3D printable elastomers with exceptional strength and toughness.

Three-dimensional (3D) printing has emerged as an attractive manufacturing technique because of its exceptional freedom in accessing geometrically complex customizable products. Its potential for mass manufacturing, however, is hampered by its low manufacturing efficiency (print speed) and insufficient product quality (mechanical properties). Recent progresses in ultra-fast 3D printing of photo-polymers1-5 have alleviated the issue of manufacturing efficiency, but the mechanical performance of typical printed polymers still falls far behind what is achievable with conventional processing techniques. This is because of the printing requirements that restrict the molecular design towards achieving high mechanical performance. Here we report a 3D photo-printable resin chemistry that yields an elastomer with tensile strength of 94.6 MPa and toughness of 310.4 MJ m-3, both of which far exceed that of any 3D printed elastomer6-10. Mechanistically, this is achieved by the dynamic covalent bonds in the printed polymer that allow network topological reconfiguration. This facilitates the formation of hierarchical hydrogen bonds (in particular, amide hydrogen bonds), micro-phase separation and interpenetration architecture, which contribute synergistically to superior mechanical performance. Our work suggests a brighter future for mass manufacturing using 3D printing.

Development and experimental validation of analytical models for water and mud inrushes through a filled karst conduit.

Water or mud inrush has become a common geological disaster during tunnel construction in karst areas. To study forming process and mechanism of water and mud inrushes through a filled karst conduit, water inrush and mud inrush model tests were carried out with a self-developed 3D model test system. The results show that the forming processes of water inrush and mud inrush have different forming modes. For water inrush, the forming process follows: flowing instability of filling material particles-formation of water inrush channel-water inrush occurring; while for mud inrush, the forming process follows: stability-sliding instability of the whole filling material suddenly-mud inrush occurring. Accordingly, a local instability model of critical hydraulic pressure causing water inrush and an integral sliding instability model of critical hydraulic pressure causing mud inrush were established respectively. The two analytical models reveal the mechanism of water inrush and mud inrush experiments to an extent. The calculated critical hydraulic pressures for water inrush and mud inrush are in good agreement with the test results. The distinguishment of water inrush and mud inrush through a karst conduit was discussed based on the critical hydraulic pressure and the evolution law of seepage water pressure in tests, and a criterion was given. The research results might provide guidance for the forecast of water and mud inrush disasters during the construction of tunnels in karst area.

Inhibition of Heterogeneous Nucleation in Water by Hydrogel Coating.

Heterogeneous nucleation plays a critical role in the phase transition of water, which can cause damage in various systems. Here, we report that heterogeneous nucleation can be inhibited by utilizing hydrogel coatings to isolate solid surfaces and water. Hydrogels, which contain over 90% water when fully swelled, exhibit a high degree of similarity to water. Due to this similarity, there is a great energy barrier for heterogeneous nucleation along the water-hydrogel interface. Additionally, hydrogel coatings, which possess polymer networks, exhibit higher fracture energy and more robust adhesion to solid surfaces compared to water. This high fracture and adhesion energy acts as a deterrent for fracture nucleation within the hydrogel or along the hydrogel-solid interface. With a hydrogel layer approximately 100 µm thick, the boiling temperature of water under atmospheric pressure can be raised from 100 to 108 °C. Notably, hydrogel coatings also result in remarkable reductions in cavitation pressure on multiple solid surfaces. We have demonstrated the efficacy of hydrogel coatings in preventing damages resulting from acceleration-induced cavitation. Hydrogel coatings have the potential to alter the energy landscape of heterogeneous nucleation on the water-solid interface, making them an exciting avenue for innovation in heat transfer and fluidic systems.

Tunable Folding Assembly Strategy for Soft Pneumatic Actuators.

