# Radiation Damage of Polydimethylsiloxane and Polyimide by X-ray Free-Electron Laser
## 1 Introduction  
X-ray free-electron lasers (XFELs) produce intense X-rays with ultrashort pulse widths, which are beneficial for understanding the time-resolved molecular dynamics ranging from small molecules to macromolecules [1]. The serial femtosecond crystallography (SFX) technique using XFEL allows for determining the room-temperature structure of small molecules or macromolecules while minimizing radiation damage [2; 3; 4; 5]. In addition, SFX with pump-probe or mix-and-inject techniques enables the visualization of the time-resolved molecular dynamics of target molecules [6; 7; 8]. In SFX experiments, a sample delivery system is essential for continuously and reliably delivering the crystals to the X-ray interaction point [9; 10; 11]. Various sample delivery methods, such as injector [12; 13; 14], injector/syringe with viscous medium [15; 16; 17; 18; 19; 20; 21], fixed-target scanning [22; 23; 24; 25; 26; 27; 28],capillary methods [29; 30], microfluidic devices [31; 32; 33; 34], and hybrid methods of injection-and-transfer [35], have been developed and successfully applied in serial crystallography experiments. In SFX experiments, depending on the sample delivery approach, the crystal sample is directly exposed to the XFEL, or the crystal sample and sample delivery device are simultaneously exposed to the XFEL by enclosing or depositing the crystal sample in a film. The sample delivery method using an injector/syringe is a representative experimental method in which a crystal suspension (or crystals embedded in a viscous medium) is directly exposed to an XFEL. Among these, liquid jet injectors minimize X-ray background scattering by extruding an injection stream composed of a crystal suspension [12]. This system delivers the crystal suspension at a high flow rate to produce a stable injection stream, which is beneficial for applying the XFEL facility with a high repetition rate. Contrarily, when a liquid jet injector is applied to an XFEL facility with a low repetition rate, non-hit crystals are overwhelmingly wasted compared to crystals exposed to XFEL; therefore, this delivery method is inefficient in terms of sample consumption. An alternative is to embed the crystals in a viscous medium and deliver them at a low flow rate to maintain a stable injection stream [11; 13; 15; 36]. However, crystals are often physically damaged during mixing with a viscous medium or dissolved by a chemical reaction between the crystals and viscous medium, which can impair the diffraction quality of the crystals [11]. Therefore, this method often requires effort to find a suitable viscous material depending on the crystal sample. Another crystal delivery approach is to enclose or deposit the crystals using a polymer film or micro-patterned chips, such as the fixed-target scanning method [22; 23; 24; 25; 26; 27] or a hybrid method such as drop-on-drop [35] or a combination of an inject-and-transfer system (BITS) [37]. In these methods, crystals are enclosed or deposited on a film and delivered to an X-ray interaction point using a programmed translation stage or conveyer approach. These methods have the advantage of lower sample consumption than the injector method [27]. In addition, these methods cause minimal physical damage to the crystals during sample delivery compared to the injector method [27]. In previous SFX experiments, polyimide (PI) films [25; 27] and Mylar films [38] have been applied to deliver crystal samples using a fixed-target scanning approach or BITS system approach. When the XFEL penetrates the film, the XFEL radiation damage creates a hole in the film where the XFEL is transmitted. This indicates that radiation damage studies using XFEL are required in advance for attractive materials that can potentially be applied for developing SFX sample delivery devices. Polydimethylsiloxane (PDMS) is a high-performance polymer with thermo-tolerance, resistance to oxidation, ease of fabrication, and tunable hardness [39].