Innovative 3D Self-Assembly Using Polymer Cross-Link Gradients

SU-8 photoresist has been utilized for decades for diverse applications including small scale patterning, microfluidic channels and MEMS. However, the intrinsic thermal stress generated during the post-baking process causes mechanical failures in the SU-8 film. Numerous literatures have been studied to minimize stress generation. Here, conversely, this intrinsic stress has been leveraged for the deformation of a SU-8 thin film in multi-directions including z-axis, which enables the creation of complex 3D micro architectures. To realize 3D structures, the SU-8 photoresist is intentionally exposed to an ultraviolet (UV) dose that is 25% lower than the recommended dose by SU-8 manufacturer. The low dose induces a cross-linked gradient along the exposure direction, generating stress gradient in the same direction. However, the structure remains flat even after removing a sacrificial layer underneath. This is because the elastic restoring force (stiffness) of a SU-8 layer, at room temperature, is larger than its bending stress generated from the stress gradient. Therefore, to reduce the stiffness and fully release the stress, the SU-8 (essentially a thermal epoxy) is heated in water. The softened SU-8 diminishes its stiffness, and the bending stress, induced by the cross-linked gradient, becomes adequate to overcome the elastic restoring force at around 70 ~ 80 degrees C, resulting in a lasting curvature shape with the directions preprogramed by the 2D patterns of the UV exposure. To support this new finding mechanism, mechanical parameters, stiffnesses and stress gradient varied with temperature and dose, are characterized. Beyond simple curved architectures, with a careful intention on the dose control, complicated 3D structures like vertically aligned helix and tubes are also able to realize. Moreover, this photo-sensitive material property also enables a new way of patterning, such as pre-programming metals and 2D materials on it, and subsequently self-assemble together with the SU-8 itself. The proposed technique may promote further development of complex micro robotics, 3D sensors, and e-muscles.