Ultracompact 3D microfluidics for time-resolved structural biology

SARS-CoV-2 Mpro responds to oxidation by forming disulfide and NOS/SONOS bonds

The main protease (Mpro) of SARS-CoV-2 is critical for viral function and a key drug target. Mpro is only active when reduced; turnover ceases upon oxidation but is restored by re-reduction. This suggests the system has evolved to survive periods in an oxidative environment, but the mechanism of this protection has not been confirmed. Here, we report a crystal structure of oxidized Mpro showing a disulfide bond between the active site cysteine, C145, and a distal cysteine, C117. Previous work proposed this disulfide provides the mechanism of protection from irreversible oxidation. Mpro forms an obligate homodimer, and the C117-C145 structure shows disruption of interactions bridging the dimer interface, implying a correlation between oxidation and dimerization. We confirm dimer stability is weakened in solution upon oxidation. Finally, we observe the protein's crystallization behavior is linked to its redox state. Oxidized Mpro spontaneously forms a distinct, more loosely packed lattice. Seeding with crystals of this lattice yields a structure with an oxidation pattern incorporating one cysteine-lysine-cysteine (SONOS) and two lysine-cysteine (NOS) bridges. These structures further our understanding of the oxidative regulation of Mpro and the crystallization conditions necessary to study this structurally.

Direct laser writing-enabled 3D printing strategies for microfluidic applications

Over the past decade, additive manufacturing-or "three-dimensional (3D) printing"-has attracted increasing attention in the Lab on a Chip community as a pathway to achieve sophisticated system architectures that are difficult or infeasible to fabricate via conventional means. One particularly promising 3D manufacturing technology is "direct laser writing (DLW)", which leverages two-photon (or multi-photon) polymerization (2PP) phenomena to enable high geometric versatility, print speeds, and precision at length scales down to the 100 nm range. Although researchers have demonstrated the potential of using DLW for microfluidic applications ranging from organ on a chip and drug delivery to micro/nanoparticle processing and soft microrobotics, such scenarios present unique challenges for DLW. Specifically, microfluidic systems typically require macro-to-micro fluidic interfaces (e.g., inlet and outlet ports) to facilitate fluidic loading, control, and retrieval operations; however, DLW-based 3D printing relies on a micron-to-submicron-sized 2PP volume element (i.e., "voxel") that is poorly suited for manufacturing these larger-scale fluidic interfaces. In this Tutorial Review, we highlight and discuss the four most prominent strategies that researchers have developed to circumvent this trade-off and realize macro-to-micro interfaces for DLW-enabled microfluidic components and systems. In addition, we consider the possibility that-with the advent of next-generation commercial DLW printers equipped with new dynamic voxel tuning, print field, and laser power capabilities-the overall utility of DLW strategies for Lab on a Chip fields may soon expand dramatically.

Efficient in situ screening of and data collection from microcrystals in crystallization plates

A considerable bottleneck in serial crystallography at XFEL and synchrotron sources is the efficient production of large quantities of homogenous, well diffracting microcrystals. Efficient high-throughput screening of batch-grown microcrystals and the determination of ground-state structures from different conditions is thus of considerable value in the early stages of a project. Here, a highly sample-efficient methodology to measure serial crystallography data from microcrystals by raster scanning within standard in situ 96-well crystallization plates is described. Structures were determined from very small quantities of microcrystal suspension and the results were compared with those from other sample-delivery methods. The analysis of a two-dimensional batch crystallization screen using this method is also described as a useful guide for further optimization and the selection of appropriate conditions for scaling up microcrystallization.

pH-dependent reaction triggering in PmHMGR crystals for time-resolved crystallography

Serial crystallography and time-resolved data collection can readily be employed to investigate the catalytic mechanism of Pseudomonas mevalonii 3-hydroxy-3-methylglutaryl (HMG)-coenzyme-A (CoA) reductase (PmHMGR) by changing the environmental conditions in the crystal and so manipulating the reaction rate. This enzyme uses a complex mechanism to convert mevalonate to HMG-CoA using the co-substrate CoA and cofactor NAD+. The multi-step reaction mechanism involves an exchange of bound NAD+ and large conformational changes by a 50-residue subdomain. The enzymatic reaction can be run in both forward and reverse directions in solution and is catalytically active in the crystal for multiple reaction steps. Initially, the enzyme was found to be inactive in the crystal starting with bound mevalonate, CoA, and NAD+. To observe the reaction from this direction, we examined the effects of crystallization buffer constituents and pH on enzyme turnover, discovering a strong inhibition in the crystallization buffer and a controllable increase in enzyme turnover as a function of pH. The inhibition is dependent on ionic concentration of the crystallization precipitant ammonium sulfate but independent of its ionic composition. Crystallographic studies show that the observed inhibition only affects the oxidation of mevalonate but not the subsequent reactions of the intermediate mevaldehyde. Calculations of the pKa values for the enzyme active site residues suggest that the effect of pH on turnover is due to the changing protonation state of His381. We have now exploited the changes in ionic inhibition in combination with the pH-dependent increase in turnover as a novel approach for triggering the PmHMGR reaction in crystals and capturing information about its intermediate states along the reaction pathway.

Droplet microfluidics for time-resolved serial crystallography

Serial crystallography requires large numbers of microcrystals and robust strategies to rapidly apply substrates to initiate reactions in time-resolved studies. Here, we report the use of droplet miniaturization for the controlled production of uniform crystals, providing an avenue for controlled substrate addition and synchronous reaction initiation. The approach was evaluated using two enzymatic systems, yielding 3 µm crystals of lysozyme and 2 µm crystals of Pdx1, an Arabidopsis enzyme involved in vitamin B6 biosynthesis. A seeding strategy was used to overcome the improbability of Pdx1 nucleation occurring with diminishing droplet volumes. Convection within droplets was exploited for rapid crystal mixing with ligands. Mixing times of <2 ms were achieved. Droplet microfluidics for crystal size engineering and rapid micromixing can be utilized to advance time-resolved serial crystallography.

