Rapid Volatilization Induced Mechanically Robust Shape-Morphing Structures toward 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.

Machine learning-enabled forward prediction and inverse design of 4D-printed active plates

Shape transformations of active composites (ACs) depend on the spatial distribution of constituent materials. Voxel-level complex material distributions can be encoded by 3D printing, offering enormous freedom for possible shape-change 4D-printed ACs. However, efficiently designing the material distribution to achieve desired 3D shape changes is significantly challenging yet greatly needed. Here, we present an approach that combines machine learning (ML) with both gradient-descent (GD) and evolutionary algorithm (EA) to design AC plates with 3D shape changes. A residual network ML model is developed for the forward shape prediction. A global-subdomain design strategy with ML-GD and ML-EA is then used for the inverse material-distribution design. For a variety of numerically generated target shapes, both ML-GD and ML-EA demonstrate high efficiency. By further combining ML-EA with a normal distance-based loss function, optimized designs are achieved for multiple irregular target shapes. Our approach thus provides a highly efficient tool for the design of 4D-printed active composites.

Active Materials for Functional Origami

In recent decades, origami has been explored to aid in the design of engineering structures. These structures span multiple scales and have been demonstrated to be used toward various areas such as aerospace, metamaterial, biomedical, robotics, and architectural applications. Conventionally, origami or deployable structures have been actuated by hands, motors, or pneumatic actuators, which can result in heavy or bulky structures. On the other hand, active materials, which reconfigure in response to external stimulus, eliminate the need for external mechanical loads and bulky actuation systems. Thus, in recent years, active materials incorporated with deployable structures have shown promise for remote actuation of light weight, programmable origami. In this review, active materials such as shape memory polymers (SMPs) and alloys (SMAs), hydrogels, liquid crystal elastomers (LCEs), magnetic soft materials (MSMs), and covalent adaptable network (CAN) polymers, their actuation mechanisms, as well as how they have been utilized for active origami and where these structures are applicable is discussed. Additionally, the state-of-the-art fabrication methods to construct active origami are highlighted. The existing structural modeling strategies for origami, the constitutive models used to describe active materials, and the largest challenges and future directions for active origami research are summarized.

Single-vat single-cure grayscale digital light processing 3D printing of materials with large property difference and high stretchability

Multimaterial additive manufacturing has important applications in various emerging fields. However, it is very challenging due to material and printing technology limitations. Here, we present a resin design strategy that can be used for single-vat single-cure grayscale digital light processing (g-DLP) 3D printing where light intensity can locally control the conversion of monomers to form from a highly stretchable soft organogel to a stiff thermoset within in a single layer of printing. The high modulus contrast and high stretchability can be realized simultaneously in a monolithic structure at a high printing speed (z-direction height 1 mm/min). We further demonstrate that the capability can enable previously unachievable or hard-to-achieve 3D printed structures for biomimetic designs, inflatable soft robots and actuators, and soft stretchable electronics. This resin design strategy thus provides a material solution in multimaterial additive manufacture for a variety of emerging applications.

Multiscale and hierarchical wrinkle enhanced graphene/Ecoflex sensors integrated with human-machine interfaces and cloud-platform

Current state-of-the-art stretchable/flexible sensors have received stringent demands on sensitivity, flexibility, linearity, and wide-range measurement capability. Herein, we report a methodology of strain sensors based on graphene/Ecoflex composites by modulating multiscale/hierarchical wrinkles on flexible substrates. The sensor shows an ultra-high sensitivity with a gauge factor of 1078.1, a stretchability of 650%, a response time of ~140 ms, and a superior cycling durability. It can detect wide-range physiological signals including vigorous body motions, pulse monitoring and speech recognition, and be used for monitoring of human respirations in real-time using a cloud platform, showing a great potential for the healthcare internet of things. Complex gestures/sign languages can be precisely detected. Human-machine interface is demonstrated by using a sensor-integrated glove to remotely control an external manipulator to remotely defuse a bomb. This study provides strategies for real-time/long-range medical diagnosis and remote assistance to perform dangerous tasks in industry and military fields.

