Review on recent advances in 4D printing of shape memory polymers

Design-encoded dual shape-morphing and shape-memory in 4D printed polymer parts toward cellularized vascular grafts

Current additive manufacturing technologies wherein as-printed simple two-dimensional (2D) structures morph into complex tissue mimetic three-dimensional (3D) shapes are limited to multi-material hydrogel systems, which necessitates multiple fabrication steps and specific materials. This work utilizes a single shape memory thermoplastic polymer (SMP), PLMC (polylactide-co-trimethylene carbonate), to achieve programmable shape deformation through anisotropic design and infill angles encoded during 3D printing. The shape changes were first computationally predicted through finite element analysis (FEA) simulations and then experimentally validated through quantitative correlation. Rectangular 2D sheets could self-roll into complete hollow tubes of specific diameters (ranging from ≈6 mm to ≈10 mm) and lengths (as long as 40 mm), as quantitatively predicted from FEA simulations within one minute at relatively lower temperatures (≈80 °C). Furthermore, shape memory properties were demonstrated post-shape change to exhibit dual shape morphing at temperatures close to physiological levels. The tubes (retained as the permanent shape) were deformed into flat sheets (temporary shape), seeded with endothelial cells (at T < Tg), and thereafter triggered at ≈37 °C back into tubes (permanent shape), utilizing the shape memory properties to yield bioresorbable tubes with cellularized lumens for potential use as vascular grafts with improved long-term patency. Additionally, out-of-plane bending and twisting deformation were demonstrated in complex structures by careful control of infill angles that can unprecedently expand the scope of cellularized biomimetic 3D shapes. This work demonstrates the potential of the combination of shape morphing and SMP behaviors at physiological temperatures to yield next-generation smart implants with precise control over dimensions for tissue repair and regeneration.

4D printing light-driven actuator with lignin photothermal conversion module

Light-responsive shape memory polymers are attractive as they can be activated through remote and spatially-controlled light. In this work, 4D printing of poly(lactic acid) (PLA) composites with a near-infrared light-responsive was achieved by using the simple melt blending method and adding 3 wt% of lignin. Lignin with a conjugated structure was used as the photothermal conversion module. The composites exhibited significant photothermal effects under near-infrared (808 nm) laser irradiation, and the laser irradiation was also effective in initiating and controlling the shape memory. The structure of lignin can be improved by the action of dicumyl peroxide (DCP) to enhance the interfacial adhesion between polyamide elastomer (PAE) and polylactic acid (PLA), reduce the size of dispersed phases, and serve as an effective rheological modifier to exhibit the ideal melt viscosity required for 3D printing of composites. The good mechanical, thermal stability, and rheological properties provide assurance for the 4D printing of composites. This research provides an environmentally friendly and practical method for creating composites that have the potential to serve as ideal actuator components in a range of applications.

From Static to Dynamic: Smart Materials Pioneering Additive Manufacturing in Regenerative Medicine

The emerging field of regenerative medicine holds immense promise for addressing complex tissue and organ regeneration challenges. Central to its advancement is the evolution of additive manufacturing techniques, which have transcended static constructs to embrace dynamic, biomimetic solutions. This manuscript explores the pivotal role of smart materials in this transformative journey, where materials are endowed with dynamic responsiveness to biological cues and environmental changes. By delving into the innovative integration of smart materials, such as shape memory polymers and stimulus-responsive hydrogels, into additive manufacturing processes, this research illuminates the potential to engineer tissue constructs with unparalleled biomimicry. From dynamically adapting scaffolds that mimic the mechanical behavior of native tissues to drug delivery systems that respond to physiological cues, the convergence of smart materials and additive manufacturing heralds a new era in regenerative medicine. This manuscript presents an insightful overview of recent advancements, challenges, and future prospects, underscoring the pivotal role of smart materials as pioneers in shaping the dynamic landscape of regenerative medicine and heralding a future where tissue engineering is propelled beyond static constructs towards biomimetic, responsive, and regenerative solutions.

