Stimuli-responsive materials in additive manufacturing

Fabrication of Large-Area Multi-Stimulus Responsive Thin Films via Interfacially Confined Irreversible Katritzky Reaction

Fabrication of large-area thin films through irreversible reactions remains a formidable task. This study reports a breakthrough strategy for in situ synthesis of large-area, free-standing, robust and multi-stimulus responsive thin films through a catalyst-free and irreversible Katritzky reaction at a liquid-liquid interface. The as resulted films are featured with adjustable thickness of 1-3 μm and an area up to 50 cm2. The thin films exhibit fast photo-mechanical motions (a response time of ca 0.1 s), vapor-mechanical motions, as well as photo-chromic and solvato-chromic behaviors. It was revealed that the reason behind the observable motions is proton transfer from the imine groups to the carbonyl structures within the film induced by photo- and/or dimethyl sulfoxide-stimulus. In addition, the films can harvest anionic radicals and the radicals as captured can be efficiently degraded under UV light illumination. This study provides a new strategy for fabricating smart thin films via interfacially confined irreversible Katritzky reaction.

Mechanically Strengthened Aerogels through Multiscale, Multicompositional, and Multidimensional Approaches: A Review

In recent decades, aerogels have attracted tremendous attention in academia and industry as a class of lightweight and porous multifunctional nanomaterial. Despite their wide application range, the low mechanical durability hinders their processing and handling, particularly in applications requiring complex physical structures. "Mechanically strengthened aerogels" have emerged as a potential solution to address this drawback. Since the first report on aerogels in 1931, various modified synthesis processes have been introduced in the last few decades to enhance the aerogel mechanical strength, further advancing their multifunctional scope. This review summarizes the state-of-the-art developments of mechanically strengthened aerogels through multicompositional and multidimensional approaches. Furthermore, new trends and future directions for as prevailed commercialization of aerogels as plastic materials are discussed.

Single-step precision programming of decoupled multiresponsive soft millirobots

Stimuli-responsive soft robots offer new capabilities for the fields of medical and rehabilitation robotics, artificial intelligence, and soft electronics. Precisely programming the shape morphing and decoupling the multiresponsiveness of such robots is crucial to enable them with ample degrees of freedom and multifunctionality, while ensuring high fabrication accuracy. However, current designs featuring coupled multiresponsiveness or intricate assembly processes face limitations in executing complex transformations and suffer from a lack of precision. Therefore, we propose a one-stepped strategy to program multistep shape-morphing soft millirobots (MSSMs) in response to decoupled environmental stimuli. Our approach involves employing a multilayered elastomer and laser scanning technology to selectively process the structure of MSSMs, achieving a minimum machining precision of 30 μm. The resulting MSSMs are capable of imitating the shape morphing of plants and hand gestures and resemble kirigami, pop-up, and bistable structures. The decoupled multistimuli responsiveness of the MSSMs allows them to conduct shape morphing during locomotion, perform logic circuit control, and remotely repair circuits in response to humidity, temperature, and magnetic field. This strategy presents a paradigm for the effective design and fabrication of untethered soft miniature robots with physical intelligence, advancing the decoupled multiresponsive materials through modular tailoring of robotic body structures and properties to suit specific applications.

3D Printing of Self-Healing Materials

This review article presents a comprehensive overview of the latest advances in the field of 3D printable structures with self-healing properties. Three-dimensional printing (3DP) is a versatile technology that enables the rapid manufacturing of complex geometric structures with precision and functionality not previously attainable. However, the application of 3DP technology is still limited by the availability of materials with customizable properties specifically designed for additive manufacturing. The addition of self-healing properties within 3D printed objects is of high interest as it can improve the performance and lifespan of structural components, and even enable the mimicking of living tissues for biomedical applications, such as organs printing. The review will discuss and analyze the most relevant results reported in recent years in the development of self-healing polymeric materials that can be processed via 3D printing. After introducing the chemical and physical self-healing mechanism that can be exploited, the literature review here reported will focus in particular on printability and repairing performances. At last, actual perspective and possible development field will be critically discussed.

3D bioprinting in bioremediation: a comprehensive review of principles, applications, and future directions

Bioremediation is experiencing a paradigm shift by integrating three-dimensional (3D) bioprinting. This transformative approach augments the precision and versatility of engineering with the functional capabilities of material science to create environmental restoration strategies. This comprehensive review elucidates the foundational principles of 3D bioprinting technology for bioremediation, its current applications in bioremediation, and the prospective avenues for future research and technological evolution, emphasizing the intersection of additive manufacturing, functionalized biosystems, and environmental remediation; this review delineates how 3D bioprinting can tailor bioremediation apparatus to maximize pollutant degradation and removal. Innovations in biofabrication have yielded bio-based and biodegradable materials conducive to microbial proliferation and pollutant sequestration, thereby addressing contamination and adhering to sustainability precepts. The review presents an in-depth analysis of the application of 3D bioprinted constructs in enhancing bioremediation efforts, exemplifying the synergy between biological systems and engineered solutions. Concurrently, the review critically addresses the inherent challenges of incorporating 3D bioprinted materials into diverse ecological settings, including assessing their environmental impact, durability, and integration into large-scale bioremediation projects. Future perspectives discussed encompass the exploration of novel biocompatible materials, the automation of bioremediation, and the convergence of 3D bioprinting with cutting-edge fields such as nanotechnology and other emerging fields. This article posits 3D bioprinting as a cornerstone of next-generation bioremediation practices, offering scalable, customizable, and potentially greener solutions for reclaiming contaminated environments. Through this review, stakeholders in environmental science, engineering, and technology are provided with a critical appraisal of the current state of 3D bioprinting in bioremediation and its potential to drive forward the efficacy of environmental management practices.