Kilohertz droplet-on-demand serial femtosecond crystallography at the European XFEL station FXE

X-ray Free Electron Lasers (XFELs) allow the collection of high-quality serial femtosecond crystallography data. The next generation of megahertz superconducting FELs promises to drastically reduce data collection times, enabling the capture of more structures with higher signal-to-noise ratios and facilitating more complex experiments. Currently, gas dynamic virtual nozzles (GDVNs) stand as the sole delivery method capable of best utilizing the repetition rate of megahertz sources for crystallography. However, their substantial sample consumption renders their use impractical for many protein targets in serial crystallography experiments. Here, we present a novel application of a droplet-on-demand injection method, which allowed operation at 47 kHz at the European XFEL (EuXFEL) by tailoring a multi-droplet injection scheme for each macro-pulse. We demonstrate a collection rate of 150 000 indexed patterns per hour. We show that the performance and effective data collection rate are comparable to GDVN, with a sample consumption reduction of two orders of magnitude. We present lysozyme crystallographic data using the Large Pixel Detector at the femtosecond x-ray experiment endstation. Significant improvement of the crystallographic statistics was made by correcting for a systematic drift of the photon energy in the EuXFEL macro-pulse train, which was characterized from indexing the individual frames in the pulse train. This is the highest resolution protein structure collected and reported at the EuXFEL at 1.38 Å resolution.

Enhancing electrospray ionization efficiency for particle transmission through an aerodynamic lens stack

This work investigates the performance of the electrospray aerosol generator at the European X-ray Free Electron Laser (EuXFEL). This generator is, together with an aerodynamic lens stack that transports the particles into the X-ray interaction vacuum chamber, the method of choice to deliver particles for single-particle coherent diffractive imaging (SPI) experiments at the EuXFEL. For these experiments to be successful, it is necessary to achieve high transmission of particles from solution into the vacuum interaction region. Particle transmission is highly dependent on efficient neutralization of the charged aerosol generated by the electrospray mechanism as well as the geometry in the vicinity of the Taylor cone. We report absolute particle transmission values for different neutralizers and geometries while keeping the conditions suitable for SPI experiments. Our findings reveal that a vacuum ultraviolet ionizer demonstrates a transmission efficiency approximately seven times greater than the soft X-ray ionizer used previously. Combined with an optimized orifice size on the counter electrode, we achieve >40% particle transmission from solution into the X-ray interaction region. These findings offer valuable insights for optimizing electrospray aerosol generator configurations and data rates for SPI experiments.

Time resolution in cryo-EM using a PDMS-based microfluidic chip assembly and its application to the study of HflX-mediated ribosome recycling

The rapid kinetics of biological processes and associated short-lived conformational changes pose a significant challenge in attempts to structurally visualize biomolecules during a reaction in real time. Conventionally, on-pathway intermediates have been trapped using chemical modifications or reduced temperature, giving limited insights. Here, we introduce a time-resolved cryo-EM method using a reusable PDMS-based microfluidic chip assembly with high reactant mixing efficiency. Coating of PDMS walls with SiO2 virtually eliminates non-specific sample adsorption and ensures maintenance of the stoichiometry of the reaction, rendering it highly reproducible. In an operating range from 10 to 1,000 ms, the device allows us to follow in vitro reactions of biological molecules at resolution levels in the range of 3 Å. By employing this method, we show the mechanism of progressive HflX-mediated splitting of the 70S E. coli ribosome in the presence of the GTP via capture of three high-resolution reaction intermediates within 140 ms.

Advanced manufacturing provides tailor-made solutions for crystallography with x-ray free-electron lasers

Serial crystallography at large facilities, such as x-ray free-electron lasers and synchrotrons, evolved as a powerful method for the high-resolution structural investigation of proteins that are critical for human health, thus advancing drug discovery and novel therapies. However, a critical barrier to successful serial crystallography experiments lies in the efficient handling of the protein microcrystals and solutions at microscales. Microfluidics are the obvious approach for any high-throughput, nano-to-microliter sample handling, that also requires design flexibility and rapid prototyping to deal with the variable shapes, sizes, and density of crystals. Here, we discuss recent advances in polymer 3D printing for microfluidics-based serial crystallography research and present a demonstration of emerging, large-scale, nano-3D printing approaches leading into the future of 3D sample environment and delivery device fabrication from liquid jet gas-dynamic virtual nozzles devices to fixed-target sample environment technology.

A 3D-MICROPRINTED COAXIAL NOZZLE FOR FABRICATING LONG, FLEXIBLE MICROFLUIDIC TUBING

A variety of emerging applications, particularly those in medical and soft robotics fields, are predicated on the ability to fabricate long, flexible meso/microfluidic tubing with high customization. To address this need, here we present a hybrid additive manufacturing (or "three-dimensional (3D) printing") strategy that involves three key steps: (i) using the "Vat Photopolymerization (VPP) technique, "Liquid-Crystal Display (LCD)" 3D printing to print a bulk microfluidic device with three inlets and three concentric outlets; (ii) using "Two-Photon Direct Laser Writing (DLW)" to 3D microprint a coaxial nozzle directly atop the concentric outlets of the bulk microdevice, and then (iii) extruding paraffin oil and a liquid-phase photocurable resin through the coaxial nozzle and into a polydimethylsiloxane (PDMS) channel for UV exposure, ultimately producing the desired tubing. In addition to fabricating the resulting tubing-composed of polymerized photomaterial-at arbitrary lengths (e.g., > 10 cm), the distinct input pressures can be adjusted to tune the inner diameter (ID) and outer diameter (OD) of the fabricated tubing. For example, experimental results revealed that increasing the driving pressure of the liquid-phase photomaterial from 50 kPa to 100 kPa led to fluidic tubing with IDs and ODs of 291±99 μm and 546±76 μm up to 741±31 μm and 888±39 μm, respectively. Furthermore, preliminary results for DLW-printing a microfluidic "M" structure directly atop the tubing suggest that the tubing could be used for "ex situ DLW (esDLW)" fabrication, which would further enhance the utility of the tubing.

