Jean-Christophe Galas

Microfluidic tunable dye laser with integrated mixer and ring resonator

We report on results of design and fabrication of a microfluidic dye laser that consists of a ring resonator, a waveguide for laser emission output, and microfluidic elements for flow control, all integrated on a chip. The optical resonator and the waveguide were obtained by photolithography, whereas microfluidic elements such as channels, valves, and pumps were fabricated by multilayer soft lithography. As results, the prototype device worked with a few nanoliters of Rhodamine 6G dye molecules in ethanol solution and showed a laser threshold of ∼15μJ∕mm2 when optically pumped with a frequency doubled Nd:YAG laser at 532nm wavelength. The modification of the laser output intensity due to photobleaching effect was characterized by changing the dye flow velocity through the cavity. In addition, the emission wavelength of the laser could be easily tuned by changing the dye molecule concentration with the integrated microfluidic elements.

Microfluidic patterning of miniaturized DNA arrays on plastic substrates.

This paper describes the patterning of DNA arrays on plastic surfaces using an elastomeric, two-dimensional microcapillary system (muCS). Fluidic structures were realized through hot-embossing lithography using Versaflex CL30. Like elastomers based on poly(dimethylsiloxane), this thermoplastic block copolymer is able to seal a surface in a reversible manner, making it possible to confine DNA probes with a level of control that is unparalleled using standard microspotting techniques. We focus on muCSs that support arrays comprising up to 2 x 48 spots, each being 45 mum in diameter. Substrates were fabricated from two hard thermoplastic materials, poly(methylmethacrylate) and a polycyclic olefin (e.g., Zeonor 1060R), which were both activated with 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride and N-hydroxysuccinimide to mediate covalent attachment of DNA molecules. The approach was exemplified by using 0.25-32 muM solutions of amino-modified oligonucleotides labeled with either Cy3 or Cy5 fluorescent dye in phosphate-buffered saline, allowing for a direct and sensitive characterization of the printed arrays. Solutions were incubated for durations of 1 to >48 h at 22, 30, and 40 degrees C to probe the conditions for obtaining uniform spots of high fluorescence intensity. The length (l) and depth (d) of microfluidic supply channels were both important with respect to depletion as well as evaporation of the solvent. While selective activation of the substrate proved helpful to limit unproductive loss of oligonucleotides along trajectories, incubation of solution in a humid environment was necessary to prevent uncontrolled drying of the liquid, keeping the immobilization process intact over extended periods of time. When combined, these strategies effectively promoted the formation of high-quality DNA arrays, making it possible to arrange multiple probes in parallel with a high degree of uniformity. Moreover, we show that resultant arrays are compatible with standard hybridization protocols, which allowed for reliable discrimination of individual strands when exposed to a specific ssDNA target molecule.

Active connectors for microfluidic drops on demand

We introduce a simple and versatile microfluidic drop-on-demand solution that enables independent and dynamical control of both the drop size and the drop production rate. To do so, we combine a standard microfluidic T- junction and a novel active switching component that connects the microfluidic channel to the macroscopic liquid reservoirs. Firstly, we explain how to make this simple but accurate drop-on-demand device. Secondly, we carefully characterize its dynamic response and its range of operations. Finally, we show how to generate complex two-dimensional drop patterns dynamically in single or multiple synchronized drop-on-demand devices.

Fabrication of three-dimensional microstructures using standard ultraviolet and electron-beam lithography

We have designed and fabricated shape-controlled three-dimensional structures combining electron- beam and ultraviolet (UV) lithography. Starting with a tiff file format, a gray scale pattern using a high energy beam sensitive mask is produced with electron-beam direct writing. In only a one step UV exposure, an optical lens and a photonic band gap structure in 16.5 μm thick AZ 4562 positive photoresist, with controlled profile and smooth resist surface, were fabricated. A three-dimensional grid structure was also transferred to a 100 μm thick SU-8 negative photoresist.

Microfluidic dye laser intracavity absorption

The authors report absorption measurements on low concentration analytes using a microfluidic dye laser. The laser cavity is made of two gold mirrors coated on the end faces of two optical fibers inserted in a chip. Rhodamine 6G dye molecules dissolved in ethanol are used for laser amplification and absorption measurements are done with methylene blue dye solutions. When optically pumped with a frequency doubled Nd:YAG laser at 532nm wavelength, the device shows a laser output emission at 565nm and a high sensitivity of the lasing output to the losses in the cavity, in good agreement with the results of numerical calculations.

