4 - 4D-printed low-voltage electroactive polymers modeling and fabrication

Four-dimensional (4D) printing technology, as a next-generation additive manufacturing method, enables printed objects to further change their shapes, functionalities, or properties upon exposure to external stimuli. The 4D printing of programmable and deformable materials such as thermo-responsive shape-memory polymers (trSMPs), which possess the ability to change shape by exposure to heat, has attracted particular interest in recent years. Three-dimensional objects based on SMPs have been proposed for various potential applications in different fields, including soft robotics, smart actuators, biomedical and electronics. To enable the manufacturing of complex multifunctional 3D objects, SMPs are often coupled with other functional polymers or fillers during or before the 3D printing process. This review highlights the 4D printing of state-of-the-art multi-component SMP formulations. Commonly used 4D printing technologies such as material extrusion techniques including fused filament fabrication (FFF) and direct ink writing (DIW), as well as vat photopolymerization techniques such as stereolithography (SLA), digital light processing (DLP), and multi-photon polymerization (MPP), are discussed. Different multicomponent SMP systems, their actuation methods, and potential applications of the 3D printed objects are reviewed. Finally, current challenges and prospects for 4D printing technology are summarized.

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].

The existing 3D printing methods exhibit certain fabrication-dependent limitations for printing curved constructs that are relevant for many tissues. Four-dimensional (4D) printing is an emerging technology that is expected to revolutionize the field of tissue engineering and regenerative medicine (TERM). 4D printing is based on 3D printing, featuring the introduction of time as the fourth dimension, in which there is a transition from a 3D printed scaffold to a new, distinct, and stable state, upon the application of one or more stimuli. Here, we present an overview of the current developments of the 4D printing technology for TERM, with a focus on approaches to achieve temporal changes of the shape of the printed constructs that would enable biofabrication of highly complex structures. To this aim, the printing methods, types of stimuli, shape-shifting mechanisms, and cell-incorporation strategies are critically reviewed. Furthermore, the challenges of this very recent biofabrication technology as well as the future research directions are discussed. Our findings show that the most common printing methods so far are stereolithography (SLA) and extrusion bioprinting, followed by fused deposition modelling, while the shape-shifting mechanisms used for TERM applications are shape-memory and differential swelling for 4D printing and 4D bioprinting, respectively. For shape-memory mechanism, there is a high prevalence of synthetic materials, such as polylactic acid (PLA), poly(glycerol dodecanoate) acrylate (PGDA), or polyurethanes. On the other hand, different acrylate combinations of alginate, hyaluronan, or gelatin have been used for differential swelling-based 4D transformations. TERM applications include bone, vascular, and cardiac tissues as the main target of the 4D (bio)printing technology. The field has great potential for further development by considering the combination of multiple stimuli, the use of a wider range of 4D techniques, and the implementation of computational-assisted strategies.

The advancement of four-dimensional (4D) printing has been fueled by the rise in demand for additive manufacturing and the expansion in shape-memory materials. The printing of smart substances that respond to external stimuli is known as 4D printing. 4D printing allows highly controlled shapes to simulate the physiological milieu by adding time dimensions. The 4D printing is suitable with current progress in smart compounds, printers, and its mechanism of action. The 4D printing paradigm, a revolutionary enhancement of 3D printing, was anticipated by various engineering disciplines. Tissue engineering, medicinal, consumer items, aerospace, and organ engineering use 4D printing technology. The current review mainly focuses on the basics of 4D printing and the methods used therein. It also discusses the time-dependent behavior of stimulus-sensitive compounds, which are widely used in 4D printing. In addition, this review highlights material aspects, specifically related to shape-memory polymers, stimuli-responsive materials (classified as physical, chemical, and biological), and modified materials, the backbone of 4D printing technology. Finally, potential applications of 4D printing in the biomedical sector are also discussed with challenges and future perspectives.

4D printing: Pragmatic progression in biofabrication

4D printing applications in medical field: A brief review

Compression property and energy absorption capacity of 4D-printed deformable honeycomb structure

Four-dimensional (4D) printing is used to describe three-dimensional (3D)-printed objects with properties that change over time. Shape memory polymers (SMPs) are representative materials for 4D printing technologies. The ability to print geometrically complex, free-standing forms with SMPs is crucial for successful 4D printing. In this study, an SMP capable of frontal polymerization featuring exothermic self-propagation was synthesized by adding cyclooctene to a poly(dicyclopentadiene) network, resulting in switching segments. The rheological properties of this SMP were controlled by adjusting incubation time. A nozzle system was designed such that the SMP could be printed with simultaneous polymerization to yield a free-standing structure. The printing speed was set to 3 cm min−1 according to the frontal polymerization speed. A free-standing, hexagonal spiral was successfully printed and printed spiral structure showed excellent shape memory performance with a fixity ratio of about 98% and a recovery ratio of 100%, thereby demonstrating the 3D printability and shape memory performance of frontally polymerizable SMPs.

