DNA compaction in microfluidic devices
In collaboration with the Institute of Bioengineering and Nanotechnology in Singapore, we have devised a series of microfluidic devices that enabled to tune the mixing time of two fluids with respect to the adsorption time between oppositely-charged molecules. By adjusting the flow rates between a central stream focused by two side streams, each of which contained either DNA, surfactants or solvent, the self-assembly process could be directed to yield nanoparticles with small sized. We thus associated by slow diffusion cationic surfactants with 2,000-base-pairs DNA molecules and formed nanoparticles with diameters close to 30 nm. So small objects likely contained one or two DNA molecules at most. This microfluidics-driven strategy is quite general and can presumably be applied to systems with diverse self-assembly kinetics. It might open attractive opportunities for the directed self-assembly of complex soft nanomaterials, especially for gene delivery applications.
(Left) Optical image of the microchannels at the confluence of the three streams containing DNA, water, and surfactant.(Middle) Electron micrographs of surfactant-DNA nanoparticles. (Right) Schematics of a nanoparticle made up of DNA (in blue) compacted by cationic surfactants (in red). [Chem. Mater. 27 (2015) 8193-8197]
How to direct the self assembly of oppositely-charged molecules by using microfluidics
In collaboration with researchers in Singapore and Romania, we have used hydrodynamic flow focusing in microfluidic devices to direct the association of polyelectrolytes with surfactants (Figure 1). This method controls the convective-diffusive mixing of miscible liquids at nanometer length scales which determine the formation of nanoparticles. It has previously allowed the nanoprecipitation of diblock copolymers with narrow size distributions but it was never applied to oppositely charged molecules. We have thus demonstrated the importance of kinetic effects : short mixing times systematically lead to the formation of small nanoparticles. Moreover, microfluidics permits an excellent repeatability from batch to batch and nanoparticle sizes comprised between 50 and 300 nm were obtained simply by varying the ratio of flow rates of the streams carrying the reactants. We anticipate that microfluidics will play an increasing role in the control over the self-assembly kinetics of complex nanoparticles.
Microfluidic device for hydrodynamic flow focusing. (A) Exploded view depicting the various parts of the device made in a combination of glass and silicon. (B) Layouts of the microfluidic channels for both versions of the device. (C) Photograph showing the bottom of the device and its fluidic connections. [Anal. Chem. 85 (2013) 5850-5856]
Lipid vesicles and cells in microfluidic devices
Manipulation, treatment, and analysis of nanostructures and macromolecules within a microfluidic system can be of great interest. To take advantage of micro total analysis systems while avoiding resorting to elaborate tools, we have proposed enclosing the submicrometer species of interest into cell-sized liposomes. Liposomes are synthetic lipid-bilayer vesicles that are used as biocompatible spherical containers. Their ability to encapsulate a wide range of materials makes them attractive as carriers and ultrasmall reactors approximating the in vivo conditions under which biochemical reactions occur. Below, a biocompatible capsule containing a drug, macromolecules, or fabricated nanostructures is displaced over a microarray of electrodes through a flow channel. That capsule is large enough to be detected by conventional means possibly involving fluorescence.
Conceptual schematic for vesicle-based manipulation of macromolecules. One or several molecules, DNA in this case, are enclosed inside a cell-sized liposome which is in turn actuated by an array of asymmetric electrodes, inducing electroosmotic flow through a fluidic channel. [Anal. Chem. 77 (2005) 2795]
In response to an intense external electric field, biological membranes form submicrometer pores, provided their transmembrane potential exceeds a critical breakdown value. Upon the formation of pores, vesicles initially in contact can reconnect their membranes and produce a hybrid vesicle thanks to the surface tension and the fluidity of their membranes. The figure below shows the sequence of events that take place during electrofusion : two loaded vesicles in contact by dielectrophoretic alignment merged into a larger one upon the application of electric pulses identical in duration to those described for electroporation. The resulting vesicle was viable and kept enclosed the fluorescent marker after the pulses ended.
Electrofusion of loaded lipid vesicles. (a) Principle of vesicle fusion depicting the pore reconnection of membranes in contact. (b) Fusion of two vesicles in a 100-µm-wide channel. The arrows indicate the vesicles that fuse and the dashed lines delimit the channel. [Appl. Phys. Lett. 90 (2007) 173901]
In collaboration with Ciprian Iliescu and Guolin Xu at the Institute of Bioengineering and Nanotechnology in Singapore, we have devised field-flow separation techniques under continuous flow in dielectrophoresis (DEP) chips with 3D silicon electrodes. The techniques were made possible by the unique design of the devices, where the electrodes also define the walls of the microfluidic channel (see below). The geometry of the electrodes generates a uniform force in the vertical direction and the force that acts on the particles is uniform along the vertical axis. This is in contrast to DEP by planar electrodes, where the dielectrophoretic force decreases exponentially with the distance from the planar electrodes. The device also offers other important advantages, such as the completely enclosed design and the small dimensions and working volume of the chip. The use of silicon electrodes eliminates electrochemical effects of multimetal electrodes.
Schematic view of the DEP chip and separation method. [Appl. Phys. Lett. 90 (2007) 234104]
- M. NI, G. TRESSET, C. ILIESCU (2017) Self-assembled polysulfone nanoparticles using microfluidic chip. Sens. Actuator B 252 458-462
- C. ILIESCU, G. TRESSET (2015) Microfluidics-driven strategy for size-controlled DNA compaction by slow diffusion through water stream. Chem. Mater. 27 8193-8197
- G. TRESSET, C. MARCULESCU, A. SALONEN, M. NI, C. ILIESCU (2013) Fine control over the size of surfactant-polyelectrolyte nanoparticles by hydrodynamic flow focusing. Anal. Chem. 85 5850-5856
- C. ILIESCU, G. TRESSET, G. XU (2007) Continuous field-flow separation of particle populations in a dielectrophoretic chip with three dimensional electrodes. Appl. Phys. Lett. 90 234104
- G. TRESSET, C. ILIESCU (2007) Electrical control of loaded biomimetic femtoliter vesicles in microfluidic system. Appl. Phys. Lett. 90 173901
- G. TRESSET, S. TAKEUCHI (2005) Utilization of cell-sized lipid containers for nanostructures and macromolecules handling in microfabricated devices. Anal. Chem. 77 2795-2801