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Capillary driven microfluidic devices rely on hydrophilic surfaces to facilitate and control the fluid flow inside microchannels. This can be accomplished through the deposition of a highly functional plasma polymer film (PPF) on the surface of the device. Highly functional plasma polymer films exhibit ageing effects characterized by oxidation, degradation, swelling, dissolution, and restructuring at the surface to minimize the surface energy. The application of plasma polymer films in microfluidics is, therefore, limited by this instability. To overcome this limitation, plasma polymer films in the form of a vertical chemical gradient structure were deposited on the surface of microfluidic devices designed to perform immunodiagnostics for use during space missions. The vertical chemical gradient PPFs were composed of a highly cross-linked base layer with gradually less cross-linked and more functional surface layers. The highly cross-linked base layer of the gradient structure effectively hindered restructuring at the surface of the film, resulting in a more stable surface composition and surface wettability than that of the more traditional non-gradient structure. In addition, the microfluidics prepared using a gradient structure showed more stable chip runtimes. The stability of the gradient plasma polymer films will allow for their implementation in microfluidics, as well as a diverse range of biomedical applications such as drug delivery and the generation of antibacterial coatings.

Microfluidic capillary systems operate using capillary forces only and require capillary pumps (CPs) to transport, regulate, and meter the flow of small quantity of liquids. Common CP architectures include arrays of posts or tree-like branched conduits that offer many parallel flow paths. However, both designs are susceptible to bubble entrapment and consequently variation in the metered volume. Here, we present two novel CP architectures that deterministically guide the filling front along rows while preventing the entrapment of bubbles using variable gap sizes between posts. The first CP (serpentine pump) guides the filling front following a serpentine path, and the second one (leading-edge pump) directs the liquid along one edge of the CP from where liquid fills the pump row-by-row. We varied the angle of the rows with respect to the pump perimeter and observed filling with minimal variation in pumping pressure and volumetric flow rate, confirming the robustness of this design. In the case of serpentine CP, the flow resistance for filling the first row is high and then decreases as subsequent rows are filled and many parallel flow paths are formed. In the leading-edge pump, parallel flow paths are formed almost immediately, and hence, no spike in flow resistance is observed; however, there is a higher tendency of the liquid for skipping a row, requiring more stringent fabrication tolerances. The flow rate within a single CP was adjusted by tuning the gap size between the microposts within the CP so as to create a gradient in pressure and flow rate. In summary, CP with deterministic guidance of the filling front enables precise and robust control of flow rate and volume.

A novel method is introduced for ultrahigh throughput and ultralow cost patterning of biomolecules with nanometer resolution and novel 2D digital nanodot gradients (DNGs) with mathematically defi ned slopes are created. The technique is based on lift-off nanocontact printing while using high-resolution photopolymer stamps that are rapidly produced at a low cost through double replication from Si originals. Printed patterns with 100 nm features are shown. DNGs with varying spacing between the dots and a record dynamic range of 4400 are produced; 64 unique DNGs, each with hundreds of thousands of dots, are inked and printed in 5.5 min. The adhesive response and haptotaxis of C2C12 myoblast cells on DNGs demonstrated their biofunctionality. The great fl exibility in pattern design, the massive parallel ability, the ultra low cost, and the extreme ease of polymer lift-off nanocontact printing will facilitate its use for various biological and medical applications.

Microfluidic capillary systems employ surface tension effects to manipulate liquids, and are thus self-powered and self-regulated as liquid handling is structurally and chemically encoded in microscale conduits. However, capillary systems have been limited to perform simple fluidic operations. Here, we introduce complex capillary flow circuits that encode sequential flow of multiple liquids with distinct flow rates and flow reversal. We first introduce two novel microfluidic capillary elements including (i) retention burst valves and (ii) robust low aspect ratio trigger valves. These elements are combined with flow resistors, capillary retention valves, capillary pumps, and open and closed reservoirs to build a capillary circuit that, following sample addition, autonomously delivers a defined sequence of multiple chemicals according to a preprogrammed and predetermined flow rate and time. Such a circuit was used to measure the concentration of C-reactive protein. This work illustrates that as in electronics, complex capillary circuits may be built by combining simple capillary elements. We define such circuits as "capillarics", and introduce symbolic representations. We believe that more complex circuits will become possible by expanding the library of building elements and formulating abstract design rules.

