Degradable and Fully Recyclable Dynamic Thermoset Elastomer for 3D-Printed Wearable Electronics

Wearable electronics have become an important part of daily lives. However, its rapid development results in the problem of electronic waste (e-waste). Consequently, recyclable materials suitable for wearable electronics are highly sought after. In this study, a conductive recyclable composite (PFBC) is designed based on a dynamic covalently cross-linked elastomer and hierarchical hybrid nanofillers. The PFBC shows excellent wide-ranging properties including processability, elasticity, conductivity, and stability, which are superior to previous materials used for recyclable electronics, and exhibits outstanding mechanical properties and environmental tolerance including high temperature, high humidity, brine, and ethanol owing to its covalent cross-linking. Reversible dissociation of Diels–Alder networks allows for convenient processing and recycling. After three recycles, the toughness of the PFBC remained at 10.1 MJ m⁻³, which is conspicuous among the reported recyclable electronic materials. Three types of PFBC-based wearable electronics including a triboelectric nanogenerator, a capacitive pressure sensor, and a flexible keyboard, are successfully 3D printed with excellent performance. The PFBC possessed both recyclability and degradability, the combination of which provides a new way to reduce e-waste. This is the first work to recycle electronics using direct 3D printing and presents promising new design principles and materials for wearable electronics.     

A Tailorable Family of Elastomeric‐to‐Rigid, 3D Printable,Interbonding Polymer Networks

Soft materials with widely tailorable mechanical properties throughout the material's volume can shape the future of soft robotics and wearable electronics, impacting both consumer and defense sectors. Herein, a platform of 3D printable soft polymer networks with unprecedented tunability of stiffness of nearly three orders of magnitude (MPa to GPa) and an inherent capability to interbond is reported. The materials are based on dynamic covalent polymer networks with variable density of crosslinkers attached to prepolymer backbones via a temperature‐reversible Diels–Alder (DA) reaction. Inherent flexibility of the prepolymer chains and controllable crosslinking density enable 3D printed networks with glass transition temperatures ranging from just a few degrees to several tens of degrees Celsius. Materials with an elastomeric network demonstrate a fast and spontaneous self‐healing behavior at room temperature both in air and under water—a behavior difficult to achieve with other crosslinked materials. Reversible dissociation of DA networks at temperatures exceeding ≈120 °C allows for reprintability, while control of the stereochemistry of DA attachments enables reprogrammable shape memory behavior. The introduced platform addresses current major challenges including control of polymer interbonding, enhanced mechanical performance of printed parts, and reprocessability of 3D‐printed crosslinked materials in the absence of solvent.     

New Dielectric Elastomers with Variable Moduli

Dielectric elastomers have been widely investigated for muscle‐like soft actuators and capacitive sensors. Mechanical properties play a central role in the performances of the active material. Most elastomers have specific moduli pre‐determined by the polymers' molecular structures, which are not suitable for applications in changing working conditions as natural muscles are capable of. Here new dielectric elastomers are described exhibiting variable moduli controlled via thermal treatment. The elastomers contain furan‐maleimide Diels–Alder adduct moieties to administer the crosslinking densities of the elastomeric networks via reversible Diels–Alder/retro‐Diels–Alder cycloaddition reaction, resulting in changes in the elastomers' moduli. One of the synthesized elastomers has moduli that can be controlled between 0.17 and 0.52 MPa incrementally and reversibly. Capacitive strain sensors based on this elastomer can be operated in both rigid and soft modes to achieve variable sensing response up to 30% linear strain. Actuators were fabricated and operated in both high strain mode (35% actuation area strain at 65 MV m^?1) and high force output mode (0.55 MPa at 104 MV m^?1). The elastomers can exhibit a range of stress–strain outputs in similar fashion as muscle.     

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.     

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.     

