Gavin D. M. Jeffries

Fabrication of thermoset polyester microfluidic devices and embossing masters using rapid prototyped polydimethylsiloxane molds

Plastics are increasingly being used for the fabrication of Lab-on-a-Chip devices due to the variety of beneficial material properties, affordable cost, and straightforward fabrication methods available from a range of different types of plastics. Rapid prototyping of polydimethylsiloxane (PDMS) devices has become a well-known process for the quick and easy fabrication of microfluidic devices in the research laboratory; however, PDMS is not always an appropriate material for every application. This paper describes the fabrication of thermoset polyester microfluidic devices and masters for hot embossing using replica molding techniques. Rapid prototyped PDMS molds are convienently used for the production of non-PDMS polymeric devices. The recessed features in the cast polyester can be bonded to a second polyester piece to form an enclosed microchannel. Thermoset polyester can withstand moderate amounts of pressure and elevated temperature; therefore, the cast polyester piece also can be used as a master for embossing polymethylmethacrylate (PMMA) microfluidic systems. Examples of enclosed polyester and PMMA microchannels are presented, and we discuss the electroosmotic properties of both types of channels, which are important for analytical applications such as capillary electrophoresis.

Tunable generation of Bessel beams with a fluidic axicon

This paper describes a tunable fluidic conical lens, or axicon, for the generation and dynamic reconfiguration of Bessel beams. When illuminated with a Gaussian laser beam, our fluidic axicon generates a diverging beam with an annular cross section. By varying the refractive index of the solution that fills our device, we can vary easily the spatial properties of the resulting Bessel beam.

Optical gradient flow focusing

This paper describes a new method for carrying out flow cytometry, which employs optical gradient forces to guide and focus particles in the fluid flow. An elliptically shaped Gaussian beam was focused at the center of a microchannel to exert radiation pressure on suspended nanoparticles that are passing through the channel, such that these particles are guided to the center of the channel for efficient detection and sorting. To verify the efficiency of this optical-gradient-flow-focusing method, we present numerical simulations of the trajectories of the nanoparticles in both electroosmotic flow (EOF) and pressure-driven flow (PDF).

Simultaneous Generation of Multiple Aqueous Droplets in a Microfluidic Device

This paper describes a microfluidic platform for the on-demand generation of multiple aqueous droplets, with varying chemical contents or chemical concentrations, for use in droplet based experiments. This generation technique was developed as a complement to existing techniques of continuous-flow (streaming) and discrete-droplet generation by enabling the formation of multiple discrete droplets simultaneously. Here sets of droplets with varying chemical contents can be generated without running the risk of cross-contamination due to the isolated nature of each supply inlet. The use of pressure pulses to generate droplets in parallel is described, and the effect of droplet size is examined in the context of flow rates and surfactant concentrations. To illustrate this technique, an array of different dye-containing droplets was generated, as well as a set of droplets that displayed a concentration gradient of a fluorescent dye.

Quantitative force mapping of an optical vortex trap

This paper describes the quantitative force mapping of micron-sized particles held in an optical vortex trap. We present a simple and efficient model, which accounts for the diffraction of the strongly localized optical field of the tightly focused laser beam, the spherical aberration introduced by the dielectric glass-to-water interface, employs the multidipole approximation for force calculations, and is able to reproduce, with quantitative agreement, the experimentally measured force map.

Fabrication improvements for thermoset polyester (TPE) microfluidic devices

Thermoset polyester (TPE) microfluidic devices were previously developed as an alternative to poly(dimethylsiloxane) (PDMS) devices, fabricated similarly by replica molding, yet offering stable surface properties and good chemical compatibility with some organics that are incompatible with PDMS. This paper describes a number of improvements in the fabrication of TPE chips. Specifically, we describe methods to form TPE devices with a thin bottom layer for use with high numerical aperture (NA) objectives for sensitive fluorescence detection and optical manipulation. We also describe plasma-bonding of TPE to glass to create hybrid TPE-glass devices. We further present a simple master-pretreatment method to replace our original technique that required the use of specialized equipment.

A rapid and economical method for profiling feature heights during microfabrication

Quality control is an important and integral part to any microfabrication process. While the widths of features often can be easily assessed by light microscopy, the heights of the fabricated structures are more difficult to determine. Here, we present a rapid, accurate, and low-cost method to measure the heights of microfabricated structures during and after the fabrication process. This technique is based on white-light interferometry, which offers accuracy on the submicrometre scale.

Optofluidic generation of Laguerre-Gaussian beams

Laguerre-Gaussian (LG) beams have been extensively studied due to their unique structure, characterized by a phase singularity at the center of the beam. Common methods for generating such beams include the use of diffractive optical elements and spatial light modulators, which although offering excellent versatility, suffers from several drawbacks, including in many cases a low power damage threshold as well as complexity and expense. This paper presents a simple, low cost method for the generation of high-fidelity LG beams using rapid prototyping techniques. Our approach is based on a fluidic-hologram concept, whereby the properties of the LG beam can be finely controlled by varying the refractive-index of the fluid that flows through the hologram. This simple approach, while optimized here for LG beam generation, is also expected to find applications in the production of tunable fluidic optical trains.

Controlled fusion of femtoliter-volume aqueous droplets using holographic optical tweezers

This paper describes the use of Laguerre-Gaussian beams for the optical manipulation and fusion of single femtoliter-volume aqueous droplets, a technique developed to facilitate new applications in droplet microfluidics. In addition, this paper describes the possibility of extending this work by using more complex holographic fields generated by computer controlled spatial light modulators (i.e. dynamic holographic optical tweezers) which should lead to the control of larger arrays of droplets. Finally this paper discusses possible applications in the fields of digital microfluidics, with specific reference to the manipulation of cellular material within femtolitre test tubes.