or passive microvalves with a solid barrier constructed in a microchannel, which allows it to change the flow behaviors of the fluid without consuming extra energy [11], [12]. However, the latter were the disadvantages of low performance, complex fabrication process and being able to control the fluid only with one-way movement. Thus, they are not widely used in microfluidic manipulation [13]. In contrast, micropumps with MEMS technology have wide application in microfluidic devices, such as the handling of volume flow rate in bio-medical inspection. But the miniaturized micropump offers only low driving efficiency due to limitations of its driving framework. Therefore, the fluid driven in microchannel routes must rely on an external apparatus, which causes system complexity and assembly difficultly. Most current micropumps are categorized into one of two driving methods: mechanical micropumps with moving parts [14], [15], [16], and non-mechanical micropumps with no moving components [17], [18], [19]. The former uses a suspension structure with piezoelectric, pneumatic, or thermal energies to force the liquid to flow; and the other uses action at a distance or field forces such as ultrasound, bubble expansion, and surface tension, or capillarity phenomenon. However, the actuated components have the problems of high failure rate and low fatigue. The latter approach gives reliable and stable switching since the formation of vapor bubble and ultrasonic wave are reversible and controllable. Thus, the external energy implementations of microswitches share many benefits such as ease of design and fabrication, especially for thermal bubble driving. Each design inherently possesses a number of strengths and weakness which must be considered in choosing an appropriate design a given biomedical application.

The micromixer used for the mixing of heterogeneous fluids in a microfluidic system also plays an important role. However, fluids with multi-layer flow are difficult to reach homogenization mixing by using the difference in concentration and molecular diffusion, since the fluids in microchannels have relatively low Reynolds numbers. An external disturbance approach to improve the mixing efficiency is required, such as using exterior energy pulses to disturb the fluid [20], [21], [22], [23], or through the layout of microchannel routing to change the flow behavior [24], [25], [26], [27]. However, the completion of the inspection process must thus rely on the assistance of external equipments. Although some studies have integrated different actuating elements to achieve multifunctional requirements, a complex fabrication process is required and low efficiency is resulted. For example, Meng et al. proposed a new mechanism to pump liquid in microchannels with virtual check valve, using H2 and O2 gas bubbles continuously generated by electrolysis. Compared with the commonly used thermal bubble driven micropumps, the electrolytic-bubble-driven achieved a similar volumetric flow rate with higher power efficiency. In addition, electrolysis is more compatible with biological solution over boiling. But this method needs a specific working fluid, such as Na2SO4 aqueous solution, to generate gas bubbles by electrolysis [28].

In order to improve above problems, this paper presents a thermal bubble actuated microfluidic chip with microvalve, micropump and micromixer, based on a simple process with silicon-on-isolation (SOI) wafer. A resistive bulk microheater with vertical sidewall is utilized to generate reliable thermal bubbles and thereby implements the functions of microvalve, micropump and micromixer, simultaneously. The performance of these components has verified with experiments and discussion in this paper. It is expected that the proposed biochip can be applied on cell sorting, such as bubble-jet cell sorter and the lysis of single cells [29], [30].

Section snippets

Design concept

A schematic view of the thermal bubble actuated microfluidic chip with microvalve, micropump, and micromixer is shown in Fig. 1. It consists of inverted T-shaped microchannel routes, three reservoirs and several arch-type microheaters. Two reservoirs below the chip are used to control the quantity of the injected fluids (F1 and F2); the reservoir on top of the chip is used to collect the fluid after mixing. The SOI-based microfluidic device consists of a single crystal silicon base layer, a

Fabrication

According to the microchannel layout and working principles in Fig. 1, a microfluidic chip is implemented based on a SOI wafer consisting of only two photolithography masks: the first mask is for the metal pad patterning, and the other is for inductively coupled plasma (ICP) etching to fabricate microheater, microchannel and liquid inlet/outlet holes. The detailed fabrication process is shown in Fig. 4. The process begins with a highly doped SOI wafer with a structure layer of 50 μm buried in an

Microheater performance

Electro-thermal properties of the microheater were measured and verified by an infrared thermal imaging radiometer. According to the fabrication results in Fig. 5, the measured temperature distribution, under applied voltages from 0 V to 8 V, are shown in Fig. 6(a)–(c), respectively. During this experiment, the deionized (DI) water has filled in the microchannel and the sample-carrying stage has an initial heating temperature of 75 °C to reduce the noise effect, as shown in Fig. 6(a). Fig. 6(b)

Conclusion

A thermal bubble actuated microfluidic chip with microvalves, micropumps, and micromixers, based on an SOI wafer have successfully been developed. A SOI-MEMS technology with a simple process has implemented to realize the chip basis with an arch-type resistive microheater and high-aspect-ratio microchannel. The characteristics for each component also were verified by simulation and experiment. Specific microheaters were easy to design by bulk geometry and intrinsic resistance, such as a

Acknowledgment

This project was supported by the National Science Council of Taiwan under grant of NSC-100-2628-E-035-008. The authors appreciate the Precision Instrument Support Center of Feng Chia University, Nano Facility Center of National Chiao Tung University, Instrument Technology Research Center (ITRC) and the NSC National Nano Device Laboratory (NDL) in providing the fabrication facilities.

Chenghan Huang received the M.S. degree in engineering from the Automatic Control Engineering Department, Feng Chia University, Taichuang, Taiwan, R.O.C., in 2006. He is now working toward the Ph.D degree in the Graduate Institute of Electrical and Communications Engineering, Feng Chia University, Taichuang, Taiwan, R.O.C. His current research interest is the development of microfluidic devices and systems by microelectromechanical systems.

References (32)

  • H. Hoefemann et al.

    Sorting and lysis of single cells by BubbleJet technology

    Sens. Actuators B: Chem.

    (2012)
  • L. Luan et al.

    Integrated optical sensor in a digital microfluidic platform

    IEEE Sens. J.

    (2008)
  • K.H. Han et al.

    Self-balanced navigation-grade capacitive microaccelerometers using branched finger electrodes and their performance for varying sense voltage and pressure

    J. Microelectromech. Syst.

    (2003)
  • M. Washizu

    Electrostatic actuation of liquid droplets for micro-reactor applications

    IEEE Trans. Ind. Appl.

    (1998)
  • N. Godino et al.

    Centrifugally enhanced paper microfluidics

  • S.K. Fan et al.

    Digital microfluidics with bubble manipulations by dielectrophoresis

    Auto. Smart Tech.

    (2012)
  • Cited by (56)

    View all citing articles on Scopus

    Chenghan Huang received the M.S. degree in engineering from the Automatic Control Engineering Department, Feng Chia University, Taichuang, Taiwan, R.O.C., in 2006. He is now working toward the Ph.D degree in the Graduate Institute of Electrical and Communications Engineering, Feng Chia University, Taichuang, Taiwan, R.O.C. His current research interest is the development of microfluidic devices and systems by microelectromechanical systems.

    Chingfu Tsou received the M.S., and Ph.D. degrees in power mechanical engineering from National Tsing Hua University (NTHU), Hsinchu, Taiwan, Republic of China, in 1998 and 2003, respectively. Currently, he is a Professor of the Department of Automatic Control Engineering, Feng Chia University, Taiwan, and has more than two years of working experience in the field of MEMS and fingerprint sensor. His research interests include MEMS devices and systems, as well as MEMS/IC packaging technology.

    View full text