Until recently, multi-stable mechanical metamaterials have been primarily used in passive energy absorption systems. However, the ability to actively program these structures has gained significant interest, expanding their functionality to enable on-demand adaptive deformation. While existing active programming methods effectively induce global state transitions, localized actuation remains largely unexplored. This study introduces a novel approach to actively programming multi-stable metamaterials via local thermal stiffness modulation at boundary conditions. Using a polymer bi-material design with distinct glass transition temperatures between the beam and boundary supports, the system can transition from a bi-stable to a mono-stable state, enabling controlled snap-back behaviour after deformation. An analytical model is developed to characterize the snap-through behaviour of the unit cell, providing insight into the geometric interactions and sensitivities associated with various design parameters. Experimental implementation, using multiple additive manufacturing techniques, revealed key limitations and design considerations. In particular, the importance of constraining the second buckling mode and careful material selection emerged as fundamental design requirements for ensuring functionality. This work contributes to the growing field of actively programmable mechanical metamaterials, with implications for compact motion systems in future work.

This research successfully establishes a proof of concept for the hot forming of PEEK sheets using resin-printed polymer molds.
Mechanical metamaterials, through their architected inner structures, offer unique properties that enable unparalleled functionalities. In the MECOMOS project, a novel manufacturing method is proposed to fabricate a multi-stable 3-DoF PEEK metamaterial that enables tip, tilt, and z-translation. The method involves forming PEEK layers to create the geometries of the unit cells, followed by joining these formed layers to produce a monolithic metamaterial.
In this research, a new forming method, hot forming, is developed and analyzed for shaping these layers. This method creates microscale features in PEEK sheets by implementing the process steps of hot embossing into matched die thermoforming.
Polymer molds with various geometries, including rounded cylinders, blocks and trapezoids, are successfully resin printed. Key variables influencing the mold output in resin printing include the cleaning procedure, post-curing, and layer thickness. The smallest printable features measure 400x400x100 µm, and the printing errors range from 40 to 150 µm and are predominantly independent of feature size.
The proof of concept for the hot forming process reveals its strong potential as a manufacturing method.The forming parameters are highly forgiving, with a broad range of process parameters still yielding high replication quality. The suitable forming temperature spans from 100 to 150°C, and pressing forces between 750 and 3000 N prove effective.

For the design of new manufacturing methods for mechanical metamaterials in compact motion systems, a new bonding technique has been developed. The manufacturing technique requires thermoformed PEEK films to be bonded together. Literature review has shown that laser welding, and specifically laser transmission welding, is a valuable method for this manufacturing technique, particularly for bonding materials that are tens of microns thick, such as thermoformed and PEEK films. This research demonstrates how to define a viable process and how to optimize laser parameters to achieve maximum joint strength and identifies factors that can diminish the quality of the bond, thereby affecting optimal speed.

Further investigation of the PEEK and absorptive coating spectral curves has revealed that absorption coating is effective in increasing the absorbance used in this research. Welding with an infrared laser around one µm wavelength can rapidly produce strong welds, where the tensile strength of the welds exceeds that of the material. A fluence of around 20 to 30 mJ/mm^2 results in optimal weld peel strengths.

The optimal parameters for 125 µm lap welding include a speed between 10 and 15 millimeters per second and a defocussing distance between 4 and 7 millimeters. The thickness of the welded films range from 25 to 125 microns. It has also been observed that thicknesses of 25 and 50 micrometers can produce welds with 125 thick thermoformed PEEK films, allowing for 2.5D film welding capabilities.