4D biofabrication via instantly generated graded hydrogel scaffolds

Formation of graded biomaterials to render shape-morphing scaffolds for 4D biofabrication holds great promise in fabrication of complex structures and the recapitulation of critical dynamics for tissue/organ regeneration. Here we describe a facile generation of an adjustable and robust gradient using a single- or multi-material one-step fabrication strategy for 4D biofabrication. By simply photocrosslinking a mixed solution of a photocrosslinkable polymer macromer, photoinitiator (PI), UV absorber and live cells, a cell-laden gradient hydrogel with pre-programmable deformation can be generated. Gradient formation was demonstrated in various polymers including poly(ethylene glycol) (PEG), alginate, and gelatin derivatives using various UV absorbers that present overlap in UV spectrum with that of the PI UV absorbance spectrum. Moreover, this simple and effective method was used as a universal platform to integrate with other hydrogel-engineering techniques such as photomask-aided microfabrication, photo-patterning, ion-transfer printing, and 3D bioprinting to fabricate more advanced cell-laden scaffold structures. Lastly, proof-of-concept 4D tissue engineering was demonstrated in a study of 4D bone-like tissue formation. The strategy's simplicity along with its versatility paves a new way in solving the hurdle of achieving temporal shape changes in cell-laden single-component hydrogel scaffolds and may expedite the development of 4D biofabricated constructs for biological applications.

Recent Progress in Active Mechanical Metamaterials and Construction Principles

Active mechanical metamaterials (AMMs) (or smart mechanical metamaterials) that combine the configurations of mechanical metamaterials and the active control of stimuli-responsive materials have been widely investigated in recent decades. The elaborate artificial microstructures of mechanical metamaterials and the stimulus response characteristics of smart materials both contribute to AMMs, making them achieve excellent properties beyond the conventional metamaterials. The micro and macro structures of the AMMs are designed based on structural construction principles such as, phase transition, strain mismatch, and mechanical instability. Considering the controllability and efficiency of the stimuli-responsive materials, physical fields such as, the temperature, chemicals, light, electric current, magnetic field, and pressure have been adopted as the external stimuli in practice. In this paper, the frontier works and the latest progress in AMMs from the aspects of the mechanics and materials are reviewed. The functions and engineering applications of the AMMs are also discussed. Finally, existing issues and future perspectives in this field are briefly described. This review is expected to provide the basis and inspiration for the follow-up research on AMMs.

Design of MXene Composites with Biomimetic Rapid and Self-Oscillating Actuation under Ambient Circumstances

Although responsive actuators have been intensively investigated, it remains challenging to enable rapid and self-oscillating actuation under ambient circumstances without human intervention analogous to living organisms. By hybridizing a unique type of two-dimensional nanomaterials (i.e., MXene) with a particular hydrophilic polymer, a smart and flexible conductive composite was produced with rapid actuation and spontaneous oscillation near a moist surface. Due to the presence of layered microstructures and the moisture-sensitivity improved by surface roughness and intercalated polymeric layers, the composites could reversibly bend up to 180° in 2 s or 210° in 10 s on demand when the circumstantial humidity was varied, being superior or comparable to many actuators in the literature. More importantly, the composite was capable not only of flipping upside down repeatedly on the moist surface but also of self-oscillating ceaselessly under ambient gradient humidity without human intervention, e.g., an oscillation between 30 and 100° with an oscillation frequency of 0.08 Hz. This self-oscillation resulted from the occurrence of rapid asymmetrical hydration and dehydration of the composite between the regions of high and low humidity, which could further be modulated both by different hydrophilic polymers and by photoradiation owing to the photothermal effect of MXene nanosheets. Because of the ubiquitous presence of humidity gradient near the moist surface, this type of smart composite may not only offer a strategy for designing artificial materials that are capable of spontaneous actuation under ambient circumstance without human intervention but also promise potential applications in artificial muscles, autonomous robotics, and energy harvesting from environments.

4D Printing of Glass Fiber-Regulated Shape Shifting Structures with High Stiffness

4D printing allows 3D printed structures to change their shapes overtime under external stimuli, finding a wide range of potential applications in actuators, soft robotics, active metamaterials, flexible electronics, and biomedical devices. However, most 4D printing uses soft polymers to accommodate large strain shape-changing capability at the price of low stiffness, which impedes their engineering applications. Here, we demonstrate an approach to design and manufacture self-morphing structures with large deformation and high modulus (∼4.8 GPa). The structures are printed by multimaterial direct ink writing (DIW) using composite inks that contain a high volume fraction of solvent, photocurable polymer resin, and short glass fibers as well as fumed silica. During printing, the glass fibers undergo shear-induced alignment through the nozzle, leading to highly anisotropic mechanical properties. The solvent is then evaporated, during which the aligned glass fibers enable anisotropic shrinkage in the parallel and perpendicular directions to the fiber alignment for shape shifting. A final postphotocuring step is applied to further increase the stiffness of the composite from ∼300 MPa to ∼4.8 GPa. A finite element analysis (FEA) model is developed to predict the influence of the solvent, fiber contents, and fiber orientation on the shape shifting. We demonstrate the anisotropic volume shrinkage of the structures can be used as active hinges to transform printed two-dimensional structures into complex three-dimensional structures with large shape-shifting and outstanding mechanical properties. This strategy for fabricating composite structures with programmable architectures and excellent mechanical properties shows potential applications in morphing lightweight structures with load-bearing capabilities.