Structure of the Lysinibacillus sphaericus Tpp49Aa1 pesticidal protein elucidated from natural crystals using MHz-SFX

The Lysinibacillus sphaericus proteins Tpp49Aa1 and Cry48Aa1 can together act as a toxin toward the mosquito Culex quinquefasciatus and have potential use in biocontrol. Given that proteins with sequence homology to the individual proteins can have activity alone against other insect species, the structure of Tpp49Aa1 was solved in order to understand this protein more fully and inform the design of improved biopesticides. Tpp49Aa1 is naturally expressed as a crystalline inclusion within the host bacterium, and MHz serial femtosecond crystallography using the novel nanofocus option at an X-ray free electron laser allowed rapid and high-quality data collection to determine the structure of Tpp49Aa1 at 1.62 Å resolution. This revealed the packing of Tpp49Aa1 within these natural nanocrystals as a homodimer with a large intermolecular interface. Complementary experiments conducted at varied pH also enabled investigation of the early structural events leading up to the dissolution of natural Tpp49Aa1 crystals-a crucial step in its mechanism of action. To better understand the cooperation between the two proteins, assays were performed on a range of different mosquito cell lines using both individual proteins and mixtures of the two. Finally, bioassays demonstrated Tpp49Aa1/Cry48Aa1 susceptibility of Anopheles stephensi, Aedes albopictus, and Culex tarsalis larvae-substantially increasing the potential use of this binary toxin in mosquito control.

Monolithic affinity columns in 3D printed microfluidics for chikungunya RNA detection

Mosquito-borne pathogens plague much of the world, yet rapid and simple diagnosis is not available for many affected patients. Using a custom stereolithography 3D printer, we created microfluidic devices with affinity monoliths that could retain, noncovalently attach a fluorescent tag, and detect oligonucleotide and viral RNA. We optimized the fluorescent binding and sample load times using an oligonucleotide sequence from chikungunya virus (CHIKV). We also tested the specificity of CHIKV capture relative to genetically similar Sindbis virus. Moreover, viral RNA from both viruses was flowed through capture columns to study the efficiency and specificity of the column for viral CHIKV. We detected ~107 loaded viral genome copies, which was similar to levels in clinical samples during acute infection. These results show considerable promise for development of this platform into a rapid mosquito-borne viral pathogen detection system.

Thinner than a knife's edge: 3D-printed liquid sheet jet technology for solution phase XFEL experiments

In this commentary, we explore the pioneering implementation of 3D-printed thin liquid sheet devices for advanced X-ray scattering and spectroscopy experiments at high-repetition rate XFELs.

Experimental capabilities for liquid jet samples at sub-MHz rates at the FXE Instrument at European XFEL

The Femtosecond X-ray Experiments (FXE) instrument at the European X-ray Free-Electron Laser (EuXFEL) provides an optimized platform for investigations of ultrafast physical, chemical and biological processes. It operates in the energy range 4.7-20 keV accommodating flexible and versatile environments for a wide range of samples using diverse ultrafast X-ray spectroscopic, scattering and diffraction techniques. FXE is particularly suitable for experiments taking advantage of the sub-MHz repetition rates provided by the EuXFEL. In this paper a dedicated setup for studies on ultrafast biological and chemical dynamics in solution phase at sub-MHz rates at FXE is presented. Particular emphasis on the different liquid jet sample delivery options and their performance is given. Our portfolio of high-speed jets compatible with sub-MHz experiments includes cylindrical jets, gas dynamic virtual nozzles and flat jets. The capability to perform multi-color X-ray emission spectroscopy (XES) experiments is illustrated by a set of measurements using the dispersive X-ray spectrometer in von Hamos geometry. Static XES data collected using a multi-crystal scanning Johann-type spectrometer are also presented. A few examples of experimental results on ultrafast time-resolved X-ray emission spectroscopy and wide-angle X-ray scattering at sub-MHz pulse repetition rates are given.

3D-printed sheet jet for stable megahertz liquid sample delivery at X-ray free-electron lasers

X-ray free-electron lasers (XFELs) can probe chemical and biological reactions as they unfold with unprecedented spatial and temporal resolution. A principal challenge in this pursuit involves the delivery of samples to the X-ray interaction point in such a way that produces data of the highest possible quality and with maximal efficiency. This is hampered by intrinsic constraints posed by the light source and operation within a beamline environment. For liquid samples, the solution typically involves some form of high-speed liquid jet, capable of keeping up with the rate of X-ray pulses. However, conventional jets are not ideal because of radiation-induced explosions of the jet, as well as their cylindrical geometry combined with the X-ray pointing instability of many beamlines which causes the interaction volume to differ for every pulse. This complicates data analysis and contributes to measurement errors. An alternative geometry is a liquid sheet jet which, with its constant thickness over large areas, eliminates the problems related to X-ray pointing. Since liquid sheets can be made very thin, the radiation-induced explosion is reduced, boosting their stability. These are especially attractive for experiments which benefit from small interaction volumes such as fluctuation X-ray scattering and several types of spectroscopy. Although their use has increased for soft X-ray applications in recent years, there has not yet been wide-scale adoption at XFELs. Here, gas-accelerated liquid sheet jet sample injection is demonstrated at the European XFEL SPB/SFX nano focus beamline. Its performance relative to a conventional liquid jet is evaluated and superior performance across several key factors has been found. This includes a thickness profile ranging from hundreds of nanometres to 60 nm, a fourfold increase in background stability and favorable radiation-induced explosion dynamics at high repetition rates up to 1.13 MHz. Its minute thickness also suggests that ultrafast single-particle solution scattering is a possibility.

Anaerobic fixed-target serial crystallography using sandwiched silicon nitride membranes

In recent years, the emergence of serial crystallography, initially pioneered at X-ray free-electron lasers (XFELs), has sparked a growing interest in collecting macromolecular crystallographic data at room temperature. Various fixed-target serial crystallography techniques have been developed, ranging from commercially available chips to in-house designs implemented at different synchrotron facilities. Nevertheless, there is currently no commercially available chip (known to the authors) specifically designed for the direct handling of oxygen-sensitive samples. This study presents a methodology employing silicon nitride chips arranged in a `sandwich' configuration, enabling reliable room-temperature data collection from oxygen-sensitive samples. The method involves the utilization of a custom-made 3D-printed assembling tool and a MX sample holder. To validate the effectiveness of the proposed method, deoxyhemoglobin and methemoglobin samples were investigated using the BioMAX X-ray macromolecular crystallography beamline, the Balder X-ray absorption spectroscopy beamline and UV-Vis absorption spectroscopy.

