Multi-Stimuli-Responsive Weldable Bilayer Actuator with Programmable Patterns and 3D Shapes

Soft actuators can harvest environmental energy and convert it into kinetic energy for motions like bending, twisting, stretching, and contracting. However, it remains challenging to design soft film actuators for complex and programmable deformation in three dimensions. Herein, a weldable and patternable multi-stimuli-responsive bilayer soft actuator is developed by a mask-assisted spraying coating process, and its 3D geometries are achieved by welding the sodium alginate (SA) layer using water. The intrinsic hygroscopicity of SA film and the magnetic and photothermal properties of Fe₃O₄ nanoparticles enable reversible deformation of the bilayer actuator under three different external stimuli: moisture, magnetic field, and sunlight. Based on these properties, a variety of multi-stimuli-responsive intelligent devices are developed including smart curtains, smart grippers, biomimetic walkers, rolling actuators, swimmers, and windmill rotators. All these actuating stimulations are derived from naturally renewable energy without the consumption of any artificial energy, providing important enlightenment for green and sustainable applications of soft actuators.     

Ultrasensitive and Stable Au Dimer‐Based Colorimetric Sensors Using the Dynamically Tunable Gap‐Dependent Plasmonic Coupling Optical Properties

A novel Au dimer‐based colorimetric sensor is reported that consists of Au dimers to a chitosan hydrogel film. It utilizes the ultrasensitively gap‐dependent properties of plasmonic coupling (PC) peak shift, which is associated with the dynamical tuning of the interparticle gap of the Au dimer driven by the volume swelling of the chitosan hydrogel film. The interparticle gap and PC peak shift of the Au dimer can be precisely and extensively controlled through the pH‐driven volume change of chitosan hydrogel film. This colorimetric sensor exhibits a high optical sensitivity and stability, and it works in a completely reversible manner at high pH values. Importantly, the sensitivity of the composite film can be tuned by controlling the crosslinking time of the composite film, and thus leading to a wide dynamic tuning sensitive range for different applications. This presented strategy paves a way to achieve the construction of high‐quality colorimetric sensors with ultrahigh sensitivity, stability and wide dynamic tuning sensitive range.     

Light‐Induced Swelling‐Responsive Conductive,Adhesive, and Stretchable Wireless Film Hydrogel as Electronic Artificial Skin

Light‐induced wireless soft electronic skin hydrogels with excellent mechanical and electronic properties are important for several applications, such as soft robotics and intelligent wearable devices. Precise control of reversible stretchability and capacitive properties depending on intermolecular interaction and surface characteristics remains a challenge. Here, a thin‐film hydrogel is designed based on titanium oxide (TiO₂) polydopamine–perfluorosilica carbon dot‐conjugated chitosan–polyvinyl alcohol‐loaded tannic acid with controllable hydrophobic–hydrophilic transition in the presence of UV–vis light irradiation. The shifting of surface wettability from hydrophobic to hydrophilic by irradiation affects thin‐film water permeability and swelling ratio. This allows the penetration of water into the matrix to change its mechanical strength, electronic properties, and adhesive behavior. Specifically, the hydrogel displays mechanical strain as high as 278% in response to light stimuli and demonstrates the ability to regain its initial state determining the elasticity of the fabricated material. Moreover, the thin‐film hydrogel shows an increase in conductivity to 1.096 × 10^?3 and 1.026 × 10^?3 S cm^?1 when irradiated with UV and visible light, respectively. The hydrogel exhibits capacitive reversibility that follows finger motion which can be identified directly or remotely using wireless connection, indicative of its possible applications as an artificial electronic skin.     

Surface Patterning of Hydrogels for Programmable and Complex Shape Deformations by Ion Inkjet Printing

