Self-assembled monolayer enhanced support-free two-photon polymerisation for microscale kinematic pairs

The dynamic of functional microsystems, such as microrobotics, depends on the precise monolithic fabrication and functional integration of microscale kinematic pairs. Although point-scanning technology endows two-photon lithography (TPL) with unparalleled geometric freedom and manufacturing precision, structures fabricated in liquid photoresists inherently suffer from mechanical instability while lacking mechanical support during the forming process. This fundamental constraint hinders the realisation of truly freeform three-dimensional structures capable of a high degree of freedom (DOF) relative motion. To overcome this bottleneck, we have developed a high-viscosity epoxy photoresist with substantial yield stress for the temporary immobilisation of unsupported components. This epoxy photoresist exhibits solid-like behaviour to provide robust support for overhanging structures and skips the fully continuous stacking, while a surface fluorosilane grafting ensures controlled relative motion. A testing array of the hollow cone and suspended sphere structure was constructed to determine the minimum accuracy between dynamic components and static constraint. The monolithically integrated 3D kinematic pair was demonstrated, including multilayer suspended ball shells, nested hollow balls, gear bearings, and a bio-inspired swinging hair follicle, which are challenging to achieve with liquid two-photon resists. This work provides a robust materials strategy for two-photon volumetric 3D printing, enabling the creation of ready-to-use integrated micro-mechanical devices.