With intrinsic compliance, soft pneumatic actuators are widely utilized in delicate tasks. However, complex fabrication approaches and limited tunability are still problems. Here, we propose a tunable folding assembly strategy to design and fabricate soft pneumatic actuators called FASPAs (folding assembly soft pneumatic actuators). A FASPA consists only of a folded silicone tube constrained by rubber bands. By designing local stiffness and folding manner, the FASPA can be designed to achieve four configurations, pure bending, discontinuous-curvature bending, helix, and discontinuous-curvature helix. Analytical models are developed to predict the deformation and the tip trajectory of different configurations. Meanwhile, experiments are performed to verify the models. The stiffness, load capacity, output force, and step response are measured, and fatigue tests are performed. Further, grippers with single, double, and triple fingers are assembled by utilizing different types of FASPAs. As such, objects with different shapes, sizes, and weights can be easily grasped. The folding assembly strategy is a promising method to design and fabricate soft robots with complex configurations to complete tough tasks in harsh environments.

Sustainable Approach for the Synthesis of a Semicrystalline Polymer with a Reversible Shape-Memory Effect.

Shape-memory polymers (SMPs) have demonstrated potential for use in automotive, biomedical, and aerospace industries. However, ensuring the sustainability of these materials remains a challenge. Herein, a sustainable approach to synthesize a semicrystalline polymer using biomass-derivable precursors via catalyst-free polyesterification is presented. The synthesized biodegradable polymer, poly(1,8-octanediol-co-1,12-dodecanedioate-co-citrate) (PODDC), exhibits excellent shape-memory properties, as evidenced by good shape fixity and shape recovery ratios of 98%, along with a large reversible actuation strain of 28%. Without the use of a catalyst, the mild polymerization enables the reconfiguration of the partially cured two-dimensional (2D) film to a three-dimensional (3D) geometric form in the middle process. This study appears to be a step forward in developing sustainable SMPs and a simple way for constructing a 3D structure of a permanent shape.

Adhesive cryogel particles for bridging confined and irregular tissue defects.

BACKGROUND: Reconstruction of damaged tissues requires both surface hemostasis and tissue bridging. Tissues with damage resulting from physical trauma or surgical treatments may have arbitrary surface topographies, making tissue bridging challenging. METHODS: This study proposes a tissue adhesive in the form of adhesive cryogel particles (ACPs) made from chitosan, acrylic acid, 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The adhesion performance was examined by the 180-degree peel test to a collection of tissues including porcine heart, intestine, liver, muscle, and stomach. Cytotoxicity of ACPs was evaluated by cell proliferation of human normal liver cells (LO2) and human intestinal epithelial cells (Caco-2). The degree of inflammation and biodegradability were examined in dorsal subcutaneous rat models. The ability of ACPs to bridge irregular tissue defects was assessed using porcine heart, liver, and kidney as the ex vivo models. Furthermore, a model of repairing liver rupture in rats and an intestinal anastomosis in rabbits were established to verify the effectiveness, biocompatibility, and applicability in clinical surgery. RESULTS: ACPs are applicable to confined and irregular tissue defects, such as deep herringbone grooves in the parenchyma organs and annular sections in the cavernous organs. ACPs formed tough adhesion between tissues [(670.9 ± 50.1) J/m2 for the heart, (607.6 ± 30.0) J/m2 for the intestine, (473.7 ± 37.0) J/m2 for the liver, (186.1 ± 13.3) J/m2 for muscle, and (579.3 ± 32.3) J/m2 for the stomach]. ACPs showed considerable cytocompatibility in vitro study, with a high level of cell viability for 3 d [(98.8 ± 1.2) % for LO2 and (98.3 ± 1.6) % for Caco-2]. It has comparable inflammation repair in a ruptured rat liver (P = 0.58 compared with suture closure), the same with intestinal anastomosis in rabbits (P = 0.40 compared with suture anastomosis). Additionally, ACPs-based intestinal anastomosis (less than 30 s) was remarkably faster than the conventional suturing process (more than 10 min). When ACPs degrade after surgery, the tissues heal across the adhesion interface. CONCLUSIONS: ACPs are promising as the adhesive for clinical operations and battlefield rescue, with the capability to bridge irregular tissue defects rapidly.

Dielectric Polymer with Designable Large Motion under Low Electric Field.