4D-Printed Hydrogel Actuators through Diffusion-Path Architecture Design

Recently, smart hydrogels have garnered considerable attention as biomedical devices, and several approaches have been introduced for their fabrication, including the incorporation of stimulus-responsive additives, utilization of molecular imprinting techniques, and application of multilayered hydrogels. However, the nonuniform properties resulting from these approaches limit the practical applications of hydrogels by causing inconsistent performance and behavior. In this study, we propose a novel approach to manipulating the swelling kinetics of hydrogels by engineering their diffusion-path architecture. By simply adjusting the diffusion path length within the hydrogel, we achieved a significant change in swelling kinetics. This approach enables precise control over the diffusion and transport processes within the hydrogel, resulting in enhanced swelling kinetics when reducing the diffusion path length. Furthermore, by strategically designing the diffusion-path architecture of a 3D-printed hydrogel specimen, we can fabricate smart hydrogel actuators that exhibit reversible shape transformations during swelling and deswelling through a nonequilibrium differential swelling. The proposed approach eliminates the need to modify the spatial properties of hydrogel structures such as cross-linking density, polymer, or additive compositions, thereby achieving uniform properties throughout the hydrogel and creating new possibilities for the development of advanced 4D-printed biomedical devices.

Four-Dimensional (4D) Printing of Dynamic Foods-Definitions, Considerations, and Current Scientific Status

Since its conception, the application of 3D printing in the structuring of food materials has been focused on the processing of novel material formulations and customized textures for innovative food applications, such as personalized nutrition and full sensory design. The continuous evolution of the used methods, approaches, and materials has created a solid foundation for technology to process dynamic food structures. Four-dimensional food printing is an extension of 3D printing where food structures are designed and printed to perform time-dependent changes activated by internal or external stimuli. In 4D food printing, structures are engineered through material tailoring and custom designs to achieve a transformation from one configuration to another. Different engineered 4D behaviors include stimulated color change, shape morphing, and biological growth. As 4D food printing is considered an emerging application, imperatively, this article proposes new considerations and definitions in 4D food printing. Moreover, this article presents an overview of 4D food printing within the current scientific progress, status, and approaches.

4D Printed Programmable Shape-Morphing Hydrogels as Intraoperative Self-Folding Nerve Conduits for Sutureless Neurorrhaphy

There are only a few reports of implantable 4D printed biomaterials, most of which exhibit slow deformations rendering them unsuitable for in situ surgical deployment. In this study, a hydrogel system is engineered with defined swelling behaviors, which demonstrated excellent printability in extrusion-based 3D printing and programmed shape deformations post-printing. Shape deformations of the spatially patterned hydrogels with defined infill angles are computationally predicted for a variety of 3D printed structures, which are subsequently validated experimentally. The gels are coated with gelatin-rich nanofibers to augment cell growth. 3D-printed hydrogel sheets with pre-programmed infill patterns rapidly self-rolled into tubes in vivo to serve as nerve-guiding conduits for repairing sciatic nerve defects in a rat model. These 4D-printed hydrogels minimized the complexity of surgeries by tightly clamping the resected ends of the nerves to assist in the healing of peripheral nerve damage, as revealed by histological evaluation and functional assessments for up to 45 days. This work demonstrates that 3D-printed hydrogels can be designed for programmed shape changes by swelling in vivo to yield 4D-printed tissue constructs for the repair of peripheral nerve damage with the potential to be extended in other areas of regenerative medicine.

4D Printing of Body Temperature-Responsive Hydrogels Based on Poly(acrylic acid) with Shape-Memory and Self-Healing Abilities

Additive manufacturing of smart materials that can be dynamically programmed with external stimuli is known as 4D printing. Among the 4D printable materials, hydrogels are the most extensively studied materials in various biomedical areas because of their hierarchical structure, similarity to native human tissues, and supreme bioactivity. However, conventional smart hydrogels suffer from poor mechanical properties, slow actuation speed, and instability of actuated shape. Herein, we present 4D-printed hydrogels based on poly(acrylic acid) that can concurrently possess shape-memory and self-healing properties. The printing of the hydrogels is achieved by solvent-free copolymerization of the hydrophilic acrylic acid (AAc) and hydrophobic hexadecyl acrylate (C16A) monomers in the presence of TPO photoinitiator using a stereolithography-based commercial resin printer followed by swelling in water. The printed hydrogels undergo a reversible strong-to-weak gel transition below and above human body temperature due to the melting and crystallization of the hydrophobic C16A domains. In this way, the shape-memory and self-healing properties of the hydrogels can be magically actuated near the body temperature by adjusting the molar ratio of the monomers. Furthermore, the printed hydrogels display a high Young's modulus (up to ∼215 MPa) and high toughness (up to ∼7 MJ/m³), and their mechanical properties can be tuned from brittle to ductile by reducing the molar fraction of C16A, or the deformation speed. Overall, the developed 4D printable hydrogels have great potential for various biomedical applications.

