Four-dimensional (4D) printing represents a paradigm shift in additive manufacturing by incorporating time-dependent, stimuli-responsive behavior into fabricated objects. Inspired by adaptive biological systems—including hygroscopic pine cone opening, cephalopod camouflage, and vascular self-healing—bio-inspired 4D printing combines smart materials, programmed anisotropy, and precise fabrication control to produce dynamic, functional structures. This review synthesizes over 140 high-impact studies, covering historical evolution, biomimetic design principles, advanced material platforms, fabrication strategies, computational modeling, crossdisciplinary applications, key challenges, and a detailed 2025–2035 roadmap. Emphasis is placed on biomedical implants, soft robotics, adaptive architecture, and sustainable systems. Despite remarkable progress in multi-material printing and multi-stimuli responsiveness, significant hurdles persist in scalability, long-term stability, and regulatory approval. The integration of artificial intelligence, living bioinks, and autonomous large-scale fabrication is anticipated to drive the next transformative decade.

The application of three-dimensional (3D) printing in the medical field is recognized as a significant innovation in healthcare technology. This technology has revolutionized medical science by enhancing diagnostic methods, treatment approaches, and the production of human body components. The fabrication of bone tissue prostheses holds particular importance, with the reduction of production time and costs being a critical research priority. The primary objective of this study is to design and analyze the mechanical behavior of a femur bone produced via 3D printing. The analysis encompasses evaluating the mechanical performance of the bone with a porous structure resembling natural bone under compression testing, dynamic simulation, fatigue assessment, and comparison with other porosity configurations. The average results from compression tests, in terms of fracture force (kN), fracture stress (MPa), fracture strain (mm), and fracture energy (kJ), were 9.456, 11.76, 8.68, and 42.6, respectively. The designed bone with bone-like porosity exhibited excellent resistance to compression and fracture, outperforming a comparable sample with differing porosity. Fatigue simulation indicated an infinite lifespan for the designed bone in in vivo applications. Additionally, in the fracture region identified from the compression test (outer ridge of the specimen) and the region derived from fatigue simulation (inner ridge of the specimen), a 5 mm diameter crack was applied, yielding stresses of 12.82 and 11.89 MPa, respectively, in fatigue simulation for these two samples.

Four-dimensional (4D) printing, an evolution of additive manufacturing (AM), integrates stimuli-responsive materials like thermal-induced shape memory polymers (TSMPs) to enable programmable, time-dependent transformations in structures. This review systematically examines recent advancements in TSMPs, 4D printing technologies, and fabrication strategies, emphasizing their interdisciplinary convergence and applications. TSMPs, with their unique phase-separated microstructures, modulus disparity, and tailorable recovery behaviors, form the foundation of 4D-printed systems, enabling innovations in aerospace, healthcare, and soft robotics [1–3, 33–45]. Key printing methods—including fused deposition modeling (FDM), direct ink writing (DIW), PolyJet, stereolithography (SLA), and digital light processing (DLP)—are analyzed for their strengths and limitations. FDM dominates due to cost-effectiveness but faces challenges in resolution and anisotropy [88, 91–93], while SLA and DLP offer high precision and speed but require material optimization [105, 111–115]. Advanced fabrication strategies, such as localized TSMP printing, composite reinforcement with carbon nanotubes or Fe 3O 4 particles, and sacrificial mold templating, expand design possibilities for multi-functional and multi-scale structures [117–120, 140–144]. Despite progress, challenges persist, including environmental sensitivity of TSMPs, interlayer bonding in AM, and dynamic modeling of non-equilibrium thermal responses [74, 77, 146]. Future directions focus on multi-responsive materials, machine learning-driven constitutive models, and sustainable bio-based formulations to address scalability and cyclic durability [31, 77, 147–149]. By bridging material science, computational modeling, and advanced manufacturing, this work provides a roadmap for harnessing 4D-printed TSMPs in real-world applications, from self-deploying biomedical devices to adaptive aerospace systems [28, 85, 137].