Liquid Crystal Elastomer Artificial Tendrils with Asymmetric Core-Sheath Structure Showing Evolutionary Biomimetic Locomotion

The sophisticated and complex haptonastic movements in response to environmental-stimuli of living organisms have always fascinated scientists. However, how to fundamentally mimic the sophisticated hierarchical architectures of living organisms to provide the artificial counterparts with similar or even beyond-natural functions based on the underlying mechanism remains a major scientific challenge. Here,  liquid crystal elastomer (LCE) artificial tendrils showing evolutionary biomimetic locomotion are developed following the structure-function principle that is used in nature to grow climbing plants. These elaborately designed tendril-like LCE actuators possess an asymmetric core-sheath architecture which shows a higher-to-lower transition in the degree of LC orientation from the sheath-to-core layer across the semi-ellipse cross-section. Upon heating and cooling, the LCE artificial tendril can undergo reversible tendril-like shape-morphing behaviors, such as helical coiling/winding, and perversion. The fundamental mechanism of the helical shape-morphing of the artificial tendril is revealed by using theoretical models and finite element simulations. Besides, the incorporation of metal-ligand coordination into the LCE network provides the artificial tendril with reconfigurable shape-morphing performances such as helical transitions and rotational deformations. Finally, the abilities of helical and rotational deformations are integrated into a new reprogrammed flagellum-like architecture to perform evolutionary locomotion mimicking the haptonastic movements of the natural flagellum.

Correlating structural changes in thermoresponsive hydrogels to the optical response of embedded plasmonic nanoparticles

Stimuli-responsive microgels, composed of small beads with soft, deformable polymer networks swollen through a combination of synthetic control over the polymer and its interaction with water, form a versatile platform for development of multifunctional and biocompatible sensors. The interfacial structural variation of such materials at a nanometer length scale is essential to their function, but not yet fully comprehended. Here, we take advantage of the plasmonic response of a gold nanorod embedded in a thermoresponsive microgel (AuNR@PNIPMAm) to monitor structural changes in the hydrogel directly near the nanorod surface. By direct comparison of the plasmon response against measurements of the hydrogel structure from dynamic light scattering and nuclear magnetic resonance, we find that the microgel shell of batch-polymerized AuNR@PNIPMAm exhibits a heterogeneous volume phase transition reflected by different onset temperatures for changes in the hydrodyanmic radius (RH) and plasmon resonance, respectively. The new approach of contrasting plasmonic response (a measure of local surface hydrogel structure) with RH and relaxation times paves a new path to gain valuable insight for the design of plasmonic sensors based on stimuli-responsive hydrogels.

Recent Progress in Stimuli-Responsive Antimicrobial Electrospun Nanofibers

Electrospun nanofibrous membranes have garnered significant attention in antimicrobial applications, owing to their intricate three-dimensional network that confers an interconnected porous structure, high specific surface area, and tunable physicochemical properties, as well as their notable capacity for loading and sustained release of antimicrobial agents. Tailoring polymer or hybrid-based nanofibrous membranes with stimuli-responsive characteristics further enhances their versatility, enabling them to exhibit broad-spectrum or specific activity against diverse microorganisms. In this review, we elucidate the pivotal advancements achieved in the realm of stimuli-responsive antimicrobial electrospun nanofibers operating by light, temperature, pH, humidity, and electric field, among others. We provide a concise introduction to the strategies employed to design smart electrospun nanofibers with antimicrobial properties. The core section of our review spotlights recent progress in electrospun nanofiber-based systems triggered by single- and multi-stimuli. Within each stimulus category, we explore recent examples of nanofibers based on different polymers and antimicrobial agents. Finally, we delve into the constraints and future directions of stimuli-responsive nanofibrous materials, paving the way for their wider application spectrum and catalyzing progress toward industrial utilization.

Self-Healing Polymeric Materials and Composites for Additive Manufacturing

Self-healing polymers have received widespread attention due to their ability to repair damage autonomously and increase material stability, reliability, and economy. However, the processability of self-healing materials has yet to be studied, limiting the application of rich self-healing mechanisms. Additive manufacturing effectively improves the shortcomings of conventional processing while increasing production speed, accuracy, and complexity, offering great promise for self-healing polymer applications. This article summarizes the current self-healing mechanisms of self-healing polymers and their corresponding additive manufacturing methods, and provides an outlook on future developments in the field.

4D Printing Shape-Morphing Hybrid Biomaterials for Advanced Bioengineering Applications

Four-dimensional (4D) printing is an innovative additive manufacturing technology used to fabricate structures that can evolve over time when exposed to a predefined environmental stimulus. 4D printed objects are no longer static objects but programmable active structures that accomplish their functions thanks to a change over time in their physical/chemical properties that usually displays macroscopically as a shapeshifting in response to an external stimulus. 4D printing is characterized by several entangled features (e.g., involved material(s), structure geometry, and applied stimulus entities) that need to be carefully coupled to obtain a favorable fabrication and a functioning structure. Overall, the integration of micro-/nanofabrication methods of biomaterials with nanomaterials represents a promising approach for the development of advanced materials. The ability to construct complex and multifunctional triggerable structures capable of being activated allows for the control of biomedical device activity, reducing the need for invasive interventions. Such advancements provide new tools to biomedical engineers and clinicians to design dynamically actuated implantable devices. In this context, the aim of this review is to demonstrate the potential of 4D printing as an enabling manufacturing technology to code the environmentally triggered physical evolution of structures and devices of biomedical interest.