Convenient patterning and precisely programmable shape deformations are crucial for the practical applications of shape deformable hydrogels. Here, a facile and versatile computer‐assisted ion inkjet printing technique is described that enables the direct printing of batched, very complicated patterns, especially those with well‐defined, programmable variation in cross‐linking densities, on one or both surfaces of a large‐sized hydrogel sample. A mechanically strong hydrogel containing poly(sodium acrylate) is first prepared, and then digital patterns are printed onto the hydrogel surfaces by using a commercial inkjet printer and an aqueous ferric solution. The complexation between the polyelectrolyte and ferric ions increases the cross‐linking density of the printed regions, and hence the gel sample can undergo shape deformation upon swelling/deswelling. The deformation rates and degrees of the hydrogels can be conveniently adjusted by changing the printing times or the different/gradient grayscale distribution of designed patterns. By printing appropriate patterns on one or both surfaces of the hydrogel sheets, many complex 3D shapes are obtained from shape deformations upon swelling/deswelling, such as cylindrical shell and forsythia flower (patterns on one surface), ding (patterns on both surfaces), blooming flower (different/gradient grayscale distributive patterns on one surface), and non‐Euclidean plates (different/gradient grayscale distributive patterns on both surfaces).     

Novel Biocompatible Polysaccharide‐Based Self‐Healing Hydrogel

A novel biocompatible polysaccharide‐based self‐healing hydrogel, CEC‐l‐OSA‐l‐ADH hydrogel (“l” means “linked‐by”), is developed by exploiting the dynamic reaction of N‐carboxyethyl chitosan (CEC) and adipic acid dihydrazide (ADH) with oxidized sodium alginate (OSA). The self‐healing ability, as demonstrated by rheological recovery, macroscopic observation, and beam‐shaped strain compression measurement, is attributed to the coexistence of dynamic imine and acylhydrazone bonds in the hydrogel networks. The CEC‐l‐OSA‐l‐ADH hydrogel shows excellent self‐healing ability under physiological conditions with a high healing efficiency (up to 95%) without need for any external stimuli. In addition, the CEC‐l‐OSA‐l‐ADH hydrogel exhibits good cytocompatibility and cell release as demonstrated by three‐dimensional cell encapsulation. With these superior properties, the developed hydrogel holds great potential for applications in various biomedical fields, e.g., as cell or drug delivery carriers.     

Controlled Degradability of Polysaccharide Multilayer Films In Vitro and In Vivo

This article demonstrates the possibility of tuning the degradability of polysaccharide multilayer films in vitro and in vivo. Chitosan and hyaluronan multilayer films (CHI/HA) were either native or crosslinked using a water soluble carbodiimide, 1‐ethyl‐3‐(3‐dimethylamino‐propyl)carbodiimide (EDC) at various concentrations in combination with N‐hydroxysulfosuccinimide. The in‐vitro degradation of the films in contact with lysozyme and hyaluronidase was followed by quartz crystal microbalance measurements, fluorimetry, and confocal laser scanning microscopy after labeling of the chitosan with fluorescein isothiocyanate (CHI^FITC). The native films were subjected to degradation by these enzymes, and the crosslinked films were more resistant to enzymatic degradation. Films made of chitosan of medium molecular weight were more resistant than films made of chitosan‐oligosaccharides. The films were also brought in contact with plasma, which induced a change in film structure for the native film but did not have any effect on the crosslinked film. The in‐vitro study shows that macrophages can degrade all types of films and internalize the chitosan. The in‐vivo degradation of the films implanted in mouse peritoneal cavity for a week again showed an almost complete degradation of the native films, whereas the crosslinked films were only partially degraded. Taken together, these results suggest that polysaccharide multilayer films are of potential interest for in‐vivo applications as biodegradable coatings, and that degradation can be tuned by using chitosan of different molecular weights and by controlling film crosslinking.     

Reversible Programing of Soft Matter with Reconfigurable Mechanical Properties

Biology uses various cross‐linking mechanisms to tailor material properties, and this is inspiring technological efforts to couple independent cross‐linking mechanisms to create hydrogels with complex mechanical properties. Here, it is reported that a hydrogel formed from a single polysaccharide can be triggered to reversibly switch cross‐linking mechanisms and switch between elastic and viscoelastic properties. Specifically, the pH‐responsive self‐assembling aminopolysaccharide chitosan is used. Under acidic conditions, chitosan is polycationic and can be electrostatically cross‐linked by sodium dodecyl sulfate (SDS) micelles to confer viscoelastic and self‐healing properties. Under basic conditions, chitosan becomes neutral, the electrostatic SDS–chitosan interactions are no longer operative, and chitosan chains can self‐assemble to form crystalline network junctions that serve as strong physical cross‐links that confer elastic properties. Mechanical measurements performed in water demonstrate these different mechanical behaviors and the repeated pH‐induced switching between these behaviors. Printing of SDS micelles onto a neutral chitosan film allows the cross‐linking mechanisms to be spatially programed to confer anisotropic mechanical properties. The reversibility of these cross‐linking mechanisms allows the patterned films to be erased and reprogramed with reconfigured mechanical properties. Potentially, the ability to reversibly program hydrogel networks enables fabrication of the dynamically reconfigurable networks required for soft machines.     

