Ian Papautsky

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).

Laminar fluid behavior in microchannels using micropolar fluid theory

Abstract In this paper, we describe microchannel fluid behavior using a numerical model based on micropolar fluid theory and experimentally verify the model using micromachined channels. The micropolar fluid theory augments the laws of classical continuum mechanics by incorporating the effects of fluid molecules on the continuum. The behavior of fluids was studied using surface micromachined rectangular metallic pipette arrays. Each array consisted of 5 or 7 pipettes with widths varying from 50 to 600 μm and heights ranging from 20 to 30 μm. A downstream port for static pressure measurement was used to eliminate entrance effects. A controllable syringe pump was used to provide flow while a differential pressure transducer was used to record pressure drop. The experimental data obtained for water showed an increase in the Darcy friction factor when compared to traditional macroscale theory, especially at the lower Reynolds number flows. The numerical model of the micropolar fluid theory predicted experimental data better than the classical Navier–Stokes theory and the model compares favorably with the currently available experimental data.

Enhanced particle filtration in straight microchannels using shear-modulated inertial migration

In this work, we introduce a novel method for enhanced particle filtration using shear-modulated inertial migration in straight microchannels. Depending on their size, inertial lift causes particles to migrate toward microchannel walls. Using microchannels with high aspect ratio cross sections, the fluidic shear can be modulated, resulting in preferential equilibration of particles along the longer microchannel walls. Due to large lift forces generated in these high aspect ratio channels, complete particle filtration can be achieved in short distances even at low flow rates (Re<50). Based on this principle, we use a straight microfluidic channel with a rectangular cross section to passively and continuously filter 1.9 μm polystyrene particles. Overall, the proposed technique is versatile and can be easily integrated with on-chip microfluidic systems for filtration of a wide range of particle sizes, from micro- to nanoparticles.

POLYMER EMBOSSING TOOLS FOR RAPID PROTOTYPING OF PLASTIC MICROFLUIDIC DEVICES

In this paper, we describe a simple and rapid method of fabricating hot embossing tools using polydimethylsiloxane (PDMS), which are then used to rapidly fabricate microchannels in polymethylmethacrylate (PMMA). A negative photoepoxy SU-8 or thick positive photoresist AZ4620 on silicon was used for molding during PDMS casting. Fabrication time of these PDMS tools was considerably less than those using conventional techniques and remains the same regardless of the aspect ratio of features. The described approach was used to emboss microchannels in PMMA of aspect ratios up to 2, with depths from 5 to 250 µm and widths over 40 µm. The technique was also applied to fabricating orthogonal 3D microchannels using multiple lithography steps. The use of a soft tool material increased cycle time and limited the tool lifetime to approximately 20 cycles. Our successful demonstration of PDMS embossing tools presents an alternative approach for rapidly prototyping microfluidic biochips when fast processing and low cost are important and the number of samples is relatively low.

Adhesive RFID Sensor Patch for Monitoring of Sweat Electrolytes

Wearable digital health devices are dominantly found in rigid form factors such as bracelets and pucks. An adhesive radio-frequency identification (RFID) sensor bandage (patch) is reported, which can be made completely intimate with human skin, a distinct advantage for chronological monitoring of biomarkers in sweat. In this demonstration, a commercial RFID chip is adapted with minimum components to allow potentiometric sensing of solutes in sweat, and surface temperature, as read by an Android smartphone app with 96% accuracy at 50 mM Na+ (in vitro tests). All circuitry is solder-reflow integrated on a standard Cu/polyimide flexible-electronic layer including an antenna, but while also allowing electroplating for simple integration of exotic metals for sensing electrodes. Optional paper microfluidics wick sweat from a sweat porous adhesive allowing flow to the sensor, or the sensor can be directly contacted to the skin. The wearability of the patch has been demonstrated for up to seven days, and includes a protective textile which provides a feel and appearance similar to a standard Band-Aid. Applications include hydration monitoring, but the basic capability is extendable to other mM ionic solutes in sweat (Cl-, K+, Mg2+, NH4+, and Zn2+). The design and fabrication of the patch are provided in full detail, as the basic components could be useful in the design of other wearable sensors.

Continuous separation of blood cells in spiral microfluidic devices

Blood cell sorting is critical to sample preparation for both clinical diagnosis and therapeutic research. The spiral inertial microfluidic devices can achieve label-free, continuous separation of cell mixtures with high throughput and efficiency. The devices utilize hydrodynamic forces acting on cells within laminar flow, coupled with rotational Dean drag due to curvilinear microchannel geometry. Here, we report on optimized Archimedean spiral devices to achieve cell separation in less than 8 cm of downstream focusing length. These improved devices are small in size (<1 in.(2)), exhibit high separation efficiency (∼95%), and high throughput with rates up to 1 × 10(6) cells per minute. These device concepts offer a path towards possible development of a lab-on-chip for point-of-care blood analysis with high efficiency, low cost, and reduced analysis time.