Hydroxyapatite-based carriers for tumor targeting therapy

At present, targeted drug delivery is regarded as the most effective means of tumor treatment, overcoming the lack of conventional chemotherapeutics that are difficult to reach or enter into cancer cells. Hydroxyapatite (HAP) is the main component of biological hard tissue, which can be regarded as a suitable drug carrier due to its biocompatibility, nontoxicity, biodegradation, and absorbability. This review focuses on the cutting edge of HAP as a drug carrier in targeted drug delivery systems. HAP-based carriers can be obtained by doping, modification, and combination, which benefit to improve the loading efficiency of drugs and the response sensitivity of the microenvironment in the synthesis process. The drug adsorbed or in situ loaded on HAP-based carriers can achieve targeted drug delivery and precise treatment through the guidance of the in vivo microenvironment and the stimulation of the in vitro response. In addition, HAP-based drug carriers can improve the cellular uptake rate of drugs to achieve a higher treatment effect. These advantages revealed the promising potential of HAP-based carriers from the perspective of targeted drug delivery for tumor treatment.

Research on Temperature-Switched Dopamine Electrochemical Sensor Based on Thermosensitive Polymers and MWCNTs

A temperature-controlled electrochemical sensor was constructed based on a composite membrane composed of temperature-sensitive polymer poly (N-isopropylacrylamide) (PNIPAM) and carboxylated multi-walled carbon nanotubes (MWCNTs-COOH). The sensor has good temperature sensitivity and reversibility in detecting Dopamine (DA). At low temperatures, the polymer is stretched to bury the electrically active sites of carbon nanocomposites. Dopamine cannot exchange electrons through the polymer, representing an “OFF” state. On the contrary, in a high-temperature environment, the polymer shrinks to expose electrically active sites and increases the background current. Dopamine can normally carry out redox reactions and generate response currents, indicating the “ON” state. In addition, the sensor has a wide detection range (from 0.5 μM to 150 μM) and low LOD (193 nM). This switch-type sensor provides new avenues for the application of thermosensitive polymers.

Adaptive 2D and Pseudo-2D Systems: Molecular, Polymeric, and Colloidal Building Blocks for Tailored Complexity

Two-dimensional and pseudo-2D systems come in various forms. Membranes separating protocells from the environment were necessary for life to occur. Later, compartmentalization allowed for the development of more complex cellular structures. Nowadays, 2D materials (e.g., graphene, molybdenum disulfide) are revolutionizing the smart materials industry. Surface engineering allows for novel functionalities, as only a limited number of bulk materials have the desired surface properties. This is realized via physical treatment (e.g., plasma treatment, rubbing), chemical modifications, thin film deposition (using both chemical and physical methods), doping and formulation of composites, or coating. However, artificial systems are usually static. Nature creates dynamic and responsive structures, which facilitates the formation of complex systems. The challenge of nanotechnology, physical chemistry, and materials science is to develop artificial adaptive systems. Dynamic 2D and pseudo-2D designs are needed for future developments of life-like materials and networked chemical systems in which the sequences of the stimuli would control the consecutive stages of the given process. This is crucial to achieving versatility, improved performance, energy efficiency, and sustainability. Here, we review the advancements in studies on adaptive, responsive, dynamic, and out-of-equilibrium 2D and pseudo-2D systems composed of molecules, polymers, and nano/microparticles.

Advances in Stimuli-responsive Hydrogels for Tissue Engineering and Regenerative Medicine Applications: A Review Towards Improving Structural Design for 3D Printing

The physicochemical properties of polymeric hydrogels render them attractive for the development of 3D printed prototypes for tissue engineering in regenerative medicine. Significant effort has been made to design hydrogels with desirable attributes that facilitate 3D printability. In addition, there is significant interest in exploring stimuli-responsive hydrogels to support automated 3D printing into more structurally organised prototypes such as customizable bio-scaffolds for regenerative medicine applications. Synthesizing stimuli-responsive hydrogels is dependent on the type of design and modulation of various polymeric materials to open novel opportunities for applications in biomedicine and bio-engineering. In this review, the salient advances made in the design of stimuli-responsive polymeric hydrogels for 3D printing in tissue engineering are discussed with a specific focus on the different methods of manipulation to develop 3D printed stimuli-responsive polymeric hydrogels. Polymeric functionalisation, nano-enabling and crosslinking are amongst the most common manipulative attributes that affect the assembly and structure of 3D printed bio-scaffolds and their stimuli- responsiveness. The review also provides a concise incursion into the various applications of stimuli to enhance the automated production of structurally organized 3D printed medical prototypes.

Emerging Magnetic Fabrication Technologies Provide Controllable Hierarchically-Structured Biomaterials and Stimulus Response for Biomedical Applications

Multifunctional nanocomposites which exhibit well-defined physical properties and encode spatiotemporally-controlled responses are emerging as components for advanced responsive systems. For biomedical applications magnetic nanocomposite materials have attracted significant attention due to their ability to respond to spatially and temporally varying magnetic fields. The current state-of-the-art in development and fabrication of magnetic hydrogels toward biomedical applications is described. There is accelerating progress in the field due to advances in manufacturing capabilities. Three categories can be identified: i) Magnetic hydrogelation, DC magnetic fields are used during solidification/gelation for aligning particles; ii) additive manufacturing of magnetic materials, 3D printing technologies are used to develop spatially-encoded magnetic properties, and more recently; iii) magnetic additive manufacturing, magnetic responses are applied during the printing process to develop increasingly complex structural arrangement that may recapitulate anisotropic tissue structure and function. The magnetic responsiveness of conventionally and additively manufactured magnetic hydrogels are described along with recent advances in soft magnetic robotics, and the categorization is related to final architecture and emergent properties. Future challenges and opportunities, including the anticipated role of combinatorial approaches in developing 4D-responsive functional materials for tackling long-standing problems in biomedicine including production of 3D-specified responsive cell scaffolds are discussed.