Dielectric elastomers (DEs) can demonstrate fast and large in-plane expansion/contraction due to electric field (e-field)-induced Maxwell stress. For robotic applications, it is often necessary that the in-plane actuation is converted into out-of-plane motions with mechanical frames. Despite their performance appeal, their high driving e-field (20-100 V µm-1 ) demands bulky power accessories and severely compromises their durability. Here, a dielectric polymer that can be programmed into diverse motions actuated under a low e-field (2-10 V µm-1 ) is reported. The material is a crystalline dynamic covalent network that can be reconfigured into arbitrary 3D geometries. This gives rise to a geometric effect that markedly amplifies the actuation, leading to designable large motions when the dielectric polymer is heated above its melting temperature to become a DE. Additionally, the crystallization transition enables dynamic multimodal motions and active deployability. These attributes result in unique design versatility for soft robots.

Primary support optimization of large-span and shallow buried hard rock tunnels based on the active support concept.

Considering the large-span underground excavation subway station of Qingdao Metro Line 6 of China for analysis, it is necessary to optimize the traditional support system by investigating relevant codes and other tunnel projects. Based on the active support concept, a high prestressed rock bolt support system is proposed, and the optimization direction is defined to apply a high prestress force to rock bolts, increasing the appropriate spacing between supporting arches and strengthening support at key parts such as the large arch foot area, sidewalls and junctions. Numerical calculations and field monitoring are performed to analyze and evaluate the new support system. Numerical simulation results show that the new support system can effectively improve the stress state of the surrounding rock; the tensile stress area markedly decreases in size or disappears; and the plastic area also decreases in size. Field monitoring results show that the settlement of the arch crown is concentrated at 2-5 mm and the deformation rates are less than 0.5 mm/day. The supporting arches, shotcrete and rock bolts are all less than the yield strength and a high safety reserve. These results verify the safety and rationality of the proposed support system, which can be used as a reference for similar projects.

Culture of patient-derived multicellular clusters in suspended hydrogel capsules for pre-clinical personalized drug screening.

A personalized medication regimen provides precise treatment for an individual and can be guided by pre-clinical drug screening. The economical and high-efficiency simulation of the liver tumor microenvironment (TME) in a drug-screening model has high value yet challenging to accomplish. Herein, we propose a simulation of the liver TME with suspended alginate-gelatin hydrogel capsules encapsulating patient-derived liver tumor multicellular clusters, and the culture of patient-derived tumor organoids(PDTOs) for personalized pre-clinical drug screening. The hydrogel capsule offers a 3D matrix environment with mechanical and biological properties similar to those of the liver in vivo. As a result, 18 of the 28 patient-derived multicellular clusters were successfully cultured as PDTOs. These PDTOs, along with hepatocyte growth factor (HGF) of non-cellular components, preserve stromal cells, including cancer-associated fibroblasts (CAFs) and vascular endothelial cells (VECs). They also maintain stable expression of molecular markers and tumor heterogeneity similar to those of the original liver tumors. Drugs, including cabazitaxel, oxaliplatin, and sorafenib, were tested in PDTOs. The sensitivity of PDTOs to these drugs differs between individuals. The sensitivity of one PDTO to oxaliplatin was validated using magnetic resonance imaging (MRI) and biochemical tests after oxaliplatin clinical treatment of the corresponding patient. Therefore, this approach is promising for economical, accurate, and high-throughput drug screening for personalized treatment.

Versatile and Simple Strategy for Preparing Bilayer Hydrogels with Janus Characteristics.

Bilayer hydrogels are attracting tremendous attention for their capability to integrate several different functions on the two sides of the gel, that is, imparting the gel with Janus characteristics, which is highly desired in many engineering and biomedical applications including soft actuators, hydrogel patches, and wearable electronics. However, the preparation process of the bilayer materials usually involves several complicated steps and is time-consuming, while the interfacial bonding is another main concern. Here, a simple and versatile method is proposed to obtain bilayer hydrogels within just one step based on the method of introducing viscosity contrast of the precursors for different layers. The bilayer structure can be well maintained during the whole preparation process with a constrained interfacial molecular exchange to ensure the strong bonding strength. The key requirements for forming distinct bilayer structures in situ are studied and discussed in detail. Bilayer hydrogels with different chemical designs are prepared via this strategy to tailor the good distribution of desired functions for soft actuators, wound healing patches, and wearable electronics. We believe that the strategy illustrated here will provide new insights into the preparation and application of bilayer materials.

