# Radiation Damage of Polydimethylsiloxane and Polyimide by X-ray Free-Electron Laser
## 4 Discussion  
In SFX experiments, a sample delivery device is necessary. Among various sample delivery devices, a polymer film is often used as a material for the sample delivery device. To expand our knowledge about radiation damage of polymer films for XFEL and understand their potential applications, we investigated the XFEL-induced radiation damage of PDMS and PI materials, which are widely used as materials for sample delivery devices in synchrotron X-ray research due to their lack of radiation damage. Upon irradiation with XFEL, both PDMS and PI materials were physically damaged, resulting in holes in the  
Figure 4: Radiation damage of PDMS- and PI-based microfluidic devices by XFEL. (**A**) The structure of the PDMS-based microfluidics is destroyed by XFEL exposure. (**B**) The structure of the PI-based microfluidic device is not physically destroyed by XFEL; however, the bubbles generated from the solution by XFEL radiation damage are accumulated in the inner channel and interrupt the sample path in the inner channel. film at the position penetrated by XFEL; however, the pattern of holes generated by XFEL radiation damage differed between the PDMS and PI films. The XFEL-exposed PDMS film exhibited holes of various sizes and shapes. Consequently, the shape of the hole caused by physical radiation damage in the PDMS film exposed to the XFEL was not consistent with the shape of each XFEL-exposed point. In particular, because the size of the hole was significantly larger than the size of the beam, it temporarily affected the surrounding sample when scanning at short intervals in the raster scanning experiment. Meanwhile, the XFEL-induced holes on the PI film showed relatively similar sized and shaped holes compared to the PDMS film. Overall, the patterns of holes generated by physical radiation damage varied across all types of films exposed to XFEL. We consider three possibilities as to why each shot on the film causes different levels of radiation damage: (1) Because the X-ray intensity of a natural XFEL pulse varies from shot to shot, radiation damage patterns in the holes of PDMS and PI films cannot be identical. Considering the natural characteristics of the XFEL pulses, the difference in the size and shape of holes observed in the PI film can be attributed to the XFEL energy fluctuation. (2) Depending on the uniformity and thickness of the film, the effect of physical radiation damage on the film may differ when exposed to an XFEL. If the XFEL exposes a film with a uniform interior and surface, the pattern of the generated hole will be similar; however, the physical radiation damage in PDMS is quite variable. In addition, the physical impact will vary depending on the thickness of the film; the thinner the film, the more the radiation damage is restricted to the part through which the XFEL is transmitted. Overall, because the PDMS film used in this experiment was directly processed and manufactured, we assume that its uniformity was lower than that of the commercially processed PI film. (3) The physical radiation damage differs according to the intrinsic properties of the film material. The PDMS and PI films used in this experiment differed in various physical properties, such as material composition, structure, strength, X-ray absorption, and thermal stability. For example, [PERSON] et al. used a 1 \(\upmu\)m-focused XFEL beam to investigate the ablation thresholds of optical materials [59]. When optical materials are exposed to an XFEL, the imprint area generated on their surfaces varies depending on the energy exposed and for each material. The measured threshold fluences of the uncoated Si, uncoated SiO\({}_{2}\), Si under a Pt coating layer, SiO\({}_{2}\) under a Pt coating layer, Pt, and Rh were 0.78 \(\pm\) 0.04, 4.5 \(\pm\) 0.7, 0.065 \(\pm\) 0.008, 0.11 \(\pm\) 0.03, 0.023 \(\pm\) 0.004, and 0.072 \(\pm\) 0.007 \(\upmu\)J/\(\upmu\)m\({}^{2}\), respectively [59].