We present and characterize cotton yarn and knots as building blocks for making microfluidic circuits from the bottom up. The yarn used is made up of 200-300 fibres, each with a lumen. Liquid applied at the extremity of the yarn spontaneously wets the yarn, and the wetted length increases linearly over time in untreated yarn, but progresses according to a square root relationship as described by Washburn's equation upon plasma activation of the yarn. Knots are proposed for combining, mixing and splitting streams of fluids. Interestingly, the topology of the knot controls the mixing ratio of two inlet streams into two outlet yarns, and thus the ratio can be adjusted by choosing a specific knot. The flow resistance of a knot is shown to depend on the force used to tighten it and the flow resistance rapidly increases for single-stranded knots, but remains low for double-stranded knots. Finally, a serial dilutor is made with a web made of yarns and double-stranded overhand knots. These results suggest that yarn and knots may be used to build low cost microfluidic circuits.

SU-8 can be patterned with high resolution, is flexible and tough. These characteristics qualify SU-8 as a material for making spotting pins for printing DNA and protein microarrays, and it can potentially replace the commonly used silicon and steel pins that are expensive, brittle in the case of silicon and can damage the substrate during the printing process. SU-8, however, accumulates large internal stress during fabrication and, as a consequence, thin and long SU-8 structures bend and coil up, which precludes using it for long, freestanding structures such as pins. Here we introduce (i) a novel fabrication process that allows the making of 30 mm long, straight spotting pins that feature (ii) a new design and surface chemistry treatments for better capillary flow control and more homogeneous spotting. A key innovation for the fabrication is a post-processing annealing step with slow temperature ramping and mechanical clamping between two identical substrates to minimize stress buildup and render it symmetric, respectively, which together yield a straight SU-8 structure. SU-8 pins fabricated using this process are compliant and resilient and can buckle without damage during printing. The pins comprise a novel flow stop valve for accurate metering of fluids, and their surface was chemically patterned to render the outside of the pin hydrophobic while the inside of the slit is hydrophilic, and the slit thus spontaneously fills when dipped into a solution while preventing droplet attachment on the outside. A single SU-8 pin was used to print 1392 protein spots in one run. SU-8 pins are inexpensive, straightforward to fabricate, robust and may be used as disposable pins for microarray fabrication. These pins serve as an illustration of the potential application of ultralow stress SU-8 for making freestanding microfabricated polymer microstructures.

High resolution live cell microscopy is increasingly used to detect cellular dynamics in response to drugs and chemicals, but it depends on complex and expensive liquid handling devices that have limited its wider adoption. Here, we present a microfluidic perfusion system that is built without using specialized microfabrication infrastructure, simple to use because only a pipette is needed for liquid handling, and yet allows for rapid media exchange and simultaneous fluorescence microscopy imaging. Yeast cells may be introduced from a culture, or spotted as arrays on a coverslip, and are sandwiched with a 20 mum thick track-etched membrane. A second coverslip and a mesh with 120 mum porosity are placed on top, forming a microfluidic conduit for lateral flow of solutions by capillary effects. Solutions introduced through the inlet flow through the mesh and chemicals diffuse vertically across the membrane to the cells trapped below. Solutions are exchanged by adding a new sample to the inlet. Using this system, we studied the dynamic response of F-actin in living yeast expressing Sac6-EGFP-a protein associated with discrete F-actin structures called "patches"-to the drug latrunculin A, a well known inhibitor of actin polymerization. We observed that the patches disappeared in 85% of the cells within 5 min, and re-assembled in 45 min following exchange of the drug with media. The perfusion system presented here is a simple, inexpensive device suited for analysis of drug dose-response and regeneration of single cells and arrays of cells.