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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. [ABSTRACT FROM AUTHOR]

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 μm thick track-etched membrane. A second coverslip and a mesh with 120 μm 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. [ABSTRACT FROM AUTHOR]

Wet etching of glass by hydrofluoric acid is widely used in microfabrication, but is limited by the isotropic nature of the process that leads to rounded sidewalls and a 90° angle between the etch front and the surface of the substrate. For many applications such as microvalving, or for further processing such as spin-coating, well controlled, gently sloping sidewalls are often preferred. Here, we present a new approach for forming straight facets and for adjusting the sidewall angle in wet-etched glass substrates by controlling the lateral dissolution of an etch mask during etching. The etch mask comprises a Ti–Au bilayer where Au serves to protect the Ti. During isotropic etching of glass by HF the Ti layer is etched away laterally at the same time, which leads to straight, gently sloping sidewalls. We introduce two methods for controlling the sidewall angle. The first one is based on adjusting the thickness of Ti which controls the lateral etch rate, and thus the angle; the thinner the Ti, the slower its lateral etch rate and the steeper the angle in the etched glass. The second method involves a cathodic bias applied to the Ti–Au etch mask which again regulates the dissolution rate of Ti; the more negative the bias the slower the lateral etch rate. Both methods offer accurate control of the sidewall angle over a wide range, can be readily integrated into existing fabrication processes, and will be particularly useful for making channels with trapezoidal cross-sections, valve seats with gentle profiles, or for patterning electrodes across and inside of microfluidic channels. [ABSTRACT FROM AUTHOR]