Currently, polymeric foams are widely utilized in the creation of passive support surfaces such as mattresses, cushions, and seating. However, these materials encounter challenges in prolonged use, including diminished performance, permanent deformation, reduction in thickness, and nonuniform pressure distribution. These issues can lead to pressure concentration in sensitive bodily areas, particularly the gluteal region, thereby increasing the risk of pressure ulcers. Advances in additive manufacturing technology, alongside the capability to design engineered structures with controllable mechanical properties, have directed researchers’ attention toward employing this method as an alternative to traditional foams. Among these, auxetic structures have garnered interest for applications related to skin wound healing due to their unique mechanical characteristics. In this study, re-entrant auxetic structure samples were numerically designed using the finite element method and subsequently fabricated via the fused deposition modeling (FDM[1](#fn-0002)) additive manufacturing process, utilizing thermoplastic polyurethane (TPU[2](#fn-0003)). The mechanical performance of these structures was assessed through compression testing, in accordance with ISO 3386-1, and fatigue testing. These analyses investigated the impact of parameters such as unit cell dimensions and cell angle on the compressive stress and resilience of the structure. The results indicated that the designed auxetic structure, when utilizing TPU with A95 hardness, could achieve a compressive stress between 7 and 8 kPa at 40% compression. Furthermore, fatigue tests demonstrated that the structure’s resilience is dependent on the amount of strain, whereas the loading duration did not significantly affect its rebound behavior. These findings underscore the high potential of auxetic structures in designing support surfaces with customizable mechanical performance, tailored to the biomechanical needs of the body.

Polylactic acid (PLA), a biodegradable polymer, is gaining attention as a sustain- able alternative to steel in civil engineering, yet its fracture behavior under complex loading remains underexplored. This study examines crack propagation in PLA sam- ples with dual keyhole notches under tensile loading, integrating experimental tests, finite element simulations, and machine learning predictions. Six PLA specimens (200×50×10 mm, crack length 10 mm, angles 60° and 70°, notch radii 0.5–2 mm) were tested experimentally, while 400 samples (angles 1°–80°, radii 0.5–4 mm) were simu- lated in ABAQUS. Artificial Neural Networks (ANN) and Long Short-Term Memory (LSTM) models, implemented in MATLAB, analyzed the results. Experimental peak stress reached 33.3 N/mm 2 (0.5 mm notch, 60°), while simulations predicted up to 65.225 N/mm 2 (0.5 mm, 1°). ANN with Bayesian Regularization outperformed other models, offering precise predictions of crack behavior. These findings provide a fracture criterion for PLA, advancing its potential in sustainable structural applications.

Currently, polymeric foams are widely utilized in the creation of passive support surfaces such as mattresses, cushions, and seating. However, these materials encounter challenges in prolonged use, including diminished performance, permanent deformation, reduction in thickness, and nonuniform pressure distribution. These issues can lead to pressure concentration in sensitive bodily areas, particularly the gluteal region, thereby increasing the risk of pressure ulcers. Advances in additive manufacturing technology, alongside the capability to design engineered structures with controllable mechanical properties, have directed researchers’ attention toward employing this method as an alternative to traditional foams. Among these, auxetic structures have garnered interest for applications related to skin wound healing due to their unique mechanical characteristics. In this study, re-entrant auxetic structure samples were numerically designed using the finite element method and subsequently fabricated via the fused deposition modeling (FDM[1](#fn-0002)) additive manufacturing process, utilizing thermoplastic polyurethane (TPU[2](#fn-0003)). The mechanical performance of these structures was assessed through compression testing, in accordance with ISO 3386-1, and fatigue testing. These analyses investigated the impact of parameters such as unit cell dimensions and cell angle on the compressive stress and resilience of the structure. The results indicated that the designed auxetic structure, when utilizing TPU with A95 hardness, could achieve a compressive stress between 7 and 8 kPa at 40% compression. Furthermore, fatigue tests demonstrated that the structure’s resilience is dependent on the amount of strain, whereas the loading duration did not significantly affect its rebound behavior. These findings underscore the high potential of auxetic structures in designing support surfaces with customizable mechanical performance, tailored to the biomechanical needs of the body.