Recent advances in X-ray compatible microfluidics for applications in soft materials and life sciences

Recent advances in synchrotron scattering methods for probing the structure and dynamics of colloids

Recent progress in synchrotron based X-ray scattering methods applied to colloid science is reviewed. An important figure of merit of these techniques is that they enable in situ investigations of colloidal systems under the desired thermophysical and rheological conditions. An ensemble averaged simultaneous structural and dynamical information can be derived albeit in reciprocal space. Significant improvements in X-ray source brilliance and advances in detector technology have overcome some of the limitations in the past. Notably coherent X-ray scattering techniques have become more competitive and they provide complementary information to laboratory based real space methods. For a system with sufficient scattering contrast, size ranges from nm to several μm and time scales down to μs are now amenable to X-ray scattering investigations. A wide variety of sample environments can be combined with scattering experiments further enriching the science that could be pursued by means of advanced X-ray scattering instruments. Some of these recent progresses are illustrated via representative examples. To derive quantitative information from the scattering data, rigorous data analysis or modeling is required. Development of powerful computational tools including the use of artificial intelligence have become the emerging trend.

On-Chip Photonic Detection Techniques for Non-Invasive In Situ Characterizations at the Microfluidic Scale

Microfluidics has emerged as a robust technology for diverse applications, ranging from bio-medical diagnostics to chemical analysis. Among the different characterization techniques that can be used to analyze samples at the microfluidic scale, the coupling of photonic detection techniques and on-chip configurations is particularly advantageous due to its non-invasive nature, which permits sensitive, real-time, high throughput, and rapid analyses, taking advantage of the microfluidic special environments and reduced sample volumes. Putting a special emphasis on integrated detection schemes, this review article explores the most relevant advances in the on-chip implementation of UV-vis, near-infrared, terahertz, and X-ray-based techniques for different characterizations, ranging from punctual spectroscopic or scattering-based measurements to different types of mapping/imaging. The principles of the techniques and their interest are discussed through their application to different systems.

RNA structures and dynamics with Å resolution revealed by x-ray free-electron lasers

RNA macromolecules, like proteins, fold to assume shapes that are intimately connected to their broadly recognized biological functions; however, because of their high charge and dynamic nature, RNA structures are far more challenging to determine. We introduce an approach that exploits the high brilliance of x-ray free-electron laser sources to reveal the formation and ready identification of angstrom-scale features in structured and unstructured RNAs. Previously unrecognized structural signatures of RNA secondary and tertiary structures are identified through wide-angle solution scattering experiments. With millisecond time resolution, we observe an RNA fold from a dynamically varying single strand through a base-paired intermediate to assume a triple-helix conformation. While the backbone orchestrates the folding, the final structure is locked in by base stacking. This method may help to rapidly characterize and identify structural elements in nucleic acids in both equilibrium and time-resolved experiments.

RNA structures and dynamics with Å resolution revealed by x-ray free electron lasers

RNA macromolecules, like proteins, fold to assume shapes that are intimately connected to their broadly recognized biological functions; however, because of their high charge and dynamic nature, RNA structures are far more challenging to determine. We introduce an approach that exploits the high brilliance of x-ray free electron laser sources to reveal the formation and ready identification of Å scale features in structured and unstructured RNAs. New structural signatures of RNA secondary and tertiary structures are identified through wide angle solution scattering experiments. With millisecond time resolution, we observe an RNA fold from a dynamically varying single strand through a base paired intermediate to assume a triple helix conformation. While the backbone orchestrates the folding, the final structure is locked in by base stacking. In addition to understanding how RNA triplexes form and thereby function as dynamic signaling elements, this new method can vastly increase the rate of structure determination for these biologically essential, but mostly uncharacterized macromolecules.

In-depth analysis of biocatalysts by microfluidics: An emerging source of data for machine learning

Nowadays, the vastly increasing demand for novel biotechnological products is supported by the continuous development of biocatalytic applications which provide sustainable green alternatives to chemical processes. The success of a biocatalytic application is critically dependent on how quickly we can identify and characterize enzyme variants fitting the conditions of industrial processes. While miniaturization and parallelization have dramatically increased the throughput of next-generation sequencing systems, the subsequent characterization of the obtained candidates is still a limiting process in identifying the desired biocatalysts. Only a few commercial microfluidic systems for enzyme analysis are currently available, and the transformation of numerous published prototypes into commercial platforms is still to be streamlined. This review presents the state-of-the-art, recent trends, and perspectives in applying microfluidic tools in the functional and structural analysis of biocatalysts. We discuss the advantages and disadvantages of available technologies, their reproducibility and robustness, and readiness for routine laboratory use. We also highlight the unexplored potential of microfluidics to leverage the power of machine learning for biocatalyst development.

Sample-minimizing co-flow cell for time-resolved pump-probe X-ray solution scattering

A fundamental problem in biological sciences is understanding how macromolecular machines work and how the structural changes of a molecule are connected to its function. Time-resolved techniques are vital in this regard and essential for understanding the structural dynamics of biomolecules. Time-resolved small- and wide-angle X-ray solution scattering has the capability to provide a multitude of information about the kinetics and global structural changes of molecules under their physiological conditions. However, standard protocols for such time-resolved measurements often require significant amounts of sample, which frequently render time-resolved measurements impossible. A cytometry-type sheath co-flow cell, developed at the BioCARS 14-ID beamline at the Advanced Photon Source, USA, allows time-resolved pump-probe X-ray solution scattering measurements to be conducted with sample consumption reduced by more than ten times compared with standard sample cells and protocols. The comparative capabilities of the standard and co-flow experimental setups were demonstrated by studying time-resolved signals in photoactive yellow protein.

Serial small- and wide-angle X-ray scattering with laboratory sources

Recent advances in X-ray instrumentation and sample injection systems have enabled serial crystallography of protein nanocrystals and the rapid structural analysis of dynamic processes. However, this progress has been restricted to large-scale X-ray free-electron laser (XFEL) and synchrotron facilities, which are often oversubscribed and have long waiting times. Here, we explore the potential of state-of-the-art laboratory X-ray systems to perform comparable analyses when coupled to micro- and millifluidic sample environments. Our results demonstrate that commercial small- and wide-angle X-ray scattering (SAXS/WAXS) instruments and X-ray diffractometers are ready to access samples and timescales (≳5 ms) relevant to many processes in materials science including the preparation of pharmaceuticals, nanoparticles and functional crystalline materials. Tests of different X-ray instruments highlighted the importance of the optical configuration and revealed that serial WAXS/XRD analysis of the investigated samples was only possible with the higher flux of a microfocus setup. We expect that these results will also stimulate similar developments for structural biology.