Advanced Formulations Based on Poly(ionic liquid) Materials for Additive Manufacturing

Innovation in materials specially formulated for additive manufacturing is of great interest and can generate new opportunities for designing cost-effective smart materials for next-generation devices and engineering applications. Nevertheless, advanced molecular and nanostructured systems are frequently not possible to integrate into 3D printable materials, thus limiting their technological transferability. In some cases, this challenge can be overcome using polymeric macromolecules of ionic nature, such as polymeric ionic liquids (PILs). Due to their tuneability, wide variety in molecular composition, and macromolecular architecture, they show a remarkable ability to stabilize molecular and nanostructured materials. The technology resulting from 3D-printable PIL-based formulations represents an untapped array of potential applications, including optoelectronic, antimicrobial, catalysis, photoactive, conductive, and redox applications.

Biodegradable and Light-Responsive Polymeric Nanoparticles for Environmentally Safe Herbicide Delivery

The low utilization efficiency of pesticides exerts an adverse impact on the environment and human health. Polymer-related controlled-release nanosized pesticide systems provide a promising and efficient way to overcome the problem. In this work, a biodegradable and light-responsive amphiphilic polymer was synthesized via 1,1,3,3-tetramethylguanidine-promoted polyesterification under mild conditions (low temperature, no vacuum, and no inert gas protection). We used this polymer to fabricate a light-triggered controlled-release nanosized pesticide system. The herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), was selected as a model drug to show its potential as a controlled-release pesticide system. It was found that the 2,4-D-loaded polymeric nanoparticles were stable without the treatment of UV, while the release rate of 2,4-D from the nanoparticles gradually increased after treatment with UV light. Pot trial showed that the 2,4-D-loaded polymer nanoparticles showed a good herbicidal effect. Finally, toxicity studies suggested that the polymer can reduce toxicity to nontarget organisms.

Piezoelectric nanocomposite bioink and ultrasound stimulation modulate early skeletal myogenesis

Despite the significant progress in bioprinting for skeletal muscle tissue engineering, new stimuli-responsive bioinks to boost the myogenesis process are highly desirable. In this work, we developed a printable alginate/Pluronic-based bioink including piezoelectric barium titanate nanoparticles (nominal diameter: ∼60 nm) for the 3D bioprinting of muscle cell-laden hydrogels. The aim was to investigate the effects of the combination of piezoelectric nanoparticles with ultrasound stimulation on early myogenic differentiation of the printed structures. After the characterization of nanoparticles and bioinks, viability tests were carried out to investigate three nanoparticle concentrations (100, 250, and 500 μg mL^-1) within the printed structures. An excellent cytocompatibility was confirmed for nanoparticle concentrations up to 250 μg mL^-1. TEM imaging demonstrated the internalization of BTNPs in intracellular vesicles. The combination of piezoelectric nanoparticles and ultrasound stimulation upregulated the expression of MYOD1, MYOG, and MYH2 and enhanced cell aggregation, which is a crucial step for myoblast fusion, and the presence of MYOG in the nuclei. These results suggest that the direct piezoelectric effect induced by ultrasound on the internalized piezoelectric nanoparticles boosts myogenesis in its early phases.

Strain Release Behaviour during Crack Growth of a Polymeric Beam under Elastic Loads for Self-Healing

The response of polymeric beams made of Acrylonitrile butadiene styrene (ABS) and thermoplastic polyurethane (TPU) in the form of 3D printed beams is investigated to test their elastic and plastic responses under different bending loads. Two types of 3D printed beams were designed to test their elastic and plastic responses under different bending loads. These responses were used to develop an origami capsule-based novel self-healing mechanism that can be triggered by crack propagation due to strain release in a structure. Origami capsules of TPU in the form of a cross with four small beams, either folded or elastically deformed, were embedded in a simple ABS beam. Crack propagation in the ABS beam released the strain, and the TPU capsule unfolded with the arms of the cross in the direction of the crack path, and this increased the crack resistance of the ABS beam. This increase in the crack resistance was validated in a delamination test of a double cantilever specimen under quasi-static load conditions. Repeated test results demonstrated the effect of self-healing on structural crack growth. The results show the potential of the proposed self-healing mechanism as a novel contribution to existing practices which are primarily based on external healing agents.

Artificial Intelligence-Empowered 3D and 4D Printing Technologies toward Smarter Biomedical Materials and Approaches

In the last decades, 3D printing has played a crucial role as an innovative technology for tissue and organ fabrication, patient-specific orthoses, drug delivery, and surgical planning. However, biomedical materials used for 3D printing are usually static and unable to dynamically respond or transform within the internal environment of the body. These materials are fabricated ex situ, which involves first printing on a planar substrate and then deploying it to the target surface, thus resulting in a possible mismatch between the printed part and the target surfaces. The emergence of 4D printing addresses some of these drawbacks, opening an attractive path for the biomedical sector. By preprogramming smart materials, 4D printing is able to manufacture structures that dynamically respond to external stimuli. Despite these potentials, 4D printed dynamic materials are still in their infancy of development. The rise of artificial intelligence (AI) could push these technologies forward enlarging their applicability, boosting the design space of smart materials by selecting promising ones with desired architectures, properties, and functions, reducing the time to manufacturing, and allowing the in situ printing directly on target surfaces achieving high-fidelity of human body micro-structures. In this review, an overview of 4D printing as a fascinating tool for designing advanced smart materials is provided. Then will be discussed the recent progress in AI-empowered 3D and 4D printing with open-loop and closed-loop methods, in particular regarding shape-morphing 4D-responsive materials, printing on moving targets, and surgical robots for in situ printing. Lastly, an outlook on 5D printing is given as an advanced future technique, in which AI will assume the role of the fifth dimension to empower the effectiveness of 3D and 4D printing for developing intelligent systems in the biomedical sector and beyond.