Biomorphic Actuation Driven Via On-Chip Buckling of Photoresponsive Hydrogel Films

Remotely controllable photoresponsive hydrogel actuators are promising for applications in multiple fields. However, simple deformation mechanisms, which rely on the general swelling/deswelling, limit their performance and application. Herein, we report a displacement amplification mechanism based on the buckling deformation of photoresponsive hydrogel film. The on-chip buckled architecture of the hydrogel enables actuation between a flat 2D shape and tubular 3D buckled shape with remarkable performances, including high deformation ratio (height ratio: ≈360%), tunable cycle motion frequency (0.1—1 Hz) and high cyclic stability. Moreover, localized buckling deformation, such as tube opening and closing, can be controlled in response to photostimulation. Inspired by these biomorphic shapes and motions, an intestine-mimetic device and demonstrate segmentation with substance crushing and peristalsis motion with substance propelling were further fabricated. This study will provide a useful design principle for hydrogel actuators and shed light on diverse applications in soft robotics, dynamic microfluidics and organs-on-chips.     

Redox‐Active Iron‐Citrate Complex Regulated Robust Coating‐Free Hydrogel Microfiber Net with High Environmental Tolerance and Sensitivity

Stretchable hydrogel microfibers as a novel type of ionic conductors are promising in gaining skin‐like sensing applications in more diverse scenarios. However, it remains a great challenge to fabricate coating‐free but water‐retaining conductive hydrogel microfibers with a good balance of spinnability and mechanical strength. Here the old yet significant redox chemistry of Fe‐citrate complex is employed to solve this issue in the continuous draw‐spinning process of poly(acrylamide‐co‐sodium acrylate) hydrogel microfibers and microfiber nets from a water/glycerol solution. The resultant microfibers are ionically conductive, highly stretchable, and uniform with tunable diameters. Furthermore, the presence of redox‐reversible Fe‐citrate complex and glycerol endows the fibers with good anti‐freezing, water‐retaining, and environmentally intelligent properties. Humidity and UV light can finely mediate the stiffness of hydrogel microfibers; conversely, the ionic conductance of microfibers is also responsive to light, humidity, and strain, which enables the highly sensitive perception of environmental changes. The present draw‐spinning strategy provides more possibilities for coating‐free conductive hydrogel microfibers with a variety of responsive and sensing applications.     

In‐Film Bioprocessing and Immunoanalysis with Electroaddressable Stimuli‐Responsive Polysaccharides

Advances in thin‐film fabrication are integral to enhancing the power of microelectronics while fabrication methods that allow the integration of biological molecules are enabling advances in bioelectronics. A thin‐film‐fabrication method that further extends the integration of biology with microelectronics by allowing living biological systems to be assembled, cultured, and analyzed on‐chip with the aid of localized electrical signals is described. Specifically, the blending of two stimuli‐responsive film‐forming polysaccharides for electroaddressing is reported. The first, alginate, can electrodeposit by undergoing a localized sol–gel transition in response to electrode‐imposed anodic signals. The second, agarose, can be co‐deposited with alginate and forms a gel upon a temperature reduction. Electrodeposition of this dual polysaccharide network is observed to be a simple, rapid, and spatially selective means for assembly. The bioprocessing capabilities are examined by co‐depositing a yeast clone engineered to display a variable lymphocyte receptor protein on the cell surface. Results demonstrate the in‐film expansion and induction of this cell population. Analysis of the cells' surface proteins is achieved by the electrophoretic delivery of immunoreagents into the film. These results demonstrate a simple and benign means to electroaddress hydrogel films for in‐film bioprocessing and immunoanalysis.     