Self-powered soft robot in the Mariana Trench.

The deep sea remains the largest unknown territory on Earth because it is so difficult to explore1-4. Owing to the extremely high pressure in the deep sea, rigid vessels5-7 and pressure-compensation systems8-10 are typically required to protect mechatronic systems. However, deep-sea creatures that lack bulky or heavy pressure-tolerant systems can thrive at extreme depths11-17. Here, inspired by the structure of a deep-sea snailfish15, we develop an untethered soft robot for deep-sea exploration, with onboard power, control and actuation protected from pressure by integrating electronics in a silicone matrix. This self-powered robot eliminates the requirement for any rigid vessel. To reduce shear stress at the interfaces between electronic components, we decentralize the electronics by increasing the distance between components or separating them from the printed circuit board. Careful design of the dielectric elastomer material used for the robot's flapping fins allowed the robot to be actuated successfully in a field test in the Mariana Trench down to a depth of 10,900 metres and to swim freely in the South China Sea at a depth of 3,224 metres. We validate the pressure resilience of the electronic components and soft actuators through systematic experiments and theoretical analyses. Our work highlights the potential of designing soft, lightweight devices for use in extreme conditions.

Actuating Supramolecular Shape Memorized Hydrogel Toward Programmable Shape Deformation.

Inspired by nature, diverse biomimetic hydrogel actuators are fabricated and become one of the most essential components of bionics research. Usually, the anisotropic structure of a hydrogel actuator is generated at the early fabrication process, only a specific shape transformation behavior can be produced under external stimuli, and thus has limited the development of hydrogel actuators toward the biomimetic shape deformation behavior. Herein, a novel bilayer hydrogel having a thermoresponsive actuating layer and a metal ion-responsive memorizing layer is proposed, therefore, a 2D hydrogel film can be fixed into various 3D shapes via supramolecular metal-ligand coordination, with further realizing programmable 4D shape deformation under the stimulus of temperature. By manipulating the temporary shapes via shape memory behavior, various temporary anisotropic structures can be obtained via the bilayer hydrogel, thus producing diverse reversible shape deformation performances, which is expected to promote the development of intelligent polymeric materials.

Stretchable Electrets: Nanoparticle-Elastomer Composites.

Manipulating charges is fundamental to numerous systems, and this ability is achieved through materials of diverse characteristics. Electrets are dielectrics that trap charges or dipoles. Applications include electrophotography, microphones, air filters, and energy harvesters. To trap charges or dipoles for a long time, electrets are commonly made of hard dielectrics. Stretchable dielectrics are short-lived electrets. The two properties, longevity and stretchability, conflict; existing electrets struggle to attain both. This work describes an approach to developing stretchable electrets. Nanoparticles of a hard electret are immobilized in a matrix of dielectric elastomer. The composite divides the labor of two functions: the particles trap charges with longevity, and the matrix enables stretchability. The design considerably broadens the choice of materials to enable stretchable electrets. Silica nanoparticles in the polydimethylsiloxane elastomer achieve a charge density ∼ 4 × 10-5 C m-2 and a lifetime beyond 60 days. Long-lived, stretchable electrets open extensive opportunities.

Hydrogel Paint.

For a hydrogel coating on a substrate to be stable, covalent bonds polymerize monomer units into polymer chains, crosslink the polymer chains into a polymer network, and interlink the polymer network to the substrate. The three processes-polymerization, crosslinking, and interlinking-usually concur. This concurrency hinders widespread applications of hydrogel coatings. Here a principle is described to create hydrogel paints that decouple polymerization from crosslinking and interlinking. Like a common paint, a hydrogel paint divides the labor between the paint maker and the paint user. The paint maker formulates the hydrogel paint by copolymerizing monomer units and coupling agents into polymer chains, but does not crosslink them. The paint user applies the paint on various materials (elastomer, plastic, glass, ceramic, or metal), and by various operations (brush, cast, dip, spin, or spray). During cure, the coupling agents crosslink the polymer chains into a network and interlink the polymer network to the substrate. As an example, hydrogels with thickness in the range of 2-20 µm are dip coated on medical nitinol wires. The coated wires reduce friction by eightfold, and remain stable over 50 test cycles. Also demonstrated are several proof-of-concept applications, including stimuli-responsive structures and antifouling model boats.