Advances in 4D printing of liquid crystalline elastomers: materials, techniques, and applications

Liquid crystalline elastomers (LCEs) are polymer networks exhibiting anisotropic liquid crystallinity while maintaining elastomeric properties. Owing to diverse polymeric forms and self-alignment molecular behaviors, LCEs have fascinated state-of-the-art efforts in various disciplines other than the traditional low-molar-mass display market. By patterning order to structures, LCEs demonstrate reversible high-speed and large-scale actuations in response to external stimuli, allowing for close integration with 4D printing and architectures of digital devices, which is scarcely observed in homogeneous soft polymer networks. In this review, we collect recent advances in 4D printing of LCEs, with emphases on synthesis and processing methods that enable microscopic changes in the molecular orientation and hence macroscopic changes in the properties of end-use objects. Promising potentials of printed complexes include fields of soft robotics, optics, and biomedical devices. Within this scope, we elucidate the relationships among external stimuli, tailorable morphologies in mesophases of liquid crystals, and programmable topological configurations of printed parts. Lastly, perspectives and potential challenges facing 4D printing of LCEs are discussed.

A Review of Multi-Material 3D Printing of Functional Materials via Vat Photopolymerization

Additive manufacturing or 3D printing of materials is a prominent process technology which involves the fabrication of materials layer-by-layer or point-by-point in a subsequent manner. With recent advancements in additive manufacturing, the technology has excited a great potential for extension of simple designs to complex multi-material geometries. Vat photopolymerization is a subdivision of additive manufacturing which possesses many attractive features, including excellent printing resolution, high dimensional accuracy, low-cost manufacturing, and the ability to spatially control the material properties. However, the technology is currently limited by design strategies, material chemistries, and equipment limitations. This review aims to provide readers with a comprehensive comparison of different additive manufacturing technologies along with detailed knowledge on advances in multi-material vat photopolymerization technologies. Furthermore, we describe popular material chemistries both from the past and more recently, along with future prospects to address the material-related limitations of vat photopolymerization. Examples of the impressive multi-material capabilities inspired by nature which are applicable today in multiple areas of life are briefly presented in the applications section. Finally, we describe our point of view on the future prospects of 3D printed multi-material structures as well as on the way forward towards promising further advancements in vat photopolymerization.

Direct-Ink Write 3D Printing Multistimuli-Responsive Hydrogels and Post-Functionalization Via Disulfide Exchange

Herein, we describe a multi-stimuli-responsive hydrogel that can be 3D printed via a direct-ink write process to afford cross-linked hydrogel networks that can be post-functionalized with thiol-bearing molecules. Poly(alkyl glycidyl ether)s with methacrylate groups at their termini were synthesized and self-assembled into hydrogels with three key stimuli-responsive behaviors necessary for extrusion based 3D printing: a sol-gel temperature response, shear-thinning behavior, and the ability to be photochemically crosslinked. In addition, the chemically crosslinked hydrogels demonstrated a temperature dependent swelling consistent with an LCST behavior. Pyridyl disulfide urethane methacrylate (PDS-UM) monomers were introduced into the network as a thiol-reactive handle for post-functionalization of the hydrogel. The reactivities of these hydrogels were investigated at different temperatures (5, 25, 37 °C) and swelling statuses (as-cured versus preswollen) using glutathione as a reactive probe. To illustrate the versatility of the platform, a number of additional thiol-containing probes such as proteins, polymers, and small molecules were conjugated to the hydrogel network at different temperatures, pH's, and concentrations. In a final demonstration of the multi-stimuli-responsive hydrogel platform, a customized DIW 3D printer was used to fabricate a printed object that was subsequently conjugated with a fluorescent tag and displayed the ability to change in size with environmental temperature.

Dual Thermo-Responsive and Strain-Responsive Ionogels for Smart Windows and Temperature/Motion Monitoring

In this work, a stretchable, dual thermo-responsive and strain-responsive ionogel has been synthesized by one-step photopolymerization. The obtained ionogel shows an ultrahigh stretchability (∼3000%), a high ionic conductivity (up to 3.1 mS/cm), and a good temperature tolerance (-40 to 300 °C). Importantly, these ionogels show an upper critical solution temperature-type phase transition with a wide tunable phase-transition temperature (17.5-42.5 °C) and reversible color/transparency switching. In particular, the as-prepared ionogel-based flexible/wearable temperature monitors and smart windows show an excellent designability and programmability, temperature modulation ability, and thermal responsiveness. Moreover, the ionogels-based strain sensors have temperature- and strain-dual responsibility and a broad strain-sensing range (1-700%), which can effectively monitor various motions. This strategy of fabricating dual thermo- and strain-responsive ionogels by using a one-step method and only one polymer holds great promise for the next generation of multifunctional stimuli-responsive materials.