Additive Manufacture of Dynamic Thiol-ene Networks Incorporating Anhydride-Derived Reversible Thioester Links

A photoprintable dynamic thiol-ene resin was developed based on commercially available anhydride, thiol, and ene monomers. The dynamic chemistry chosen for this study relied on the thermal reversibility of the in situ generated thioester-anhydride links. The resin's rheological and curing properties were optimized to enable 3D printing using the masked stereolithography (MSLA) technique. To achieve a desirable depth of cure of 200 μm, a combination of radical photoinitiator (BAPO) and inhibitor (pyrogallol) were used at a weight ratio of 0.5 to 0.05, resulting in more than 90% thiol-ene conversion within 12 s curing time. In a series of stress relaxation and creep experiments, the dynamic reversible exchange was characterized and yielded rapid exchange rates ranging from minutes to seconds at temperatures of 80-140 °C. Little to no exchange was observed at temperatures below 60 °C. Various 3D geometries were 3D printed, and the printed objects were shown to be reconfigurable above 80 °C and depolymerizable at or above 120 °C. By deactivation of the exchange catalyst (DMAP), the stimuli responsiveness was demonstrated to be erasable, allowing for a significant shift in the actuation threshold. These highly enabling features of the dynamic chemistry open up new possibilities in the field of shape memory and 4D printable functional materials.

4D-Printable Liquid Metal-Liquid Crystal Elastomer Composites

Soft actuators that undergo programmable shape change in response to a stimulus are enabling components of future soft robots and other soft machines. Strategies to power these actuators often require the incorporation of rigid, electrically conductive materials into the soft actuator, thus limiting the compliance and shape change of the material. In this study, we develop a 4D-printable composite composed of liquid crystal elastomer (LCE) matrix with dispersed droplets of eutectic gallium indium alloy (EGaIn). Using deformable EGaIn droplets in place of rigid conductive fillers preserves the compliance and shape-morphing properties of the LCE. The process enables 4D-printed LCE actuators capable of photothermal and electrothermal actuation. At low liquid metal (LM) concentrations (71 wt %), the composite actuator exhibits a photothermal response upon irradiation of near-IR light. Printed actuators with a twisted nematic configuration are capable of bending angles of 150° at 800 mW cm^-2. At higher LM concentrations (88 wt %), the embedded LM droplets can form percolating networks that conduct electricity and enable electrical Joule heating of the LCE. Actuation strain ranging from 5 to 12% is controlled by the amount of electrical power that is delivered to the composite. We also introduce a method for multimaterial printing of monolithic structures where the LM filler loading is spatially varied. These multifunctional materials exhibit innate responsivity where the actuator behaves as an electrical switch and can report one of two states (on/off). These multiresponsive, 4D-printable composites enable multifunctional, mechanically active structures that can be powered with IR light or low DC voltages.

Processing advances in liquid crystal elastomers provide a path to biomedical applications

Liquid crystal elastomers (LCEs) are a class of stimuli-responsive polymers that undergo reversible shape-change in response to environmental changes. The shape change of LCEs can be programmed during processing by orienting the liquid crystal phase prior to crosslinking. The suite of processing techniques that has been developed has resulted in a myriad of LCEs with different shape-changing behavior and mechanical properties. Aligning LCEs via mechanical straining yields large uniaxial actuators capable of a moderate force output. Magnetic fields are utilized to control the alignment within LCE microstructures. The generation of out-of-plane deformations such as bending, twisting, and coning is enabled by surface alignment techniques within thin films. 4D printing processes have emerged that enable the fabrication of centimeter-scale, 3D LCE structures with a complex alignment. The processing technique also determines, to a large extent, the potential applications of the LCE. For example, 4D printing enables the fabrication of LCE actuators capable of replicating the forces generated by human muscles. Employing surface alignment techniques, LCE films can be designed for use as coatings or as substrates for stretchable electronics. The growth of new processes and strategies opens and strengthens the path for LCEs to be applicable within biomedical device designs.

Vat photopolymerization 3D printing for advanced drug delivery and medical device applications

Three-dimensional (3D) printing is transforming manufacturing paradigms within healthcare. Vat photopolymerization 3D printing technology combines the benefits of high resolution and favourable printing speed, offering a sophisticated approach to fabricate bespoke medical devices and drug delivery systems. Herein, an overview of the vat polymerization techniques, their unique applications in the fields of drug delivery and medical device fabrication, material examples and the advantages they provide within healthcare, is provided. The challenges and drawbacks presented by this technology are also discussed. It is forecast that the adoption of 3D printing could pave the way for a personalised health system, advancing from traditional treatments pathways towards digital healthcare.