Programmable and Pixelated Solute Concentration Fields Controlled by Three-Dimensionally Networked Microfluidic Source/Sink Arrays

Membrane-integrated microfluidic platforms have played a pivotal role in understanding natural phenomena coupled with solute concentration gradients at the micro- and nanoscale, enabling on-chip microscopy in well-defined planar concentration fields. However, the standardized two-dimensional fabrication schemes in microfluidics have impeded the realization of more complex and diverse chemical environmental conditions due to the limited possible arrangements of source/sink conditions in a fluidic domain. In this study, we present a microfluidic platform with a three-dimensional microchannel network design, where discretized membranes can be integrated and individually controlled in a two-dimensional array format at any location within the entire quasi-two-dimensional solute concentration field. We elucidate the principles of the device to implement operations of the pixel-like sources/sinks and dynamically programmable control of various long-lasting solute concentration fields. Furthermore, we demonstrate the application of the generated solute concentration fields in manipulating the transport of micrometer or submicrometer particles with a high degree of freedom, surpassing conventionally available solute concentration fields. This work provides an experimental tool for investigating complex systems under high-order chemical environmental conditions, thereby facilitating the extensive development of higher-performance micro- and nanotechnologies.

Fabricating and Laminating Films with Through-Holes and Engraved/Protruding Structures for 3D Micro/Nanofluidic Platforms

Micro/nanofluidic devices have become popular for delicately processing biological, material, and chemical samples. However, their reliance on 2D fabrication schemes has hindered further innovation. Here, a 3D manufacturing method is proposed through the innovation of laminated object manufacturing (LOM), which involves the selection of building materials as well as the development of molding and lamination techniques. Fabrication of interlayer films is demonstrated with both multi-layered micro-/nanostructures and through-holes, using an injection molding approach and establishing strategic principles of film design. Utilization of the multi-layered through-hole films in LOM allows reducing the number of alignments and laminations by at least two times compared to conventional LOM. Using a dual-curing resin for film fabrication, a surface-treatment-free and collapse-free lamination technique is shown for constructing 3D multiscale micro/nanofluidic devices with ultralow aspect ratio nanochannels. The 3D manufacturing method enables the development of a nanochannel-based attoliter droplet generator capable of 3D parallelization for mass production, which implies the remarkable potential to extend numerous existing 2D micro/nanofluidic platforms into a 3D framework.

All polymer microfluidic chips-A fixed target sample delivery workhorse for serial crystallography

The development of x-ray free electron laser (XFEL) light sources and serial crystallography methodologies has led to a revolution in protein crystallography, enabling the determination of previously unobtainable protein structures and near-atomic resolution of otherwise poorly diffracting protein crystals. However, to utilize XFEL sources efficiently demands the continuous, rapid delivery of a large number of difficult-to-handle microcrystals to the x-ray beam. A recently developed fixed-target system, in which crystals of interest are enclosed within a sample holder, which is rastered through the x-ray beam, is discussed in detail in this Perspective. The fixed target is easy to use, maintains sample hydration, and can be readily modified to allow a broad range of sample types and different beamline requirements. Recent innovations demonstrate the potential of such microfluidic-based fixed targets to be an all-around "workhorse" for serial crystallography measurements. This Perspective will summarize recent advancements in microfluidic fixed targets for serial crystallography, examine needs for future development, and guide users in designing, choosing, and utilizing a fixed-target sample delivery device for their system.

Controlling amorphous silicon in scratching for fabricating high-performance micromixers

As core parts of microfluidic chip analysis systems, micromixers show robust applications in wide fields. However, restricted by the fabrication technology, it remains challenging to achieve high-quality micromixers with both delicately designed structure and efficient mixing. In this study, based on the theory of chaotic mixing, sinusoidal structures with variable phases were designed and then fabricated through scanning probe lithography (SPL) and post-selective etching. It was found that scratches with phase differences can lead to the periodic formation of amorphous silicon (a-Si), which can resist etching. Consequentially, misaligned sine channels with thick-thin alternating 3D shapes can be generated in situ from the scratched traces after the etching. Further analysis showed that a thicker a-Si layer can be obtained by reducing the line spacing in the scratching, confirmed by Raman detections and simulations. With the proposed method, the misaligned sine micromixer was achieved with higher mixing efficiency than ever. The duplicating process was also investigated for high-precision production of micromixers. The study provided strategies for the miniaturization of high-performance microfluidic chips.

XFEL Microcrystallography of Self-Assembling Silver n-Alkanethiolates

New synthetic hybrid materials and their increasing complexity have placed growing demands on crystal growth for single-crystal X-ray diffraction analysis. Unfortunately, not all chemical systems are conducive to the isolation of single crystals for traditional characterization. Here, small-molecule serial femtosecond crystallography (smSFX) at atomic resolution (0.833 Å) is employed to characterize microcrystalline silver n-alkanethiolates with various alkyl chain lengths at X-ray free electron laser facilities, resolving long-standing controversies regarding the atomic connectivity and odd-even effects of layer stacking. smSFX provides high-quality crystal structures directly from the powder of the true unknowns, a capability that is particularly useful for systems having notoriously small or defective crystals. We present crystal structures of silver n-butanethiolate (C4), silver n-hexanethiolate (C6), and silver n-nonanethiolate (C9). We show that an odd-even effect originates from the orientation of the terminal methyl group and its role in packing efficiency. We also propose a secondary odd-even effect involving multiple mosaic blocks in the crystals containing even-numbered chains, identified by selected-area electron diffraction measurements. We conclude with a discussion of the merits of the synthetic preparation for the preparation of microdiffraction specimens and compare the long-range order in these crystals to that of self-assembled monolayers.