3D and 4D Printing in the Fight against Breast Cancer

Breast cancer is the second most common cancer worldwide, characterized by a high incidence and mortality rate. Despite the advances achieved in cancer management, improvements in the quality of life of breast cancer survivors are urgent. Moreover, considering the heterogeneity that characterizes tumors and patients, focusing on individuality is fundamental. In this context, 3D printing (3DP) and 4D printing (4DP) techniques allow for a patient-centered approach. At present, 3DP applications against breast cancer are focused on three main aspects: treatment, tissue regeneration, and recovery of the physical appearance. Scaffolds, drug-loaded implants, and prosthetics have been successfully manufactured; however, some challenges must be overcome to shift to clinical practice. The introduction of the fourth dimension has led to an increase in the degree of complexity and customization possibilities. However, 4DP is still in the early stages; thus, research is needed to prove its feasibility in healthcare applications. This review article provides an overview of current approaches for breast cancer management, including standard treatments and breast reconstruction strategies. The benefits and limitations of 3DP and 4DP technologies are discussed, as well as their application in the fight against breast cancer. Future perspectives and challenges are outlined to encourage and promote AM technologies in real-world practice.

Customized protective visors enabled by closed loop controlled 4D printing

The COVID-19 pandemic makes protective visors important for protecting people in close contacts. However, the production of visors cannot be increased greatly in a short time, especially at the beginning of the pandemic. The 3D printing community contributed largely in fabricating the visor frames using the rapid and adaptive manufacturing ability. While there are many open source designs of face visors for affordable 3D printers, all these designs fabricate mono-sized frames without considering diverse users’ dimensions. Here, a new method of visor post-processing technology enabled by closed loop controlled 4D printing is proposed. The new process can further deform the printed visor to any customized size for a more comfortable user experience. FEM analysis of the customized visor also shows consistent wearing experience in different circumstances compared with the old visor design. The fabrication precision and time cost of the method is studied experimentally. A case study regarding the reducing, reusing and recycling (3R) of customized visors in classrooms is proposed to enable the customized visors manufactured in a more sustainable way.

Volumetric Additive Manufacturing of Shape Memory Polymers

Shape memory polymers (SMPs) are stimuli responsive materials with programmable recovery from a deformed state. SMP behavior is often impacted by manufacturing features like layering that can impart anisotropic responses....

Dataset on a Small-Scale Film-Coating Process Developed for Self-Expanding 4D Printed Drug Delivery Devices

Film-coating is widely applied in pharmaceutics to enhance aspect/taste and mechanical properties of dosage forms, to protect them from the environment and to modify their release performance. In this respect, a film-coating process was recently involved in the development of 4D printed prolonged-release systems intended for organ retention. During coating processes, liquid formulations are sprayed onto moving cores, whose shape, weight and surface characteristics are essential to attain a homogeneous film. Devices of complex shapes, composed of smart materials and fabricated by hot-processing techniques, such as extrusion and fused deposition modeling 3D printing, might be poorly compatible with the requirements of traditional coating methods, e.g., need for spherical substrates with smooth surface and stable under process temperatures. This work was aimed at evaluating, at a small scale level, the feasibility of a versatile equipment for film-coating of rod-shaped extruded and printed prototypes with different section. Equipment design and set up of process parameters were performed starting from polymeric solutions and suspensions and selecting as cores 50 mm-long rod-shaped samples based on shape memory poly(vinyl alcohol). Integrity and thickness of the applied layer and its impact on shape memory and release performance of prototypes were investigated.