Magnetoactive Millirobots with Ternary Phase Transition.

Magnetoactive soft millirobots have made significant advances in programmable deformation, multimodal locomotion, and untethered manipulation in unreachable regions. However, the inherent limitations are manifested in the solid-phase millirobot as limited deformability and in the liquid-phase millirobot as low stiffness. Herein, we propose a ternary-state magnetoactive millirobot based on a phase transitional polymer embedded with magnetic nanoparticles. The millirobot can reversibly transit among the liquid, solid, and viscous-fluid phases through heating and cooling. The liquid-phase millirobot has elastic deformation and mobility for unimpeded navigation in a constrained space. The viscous-fluid phase millirobot shows irreversible deformation and large ductility. The solid-phase millirobot shows good shape stability and controllable locomotion. Moreover, the ternary-state magnetoactive millirobot can achieve prominent capabilities including stiffness change and shape reconfiguration through phase transition. The millirobot can perform potential functions of navigation in complex terrain, three-dimensional circuit connection, and simulated treatment in a stomach model. This magnetoactive millirobot may find new applications in flexible electronics and biomedicine.

Flaw-insensitive fatigue resistance of chemically fixed collagenous soft tissues.

Bovine pericardium (BP) has been used as leaflets of prosthetic heart valves. The leaflets are sutured on metallic stents and can survive 400 million flaps (~10-year life span), unaffected by the suture holes. This flaw-insensitive fatigue resistance is unmatched by synthetic leaflets. We show that the endurance strength of BP under cyclic stretch is insensitive to cuts as long as 1 centimeter, about two orders of magnitude longer than that of a thermoplastic polyurethane (TPU). The flaw-insensitive fatigue resistance of BP results from the high strength of collagen fibers and soft matrix between them. When BP is stretched, the soft matrix enables a collagen fiber to transmit tension over a long length. The energy in the long length dissipates when the fiber breaks. We demonstrate that a BP leaflet greatly outperforms a TPU leaflet. It is hoped that these findings will aid the development of soft materials for flaw-insensitive fatigue resistance.

Stretchable and transparent ionogel-based heaters.

Transparent heaters (THs) are widely used for various applications, such as in smart windows, deicers, defoggers, displays, and thermotherapy pads. The rapid development of flexible electronics has led to a demand for flexible and even stretchable THs. At present, most stretchable THs are designed using a combination of electronically conductive networks and flexible polymer materials. Electronic THs still face common challenges, such as a transparency-conductance trade-off, non-uniform heating, and poor interfacial adhesion. In this work, an ionic TH is reported based on a stretchable and transparent ionogel. Joule heating from an ionic current induced by alternating voltage functions as the heating source. This ionogel-based TH exhibits excellent and steady mechanical, optical, electrical, and thermal properties, simultaneously solving the abovementioned three problems relating to electronic THs. Two simple applications of this ionogel-based TH are demonstrated: deicing and boiling water. This reported ionogel-based TH provides a new material choice and heating principle to compete with conventional electronic TH technology.

Adhesive anastomosis for organ transplantation.

The recent development of tough tissue adhesives has stimulated intense interests among material scientists and medical doctors. However, these adhesives have seldom been tested in clinically demanding surgeries. Here we demonstrate adhesive anastomosis in organ transplantation. Anastomosis is commonly conducted by dense sutures and takes a long time, during which all the vessels are occluded. Prolonged occlusion may damage organs and even cause death. We formulate a tough, biocompatible, bioabsorbable adhesive that can sustain tissue tension and pressurized flow. We expose the endothelial surface of vessels onto a gasket, press two endothelial surfaces to the adhesive using a pair of magnetic rings, and reopen the bloodstream immediately. The time for adhesive anastomosis is shortened compared to the time for sutured anastomosis. We have achieved adhesive anastomosis of a great vein in transplanting the liver of a pig. After the surgery, the adhesive is absorbed, the vein heals, and the pig lives for over one month.

3D printable, tough, magnetic hydrogels with programmed magnetization for fast actuation.