Near-digital amplification in paper improves sensitivity and speed in biplexed reactions

The simplest point-of-care assays are usually paper and plastic devices that detect proteins or nucleic acids at low cost and minimal user steps, albeit with poor limits of detection. Digital assays improve limits of detection and analyte quantification by splitting a sample across many wells (or droplets), preventing diffusion, and performing analyte amplification and detection in multiple small wells. However, truly digital nucleic acid amplification tests (NAATs) require costly consumable cartridges that are precisely manufactured, aligned, and operated to enable low detection limits. In this study, we demonstrate how to implement near-digital NAATs in low-cost porous media while approaching the low limits of detection of digital assays. The near-digital NAAT was enabled by a paper membrane containing lyophilized amplification reagents that automatically, passively meters and distributes a sample over a wide area. Performing a NAAT in the paper membrane while allowing diffusion captures many of the benefits of digital NAATs if the pad is imaged at a high spatial resolution during amplification. We show that the near-digital NAAT is compatible with a low-cost paper and plastic disposable cartridge coupled to a 2-layer rigid printed circuit board heater (the MD NAAT platform). We also demonstrate compatibility with biplexing and imaging with mobile phones with different camera sensors. We show that the near-digital NAAT increased signal-to-noise ratios by ~ 10×, improved limits of detection from above 10³ copies of methicillin-resistant Staphylococcus aureus genomic DNA to between 100 and 316 copies in a biplexed reaction containing 10⁵ copies of co-amplifying internal amplification control DNA, and reduced time-to-result from 45 min of amplification to 15-20 min for the positive samples.

Manufacturing of Ultra-Thin X-ray-Compatible COC Microfluidic Devices for Optimal In Situ Macromolecular Crystallography Experiments

Cyclic-olefin-copolymer (COC)-based microfluidic devices are increasingly becoming the center of highly valuable research for in situ X-ray measurements due to their compatibility with X-rays, biological compounds, chemical resistance, optical properties, low cost, and simplified handling. COC microfluidic devices present potential solutions to challenging biological applications such as protein binding, folding, nucleation, growth kinetics, and structural changes. In recent years, the techniques applied to manufacturing and handling these devices have capitalized on enormous progress toward small-scale sample probing. Here, we describe the new and innovative design aspects, fabrication, and experimental implementation of low-cost and micron-sized X-ray-compatible microfluidic sample environments that address diffusion-based crystal formation for crystallographic characterization. The devices appear fully compatible with crystal growth and subsequent X-ray diffraction experiments, resulting in remarkably low background data recording. The results highlighted in this research demonstrate how the engineered microfluidic devices allow the recording of accurate crystallographic data at room temperature and structure determination at high resolution.

3D-printed SAXS chamber for controlled in situ dialysis and optical characterization

3D printing changes the scope of how samples can be mounted for small-angle X-ray scattering (SAXS). In this paper a 3D-printed X-ray chamber, which allows for in situ exchange of buffer and in situ optical transmission spectroscopy, is presented. The chamber is made of cyclic olefin copolymers (COC), including COC X-ray windows providing ultra-low SAXS background. The design integrates a membrane insert for in situ dialysis of the 100 µl sample volume against a reservoir, which enables measurements of the same sample under multiple conditions using an in-house X-ray setup equipped with a 17.4 keV molybdenum source. The design's capabilities are demonstrated by measuring reversible structural changes in lipid and polymer systems as a function of salt concentration and pH. In the same chambers optical light transmission spectroscopy was carried out measuring the optical turbidity of the mesophases and local pH values using pH-responsive dyes. Microfluidic exchange and optical spectroscopy combined with in situ X-ray scattering enables vast applications for the study of responsive materials.

Radiotherapy on-chip: microfluidics for translational radiation oncology

The clinical importance of radiotherapy in the treatment of cancer patients justifies the development and use of research tools at the fundamental, pre-clinical, and ultimately clinical levels, to investigate their toxicities and synergies with systemic agents on relevant biological samples. Although microfluidics has prompted a paradigm shift in drug discovery in the past two decades, it appears to have yet to translate to radiotherapy research. However, the materials, dimensions, design versatility and multiplexing capabilities of microfluidic devices make them well-suited to a variety of studies involving radiation physics, radiobiology and radiotherapy. This review will present the state-of-the-art applications of microfluidics in these fields and specifically highlight the perspectives offered by radiotherapy on-a-chip in the field of translational radiobiology and precision medicine. This body of knowledge can serve both the microfluidics and radiotherapy communities by identifying potential collaboration avenues to improve patient care.

X-ray compatible microfluidics for in situ studies of chemical state, transport and reaction of light elements in an aqueous environment using synchrotron radiation

This paper presents an X-ray compatible microfluidic platform for in situ characterization of chemical reactions at synchrotron light sources. We demonstrate easy to implement techniques to probe reacting solutions as they first come into contact, and study the very first milliseconds of their reaction in real-time through X-ray absorption spectroscopy (XAS). The devices use polydimethylsiloxane (PDMS) microfluidic channels sandwiched between ultrathin, X-ray transparent silicon nitride observation windows and rigid substrates. The new approach has three key advantages: i) owing to the assembly techniques employed, the devices are suitable for both high energy and tender (1-5 keV) X-rays; ii) they can operate in a vacuum environment (a must for low energy X-rays) and iii) they are robust enough to survive a full 8 hour shift of continuous scanning with a micro-focused beam, providing higher spatial and thus greater time resolution than previous studies. The combination of these opens new opportunities for in situ studies. This has so far not been possible with Kapton or glass-based flow cells due to increased attenuation of the low energy beam passing through these materials. The devices provide a well-defined mixing region to collect spatial maps of spatially stable concentration profiles, and XAS point spectra to elucidate the chemical structure and characterize the chemical reactions. The versatility of the approach is demonstrated through in situ XAS measurements on the mixing of two reactants in a microfluidic laminar flow device, as well as a segmented droplet based system for time resolved analysis.

3D printed devices and infrastructure for liquid sample delivery at the European XFEL

The Sample Environment and Characterization (SEC) group of the European X-ray Free-Electron Laser (EuXFEL) develops sample delivery systems for the various scientific instruments, including systems for the injection of liquid samples that enable serial femtosecond X-ray crystallography (SFX) and single-particle imaging (SPI) experiments, among others. For rapid prototyping of various device types and materials, sub-micrometre precision 3D printers are used to address the specific experimental conditions of SFX and SPI by providing a large number of devices with reliable performance. This work presents the current pool of 3D printed liquid sample delivery devices, based on the two-photon polymerization (2PP) technique. These devices encompass gas dynamic virtual nozzles (GDVNs), mixing-GDVNs, high-viscosity extruders (HVEs) and electrospray conical capillary tips (CCTs) with highly reproducible geometric features that are suitable for time-resolved SFX and SPI experiments at XFEL facilities. Liquid sample injection setups and infrastructure on the Single Particles, Clusters, and Biomolecules and Serial Femtosecond Crystallography (SPB/SFX) instrument are described, this being the instrument which is designated for biological structure determination at the EuXFEL.

