Microfluidic Electrokinetic Sample Holder for Serial Femtosecond Crystallography

We present a microfluidic electrokinetic sample holder (MESH) which delivers suspensions of protein crystals to a hard X-ray free electron laser (XFEL) source at the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Lab, for structural determination by serial femtosecond crystallography (SFX). The MESH allows for low sample consumption rates on the order of hundreds of nanoliters per minute of protein suspension; 10x to 100x lower than current sample delivery technologies, such as hydrodynamically focused jets and aerodynamically focused particle streams. The applied electric field and added viscosity, necessary for operating the MESH, diminish the background signal and reduce the flowrate, while not affecting the structure of protein crystals as observable down to better than 4 Angstroms. Simple, off-the-shelf components combined with small sample consumption rates on the order of micrograms, make MESH an ideal sample delivery method for protein structural determination at hard X-ray sources, from simple protein standards like thermolysin, to complex membrane proteins like photosystem II.