Review of Electronics-Free Robotics: Toward a Highly Decentralized Control Architecture

In recent years, conventional model-based motion control has become more challenging owing to the continuously increasing complexity of areas in which robots must operate and navigate. A promising approach for solving this issue is by employing interaction-based robotics, which includes behavior-based robotics, morphological computations, and soft robotics that generate control and computation functions based on interactions between the robot body and environment. These control strategies, which incorporate the diverse dynamics of the environment to generate control and computation functions, may alleviate the limitations imposed by the finite physical and computational resources of conventional robots. However, current interaction-based robots can only perform a limited number of actions compared with conventional robots. To increase the diversity of behaviors generated from body–environment interactions, a robotic body design methodology that can generate appropriate behaviors depending on the various situations and environmental stimuli that arise from them is necessitated. Electronics-free robotics is reviewed herein as a paradigm for designing robots with control and computing functions in each part of the body. In electronics-free robotics, instead of using electrical sensors or computers, a control system is constructed based on only mechanical or chemical reactions. Robotic bodies fabricated using this approach do not require bulky electrical wiring or peripheral circuits and can perform control and computational functions by obtaining energy from a central source. Therefore, by distributing these electronics-free controllers throughout the body, we hope to design autonomous and highly decentralized robotic bodies than can generate various behaviors in response to environmental stimuli. This new paradigm of designing and controlling robot bodies can enable realization of completely electronics-free robots as well as expand the range of conventional electronics-based robot designs.

Mechanochromophore-linked Polymeric Materials with Visible Color Changes

Mechanical force as a type of stimuli for smart materials has obtained much attention in the past decade. Color-changing materials in response to mechanical stimuli have shown great potential in the applications such as sensors and displays. Mechanochromophore-linked polymeric materials, which are a growing sub-class of these materials, are discussed in detail in this review. Two main types of mechanochromophores which exhibit visible color change, summarized herein, involve either isomerization or radical generation mechanisms. This review focuses on their synthesis and incorporation into polymer matrices, the type of mechanical force used, factors affecting the mechanochromic properties, and their applications. This article is protected by copyright. All rights reserved.

Integrating Boronic Esters and Anthracene into Covalent Adaptable Networks toward Stimuli-Responsive Elastomers

Stimuli-responsive polymer materials have a promising potential application in many areas. However, integrating multi-stimuli into one elastomer is still a challenge. Here, we utilized boronic esters and anthracene to prepare a cross-linked poly(styrene-butadiene-styrene) (SBS) which was endowed with responsiveness to three stimuli (light, heat, and alcohols). SBS was first functionalized with a certain amount of dihydroxyl groups via a thiol-ene “click” reaction between unsaturated double bonds in PB block and thioglycerol. Then, 9-anthraceneboronic acid was applied to form a cross-linked SBS network upon heat and ultraviolet radiation (λ = 365 nm). The prepared elastomer was demonstrated to be stimuli-responsive based on the dynamic nature of boronic esters and the reversible dimerization of anthracene. In addition, the mechanical properties of the elastomer could be regulated continuously owing to the stimulus responsiveness to ultraviolet or heat.

Smart and Biomimetic 3D and 4D Printed Composite Hydrogels: Opportunities for Different Biomedical Applications

In recent years, smart/stimuli-responsive hydrogels have drawn tremendous attention for their varied applications, mainly in the biomedical field. These hydrogels are derived from different natural and synthetic polymers but are also composite with various organic and nano-organic fillers. The basic functions of smart hydrogels rely on their ability to change behavior; functions include mechanical, swelling, shaping, hydrophilicity, and bioactivity in response to external stimuli such as temperature, pH, magnetic field, electromagnetic radiation, and biological molecules. Depending on the final applications, smart hydrogels can be processed in different geometries and modalities to meet the complicated situations in biological media, namely, injectable hydrogels (following the sol-gel transition), colloidal nano and microgels, and three dimensional (3D) printed gel constructs. In recent decades smart hydrogels have opened a new horizon for scientists to fabricate biomimetic customized biomaterials for tissue engineering, cancer therapy, wound dressing, soft robotic actuators, and controlled release of bioactive substances/drugs. Remarkably, 4D bioprinting, a newly emerged technology/concept, aims to rationally design 3D patterned biological matrices from synthesized hydrogel-based inks with the ability to change structure under stimuli. This technology has enlarged the applicability of engineered smart hydrogels and hydrogel composites in biomedical fields. This paper aims to review stimuli-responsive hydrogels according to the kinds of external changes and t recent applications in biomedical and 4D bioprinting.

Tunable Omnidirectional Antireflection Coatings Inspired by Inclined Irregular Nanostructures on Transparent Blue-Tailed Forest Hawk Dragonfly Wings

Randomly arranged inclined irregular nanostructure-covered blue-tailed forest hawk dragonfly wings are highly transparent for wide viewing angles. Inspired by the dragonfly wings, monolayer silica colloids are self-assembled on shape memory polymer-coated substrates and utilized as plasma etching masks to pattern disorderly arranged inclined irregular conical structures. The structures build gradual refractive index transitions at various angles of incidences, resulting in omnidirectional antireflection performance over the whole visible wavelength region. In comparison with a bare substrate, the optimized structure-covered substrate presents 10% higher optical transmission at 0° and even 41% higher optical transmission at an angle of incidence of 75°. Importantly, by manipulating the structural configuration of the shape memory polymer-based structures, the corresponding antireflection characteristics can be instantaneously and reversibly eliminated and recovered after drying out of common household liquids or applying contact pressures in ambient environments. The tunable omnidirectional antireflection coatings are prospective candidates for realizing optical modulation, which exhibits an enormous application value in smart windows, intelligent display screens, optical components, and novel optoelectronic devices.