Optimization of a Paper-Based ELISA for a Human Performance Biomarker

Monitoring aspects of human performance during various activities has recently become a highly investigated research area. Many new commercial products are available now to monitor human physical activity or responses while performing activities ranging from playing sports, to driving, and even sleeping. However, monitoring cognitive performance biomarkers, such as neuropeptides, is still an emerging field due to the complicated sample collection and processing, as well as the need for a clinical lab to perform analysis. Enzyme-linked immunosorbent assays (ELISAs) provide specific detection of biomolecules with high sensitivity (picomolar concentrations). Even with the advantage of high sensitivity, most ELISAs need to be performed in a laboratory setting and require around 6 h to complete. Transitioning this assay to a platform where it reduces cost, shortens assay time, and is able to be performed outside a lab is invaluable. Recently developed paper diagnostics provide an inexpensive platform on which to perform ELISAs; however, the major limiting factor for moving out of the laboratory environment is the measurement and analysis instrumentation. Using something as simple as a digital camera or camera-enabled Windows- or Android-based tablets, we are able to image paper-based ELISAs (P-ELISAs), perform image analysis, and produce response curves with high correlation to target biomolecule concentration in the 10 pM range. Neuropeptide Y detection was performed. Additionally, silver enhancement of Au NPs conjugated with IgG antibodies showed a concentration-dependent response to IgG, thus eliminating the need for an enzyme-substrate system. Automated image analysis and quantification of antigen concentrations are able to be performed on Windows- and Android-based mobile platforms.

A low-temperature IC-compatible process for fabricating surface-micromachined metallic microchannels

In this paper, a low-temperature integrated-circuit (IC)-compatible process for fabricating metallic microchannels is described. Arrays of 1-100 metallic microchannels have been fabricated on silicon and glass substrates. The process can be extended to many planar substrate materials including polymers and ceramics. The microchannels are formed using microelectro-formed metals. The microchannels demonstrated in this paper use nickel as the structural material and gold as the surface coating on the inside walls of the microchannels. The inner dimensions of the individual microchannels fabricated to date range from 30 /spl mu/m to 1.5 mm in width, 0.5 mm to several centimeters in length, and 5-100 /spl mu/m in thickness. The wall thickness ranges from 5 to 50 /spl mu/m. The microchannel fabrication technology enables the fabrication of surface microchannels with a relatively large cross-sectional area. The metallic microchannels can be fabricated to extend from the substrate edge. Interfacing schemes are given for attaching external pressure feeds.

Enhancing particle dispersion in a passive planar micromixer using rectangular obstacles

In this paper, we numerically and experimentally examine the effectiveness of conventional fluidic micromixers for the mixing of particle flows and demonstrate that microfluidic mixers designed for efficient fluidic mixing are not necessarily efficient at mixing particles. To enhance particle mixing, particles must be forced to move laterally in microchannels. We accomplish this by introducing obstructions within the microfluidic channel to yield a simple planar passive micromixer design. The micromixer performance was benchmarked against a conventional Y-mixer and the modified Tesla mixer, which has been shown to work well with fluids. Simulations using CFD-ACE+ and experiments using fluorescently labeled 590 nm polystyrene particles show that the obstruction micromixer of this work exhibits excellent mixing performance, achieving ~90% fluid mixing in 5 mm and ~90% particle dispersion in 3 mm for Re = 0.05. Overall, the micromixer has a simple planar structure, thus resulting in easy realization and integration with on-chip components in microfluidic lab-on-a-chip (LOC) systems.

Effects of rectangular microchannel aspect ratio on laminar friction constant

In this paper, the effects of rectangular microchannel aspect ratio on laminar friction constant are described. The behavior of fluids was studied using surface micromachined rectangular metallic pipette arrays. Each array consisted of 5 or 7 pipettes with widths varying from 150 micrometers to 600 micrometers and heights ranging from 22.71 micrometers to 26.35 micrometers . A downstream port for static pressure measurement was used to eliminate entrance effects. A controllable syringe pump was used to provide flow while a differential pressure transducer was used to record the pressure drop. The experimental data obtained for water for flows at Reynolds numbers below 10 showed an approximate 20% increase in the friction constant for a specified driving potential when compared to macroscale predictions from the classical Navier-Stokes theory. When the experimental data are studied as a function of aspect ratio, a 20% increase in the friction constant is evident at low aspect ratios. A similar increase is shown by the currently available experimental data for low Reynolds number (< 100) flows of water.