Scaffolded Thermally Remendable Hybrid Polymer Networks

Step‐growth Diels–Alder (DA) networks using furan and maleimide groups are particularly useful in forming thermally remendable crosslinked polymers, due to the dramatic shift in equilibrium over a relatively low temperature range as compared with other diene‐dienophile pairs. However, the efficient healing observed in these materials at high temperature is directly tied to their ability to depolymerize and flow, and thermal treatment often results in deformation of the original shape. To overcome this limitation, a hybrid network material is developed, which consists of orthogonal Diels–Alder and polyurethane networks. Both step‐growth networks form simultaneously at elevated temperature without the presence of a catalyst. At high temperatures, the Diels–Alder network depolymerizes and flows into fractures through capillary action, while the polyurethane serves as a scaffold to maintain the overall shape of the sample. The DA network then repolymerizes at lower temperatures, creating a crosslinked, scar‐like “patch” throughout the crack. This healing process is repeatable without concern of monomer depletion. During heating through the glass transition, a shape memory “assist” is observed, which reverses some of the localized damage by bringing broken edges closer together. Samples are repeatedly damaged and then healed through temperature cycling, as evidenced through tensile fracture tests and electrochemical conductivity tests.     

Printed Transistors on Paper: Towards Smart Consumer Product Packaging

The integration of fully printed transistors on low cost paper substrates compatible with roll‐to‐roll processes is demonstrated here. Printed electronics promises to enable a range of technologies on paper including printed sensors, RF tags, and displays. However, progress has been slow due to the paper roughness and ink absorption. This is solved here by employing gravure printing to print local smoothing pads that also act as an absorption barrier. This innovative local smoothing process retains desirable paper properties such as foldability, breathability, and biodegradability outside of electronically active areas. Atomic force microscopy measurements show significant improvements in roughness. The polymer ink and printing parameters are optimized to minimize ink absorption and printing artifacts when printing the smoothing layer. Organic thin film transistors (OTFT) are fabricated on top of this locally smoothed paper. OTFTs exhibit performance on par with previously reported printed transistors on plastic utilizing the same materials system (pBTTT semiconductor, poly‐4‐vinylphenol dielectric). OTFTs deliver saturation mobility approaching 0.1 cm²V^–1s^–1 and on‐off‐ratio of 3.2 × 10⁴. This attests to the quality of the local smoothing, and points to a promising path for realizing electronics on paper.     

Materials Fabrication from Native and Recombinant Thermoplastic Squid Proteins

Natural elastomers made from protein extracts have received significant interest as eco‐friendly functional materials due to their unique mechanical and optical properties emanating from secondary structures. The next generation sequencing approach is used to identify protein sequences in a squid ring teeth complex extracted from Loligo vulgaris and the use of recombinant expression is demonstrated in the fabrication of a new generation of thermoplastic materials. Native and recombinant thermoplastic squid proteins exhibit reversible solid to melt phase transition, enabling them to be thermally shaped into 3D geometries such as fibers, colloids, and thin films. Direct extraction or recombinant expression of protein based thermoplastics opens up new avenues for materials fabrication and synthesis, which will eventually be competitive with the high‐end synthetic oil based plastics.     

Room‐Temperature Printing of Organic Thin‐Film Transistors with π‐Junction Gold Nanoparticles

Printing semiconductor devices under ambient atmospheric conditions is a promising method for the large‐area, low‐cost fabrication of flexible electronic products. However, processes conducted at temperatures greater than 150 °C are typically used for printed electronics, which prevents the use of common flexible substrates because of the distortion caused by heat. The present report describes a method for the room‐temperature printing of electronics, which allows thin‐film electronic devices to be printed at room temperature without the application of heat. The development of π‐junction gold nanoparticles as the electrode material permits the room‐temperature deposition of a conductive metal layer. Room‐temperature patterning methods are also developed for the Au ink electrodes and an active organic semiconductor layer, which enables the fabrication of organic thin‐film transistors through room‐temperature printing. The transistor devices printed at room temperature exhibit average field‐effect mobilities of 7.9 and 2.5 cm² V^?1 s^?1 on plastic and paper substrates, respectively. These results suggest that this fabrication method is very promising as a core technology for low‐cost and high‐performance printed electronics.     