Agile and Resilient Insect-Scale Robot.

A key challenge in bioinspired insect-scale running robots is to make them both agile and resilient. In this study, we develop a dielectric elastomer actuated soft robot that mimics inchworms. We use an elastomer to make the soft body, a stretchable dielectric to provide electrostatic actuation of high power density, and multizone actuation to achieve ratcheting locomotion. We fabricate the body, muscles, and feet in a single piece, with no internal open space. The robot runs four times its body length per second and turns at a radius about three times its body length in 0.3 s. The robot survives compression 30,000 times its own weight and survives collision with a rigid surface at a speed of 30 m/s. The robot can climb a slope of 30°. Walking on a horizontal plane, the robot carries a payload four times its own weight. The robot can operate on land, underwater, and in vacuum. The simplicity in design and fabrication will enable the robot to serve as a model system to investigate insect-scale actuation and locomotion, as well as the social behavior of swarms of robots. The robot also provides a platform to integrate wireless charging, mobile communication, and stretchable electronics.

Soft Artificial Bladder Detrusor.

Developing soft devices for invasive procedures bears great importance for human health. The softness and large strain actuation of responsive hydrogels promise the potential to fabricate soft devices, which can attach on and assist to the function of organs. The key challenges lie in the fabrication of soft devices with robust actuating ability and biocompatibility to the attached organ. This paper presents a solution that integrates the thermoresponsive hydrogel membrane with flexible electronics and silk scaffold into a balloon-like soft device. As an example, the actuation assisting function of this soft device for shrinking an animal bladder is presented. The mechanical behaviors of the balloon-like soft device are experimentally and theoretically investigated. The concepts are applicable to other applications such as soft implants, soft robotics, and microfluidics.

Thermoplastic Dielectric Elastomer of Triblock Copolymer with High Electromechanical Performance.

Dielectric elastomer (DE) actuators have been shown to have promising applications as soft electromechanical transducers in many emerging technologies. The DE actuators, which are capable of large actuation strain over a wide range of excitation frequencies, are highly desirable. Here, the first single-component DE of a triblock copolymer with attractive electromechanical performance is reported. Symmetric poly(styrene-b-butyl acrylate-b-styrene) (SBAS) is designed and synthesized. The SBAS actuator exhibits about 100% static actuation area strain and excellent dynamic performance, as evidenced by a wide half bandwidth of 300 Hz and a very high specific power of 1.2 W g-1 within the excitation frequency range of 300-800 Hz.

Fast-moving soft electronic fish.

Soft robots driven by stimuli-responsive materials have unique advantages over conventional rigid robots, especially in their high adaptability for field exploration and seamless interaction with humans. The grand challenge lies in achieving self-powered soft robots with high mobility, environmental tolerance, and long endurance. We are able to advance a soft electronic fish with a fully integrated onboard system for power and remote control. Without any motor, the fish is driven solely by a soft electroactive structure made of dielectric elastomer and ionically conductive hydrogel. The electronic fish can swim at a speed of 6.4 cm/s (0.69 body length per second), which is much faster than previously reported untethered soft robotic fish driven by soft responsive materials. The fish shows consistent performance in a wide temperature range and permits stealth sailing due to its nearly transparent nature. Furthermore, the fish is robust, as it uses the surrounding water as the electric ground and can operate for 3 hours with one single charge. The design principle can be potentially extended to a variety of flexible devices and soft robots.

Supramolecular Lego assembly towards three-dimensional multi-responsive hydrogels.

Inspired by the assembly of Lego toys, hydrogel building blocks with heterogeneous responsiveness are assembled utilizing macroscopic supramolecular recognition as the adhesion force. The Lego hydrogel provides 3D transformation upon pH variation. After disassembly of the building blocks by changing the oxidation state, they can be re-assembled into a completely new shape.