Microscale Diffusiophoresis of Proteins.

Living systems are characterized by their spatially highly inhomogeneous nature which is susceptible to modify fundamentally the behavior of biomolecular species, including the proteins that underpin biological functionality in cells. Spatial gradients in chemical potential are known to lead to strong transport effects for colloidal particles, but their effect on molecular scale species such as proteins has remained largely unexplored. Here, we improve on existing diffusiophoresis microfluidic technique to measure protein diffusiophoresis in real space. The measurement of proteins is made possible by two ameliorations. First, a label-free microscope is used to suppress label interference. Second, improvements in numerical methods are developed to meet the particular challenges posed by small molecules. We demonstrate that individual proteins can undergo strong diffusiophoretic motion in salt gradients in a manner which is sufficient to overcome diffusion and which leads to dramatic changes in their spatial organization on the scale of a cell. Moreover, we demonstrate that this phenomenon can be used to control the motion of proteins in microfluidic devices. These results open up a path towards a physical understanding of the role of gradients in living systems in the spatial organization of macromolecules and highlight novel routes towards protein sorting applications on device.

Multidimensional protein characterisation using microfluidic post-column analysis.

The biological function of proteins is dictated by the formation of supra-molecular complexes that act as the basic machinery of the cell. As such, measuring the properties of protein species in heterogeneous mixtures is of key importance for understanding the molecular basis of biological function. Here, we describe the combination of analytical microfluidic tools with liquid chromatography for multidimensional characterisation of biomolecules in complex mixtures in the solution phase. Following chromatographic separation, a small fraction of the flow-through is distributed to multiple microfluidic devices for analysis. The microfluidic device developed here allows the simultaneous determination of the hydrodynamic radius, electrophoretic mobility, effective molecular charge and isoelectric point of isolated protein species. We demonstrate the operation principle of this approach with a mixture of three unlabelled model proteins varying in size and charge. We further extend the analytical potential of the presented approach by analysing a mixture of interacting streptavidin with biotinylated BSA and fluorophores, which form a mixture of stable complexes with diverse biophysical properties and stoichiometries. The presented microfluidic device positioned in-line with liquid chromatography presents an advanced tool for characterising multidimensional physical properties of proteins in biological samples to further understand the assembly/disassembly mechanism of proteins and the nature of complex mixtures.

Attoliter protein nanogels from droplet nanofluidics for intracellular delivery.

Microscale hydrogels consisting of macromolecular networks in aqueous continuous phases have received increasing attention because of their potential use in tissue engineering, cell encapsulation and for the storage and release of cargo molecules. However, for applications targeting intracellular delivery, their micrometer-scale size is unsuitable for effective cellular uptake. Nanoscale analogs of such materials are thus required for this key area. Here, we describe a microfluidics/nanofluidics-based strategy for generating monodisperse nanosized water-in-oil emulsions with controllable sizes ranging from 2500 ± 110 nm down to 51 ± 6 nm. We demonstrate that these nanoemulsions can act as templates to form protein nanogels stabilized by supramolecular fibrils from three different proteins. We further show that these nanoparticles have the ability to penetrate mammalian cell membranes and deliver intracellular cargo. Due to their biocompatibility and lack of toxicity, natural protein-based nanoparticles present advantageous characteristics as vehicles for cargo molecules in the context of pharmaceutical and biomedical applications.

Low-Cost Microfabrication Tool Box.

