4D printing of reconfigurable metamaterials and devices

Shape-shifting materials offer significant potential for creating reconfigurable materials and devices. These materials change shape and properties upon activation, enabling advanced functionalities. While 2D-to-3D shape-shifting (flat to 3D) is relatively well-established, 3D-to-3D shape-shifting (3D to a different 3D shape) remains challenging due to the layer-by-layer nature of typical 3D printing processes. This limitation restricts the possible shape-shifting modes. Existing techniques often rely on active materials that change dimensions upon activation (e.g., swelling hydrogels or shape-memory polymers). 4D printing, which integrates the time dimension through the use of responsive materials, offers a promising route to creating shape-shifting structures, but methods for 3D-to-3D transformations are limited. Previous research by the authors demonstrated the use of hobbyist FDM 3D printers to program complex 2D-to-3D shape-shifting behaviors by introducing spatially varying anisotropies in the material during printing. However, this approach is insufficient for creating reconfigurable materials due to the in-plane nature of the anisotropies. This paper addresses this limitation by introducing a novel method that allows for printing on curved surfaces, thereby enabling the creation of more complex 3D-to-3D shape-shifting behaviors.

The literature extensively covers shape-shifting materials, including 2D-to-3D and 3D-to-3D transformations. Numerous techniques utilize active materials such as hydrogels and shape-memory polymers. 4D printing has emerged as a method for single-step fabrication of complex shapes. Several strategies have been explored, focusing on introducing anisotropy in the material during printing. Previous work by the authors and others has shown success in 2D-to-3D shape-shifting using FDM 3D printing, but these approaches lacked the capability to produce reconfigurable materials due to the intrinsic limitations of in-plane anisotropy. The current work builds on this foundation, seeking to overcome the limitations of in-plane anisotropy by enabling printing on curved surfaces. This approach enables the creation of 3D-to-3D shape-shifting behaviors that are difficult or impossible to achieve with existing techniques.

The researchers modified a commercially available FDM 3D printer by adding a simple add-on device consisting of a stepper motor-driven rotating drum. This allows the printer to deposit material onto curved surfaces. Polylactic acid (PLA) filaments were used as the material. The thermomechanical properties of PLA were characterized using dynamic mechanical analysis (DMA) to determine its glass transition temperature (Tg) and to develop a viscoelastic material model for finite element analysis (FEA). The influence of printing parameters (layer thickness, extrusion temperature) on filament shrinkage was investigated experimentally. The effects of printing path on shape-shifting were also studied. FEA simulations were used to model the shape-shifting behavior of the printed structures. Basic shape-shifting elements, including in-plane and out-of-plane bending elements, were designed and fabricated. Three design strategies were employed to create more complex reconfigurable structures: (1) spatial variation in printing path orientation, (2) in-plane bending elements within lattice structures, and (3) arrangements of out-of-plane elements connected by semi-passive cylinders. The mechanical properties (stiffness and Poisson's ratio) of the resulting reconfigurable materials were characterized experimentally and through FEA. Finally, the method was used to fabricate deployable medical devices, including self-expandable stents and bifurcation stents. The activation of the shape-shifting behavior was achieved using a hot water bath (90°C), with experiments also conducted using hot air to assess the influence of the activation medium. The fabrication process involved custom MATLAB programs for generating printing paths and G-code files. The FEA was performed using Abaqus software, employing a transient coupled temperature-displacement analysis.

The modified FDM 3D printer successfully enabled the fabrication of curved shape-shifting structures. The experiments and FEA showed good agreement in predicting the shape-shifting behavior. The printing parameters significantly affected filament shrinkage, with lower printing temperatures and smaller layer thicknesses leading to increased shrinkage. The printing path had a noticeable impact on shape-shifting only for short, parallel filaments. The basic bending elements demonstrated predictable curvature based on their dimensions. The three design strategies yielded a wide variety of complex shape-changing behaviors in the reconfigurable materials, including buckling, bending, and unfolding. The adaptive materials showed a significant change in stiffness upon activation, with a 30-fold increase observed in tension and over two orders of magnitude increase in compression. A switch in Poisson's ratio from positive (conventional) to negative (auxetic) was demonstrated in a re-entrant honeycomb structure. Deployable cylinders with different expansion patterns were successfully fabricated by varying the design parameters. Miniaturized and bifurcation stents were produced, demonstrating the potential for complex medical device fabrication. The activation process was completed within 30 seconds, regardless of the activation medium. The study found that the shape-shifting behavior was largely independent of the activation method, be it water or air.

This work significantly advances 4D printing capabilities by overcoming the limitations of in-plane anisotropy in existing methods. The ability to print on curved surfaces opens up new possibilities for creating complex, reconfigurable materials and devices. The demonstrated ability to create adaptive materials with switchable stiffness and Poisson's ratio is highly significant, offering new opportunities for developing advanced materials with tunable mechanical properties. The successful fabrication of deployable stents highlights the potential of this approach for biomedical applications. The agreement between experimental results and FEA simulations validates the design methodology and provides a useful tool for predicting shape-shifting behavior. The relative simplicity and low cost of the modified FDM printer make this technology accessible, potentially democratizing 4D printing and promoting its use in diverse applications.

This research successfully demonstrated a simple yet highly effective method for 4D printing of reconfigurable materials and devices. By modifying a standard FDM 3D printer to print on curved surfaces, the researchers achieved unprecedented control over 3D-to-3D shape-shifting behaviors. The demonstrated applications in adaptive materials and deployable medical devices showcase the broad potential of this technology. Future research could explore the use of different shape memory polymers with varying Tg values and investigate further applications in diverse fields.

The study focused on PLA filaments as the material, which may limit the generalizability of the findings to other materials. The FEA models simplified certain aspects of the 3D printing process, such as porosity, which could affect the accuracy of the simulations. The scope of medical device applications was limited to stents; further research is needed to explore its applicability to other medical devices. While the hot water and hot air activation methods yielded similar results, further investigation is warranted for activation methods not involving liquids.