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

Toward sub-second solution exchange dynamics in flow reactors for liquid-phase transmission electron microscopy

Liquid-phase transmission electron microscopy is a burgeoning experimental technique for monitoring nanoscale dynamics in a liquid environment, increasingly employing microfluidic reactors to control the composition of the sample solution. Current challenges comprise fast mass transport dynamics inside the central nanochannel of the liquid cell, typically flow cells, and reliable fixation of the specimen in the limited imaging area. In this work, we present a liquid cell concept - the diffusion cell - that satisfies these seemingly contradictory requirements by providing additional on-chip bypasses to allow high convective transport around the nanochannel in which diffusive transport predominates. Diffusion cell prototypes are developed using numerical mass transport models and fabricated on the basis of existing two-chip setups. Important hydrodynamic parameters, i.e., the total flow resistance, the flow velocity in the imaging area, and the time constants of mixing, are improved by 2-3 orders of magnitude compared to existing setups. The solution replacement dynamics achieved within seconds already match the mixing timescales of many ex-situ scenarios, and further improvements are possible. Diffusion cells can be easily integrated into existing liquid-phase transmission electron microscopy workflows, provide correlation of results with ex-situ experiments, and can create additional research directions addressing fast nanoscale processes.

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.

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.

Microfluidic Nanomaterial Synthesis and In Situ SAXS, WAXS, or SANS Characterization: Manipulation of Size Characteristics and Online Elucidation of Dynamic Structural Transitions

With the ability to cross biological barriers, encapsulate and efficiently deliver drugs and nucleic acid therapeutics, and protect the loaded cargos from degradation, different soft polymer and lipid nanoparticles (including liposomes, cubosomes, and hexosomes) have received considerable interest in the last three decades as versatile platforms for drug delivery applications and for the design of vaccines. Hard nanocrystals (including gold nanoparticles and quantum dots) are also attractive for use in various biomedical applications. Here, microfluidics provides unique opportunities for the continuous synthesis of these hard and soft nanomaterials with controllable shapes and sizes, and their in situ characterization through manipulation of the flow conditions and coupling to synchrotron small-angle X-ray (SAXS), wide-angle scattering (WAXS), or neutron (SANS) scattering techniques, respectively. Two-dimensional (2D) and three-dimensional (3D) microfluidic devices are attractive not only for the continuous production of monodispersed nanomaterials, but also for improving our understanding of the involved nucleation and growth mechanisms during the formation of hard nanocrystals under confined geometry conditions. They allow further gaining insight into the involved dynamic structural transitions, mechanisms, and kinetics during the generation of self-assembled nanostructures (including drug nanocarriers) at different reaction times (ranging from fractions of seconds to minutes). This review provides an overview of recently developed 2D and 3D microfluidic platforms for the continuous production of nanomaterials, and their simultaneous use in in situ characterization investigations through coupling to nanostructural characterization techniques (e.g., SAXS, WAXS, and SANS).

Structural Evolution of a Polystyrene-Block-Poly(Ethylene Oxide) Block Copolymer in Tetrahydrofuran/Water Cosolvents

This study aims to quantitatively investigate the effect of water content on the self-assembly behavior of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) in tetrahydrofuran/water cosolvents by small-angle X-ray scattering. PS-b-PEO chains preferentially form fractal aggregates at a dilute concentration in neat tetrahydrofuran (THF). By adding a small amount of water into THF, PS-b-PEO forms gelled networks. The gelled networks have correlated inhomogeneities, which were generated through mesophase separation. These gelled networks are not present when PS-b-PEO is dissolved in THF/methanol and THF/ethanol cosolvents. The substitution of water with 12 M HCl reduces the viscosity of the gelled networks. Those results indicate that the gelled networks of PS-b-PEO need hydrogen bonds formed from surrounding water molecules to be bridging agents, which connect different PEO block chains together. Upon increasing the water content in THF/water cosolvents, dispersed micelles with a core-shell conformation or aggregated micelles preferentially coexist with fractal aggregates.

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.

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.

X-ray-Based Techniques to Study the Nano-Bio Interface

X-ray-based analytics are routinely applied in many fields, including physics, chemistry, materials science, and engineering. The full potential of such techniques in the life sciences and medicine, however, has not yet been fully exploited. We highlight current and upcoming advances in this direction. We describe different X-ray-based methodologies (including those performed at synchrotron light sources and X-ray free-electron lasers) and their potentials for application to investigate the nano-bio interface. The discussion is predominantly guided by asking how such methods could better help to understand and to improve nanoparticle-based drug delivery, though the concepts also apply to nano-bio interactions in general. We discuss current limitations and how they might be overcome, particularly for future use in vivo.

Microfluidic polyimide gas dynamic virtual nozzles for serial crystallography

Free liquid jets are a common sample delivery method in serial femtosecond x-ray (SFX) crystallography. Gas dynamic virtual nozzles (GDVNs) use an outer gas stream to focus a liquid jet down to a few micrometers in diameter. Such nozzles can be fabricated through various methods (capillary grinding, soft lithography, digital light processing, and two-photon polymerization) and materials, such as glass, polydimethylsiloxane, and photosensitive polyacrylates. Here, we present a broadly accessible, rapid prototyping laser ablation approach to micromachine solvent-resistant and inert Kapton polyimide foils with highly reproducible geometric features that result in 3D flow-focused GDVNs suitable for crystallography experiments at synchrotrons and free-electron laser facilities.