Generation of stable complex gradients across two-dimensional surfaces and three-dimensional gels.

Many chemical and biological processes are dependent on molecular gradients. We describe a new microfluidic approach that can be used to produce spatiotemporal gradients across two-dimensional surfaces and three-dimensional gels under flow-free conditions. Free diffusion between dynamically replenished flow channels acting as a sink and source is utilized to give rise to stable steady-state gradient profiles. The gradient profile is dictated by the engineered design of the device's gradient-generating region. Different designs can yield both linear and non-linear gradients of varying profiles. More complex gradients can be made by juxtaposing different designs within a single gradient-generating region. By fabricating an array of designs along the gradient-generating region, different gradient profiles can be generated simultaneously, allowing for parallel analysis. Additionally, simple methods of localizing gels into microdevices are demonstrated. The device was characterized by experimentally obtained gradient profiles of fluorescent molecules that corroborated closely with a simulated finite element model.

Vertically aligned carbon-nanotube arrays showing Schottky behavior at room temperature.

Vertically aligned carbon-nanotube (CNT) arrays were fabricated in the thin-film anodic aluminum oxide (AAO) templates on silicon wafers utilizing a niobium (Nb) thin film as the source electrode. The average diameter of the CNTs was 25 nm, and the number density was 3 x 10(10) cm(-2). The CNT arrays synthesized at 700 degrees C and above exhibited Schottky behavior even at 300 K, with energy gaps between 0.2 eV and 0.3 eV. However, individual CNTs obtained by removal of the template behaved as resistors at 300 K. The CNT/Nb oxide/Nb junction is thought to be responsible for the Schottky behavior. This structure can be a useful cornerstone in the fabrication of nanotransistors operating at room temperature.