Polymer Microstructures through Two‐Photon Crosslinking

Two‐photon crosslinking of polymers (2PC) is proposed as a novel method for the fabrication of freestanding microstructures via two‐photon lithography. During this process in the confocal volume, two‐photon absorption leads to (formal) C,H‐insertion reactions, and consequently to a strictly localized crosslinking of the polymer. To achieve this, the polymer is coated as a solvent‐free (glassy) film onto an appropriate substrate, and the desired microstructure is written by 2PC into this glass. In all regions outside of the focal volume where no two‐photon process occurs, the polymer remains uncrosslinked and can be washed away during a developing process. Using a self‐assembled monolayer containing the same photoreactive group allows covalent attachment of the forming freestanding structures to the substrate, and thus guarantees an improved stability of these structures against shear‐induced detachment. As the two photon process is carried out in the glassy state, in a simple way, multilayer structures can be used to write structures having a varying chemical composition perpendicular to the surface. As an example, the 2PC process is used to build a structure from both protein‐repellent and protein‐adsorbing polymers so that the resulting 3D structure exhibits spatially controlled protein adsorption.