Tailored nanostructured titania integrated on titanium micropillars with outstanding wicking properties.

A novel wicking material using nanostructured titania grown on high aspect ratio titanium micropillars is demonstrated. High aspect ratio titanium micropillars were micromachined from bulk titanium sheets. Nanostructured titania was then grown on the surface of titanium micropillars by oxidation in aqueous hydrogen peroxide solution followed by thermal annealing. The nanostructured titania formed has an open porous structure with a nanoscale pore diameter and wall thickness. X-ray diffraction and pole figure studies indicate the formation of anatase phase of titania and the absence of a preferred orientation in the porous film. The hybrid nanostructured titania on titanium micropillars has excellent hydrophilic properties with a water capillary speed comparable to or exceeding that of conventional wick materials commonly used in heat pipes for the thermal management of electronic devices.

Superhydrophilicity on microstructured titanium surfaces via a superficial titania layer with interconnected nanoscale pores.

Microstructured titanium (Ti) surfaces often suffer from poor hydrophilicity which makes the realization of open microfluidic devices difficult. Here, we investigate the effect of a superficial porous titania (TiO2) layer on the hydrophilicity of microstructured surfaces. High aspect ratio Ti micropillars were micromachined from bulk Ti sheets. Porous TiO2 was subsequently grown on Ti micropillars by a wet oxidation route followed by thermal annealing. Porous TiO2 was characterized using atomic force microscopy, x-ray diffraction and x-ray photoelectron spectroscopy. Detailed morphology study and pore size analysis were carried using focused ion beam machining coupled with scanning electron microscopy. Static contact angle and dynamic spreading studies clearly demonstrate enhanced hydrophilicity of microstructured Ti surfaces with a superficial porous TiO2 layer. Such enhancement promises interesting applications in the microfluidics and microsystems fields.

A unified scaling model for flow through a lattice of microfabricated posts.

A scaling model is presented for low Reynolds number viscous flow within an array of microfabricated posts. Such posts are widely used in several lab-on-a-chip applications such as heat pipes, antibody arrays and biomolecule separation columns. Finite element simulations are used to develop a predictive model for pressure driven viscous flow through posts. The results indicate that the flow rate per unit width scales as approximately h1.17g1.33/d0.5 where h is the post height, d post diameter and g is the spacing between the posts. These results compare favorably to theoretical limits. The scaling is extended to capillary pressure driven viscous flows. This unified model is the first report of a scaling that incorporates both viscous and capillary forces in the microfabricated post geometry. The model is consistent with Washburn dynamics and was experimentally validated to within 8% using wetting on microfabricated silicon posts.

Improved endothelial cell adhesion and proliferation on patterned titanium surfaces with rationally designed, micrometer to nanometer features.

Previous in vitro studies have demonstrated increased vascular endothelial cell adhesion on random nanostructured titanium (Ti) surfaces compared with conventional (or nanometer smooth) Ti surfaces. These results indicated for the first time the potential nanophase metals have for improving vascular stent efficacy. However, considering the structural properties of the endothelium, which is composed of elongated vascular endothelial cells aligned with the direction of blood flow, it has been speculated that rationally designed, patterned nano-Ti surface features could further enhance endothelial cell functions by promoting a more native cellular morphology. To this end, patterned Ti surfaces consisting of periodic arrays of grooves with spacings ranging from 750 nm to 100 microm have been successfully fabricated in the present study by utilizing a novel plasma-based dry etching technique that enables machining of Ti with unprecedented resolution. In vitro rat aortic endothelial cell adhesion and growth assays performed on these substrates demonstrated enhanced endothelial cell coverage on nanometer-scale Ti patterns compared with larger micrometer-scale Ti patterns, as well as controls consisting of random nanostructured surface features. Furthermore, nanometer-patterned Ti surfaces induced endothelial cell alignment similar to the natural endothelium. Since the re-establishment of the endothelium on vascular stent surfaces is critical for stent success, the present study suggests that nanometer to submicrometer patterned Ti surface features should be further investigated for improving vascular stent efficacy.

Beam-supported AlN thin film bulk acoustic resonators.

A novel, suspended thin film bulk acoustic wave resonator (SFBAR) has been fabricated from an aluminum nitride film sputtered directly on a (100) silicon substrate. The suspended membrane design uses thin beams to support, as well as electrically connect, the resonator and has been fabricated using both thin film processing and bulk silicon micromachining. The quality factor and the effective electromechanical coupling coefficient were characterized as a function of the number and the length of the support beams. The length of the support beams was found to affect neither the quality factor at resonance nor the effective electromechanical coupling factor. However, longer support beams did facilitate better frequency pair response. Device performance varied with the number of support beams: 70% of the resonators tested showed a higher figure of merit with eight support beams than with four support beams.

Control of particles in microelectrode devices.

The impact of the convective fluid motion induced by the electric fields on the dielectrophoretic manipulation of particles is investigated theoretically and experimentally. By means of a simplified model a channel with a periodic array of microelectrodes we show that electroconvective flows induce the formation of traps for particles, providing a dynamical mechanism to control microparticles in such devices. We demonstrate experimentally the theoretically predicted dynamical phenomena.

High-aspect-ratio bulk micromachining of titanium.

Recent process developments have permitted the highly anisotropic bulk micromachining of titanium microelectromechanical systems (MEMS). By using the metal anisotropic reactive ion etching with oxidation (MARIO) process, arbitrarily high-aspect-ratio structures with straight sidewalls and micrometre-scale features have been bulk micromachined into titanium substrates of various thicknesses, ranging from 0.5-mm sheet down to 10-microm free-standing titanium foils. Bulk micromachined structures are generally free of residual stresses and are preferred when large, rigid, flat and/or high-force actuators are desired. However, so far there has been a limited ability to select materials on the basis of specific application in bulk micromachining, primarily because of the predominance of MEMS processes dedicated to single-crystal silicon, such as silicon deep reactive ion etching. The MARIO process permits the creation of bulk titanium MEMS, which offers potential for the use of a set of material properties beyond those provided by traditional semiconductor-based MEMS. Consequently, the MARIO process enables the fabrication of novel devices that capitalize on these assets to yield enhanced functionalities that would not be possible with traditional micromechanical material systems.