Physicochemical Characterization of Asphaltenes Using Microfluidic Analysis

Crude oils are complex mixtures of organic molecules, of which asphaltenes are the heaviest component. Asphaltene precipitation and deposition have been recognized to be a significant problem in oil production, transmission, and processing facilities. These macromolecular aromatics are challenging to characterize due to their heterogeneity and complex molecular structure. Microfluidic devices are able to capture key characteristics of reservoir rocks and provide new insights into the transport, reactions, and chemical interactions governing fluids used in the oil and gas industry. Understanding the microscale phenomena has led to better design of macroscale processes used by the industry. One area that has seen significant growth is in the area of chemical analysis under flowing conditions. Microfluidics and microscale analysis have advanced the understanding of complex mixtures by providing in situ imaging that can be combined with other chemical characterization methods to give details of how oil, water, and added chemicals interface with pore-scale detail. This review article aims to showcase how microfluidic devices offer new physical, chemical, and dynamic information on the behavior of asphaltenes. Specifically, asphaltene deposition and related flow assurance problems, interfacial properties and rheology, and evaluation of remediation strategies studied in microchannels and microfluidic porous media are presented. Examples of successful applications that address key asphaltene-related problems highlight the advances of microscale systems as a tool for advancing the physicochemical characterization of complex fluids for the oil and gas industry.

A Review of Microfluidic Devices for Rheological Characterisation

The rheological characterisation of liquids finds application in several fields ranging from industrial production to the medical practice. Conventional rheometers are the gold standard for the rheological characterisation; however, they are affected by several limitations, including high costs, large volumes required and difficult integration to other systems. By contrast, microfluidic devices emerged as inexpensive platforms, requiring a little sample to operate and fashioning a very easy integration into other systems. Such advantages have prompted the development of microfluidic devices to measure rheological properties such as viscosity and longest relaxation time, using a finger-prick of volumes. This review highlights some of the microfluidic platforms introduced so far, describing their advantages and limitations, while also offering some prospective for future works.

Development of a sticker sealed microfluidic device for in situ analytical measurements using synchrotron radiation

Shedding synchrotron light on microfluidic systems, exploring several contrasts in situ/operando at the nanoscale, like X-ray fluorescence, diffraction, luminescence, and absorption, has the potential to reveal new properties and functionalities of materials across diverse areas, such as green energy, photonics, and nanomedicine. In this work, we present the micro-fabrication and characterization of a multifunctional polyester/glass sealed microfluidic device well-suited to combine with analytical X-ray techniques. The device consists of smooth microchannels patterned on glass, where three gold electrodes are deposited into the channels to serve in situ electrochemistry analysis or standard electrical measurements. It has been efficiently sealed through an ultraviolet-sensitive sticker-like layer based on a polyester film, and The burst pressure determined by pumping water through the microchannel(up to 0.22 MPa). Overall, the device has demonstrated exquisite chemical resistance to organic solvents, and its efficiency in the presence of biological samples (proteins) is remarkable. The device potentialities, and its high transparency to X-rays, have been demonstrated by taking advantage of the X-ray nanoprobe Carnaúba/Sirius/LNLS, by obtaining 2D X-ray nanofluorescence maps on the microchannel filled with water and after an electrochemical nucleation reaction. To wrap up, the microfluidic device characterized here has the potential to be employed in standard laboratory experiments as well as in in situ and in vivo analytical experiments using a wide electromagnetic window, from infrared to X-rays, which could serve experiments in many branches of science.

An innovative data processing method for studying nanoparticle formation in droplet microfluidics using X-rays scattering

X-ray scattering techniques provide a powerful means of characterizing the formation of nanoparticles in solution. Coupling these techniques to segmented-flow microfluidic devices that offer well-defined environments gives access to in situ time-resolved analysis, excellent reproducibility, and eliminates potential radiation damage. However, analysis of the resulting datasets can be extremely time-consuming, where these comprise frames corresponding to the droplets alone, the continuous phase alone, and to both at their interface. We here describe a robust, low-cost, and versatile droplet microfluidics device and use it to study the formation of magnetite nanoparticles with simultaneous synchrotron SAXS and WAXS. Lateral outlet capillaries facilitate the X-ray analysis and reaction times of between a few seconds and minutes can be accommodated. A two-step data processing method is then described that exploits the unique WAXS signatures of the droplets, continuous phase, and interfacial region to identify the frames corresponding to the droplets. These are then sorted, and the background scattering is subtracted using an automated frame-by-frame approach, allowing the signal from the nanoparticles to be isolated from the raw data. Modeling these data gives quantitative information about the evolution of the sizes and structures of the nanoparticles, in agreement with TEM observations. This versatile platform can be readily employed to study a wide range of dynamic processes in heterogeneous systems.

Fabrication and characterisation of a silicon-borosilicate glass microfluidic device for synchrotron-based hard X-ray spectroscopy studies

Some of the most fundamental chemical building blocks of life on Earth are the metal elements. X-ray absorption spectroscopy (XAS) is an element-specific technique that can analyse the local atomic and electronic structure of, for example, the active sites in catalysts and energy materials and allow the metal sites in biological samples to be identified and understood. A microfluidic device capable of withstanding the intense hard X-ray beams of a 4th generation synchrotron and harsh chemical sample conditions is presented in this work. The device is evaluated at the K-edges of iron and bromine and the L ₃-edge of lead, in both transmission and fluorescence mode detection and in a wide range of sample concentrations, as low as 0.001 M. The device is fabricated in silicon and glass with plasma etched microchannels defined in the silicon wafer before anodic bonding of the glass wafer into a complete device. The device is supported with a well-designed printed chip holder that made the microfluidic device portable and easy to handle. The chip holder plays a pivotal role in mounting the delicate microfluidic device on the beamline stage. Testing validated that the device was sufficiently robust to contain and flow through harsh acids and toxic samples. There was also no significant radiation damage to the device observed, despite focusing with intense X-ray beams for multiple hours. The quality of X-ray spectra collected is comparable to that from standard methods; hence we present a robust microfluidic device to analyse liquid samples using synchrotron XAS.

