New Flow Focusing Technology Facilitates Success of First Experiment Performed by European XFEL Project

The Flow Focusing technology (also referred to as Gas Dynamic Virtual Nozzle [GDVN]), has been one of the core elements in the triumph of the first experiments performed by the European XFEL project, the largest source of X-rays in the world currently. It is a tool that was designed and developed by the Fluid Mechanics teacher Alfonso Gañán Calvo, of the Higher Technical School of Engineering at the University of Seville. This technology, which has been used for the research of microscopic biological samples, places the University of Seville among the institutions at the global frontline in the application of X-Ray Free-Electron Laser (XFEL).

Water jet pulses. (Image credit: University of Seville)

The major advancement obtained in the European XFEL project is the increase in the amount of data that can be acquired per second in the examination of a sample. This feat is possible owing to the use of a frequency of pulses that had not been applied before, more than a megahertz. For that, a high rate of renewal is essential—that is to say, that each pulse has clean samples, not influenced by the earlier pulse. Thus, they have to be given enough velocity.

This means photographing or “hunting” the molecules using an ultra-rapid, and ultra-powerful, flash, before the samples disintegrate due to the intensity of the ionising radiation that they receive.

Alfonso Gañán Calvo, Fluid Mechanics Teacher, Higher Technical School of Engineering, University of Seville.

The biological samples (typically protein microcrystals) have to be in an aqueous setting. The difficult task has been to present them in the right way for them to be intercepted by X-ray pulses that are barely a few microns in diameter and last for less than ten femtoseconds (a hundredth of one thousand billionth of a second), and to produce the clearest and most comprehensible diffraction pattern possible.

To achieve this, the GDVN technology has been capable of producing jets of liquid of less than 2.5 µm in diameter (1 µm = one thousandth of a millimeter) with speeds that touch 100 m per second (360 km per hour), enough to convey protein microcrystals and repeatedly renew them at the point of impact without them being influenced by the earlier pulses.

This has been accomplished as a result of using helium as the focusing gas for the micro-jet, as helium with its physical properties, gains expansion speeds three times greater than those of air. Furthermore, an extremely precise technique of nano-fabrication has been used (3D nano-printing) for the device that releases the jet.

The combination of XFEL technology (trains of ultra-short and ultra-powerful X-ray pulses) with the Flow Focusing vehicle (GDVN) has given birth to what is nowadays called ‘Serial Femtosecond Crystallography’ (SFX), a transformation in molecular biology.

GDVN technology has been accepted as the most efficient, strong, and reproducible (standard) technique for the introduction of samples for Serial Femtosecond Crystallography (SFX) y time-resolved SFX at the European XFEL (Hamburg, Germany), SACLA (Japan), LCLS (Stanford, USA), SwissFEL (Zürich, Switzerland), and the recently constructed Chinese and Korean XFELs, among others.

The European XFEL has been a pioneer, by the way, followed by LINAC LCLS III, which is still not functioning. The so-called “impact index” is maximized with this frequency. This measures the percentage of samples that are efficiently intercepted by the X-ray. The scientific review Nature Communications has reported an article which has showcased the chief advances.

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