Mix-and-extrude: high-viscosity sample injection towards time-resolved protein crystallography

Time-resolved crystallography enables the visualization of protein molecular motion during a reaction. Although light is often used to initiate reactions in time-resolved crystallography, only a small number of proteins can be activated by light. However, many biological reactions can be triggered by the interaction between proteins and ligands. The sample delivery method presented here uses a mix-and-extrude approach based on 3D-printed microchannels in conjunction with a micronozzle. The diffusive mixing enables the study of the dynamics of samples in viscous media. The device design allows mixing of the ligands and protein crystals in 2 to 20 s. The device characterization using a model system (fluorescence quenching of iq-mEmerald proteins by copper ions) demonstrated that ligand and protein crystals, each within lipidic cubic phase, can be mixed efficiently. The potential of this approach for time-resolved membrane protein crystallography to support the development of new drugs is discussed.

Science and Technology of Additive Manufacturing Progress: Processes, Materials, and Applications

As a special review article, several significant and applied results in 3D printing and additive manufacturing (AM) science and technology are reviewed and studied. Which, the reviewed research works were published in 2020. Then, we would have another review article for 2021 and 2022. The main purpose is to collect new and applied research results as a useful package for researchers. Nowadays, AM is an extremely discussed topic and subject in scientific and industrial societies, as well as a new vision of the unknown modern world. Also, the future of AM materials is toward fundamental changes. Which, AM would be an ongoing new industrial revolution in the digital world. With parallel methods and similar technologies, considerable developments have been made in 4D in recent years. AM as a tool is related to the 4th industrial revolution. So, AM and 3D printing are moving towards the fifth industrial revolution. In addition, a study on AM is vital for generating the next developments, which are beneficial for human beings and life. Thus, this article presents the brief, updated, and applied methods and results published in 2020.

Graphical Abstract

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.

Chaotic advection mixer for capturing transient states of diverse biological macromolecular systems with time-resolved small-angle X-ray scattering

Advances in time-resolved structural techniques, mainly in macromolecular crystallography and small-angle X-ray scattering (SAXS), allow for a detailed view of the dynamics of biological macromolecules and reactions between binding partners. Of particular promise, are mix-and-inject techniques, which offer a wide range of experimental possibility as microfluidic mixers are used to rapidly combine two species just prior to data collection. Most mix-and-inject approaches rely on diffusive mixers, which have been effectively used within crystallography and SAXS for a variety of systems, but their success is dependent on a specific set of conditions to facilitate fast diffusion for mixing. The use of a new chaotic advection mixer designed for microfluidic applications helps to further broaden the types of systems compatible with time-resolved mixing experiments. The chaotic advection mixer can create ultra-thin, alternating layers of liquid, enabling faster diffusion so that even more slowly diffusing molecules, like proteins or nucleic acids, can achieve fast mixing on timescales relevant to biological reactions. This mixer was first used in UV-vis absorbance and SAXS experiments with systems of a variety of molecular weights, and thus diffusion speeds. Careful effort was also dedicated to making a loop-loading sample-delivery system that consumes as little sample as possible, enabling the study of precious, laboratory-purified samples. The combination of the versatile mixer with low sample consumption opens the door to many new applications for mix-and-inject studies.

JINXED: just in time crystallization for easy structure determination of biological macromolecules

Macromolecular crystallography is a well established method in the field of structural biology and has led to the majority of known protein structures to date. After focusing on static structures, the method is now under development towards the investigation of protein dynamics through time-resolved methods. These experiments often require multiple handling steps of the sensitive protein crystals, e.g. for ligand-soaking and cryo-protection. These handling steps can cause significant crystal damage, and hence reduce data quality. Furthermore, in time-resolved experiments based on serial crystallography, which use micrometre-sized crystals for short diffusion times of ligands, certain crystal morphologies with small solvent channels can prevent sufficient ligand diffusion. Described here is a method that combines protein crystallization and data collection in a novel one-step process. Corresponding experiments were successfully performed as a proof-of-principle using hen egg-white lysozyme and crystallization times of only a few seconds. This method, called JINXED (Just IN time Crystallization for Easy structure Determination), promises high-quality data due to the avoidance of crystal handling and has the potential to enable time-resolved experiments with crystals containing small solvent channels by adding potential ligands to the crystallization buffer, simulating traditional co-crystallization approaches.

Microfluidic Methods for Generation of Submicron Droplets: A Review

Submicron droplets are ubiquitous in nature and widely applied in fields such as biomedical diagnosis and therapy, oil recovery and energy conversion, among others. The submicron droplets are kinetically stable, their submicron size endows them with good mobility in highly constricted pathways, and the high surface-to-volume ratio allows effective loading of chemical components at the interface and good heat transfer performance. Conventional generation technology of submicron droplets in bulk involves high energy input, or relies on chemical energy released from the system. Microfluidic methods are widely used to generate highly monodispersed micron-sized or bigger droplets, while downsizing to the order of 100 nm was thought to be challenging because of sophisticated nanofabrication. In this review, we summarize the microfluidic methods that are promising for the generation of submicron droplets, with an emphasize on the device fabrication, operational condition, and resultant droplet size. Microfluidics offer a relatively energy-efficient and versatile tool for the generation of highly monodisperse submicron droplets.

Development of a 3D Printed Micro Simulated Moving Bed Chromatography System

In the 1960s, chromatography processes were revolutionized by the invention of simulated moving bed chromatography. This method not only enhances the separation performance and resin utilization in comparison to batch-chromatography, it has also a much lower buffer consumption. While simulated moving bed chromatography nowadays is applied for a wide range of industrial applications, it was never transferred to the micro-scale (in regards to column and system volume). In our opinion a micro simulated moving bed chromatography system (µSMB) would be a useful tool for many applications, ranging from early process development and long term studies to downstream processing of speciality products. We implemented such a µSMB with a 3D printed central rotary valve and a microfluidic flow controller as flow source. We tested the system with a four zone open loop setup for the separation of bovine serum albumin and ammonium sulfate with size exclusion chromatography. We used four process points and could achieve desalting levels of BSA ranging from 94% to 99%, with yields ranging form 65% to 88%. Thus, we were able to achieve comparable results to common lab scale processes. With a total dead volume of 358 µL, including all sensors, connections and the valve, this is, to the best of our knowledge, the smallest SMB system that was ever built and we were able to perform experiments with feed flow rates reaching as low as 15 µL/min.

