This review describes the design and fabrication of microfluidic systems in poly(dimethylsiloxane) (PDMS). PDMS is a soft polymer with attractive physical and chemical properties: elasticity, optical transparency, flexible surface chemistry, low permeability to water, and low electrical conductivity. Soft lithography makes fabrication of microfluidic systems in PDMS particularly easy. Integration of components, and interfacing of devices with the user, is also convenient and simpler in PDMS than in systems made in hard materials. Fabrication of both single and multilayer microfluidic systems is straightforward in PDMS. Several components are described in detail: a passive chaotic mixer, pneumatically actuated switches and valves, a magnetic filter, functional membranes, and optical components.
This report describes a miniaturized, microfluidic version of a serial-dilution fluorescent immunoassay. 1 This assay is capable of analyzing multiple antibodies quantitatively and in parallel in small volumes (<1 µL) of liquids in one experiment. It uses a network of microfluidic channels to achieve serial dilution; we call this system of microchannels a microdilutor network (µDN, Figure 1). The branching structures of the µDN serially dilute one stream with a second (so long as proper mixing occurs at each stage). Flow in microchannels is normally laminar; 2 to ensure complete mixing, we incorporate chaotic advective mixers (CAMs) into the µDN. 3 In this assay, we use this device to dilute a sample serially with buffer.We illustrate this assay by determining the concentrations of antibodies (IgGs in this case) in HIV+ human serum (anti-gp41 and anti-gp120). In the assay, serially diluted solutions of serum flow in channels across orthogonal, parallel strips of HIV ENV proteins (antigens, gp41, and pg120) adsorbed on a polycarbonate membrane. The soluble antibodies bind to these adsorbed antigens and are themselves immobilized; the quantity of adsorbed antibody can be measured using a second, fluorescent antibody.We believe that this method provides a new and general approach to one of the most common bioanalytical procedures. 1 The µDN replaces the set of microwells used in manual serial dilutions; the format of this assay is the same as that of the traditional ELISAtype assays for HIV; this format includes the sequential adsorption and immobilization of antigen, antibody, and secondary antibody. 4 This method is, we believe, generalizable to the analysis of 10-100 antibodies (as long as there is no cross-reactivity between these antibodies), although we have analyzed only two.The microfluidic device has two components ( Figure 1): the first component is the µDN that dilutes the analyte (serum containing HIV antibodies) serially using CAMs (the top portion of Figure 1) to achieve mixing; the second component is a membrane that presents stripes of immobilized proteins (in this case, gp41 and gp120; bottom portion of Figure 1 and Supporting Information Figure S1). The dilutor mixes and dilutes the serum with a buffer containing 5% bovine serum albumin (BSA), used to block nonspecific adsorption of antibodies on the surfaces of the device); this procedure generates a series of solutions containing exponentially decreasing concentrations of antibodies. The design in Figure 1 5,6 ). We used an array of microchannels to deliver gp120 and gp41 to a polycarbonate membrane. 5 The membrane contained pores of diameter ∼200 nm; these pores presented large surface areas that both adsorbed amounts of proteins sufficient to give good sensitivity and also maintained the proteins in hydrated and active forms when the membrane was momentarily dried during assembly of the system. The antigen-immobilized membrane was sandwiched between the two pieces of PDMS that formed the microchannels. (See the Supporting Information for the ...
This paper describes microfluidic systems that can be used to investigate multiple chemical or biochemical interactions in a parallel format. These three-dimensional systems are generated by crossing two sets of microfluidic channels, fabricated in two different layers, at right angles. Solutions of the reagents are placed in the channels; in different modes of operation, these solutions can be either flowing or stationary-the latter is important when one set of channels is filled with viscous gels with immobilized reagents. At every crossing, the channels are separated either by a single membrane or by a composite separator comprising a membrane, a microwell, and a second membrane. These components allow diffusive mass transport and minimize convective transport through the crossing. Polycarbonate membranes with 0.1-1-microm vertical pores were used to fabricate the devices. Each crossing of parallel channels serves as an element in which chemical or biochemical interactions can take place; interactions can be detected by monitoring changes in fluorescence and absorbance. These all-organic systems are straightforward to fabricate and to operate and may find applications as portable microanalytical systems and as tools in combinatorial research.
This paper describes a dynamic system-a system that develops order only when dissipating energy-comprising millimeter to centimeter scale gears that self-assemble into a simple machine at a fluid/air interface. The gears are driven externally and indirectly by magnetic interactions; they are made of poly(dimethylsiloxane) (PDMS) or magnetically doped PDMS, and fabricated by soft lithography. Transfer of torque between gears can take place through three different mechanisms: mechanical interaction, hydrodynamic shear, and capillarity/overlap of menisci. Interplay between these forces allows interactions and motions that are not possible with conventional systems of gears.
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