In Situ X-ray Absorption Spectroscopy and Droplet-Based Microfluidics: An Analysis of Calcium Carbonate Precipitation

Droplet-based microfluidic systems are ideally suited for the investigation of nucleation and crystallization processes. To best leverage the features of such platforms (including exquisite time resolution and high-throughput operation), sensitive and in situ detection schemes are needed to extract real-time chemical information about all species of interest. In this regard, the extension of conventional (UV, visible, and infrared) optical detection schemes to the X-ray region of the electromagnetic spectrum is of high current interest, as techniques such as X-ray absorption spectroscopy (XAS) provide for the element-specific investigation of the local chemical environment. Accordingly, herein, we report for the first time the integration of millisecond droplet-based microfluidics with XAS. Such a platform allows for the sensitive acquisition of X-ray absorption data from picoliter-volume droplets moving at high linear velocities. Significantly, the high-temporal resolution of the droplet-based microfluidic platform enables unprecedented access to the early stages of the reaction. Using such an approach, we demonstrate in situ monitoring of calcium carbonate precipitation by extracting XAS spectra at the early time points of the reaction with a dead time as low as 10 ms. We obtain insights into the kinetics of the formation of amorphous calcium carbonate (ACC) as a first species during the crystallization process by monitoring the proportion of calcium ions converted into ACC. Within the confined and homogeneous environment of picoliter-volume droplets, the ACC content reaches 60% over the first 130 ms. More generally, the presented method offers new opportunities for the real-time monitoring of fast chemical and biological processes.

Stable sample delivery in a viscous medium via a polyimide-based single-channel microfluidic chip for serial crystallography

Serial crystallography (SX) provides room-temperature crystal structures with minimal radiation damage and facilitates the comprehension of molecular dynamics through time-resolved studies. In SX experiments, it is important to deliver a large number of crystal samples to the X-ray interaction point in a serial and stable manner. The advantage of crystal delivery in a viscous medium via a capillary is the ability to deliver all of the crystal samples to the X-ray interaction point at a low flow rate; however, the capillary often breaks during handling and high X-ray absorption can occur at low energy states. This study aimed to develop a stable system for sample delivery in a viscous medium via a polyimide-based single-channel microfluidic (PSM) chip for SX. Since this microfluidic chip comprises a polyimide film, it has high tensile strength and higher X-ray transmittance than a quartz capillary. The PSM chip was connected to a syringe containing the microcrystals embedded in viscous medium. The channel of the PSM chip was aligned to the X-ray path, and the viscous medium containing lysozyme crystals was stably delivered using a syringe pump at a flow rate of 100 nl min−1. Room-temperature lysozyme crystal structures were successfully determined at 1.85 Å resolution. This method would greatly facilitate sample delivery for SX experiments using synchrotron X-rays.

Millisecond timescale reactions observed via X-ray spectroscopy in a 3D microfabricated fused silica mixer

Determination of electronic structures during chemical reactions remains challenging in studies which involve reactions in the millisecond timescale, toxic chemicals, and/or anaerobic conditions. In this study, a three-dimensionally (3D) microfabricated microfluidic mixer platform that is compatible with time-resolved X-ray absorption and emission spectroscopy (XAS and XES, respectively) is presented. This platform, to initiate reactions and study their progression, mixes a high flow rate (0.50-1.5 ml min^-1) sheath stream with a low-flow-rate (5-90 µl min^-1) sample stream within a monolithic fused silica chip. The chip geometry enables hydrodynamic focusing of the sample stream in 3D and sample widths as small as 5 µm. The chip is also connected to a polyimide capillary downstream to enable sample stream deceleration, expansion, and X-ray detection. In this capillary, sample widths of 50 µm are demonstrated. Further, convection-diffusion-reaction models of the mixer are presented. The models are experimentally validated using confocal epifluorescence microscopy and XAS/XES measurements of a ferricyanide and ascorbic acid reaction. The models additionally enable prediction of the residence time and residence time uncertainty of reactive species as well as mixing times. Residence times (from initiation of mixing to the point of X-ray detection) during sample stream expansion as small as 2.1 ± 0.3 ms are also demonstrated. Importantly, an exploration of the mixer operational space reveals a theoretical minimum mixing time of 0.91 ms. The proposed platform is applicable to the determination of the electronic structure of conventionally inaccessible reaction intermediates.

Recent Advances and Future Perspectives on Microfluidic Mix-and-Jet Sample Delivery Devices

The integration of the Gas Dynamic Virtual Nozzle (GDVN) and microfluidic technologies has proven to be a promising sample delivery solution for biomolecular imaging studies and has the potential to be transformative for a range of applications in physics, biology, and chemistry. Here, we review the recent advances in the emerging field of microfluidic mix-and-jet sample delivery devices for the study of biomolecular reaction dynamics. First, we introduce the key parameters and dimensionless numbers involved in their design and characterisation. Then we critically review the techniques used to fabricate these integrated devices and discuss their advantages and disadvantages. We then summarise the most common experimental methods used for the characterisation of both the mixing and jetting components. Finally, we discuss future perspectives on the emerging field of microfluidic mix-and-jet sample delivery devices. In summary, this review aims to introduce this exciting new topic to the wider microfluidics community and to help guide future research in the field.

Shear-free mixing to achieve accurate temporospatial nanoscale kinetics through scanning-SAXS: ion-induced phase transition of dispersed cellulose nanocrystals

Time-resolved in situ characterization of well-defined mixing processes using small-angle X-ray scattering (SAXS) is usually challenging, especially if the process involves changes of material viscoelasticity. In specific, it can be difficult to create a continuous mixing experiment without shearing the material of interest; a desirable situation since shear flow both affects nanoscale structures and flow stability as well as resulting in unreliable time-resolved data. Here, we demonstrate a flow-focusing mixing device for in situ nanostructural characterization using scanning-SAXS. Given the interfacial tension and viscosity ratio between core and sheath fluids, the core material confined by sheath flows is completely detached from the walls and forms a zero-shear plug flow at the channel center, allowing for a trivial conversion of spatial coordinates to mixing times. With this technique, the time-resolved gel formation of dispersed cellulose nanocrystals (CNCs) was studied by mixing with a sodium chloride solution. It is observed how locally ordered regions, so called tactoids, are disrupted when the added monovalent ions affect the electrostatic interactions, which in turn leads to a loss of CNC alignment through enhanced rotary diffusion. The demonstrated flow-focusing scanning-SAXS technique can be used to unveil important kinetics during structural formation of nanocellulosic materials. However, the same technique is also applicable in many soft matter systems to provide new insights into the nanoscale dynamics during mixing.

