Cold-programmed shape-morphing structures based on grayscale digital light processing 4D printing

The study addresses how to fabricate reconfigurable, shape-morphing structures that can be easily and locally programmed at room temperature. Conventional 4D-printed shape memory polymer (SMP) systems rely primarily on hot-programming: heating above a transition temperature (e.g., Tg), deforming, and then cooling under load. While hot-programming offers low forces, it typically requires global heating and has limited capability for local, independent programming. Local programming via localized heating adds hardware complexity, and global deformation strategies program all features simultaneously. Cold-programming, which deforms SMPs below their transition temperature, promises simple, local programming at room temperature but requires ductile glassy materials that can undergo large strains without fracture. This work hypothesizes that a single-vat grayscale DLP (g-DLP) printing platform can produce a monolithic structure with spatially varying properties (from glassy ductile thermosets to soft organogels) enabling cold-programming-based, modular hinge units for controllable, independent, and multistage morphing. The purpose is to demonstrate material design, hinge mechanics, and system-level demonstrations that validate cold-programmable shape morphing with high fixity and recovery, accurate modeling, and practical applications.

4D printing merges additive manufacturing with stimuli-responsive materials to realize time-dependent morphing. SMPs are widely studied due to programmable shape and thermal recovery behavior. Traditional hot-programming commonly involves global heating in baths or ovens, limiting local programmability and increasing energy costs. Localized actuation can be achieved via embedded heaters or photothermal elements but adds complexity. Alternatives like hinges in printed origami concentrate local deformation but are still influenced by global loads. Active materials such as liquid crystal elastomers and hydrogels face similar global/local actuation tradeoffs. Cold-programming of SMPs, less explored, relies on stress-accelerated relaxation: at high stress below Tg, glassy polymers can undergo viscoplastic deformation that is recoverable upon heating. However, cold-programming demands sufficient ductility in the glassy state to avoid fracture. Prior modeling efforts have described viscoelastic/viscoplastic behavior and stress/temperature-dependent relaxation and glass transition behavior in SMPs, motivating material systems that can exploit cold-programming effectively.

