*Microfluidics for Time-Resolved Small-Angle X-Ray Scattering DOI: http://dx.doi.org/10.5772/intechopen.95059*

characteristics of the device. For example, NOA 81 is a turbid, commercially available, UV curable polymer mixture from Norland Optical Adhesive, which is relatively easy to work with. However, devices made from NOA 81 are thin and relatively flexible, even after sealing top and bottom half, so it should be avoided if stiff or thick devices are required. In comparison, SIFEL (SIFEL2610) is a fluorinated polymer distributed by Shin-Etsu that is liquid at room temperature and hardens at higher temperature, and is stable against all tested organic solvents. The device fabrication however, is time consuming, requiring the additional step of sputtering the silicon wafer with an inert chemical layer to allow release of the SIFEL device from the mould.

Materials can also dictate the method of fabrication, for example THVs (fluorothermoplastics of blended tetrafluoro ethylene, hexafluoro propylene and vinylidene) must be fabricated by hot embossing. Glass or hard material devices are made with difficult fabrication techniques, like etching. In many cases prototypes are made with cheap, easy to fabricate materials, with the more difficult fabrication only for the final working devices where needed.

**Optical and X-ray transparency.** The most commonly used method for alignment of device parts and analysis of ongoing reactions is optical microscopy. Hence, the optical properties, e.g. transparency, of the microfluidic devices should be considered. Further, the final measurement modality must be considered in material selection. For example, if three-dimensional confocal microscopic investigations of the whole channel volume are required, the selected device material should provide a low absorption behaviour in the range of the sample-specific selected laser wavelength, and low fluorescence background. Or as the focus of this chapter is SAXS, the material of the device should have high X-ray transmission, and low scattering in the q-range of interest. The material should also be able to withstand the X-ray radiation, which is present on high flux SAXS beamlines. In our experience, the lowest background scattering for higher q measurements above 0.05 Å<sup>1</sup> were achieved with glass, NOA81, PDMS and Kapton. Other polymers such as THV and TOPAS showed diffraction and correlation peaks in the high q region >0.1 Å <sup>1</sup> , that interfere with background subtraction, and worsen signal to noise. For measurements at low scattering angles with *q* values under 0.05 Å<sup>1</sup> , glass, PMMA, PS, NOA81 and TOPAS display flat scattering curves. All other tested materials at this q-range showed significant scattering signals from the device [23]. Furthermore, although showing a low scattering background at high scattering vectors, PDMS was extremely sensitive to radiation, deforming the channel and showing an increasing and changing scattering profile with exposure. This material is typically unsuitable for SAXS measurements.

Hybrid microfluidic devices can marry the best characteristics of materials, to achieve a successful device. For example, the complex mixing cross section can be made from easy to handle materials, e.g. PDMS, and a robust X-ray transparent, low background scattering material inserted as an outlet channel after the last crosssection, e.g. a glass capillary (**Figure 2K**). These devices have the advantage of high optical and X-ray transparency in the measurement region, while allowing adjustment to the mixing cross design in the polymer part [23, 24].