Magnetic hydrogels have demonstrated great potential in soft robots, drug delivery, and bioengineering, and their functions are usually determined by the deforming capability. However, most magnetic hydrogels are embedded with soft magnetic particles (e.g. Fe3O4), where the magnetic domains cannot be programmed and retained under external magnetic fields. Here, we present a strategy to prepare a microgel-reinforced magnetic hydrogel, embedded with hard magnetic NdFeB particles. These magnetic hydrogels show outstanding mechanical properties (ultimate stretching ratio >15 and fracture toughness >15 000 J m-2) and fast actuation speed under external magnetic fields. We use direct ink writing to fabricate magnetic hydrogels with sophisticated geometry and program their magnetization to achieve complex deformations. Fast, reversible, shape-changing structures have been demonstrated with printed magnetic hydrogels. It is hoped that this material system of hard magnetic hydrogels can open opportunities for wide applications.

Fabrication of patterned magnetic hydrogels by ion transfer printing.

Magnetic hydrogels have found a myriad of applications in bioengineering and soft robotics. As the function of magnetic hydrogels is affected by the distribution of magnetic nanoparticles, it is imperative to propose a strategy for fabricating patterned magnetic hydrogels. However, previous strategies can only achieve very simple distribution by using external magnetic fields to guide the chain-like assembly of nanoparticles. It remains challenging to realize the complex distribution of magnetic nanoparticles in a hydrogel. Here we propose an ion transfer printing strategy to prepare patterned magnetic hydrogels, taking advantage of the ion permeation and nanoparticle precipitation in the hydrogel. The polyacrylamide (PAAm) hydrogel is loaded with Fe2+/Fe3+ ions and covered with a patterned filter paper with OH- ions to generate Fe3O4 nanoparticles locally. The effect of the ion concentration and covering time on the generation of nanoparticles is investigated by using a reaction-diffusion model. Furthermore, the magnetothermal response of the patterned magnetic hydrogels has been characterized to reveal the distribution and thermogenesis of magnetic nanoparticles. We hope that the fabricated magnetic hydrogels with complex patterns can open up new opportunities for applications.

Shape Morphing of Hydrogels in Alternating Magnetic Field.

Shape-morphing hydrogels have found a myriad of applications in biomimetics, soft robotics, and biomedical engineering. A magnetic field is favorable for specific applications of hydrogels, since it is noncontact and biocompatible at high field strengths. However, most magnetosensitive shape-morphing structures are made of elastomers rather than hydrogels because the magnetization of magnetic hydrogels is usually too low to be actuated under a static magnetic field. Here, we propose a strategy to achieve the shape morphing of magnetic hydrogels. We actuate magnetothermal sensitive hydrogels by an alternating magnetic field (AMF), where magnetic poly( N-isopropylacrylamide) hydrogels can be heated by the AMF and can undergo giant volume shrinkage under high temperature. We design the distributing pattern of magnetic hydrogel strips on an elastomer substrate to realize various two-dimensional and three-dimensional shapes such as heart-shape, truss, tube, and helix. Complex three-dimensional origami structures have been demonstrated using elastomer-magnetic hydrogels as hinges. We further demonstrate the combination of magnetic navigation and magnetic shape morphing, by applying both a direct magnetic field and an alternating magnetic field. The strategy may open new opportunities for the shape morphing of functional hydrogels.

Strong and Degradable Adhesion of Hydrogels.

Adhesives potentially offer convenient means to close wounds, but existing adhesives do not fulfill many common requirements. Here we demonstrate an approach to develop hydrogel adhesives that are strong initially, remain soft when adhered to soft tissues, and degrade over time. We demonstrate the approach by using a hydrogel that dissipates a large amount of energy during separation, forms strong and interlinks with the soft tissues, and degrades by breaking cross-links. The hydrogel achieves initial adhesion energies of 300-700 J/m2 when adhered to different tissues, can bear huge pressure, and completely degrades in 5 weeks under simulated physiological conditions.

Fracture Toughness and Fatigue Threshold of Tough Hydrogels.

Hydrogels of numerous chemical compositions have achieved high fracture toughness on the basis of one physical principle. As a crack advances in such a hydrogel, a polymer network of strong bonds ruptures at the front of the crack and elicits energy dissipation in the bulk of the hydrogel. The constituent that dissipates energy in the bulk of the hydrogel is called a toughener. A hypothesis has emerged recently that tougheners increase fracture toughness greatly but contribute little to fatigue threshold. Here we ascertain this hypothesis by studying hydrogels of two kinds, identical in all aspects except for tougheners. A Ca-alginate/polyacrylamide hydrogel has ionic bonds, which act as tougheners, resulting in a toughness of 3375 J/m2 and a threshold of 35 J/m2. A Na-alginate/polyacrylamide hydrogel has no ionic bonds, resulting in a toughness of 169 J/m2 and a threshold of 17 J/m2. These results motivate a discussion on the development of fatigue-resistant hydrogels.