Addressing the Silent Spread of Monkeypox Disease with Advanced Analytical Tools

Monkeypox disease is caused by a virus which belongs to the orthopoxvirus genus of the poxviridae family. This disease has recently spread out to several non-endemic countries. While some cases have been linked to travel from endemic regions, more recent infections are thought to have spread in the community without any travel links, raising the risks of a wider outbreak. This state of public health represents a highly unusual event which requires urgent surveillance. In this context, the opportunities and technological challenges of current bio/chemical sensors, nanomaterials, nanomaterial characterization instruments, and artificially intelligent biosystems collectively called "advanced analytical tools" are reviewed here, which will allow early detection, characterization, and inhibition of the monkeypox virus (MPXV) in the community and limit its expansion from endemic to pandemic. A summary of background information is also provided from biological and epidemiological perspective of monkeypox to support the scientific case for its holistic management using advanced analytical tools.

An experimental study of liquid micro-jets produced with a gas dynamic virtual nozzle under the influence of an electric field

The results of an experimental study of micro-jets produced with a gas dynamic virtual nozzle (GDVN) under the influence of an electric field are provided and discussed for the first time. The experimental study is performed with a 50% volume mixture of water and ethanol, and nitrogen focusing gas. The liquid sample and gas Reynolds numbers range from 0.09-5.4 and 0-190, respectively. The external electrode was positioned 400-500 μm downstream of the nozzle tip and an effect of electric potential between the electrode and the sample liquid from 0-7 kV was investigated. The jetting parametric space is examined as a function of operating gas and liquid flow rates, outlet chamber pressure, and an external electric field. The experimentally observed jet diameter, length and velocity ranged from 1-25 μm, 50-500 μm and 0.5-10 m/s, respectively. The jetting shape snapshots were processed automatically using purposely developed computer vision software. The velocity of the jet was calculated from the measured jet diameter and the sample flow rate. It is found that micro-jets accelerate in the direction of the applied electric field in the downstream direction at a constant acceleration as opposed to the standard GDVNs. New jetting modes were observed, where either the focusing gas or the electric forces dominate, encouraging further theoretical and numerical studies towards optimized system design. The study shows the potential to unlock a new generation of low background sample delivery for serial diffraction measurements of weakly scattering objects.

Speckle contrast of interfering fluorescence X-rays

With the development of X-ray free-electron lasers (XFELs), producing pulses of femtosecond durations comparable with the coherence times of X-ray fluorescence, it has become possible to observe intensity-intensity correlations due to the interference of emission from independent atoms. This has been used to compare durations of X-ray pulses and to measure the size of a focusedX-ray beam, for example. Here it is shown that it is also possible to observe the interference of fluorescence photons through the measurement of the speckle contrast of angle-resolved fluorescence patterns. Speckle contrast is often used as a measure of the degree of coherence of the incident beam or the fluctuations of the illuminated sample as determined from X-ray diffraction patterns formed by elastic scattering, rather than from fluorescence patterns as addressed here. Commonly used approaches to estimate speckle contrast were found to suffer when applied to XFEL-generated fluorescence patterns due to low photon counts and a significant variation of the excitation pulse energy from shot to shot. A new method to reliably estimate speckle contrast under such conditions, using a weighting scheme, is introduced. The method is demonstrated by comparing the speckle contrast of fluorescence observed with pulses of 3 fs to 15 fs duration.

Mix-and-Inject Serial Femtosecond Crystallography to Capture RNA Riboswitch Intermediates

Time-resolved structure determination of macromolecular conformations and ligand-bound intermediates is extremely challenging, particularly for RNA. With rapid technological advances in both microfluidic liquid injection and X-ray free electron lasers (XFEL), a new frontier has emerged in time-resolved crystallography whereby crystals can be mixed with ligand and then probed with X-rays (mix-and-inject) in real time and at room temperature. This chapter outlines the basic setup and procedures for mix-and-inject experiments for recording time-resolved crystallographic data of riboswitch RNA reaction states using serial femtosecond crystallography (SFX) and an XFEL.

Automatic bad-pixel mask maker for X-ray pixel detectors with application to serial crystallography

X-ray crystallography has witnessed a massive development over the past decade, driven by large increases in the intensity and brightness of X-ray sources and enabled by employing high-frame-rate X-ray detectors. The analysis of large data sets is done via automatic algorithms that are vulnerable to imperfections in the detector and noise inherent with the detection process. By improving the model of the behaviour of the detector, data can be analysed more reliably and data storage costs can be significantly reduced. One major requirement is a software mask that identifies defective pixels in diffraction frames. This paper introduces a methodology and program based upon concepts of machine learning, called robust mask maker (RMM), for the generation of bad-pixel masks for large-area X-ray pixel detectors based on modern robust statistics. It is proposed to discriminate normally behaving pixels from abnormal pixels by analysing routine measurements made with and without X-ray illumination. Analysis software typically uses a Bragg peak finder to detect Bragg peaks and an indexing method to detect crystal lattices among those peaks. Without proper masking of the bad pixels, peak finding methods often confuse the abnormal values of bad pixels in a pattern with true Bragg peaks and flag such patterns as useful regardless, leading to storage of enormous uninformative data sets. Also, it is computationally very expensive for indexing methods to search for crystal lattices among false peaks and the solution may be biased. This paper shows how RMM vastly improves peak finders and prevents them from labelling bad pixels as Bragg peaks, by demonstrating its effectiveness on several serial crystallography data sets.

Acoustic Focusing of Protein Crystals for In-Line Monitoring and Up-Concentration during Serial Crystallography

Serial femtosecond crystallography (SFX) has become one of the standard techniques at X-ray free-electron lasers (XFELs) to obtain high-resolution structural information from microcrystals of proteins. Nevertheless, reliable sample delivery is still often limiting data collection, as microcrystals can clog both field- and flow-focusing nozzles despite in-line filters. In this study, we developed acoustic 2D focusing of protein microcrystals in capillaries that enables real-time online characterization of crystal size and shape in the sample delivery line after the in-line filter. We used a piezoelectric actuator to create a standing wave perpendicular to the crystal flow, which focused lysozyme microcrystals into a single line inside a silica capillary so that they can be imaged using a high-speed camera. We characterized the acoustic contrast factor, focus size, and the coaxial flow lines and developed a splitting union that enables up-concentration to at least a factor of five. The focus size, flow rates, and geometry may enable an upper limit of up-concentration as high as 200 fold. The novel feedback and concentration control could be implemented for serial crystallography at synchrotrons with minor modifications. It will also aid the development of improved sample delivery systems that will increase SFX data collection rates at XFELs, with potential applications to many proteins that can only be purified and crystallized in small amounts.

