Different crystal shapes were observed, including single needles, star shapes with few needles, and star shapes with many needles (Figures 157a and 157b). The crystals grew centred in the droplets, demonstrating that the droplet approach avoids heterogeneous nucleation on the channel walls. The crystals persisted over several weeks within sealed devices stored at ambient conditions. Crystals of lysozyme (Figure 157c) and thaumatin (Figure 157d) were grown within droplets using similar devices.
Two tests were done to evaluate the possibility to use the microfluidic devices with beam to obtain in-situ diffraction data from micro- crystals trapped inside the channels. For the first test, a thaumatin crystal grown with the hanging drop method was placed on a slab of 0.6 mm-thick 3D-printed material positioned in the beam of ID30A-3. Scattering of the resin (Figure 157e, left panel) is characterised by a broad ring related to real space distances ranging between 5 and 6 Å, while the diffraction peaks (Figure 157e, right panel) can be seen with the naked eye. For the second test, a complete device containing crystals grown within droplets trapped in the channels was positioned in the beam of ID30B and mesh-scanned to find diffracting crystals. The diffraction peaks were less clearly visible, but could be easily identified by software routines even in the regions of higher background intensity originated by the resin. The integration of thin X-ray-compatible windows can further mitigate the background scattering issues.
In conclusion, minor modifications of a desktop DLP printer and resin have allowed remarkable improvements in the quality of 3D-printed microfluidic devices. The whole path, from modification of a design to printing and use of a device, can be accomplished within a single day. The as-printed devices are suitable for diffraction of crystals grown inside the channels. The developed method is thus particularly suitable for X-ray studies.
Fig. 156: a) DLP printer. b) LED spectra with and without filter. c) Light intensity versus depth with and without filter.
Accurate and rapid 3D printing of microfluidic devices using wavelength selection on a DLP printer,
P.J. van der Linden (a), A.M. Popov (a) and D. Pontoni (a), Lab. Chip 20 4128-4140 (2020);
https://doi.org/10.1039/d0lc00767f. (a) ESRF
 S. Köster & T. Pfohl, Mod. Phys. Lett. B 26, 1230018 (2012).  A. Ghazal et al., Lab. Chip 16, 4263 (2016).  N. Junius et al., Lab. Chip 20, 296 (2020).  C. Gosse et al., Lab. Chip 20, 3213 (2020).
PRINCIPAL PUBLICATION AND AUTHORS
Fig. 157: a) and (b) gypsum crystals. c) Lysozyme crystals. d) Thaumatin crystals.
e) Left panel: Scattering of 0.6 mm of printed resin, right panel: diffraction of a thaumatin crystal.