Synchrotron-generated X-ray crystallography have been used for identifying structures of proteins, but it has not been able to solve many kinds of structures. X-ray free-electron lasers (XFELs) is emerging as a next-generation technology for solving structures of proteins. XFELs have extremely high luminance, femtosecond-region X-ray pulses, and perfect optical coherence in contrast to synchrotron-generated X-rays. The femtosecond X-ray pulse is tremendously short, so high-resolution diffraction patterns of proteins can be obtained without deformation or destruction of proteins. Protein crystallography by using XFELs is based on serial femtosecond crystallography (SFX), collecting huge amount of single diffraction pattern of proteins in femtoseconds. Therefore, SFX can investigate crystal structures of proteins that have not solved before. Sample delivery system is one of the significant issue for collecting diffraction data in SFX. Injection systems, such as lipidic cubic phase (LCP) injector, liquid jet or acoustic injector are currently used in SFX experiments. The velocity of injected solution must be very high because of thin flow of crystals, therefore most of crystals are wasted without hitting by X-ray beam, that indicates low hitting rates of X-rays. Besides, injector exerts high pressure on the mother liquor to maintain the thin flow of crystals, so it can be the damage to the protein crystals. Low hitting rates of X-ray beam and the waste of the protein crystals are huge barrier for protein crystallography.
In this work, we implemented an attractive sample delivery system of SFX to reduce the waste of crystals and to improve hitting rates of X-ray beam, called fixed-target matrix. Fixed-target matrix is successfully fabricated by low-cost fabrication process, using ultra violet photolithography and chemical etching. Additionally, surface treatment using oxygen plasma is applied to fixed-target matrix for hydrophilic surface. Polar energy of the matrix is drastically enhanced from 3.25mN/m to 40.14mN/m, resulting in zero contact angle of water. Therefore, mother liquor with protein crystals can be spread well, and protein crystals can enter the hole efficiently. This fixed-target matrix can attend significant role to demonstrate the crystal structure of proteins.