Megahertz pulse trains enable multi-hit serial femtosecond crystallography experiments at X-ray free electron lasers

The European X-ray Free Electron Laser (XFEL) and Linac Coherent Light Source (LCLS) II are extremely intense sources of X-rays capable of generating Serial Femtosecond Crystallography (SFX) data at megahertz (MHz) repetition rates. Previous work has shown that it is possible to use consecutive X-ray pulses to collect diffraction patterns from individual crystals. Here, we exploit the MHz pulse structure of the European XFEL to obtain two complete datasets from the same lysozyme crystal, first hit and the second hit, before it exits the beam. The two datasets, separated by <1 µs, yield up to 2.1 Å resolution structures. Comparisons between the two structures reveal no indications of radiation damage or significant changes within the active site, consistent with the calculated dose estimates. This demonstrates MHz SFX can be used as a tool for tracking sub-microsecond structural changes in individual single crystals, a technique we refer to as multi-hit SFX.

Free-electron lasers are capable of high repetition rates and it is assumed that protein crystals often do not survive the first X-ray pulse. Here the authors address these issues with a demonstration of multi-hit serial crystallography in which multiple FEL pulses interact with the sample without destroying it.

Towards real-time analysis of liquid jet alignment in serial femtosecond crystallography

A novel strategy is presented for sample jet alignment using machine vision for liquid-jet-based sample delivery systems. Feedback using height-resolution images from an optical microscope positioned perpendicular to the path of the X-ray beam enables tracking of the relative alignment of the liquid jet and X-ray beam.

Liquid sample delivery systems are used extensively for serial femtosecond crystallography at X-ray free-electron lasers (XFELs). However, misalignment of the liquid jet and the XFEL beam leads to the X-rays either partially or completely missing the sample, resulting in sample wastage and a loss of experiment time. Implemented here is an algorithm to analyse optical images using machine vision to determine whether there is overlap of the X-ray beam and liquid jet. The long-term goal is to use the output from this algorithm to implement an automated feedback mechanism to maintain constant alignment of the X-ray beam and liquid jet. The key elements of this jet alignment algorithm are discussed and its performance is characterized by comparing the results with a manual analysis of the optical image data. The success rate of the algorithm for correctly identifying hits is quantified via a similarity metric, the Dice coefficient. In total four different nozzle designs were used in this study, yielding an overall Dice coefficient of 0.98.

Characterization of a 30 µm pixel size CLIP-based 3D printer and its enhancement through dynamic printing optimization

Resolving microscopic and complex 3D polymeric structures while maintaining high print speeds in additive manufacturing has been challenging. To achieve print precision at micrometer length scales for polymeric materials, most 3D printing technologies utilize the serial voxel printing approach that has a relatively slow print speed. Here, a 30-µm-resolution continuous liquid interface production (CLIP)-based 3D printing system for printing polymeric microstructures is described. This technology combines the high-resolution from projection microstereolithography and the fast print speed from CLIP, thereby achieving micrometer print resolution at x10³ times faster than other high-resolution 3D printing technologies. Print resolutions in both lateral and vertical directions were characterized, and the printability of minimum 30 µm features in 2D and 3D has been demonstrated. Through dynamic printing optimization, a method that varies the print parameters (e.g. exposure time, UV intensity, and dark time) for each print layer, overhanging struts at various thicknesses spanning 1 order of magnitude (25 µm - 200 µm) in a single print are resolvable. Taken together, this work illustrates that the micro-CLIP 3D printing technology, in combination with dynamic printing optimization, has the high resolution needed to enable manufacturing of exquisitely detailed and gradient 3D structures, such as terraced microneedle arrays and micro-lattice structures, while maintaining high print speeds.

Microfluidic assisted recognition of miRNAs towards point-of-care diagnosis: Technical and analytical overview towards biosensing of short stranded single non-coding oligonucleotides

MiRNAs are short stranded single non-coding oligonucleotides that play an important role in regulating gene expression. MiRNAs are stable in RNase enriched environments such as human body fluids and their dysregulation or abnormal abundance in human body fluids as a diagnostic biomarker has been associated with several diseases. Due to the low concentration of miRNAs, it is difficult to detect using interactive methods (ideal detection limit is femtomolar range). However, clinicians lack sensitive and reliable methods for quantifying miRNA. Microfluidic devices integrated with electrochemical, optical (fluorometric, SERs, FRET, colorimetric), electrochemiluminescence and photoelectrochemical signal readout led to development innovative diagnostic device test, can probably overcome the limitations of the traditional methods. In the present review, microfluid methods for the sensitive and selective recognition of miRNA in various biological matrices are surveyed. Also, advantages and limitation of recognition methods on the performance and efficiency of microfluidic based biosensing of miRNAs are critically investigated. Finally, the future perspectives on the diagnosis of disease based on microfluidic analysis of miRNAs are provided.

X-ray Fourier transform holography with beam shaping optical elements

Holography is a powerful method for achieving 3D images of objects. Extending this method to short wavelengths potentially offers significantly higher resolution than visible light holography. However, current X-ray holography setups employ nanoscale pinholes to form the reference wave. This approach is relatively inefficient and limited to very small sample size. Here, we propose a new setup for X-ray holography based on a binary diffractive optical element (DOE), which forms at the same time the object illumination and the reference wave. This optic is located separately from the sample plane, which permits investigation of larger sample areas. Using an extended test sample, we demonstrate a resolution of 90 nm (half-pitch) at an undulator beamline at BESSY II. The new holography setup can be directly transferred to free electron laser sources enabling time-resolved nanoscale imaging for ultra-fast processes.