Patterned Carbon Nitride–Based Hybrid Aerogel Membranes via 3D Printing for Broadband Solar Wastewater Remediation

Controlled scalable assembly of 2D building blocks into macroscopic 3D architectures is highly significant. However, the assembly of g‐C₃N₄ into tailored, 3D architectures is not yet reported. Here, a 3D printing methodology to enable the programmable construction of carbon nitride–based hybrid aerogel membranes with patterned macroscopic architectures is proposed. g‐C₃N₄ nanosheets (CNNS) are used as the building block, and sodium alginate (SA) increases the viscosity of the ink to obtain the desired rheological properties. Three printing routes, including printing directly in air and in the supporting reservoirs composed of CaCl₂/glycerol solution or Pluronic F127, are demonstrated for printing versatility. The printed Au nanobipyramid–CNNS–SA hybrid aerogels exhibit broadband visible‐light absorption and superior solar wastewater remediation performance with excellent cyclic stability and easy manipulation features. Remarkably, the activity of the 3D‐printed aerogel is about 2.5 times of that of the contrast sample, attributing to the enhanced liquid velocity and solution diffusion efficiency because of the 3D‐printed structure, which is demonstrated by experimental and theoretical simulations. This approach can be extended to the macroscopic assembly of other 2D materials for myriad applications.     

Self‐Healing Materials via Reversible Crosslinking of Poly(ethylene oxide)‐Block‐Poly(furfuryl glycidyl ether) (PEO‐b‐PFGE) Block Copolymer Films

The application of well‐defined poly(furfuryl glycidyl ether) (PFGE) homopolymers and poly(ethylene oxide)‐b‐poly(furfuryl glycidyl ether) (PEO‐b‐PFGE) block copolymers synthesized by living anionic polymerization as self‐healing materials is demonstrated. This is achieved by thermo‐reversible network formation via (retro) Diels‐Alder chemistry between the furan groups in the side‐chain of the PFGE segments and a bifunctional maleimide crosslinker within drop‐cast polymer films. The process is studied in detail by differential scanning calorimetry (DSC), depth‐sensing indentation, and profilometry. It is shown that such materials are capable of healing complex scratch patterns, also multiple times. Furthermore, microphase separation within PEO‐b‐PFGE block copolymer films is indicated by small angle X‐ray scattering (lamellar morphology with a domain spacing of approximately 19 nm), differential scanning calorimetry, and contact angle measurements.     

Development of Thermoinks for 4D Direct Printing of Temperature-Induced Self-Rolling Hydrogel Actuators

4D printing has emerged as an important technique for fabricating 3D objects from programmable materials capable of time-dependent reshaping. In the present investigation, novel 4D thermoinks composed of laponite (LAP), an interpenetrating network of poly(N-isopropylacrylamide) (PNIPAAm), and alginate (ALG) are developed for direct printing of shape-morphing structures. This approach consists of the design and fabrication of 3D honeycomb-patterned hydrogel discs self-rolling into tubular constructs under the stimulus of temperature. The shape morphing behavior of hydrogels is due to shear-induced anisotropy generated via 3D printing. The compositionally tunable hydrogel discs can be programmed to exhibit different actuation behaviors at different temperatures. Upon immersion in 12 °C water, singly crosslinked sheets roll up into a tubular construct. When transferred to 42 °C water, the tubes first rapidly unfold and then slightly curve up in the opposite direction. Through a dual photocrosslinking of PNIPAAm, it is possible to inverse temperature-dependent shape morphing and induce self-folding at higher and unrolling at lower temperatures. The extensive self-assembling motion is essential to developing thermal actuators with broad applications in, e.g., soft robotics and active implantology, whereas controllable self-rolling of planar hydrogels is of the highest interest to biomedical engineering as it allows for effective fabrication of hollow tubes.     