Graphene‐Directed Supramolecular Assembly of Multifunctional Polymer Hydrogel Membranes

Polymer‐based nanoporous hydrogel membranes hold great potential for a range of applications including molecular filtration/separation, controlled drug release, and as sensors and actuators. However, to be of practical utility, polymer membranes generally need to be fabricated as ultrathin yet mechanically robust, have a large‐area yet be defect‐free and in some cases, their structure needs the capability to adapt to certain stimuli. These stringent and sometimes self‐conflicting requirements make it very challenging to manufacture such bulk nanostructures in a controllable, scalable and cost‐effective manner. Here, a versatile approach to the fabrication of multifunctional polymer‐based hydrogel membranes is demonstrated by a single step involving filtration of an aqueous dispersion containing chemically converted graphene (CCG) and a polymer. With CCG uniquely serving as a membrane‐ and pore‐forming directing agent and as a physical cross‐linker, a range of water soluble polymers can be readily processed into nanoporous hydrogel membranes through supramolecular interactions. With the interconnected CCG network as a robust and porous scaffold, the membrane nanostructure can easily be fine‐tuned to suit different applications simply by controlling the chemistry and concentration of the incorporated polymer. This work provides a simple and versatile platform for the design and fabrication of new adaptive supramolecular membranes for a variety of applications.     

Bio‐Inspired,Water‐Soluble to Insoluble Self‐Conversion for Flexible,Biocompatible, Transparent,Catecholamine Polysaccharide Thin Films

In nature, a variety of functional water‐insoluble organic materials are biologically synthesized in aqueous conditions without chemical additives and organic solvents. Insect cuticle, crustacean shells, and many others are representative examples. The insoluble materials are prepared by enzyme reactions and programmed self‐assembly in water from water‐soluble precursors. If the water‐basis could be adapted, environment‐friendly strategy developed in nature, many problems caused by the vast consumption of petroleum‐based olefin materials could be solved or significantly attenuated. Here, the spontaneous formation of water‐insoluble, biocompatible films from a water‐soluble polymer is demonstrated without using any chemical additives and organic solvents. It is found that a water‐soluble chitosan–catechol polymeric precursor is spontaneously self‐converted to flexible water‐insoluble thin film by simple dehydration. The preparation of mechanically robust, water‐insoluble, flexible, transparent chitosan–catechol film is a completely unexpected result because most water‐soluble polymers exist as powders when dehydrated. The film can be used as a bag similar to polyvinyl one and is multifunctional and biocompatible for drug delivery depots and tissue engineering applications.     

Combining 3D Printing with Electrospinning for Rapid Response and Enhanced Designability of Hydrogel Actuators

Porous structures have emerged as a breakthrough of shape‐morphing hydrogels to achieve a rapid response. However, these porous actuators generally suffer from a lack of complexity and diversity in obtained 3D shapes. Herein, a simple yet versatile strategy is developed to generate shape‐morphing hydrogels with both fast deformation and enhanced designability in 3D shapes by combining two promising technologies: electrospinning and 3D printing. Elaborate patterns are printed on mesostructured stimuli‐responsive electrospun membranes, modulating in‐plane and interlayer internal stresses induced by swelling/shrinkage mismatch, and thus guiding morphing behaviors of electrospun membranes to adapt to changes of the environment. With this strategy, a series of fast deformed hydrogel actuators are constructed with various distinctive responsive behaviors, including reversible/irreversible formations of 3D structures, folding of 3D tubes, and formations of 3D structures with multi low‐energy states. It is worth noting that although poly(N‐isopropyl acrylamide) is chosen as the model system in the present research, our strategy is applicable to other stimuli‐responsive hydrogels, which enriches designs of rapid deformed hydrogel actuators.     

Generation of Compositional‐Gradient Structures in Biodegradable,Immiscible, Polymer Blends by Intermolecular Hydrogen‐Bonding Interactions

A biodegradable, immiscible poly(butylenes adipate‐co‐butylenes terephthalate) [P(BA‐co‐BT)]/poly(ethylene oxide) (PEO) polymer blend film with compositional gradient in the film‐thickness direction has been successfully prepared in the presence of a low‐molecular‐weight compound 4,4′‐thiodiphenal (TDP), which is used as a miscibility‐enhancing agent. The miscibilities of the P(BA‐co‐BT)/PEO/TDP ternary blend films and the P(BA‐co‐BT)/PEO/TDP gradient film were investigated by differential scanning calorimetry (DSC). The compositional gradient structure of the P(BA‐co‐BT)/PEO/TDP (46/46/8 w/w/w) film has been confirmed by microscopic mapping measurement of Fourier‐transform infrared spectra and dynamic mechanical thermal analysis. We have developed a new strategy for generating gradient‐phase structures in immiscible polymer‐blend systems by homogenization, i.e., adding a third agent that can enhance the miscibility of the two immiscible polymers through simultaneous formation of hydrogen bonds with two component polymers.     

