R.H. Liu

University of Wisconsin, Madison, Madison, MS, United States

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Publications (7)50.68 Total impact

  • R.H. Liu, Qing Yu, D.J. Beebe
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    ABSTRACT: Several microvalves utilizing stimuli-responsive hydrogel materials have been developed. The hydrogel components are fabricated inside microchannels using a liquid phase polymerization process. In-channel processing greatly simplifies device construction, assembly, and operation since the functional components are fabricated in situ and can perform both sensing and actuation functions. Two in situ photopolymerization techniques, "laminar stream mode" and "mask mode," have been explored. Three two-dimensional (2-D) valves were fabricated and tested (response time, pressure drop, maximum differential pressure). In addition, a hydrogel/PDMS three-dimensional (3-D) hybrid valve that physically separates the sensing and regulated streams was demonstrated. Analytical modeling was performed on the 3-D valve. Hydrogel-based microvalves have a number of advantages over conventional microvalves, including relatively simple fabrication, no external power requirement, no integrated electronics, large displacement (185 μm), and large force generation (22 mN)
    Journal of Microelectromechanical Systems 03/2002; · 2.13 Impact Factor
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    ABSTRACT: In this paper we present an organic feedback scheme that merges microfluidics and responsive materials to address several limitations of current microfluidic systems. By using in situ fabrication and by taking advantage of microscale phenomena (e.g., laminar flow, short diffusion times), we have demonstrated feedback control of the output pH in a completely organic system. The system autonomously regulates an output stream at pH 7 under a range of input flow conditions. A single responsive hydrogel component performs the functionality of traditional feedback system components. Vertically stacked laminar flow is used to improve the time response of the hydrogel actuator. A star shaped orifice is utilized to improve the flow characteristics of the membrane/orifice valve. By changing the chemistry of the hydrogel component, the system can be altered to regulate flow based on hydrogels sensitive to temperature, light, biological/molecular, and others.
    Lab on a Chip 01/2002; 1(2):96-9. · 5.70 Impact Factor
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    ABSTRACT: A three-dimensional serpentine microchannel design with a “C shaped” repeating unit is presented in this paper as a means of implementing chaotic advection to passively enhance fluid mixing. The device is fabricated in a silicon wafer using a double-sided KOH wet-etching technique to realize a three-dimensional channel geometry. Experiments using phenolphthalein and sodium hydroxide solutions demonstrate the ability of flow in this channel to mix faster and more uniformly than either pure molecular diffusion or flow in a “square-wave” channel for Reynolds numbers from 6 to 70. The mixing capability of the channel increases with increasing Reynolds number. At least 98% of the maximum intensity of reacted phenolphthalein is observed in the channel after five mixing segments for Reynolds numbers greater than 25. At a Reynolds number of 70, the serpentine channel produces 16 times more reacted phenolphthalein than a straight channel and 1.6 times more than the square-wave channel. Mixing rates in the serpentine channel at the higher Reynolds numbers are consistent with the occurrence of chaotic advection. Visualization of the interface formed in the channel between streams of water and ethyl alcohol indicates that the mixing is due to both diffusion and fluid stirring
    Journal of Microelectromechanical Systems 07/2000; · 2.13 Impact Factor
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    ABSTRACT: Hydrogels have been developed to respond to a wide variety of stimuli, but their use in macroscopic systems has been hindered by slow response times (diffusion being the rate-limiting factor governing the swelling process). However, there are many natural examples of chemically driven actuation that rely on short diffusion paths to produce a rapid response. It is therefore expected that scaling down hydrogel objects to the micrometre scale should greatly improve response times. At these scales, stimuli-responsive hydrogels could enhance the capabilities of microfluidic systems by allowing self-regulated flow control. Here we report the fabrication of active hydrogel components inside microchannels via direct photopatterning of a liquid phase. Our approach greatly simplifies system construction and assembly as the functional components are fabricated in situ, and the stimuli-responsive hydrogel components perform both sensing and actuation functions. We demonstrate significantly improved response times (less than 10 seconds) in hydrogel valves capable of autonomous control of local flow.
    Nature 05/2000; 404(6778):588-90. · 38.60 Impact Factor
  • R.H. Liu, M.J. Vasile, D.J. Beebe
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    ABSTRACT: This paper presents a new application of spin-on glass to fabricate nonplanar dielectric structures (channel plate microstructures) with aspect ratios (i.e., ratio of channel length to channel width) of 20:1. A variety of microchannel geometries have been fabricated. The LIGA process is used to make nickel molds up to 150 μm in height with mechanically planarized surfaces. Spin-on glass (SOG) is applied to obtain glass structures in nickel molds. A multiple dispensing/drying/curing process was developed resulting in crack-free SOG structures. Reverse electroplating is used to remove the nickel mold and release the glass structures. The resulting freestanding glass microchannel plates (>100 μm in height) demonstrated good electrical properties (400-V/μm breakdown voltage) and good spatial definition
    Journal of Microelectromechanical Systems 07/1999; · 2.13 Impact Factor
  • R.H. Liu, Lin Wang, D.J. Beebe
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    ABSTRACT: This paper describes progress towards the development of a force sensitive skin (“smart skin”) capable of measuring force distributions on the human palmer and finger surface. Closely spaced normal force tactile sensing elements in a flexible skin have been successfully designed and fabricated based on silicon piezoresistive effect using MEMS (microelectromechanical systems) techniques. The sensing array consists of five elements each 4.4 mm in diameter, and arranged within a 16 mm circle designed to fit on a human finger tip. Preliminary bench testing shows the sensor performance is excellent with negligible hysteresis and good linearity (R=0.997). Normal loads of up to 180 N have been measured. A shear sensitive tactile sensor prototype was also developed. The preliminary results demonstrate that the prototype can sense both normal and shear forces with good linearity and repeatability
    Engineering in Medicine and Biology Society, 1998. Proceedings of the 20th Annual International Conference of the IEEE; 01/1998
  • [show abstract] [hide abstract]
    ABSTRACT: This paper describes the development of microchannel plate prototypes with aspect ratios of 20:1 and a variety of microchannel geometries. The LIGA process is utilized to fabricate nickel molds up to 150 μm in height with planarized surfaces. A novel application of spin-on glass is developed to obtain glass structures in Ni molds. A reverse electroplating is used to remove the Ni mold and release the glass structures with height of more than 100 μm
    Solid State Sensors and Actuators, 1997. TRANSDUCERS '97 Chicago., 1997 International Conference on; 07/1997

Publication Stats

1k Citations
74 Downloads
424 Views
50.68 Total Impact Points

Institutions

  • 2002
    • University of Wisconsin, Madison
      • Department of Biomedical Engineering
      Madison, MS, United States
  • 1997–2002
    • University of Illinois, Urbana-Champaign
      • • Beckman Institute for Advanced Science and Technology
      • • Department of Electrical and Computer Engineering
      Urbana, IL, United States