Ionic-surfactant-mediated electro-dewetting for digital microfluidics.

The ability to manipulate droplets on a substrate using electric signals1-known as digital microfluidics-is used in optical2,3, biomedical4,5, thermal6 and electronic7 applications and has led to commercially available liquid lenses8 and diagnostics kits9,10. Such electrical actuation is mainly achieved by electrowetting, with droplets attracted towards and spreading on a conductive substrate in response to an applied voltage. To ensure strong and practical actuation, the substrate is covered with a dielectric layer and a hydrophobic topcoat for electrowetting-on-dielectric (EWOD)11-13; this increases the actuation voltage (to about 100 volts) and can compromise reliability owing to dielectric breakdown14, electric charging15 and biofouling16. Here we demonstrate droplet manipulation that uses electrical signals to induce the liquid to dewet, rather than wet, a hydrophilic conductive substrate without the need for added layers. In this electrodewetting mechanism, which is phenomenologically opposite to electrowetting, the liquid-substrate interaction is not controlled directly by electric field but instead by field-induced attachment and detachment of ionic surfactants to the substrate. We show that this actuation mechanism can perform all the basic fluidic operations of digital microfluidics using water on doped silicon wafers in air, with only ±2.5 volts of driving voltage, a few microamperes of current and about 0.015 times the critical micelle concentration of an ionic surfactant. The system can also handle common buffers and organic solvents, promising a simple and reliable microfluidic platform for a broad range of applications.

Cell interaction with three-dimensional sharp-tip nanotopography.

Cells in their native microenvironment interact with three-dimensional (3D) nanofeatures. Despite many reports on the effects of substrate nanotopography on cells, the independent effect of 3D parameters has not been investigated. Recent advances in nanofabrication for precise control of nanostructure pattern, periodicity, shape, and height enabled this systematic study of cell interactions with 3D nanotopographies. Two distinct nanopatterns (posts and grates) with varying three-dimensionalities (50-600 nm in nanostructure height) were created, while maintaining the pattern periodicity (230 nm in pitch) and tip shape (needle- or blade-like sharp tips). Human foreskin fibroblasts exhibited significantly smaller cell size and lower proliferation on needle-like nanoposts, and enhanced elongation with alignment on blade-like nanogrates. These phenomena became more pronounced as the nanotopographical three-dimensionality (structural height) increased. The nanopost and nanograte architectures provided the distinct contact guidance for both filopodia extension and the formation of adhesion molecules complex, which was believed to lead to the unique cell behaviors observed.