Microfluidic blood vasculature replicas using backside lithography.

Blood vessels in living tissues are an organized and hierarchical network of arteries, arterioles, capillaries, veinules and veins. Their sizes, lengths, shapes and connectivity are set up for an optimum perfusion of the tissues in which they deploy. In order to study the hemodynamics and hemophysics of blood flows and also to investigate artificial vasculature for organs on a chip, it is essential to reproduce most of these geometric features. Common microfluidic techniques produce channels with a uniform height and a rectangular cross section that do not capture the size hierarchy observed in vivo. This paper presents a new single-mask photolithography process using an optical diffuser to produce a backside exposure leading to microchannels with both a rounded cross section and a direct proportionality between local height and local width, allowing a one-step design of intrinsically hierarchical networks.

Study of Surface Charge Instabilities by EOF Measurements on a Chip: A Real-Time Hysteresis and Peptide Adsorption Based Methodology.

This paper describes the measurement of the electroosmotic mobility (EOF) in a Wheatstone fluidic bridge (µFWB) as a direct probe of the surface instability. The variation of EOF known as one major contribution of the electrokinetic migration has been determined with a real-time measurement platform after different conditionings on chips. We also scan the pH of the background electrolytes with three different ionic strengths to evaluate the dependencies of the EOF as a function of the pH. A hysteresis methodology has been developed for probing the surface charge instabilities. EOF mobility has been recorded during on-a-chip electrophoresis to estimate the effect of such instability on the analytical performance. As expected, our experimental curves show that a decrease in the ionic strength increases the surface charge stability of the hybrid microchip. This result demonstrates that ionic exchanges between the surface and the fluid are clearly involved in the stability of the surface charge. With this original method based on real-time EOF measurement, the surface state can be characterized after hydrodynamic and electrophoresis sequences to mimic any liquid conditioning and separation steps. Finally, as a demonstrative application, isotherms of the adsorption of insulin have been recorded showing the change in surface charge by unspecific adsorption of this biomolecule onto the microfluidic channel's wall. These methodologies and findings could be particularly relevant to investigating various analytical pathways and to understanding the molecular mechanisms at solid/liquid interfaces.

Electrochemiluminescence on-a-chip: towards a hand-held electrically powered optofluidic source.

We report a microfluidic platform that integrates several parallel optical sources based on electrochemiluminescence (ECL) of 9,10-diphenylanthracene (DPA) as luminophore agent. The annihilation of DPA radicals provides a low wavelength emission at λ=430 nm in the blue-visible range. By varying the distance between electrodes for each ECL integrated source, this glass/PDMS/glass platform enabled a systematic investigation of the main electrochemical parameters involved in ECL. These parameters have been studied either in a static mode or in a dynamic one. Even at slow flow rate (~2 µl s(-1)), the renewal of electroactive species could be easily promoted inside the microfluidic channel which gives rise to a stable optical intensity for several minutes. Compared with traditional optically pumped dye sources, this microfluidic system demonstrates that ECL can be easily implemented on chip for producing much compact optofluidic sources. Such simply electrically powered system-on-chip would surely encourage the future of hand-held µTAS devices with integrated fast detection and embedded electronics.

Fast microfluidic temperature control for high resolution live cell imaging.

One major advantage of using genetically tractable model organisms such as the fission yeast Schizosaccharomyces pombe is the ability to construct temperature-sensitive mutations in a gene. The resulting gene product or protein behaves as wildtype at permissive temperatures. At non-permissive or restrictive temperatures the protein becomes unstable and some or all of its functions are abrogated. The protein regains its function when returning to a permissive temperature. In principle, temperature-sensitive mutation enables precise temporal control of protein activity when coupled to a fast temperature controller. Current commercial temperature control devices do not have fast switching capability over a wide range of temperatures, making repeated temperature changes impossible or impractical at the cellular timescale of seconds or minutes. Microfabrication using soft-lithography is emerging as a powerful tool for cell biological research. We present here a simple disposable polydimethylsiloxane (PDMS) based microfluidic device capable of reversibly switching between 5 °C and 45 °C in less than 10 s. This device allows high-resolution live cell imaging with an oil immersion objective lens. We demonstrate the utility of this device for studying microtubule dynamics throughout the cell cycle.