Materials and ink formulation: A single-vat g-DLP resin comprising isobornyl acrylate (IBOA, stiff segment), 2-hydroxyethyl acrylate (2-HEA, soft segment), and aliphatic urethane diacrylate (AUD, crosslinker) in a 60:20:20 wt% ratio was prepared. Photoinitiator Irgacure 819 (1.2 wt%) and photoabsorber Sudan I (0.1 wt%) were added. The ink design provides multiple hydrogen-bond donors and acceptors, enabling organogel formation at low degree of cure (DoC) and glassy thermoset at high DoC. g-DLP printing process: A bottom-up DLP printer with a 385 nm UV-LED projector and oxygen-permeable vat window was used. 3D models were sliced into 0.05 mm layers, converted to grayscale images to locally modulate UV intensity per 50 µm pixel. Continuous liquid interface production (CLIP) was used at ~3 s/layer. Projector brightness controlled local intensity, enabling three representative materials: B1 (100% brightness, 23.6 mW/cm2), B2 (80%, 15.4 mW/cm2), B3 (40%, 3.1 mW/cm2). Light intensity was calibrated by photometer. The single-vat approach integrates soft organogels and stiff glassy thermosets in one monolithic build. Material states and characterization: Three grayscale-defined materials were studied: B1 and B2 (glassy, ductile thermosets at room temperature) and B3 (soft, highly stretchable organogel). Uniaxial tensile testing provided stress–strain behavior; dynamic mechanical analysis (DMA) provided storage modulus and tan delta to determine Tg and onset temperatures (Te). B1 and B2 exhibited linear elasticity, yielding, post-yield softening, and strain stiffening; B3 was soft and highly extensible. Thermal zones were defined: cold (below Te), warm (glass transition region), and hot (rubbery region). Programming protocols: Hot-programming involved stretching above Tc/Tg (e.g., 100 °C), cooling to 25 °C under load, then unloading to measure fixity and subsequent free recovery upon reheating. Cold-programming involved stretching at room temperature without heating, then free recovery upon heating. Demonstrations included graded strips (glassy ends with rubbery middle) and graded lattices, with staged recoveries at 50 °C and 80 °C. Hinge design and fabrication: A cold-draw programmable heterogeneous hinge was designed: glassy SMP fibers (B1) embedded within a rubbery matrix (B3), printed monolithically via g-DLP. The hinge is connected to glassy panels. Stretching at room temperature induces viscoplastic deformation in the glassy fibers while the rubbery matrix remains elastic, creating a mismatch upon release that causes bending toward the rubbery side. Heating above Tg recovers the fiber and resets the hinge. Variants included H13 (B1 fibers in B3 matrix), H12 (B1 fibers in B2 matrix), and H23 (B2 fibers in B3 matrix) to enable temperature-dependent activation states. Modeling: Analytical expressions linked folding angle to curvature, hinge length, and neutral plane strain, incorporating shape fixity as a function of programming strain. Finite element analysis (ABAQUS) used a multi-branch viscoelastic model (neo-Hookean plus Prony series with temperature- and stress-dependent shift factors) for glassy B1/B2 and incompressible neo-Hookean behavior for rubbery B3. 3D C3D8H elements and implicit dynamics quasi-static solver captured stretch-induced yielding and morphing. Material parameters were calibrated against tensile and DMA data; boundary conditions matched experiments. Applications and assemblies: g-DLP printed strips and sheets with arrays of hinges enabled local, independent morphing. Microchannels (400 µm) were printed and filled with liquid metal (EGaIn) for conductive morphing devices. Through-hole features supported interlocking assemblies for more complex 3D architectures. Dual-direction and uni-direction transformable panels were designed to fold into prescribed 3D shapes depending on applied strain direction(s).

- Multi-property printing via a single-vat g-DLP process produced materials from soft organogel to glassy thermoset in one print by grayscale control of UV intensity. - Mechanical and thermal properties: • B3 organogel: exceptionally soft (Young’s modulus ~0.016 MPa) and highly stretchable (up to ~1500%). • B1/B2 glassy thermosets: ductile, with yield followed by strain softening and later strain stiffening; stretchable to >300% strain at room temperature; higher elastic stretchability at elevated temperature. • Tg values: B1 ~87 °C, B2 ~66 °C, B3 ~−14 °C (DMA, tan delta peak). Onset temperatures Te: B1 ~63 °C, B2 ~42 °C. • High DoC yields a glassy thermoset with high modulus (~478 MPa for B1). - Shape memory performance of B1: • Hot-programming at 100 °C with 100% programming strain: shape fixity rf ~96.9%; shape recovery rr ~100% upon heating. • Cold-programming at room temperature with 100% strain: rf ~95.8%; rr ~99.4% upon heating. • FEA accurately reproduced stress–strain–temperature histories. - Graded demonstrations validated sequential recovery: • Strip with B1/B2 ends and B3 middle: two-stage recovery at 50 °C (B2) and 80 °C (B1); rubbery middle recovers instantly at room temperature and stretches to ~1000% elastically. • Graded 3D lattice: three-stage behavior apparent in force–strain during room-temperature compression; after unloading, only B3 recovers; subsequent recoveries at 50 °C (B2) and 80 °C (B1). - Cold-draw programmable heterogeneous hinge modules (glassy B1 fibers in rubbery B3 matrix) enabled low-force programming (~2 N) and large, controllable bending; achieved fully folded 180° bending at ~120% strain; deformation remained stable for months at room temperature. - Analytical and FEA models predicted hinge folding angle versus applied strain with good agreement; empirical relation for shape fixity versus strain supported design. - Modular, independently addressable morphing: • Structures with alternating hinge orientations formed M or square shapes on-demand and recovered upon heating (e.g., 80 °C). • Orientation of hinge layers allowed upward/downward bending; angled hinge arrays (e.g., 30°) produced 3D helices with pitch controlled by applied strain (e.g., 30% vs 60%). • Arrays of hinges permitted interactive, local configuration with real-time feedback. - Functional integration: • 400 µm microchannels filled with EGaIn yielded conductive, shape-morphing electronic strips that maintained conductivity during smooth transitions. • Transformable panels: dual-direction sheets encoded to form open tube (y-strain), closed tube (x-strain), or combined shapes (x and y); symmetric designs switched between buckled-up and buckled-down by flipping hinge orientations; through-hole interlocking enabled complex 3D architectures. - Multistage smart morphing via thermal logic of hinge variants: • H13 (B1/B3) active at room temperature and 50 °C (on in both conditions). • H12 (B1/B2) off at room temperature, on at 50 °C. • H23 (B2/B3) on at room temperature, off at 50 °C. • Hybrid assemblies autonomously transitioned between pre-programmed shapes with temperature changes (e.g., M shape at RT via H23 then S shape at 50 °C via H12).