Microsystems are key enabling technologies, with applications found in almost every industrial field, including in vitro diagnostic, energy harvesting, automotive, telecommunication, drug screening, etc. Microsystems, such as microsensors and actuators, are typically made up of components below 1000 microns in size that can be manufactured at low unit cost through mass-production. Yet, their development for commercial or educational purposes has typically been limited to specialized laboratories in upper-income countries due to the initial investment costs associated with the microfabrication equipment and processes. However, recent technological advances have enabled the development of low-cost microfabrication tools. In this paper, we describe a range of low-cost approaches and equipment (below £1000), developed or adapted and implemented in our laboratories. We describe processes including photolithography, micromilling, 3D printing, xurography and screen-printing used for the microfabrication of structural and functional materials. The processes that can be used to shape a range of materials with sub-millimetre feature sizes are demonstrated here in the context of lab-on-chips, but they can be adapted for other applications. We anticipate that this paper, which will enable researchers to build a low-cost microfabrication toolbox in a wide range of settings, will spark a new interest in microsystems.

Nucleation and Growth of Amino Acid and Peptide Supramolecular Polymers through Liquid-Liquid Phase Separation.

The transition of peptides and proteins from the solution phase into fibrillar structures is a general phenomenon encountered in functional and aberrant biology and is increasingly exploited in soft materials science. However, the fundamental molecular events underpinning the early stages of their assembly and subsequent growth have remained challenging to elucidate. Here, we show that liquid-liquid phase separation into solute-rich and solute-poor phases is a fundamental step leading to the nucleation of supramolecular nanofibrils from molecular building blocks, including peptides and even amphiphilic amino acids. The solute-rich liquid droplets act as nucleation sites, allowing the formation of thermodynamically favorable nanofibrils following Ostwald's step rule. The transition from solution to liquid droplets is entropy driven while the transition from liquid droplets to nanofibrils is mediated by enthalpic interactions and characterized by structural reorganization. These findings shed light on how the nucleation barrier toward the formation of solid phases can be lowered through a kinetic mechanism which proceeds through a metastable liquid phase.

Observation of molecular self-assembly events in massively parallel microdroplet arrays.

The self-assembly of peptide and protein molecules into nanoscale filaments is a process associated with both biological function and malfunction. Microfluidic techniques can provide powerful tools in the study of such aggregation phenomena while providing access to exploring the role of molecular interactions in disease development. Yet, a common challenge encountered in the study of protein aggregation is the difficulty in achieving spatial and temporal control of the underlying processes. Here, we present a planar (2-D) device allowing for both the generation and confinement of 10 000 monodisperse water-in-oil droplets in an array of chambers with a trapping efficiency of 99%. Due to the specific geometry of the device, droplets can be formed and immediately trapped on the same chip, without the need for continuous flow of the oil phase. Furthermore, we demonstrate the capability of this device as a platform to study the aggregation kinetics and determine stochastic molecular nanoscale self-assembly events in a highly parallel manner for the aggregation of the dipeptide, diphenylalanine, the core recognition motif of the Aß-42 peptide associated with Alzheimer's disease. The ability to reproducibly generate and confine monodisperse water-in-oil droplets with an extremely high trapping efficiency while maintaining entrapment under zero-flow conditions, on timescales compatible with observing molecular self-assembly events, renders it promising for numerous potential further applications in the biological and biophysical fields.

Enhancing the Resolution of Micro Free Flow Electrophoresis through Spatially Controlled Sample Injection.

Free flow electrophoresis is a versatile technique for the continuous separation of mixtures with both preparative and analytical applications. Microscale versions of free flow electrophoresis are particularly attractive strategies because of their fast separation times, ability to work with small sample volumes, and large surface area to volume ratios facilitating rapid heat transfer, thus minimizing the detrimental effects of Joule heating even at high voltages. The resolution of microscale free flow electrophoresis, however, is limited by the broadening of the analyte beam in the microfluidic channel, an effect that becomes especially pronounced when the analyte is deflected significantly away from its original position. Here, we describe and demonstrate how restricting spatially the sample injection and collection to the regions where the gradients in the velocity distribution of the carrier medium are the smallest allows this broadening effect to be substantially suppressed and hence the resolution of microscale free flow electrophoresis devices to be increased. To demonstrate this concept, we fabricated microfluidic free flow electrophoresis devices with spatially restricted injection nozzles implemented through the use of multilayer soft-photolithography and further integrated quartz based observation areas for fluorescent detection and imaging. With these devices, we demonstrated a 5-fold reduction in the extent of beam broadening relative to conventional free flow electrophoresis approaches with nonrestricted sample introduction. The manifold enhancement in the achievable resolution of microscale free flow electrophoresis devices opens up the possibility of rapid separation and analysis of complex mixtures.

