Erik T. K. Peterson

A passive planar micromixer with obstructions for mixing at low Reynolds numbers

Passive mixers rely on the channel geometry to mix fluids. However, many previously reported designs either work efficiently only at moderate to high Reynolds numbers (Re), or require a complex 3D channel geometry that is often difficult to fabricate. In this paper, we report design, simulation, fabrication and characterization of a planar passive microfluidic mixer capable of mixing at low Reynolds numbers. The design incorporates diamond-shaped obstructions within the microchannel to break-up and recombine the flow. Simulation and experimental results of the developed micromixer show excellent mixing performance over a wide range of flow conditions (numerically: 0.01 < Re < 100, experimentally: 0.02 < Re < 10). The micromixer is also characterized by low pressure drop, an important characteristic for integration into complex, cascading microfluidic systems. Due to the simple planar structure of the micromixer, it can be easily realized and integrated with on-chip microfluidic systems, such as micro total analysis systems (μTAS) or lab on a chip (LOC).

Passive micromixer with break-up obstructions

In this paper, we report on design and fabrication of a passive microfluidic mixer capable of mixing at low Reynolds numbers (Re). Passive mixers typically use channel geometry to mix fluids, and many previously reported designs that work only at moderate to high Reynolds numbers and are often difficult to fabricate. Our design uses diamond-shaped obstructions inside the microchannel to break up and laminate the flow, thus enhancing mixing. Both numerical and experimental studies show that the mixer is efficient at mixing fluids at low Reynolds numbers. We benchmarked our mixer design against a conventional T-mixer. Results show that the new design exhibits rapid mixing at Re < 0.1. The new mixer has a planar design which is easy to fabricate and thus will benefit a wide range of lab-on-a-chip applications.

Microtextured polydimethylsiloxane substrates for culturing mesenchymal stem cells.

Musculoskeletal tissue-engineering strategies have recently focused on the use of biomaterial scaffolds capable of guiding growth and organization of mesenchymal stem cells (MSCs), which are precursors for connective tissues. This chapter describes the methods for culturing MSCs on micropatterned polydimethylsiloxane (PDMS) substrates. MSCs are isolated from bone marrow biopsies and subcultivated before plating onto PDMS substrates. Micropatterned substrates are fabricated by casting PDMS on AZ P4620 photoresist molds. Prior to plating cells, substrates are cleaned, sterilized, and coated with fibronectin. Micropatterned growth surfaces are a useful research tool enabling the study of cell morphology and alignment in response to substrate geometry. Understanding MSC response to surface topography will assist in the design of improved scaffolds for connective-tissue repair.

Adsorption of fluorescently labeled microbeads on PDMS surfaces

Fluorescently labeled beads may be utilized in transparent microfluidic devices to facilitate a variety of immunoassay based chemical measurements. We investigate the ability to visualize, quantitate, and reduce undesirable adsorption of beads within a polydimethylsiloxane (PDMS) microchannel device. These methods are prerequisites to the design of practical bead-based microfluidic sensing devices. The PDMS microchannels were shown to be transparent enough to make accurate quantitative optical measurements, although significant adsorption was observed. Epifluorescence microscopy was employed in an attempt to quantitatively evaluate microbead adsorption to PDMS microchannel walls and bulk surfaces after different agitation, solution, and surface treatments. This microscopy method provides reproducible imaging of individual beads and allows for characterization of adsorption to PDMS microchannel walls. Solution composition seemed to play a more important role in the ability to reduce the number of adsorbed beads to the PDMS surface than agitation. The most significant reduction in bead adsorption was seen in surface treatment. The most effective surface treatment examined in this study was Teflon AF.

Passive micromixer with obstructions for lab-on-a-chip applications

A new passive micromixer has been developed with a low dependence on Reynolds number. The mixer design contains obstructions inside the mixing microchannels to breakup the flow resulting in chaotic mixing. Using CFDRC ACE+ software the mixer was modeled and was shown to completely mix water and glycerin in less than 1 cm. The micromixer was fabricated in cyclic olefin copolymer (COC) using hot embossing with polydimethylsiloxane (PDMS) tools and evaluated using epifluorescence microcopy.