Ambient‐Dried, 3D‐Printable and Electrically Conducting Cellulose Nanofiber Aerogels by Inclusion of Functional Polymers

This study presents a novel, green, and efficient way of preparing crosslinked aerogels from cellulose nanofibers (CNFs) and alginate using non‐covalent chemistry. This new process can ultimately facilitate the fast, continuous, and large‐scale production of porous, light‐weight materials as it does not require freeze‐drying, supercritical CO₂ drying, or any environmentally harmful crosslinking chemistries. The reported preparation procedure relies solely on the successive freezing, solvent‐exchange, and ambient drying of composite CNF‐alginate gels. The presented findings suggest that a highly‐porous structure can be preserved throughout the process by simply controlling the ionic strength of the gel. Aerogels with tunable densities (23–38 kg m^?3) and compressive moduli (97–275 kPa) can be prepared by using different CNF concentrations. These low‐density networks have a unique combination of formability (using molding or 3D‐printing) and wet‐stability (when ion exchanged to calcium ions). To demonstrate their use in advanced wet applications, the printed aerogels are functionalized with very high loadings of conducting poly(3,4‐ethylenedioxythiophene):tosylate (PEDOT:TOS) polymer by using a novel in situ polymerization approach. In‐depth material characterization reveals that these aerogels have the potential to be used in not only energy storage applications (specific capacitance of 78 F g^?1), but also as mechanical‐strain and humidity sensors.     

Sol‐Gel/Polyelectrolyte Active Corrosion Protection System

This work presents a new type of feed‐back active coating with inhibitor‐containing reservoirs for corrosion protection of metallic substrates. The reservoirs are composed of stratified layers of oppositely charged polyelectrolytes deposited on AA2024 aluminum alloy coated with hybrid sol‐gel film. The layer‐by‐layer assembled polyelectrolyte film with the entrapped corrosion inhibitor is constructed by sequential spray‐coating deposition of water solutions of poly(ethyleneimine), poly(sodium styrenesulfonate) and 8‐hydroxyquiniline on the top of the sol‐gel coating. The active corrosion protection of AA2024 alloy coated with SiO₂/ZrO₂ sol‐gel film and modified by polyelectrolytes is demonstrated by electrochemical impedance spectroscopy and scanning vibrating electrode technique. The results obtained here show that polyelectrolyte films deposited atop of the hybrid sol‐gel coating on AA2024 alloy remarkably improve the long‐term protection performance providing additional “intelligent” anticorrosion effect that results from delivery of inhibiting species “on demand”. This becomes possible since the configuration of the polyelectrolyte molecules depends on the presence of H⁺ ions making the polyelectrolyte film sensitive to the pH of the surrounding solution. The source of local pH changes is the corrosion process starting in the micro‐ and nano‐defects leading to increased permeability of the polyelectrolyte reservoir and, consequently, to controllable release of entrapped inhibitor.     

Biodegradable Polymer Crosslinker: Independent Control of Stiffness,Toughness, and Hydrogel Degradation Rate

Hydrogels are being increasingly studied for use in various biomedical applications including drug delivery and tissue engineering. The successful use of a hydrogel in these applications greatly relies on a refined control of the mechanical properties including stiffness, toughness, and the degradation rate. However, it is still challenging to control the hydrogel properties in an independent manner due to the interdependency between hydrogel properties. Here it is hypothesized that a biodegradable polymeric crosslinker would allow for decoupling of the dependency between the properties of various hydrogel materials. This hypothesis is examined using oxidized methacrylic alginate (OMA). The OMA is synthesized by partially oxidizing alginate to generate hydrolytically labile units and conjugating methacrylic groups. It is used to crosslink poly(ethylene glycol) methacrylate and poly(N‐hydroxymethyl acrylamide) to form three‐dimensional hydrogel systems. OMA significantly improves rigidity and toughness of both hydrogels as compared with a small molecule crosslinker, and also controls the degradation rate of hydrogels depending on the oxidation degree, without altering their initial mechanical properties. The protein‐release rate from a hydrogel and subsequent angiogenesis in vivo are thus regulated with the chemical structure of OMA. Overall, the results of this study suggests that the use of OMA as a crosslinker will allow the implantation of a hydrogel in tissue subject to an external mechanical loading with a desired protein‐release profile. The OMA synthesized in this study will be, therefore, highly useful to independently control the mechanical properties and degradation rate of a wide array of hydrogels.     