Hybrid hinge structure with elastic hinge on self-folding of 4D printing using a fused deposition modeling 3D printer

Four-dimensional (4D) printing combines shape-morphing materials and three-dimensional (3D) printing technology, enabling efficient fabrication of complex shape-changing structures. However, 4D printing of hydrogels into structures with complex shapes suffers from poor printability, which limit their practical applications. Here, we present an efficient strategy for direct 4D printing of hydrogels, leveraging intricate structural designs and highly viscous hydrogels. This strategy facilitates programmable shape-morphing through precise control of filament spacing and orientation, resulting in gradient swelling behaviours when the structures are immersed in a Ca2+-ion solution. Our study also reveals the critical role of printability in improving shape-morphing performance. On this basis, we propose a practical solution to enhance the shape-morphing capability of hydrogels with limited inherent performance by improving their printability through the addition of viscous additives such as MC or PVA. Overall, this strategy expands the list of hydrogels suitable for 4D printing, demonstrating compatibility with both synthetic and natural hydrogels, including Alginate/Methylcellulose (ALG/MC), gelatin methacryloyl (GELMA), and ALG/polyvinyl alcohol (PVA). Various sophisticated plant-inspired shape-morphing behaviours can be achieved in 4D-printed hydrogels through precise control of structural topology. The combined strategy of employing highly viscous hydrogel with intricate structural design demonstrates vast potential for applications in biomimetic soft robotics.

ABSTRACTBackground:4D printing, an evolution of 3D printing, incorporates time as the fourth dimension to enable shape-changing materials to respond to external stimuli. Its application in orthodontics is a burgeoning field, offering possibilities for self-adjusting brackets that adapt dynamically to the biomechanical requirements of tooth movement. This study evaluates the feasibility and efficacy of 4D-printed self-adjusting brackets in achieving controlled and dynamic orthodontic tooth movement.Materials and Methods:Self-adjusting orthodontic brackets were fabricated using 4D printing technology with shape-memory polymers (SMPs). The brackets were designed to respond to thermal stimuli and were programmed to apply a controlled force of 150–200 g/cm2 for optimal tooth movement. Twenty patients with malocclusions were enrolled, with each receiving self-adjusting brackets on one dental arch and conventional brackets on the opposite arch as a control. The evaluation period spanned six months, with periodic measurements of tooth movement using 3D digital scanning and bite force analysis.Results:The 4D-printed self-adjusting brackets demonstrated a 30% faster rate of tooth movement (mean: 1.5 mm/month) compared to conventional brackets (mean: 1.1 mm/month). Patients reported reduced discomfort, with a pain score of 3.2 ± 0.5 compared to 5.8 ± 0.7 in the control group. The self-adjusting brackets maintained consistent force levels, reducing the need for frequent adjustments by 40%.Conclusion:4D printing offers a revolutionary approach to orthodontic treatment through self-adjusting brackets, enhancing treatment efficiency and patient comfort. The use of SMPs enables precise and adaptive force application, reducing chairside adjustments and overall treatment duration. Further studies are needed to explore long-term outcomes and cost-effectiveness.

Despite great progress in four-dimensional (4D) printing, i.e. three-dimensional (3D) printing of active (stimuli-responsive) materials, the relatively low actuation force of the 4D printed structures often impedes their engineering applications. In this study, we use multimaterial inkjet 3D printing technology to fabricate shape memory structures, including a morphing wing flap and a deployable structure, which consist of active and flexible hinges joining rigid (non-active) parts. The active hinges, printed from a shape memory polymer (SMP), lock the structure into a second temporary shape during a thermomechanical programming process, while the flexible hinges, printed from an elastomer, effectively increase the actuation force and the load-bearing capacity of the printed structure as reflected in the recovery ratio. A broad range of mechanical properties such as modulus and failure strain can be achieved for both active and flexible hinges by varying the composition of the two base materials, i.e. the SMP and the elastomer, to accommodate large deformation induced during programming step, and enhance the recovery in the actuating step. To find the important design parameters, including local deformation, shape fixity and recovery ratio, we conduct high fidelity finite element simulations, which are able to accurately predict the nonlinear deformation of the printed structures. In addition, a coupled thermal-electrical finite element analysis was performed to model the heat transfer within the active hinges during the localized Joule heating process. The model predictions showed good agreement with the measured temperature data and were used to find the major parameters affecting temperature distribution including the applied voltage and the convection rate.

3D Printing, sometimes referred to as additive manufacturing, has made the concept of personalized medicine a reality. The primary objective of 3D and 4D printing is to produce intricate, customized pharmaceuticals at a reasonable cost. With improvements in materials, resolution, and speed, 3D printing technology is quickly developing. It includes faster construction, cost efficiency through reduced waste, design flexibility for complex structures, and sustainability through optimized material usage. An extensive literature survey was done on 3D and 4D printing of pharmaceuticals using PubMed, Elsevier, ScienceDirect, and Springer. The results were then filtered based on the titles, abstracts, and accessibility of the complete texts. The search engine Google Scholar was accessed for literature data mining. From the data mining, it was found that from the year 2009 to 2024 the number of research publications surged more than 200 times on the current topic. Even though 3-D and 4-D printing technologies have advanced significantly in a short amount of time, the most often used ones are still stereolithography, nozzle-based deposition, inkjet, and selective laser sintering. Their use has been modified for the production of nanoparticles, polypills, tablets, and implants, etc. Pharma's aspirations for tailored medications are being revolutionized by 3D printing, but cost, flexibility, and bioequivalence still need to be investigated. The present review offers a thorough analysis of various 3D and 4D printing methods and emphasizes the major advantages and disadvantages and major key challenges of 3D and 4D printing related to pharmaceuticals. Compared to 3D Printing, 4D printing offers better quality, efficacy, and functionality.

Recent 3D and 4D intelligent printing technologies: A comparative review and future perspective

Reversible energy absorbing meta-sandwiches by FDM 4D printing