A multi-million image Serial Femtosecond Crystallography dataset collected at the European XFEL

Serial femtosecond crystallography is a rapidly developing method for determining the structure of biomolecules for samples which have proven challenging with conventional X-ray crystallography, such as for membrane proteins and microcrystals, or for time-resolved studies. The European XFEL, the first high repetition rate hard X-ray free electron laser, provides the ability to record diffraction data at more than an order of magnitude faster than previously achievable, putting increased demand on sample delivery and data processing. This work describes a publicly available serial femtosecond crystallography dataset collected at the SPB/SFX instrument at the European XFEL. This dataset contains information suitable for algorithmic development for detector calibration, image classification and structure determination, as well as testing and training for future users of the European XFEL and other XFELs.

Measurement(s)	lysozyme measurement
Technology Type(s)	X-ray crystallography

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.

Potential of Time-Resolved Serial Femtosecond Crystallography Using High Repetition Rate XFEL Sources

This perspective review describes emerging techniques and future opportunities for time-resolved serial femtosecond crystallography (TR-SFX) experiments using high repetition rate XFEL sources. High repetition rate sources are becoming more available with the European XFEL in operation and the recently upgraded LCLS-II will be available in the near future. One efficient use of these facilities for TR-SFX relies on pump–probe experiments using a laser to trigger a reaction of light-responsive proteins or mix-and-inject experiments for light-unresponsive proteins. With the view to widen the application of TR-SFX, the promising field of photocaged compounds is under development, which allows the very fast laser triggering of reactions that is no longer limited to naturally light-responsive samples. In addition to reaction triggering, a key concern when performing an SFX experiment is efficient sample usage, which is a main focus of new high repetition rate-compatible sample delivery methods.

Serial X-ray Crystallography

Serial crystallography (SX) is an emerging technique to determine macromolecules at room temperature. SX with a pump–probe experiment provides the time-resolved dynamics of target molecules. SX has developed rapidly over the past decade as a technique that not only provides room-temperature structures with biomolecules, but also has the ability to time-resolve their molecular dynamics. The serial femtosecond crystallography (SFX) technique using an X-ray free electron laser (XFEL) has now been extended to serial synchrotron crystallography (SSX) using synchrotron X-rays. The development of a variety of sample delivery techniques and data processing programs is currently accelerating SX research, thereby increasing the research scope. In this editorial, I briefly review some of the experimental techniques that have contributed to advances in the field of SX research and recent major research achievements. This Special Issue will contribute to the field of SX research.

A low-cost and high-performance 3D micromixer over a wide working range and its application for high-sensitivity biomarker detection

Homogenous mixing in microfluidic devices is often required for efficient chemical and biological reactions.Passive micromixing without external energy input has attracted much research interest. We have developed a high-performance 3D...

In a flash of light: X-ray free electron lasers meet native mass spectrometry

During the last years, X-ray free electron lasers (XFELs) have emerged as X-ray sources of unparalleled brightness, delivering extreme amounts of photons in femtosecond pulses. As such, they have opened up completely new possibilities in drug discovery and structural biology, including studying high resolution biomolecular structures and their functioning in a time resolved manner, and diffractive imaging of single particles without the need for their crystallization. In this perspective, we briefly review the operation of XFELs, their immediate uses for drug discovery and focus on the potentially revolutionary single particle diffractive imaging technique and the challenges which remain to be overcome to fully realize its potential to provide high resolution structures without the need for crystallization, freezing or the need to keep proteins stable at extreme concentrations for long periods of time. As the issues have been to a large extent sample delivery related, we outline a way for native mass spectrometry to overcome these and enable so far impossible research with a potentially huge impact on structural biology and drug discovery, such as studying structures of transient intermediate species in viral life cycles or during functioning of molecular machines.

Optical bunching of particles in a liquid flow

High-speed liquid micro-jets are used to rapidly and repeatedly deliver protein microcrystals to focused and pulsed X-ray beams in the method of serial femtosecond crystallography. However, the current continuous flow of crystals is mismatched to the arrival of X-ray pulses, wasting vast amounts of an often rare and precious sample. Here, we introduce a method to address this problem by periodically trapping and releasing crystals in the liquid flow, creating locally concentrated crystal bunches, using an optical trap integrated in the microfluidic supply line. We experimentally demonstrate a 30-fold increase of particle concentration into 10 Hz bunches of 6.4 μm diameter polystyrene particles. Furthermore, using particle trajectory simulations, a comprehensive description of the optical bunching process and parameter space is presented. Adding this compact optofluidics device to existing injection systems would thereby dramatically reduce sample consumption and extend the application of serial crystallography to a greater range of protein crystal systems that cannot be produced in high abundance. Our approach is suitable for other microfluidic systems that require synchronous measurements of flowing objects.

A millisecond passive micromixer with low flow rate, low sample consumption and easy fabrication

Fast mixing of small volumes of solutions in microfluidic devices is essential for an accurate control and observation of the dynamics of a reaction in biological or chemical studies. It is often, however, a challenging task, as the Reynolds number (Re) in microscopic devices is typically < 100. In this report, we detail a novel mixer based on the “staggered herring bone” (SHB) pattern and “split-recombination” strategies with an optimized geometry, the periodic rotation of the flow structure can be controlled and recombined in a way that the vortices and phase shifts of the flow induce intertwined lamellar structures, thus increasing the contact surface and enhancing mixing. The optimization improves the mixing while using a low flow rate, hence a small volume for mixing and moderate pressure drops. The performances of the patterns were first simulated using COMSOL Multiphysics under different operating conditions. The simulation indicates that at very low flow rate (1–12 µL·min⁻¹) and Re (3.3–40), as well as a very small working volume (~ 3 nL), a very good mixing (~ 98%) can be achieved in the ms time range (4.5–78 ms). The most promising design was then visualized experimentally, showing results that are consistent with the outcomes of the simulations. Importantly, the devices were fabricated using a classical soft-lithography method, as opposed to additive manufacturing often used to generate complex mixing structures. This new device minimizes the sample consumption and could therefore be applied for studies using precious samples.