Advanced Switchable Molecules and Materials for Oil Recovery and Oily Waste Cleanup

Advanced switchable molecules and materials have shown great potential in numerous applications. These novel materials can express different states of physicochemical properties as controlled by a designated stimulus, such that the processing condition can always be maintained in an optimized manner for improved efficiency and sustainability throughout the whole process. Herein, the recent advances in switchable molecules/materials in oil recovery and oily waste cleanup are reviewed. Oil recovery and oily waste cleanup are of critical importance to the industry and environment. Switchable materials can be designed with various types of switchable properties, including i) switchable interfacial activity, ii) switchable viscosity, iii) switchable solvent, and iv) switchable wettability. The materials can then be deployed into the most suitable applications according to the process requirements. An in-depth discussion about the fundamental basis of the design considerations is provided for each type of switchable material, followed by details about their performances and challenges in the applications. Finally, an outlook for the development of next-generation switchable molecules/materials is discussed.

Preparation of flame-retardant, hydrophobic, ultraviolet protective, and luminescent transparent wood

Transparent wood with multifunctional properties has recently attracted more attention as an efficient building product. Here, we describe the development of transparent wood with long-persistent phosphorescence, tough surface, high durability, photostability, and reversibility without fatigue, and with ultraviolet shielding, superhydrophobicity, and flame-retardant activity. This long-persistent phosphorescent, or glow-in-the-dark, smart wood exhibited an ability to continue emitting light for prolonged periods of time. The photoluminescent translucent wooden substrate was prepared by immobilizing lignin-modulated wooden bulk with an admixture of methylmethacrylate (MMA), ammonium polyphosphate (APP), and lanthanide-doped strontium aluminate (LSA; SrAl₂ O₄ :Eu²⁺ ,Dy³⁺ ) phosphor nanoparticles. The photoluminescent transparent wood displayed a colour switch from colourless to bright white beneath ultraviolet (UV) light and greenish-yellow in the dark as reported by Commission Internationale de l'Éclairage laboratory colorimetric space coordinates. The generated phosphorescent wooden substrates demonstrated an absorbance band at 365 nm and an emission band at 516 nm. The phosphorescent transparent wood was improved flame-retardant properties, ultraviolet shielding, and superhydrophobic properties, as well as a reversible long-persistent phosphorescent responsiveness to UV light without fatigue. The current approach demonstrated a potential large-scale production strategy for multifunctional transparent wooden substrates for a range of applications such as smart windows, gentle indoor and outdoor lighting, and safety directional signs in buildings.

Fluorinated Metal-Organic Coatings with Selective Wettability

Surface chemistry is a major factor that determines the wettability of materials, and devising broadly applicable coating strategies that afford tunable and selective surface properties required for next-generation materials remains a challenge. Herein, we report fluorinated metal-organic coatings that display water-wetting and oil-repelling characteristics, a wetting phenomenon different from responsive wetting induced by external stimuli. We demonstrate this selective wettability with a library of metal-organic coatings using catechol-based coordination and silanization (both fluorinated and fluorine-free), enabling sensing through interfacial reconfigurations in both gaseous and liquid environments, and establish a correlation between the coating wettability and polarity of the liquids. This selective wetting performance is substrate-independent, spontaneous, durable, and reversible and occurs over a range of polar and nonpolar liquids (60 studied). These results provide insight into advanced liquid-solid interactions and a pathway toward tuning interfacial affinities and realizing robust, selective superwettability according to the surrounding conditions.

Printed miniature robotic actuators with curvature-induced stiffness control inspired by the insect wing

Stimuli-responsive actuating materials offer a promising way to power insect-scale robots, but a vast majority of these material systems are too soft for load bearing in different applications. While strategies for active stiffness control have been developed for humanoid-scale robots, for insect-scale counterparts for which compactness and functional complexity are essential requirements, these strategies are too bulky to be applicable. Here, we introduce a method whereby the same actuating material serves not only as the artificial muscles to power an insect-scale robot for load bearing, but also to increase the robot stiffness on-demand, by bending it to increase the second moment of area. This concept is biomimetically inspired by how insect wings stiffen themselves, and is realized here with manganese dioxide as a high-performing electrochemical actuating material printed on metallized polycarbonate films as the robot bodies. Using an open-electrodeposition printing method, the robots can be rapidly fabricated in one single step in ∼15 minutes, and they can be electrochemically actuated by a potential of ∼1 V to produce large bending of ∼500° in less than 5 s. With the stiffness enhancement method, fast (∼5 s) and reversible stiffness tuning with a theoretical increment by ∼4000 times is achieved in a micro-robotic arm at ultra-low potential input of ∼1 V, resulting in an improvement in load-bearing capability by about 4 times from ∼10μN to ∼41μN.

Additive manufacturing landscape and materials perspective in 4D printing

4D printing is inspired by embedded product designs to produce stimuli-responsive consumables fabricated by available commercial 3D printers. Although significant progress on smart material performance has been made and different studies have focused on new strategies and process improvements in typical additive manufacturing. Herein, the proposed review article discusses material arrangements for 4D printing, highlighting the structural evolvement mechanism, the behavior of deformation, and their prospective implementation with respect. Starting from a generalized idea, and fundamental workflow, together with a graphical manifestation of the 4D printing concept, and 4D printing for shape-memory materials (SMMs), self-fitting wearables based on shape memory alloys (SMAs) are reviewed exclusively. Furthermore, the capabilities of single and multiple materials mechanisms for shape-shifting behavior are summarized. Finally, we explored the future application potential under succeeding context: SMA-based knitted garments, transforming food, and relevant sectors wise development and proceedings with the advancement in smart materials. We determined our review by aiming our future directions such as the "dream it and make it feasible" technology.