The work demonstrates that a single-vat grayscale DLP printing platform can realize monolithic structures with spatially varying thermomechanical properties sufficient for robust cold-programming. By selecting grayscale-defined material states (ductile glassy B1/B2 and soft organogel B3), the authors created heterogeneous hinge modules that require only simple room-temperature stretching to program localized, independent bending. This addresses the challenge in classic hot-programming approaches where global heating and loading hinder local programmability and require more complex hardware for localized actuation. The combination of cold-programming and temperature-dependent recovery enables rapid, interactive reconfiguration and staged responses. Mechanical modeling (analytical and FEA) accurately predicts folding angles and temperature-dependent morphing, offering a design framework for engineering complex structures. Demonstrations, from graded lattices and strips to transformable panels and conductive devices, show the platform’s versatility and applicability. The smart, multistage behavior achieved by mixing hinge variants (H12, H13, H23) provides autonomous responses to environmental temperature shifts, expanding practical use cases such as reconfigurable electronics, antennas, and morphing metamaterials. Overall, the findings validate that g-DLP printed, cold-programmable modules can deliver controllable, repeatable, and scalable shape morphing with minimized energy and equipment complexity.

This study introduces a single-vat grayscale DLP printing strategy to fabricate cold-programmable shape-morphing structures, enabled by spatially graded materials ranging from ductile glassy thermosets to soft organogels. The authors design heterogeneous hinge modules that can be cold-programmed at room temperature with low force and that recover upon heating, achieving large, controllable bending and multi-shape configurations. Analytical and finite element models provide predictive design capability. System demonstrations include graded, sequentially recovering structures, transformable multi-directional panels, and conductive morphing devices. Smart, multistage morphing is realized by combining hinge variants with distinct thermal activation logic. Future work should focus on stabilizing organogel components against UV exposure (e.g., coatings or secondary reactions), mitigating damage from repeated high-strain cold-draw cycles (e.g., optimizing fiber density or incorporating exchangeable bonds), and developing additional functional g-DLP inks to broaden applications in transformable metamaterials, antennas, and reconfigurable electronics.

- The rubbery matrix (organogel) contains uncured photomonomers and is UV-sensitive; exposure to UV can deactivate hinge functionality. Protective measures such as UV-blocking coatings or secondary thermal reactions to consume reactive groups are needed. - Cold-draw programming at high strains can induce irreversible damage in the glassy network, degrading morphing accuracy over multiple cycles. Design optimization (e.g., fiber density and allowable strain) or incorporating exchangeable bonds to restore the network is recommended. - While demonstrated across multiple geometries and scales, long-term durability under cyclic mechanical and thermal loads and environmental exposure (e.g., humidity, solvents) requires further study.