Innovative 3D Microfluidic Tools for On-Chip Fluids and Particles Manipulation: From Design to Experimental Validation

Micromixers are essential components in lab-on-a-chip devices, of which the low efficiency can limit many bio-application studies. Effective mixing with automation capabilities is still a crucial requirement. In this paper, we present a method to fabricate a three-dimensional (3D) poly(methyl methacrylate) (PMMA) fluidic mixer by combining computer-aided design (CAD), micromilling technology, and experimental application via manipulating fluids and nanoparticles. The entire platform consists of three microfabricated layers with a bottom reservoir-shaped microchannel, a central serpentine channel, and a through-hole for interconnection and an upper layer containing inlets and outlet. The sealing process of the three layers and the high-precision and customizable methods used for fabrication ensure the realization of the monolithic 3D architecture. This provides buried running channels able to perform passive chaotic mixing and dilution functions, thanks to a portion of the pathway in common between the reservoir and serpentine layers. The possibility to plug-and-play micropumping systems allows us to easily demonstrate the feasibility and working features of our device for tracking the mixing and dilution performances of the micromixer by using colored fluids and fluorescent nanoparticles as the proof of concept. Exploiting the good transparency of the PMMA, spatial liquid composition and better control over reaction variables are possible, and the real-time monitoring of experiments under a fluorescence microscope is also allowed. The tools shown in this paper are easily integrable in more complex lab-on-chip platforms.

Accurate and rapid 3D printing of microfluidic devices using wavelength selection on a DLP printer

The use of microfluidics on synchrotron X-ray beamlines represents an advanced sample preparation and delivery platform for state-of-the-art X-ray characterization of micro-samples. The recent developments of 3D printing technologies have opened possibilities for rapid fabrication of complex microfluidic devices. One of the major challenges in 3D printing of microfluidic devices using a digital light processing (DLP) desktop printer is that the static liquid resin trapped in the channels, once the "ceiling" is printed, still receives small doses of light through the subsequently printed layers. This easily triggers partial polymerisation of the resin which impedes its flushing out of the channels after completion of the printing session. We show here that it is possible to gain better control over the resin polymerisation and improve the quality of the microfluidic devices by efficiently reducing the penetration depth of the UV LED light through wavelength selection combined with a careful choice of absorber and photo-initiator materials. We produced and tested several structures using a slightly modified desktop printer at 385 nm wavelength with 37 × 37 μm2 pixel resolution at a printed layer thickness of 25 μm. The structures include particle filters, mixers, droplet generators and droplet storage traps with features below 100 μm. We demonstrate crystallisation of model inorganic and organic compounds in trapped droplets and assess the feasibility of in-device X-ray diffraction experiments. This research opens the path for the use of 3D printed microfluidic devices on X-ray beamlines.

A novel sample delivery system based on circular motion for in situ serial synchrotron crystallography

A sample delivery system is one of the key parts of serial crystallography. It is the main limiting factor affecting the application of serial crystallography. At present, although a variety of useful sample delivery systems have been developed for serial crystallography, it still remains the focus of the field to further improve the performance and efficiency of sample delivery. In existing sample delivery technologies, samples are usually delivered in linear motion. Here we show that the samples can also be delivered using circular motion, which is a novel motion mode never tested before. In this paper, we report a microfluidic rotating-target sample delivery device, which is characterized by the circular motion of the samples, and verify the performance of the device at a synchrotron radiation facility. The microfluidic rotating-target sample delivery device consists of two parts: a microfluidic sample plate and a motion control system. Sample delivery is realized by rotating the microfluidic sample plate containing in situ grown crystals. This device offers significant advantages, including a very wide adjustable range of delivery speed, low background noise, and low sample consumption. Using the microfluidic rotating-target device, we carried out in situ serial crystallography experiments with lysozyme and proteinase K as model samples at the Shanghai Synchrotron Radiation Facility, and performed structural determination based on the serial crystallographic data. The results showed that the designed device is fully compatible with the synchrotron radiation facility, and the structure determination of proteins is successful using the serial crystallographic data obtained with the device.

Evaluation of microflow configurations for scale inhibition and serial X-ray diffraction analysis of crystallization processes

The clean and reproducible conditions provided by microfluidic devices are ideal sample environments for in situ analyses of chemical and biochemical reactions and assembly processes. However, the small size of microchannels makes investigating the crystallization of poorly soluble materials on-chip challenging due to crystal nucleation and growth that result in channel fouling and blockage. Here, we demonstrate a reusable insert-based microfluidic platform for serial X-ray diffraction analysis and examine scale formation in response to continuous and segmented flow configurations across a range of temperatures. Under continuous flow, scale formation on the reactor walls begins almost immediately on mixing of the crystallizing species, which over time results in occlusion of the channel. Depletion of ions at the start of the channel results in reduced crystallization towards the end of the channel. Conversely, segmented flow can control crystallization, so it occurs entirely within the droplet. Consequently, the spatial location within the channel represents a temporal point in the crystallization process. Whilst each method can provide useful crystallographic information, time-resolved information is lost when reactor fouling occurs and changes the solution conditions with time. The flow within a single device can be manipulated to give a broad range of information addressing surface interaction or solution crystallization.

OSTE+ for in situ SAXS analysis with droplet microfluidic devices

In recent years, microfluidic-based sample preparation techniques have emerged as a powerful tool for measurements at large scale X-ray facilities. Most often the microfluidic device was a form of hybrid system, i.e. an assembly of different materials, because a simple, versatile and inexpensive microfabrication method, on the one hand, and X-ray compatibility, on the other hand, cannot generally be achieved by the same material. The arrival of a new polymer family based on off-stoichiometric thiol-ene-epoxy (OSTE+) has recently redistributed the cards. In this context, we studied the relevance and the compatibility of OSTE+ for small-angle X-ray scattering (SAXS) studies. The material was characterized regarding its X-ray properties (transmission coefficient, attenuation coefficient, scattering pattern and polymer aging under X-ray light) and their comparison with those of the usual polymers used in microfluidics and/or for synchrotron radiation experiments. We show that OSTE+ has a better SAXS signal than polyimide, the polymer of reference in the SAXS community. Then a detailed protocol to manufacture a suitably thin full OSTE+ chip (total thickness <500 μm) is described and the potency of full OSTE+ devices for in situ SAXS studies is highlighted in two case-studies: the characterization of gold nanoparticles and the precipitation of cerium oxalate particles, both in moving droplets. Additionally, a method to analyze the scattering signals from droplet and carrier phase in a segmented flow is proposed.

