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

4D bioprinting of programmed dynamic tissues

Setting time as the fourth dimension, 4D printing allows us to construct dynamic structures that can change their shape, property, or functionality over time under stimuli, leading to a wave of innovations in various fields. Recently, 4D printing of smart biomaterials, biological components, and living cells into dynamic living 3D constructs with 4D effects has led to an exciting field of 4D bioprinting. 4D bioprinting has gained increasing attention and is being applied to create programmed and dynamic cell-laden constructs such as bone, cartilage, and vasculature. This review presents an overview on 4D bioprinting for engineering dynamic tissues and organs, followed by a discussion on the approaches, bioprinting technologies, smart biomaterials and smart design, bioink requirements, and applications. While much progress has been achieved, 4D bioprinting as a complex process is facing challenges that need to be addressed by transdisciplinary strategies to unleash the full potential of this advanced biofabrication technology. Finally, we present future perspectives on the rapidly evolving field of 4D bioprinting, in view of its potential, increasingly important roles in the development of advanced dynamic tissues for basic research, pharmaceutics, and regenerative medicine.

Programmed multimaterial assembly by synergized 3D printing and freeform laser induction

In nature, structural and functional materials often form programmed three-dimensional (3D) assembly to perform daily functions, inspiring researchers to engineer multifunctional 3D structures. Despite much progress, a general method to fabricate and assemble a broad range of materials into functional 3D objects remains limited. Herein, to bridge the gap, we demonstrate a freeform multimaterial assembly process (FMAP) by integrating 3D printing (fused filament fabrication (FFF), direct ink writing (DIW)) with freeform laser induction (FLI). 3D printing performs the 3D structural material assembly, while FLI fabricates the functional materials in predesigned 3D space by synergistic, programmed control. This paper showcases the versatility of FMAP in spatially fabricating various types of functional materials (metals, semiconductors) within 3D structures for applications in crossbar circuits for LED display, a strain sensor for multifunctional springs and haptic manipulators, a UV sensor, a 3D electromagnet as a magnetic encoder, capacitive sensors for human machine interface, and an integrated microfluidic reactor with a built-in Joule heater for nanomaterial synthesis. This success underscores the potential of FMAP to redefine 3D printing and FLI for programmed multimaterial assembly.

4D printing for biomedical applications

While three-dimensional (3D) printing excels at fabricating static constructs, it fails to emulate the dynamic behavior of native tissues or the temporal programmability desired for medical devices. Four-dimensional (4D) printing is an advanced additive manufacturing technology capable of fabricating constructs that can undergo pre-programmed changes in shape, property, or functionality when exposed to specific stimuli. In this Perspective, we summarize the advances in materials chemistry, 3D printing strategies, and post-printing methodologies that collectively facilitate the realization of temporal dynamics within 4D-printed soft materials (hydrogels, shape-memory polymers, liquid crystalline elastomers), ceramics, and metals. We also discuss and present insights about the diverse biomedical applications of 4D printing, including tissue engineering and regenerative medicine, drug delivery, in vitro models, and medical devices. Finally, we discuss the current challenges and emphasize the importance of an application-driven design approach to enable the clinical translation and widespread adoption of 4D printing.

Recent Advances in 4D Printing of Advanced Materials and Structures for Functional Applications

4D printing has attracted tremendous worldwide attention during the past decade. This technology enables the shape, property, or functionality of printed structures to change with time in response to diverse external stimuli, making the original static structures alive. The revolutionary 4D-printing technology offers remarkable benefits in controlling geometric and functional reconfiguration, thereby showcasing immense potential across diverse fields, including biomedical engineering, electronics, robotics, and photonics. Here, a comprehensive review of the latest achievements in 4D printing using various types of materials and different additive manufacturing techniques is presented. The state-of-the-art strategies implemented in harnessing various 4D-printed structures are highlighted, which involve materials design, stimuli, functionalities, and applications. The machine learning approach explored for 4D printing is also discussed. Finally, the perspectives on the current challenges and future trends toward further development in 4D printing are summarized.