Magnetic double-network hydrogels for tissue hyperthermia and drug release.

Magnetic-field driven soft materials have found extensive applications in fields such as soft robotics, shape morphing and biomedicine. Compared to magnetoactive elastomers (MAEs), magnetic hydrogels have shown significant advantages for in vivo applications, because of their better biocompatibility, as well as their soft and wet nature. However, the poor mechanical properties and ion sensitivity of conventional magnetic hydrogels will severely limit their applications especially under physiological conditions. Double network hydrogels are tough and stable, but do not respond to environmental stimuli. Here magnetic double network (M-DN) hydrogels have been developed with outstanding mechanical performances and ion-resistant stability. M-DN hydrogels show a high modulus of ∼0.4 MPa and a high toughness of ∼1500 J m-2. The volume, magnetic and mechanical properties of M-DN hydrogels show negligible deterioration in ionic solutions. M-DN hydrogels exhibit magnetic responsiveness and have been used for tissue hyperthermia and drug release by magnetic induction heating. The induction heating behavior of M-DN hydrogels can be tuned to meet the clinical requirements, by changing the magnetic field strength or the composition of magnetic hydrogels. M-DN hydrogels may be inspiring to the development of responsive DN hydrogels and expand their more potential applications in load-bearing biomedical engineering.

Stretchable Seal.

Many stretchable electronic devices require stretchable hermetic seals. However, stretchability and permeability are inextricably linked at the molecular level: stretchable, low-permeability materials do not exist. We collect data for the permeation of water and oxygen in many materials and describe the scaling relations for both flat and wrinkled seals. Whereas flat seals struggle to fulfill the simultaneous requirements of stretchability, low stiffness, and low transmissibility, wrinkled seals can fulfill them readily. We further explore the behavior of wrinkled seals under cyclic stretch using aluminum, polyethylene, and silica films on elastomer substrates. The wrinkled aluminum develops fatigue cracks after a small number of cycles, but the wrinkled polyethylene and silica maintain low transmissibility after 10 000 cycles of tensile strain.

Super tough magnetic hydrogels for remotely triggered shape morphing.

Despite their potential in various fields such as soft robots, drug delivery and biomedical engineering, magnetic hydrogels have always been limited by their poor mechanical properties. Here a universal soaking strategy has been presented to synthesize tough magnetic nanocomposite (NC) hydrogels. We can simultaneously solve two common issues for magnetic hydrogels: the poor mechanical properties and poor distribution of magnetic particles. The toughness of the magnetic NC hydrogel achieves approximately 11 000 J m-2. The outstanding properties of tough magnetic hydrogels will enable myriad applications. Here we demonstrate a new application for remotely triggered shape morphing. Heterogeneous structures based on magnetic hydrogels are shown to evolve into bio-inspired three-dimensional (3D) shapes (lotus flowers) from 2D-structured sheets. The self-folding of the structure is controlled by the magnetothermal effect in an alternating magnetic field. The capability to control the shape morphing of a multi-material system by a magnetic field may emerge as a new general strategy for programming complex soft structures.

Adhesion between highly stretchable materials.

Recently developed high-speed ionic devices require adherent laminates of stretchable and dissimilar materials, such as gels and elastomers. Adhesion between stretchable and dissimilar materials also plays important roles in medicine, stretchable electronics, and soft robots. Here we develop a method to characterize adhesion between materials capable of large, elastic deformation. We apply the method to measure the debond energy of elastomer-hydrogel bilayers. The debond energy between an acrylic elastomer and a polyacrylamide hydrogel is found to be about 0.5 J m(-2), independent of the thickness and the crosslink density of the hydrogel. This low debond energy, however, allows the bilayer to be adherent and highly stretchable, provided that the hydrogel is thin and compliant. Furthermore, we demonstrate that nanoparticles applied at the interface can improve adhesion between the elastomer and the hydrogel.