A microvolume shear cell for combined rheology and x-ray scattering experiments

An experimental setup is presented for x-ray scattering studies of soft matter under shear flow that employs a low-background coaxial capillary cell coupled to a high-resolution commercial rheometer. The rotor of the Searle type cell is attached to the rheometer shaft, which allows the application of either steady or oscillatory shear of controlled stress or rate on the sample confined in the annular space between the stator and the rotor. The shearing device facilitates ultrasmall-angle x-ray scattering and ultrasmall-angle x-ray photon correlation spectroscopy measurements with relatively low scattering backgrounds. This enables the elucidation of weak structural features otherwise submerged in the background and probes the underlying dynamics. The performance of the setup is demonstrated by means of a variety of colloidal systems subjected to different rheological protocols. Examples include shear deformation of a short-range attractive colloidal gel, dynamics of dilute colloids in shear flow, distortion of the structure factor of a dense repulsive colloidal suspension, shear induced ordering of colloidal crystals, and alignment of multilamellar microtubes formed by a surfactant-polysaccharide mixture. Finally, the new possibilities offered by this setup for investigating soft matter subjected to shear flow by x-ray scattering are discussed.

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

This article aims to provide an overview of broad range of applications of synchrotron scattering methods in the investigation of nanoscale materials. These scattering techniques allow the elucidation of the structure and dynamics of nanomaterials from sub-nm to micron size scales and down to sub-millisecond time ranges both in bulk and at interfaces. A major advantage of scattering methods is that they provide the ensemble averaged information under in situ and operando conditions. As a result, they are complementary to various imaging techniques which reveal more local information. Scattering methods are particularly suitable for probing buried structures that are difficult to image. Although, many qualitative features can be directly extracted from scattering data, derivation of detailed structural and dynamical information requires quantitative modeling. The fourth-generation synchrotron sources open new possibilities for investigating these complex systems by exploiting the enhanced brightness and coherence properties of X-rays.

Picosecond pump-probe X-ray scattering at the Elettra SAXS beamline

A new setup for picosecond pump-probe X-ray scattering at the Austrian SAXS beamline at Elettra-Sincrotrone Trieste is presented. A high-power/high-repetion-rate laser has been installed on-site, delivering UV/VIS/IR femtosecond-pulses in-sync with the storage ring. Data acquisition is achieved by gating a multi-panel detector, capable of discriminating the single X-ray pulse in the dark-gap of the Elettra hybrid filling mode. Specific aspects of laser- and detection-synchronization, on-line beam steering as well protocols for spatial and temporal overlap of laser and X-ray beam are also described. The capabilities of the setup are demonstrated by studying transient heat-transfer in an In/Al/GaAs superlattice structure and results are confirmed by theoretical calculations.

Comparison of bulk and microfluidic methods to monitor the phase behaviour of nanoparticles during digestion of lipid-based drug formulations using in situ X-ray scattering

The performance of orally administered lipid-based drug formulations is crucially dependent on digestion, and understanding the colloidal structures formed during digestion is necessary for rational formulation design. Previous studies using the established bulk pH-stat approach (Hong et al. 2015), coupled to synchrotron small angle X-ray scattering (SAXS), have begun to shed light on this subject. Such studies of digestion using in situ SAXS measurements are complex and have limitations regarding the resolution of intermediate structures. Using a microfluidic device, the digestion of lipid systems may be monitored with far better control over the mixing of the components and the application of enzyme, thereby elucidating a finer understanding of the structural progression of these lipid systems. This work compares a simple T-junction microcapillary device and a custom-built microfluidic chip featuring hydrodynamic flow focusing, with an equivalent experiment with the full scale pH-stat approach. Both microfluidic devices were found to be suitable for in situ SAXS measurements in tracking the kinetics with improved time and signal sensitivity compared to other microfluidic devices studying similar lipid-based systems, and producing more consistent and controllable structural transformations. Particle sizing of the nanoparticles produced in the microfluidic devices were more consistent than the pH-stat approach.

Laminar flow in microfluidics investigated by spatially-resolved soft X-ray absorption and infrared spectroscopy

The application of soft X-ray absorption spectroscopy (XAS) to liquid cells based on microfluidics for chemical state analysis of light elements is much more difficult than hard X-ray absorption since soft X-rays cannot deeply penetrate a microfluidic cell. In this study, we have newly developed a microfluidic cell for spatially resolved XAS, where a 100 nm thick Si₃N₄ membrane is used for the measurement window to transmit soft X-rays for keeping the microfluidic flow at a width and depth of 50 µm. The π^* peak of pyridine near the N K-edge XAS shows characteristic energy shifts near the liquid-liquid interface in a laminar flow of pyridine and water. The distributions of the molar fractions of pyridine and water near the liquid-liquid interface have been determined from the energy shifts of the π^* peak probed at different geometric positions, where pyridine is mixed in the water part of the laminar flow and vice versa. The spatial distribution of both species has also been studied by infrared microscopy, using the same microfluidic setup. The present work clearly shows that these spectroscopic techniques are easily applicable to chemical and biological reactions prepared by microfluidics.

3D printed droplet generation devices for serial femtosecond crystallography enabled by surface coating

The role of surface wetting properties and their impact on the performance of 3D printed microfluidic droplet generation devices for serial femtosecond crystallography (SFX) are reported. SFX is a novel crystallography method enabling structure determination of proteins at room temperature with atomic resolution using X-ray free-electron lasers (XFELs). In SFX, protein crystals in their mother liquor are delivered and intersected with a pulsed X-ray beam using a liquid jet injector. Owing to the pulsed nature of the X-ray beam, liquid jets tend to waste the vast majority of injected crystals, which this work aims to overcome with the delivery of aqueous protein crystal suspension droplets segmented by an oil phase. For this purpose, 3D printed droplet generators that can be easily customized for a variety of XFEL measurements have been developed. The surface properties, in particular the wetting properties of the resist materials compatible with the employed two-photon printing technology, have so far not been characterized extensively, but are crucial for stable droplet generation. This work investigates experimentally the effectiveness and the long-term stability of three different surface treatments on photoresist films and glass as models for our 3D printed droplet generator and the fused silica capillaries employed in the other fluidic components of an SFX experiment. Finally, the droplet generation performance of an assembly consisting of the 3D printed device and fused silica capillaries is examined. Stable and reproducible droplet generation was achieved with a fluorinated surface coating which also allowed for robust downstream droplet delivery. Experimental XFEL diffraction data of crystals formed from the large membrane protein complex photosystem I demonstrate the full compatibility of the new injection method with very fragile membrane protein crystals and show that successful droplet generation of crystal-laden aqueous droplets intersected by an oil phase correlates with increased crystal hit rates.