Characterization of SU-8 optical multimode waveguides for integrated optics and sensing on microchip devices

Our research group is interested in environmental sensing of heavy metals that are involved in pollution of aqueous environments. As a result, we are developing chemical sensors within integrated microfluidic systems for sensitive and selective detection of these pollutants. Our approach is to combine established chemical sensing strategies with microfluidic structures, especially in plastic devices, to achieve a total heavy metal analysis system. In this regard, the combination of three complementary techniques - optical waveguide spectroscopy, electrochemistry and chemical partitioning offers the required selectivity and sensitivity essential for many environmental samples. On-chip optical waveguide spectroscopy promises to yield the necessary high sensitivity but relies on fabrication of optical structures with a material of appropriate refractive index, optical quality, and chemical stability by methods consistent with established fabrication methods. SU-8, the epoxy-based negative photoresist, appears to satisfy these requirements and, thus, has become one of our candidate materials for waveguide fabrication on plastic microchips. Although the SU-8 has been previously used for waveguide fabrication, its optical properties and more specifically the influence of processing conditions on resultant optical properties have not been thoroughly characterized. This work presents an evaluation of SU-8-based multimode waveguides on glass and plastic substrates. Optical constants of waveguides have been characterized by spectroscopic ellipsometric and prism coupling techniques. Additionally, using the latter method, evaluation of propagation losses of various structures with different thicknesses has been made. Ellipsometric and prism coupling measurements gave comparable refractive indices for variously cured SU-8 waveguide materials. Prism coupling analyses proved to be more useful for analysis of the many SU-8 waveguide structures fabricated in the thickness range of 5 to 75 μm.

Rapid prototyping of plastic microfluidic devices in cyclic olefin copolymer (COC)

In this paper, we report on using polydimethylsiloxane (PDMS) tools to emboss cyclic olefin copolymer (COC). Positive photoresist AZP 4620 was used to fabricate 5 and 20 μm thick PDMS tools. The embossed microchannels were 10 μm to 100 μm in width at 10 μm to 100 μm in spacing. The COC embossing parameters, including temperature, force, and time were optimized to reduce replication errors. The optimized process was then successfully applied to fabrication of a passive microfluidic mixer designed and simulated using CFDRC ACE+.

Interdigitated array microelectrode capacitive sensor for detection of paraffinophilic Mycobacteria

Mycobacterium Avium Complex (MAC) is an opportunistic pathogen that threatens public health and has high clinical relevance. While culture-based and molecular biology techniques for identification are available, these methods are prone to error and require weeks to perform. There is a critical need for improved portable lab-on-a-chip sensor technology which will enable accurate and rapid point-of-care detection of these microorganisms. In this work, a new capacitive sensing strategy is explored utilizing interdigitated array (IDA) microelectrodes and exploiting the paraffinophilic nature of MAC. In this approach, paraffin wax is deposited over IDA microelectrodes to selectively extract these microorganisms from samples. As bacteria consume the dielectric paraffin layer, the charging current of the IDA capacitor changes to facilitate detection. Several IDA geometries were designed and simulated using CFD-ACE+ modeling software and compared with mathematical models. Capacitance of fabricated devices was determined using a charge-based capacitance measurement (CBCM) technique. Modeling and experimental results were in good agreement. Detection of femto-Farad changes in capacitance is possible, making this a feasible technique for sensing small changes in the paraffin for detection of paraffinophilic MAC.

A simple planar micromixer with low-pressure drop for disposable lab-on-a-chip (LOC) systems

In this work the design and fabrication of a novel passive microfluidic mixer capable of achieving mixing in shorter distances and lower Reynolds numbers (Re) is reported. Passive mixers typically rely on the channel geometry to mix fluids, and many previously reported designs work efficiently only at moderate to high Re and are often difficult to fabricate as they incorporate complex 3-D structures within the channel. The mixer design discussed in this work achieves good mixing at low Re, has planar geometry and thus is simpler to fabricate and integrate with existing labon- a-chip (LOC) technologies. The design incorporates triangular notches patterned along the channel walls to laminate the flow, thus enhancing mixing. Numerical and experimental studies to determine the effect of the notch dimensions and placement within the microchannel were carried out to optimize the mixing performance. Results show that the final mixer design is efficient at mixing fluids at low Re. The mixer is fabricated in polydimethylsiloxane (PDMS) bonded to glass slides and tested using fluorescence dyes. Results show that the new design exhibit complete mixing at Re < 0.1 within 7 mm and thus will benefit a wide range of LOC applications where space is limited.