4D Printing of Hydrogels: A Review

3D printing permits the construction of objects by layer‐by‐layer deposition of material, resulting in precise control of the dimensions and properties of complex printed structures. Although 3D printing fabricates inanimate objects, the emerging technology of 4D printing allows for animated structures that change their shape, function, or properties over time when exposed to specific external stimuli after fabrication. Among the materials used in 4D printing, hydrogels have attracted growing interest due to the availability of various smart hydrogels. The reversible shape‐morphing in 4D printed hydrogel structures is driven by a stress mismatch arising from the different swelling degrees in the parts of the structure upon application of a stimulus. This review provides the state‐of‐the‐art of 4D printing of hydrogels from the materials perspective. First, the main 3D printing technologies employed are briefly depicted, and, for each one, the required physico‐chemical properties of the precursor material. Then, the hydrogels that have been printed are described, including stimuli‐responsive hydrogels, non‐responsive hydrogels that are sensitive to solvent absorption/desorption, and multimaterial structures that are totally hydrogel‐based. Finally, the current and future applications of this technology are presented, and the requisites and avenues of improvement in terms of material properties are discussed.     

Advances in 4D Printing: Materials and Applications

4D printing has attracted tremendous interest since its first conceptualization in 2013. 4D printing derived from the fast growth and interdisciplinary research of smart materials, 3D printer, and design. Compared with the static objects created by 3D printing, 4D printing allows a 3D printed structure to change its configuration or function with time in response to external stimuli such as temperature, light, water, etc., which makes 3D printing alive. Herein, the material systems used in 4D printing are reviewed, with emphasis on mechanisms and potential applications. After a brief overview of the definition, history, and basic elements of 4D printing, the state‐of‐the‐art advances in 4D printing for shape‐shifting materials are reviewed in detail. Both single material and multiple materials using different mechanisms for shape changing are summarized. In addition, 4D printing of multifunctional materials, such as 4D bioprinting, is briefly introduced. Finally, the trend of 4D printing and the perspectives for this exciting new field are highlighted.     

MXene‐Reinforced Cellulose Nanofibril Inks for 3D‐Printed Smart Fibres and Textiles

Fibre‐based materials have received tremendous attention due to their flexibility and wearability. Although great efforts have been devoted to achieve high‐performance fibres over the past several years, it is still challenging for multifunctional macroscopic fibres to satisfy versatile applications. 2D transition metal carbides/nitrides (MXenes) with intriguing physical/chemical properties have been explored in broad application, and may be able to reinforce synthetic fibres. Inspired by natural materials, for the first time, flexible smart fibres and textiles are fabricated using a 3D printing process with hybrid inks of TEMPO (2,2,6,6‐tetramethylpiperidine‐1‐oxylradi‐cal)‐mediated oxidized cellulose nanofibrils (TOCNFs) and Ti₃C₂ MXene. The hybrid inks display good rheological properties, which allow them to achieve accurate structures and be rapidly printed. TOCNFs/Ti₃C₂ in hybrid inks self‐assemble to fibres with an aligned structure in ethanol, mimicking the features of the natural structures of plant fibres. In contrast to conventional synthetic fibres with limited functions, smart TOCNFs/Ti₃C₂ fibres and textiles exhibit significant responsiveness to multiple external stimuli (electrical/photonic/mechanical). TOCNFs/Ti₃C₂ textiles with electromechanical performance can be processed into sensitive strain sensors. Such multifunctional smart fibres and textiles will be promising in diverse applications, including wearable heating textiles, human health monitoring, and human–machine interfaces.     

Self-Healing,Photothermal-Responsive,and Shape Memory Polyurethanes for Enhanced Mechanical Properties of 3D/4D Printed Objects

Additive manufacturing is a promising technology that can directly fabricate structures with complex internal geometries, which is barely achieved by traditional manufacturing. However, the mechanical properties of fused deposition modeling (FDM)-printed objects are inferior to those of conventionally manufactured products. To improve the mechanical properties of the printed products, a series of novel thermoplastic polyurethanes with self-healing properties, intrinsic photothermal effects, and excellent printability are designed and synthesized by introducing dynamic oxime–carbamate bonds and hydrogen bonds into the polymer chains. On-demand introduction of near-infrared (NIR) irradiation, direct heating, and sunlight irradiation enhances interfacial bonding strength and thus improve the mechanical properties of the printed product. Additionally, mechanical anisotropy of the printed products can be sophistically manipulated by regulating the self-healing conditions. Support-free printing and healing of damaged printed products are also achieved owing to the self-healing properties of the material. Moreover, the as-prepared materials exhibit shape-memory properties NIR irradiation or direct heating effectively triggers shape-memory recovery and demonstrates their potential in 4D printing by printing a man-like robot. This study not only provides a facile strategy for obtaining high-performance printed products but also broadens the potential applications of FDM technology in intelligent devices.     