Exploiting Chemical Switching in a Diels–Alder Polymer for Nanoscale Probe Lithography and Data Storage

Reversibly crosslinked polymer films have properties that are beneficial to scanned‐probe data storage and lithographic applications that use thermomechanical nanoindentation as a write or expose mechanism. The novel polymer under study contains linkages based on thermally reversible Diels–Alder crosslinking. Thermomechanical properties on the nanometer scale are analyzed by indentation experiments on polymer thin films using heated tips. The underlying indentation mechanism is studied at varying tip temperatures and indentation times, revealing Arrhenius kinetics. This is in contrast to the Williams–Landau–Ferry kinetics usually observed for polymer systems. The discrepancy is explained by the reversible crosslinking incorporated into the structure of the polymer that allows switching between two different states: a rigid, highly crosslinked, low‐temperature state, and a deformable, fragmented, high‐temperature state. An individual indentation volume of less than 10^–20 L (10 000 molecule pairs) is estimated. These kinetics experiments demonstrate that a chemical reaction of only a few thousand molecules can be transduced into a mechanically measurable action. The ability to cycle between two sets of properties in these materials opens up new perspectives in lithography and data storage. Examples of data storage with densities up to 1 Tb in.^–2 and maskless lithography with resolution below 20 nm are demonstrated at writing times of 10 μs per bit/pixel.     

Complex‐Shaped Cellulose Composites Made by Wet Densification of 3D Printed Scaffolds

Cellulose is an attractive material resource for the fabrication of sustainable functional products, but its processing into structures with complex architecture and high cellulose content remains challenging. Such limitation has prevented cellulose‐based synthetic materials from reaching the level of structural control and mechanical properties observed in their biological counterparts, such as wood and plant tissues. To address this issue, a simple approach is reported to manufacture complex‐shaped cellulose‐based composites, in which the shaping capabilities of 3D printing technologies are combined with a wet densification process that increases the concentration of cellulose in the final printed material. Densification is achieved by exchanging the liquid of the wet printed material with a poor solvent mixture that induces attractive interactions between cellulose particles. The effect of the solvent mixture on the final cellulose concentration is rationalized using solubility parameters that quantify the attractive interparticle interactions. Using X‐ray diffraction analysis and mechanical tests, 3D printed composites obtained through this process are shown to exhibit highly aligned microstructures and mechanical properties significantly higher than those obtained by earlier additively manufactured cellulose‐based materials. These features enable the fabrication of cellulose‐rich synthetic structures that more closely resemble the exquisite designs found in biological materials grown by plants in nature.     

Wearable Supercapacitors Printed on Garments

Electronic garments have garnered considerable attention as a core technology for the upcoming wearable electronics era. To enable ubiquitous operation of electronic garments, they must be monolithically integrated with rechargeable power sources. Here, inspired by printing‐assisted aesthetic clothing designs, a new class of wearable supercapacitors (SCs) is demonstrated that can be directly printed on T‐shirts, which look like letters (or symbols) commonly printed on T‐shirts. The printed SCs consist of activated carbon/multiwalled carbon nanotube/ionic liquid‐based electrodes and ionic liquid/thiol‐ene polymer network skeleton/SiO₂ nanoparticle‐based gel electrolytes. The rheological properties of the electrode/electrolyte pastes are fine‐tuned by varying the colloidal network structure, which affects the printing processability and formation of the nanoscale ion/electron conduction channels. To ensure the seamless unitization and design versatility of the printed SCs, the T‐shirt is sewn with electroconductive stainless steel (SS) threads prior to the printing process. Onto the SS threads acting as shape‐directing current collectors, the electrode/electrolyte pastes are sequentially stencil‐printed and sealed with water‐proof packaging films. The printed SCs exhibit exceptional form factors, flexibility, and thermal stability. Notably, the SC‐printed T‐shirts maintain their electrochemical activity upon exposure to laundering, wringing, ironing, and folding, demonstrating their potential and practical applicability as a promising electronic garment technology.     

Inkjet Printing of Self‐Assembled 2D Titanium Carbide and Protein Electrodes for Stimuli‐Responsive Electromagnetic Shielding

2D titanium carbides (MXene) possess significant characteristics including high conductivity and electromagnetic interference shielding efficiency (EMI SE) that are important for applications in printed and flexible electronics. However, MXene‐based ink formulations are yet to be demonstrated for proper inkjet printing of MXene patterns. Here, tandem repeat synthetic proteins based on squid ring teeth (SRT) are employed as templates of molecular self‐assembly to engineer MXene inks that can be printed as stimuli‐responsive electrodes on various substrates including cellulose paper, glass, and flexible polyethylene terephthalate (PET). MXene electrodes printed on PET substrates are able to display electrical conductivity values as high as 1080 ± 175 S cm^?1, which significantly exceeds electrical conductivity values of state‐of‐the‐art inkjet‐printed electrodes composed of other 2D materials including graphene (250 S cm^?1) and reduced graphene oxide (340 S cm^?1). Furthermore, this high electrical conductivity is sustained under excessive bending deformation. These flexible electrodes also exhibit effective EMI SE values reaching 50 dB at films with thicknesses of 1.35 µm, which mainly originate from their high electrical conductivity and layered structure.     

