# Radiation Damage of Polydimethylsiloxane and Polyimide by X-ray Free-Electron Laser
## 4 Discussion
Accordingly, even if the PDMS and PI materials have the same uniformity and thickness, the physical damage caused by exposure to the XFEL will differ. Consequently, we believe that the physical radiation damage between PDMS and PI differs from shot to shot and is caused by the difference in the characteristics of the XFEL, the uniformity of the materials, and the physical properties of the two materials. Accordingly, we expect that the radiation damage pattern that appears when a different type of film is exposed to an XFEL will differ depending on the material and XFEL characteristics. Because of the large hole size of the PDMS film due to its exposure to the XFEL, raster scanning in FT-SFX may be limited to wide intervals. However, in the case of a PI film, the sample consumption can be reduced by scanning more narrowly than the 50 \(\upmu\)m interval used in the experiment. Nevertheless, PDMS can be easily and freely processed in the laboratory to any desired size, thickness, or design. For PDMS application in SFX, to minimize the physical radiation damage of the PDMS film exposed to XFEL, it is critical not only to make the PDMS film more uniform but also to make it significantly thin. We observed radiation damage by continuously exposing the PDMS and PI films to the XFEL. When the XFEL was continuously passed through the PDMS film, the size of the hole gradually increased after the hole was generated at the point where the XFEL penetrated; physical damage, considered radiation damage, was also observed in the film area around the hole. Observing this with a high-speed camera, we confirmed that the focus of the hole changed as shown in Supplementary Video S3, which tentatively indicates that the film around the PDMS hole moved fluidly when the XFEL was continuously exposed. Because the position jitter of the XFEL beam used in the experiment was <1 \(\upmu\)m per day, the gradual increase in the hole after the penetration of the XFEL in PDMS was unaffected by the movement of the main beam. Accordingly, it is considered physical damage to the PDMS chip owing to the continuous exposure of the parasitic scattering around the focused beam. Contrarily, in the PI film, no significant change occurred in the hole size after the XFEL passed through holes despite continuous exposure. We believe that the parasitic scattering around the main XFEL beam cannot affect the PI film. In a microfluidic device based on PDMS and PI, we observed unique radiation damage when the XFEL passed through the inner channel through which the sample passed. In PDMS-based microfluidics, not only the occurrence of holes due to radiation damage was observed in the PI film, but also the structural distortion of the PDMS device around the inner channel to which the XFEL was continuously exposed was observed. In addition, we observed that the solution sample delivered to the inner channel leaked through the large hole generated in the PDMS microfluidic device. Consequently, collecting diffraction data while penetrating the XFEL through the inner channel of the PDMS-based microfluidic chip is considered impossible. Contrarily, in the PI-based microfluidics chip, holes were generated by XFEL penetration; however, structural destruction of the microfluidic device was not observed, even when the XFEL was continuously exposed. Meanwhile, the solution did not leak from the holes created in the PI microfluidic device. As the internal pressure is higher than the atmospheric pressure, leaking can occur in terms of pressure. However, we assume that there is no leakage into the small hole of the solution owing to the surface tension of the solution. However, when the XFEL penetrates the inner channel of PI microfluidics, the inner channel is partially blocked by bubbles caused by radiation damage to the solution, and there is an issue of changing the route of the solution flowing through it.