4D Printing of Multi-Stimuli Responsive Protein-Based Hydrogels for Autonomous Shape Transformations

Stimuli responsive hydrogels that can change shape in response to applied external stimuli are appealing for soft robotics, biomedical devices, drug delivery, and actuators. However, existing 3D printed shape morphing materials are non-biodegradable, which limits their use in biomedical applications. Here, 3D printed protein-based hydrogels are developed and applied for programmable structural changes under the action of temperature, pH, or an enzyme. Key to the success of this strategy is the use of methacrylated bovine serum albumin (MA–BSA) as a biodegradable building block to Pickering emulsion gels in the presence of N-isopropylacrylamide or 2-dimethylaminoethyl methacrylate. These shear-thinning gels are ideal for direct ink write (DIW) 3D printing of multi-layered stimuli-responsive hydrogels. While poly(N-isopropylacrylamide) and poly(dimethylaminoethyl methacrylate) introduce temperature and pH-responsive properties into the printed objects, a unique feature of this strategy is an enzyme-triggered shape transformation based on the degradation of the bovine serum albumin network. To highlight this technique, protein-based hydrogels that reversibly change shape based on environmental temperature and pH are fabricated, and irreversibly altered by enzymatic degradation, which demonstrates the complexity that can be introduced into 4D printed systems.     

A Subtractive Photoresist Platform for Micro‐ and Macroscopic 3D Printed Structures

The selective removal of structural elements plays a decisive role in 3D printing applications enabling complex geometries. To date, the fabrication of complex structures on the microscale is severely limited by multistep processes. Herein, a subtractive photoresist platform technology that is transferable from microscopic 3D printing via direct laser writing to macroscopic structures via stereolithography is reported. All resist components are readily accessible and exchangeable, offering fast adaptation of the resist's property profile. The micro‐ and macroprinted structures can be removed in a facile fashion, without affecting objects based on standard photoresists. The cleavage is analyzed by time‐lapse optical microscopy as well as via in‐depth spectroscopic assessment. The mechanical properties of the printed materials are investigated by nanoindentation. Critically, the power of the subtractive resist platform is demonstrated by constructing complex 3D objects with flying features on the microscale.     

4D Printing at the Microscale

3D printing of adaptive and dynamic structures, also known as 4D printing, is one of the key challenges in contemporary materials science. The additional dimension refers to the ability of 3D printed structures to change their properties—for example, shape—over time in a controlled fashion as the result of external stimulation. Within the last years, significant efforts have been undertaken in the development of new responsive materials for printing at the macroscale. However, 4D printing at the microscale is still in its early stages. Thus, this progress report will focus on emerging materials for 4D printing at the microscale as well as their challenges and potential applications. Hydrogels and liquid crystalline and composite materials have been identified as the main classes of materials representing the state of the art of the growing field. For each type of material, the challenges and critical barriers in the material design and their performance in 4D microprinting are discussed. Importantly, further necessary strategies are proposed to overcome the limitations of the current approaches and move toward their application in fields such as biomedicine, microrobotics, or optics.     

The Design of 4D-Printed Hygromorphs: State-of-the-Art and Future Challenges

In recent years, 4D printing has allowed the rapid development of new concepts of multifunctional/adaptive structures. The 4D printing technology makes it possible to generate new shapes and/or property-changing capabilities by combining smart materials, multiphysics stimuli, and additive manufacturing. Hygromorphs constitute a specific class of new smart materials where their properties and morphing capabilities are dependent on the surrounding humidity, which drives actuation. Although multiple efforts have been made to fabricate hygromorph demonstrators, a comprehensive design process to produce hygromorphs by multiple 4D printing techniques is not yet available. The broad aim of this review and concept paper is to i) highlight existing scientific and technology gaps in the field of 4D-printed hygromorphs, ii) identify tools existing in other research fields for filling those gaps, and iii) discuss a series of guidelines for tackling future challenges and opportunities to develop 4D-printed composite hygromorph materials and related manufacturing processes. Accordingly, this review describes the materials and additive manufacturing techniques used for hygromorph composite fabrication. Moreover, the relevant parameters that control actuation, the models selection and performance, the design methods and the actuation measurements for customized 4D-printed hygromorph materials, are discussed.     