Highly Efficient Diels–Alder Crosslinkable Electro‐Optic Dendrimers for Electric‐Field Sensors

One of the most challenging tasks encountered in developing highly efficient electro‐optic (EO) devices is to find a material system that possesses all desirable properties such as large EO coefficients, good thermal and mechanical stability, and low optical loss. In order to meet this stringent requirement, we have developed a series of crosslinkable EO dendrimers using the standardized AJL8 ‐type chromophore as the center core and the furyl‐ and anthryl‐containing dendrons as the periphery. Upon adding a trismaleimide ( TMI ) dienophile, these dendrimers could be in‐situ crosslinked via the Diels–Alder cycloaddition and efficiently poled under a high electric field. Through this dynamic process, the spatially voided and π‐electron‐rich surrounding of the chromophore core changes into a dense and more aliphatic network, with the dipolar chromophore embedded and aligned inside. The resultant materials exhibit large EO coefficients (63–99 pm V^–1 at 1.31 μm), excellent temporal stability (the original r₃₃ values remain unchanged at 100 °C for more than 500 h), and blue‐shifted near‐IR absorption. With these combined desirable properties, a poled EOD2/TMI film could be processed through multiple lithographic and etching steps to fabricate a racetrack‐shaped micro‐ring resonator. By coupling this ring resonator with a side‐polished optical fiber, a novel broadband electric‐field sensor with high sensitivity of 100 mV m^–1 at 550 MHz was successfully demonstrated.     

A Dynamically Hybrid Crosslinked Elastomer for Room-Temperature Recyclable Flexible Electronic Devices

The rapid development of flexible electronics has resulted in serious pollution in the form of electronic waste. Accordingly, recyclability is highly desirable for these devices, but this remains a significant challenge. A dynamically hybrid crosslinked polyurethane (FPU) elastomer is designed in this study to address this challenge. Distinctive Diels–Alder adducts with suitable dissociation and reassociation dynamics are designed as crosslinking units to provide an efficient time frame for recycling. FPU is maintained in a state with a low crosslinking density after heating at 120 °C for 5 min. FPU-based electronics can therefore be dissolved in chloroform under ambient conditions to separate the electronic components and polymers for the refabrication of new electronic devices. This is the first reported thermoset elastomer that can be completely recycled at room temperature without chemical treatment to decompose the polymer chain. The design concept is applied by demonstrating the fabrication by recycling of different FPU-based flexible electronic devices: position sensor, flexible keyboard, and motion sensor. Furthermore, the FPU has many advantages as a material for flexible electronics in terms of its biomimetic mechanical properties, room-temperature self-healing, and facile processability. This study provides promising new design principles to develop materials for promoting sustainable flexible electronics.     

3D Printing of Interdigitated Dielectric Elastomer Actuators

Dielectric elastomer actuators (DEAs) are soft electromechanical devices that exhibit large energy densities and fast actuation rates. They are typically produced by planar methods and, thus, expand in‐plane when actuated. Here, reported is a method for fabricating 3D interdigitated DEAs that exhibit in‐plane contractile actuation modes. First, a conductive elastomer ink is created with the desired rheology needed for printing high‐fidelity, interdigitated electrodes. Upon curing, the electrodes are then encapsulated in a self‐healing dielectric matrix composed of a plasticized, chemically crosslinked polyurethane acrylate. 3D DEA devices are fabricated with tunable mechanical properties that exhibit breakdown fields of 25 V µm^?1 and actuation strains of up to 9%. As exemplars, printed are prestrain‐free rotational actuators and multi‐voxel DEAs with orthogonal actuation directions in large‐area, out‐of‐plane motifs.     

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.