The flow cell is considered part of the relay optomechanical system because it needs to be kept in alignment with the DMD image with an accuracy of about ±10 µm along the optical axis and about ±0.25 µm perpendicular to the optical axis. In addition, the flow cell needs to be easily removed from the optical system, as well as reinserted with an accuracy of few micrometers along the optical axis. The design of the flow cell is shown in Figure . Each side of the flow cell metal block, which constitutes the structural core of the assembly, has two sets of three Kelvintype kinematic coupling points to allow use from both sides. These engage with the ball tips of micrometer actuators held rigidly by the flow cell holder. In typical use, with a transparent glass microscope slide as a synthesis substrate, photodeprotection is performed through the slide, but if it is necessary to use a non-transparent substrate, the exposure can be performed through the 10 mm thick quartz block forming the other side of the reaction chamber. The flow cell is defined by the interior surfaces of the glass slide and the quartz block, as well as by a perfluoroelastomer (FFKM) gasket on the perimeter. This assembly is held together with a clamp attached to the flow cell metal block via four screws. Access to the flow cell for the solvents and reagents needed for the synthesis chemistry is provided by two 0.8 mm channels through the quartz block and two corresponding fluidics ports that accept 1/32″ OD, 0.4 mm ID fluorinated ethylene propylene (FEP) tubing (not shown).