3D Printed Actuators: Reversibility,Relaxation, and Ratcheting

Additive manufacturing strives to combine any combination of materials into 3D functional structures and devices, ultimately opening up the possibility of 3D printed machines. It remains difficult to actuate such devices, thus limiting the scope of 3D printed machines to passive devices or necessitating the incorporation of external actuators that are manufactured differently. Here, 3D printed hybrid thermoplast/conducter bilayers are explored, which can be actuated by differential heating caused by externally controllable currents flowing through their conducting faces. The functionality of such actuators is uncovered and it is shown that they allow to 3D print, in one pass, simple flexible robotic structures that propel forward under step‐wise applied voltages. Moreover, exploiting the thermoplasticity of the nonconducting plastic parts at elevated temperatures, it is shown that how strong driving leads to irreversible deformations—a form of 4D printing—which also enlarges the range of linear response of the actuators. Finally, it is shown that how to leverage such thermoplastic relaxations to accumulate plastic deformations and obtain very large deformations by alternatively driving both layers of a bilayer; this is called ratcheting. The strategy is scalable and widely applicable, and opens up a new approach to reversible actuation and irreversible 4D printing of arbitrary structures and machines.     

Reprintable Polymers for Digital Light Processing 3D Printing

3D printing is becoming a disruptive technology and shows great potential for various practical applications. Specially, digital light processing (DLP) 3D printing demonstrates advantages in high resolution and high efficiency. However, extensive production of infusible and insoluble thermosets in DLP printing causes serious resource waste and environmental problems after its disposal. Herein, a reprintable linear polymer is reported for repeatable DLP printing. Taking advantage of the dissolution of linear polymer in its monomer, printed objects can be recycled into liquid resin and reprinted via the same DLP. Polymerization kinetics and printing resolution of recycled resins and mechanical properties of reprinted polymers retain identical as the original. The thermoplastic nature of linear polymer endows 3D objects with welding and reshaping property. Recyclable composites are also successfully fabricated, and sustainable usage of high-value fillers comes true. This strategy helps to address environmental issues arising from unprocessable thermosets and may contribute to an efficient materials recycling.     

Biomimetic 4D Printing Catapult: From Biological Prototype to Practical Implementation

4D printing technologies are currently suffering from the inability to produce rapid motions, which limit their applications that require fast shape transformation such as rapid unlocking and deployment of aerospace equipment. Herein, inspired by the shooting mechanisms of Viola verecunda fruit for seed dispersal, the 4D-printed biomimetic catapult is developed. Based on the structure change characteristics of gradient fan-shaped cells of the fruit pods during seed ejection, the biomimetic smart catapult is processed via the programming of spatial distribution of heterogeneous materials with various storage modulus enabled by additive manufacturing. This catapult can achieve high-speed ejection with the logically stimuli of external force, temperature, light, humidity, or electricity. The proposed biomimetic 4D printing strategy has broken through the limitations in motion speed, which helps fully unleash the potential of 4D printing.     

Microstructured Actuation of Liquid Crystal Polymer Networks

Smart microstructured materials enable functions such as actuation, detection, transportation, and sensing with potential applications ranging from robotics and photonics to biomedical devices. Of the many materials systems, liquid crystal polymer networks (LCN) are fascinating owing to their ability to exhibit reversible macroscopic deformation driven by a molecular order–disorder phase transition. LCN have been increasingly explored for their utility in the design and fabrication of smart actuating devices capable of complex shape changes or motions upon external stimulation of humidity, heat, light, and other stimuli, and recent studies in this field show that their actuation complexity can be enriched and actuation performance enhanced by having some sort of microstructures. Herein, the recent progress in microstructured actuation of LCN materials with substructures in scale ranging from micrometer to millimeter is reported, placing the emphasis on the main approaches to generating a microstructure in LCN, which include patterned LC director fields, patterned chain crosslinking in LCN with uniaxial orientation of mesogens, 3D/4D printing, and replica molding. The potential applications in microstructured 3D actuators and devices as well as functional LCN surfaces are also highlighted, with an outlook on important issues and future trends in smart microstructured LCN materials and actuators.     