3D Micromachined Polyimide Mixing Devices for in Situ X-ray Imaging of Solution-Based Block Copolymer Phase Transitions

Advances in modern interface- and material sciences often rely on the understanding of a system's structure-function relationship. Designing reproducible experiments that yield in situ time-resolved structural information at fast time scales is therefore of great interest, e.g., for better understanding the early stages of self-assembly or other phase transitions. However, it can be challenging to accurately control experimental conditions, especially when samples are only available in small amounts, prone to agglomeration, or if X-ray compatibility is required. We address these challenges by presenting a microfluidic chip for triggering dynamics via rapid diffusive mixing for in situ time-resolved X-ray investigations. This polyimide/Kapton-only-based device can be used to study the structural dynamics and phase transitions of a wide range of colloidal and soft matter samples down to millisecond time scales. The novel multiangle laser ablation three-dimensional (3D) microstructuring approach combines, for the first time, the highly desirable characteristics of Kapton (high X-ray stability with low background, organic solvent compatibility) with a 3D flow-focusing geometry that minimizes mixing dispersion and wall agglomeration. As a model system, to demonstrate the performance of these 3D Kapton microfluidic devices, we selected the non-solvent-induced self-assembly of biocompatible and amphiphilic diblock copolymers. We then followed their structural evolution in situ at millisecond time scales using on-the-chip time-resolved small-angle X-ray scattering under continuous-flow conditions. Combined with complementary results from 3D finite-element method computational fluid dynamics simulations, we find that the nonsolvent mixing is mostly complete within a few tens of milliseconds, which triggers initial spherical micelle formation, while structural transitions into micelle lattices and their deswelling only occur on the hundreds of milliseconds to second time scale. These results could have an important implication for the design and formulation of amphiphilic polymer nanoparticles for industrial applications and their use as drug-delivery systems in medicine.

Microfluidic sample delivery for serial crystallography using XFELs

Serial femtosecond crystallography (SFX) with X-ray free electron lasers (XFELs) is an emerging field for structural biology. One of its major impacts lies in the ability to reveal the structure of complex proteins previously inaccessible with synchrotron-based crystallography techniques and allowing time-resolved studies from femtoseconds to seconds. The nature of this serial technique requires new approaches for crystallization, data analysis, and sample delivery. With continued advancements in microfabrication techniques, various developments have been reported in the past decade for innovative and efficient microfluidic sample delivery for crystallography experiments using XFELs. This article summarizes the recent developments in microfluidic sample delivery with liquid injection and fixed-target approaches, which allow exciting new research with XFELs. Graphical abstract.

A microfluidic flow-focusing device for low sample consumption serial synchrotron crystallography experiments in liquid flow

Serial synchrotron crystallography allows low X-ray dose, room-temperature crystal structures of proteins to be determined from a population of microcrystals. Protein production and crystallization is a non-trivial procedure and it is essential to have X-ray-compatible sample environments that keep sample consumption low and the crystals in their native environment. This article presents a fast and optimized manufacturing route to metal-polyimide microfluidic flow-focusing devices which allow for the collection of X-ray diffraction data in flow. The flow-focusing conditions allow for sample consumption to be significantly decreased, while also opening up the possibility of more complex experiments such as rapid mixing for time-resolved serial crystallography. This high-repetition-rate experiment allows for full datasets to be obtained quickly (∼1 h) from crystal slurries in liquid flow. The X-ray compatible microfluidic chips are easily manufacturable, reliable and durable and require sample-flow rates on the order of only 30 µl h^-1.

The optical stretcher as a tool for single-particle X-ray imaging and diffraction

For almost half a century, optical tweezers have successfully been used to micromanipulate micrometre and sub-micrometre-sized particles. However, in recent years it has been shown experimentally that, compared with single-beam traps, the use of two opposing and divergent laser beams can be more suitable in studying the elastic properties of biological cells and vesicles. Such a configuration is termed an optical stretcher due to its capability of applying high deforming forces on biological objects such as cells. In this article the experimental capabilities of an optical stretcher as a potential sample delivery system for X-ray diffraction and imaging studies at synchrotrons and X-ray free-electron laser (FEL) facilites are demonstrated. To highlight the potential of the optical stretcher its micromanipulation capabilities have been used to image polymer beads and label biological cells. Even in a non-optimized configuration based on a commercially available optical stretcher system, X-ray holograms could be recorded from different views on a biological cell and the three-dimensional phase of the cell could be reconstructed. The capability of the setup to deform cells at higher laser intensities in combination with, for example, X-ray diffraction studies could furthermore lead to interesting studies that couple structural parameters to elastic properties. By means of high-throughput screening, the optical stretcher could become a useful tool in X-ray studies employing synchrotron radiation, and, at a later stage, femtosecond X-ray pulses delivered by X-ray free-electron lasers.

X-ray diffraction imaging of cardiac cells and tissue

With the development of advanced focusing optics for x-rays, we can now use x-ray beams with spot sizes in the micro- or nanometer range to scan cells and large areas of tissues and continuously record the diffraction signals. From this data, x-ray scattering maps or so-called x-ray darkfield images are computed showing how different types of cells or regions of tissues differ in their diffraction intensity. At the same time a diffraction pattern is available for each scan point which encodes the local nanostructure, averaged over many contributing constituents illuminated by the beam. In this work we have exploited these new capabilities of scanning x-ray diffraction to investigate cardiac muscle cells as well as cardiac tissue. We give examples of how cardiac cells, especially living, cultured cells, can be prepared to be compatible with the instrumentation constraints of nano- or micro-diffraction instruments. Furthermore, we show how the developmental stage, ranging from neonatal to adult cells, as well as the final preparation state of the cardiomyocytes influences the recorded scattering signal and how these diffraction signals compare to the structure of a fully developed cardiac muscle.

Microfluidic devices for small-angle neutron scattering

A comparative examination is presented of materials and approaches for the fabrication of microfluidic devices for small-angle neutron scattering (SANS). Representative inorganic glasses, metals, and polymer materials and devices are evaluated under typical SANS configurations. Performance criteria include neutron absorption, scattering background and activation, as well as spatial resolution, chemical compatibility and pressure resistance, and also cost, durability and manufacturability. Closed-face polymer photolithography between boron-free glass (or quartz) plates emerges as an attractive approach for rapidly prototyped microfluidic SANS devices, with transmissions up to ∼98% and background similar to a standard liquid cell (I ≃ 10-3 cm-1). For applications requiring higher durability and/or chemical, thermal and pressure resistance, sintered or etched boron-free glass and silicon devices offer superior performance, at the expense of various fabrication requirements, and are increasingly available commercially.

