Shape-morphing structures, capable of reconfiguring their shape to adapt to various tasks, are crucial for intelligent machines across numerous fields, including soft robotics, deployable systems, wearable devices, and more. These structures often utilize composite designs to achieve shape-shifting between multiple configurations. Additive manufacturing (AM) or 3D printing offers significant fabrication flexibility, leading to the development of 4D printing, which integrates time as the fourth dimension. Shape memory polymers (SMPs) are frequently used in 4D printing due to their ability to be programmed into different shapes and recover their original shape upon external stimuli, most commonly temperature changes. Traditional hot-programming involves heating the SMP above its transition temperature, deforming it, and then cooling it while maintaining the deformed shape. This method, while offering low programming force, suffers from high energy costs and difficulties in achieving local programming. Localized heating, though allowing for local programming, necessitates additional hardware. Cold-programming, an alternative, involves deforming the SMP at low temperatures without a heating-cooling cycle, offering simplicity and local programmability. However, it requires a much larger force and places stringent demands on material properties, limiting its widespread adoption. This research explores the use of grayscale digital light processing (g-DLP) 3D printing as a method to overcome the limitations of cold-programming and enable the fabrication of complex shape-morphing structures.

The literature extensively covers shape-morphing structures and their applications in various fields. Research highlights the use of stimuli-responsive materials, particularly SMPs, in 4D printing to create shape-changing objects. Existing methods predominantly rely on hot-programming, which entails heating the SMP above its glass transition temperature to enable deformation. However, the literature acknowledges the limitations of hot-programming, including the need for global heating, high energy consumption, and challenges in achieving local control. Cold-programming has been explored as an alternative approach, offering the advantage of room-temperature programming. Nevertheless, the literature also notes the significant force requirements and material constraints associated with cold-programming. The scarcity of research on cold-programming underscores the need for innovative fabrication techniques that can overcome the challenges of this approach and unlock its potential in 4D printing.

This study employs a novel grayscale digital light processing (g-DLP) based 3D printing approach to fabricate cold-programmable shape-morphing structures. A single-vat multi-property g-DLP printing method is developed, where the grayscale modulation of UV light intensity during printing controls the degree of curing (DoC) of the UV-curable resin. This allows for the creation of materials with varying properties within a single print. A rationally designed resin, consisting of isobornyl acrylate (IBOA), 2-hydroxyethyl acrylate (2-HEA), and an aliphatic urethane diacrylate (AUD) crosslinker, is utilized. This resin formulation exhibits multiple hydrogen bonding donor and acceptor moieties, resulting in abundant intermolecular hydrogen bonds. The g-DLP process allows for the creation of a soft, rubbery organogel at low DoC and a glassy thermoset at high DoC. Three distinct DoC states, labeled B1, B2, and B3, are used, corresponding to increasing grayscale levels. These materials exhibit different mechanical properties, with B1 and B2 displaying glassy and ductile behaviors suitable for cold-programming, while B3 is a soft, stretchable organogel. The materials' thermomechanical properties are characterized using uniaxial tensile tests and dynamic mechanical analysis (DMA). A bilayer-based hinge design, incorporating glassy fibers (B1) embedded in a rubbery matrix (B3), is introduced as a cold-draw programmable unit for shape morphing. This design enables easy deformation with low force and high strain capacity. Finite element analysis (FEA) simulations using ABAQUS software are performed to model the stretching-releasing behavior of the hinge, validating the experimental results. The FEA models employ a multi-branch viscoelastic model for the glassy phases and a neo-Hookean model for the rubbery phase. The g-DLP printed structures are subjected to both hot and cold-programming tests to evaluate their shape memory behavior. The shape fixity and shape recovery ratios are calculated to quantify the performance. Multiple samples with graded properties and complex geometries, including a human hand and transformable panel structures, are printed and tested to demonstrate the versatility of the approach. Conductive microchannels filled with liquid metal are incorporated into some designs to create shape-morphing electronic devices.

The research demonstrates the successful fabrication of cold-programmable shape-morphing structures using a single-vat grayscale digital light processing (g-DLP) 3D printing platform. The g-DLP process enabled the creation of materials with tunable mechanical properties, ranging from ductile glassy thermosets to highly stretchable organogels, within a single print. The ductile glassy thermosets (B1 and B2) proved suitable for cold-programming, exhibiting excellent shape fixity and recovery ratios even under large deformations (e.g., 100% strain). The developed bilayer hinge module, comprised of glassy fibers embedded in a rubbery matrix, allowed for easy cold-programming with low forces (around 2N) and large strains (up to 120%). An analytical model was developed to predict the folding angle of the hinge module based on material properties and geometric parameters, showing good agreement with experimental and FEA results. The modular hinge design enables the creation of complex shape-morphing structures with controlled independent deformation of each hinge unit. The cold-draw programming process demonstrated the ability to produce diverse configurations, including an 'M' shape, a square, and a helix, using the same basic building blocks. Further, the integration of conductive microchannels filled with liquid metal into the structures yielded shape-morphing electronic devices with maintained conductivity. The transformable panel structures demonstrated the ability to encode multiple 3D shapes within a single 2D sheet, enabling complex shape transformations via directional stretching. Finally, the use of materials with different glass transition temperatures (Tg) allowed for the creation of multistage smart shape-morphing structures. Hybrid hinge modules, combining materials with different Tg values, enabled temperature-dependent shape transitions, showcasing the autonomous response of these structures to environmental temperature changes.

This research significantly advances the field of 4D printing by introducing a novel cold-programming strategy enabled by g-DLP 3D printing. The ability to fabricate complex shape-morphing structures at room temperature, without the need for global heating, represents a substantial improvement over existing hot-programming methods. The modular hinge design offers exceptional flexibility in designing and controlling the shape-morphing behavior. The results demonstrate the potential for creating highly reconfigurable devices with diverse functionalities. The integration of conductive materials expands the applications to electronic devices and actuators. The development of multistage smart shape-morphing structures showcases the potential for creating adaptive systems that respond autonomously to environmental changes. This technology has implications for various fields, including soft robotics, deployable systems, wearable devices, and metamaterials. Future research could focus on exploring new material combinations to expand the range of achievable properties and functionalities. Further optimization of the hinge design could lead to improved control and durability. Investigating the long-term stability and reliability of the cold-programmed structures under various conditions is also crucial.

This study successfully demonstrates a novel approach to cold-programming shape-morphing structures using g-DLP 3D printing. The integration of multi-material capabilities in g-DLP, along with the innovative hinge design, facilitates the fabrication of complex and highly reconfigurable structures. The cold-programming method eliminates the need for high-temperature processing, making it energy-efficient and suitable for various applications. Future research should focus on exploring new material systems, optimizing the hinge design for enhanced durability, and investigating the long-term stability of these structures.

The current study has a few limitations. First, the rubbery matrix in the hinge module is UV-sensitive due to the presence of uncured photo monomers, which may limit its long-term stability under UV exposure. Second, repeated large deformations could cause irreversible damage to the network, affecting the accuracy of shape morphing after multiple cycles. Further research should address these limitations by exploring UV-resistant materials or employing post-treatments to enhance the durability of the structures.