Redox Capacitor to Establish Bio‐Device Redox‐Connectivity

Electronic devices process information and transduce energy with electrons, while biology performs such operations with ions and chemicals. To establish bio‐device connectivity, we fabricate a redox‐capacitor film from a polysaccharide (i.e., chitosan) and a redox‐active catechol. We report that these films are rapidly and repeatedly charged and discharged electrochemically via a redox‐cycling mechanism in which mediators shuttle electrons between the electrode and film (capacitance ≈ 40 F/g or 2.9 mF/cm²). Further, charging and discharging can be executed under bio‐relevant conditions. Enzymatic‐charging is achieved by electron‐transfer from glucose to the film via an NADPH‐mediated redox‐cycling mechanism. Discharging occurs by electron‐donation to O₂ to generate H₂O₂ that serves as substrate for peroxidase‐mediated biochemical reactions. Thus, these films offer the capability of inter‐converting electrochemical and biochemical inputs/outputs. Among potential applications, we anticipate that catechol–chitosan redox‐capacitor films could serve as circuit elements for molecular logic operations or for transducing bio‐based chemical energy into electricity.     

A Novel Double‐Crosslinking‐Double‐Network Design for Injectable Hydrogels with Enhanced Tissue Adhesion and Antibacterial Capability for Wound Treatment

Most photocrosslinkable hydrogels have inadequacy in either mechanical performance or biodegradability. This issue is addressed by adopting a novel hydrogel design by introducing two different chitosan chains (catechol‐modified methacryloyl chitosan, CMC; methacryloyl chitosan, MC) via the simultaneous crosslinking of carbon–carbon double bonds and catechol‐Fe³⁺ chelation. This leads to an interpenetrating network of two chitosan chains with high crosslinking‐network density, which enhances mechanical performance including high compressive modulus and high ductility. The chitosan polymers not only endow the hydrogels with good biodegradability and biocompatibility, they also offer intrinsic antibacterial capability. The quinone groups formed by Fe³⁺ oxidation and protonated amino groups of chitosan polymer further enhance antibacterial property of the hydrogels. Serving as one of the two types of crosslinking mechanisms, the catechol‐Fe³⁺ chelation can covalently link with amino, thiol, and imidazole groups, which substantially enhance the hydrogel's adhesion to biological tissues. The hydrogel's adhesion to porcine skin shows a lap shear strength of 18.1 kPa, which is 6‐time that of the clinically established Fibrin Glue's adhesion. The hydrogel also has a good hemostatic performance due to the superior tissue adhesion as demonstrated with a hemorrhaging liver model. Furthermore, the hydrogel can remarkably promote healing of bacteria‐infected wound.     

Graphene Oxide‐Templated Conductive and Redox‐Active Nanosheets Incorporated Hydrogels for Adhesive Bioelectronics

2D conductive nanosheets are central to electronic applications because of their large surface areas and excellent electronic properties. However, tuning the multifunctions and hydrophilicity of conductive nanosheets are still challenging. Herein, a green strategy is developed for fabricating conductive, redox‐active, water‐soluble nanosheets via the self‐assembly of poly(3,4‐ethylenedioxythiophene) (PEDOT) on the polydopamine‐reduced and sulfonated graphene oxide (PSGO) template. The conductivity and hydrophilicity of nanosheets are highly improved by PSGO. The nanosheets are redox active due to the abundant catechol groups and can be used as versatile nanofillers in developing conductive and adhesive hydrogels. The nanosheets create a mussel‐inspired redox environment inside the hydrogel networks and endow the hydrogel with long‐term and repeatable adhesiveness. This hydrogel is biocompatible and can be implanted for biosignals detection in vivo. This mussel‐inspired strategy for assembling 2D nanosheets can be adapted for producing diverse multifunctional nanomaterials, with various potential applications in bioelectronics.