Real-Time Intrinsic Fluorescence Visualization and Sizing of Proteins and Protein Complexes in Microfluidic Devices.

Optical detection has become a convenient and scalable approach to read out information from microfluidic systems. For the study of many key biomolecules, however, including peptides and proteins, which have low fluorescence emission efficiencies at visible wavelengths, this approach typically requires labeling of the species of interest with extrinsic fluorophores to enhance the optical signal obtained - a process which can be time-consuming, requires purification steps, and has the propensity to perturb the behavior of the systems under study due to interactions between the labels and the analyte molecules. As such, the exploitation of the intrinsic fluorescence of protein molecules in the UV range of the electromagnetic spectrum is an attractive path to allow the study of unlabeled proteins. However, direct visualization using 280 nm excitation in microfluidic devices has to date commonly required the use of coherent sources with frequency multipliers and devices fabricated out of materials that are incompatible with soft lithography techniques. Here, we have developed a simple, robust, and cost-effective 280 nm LED platform that allows real-time visualization of intrinsic fluorescence from both unlabeled proteins and protein complexes in polydimethylsiloxane microfluidic channels fabricated through soft lithography. Using this platform, we demonstrate intrinsic fluorescence visualization of proteins at nanomolar concentrations on chip and combine visualization with micron-scale diffusional sizing to measure the hydrodynamic radii of individual proteins and protein complexes under their native conditions in solution in a label-free manner.

Enhanced Quality Factor Label-free Biosensing with Micro-Cantilevers Integrated into Microfluidic Systems.

Microelectromechanical systems (MEMS) have enabled the development of a new generation of sensor platforms. Acoustic sensor operation in liquid, the native environment of biomolecules, causes, however, significant degradation of sensing performance due to viscous drag and relies on the availability of capture molecules to bind analytes of interest to the sensor surface. Here, we describe a strategy to interface MEMS sensors with microfluidic platforms through an aerosol spray. Our sensing platform comprises a microfluidic spray nozzle and a microcantilever array operated in dynamic mode within a closed loop oscillator. A solution containing the analyte is sprayed uniformly through picoliter droplets onto the microcantilever surface; the micrometer-scale drops evaporate rapidly and leave the solutes behind, adding to the mass of the cantilever. This sensing scheme results in a 50-fold increase in the quality factor compared to operation in liquid, yet allows the analytes to be introduced into the sensing system from a solution phase. It achieves a 370 femtogram limit of detection, and we demonstrate quantitative label-free analysis of inorganic salts and model proteins. These results demonstrate that the standard resolution limits of cantilever sensing in dynamic mode can be overcome with the integration of spray microfluidics with MEMS.

Systematic development of small molecules to inhibit specific microscopic steps of Aß42 aggregation in Alzheimer's disease.

The aggregation of the 42-residue form of the amyloid-ß peptide (Aß42) is a pivotal event in Alzheimer's disease (AD). The use of chemical kinetics has recently enabled highly accurate quantifications of the effects of small molecules on specific microscopic steps in Aß42 aggregation. Here, we exploit this approach to develop a rational drug discovery strategy against Aß42 aggregation that uses as a read-out the changes in the nucleation and elongation rate constants caused by candidate small molecules. We thus identify a pool of compounds that target specific microscopic steps in Aß42 aggregation. We then test further these small molecules in human cerebrospinal fluid and in a Caenorhabditis elegans model of AD. Our results show that this strategy represents a powerful approach to identify systematically small molecule lead compounds, thus offering an appealing opportunity to reduce the attrition problem in drug discovery.