Bioinspired 3D flexible devices and functional systems

Flexible devices and functional systems with elaborated three-dimensional (3D) architectures can endow better mechanical/electrical performances, more design freedom, and unique functionalities, when compared to their two-dimensional (2D) counterparts. Such 3D flexible devices/systems are rapidly evolving in three primary directions, including the miniaturization, the increasingly merged physical/artificial intelligence and the enhanced adaptability and capabilities of heterogeneous integration. Intractable challenges exist in this emerging research area, such as relatively poor controllability in the locomotion of soft robotic systems, mismatch of bioelectronic interfaces, and signal coupling in multi-parameter sensing. By virtue of long-time-optimized materials, structures and processes, natural organisms provide rich sources of inspiration to address these challenges, enabling the design and manufacture of many bioinspired 3D flexible devices/systems. In this Review, we focus on bioinspired 3D flexible devices and functional systems, and summarize their representative design concepts, manufacturing methods, principles of structure-function relationship and broad-ranging applications. Discussions on existing challenges, potential solutions and future opportunities are also provided to usher in further research efforts toward realizing bioinspired 3D flexible devices/systems with precisely programmed shapes, enhanced mechanical/electrical performances, and high-level physical/artificial intelligence.

Investigation of Shape Memory Polyurethane Properties in Cold Programming Process Towards Its Applications

Thermoresponsive shape memory polymers (SMPs) with the remarkable ability to remember a temporary shape and recover their original one using temperature have been gaining more and more attention in a wide range of applications. Traditionally, SMPs are investigated using a method named often "hot-programming", since they are heated above their glass transition temperature (Tg) and after that, reshaped and cooled below Tg to achieve and fix the desired configuration. Upon reheating, these materials return to their original shape. However, the heating of SMPs above their Tg during a thermomechanical cycle to trigger a change in their shape creates a temperature gradient within the material structure and causes significant thermal expansion of the polymer sample resulting in a reduction in its shape recovery property. These phenomena, in turn, limit the application fields of SMPs, in which fast actuation, dimensional stability and low thermal expansion coefficient are crucial. This paper aims at a comprehensive experimental investigation of thermoplastic polyurethane shape memory polymer (PU-SMP) using the cold programming approach, in which the deformation of the SMP into the programmed shape is conducted at temperatures below Tg. The PU-SMP glass transition temperature equals approximately 65 °C. Structural, mechanical and thermomechanical characterization was performed, and the results on the identification of functional properties of PU-SMPs in quite a large strain range beyond yield limit were obtained. The average shape fixity ratio of the PU-SMP at room temperature programming was found to be approximately 90%, while the average shape fixity ratio at 45 °C (Tg - 20 °C) was approximately 97%. Whereas, the average shape recovery ratio was 93% at room temperature programming and it was equal to approximately 90% at 45 °C. However, the results obtained using the traditional method, the so-called hot programming at 65 °C, indicate a higher shape fixity value of 98%, but a lower shape recovery of 90%. Thus, the obtained results confirmed good shape memory properties of the PU-SMPs at a large strain range at various temperatures. Furthermore, the experiments conducted at both temperatures below Tg demonstrated that cold programming can be successfully applied to PU-SMPs with a relatively high Tg. Knowledge of the PU-SMP shape memory and shape fixity properties, estimated without risk of material degradation, caused by heating above Tg, makes them attractive for various applications, e.g., in electronic components, aircraft or aerospace structures.

Free-Form Liquid Crystal Elastomers via Embedded 4D Printing

Liquid crystal elastomers (LCEs) are a class of active materials that can generate rapid, reversible mechanical actuation in response to external stimuli. Fabrication methods for LCEs have remained a topic of intense research interest in recent years. One promising approach, termed 4D printing, combines the advantages of 3D printing with responsive materials, such as LCEs, to generate smart structures that not only possess user-defined static shapes but also can change their shape over time. To date, 4D-printed LCE structures have been limited to flat objects, restricting shape complexity and associated actuation for smart structure applications. In this work, we report the development of embedded 4D printing to extrude hydrophobic LCE ink into an aqueous, thixotropic gel matrix to produce free-standing, free-form 3D architectures without sacrificing the mechanical actuation properties. The ability to 4D print complex, free-standing 3D LCE architectures opens new avenues for the design and development of functional and responsive systems, such as reconfigurable metamaterials, soft robotics, or biomedical devices.