Microfluidic Chips for In Situ Crystal X-ray Diffraction and In Situ Dynamic Light Scattering for Serial Crystallography

This protocol describes fabricating microfluidic devices with low X-ray background optimized for goniometer based fixed target serial crystallography. The devices are patterned from epoxy glue using soft lithography and are suitable for in situ X-ray diffraction experiments at room temperature. The sample wells are lidded on both sides with polymeric polyimide foil windows that allow diffraction data collection with low X-ray background. This fabrication method is undemanding and inexpensive. After the sourcing of a SU-8 master wafer, all fabrication can be completed outside of a cleanroom in a typical research lab environment. The chip design and fabrication protocol utilize capillary valving to microfluidically split an aqueous reaction into defined nanoliter sized droplets. This loading mechanism avoids the sample loss from channel dead-volume and can easily be performed manually without using pumps or other equipment for fluid actuation. We describe how isolated nanoliter sized drops of protein solution can be monitored in situ by dynamic light scattering to control protein crystal nucleation and growth. After suitable crystals are grown, complete X-ray diffraction datasets can be collected using goniometer based in situ fixed target serial X-ray crystallography at room temperature. The protocol provides custom scripts to process diffraction datasets using a suite of software tools to solve and refine the protein crystal structure. This approach avoids the artefacts possibly induced during cryo-preservation or manual crystal handling in conventional crystallography experiments. We present and compare three protein structures that were solved using small crystals with dimensions of approximately 10-20 µm grown in chip. By crystallizing and diffracting in situ, handling and hence mechanical disturbances of fragile crystals is minimized. The protocol details how to fabricate a custom X-ray transparent microfluidic chip suitable for in situ serial crystallography. As almost every crystal can be used for diffraction data collection, these microfluidic chips are a very efficient crystal delivery method.

Advances in microfluidics for lipid nanoparticles and extracellular vesicles and applications in drug delivery systems

Lipid-based nanobiomaterials as liposomes and lipid nanoparticles (LNPs) are the most widely used nanocarriers for drug delivery systems (DDSs). Extracellular vesicles (EVs) and exosomes are also expected to be applied as DDS nanocarriers. The performance of nanomedicines relies on their components such as lipids, targeting ligands, encapsulated DNA, encapsulated RNA, and drugs. Recently, the importance of the nanocarrier sizes smaller than 100nm is attracting attention as a means to improve nanomedicine performance. Microfluidics and lab-on-a chip technologies make it possible to produce size-controlled LNPs by a simple continuous flow process and to separate EVs from blood samples by using a surface marker, ligand, or electric charge or by making a mass or particle size discrimination. Here, we overview recent advances in microfluidic devices and techniques for liposomes, LNPs, and EVs and their applications for DDSs.

Droplet microfluidic SANS

The coupling of droplet microfluidics and Small Angle Neutron Scattering (SANS) is demonstrated with a range of model systems: isotopic solvent (H₂O/D₂O) mixtures, surfactant (sodium dodecyl sulfate, SDS) solutions and colloidal (silica) suspensions. Several droplet carrier phases are evaluated and fluorinated oil emerges as a suitable fluid with minimal neutron background scattering (commensurate with air), and excellent interfacial properties. The combined effects of flow dispersion and compositional averaging caused by the neutron beam footprint are evaluated in both continuous and droplet flows and an operational window is established. Systematic droplet-SANS dilution measurements of colloidal silica suspensions enable unprecedented quantification of form and structure factors, osmotic compressibility, enhanced by constrained global data fits. Contrast variation measurements with over 100 data points are readily carried out in 10-20 min timescales, and validated for colloidal silica of two sizes, in both continuous and droplet flows. While droplet microfluidics is established as an attractive platform for SANS, the compositional averaging imposed by large (∼1 cm) beam footprints can, under certain circumstances, make single phase, continuous flow a preferable option for low scattering systems. We propose simple guidelines to assess the suitability of either approach based on well-defined system parameters.

SAXS on a chip: from dynamics of phase transitions to alignment phenomena at interfaces studied with microfluidic devices

The field of microfluidics offers attractive possibilities to perform novel experiments that are difficult (or even impossible) to perform using conventional bulk and surface-based methods. Such attractiveness comes from several important aspects inherent to these miniaturized devices. First, the flow of fluids under submillimeter confinement typically leads to a drop of inertial forces, meaning that turbulence is practically suppressed. This leads to predictable and controllable flow profiles, along with well-defined chemical gradients and stress fields that can be used for controlled mixing and actuation on the micro and nanoscale. Secondly, intricate microfluidic device designs can be fabricated using cleanroom standard procedures. Such intricate geometries can take diverse forms, designed by researchers to perform complex tasks, that require exquisite control of flow of several components and gradients, or to mimic real world examples, facilitating the establishment of more realistic models. Thirdly, microfluidic devices are usually compatible with in situ or integrated characterization methods that allow constant real-time monitoring of the processes occurring inside the microchannels. This is very different from typical bulk-based methods, where usually one can only observe the final result, or otherwise, take quick snapshots of the evolving process or take aliquots to be analyzed separately. Altogether, these characteristics inherent to microfluidic devices provide researchers with a set of tools that allow not only exquisite control and manipulation of materials at the micro and nanoscale, but also observation of these effects. In this review, we will focus on the use and prospects of combining microfluidic devices with in situ small-angle X-ray scattering (and related techniques such as small-angle neutron scattering and X-ray photon correlation spectroscopy), and their enormous potential for physical-chemical research, mainly in self-assembly and phase-transitions, and surface characterization.

A Graphene-Based Microfluidic Platform for Electrocrystallization and In Situ X-ray Diffraction

Here, we describe a novel microfluidic platform for use in electrocrystallization experiments. The device incorporates ultra-thin graphene-based films as electrodes and as X-ray transparent windows to enable in situ X-ray diffraction analysis. Furthermore, large-area graphene films serve as a gas barrier, creating a stable sample environment over time. We characterize different methods for fabricating graphene electrodes, and validate the electrical capabilities of our device through the use of methyl viologen, a redox-sensitive dye. Proof-of-concept electrocrystallization experiments using an internal electric field at constant potential were performed using hen egg-white lysozyme (HEWL) as a model system. We observed faster nucleation and crystal growth, as well as a higher signal-to-noise for diffraction data obtained from crystals prepared in the presence of an applied electric field. Although this work is focused on the electrocrystallization of proteins for structural biology, we anticipate that this technology should also find utility in a broad range of both X-ray technologies and other applications of microfluidic technology.

Cyclic olefin copolymer as an X-ray compatible material for microfluidic devices

The combination of microfluidics and X-ray methods attracts a lot of attention from researchers as it brings together the high controllability of microfluidic sample environments and the small length scales probed by X-rays. In particular, the fields of biophysics and biology have benefited enormously from such approaches. We introduce a straightforward fabrication method for X-ray compatible microfluidic devices made solely from cyclic olefin copolymers. We benchmark the performance of the devices against other devices including more commonly used Kapton windows and obtain data of equal quality using small angle X-ray scattering. An advantage of the devices presented here is that no gluing between interfaces is necessary, rendering the production very reliable. As a biophysical application, we investigate the early time points of the assembly of vimentin intermediate filament proteins into higher-order structures. This weakly scattering protein system leads to high quality data in the new devices, thus opening up the way for numerous future applications.