XFEL: Core-shell jets increase liquid sample delivery stability

Liquid jets are a well-established sample delivery method for serial femtosecond crystallography (SFX) experiments, which enable biological structure determination. A decisive factor for jet stability and therefore data collection efficiency is the design of the nozzle ejecting the liquid into the sample chamber. In a new study published in Scientific Reports, an international team of European XFEL staff, collaborators and users demonstrates how the jet can be extremely elongated and simultaneously stabilized, which enables the injection of liquids of a higher viscosity than previously possible.

Typically, liquid jets are generated by gas dynamic virtual nozzles, which are continuously improved and manufactured at European XFEL before being provided to user experiments at the SPB/SFX instrument. They use an outer stream of helium to compress and accelerate the liquid containing the microcrystal samples into a jet of a few micrometers in diameter. Double-flow focusing nozzles additionally use a sheath solution between the crystal stream and the helium. The scientists now added a low concentration of polyethylene oxide (PEO), a bio-compatible polymer, to this aqueous sheath. This produced a viscoelastic jet: The PEO makes the sheath liquid behave partly like a viscous fluid, as the stretched polymer chains form a stabilizing ‘armor’ around the sample stream and generate a long, steady jet.

The key aspect of the stabilization is “the intense converging extensional flow at the ejection point triggers a coil-stretch transition of the polymers inside the micronozzle, thereby building up large extensional viscoelastic stress. This stress does not relax in the jet even for times much longer than the polymer relaxation time. The increased extensional viscosity confers superstability on the jet”, says co-author José María Montanero, Professor of Fluid Mechanics at the Universidad de Extremadura.

Due to their millimeter length, such core-shell jets can even be considered for pump–probe SFX in order to access a timescale of a few tens of microseconds. This range is in between the previously achievable ranges accessible at XFELs (picoseconds-to-microsecond time delays) and synchrotrons (a few hundred microseconds to millisecond delays), and has so far been considered a blind spot.

“We evaluated crystal hit rate statistics for a variety of samples with biological significance, buffer compositions, beam sizes and applied flow rates. What we found is an unmatched sample-to-beam overlap that has the potential to advance the serial femtosecond X-ray crystallography data collection strategy. The advantages of this approach could become widely adopted at other high repetition rate XFELs as well as at synchrotron light sources”, says lead author Mohammad Vakili, beamline scientist at SPB/SFX.

The researchers also developed novel triple-flow focusing nozzles that introduce the described benefits to microfluidic mixing-nozzles, thus paving the way for time-resolved studies of samples that have to reside in viscous buffers. Petra Fromme, Professor in the School of Molecular Sciences, Arizona State University, says: “Viscoelastic jets represent an efficient method to deliver samples that are dispersed in both low-viscosity and medium-viscosity liquids. Finally, we may have found an injection method that caters to challenging-to-jet liquids, such as photosystem II crystal media.” Photosystem II is a protein complex that plays a key role in photosynthesis and is at the centre of many research projects. 

The multidisciplinary work —which spans X-ray diffraction methods, fluid dynamics, polymer science, and structural biology —involved close collaboration between European XFEL, the Universities of Extremadura, Sevilla and Hamburg, the Arizona State University, and Deutsches Elektronen-Synchrotron (DESY) in Hamburg.

Original Publication: https://doi.org/10.1038/s41598-026-44308-8