Microscopic agents programmed by DNA circuits

Information stored in synthetic nucleic acids sequences can be used in vitro to create complex reaction networks with precisely programmed chemical dynamics. Here, we scale up this approach to program networks of microscopic particles (agents) dispersed in an enzymatic solution. Agents may possess multiple stable states, thus maintaining a memory and communicate by emitting various orthogonal chemical signals, while also sensing the behaviour of neighbouring agents. Using this approach, we can produce collective behaviours involving thousands of agents, for example retrieving information over long distances or creating spatial patterns. Our systems recapitulate some fundamental mechanisms of distributed decision making and morphogenesis among living organisms and could find applications in cases where many individual clues need to be combined to reach a decision, for example in molecular diagnostics.

Pressure-assisted selective preconcentration in a straight nanochannel.

We investigate the preconcentration profiles of a fluorescein and bovine serum albumin derivatized with this fluorescent tag in a microfluidic chip bearing a nanoslit. A new preconcentration method in which a hydrodynamic pressure is added to both electroosmotic and electrophoretic contributions is proposed to monitor the location of the preconcentration frontline. A simple predictive model of this pressure-assisted electropreconcentration is proposed for the evolution of the flow profile along this micro/nano/microfluidic structure. We show with a small analyte such as fluorescein that the additional hydrostatic pressure mode enables to stabilize the concentration polarization (CP) effect, resulting in better control of the cathodic focusing (CF) peak. For BSA (bovine serum albumin), we exhibit that the variation of the hydrodynamic pressure can have an even more drastic effect on the preconcentration. We show that, depending on this hydrodynamic pressure, the preconcentration can be chosen, either in the cathodic side or in the anodic one. For the first time, we prove here that both anodic focusing (AF) and cathodic focusing (CF) regimes can be reached in the same structures. These results also open new routes for the detection and the quantification of low abundance biomarkers.

Amplification and Temporal Filtering during Gradient Sensing by Nerve Growth Cones Probed with a Microfluidic Assay

Nerve growth cones (GCs) are chemical sensors that convert graded extracellular cues into oriented axonal motion. To ensure a sensitive and robust response to directional signals in complex and dynamic chemical landscapes, GCs are presumably able to amplify and filter external information. How these processing tasks are performed remains however poorly known. Here, we probe the signal-processing capabilities of single GCs during γ-Aminobutyric acid (GABA) directional sensing with a shear-free microfluidic assay that enables systematic measurements of the GC output response to variable input gradients. By measuring at the single molecule level the polarization of GABA(A) chemoreceptors at the GC membrane, as a function of the external GABA gradient, we find that GCs act as i), signal amplifiers over a narrow range of concentrations, and ii), low-pass temporal filters with a cutoff frequency independent of stimuli conditions. With computational modeling, we determine that these systems-level properties arise at a molecular level from the saturable occupancy response and the lateral dynamics of GABA(A) receptors.

Compact dye laser on a chip fabricated by ultraviolet nanoimprint lithography

High aspect ratio and high resolution distributed feed back (DFB) gratings have been patterned on a fused silica plate by ultraviolet nanoimprint lithography and reactive ion etch techniques. Then, they were integrated into a microfluidic chip for optofluidic operations. The authors observed laser emission from organic dye solutions flowing through an optical resonator formed between two third order DFB gratings. Such a dye layer can operate with a picoliter dye solution. With rhodamine 6G dye molecules dissolved in ethanol and pumped by a frequency-doubled Nd:YAG pulsed laser, a laser emission of threshold of 12μJ∕mm2 has been found.

Pursuit-and-Evasion Reaction-Diffusion Waves in Microreactors with Tailored Geometry.

Out-of-equilibrium chemical systems may self-organize into structures displaying spatiotemporal order, such as traveling waves and Turing patterns. Because of its predictable chemistry, DNA has recently appeared as an interesting candidate to engineer these spatiotemporal structures. However, in addition to the intrinsic chemical parameters, initial and boundary conditions have a major impact on the final structure. Here we take advantage of microfluidics to design controlled reactors and investigate pursuit-and-evasion chemical waves generated by a DNA-based reaction network with Predator-Prey dynamics. We first propose two complementary microfabrication strategies to either control the initial condition or the two-dimensional geometry of the reactor where the waves develop. We subsequently use them to investigate the effect of curvature in wave propagation. We finally show that DNA-based waves can compute the optimal path within a maze. We thus suggest that coupling configurable microfluidics to programmable DNA-based dissipative reaction networks is a powerful route to investigate spatiotemporal order formation in chemistry.