Shape-Memory Balloon Structures by Pneumatic Multi-material 4D Printing

4D printing is an attractive approach for manufacturing structures that can adopt new shapes or functionalities after printing. However, 4D printing methods and materials that can be used to achieve structures with complex shapes and excellent mechanical properties simultaneously are still lacking. Here, a novel 4D printing is developed where multi-material digital light process 3D printing of shape memory polymers (SMPs) fabricates a structure that is later transformed into a complex 3D shape with robust mechanical properties by pneumatic manipulation. In this method, the shape change is controlled by the spatial distributions of SMPs, which is designed by finite element analysis. Experimental investigations are carried out to print various structured balloons with predefined intricate shapes, including a structure in dog-like shape and a surface with the human face contour. These structures are also endowed with robust mechanical stiffness and lightweight features, which allow this new 4D printing approach for potential applications in biomedical devices, reconfigurable structures, and metamaterials.     

Functional Textiles with Smart Properties: Their Fabrications and Sustainable Applications

Benefiting from inherent lightweight, flexibility, and good adaptability to human body, functional textiles are attracting tremendous attention to cope with wearable issues in sustainable applications around human beings. In this feature article, a comprehensive and thoughtful review is proposed regarding research activities of functional textiles with smart properties. Specifically, a brief exposition of highlighting the significance and rising demands of novel textiles throughout the human society is begun. Next, a systematic review is provided about the fabrication of functional textiles from 1D spinning, 2D modification, and 3D construction, their diverse functionality as well as sustainable applications, showing a clear picture of evolved textiles to the readers. How to engineer the compositions, structures, and properties of functional textiles is elaborated to achieve different smart properties. All these tunable, upgraded, and versatile properties make the developed textiles well suited for extensive applications ranging from environmental monitoring or freshwater access to personal protection and wearable power supply. Finally, a simple summary and critical analysis is drawn, with emphasis on the insight into remaining challenges and future directions. With worldwide efforts, advance and breakthrough in textile functionalization expounded in this review will promote the revolution of smart textiles for intelligence era.     

3D Printing of Resilin in Water by Multiphoton Absorption Polymerization

Resilin is an elastic rubber-like protein found in the cuticles of insects. It incorporates outstanding properties of high resilience and fatigue lifetime, where kinetic energy storage is needed for biological functions such as flight and jumps. Since resilin is rich in tyrosine groups, localized photopolymerization is enabled due to the ability to introduce di-tyrosine bonds by a ruthenium-based photoinitiator. Using Multiphoton Absorption Polymerization 3D printing process, objects containing 100% recombinant resilin protein are printed in water at a submicron length scale. Consequently, protein-based hydrogels with complex structures are printed using space positioning voxel polymerization. The objects are characterized by dynamic mechanical analysis using nanoindentation. Printing parameters such as printing speed and laser power are found to enable tuning the mechanical properties of the printed objects. The printed objects are soft and resilient, similar to native resilin, while presenting the highest resolution of a structure made entirely of a protein and better mechanical properties of common hydrogels and poly(dimethylsiloxane). Moreover, topography and mechanical properties enable cell growth and alignment without cell adhesion primers, thus facilitating biological applications. The fabrication of 3D resilin-based hydrogel will open the way for potential applications based on biomimicking and in creating new functional objects.     

A Multiresponsive Anisotropic Hydrogel with Macroscopic 3D Complex Deformations

As one of the most promising smart materials, stimuli‐responsive polymer hydrogels (SPHs) can reversibly change volume or shape in response to external stimuli. They thus have shown promising applications in many fields. While considerable progress of 2D deformation of SPHs has been achieved, the realization of 3D or even more complex deformation still remains a significant challenge. Here, a general strategy towards designing multiresponsive, macroscopically anisotropic SPHs (MA‐SPHs) with the ability of 3D complex deformations is reported. Through a local UV‐reduction of graphene oxide sheets (GOs) with a patterned fashion in the GO‐poly(N‐isopropylacrylamide) (GO‐PNIPAM) composite hydrogel sheet, MA‐SPHs can be achieved after the introduction of a second poly(methylacrylic acid) network in the unreduced part of GO‐PNIPAM hydrogel sheet. The resulting 3D MA‐SPHs can provide remote‐controllable light‐driven, as well as thermo‐, pH‐, and ionic strength‐triggered multiresponsive 3D complex deformations. Approaches in this study may provide new insights in designing and fabricating intelligent soft materials for bioinspired applications.     

