Use of a virtual wall valve in polydimethylsiloxane microfluidic devices for bioanalytical applications

A simple method for micromanipulation of liquids and∕or small groups of cells is presented in this study. Microfabricated sieving structures composed of PDMS (polydimethylsiloxane) were used to segregate aqueous solutions. This microfluidic valving scheme was an application of Cassie-Baxter wetting and was termed "virtual walls" as a nonsolid barrier exists at an air∕water interface. The manipulation of the virtual-air-wall valve was accomplished by controlling the strength of surface-tension and hydrostatic-pressure forces. Virtual walls with a range of feature sizes were designed and characterized by monitoring air and water displacement in response to hydrostatic pressure. Thresholds for the virtual-air-wall valves to be turned on or off were quantified. The walls could also be formed or dissipated by the focused microbeam of a pulsed laser. As an illustration of the virtual wall utility, a series of microfluidic applications were demonstrated. First, the capability of virtual walls to temporarily segregate liquids was integrated into a device utilized to establish a chemical gradient. In a second application, the arraying of nonadherent cells within individual aqueous cavities created by the virtual walls was demonstrated. Individual cells were also released from the cavities on demand using a focused microbeam. The virtual walls were simple and easy-to-fabricate without the requirement for surface treatment or precision alignment, and should find usage in bioanalytical applications.