This dissertation explores the development and application of FeCl$_3$-PEO photoreactions for precision photolithographic patterning in both conductive polymer and silicone elastomer systems. Conductive polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) are integral to advancements in optoelectronics, sensors, and bioelectronics, but current patterning methods suffer from trade-offs in adhesion, resolution, and scalability. We present a novel vapor-phase polymerization (VPP) approach that utilizes FeCl$_3$-PEO photoreactions to pattern PEDOT films with feature sizes as small as 15 $\mu$m and conductivities over 1000 S cm$^{-1}$. This process employs a photosensitive initiating film, where UV light selectively deactivates polymerization, achieving high-contrast patterns on diverse substrates. Mechanistic studies reveal that this behavior is governed by photoinitiated ligand-to-metal charge transfer between FeCl$_3$ and the polyether component. Building on this work, we extend the utility of the FeCl$_3$-PEO photoreaction to pattern biocompatible silicone elastomers, such as Sylgard 184 and Dragonskin, which are crucial materials for microfluidic devices, wearable sensors, and soft bio-MEMS. Incorporating the FeCl$_3$-PEO complex into platinum-catalyzed silicone formulations enables UV-mediated control over hydrosilylation crosslinking, allowing rapid, full-thickness patterning with low doses of 365 nm light. This approach preserves the mechanical tunability and transparency of commercial silicone formulations, offering a straightforward and reproducible method for creating microstructures. To demonstrate the versatility of the silicone photopatterning process, we fabricated microfluidic devices and cardiac microphysiological systems, showcasing the potential of this technique for advanced biomedical device engineering.