Leon M. Bellan

High quality factor resonance at room temperature with nanostrings under high tensile stress

Quality factors as high as 207 000 are demonstrated at room temperature for radio-frequency silicon nitride string resonators with cross sectional dimensions on the scale of 100nm, made with a nonlithographic technique. A product of quality factor and surface to volume ratio greater than 6000nm−1 is presented, the highest yet reported. Doubly clamped nanostring resonators are fabricated in high tensile-stress silicon nitride using a nonlithographic electrospinning process. We fabricate devices with an electron beam process, and demonstrate frequency and quality factor results identical to those obtained with the nonlithographic technique. We also compare high tensile-stress doubly clamped beams with doubly clamped and cantilever resonators made of a lower stress material, as well as cantilever beams made of the high stress material. In all cases, the doubly clamped high stress beams have the highest quality factors. We therefore attribute the high quality factors to high tensile stress. Potential dominant...

Fabrication of an artificial 3-dimensional vascular network using sacrificial sugar structures

Using sacrificial sugar structures, we have formed a 3D fluidic vascular network in a polymeric matrix. Melt-spun sugar fibers (cotton candy) were used to form channels with diameters and densities similar to those of capillaries. To interface to macroscopic fluidic systems, larger sacrificial sugar structures were used to form an artificial inlet and outlet. To verify that the channel network supported flow, we used video fluorescence microscopy to image both 2 µm fluorescent polystyrene spheres in an aqueous solution and fluorescently labeled blood. This fabrication process may be applied to a wide range of polymeric materials and is rapid, inexpensive, and highly scalable.

Control of an electrospinning jet using electric focusing and jet-steering fields

Electrospinning can be used to deposit a wide variety of nanoscale polymeric fibers that have electrical, optical, or biological properties of interest. While there have been many studies of material properties, the typical deposited nanofibers are in the form of a randomly oriented mat. The authors are interested in forming functional devices utilizing the properties of the individual nanofibers. To this end they have used electric fields to both confine and steer an electrospun polymer jet for controlled deposition of functional materials. They have used an electrode between the electrospinning tip and grounded sample to suppress the chaotic whipping mode, thereby focusing the characteristic spot size of the deposited fibers to a smaller diameter. The same electrode setup was then modified to produce a time-varying steering field. Using this system, they have deposited isolated electrospun polymer fibers in a controlled fashion. They have also demonstrated that it is possible to terminate electrospun fi...

Current trends in nanobiosensor technology

The development of tools and processes used to fabricate, measure, and image nanoscale objects has lead to a wide range of work devoted to producing sensors that interact with extremely small numbers (or an extremely small concentration) of analyte molecules. These advances are particularly exciting in the context of biosensing, where the demands for low concentration detection and high specificity are great. Nanoscale biosensors, or nanobiosensors, provide researchers with an unprecedented level of sensitivity, often to the single molecule level. The use of biomolecule-functionalized surfaces can dramatically boost the specificity of the detection system, but can also yield reproducibility problems and increased complexity. Several nanobiosensor architectures based on mechanical devices, optical resonators, functionalized nanoparticles, nanowires, nanotubes, and nanofibers have been demonstrated in the lab. As nanobiosensor technology becomes more refined and reliable, it is likely it will eventually make its way from the lab to the clinic, where future lab-on-a-chip devices incorporating an array of nanobiosensors could be used for rapid screening of a wide variety of analytes at low cost using small samples of patient material.

Measurement of the Young's moduli of individual polyethylene oxide and glass nanofibres

We have used scanned electrospinning to deposit oriented polyethylene oxide and silica glass fibres over trenches etched in silicon. We measured the Youngs moduli of the fibres using an atomic force microscope. The Youngs moduli of the glass fibres agree with values calculated from previously measured mechanical resonance frequencies of similar fibres. The Youngs moduli of the polyethylene oxide fibres are significantly larger than those reported for polyethylene oxide bulk and films, suggesting molecular orientation in the fibres.

A 3D Interconnected Microchannel Network Formed in Gelatin by Sacrificial Shellac Microfibers

3D microfluidic networks are fabricated in a gelatin hydrogel using sacrificial melt-spun microfibers made from a material with pH-dependent solubility. The fibers, after being embedded within the gel, can be removed by changing the gel pH to induce dissolution. This process is performed in an entirely aqueous environment, avoiding extreme temperatures, low pressures, and toxic organic solvents.

Stress-based vapor sensing using resonant microbridges

We demonstrate that silicon-polymer composite microbridges provide a robust means of water vapor detection at ambient pressure. Volumetric changes in the reactive polymer alter the tension in a doubly clamped structure leading to large and rapid changes in the resonance frequency. We demonstrate stress-based sensing of water vapor in ambient pressure nitrogen using doubly clamped buckled beams coated with a hygroscopic polymer. We show stress sensitivity of around 20 kPa (∼170 ppb of water vapor) and subsecond response time for coated microbridges.

Directing human embryonic stem cell differentiation by non-viral delivery of siRNA in 3D culture.

Human embryonic stem cells (hESCs) hold great potential as a resource for regenerative medicine. Before achieving therapeutic relevancy, methods must be developed to control stem cell differentiation. It is clear that stem cells can respond to genetic signals, such as those imparted by nucleic acids, to promote lineage-specific differentiation. Here we have developed an efficient system for delivering siRNA to hESCs in a 3D culture matrix using lipid-like materials. We show that non-viral siRNA delivery in a 3D scaffolds can efficiently knockdown 90% of GFP expression in GFP-hESCs. We further show that this system can be used as a platform for directing hESC differentiation. Through siRNA silencing of the KDR receptor gene, we achieve concurrent downregulation (60-90%) in genes representative of the endoderm germ layer and significant upregulation of genes representative of the mesoderm germ layer (27-90 fold). This demonstrates that siRNA can direct stem cell differentiation by blocking genes representative of one germ layer and also provides a particularly powerful means to isolate the endoderm germ layer from the mesoderm and ectoderm. This ability to inhibit endoderm germ layer differentiation could allow for improved control over hESC differentiation to desired cell types.

Direct measurement of fluid velocity in an electrospinning jet using particle image velocimetry

By observing the movement of small fluorescent particles in an electrospinning jet, we have directly measured the fluid velocity along the jet axis. The correlation between these direct velocity measurements and the velocity calculated from the jet radius using volume conservation indicates when evaporation is a significant factor. Measurements of the fluid properties of the solution used in the experiment allow us to construct a plot of Deborah number as a function of position along the jet. Our data also indicate transverse movement at the beginning of the fluid jet, potentially indicating the precursor to the macroscopic bending instability.

Nanochannels fabricated in polydimethylsiloxane using sacrificial electrospun polyethylene oxide nanofibers

The authors have used electrospun polyethylene oxide nanofibers as sacrificial templates to form nanofluidic channels in polydimethylsiloxane (PDMS). By depositing fibers on silicon templates incorporating larger structures, the authors demonstrate that these nanochannels can be integrated easily with microfluidics. They use fluorescence microscopy to image channels filled with dye solution. The utility of the hybrid micro- and nanofluidic PDMS structures for single molecule observation and manipulation was demonstrated by introducing single molecules of λ-DNA into the channels. This nanofabrication technique allows the simple construction of integrated micro- and nanofluidic PDMS structures without lithographic nanofabrication techniques.