Femtosecond Laser 4D Printing of Light-Driven Intelligent Micromachines

Intelligent micromachines that respond to external light stimuli have a broad range of potential applications, such as microbots, biomedicine, and adaptive optics. However, artificial light-driven intelligent micromachines with a low actuation threshold, rapid responsiveness, and designable and precise 3D transformation capability remain unachievable to date. Here, a single-material and one-step 4D printing strategy are proposed to enable the nanomanufacturing of agile and low-threshold light-driven 3D micromachines with programmable shape-morphing characteristics. The as-developed carbon nanotube-doped composite hydrogel simultaneously enhanced the light absorption, thermal conductivity, and mechanical modulus of the crosslinked network, thus significantly increasing the light sensitivity and response speed of micromachines. Moreover, the structural design and assembly of asymmetric microscale mechanical metamaterial unit cells enable the highly efficient additive nanomanufacturing of 3D shape-morphable micromachines with large dynamic modulation and spatiotemporal controllability. Using this strategy, the world's smallest artificial beating heart with programmable light-stimulus responsiveness for the cardiac cycle is successfully printed. This 4D printing method paves the way for the construction of multifunctional intelligent micromachines for bionics, drug delivery, integrated microsystems, and other fields.     

Conformal Printing of Graphene for Single‐ and Multilayered Devices onto Arbitrarily Shaped 3D Surfaces

Printing has drawn a lot of attention as a means of low per‐unit cost and high throughput patterning of graphene inks for scaled‐up thin‐form factor device manufacturing. However, traditional printing processes require a flat surface and are incapable of achieving patterning onto 3D objects. Here, a conformal printing method is presented to achieve functional graphene‐based patterns onto arbitrarily shaped surfaces. Using experimental design, a water‐insoluble graphene ink with optimum conductivity is formulated. Then single‐ and multilayered electrically functional structures are printed onto a sacrificial layer using conventional screen printing. The print is then floated on water, allowing the dissolution of the sacrificial layer, while retaining the functional patterns. The single‐ and multilayer patterns can then be directly transferred onto arbitrarily shaped 3D objects without requiring any postdeposition processing. Using this technique, conformal printing of single‐ and multilayer functional devices that include joule heaters, resistive deformation sensors, and proximity sensors on hard, flexible, and soft substrates, such as glass, latex, thermoplastics, textiles, and even candies and marshmallows, is demonstrated. This simple strategy promises to add new device and sensing functionalities to previously inert 3D surfaces.     

Inkjet Process for Conductive Patterning on Textiles: Maintaining Inherent Stretchability and Breathability in Knit Structures

In this work, a novel technique of inkjet printing e‐textiles with particle free reactive silver inks on knit structures is developed. The inkjet‐printed e‐textiles are highly conductive, with a sheet resistance of 0.09 Ω sq^‐1, by means of controlling the number of print passes, annealing process, and textile structures. It is notable that the inkjet process allows textiles to maintain its inherent properties, including stretchability, flexibility, breathability, and fabric hand after printing process. This is achieved by formation of ultrathin silver layers surrounding individual fibers. The silver layers coated on fibers range from 250 nm to 2.5 µm, maintaining the size of interstices and flexibility of fibers. The annealing process, structure of fibers, and printed layers significantly influence the electrical conductivity of the patterned structures on textiles. Outstanding electrical conductivity and durability are demonstrated by optimizing print passes, controlling textile structures, and incorporating an in situ annealing process. The electrical resistance dependence on the strain rate of the textiles is examined, showing the ability to maintain electrical conductivity to retain light‐emitting diode use, stable more than 500 consecutive strain cycles. Most importantly, inkjet‐printed e‐textiles maintain their characteristic washability, breathability, and fabric hands for applications in wearable technology.     

Additive Manufacturing of Batteries

Additive manufacturing, i.e., 3D printing, is being increasingly utilized to fabricate a variety of complex‐shaped electronics and energy devices (e.g., batteries, supercapacitors, and solar cells) due to its excellent process flexibility, good geometry controllability, as well as cost and material waste reduction. In this review, the recent advances in 3D printing of emerging batteries are emphasized and discussed. The recent progress in fabricating 3D‐printed batteries through the major 3D‐printing methods, including lithography‐based 3D printing, template‐assisted electrodeposition‐based 3D printing, inkjet printing, direct ink writing, fused deposition modeling, and aerosol jet printing, are first summarized. Then, the significant achievements made in the development and printing of battery electrodes and electrolytes are highlighted. Finally, major challenges are discussed and potential research frontiers in developing 3D‐printed batteries are proposed. It is expected that with the continuous development of printing techniques and materials, 3D‐printed batteries with long‐term durability, favorable safety as well as high energy and power density will eventually be widely used in many fields.