GRAPHICAL ABSTRACT

Remote Sensing and Remote Actuation via Silicone-Magnetic Nanorod Composites

The capacity for a soft material to combine remote sensing and remote actuation is highly desirable for many applications in soft robotics and wearable technologies. This work presents a silicone elastomer with a suspension of a small weight fraction of ferromagnetic nickel nanorods, which is capable of both sensing deformation and altering stiffness in the presence of an external magnetic field. Cylinders composed of silicone elastomer and 1% by weight nickel nanorods experience large increases in compressive modulus when exposed to an external magnetic field. Incremental compressions totaling 600 g of force applied to the same silicone-nanorod composites increase the magnetic field strength measured by a Hall effect sensor enabling the material to be used as a soft load cell capable of detecting the rate, duration, and magnitude of force applied. In addition, lattice structures are 3D printed using an ink composed of silicone elastomer and 1% by weight nickel nanorods, which possess the same sensing capacity.

Efficient 3D printing via photooxidation of ketocoumarin based photopolymerization

Photopolymerization-based three-dimensional (3D) printing can enable customized manufacturing that is difficult to achieve through other traditional means. Nevertheless, it remains challenging to achieve efficient 3D printing due to the compromise between print speed and resolution. Herein, we report an efficient 3D printing approach based on the photooxidation of ketocoumarin that functions as the photosensitizer during photopolymerization, which can simultaneously deliver high print speed (5.1 cm h-1) and high print resolution (23 μm) on a common 3D printer. Mechanistically, the initiating radical and deethylated ketocoumarin are both generated upon visible light exposure, with the former giving rise to rapid photopolymerization and high print speed while the latter ensuring high print resolution by confining the light penetration. By comparison, the printed feature is hard to identify when the ketocoumarin encounters photoreduction due to the increased lateral photopolymerization. The proposed approach here provides a viable solution towards efficient additive manufacturing by controlling the photoreaction of photosensitizers during photopolymerization.

Tough Multimaterial Interfaces through Wavelength-Selective 3D Printing

Strong and well-engineered interfaces between dissimilar materials are a hallmark of natural systems but have proven difficult to emulate in synthetic materials, where interfaces often act as points of failure. In this work, curing reactions that are triggered by exposure to different wavelengths of visible light are used to produce multimaterial objects with tough, well-defined interfaces between chemically distinct domains. Longer-wavelength (green) light selectively initiates acrylate-based radical polymerization, while shorter-wavelength (blue) light results in the simultaneous formation of epoxy and acrylate networks through orthogonal cationic and radical processes. The improved mechanical strength of these interfaces is hypothesized to arise from a continuous acrylate network that bridges domains. Using printed test structures, interfaces were characterized through spatial resolution of their chemical composition, localized mechanical properties, and bulk fracture strength. This wavelength-selective photocuring of interpenetrating polymer networks is a promising strategy for increasing the mechanical performance of 3D-printed objects and expanding light-based additive manufacturing technologies.

Functional Fibers and Fabrics for Soft Robotics, Wearables, and Human-Robot Interface

Soft robotics inspired by the movement of living organisms, with excellent adaptability and accuracy for accomplishing tasks, are highly desirable for efficient operations and safe interactions with human. With the emerging wearable electronics, higher tactility and skin affinity are pursued for safe and user-friendly human-robot interactions. Fabrics interlocked by fibers perform traditional static functions such as warming, protection, and fashion. Recently, dynamic fibers and fabrics are favorable to deliver active stimulus responses such as sensing and actuating abilities for soft-robots and wearables. First, the responsive mechanisms of fiber/fabric actuators and their performances under various external stimuli are reviewed. Fiber/yarn-based artificial muscles for soft-robots manipulation and assistance in human motion are discussed, as well as smart clothes for improving human perception. Second, the geometric designs, fabrications, mechanisms, and functions of fibers/fabrics for sensing and energy harvesting from the human body and environments are summarized. Effective integration between the electronic components with garments, human skin, and living organisms is illustrated, presenting multifunctional platforms with self-powered potential for human-robot interactions and biomedicine. Lastly, the relationships between robotic/wearable fibers/fabrics and the external stimuli, together with the challenges and possible routes for revolutionizing the robotic fibers/fabrics and wearables in this new era are proposed.

An Integrated Design of a Polypseudorotaxane-Based Sea Cucumber Mimic

The development of integrated systems that mimic the multi-stage stiffness change of marine animals such as the sea cucumber requires the design of molecularly tailored structures. Herein, we used an integrated biomimicry design to fabricate a sea cucumber mimic using sidechain polypseudorotaxanes with tunable nano-to-macroscale properties. A series of polyethylene glycol (PEG)-based sidechain copolymers were synthesized to form sidechain polypseudorotaxanes with α-cyclodextrins (α-CDs). By tailoring the copolymers' molecular weights and their PEG grafting densities, we rationally tuned the sizes of the formed polypseudorotaxanes crystalline domain and the physical crosslinking density of the hydrogels, which facilitated 3D printing and the mechanical adaptability to these hydrogels. After 3D printing and photo-crosslinking, the obtained hydrogels exhibited large tensile strain and broad elastic-to-plastic variations upon α-CD (de)threading. These discoveries enabled a successful fabrication of a sea cucumber mimic, demonstrating multi-stage stiffness changes.