Programmable Morphing Hydrogels for Soft Actuators and Robots: From Structure Designs to Active Functions.

Nature provides abundant inspiration and elegant paradigms for the development of smart materials that can actuate, morph, and move on demand. One remarkable capacity of living organisms is to adapt their shapes or positions in response to stimuli. Programmed deformations or movements in plant organs are mainly driven by water absorption/dehydration of cells, while versatile motions of mollusks are based on contraction/extension of muscles. Understanding the general principles of these morphing and motion behaviors can give rise to disruptive technologies for soft robotics, flexible electronics, biomedical devices, etc. As one kind of intelligent material, hydrogels with high similarity to soft biotissues and diverse responses to external stimuli are an ideal candidate to construct soft actuators and robots.The objective of this Account is to give an overview of the fundamental principles for controllable deformations and motions of hydrogels, with a focus on the structure designs and responsive functions of the corresponding soft actuators and robots. This field has been rapidly developed in recent years with a growing understanding of working principles in natural organisms and a substantial revolution of manufacturing technologies to devise bioinspired hydrogel systems with desired structures. Diverse morphing hydrogels and soft actuators/robots have been developed on the basis of several pioneering works, ranging from bending and folding deformations of bilayer hydrogels to self-shaping of non-Euclidean hydrogel surfaces, and from thermoactuated bilayer gel "hands" to electrodriven polyelectrolyte gel "worms". These morphing hydrogels have demonstrated active functions and versatile applications in biomedical and engineering fields.In this Account, we discuss recent progress in morphing hydrogels and highlight the design principles and relevant applications. First, we introduce the fundamentals of basic deformation modes, together with generic structure features, actuation strategies, and morphing mechanisms. The advantages of in-plane gradient structures are highlighted for programmable deformations by harnessing the out-of-plane buckling with bistability nature to obtain sophisticated three-dimensional configurations. Next, we give an overview of soft actuators and robots based on morphing hydrogels and focus on the working principles of the active systems with different structure designs. We discuss the advancements of hydrogel-based soft robots capable of swift locomotion with different gaits and emphasize the significances of structure control and dynamic actuation. Then we summarize versatile applications of hydrogel-based actuators and robots in biomedicines, cargo delivery, soft electronics, information encryption, and so forth. Some hydrogel robots with a built-in feedback loop and self-sensing system exhibit collaborative functions and advanced intelligence that are informative for the design of next-generation hydrogel machines. Finally, concluding remarks are given to discuss future opportunities and remaining challenges in this field. For example, miniature hydrogel-based actuators/robots with therapeutic or diagnostic functions are highly desired for biomedical applications. The morphing mechanisms summarized in this Account should be applicable to other responsive materials. We hope that this Account will inspire more scientists to be involved in this emerging area and make contributions to reveal novel working principles, design multifunctional soft machines, and explore applications in diverse fields.

Programmable Deformations of Biomimetic Composite Hydrogels Embedded with Printed Fibers.

Shape deformations are prevalent in nature, which are closely related to the heterogeneous structures with a feature of fibrous elements embedded in a matrix. The microfibers with specific orientations act as either passive geometrical constraints in an active matrix or active elements in a passive matrix, which generate programmed internal stresses and drive shape morphing under external stimuli. Morphing materials can be designed in a biomimetic way, yet it is challenging to fabricate composite hydrogels with well-distributed fibers by a facile strategy. Here, we demonstrate the fabrication of microfiber-embedded hydrogels facilitated by the extrusion-based printing technology. Programmed deformations are achieved in these hydrogels with microfibers distributed in the upper and/or bottom layers of the gel matrix. Under external stimuli, the microfibers and the gel matrix have different responses that produce internal stresses and result in programmable deformations of the composite gel. Multiple shape transformations are realized in the hydrogel by embedding multiple types of responsive microfibers in the passive or active matrix, which is fabricated with the assistance of multinozzle printing. A soft hook is designed to show the capacity of the composite hydrogel to hold and move an object in a saline solution. This facile and versatile strategy provides an alternative way to prepare biomimetic hydrogels with potential applications in biomedical devices, flexible electronics, and soft robots.

Fabrication of a mechanically anisotropic poly(glycerol sebacate) membrane for tissue engineering.

Poly(glycerol sebacate) (PGS) has been used successfully as a scaffolding material for soft tissue engineering. PGS scaffolds, however, are usually mechanically isotropic, which may restrict their use in tissue repairs as many soft tissues in the body have anisotropic mechanical behaviors. Although various methods have been used to fabricate anisotropic scaffolds, it remains challenging to make anisotropic scaffolds from thermoset PGS. Here a new, simple method to fabricate an anisotropic PGS membrane which can then be used to construct thicker three-dimensional anisotropic scaffolds was developed. First, an aligned sacrificial poly(vinyl alcohol) fibrous membrane was prepared by electrospinning. The fibrous membrane was then partially immersed in PGS prepolymer solution, resulting in a composite membrane upon drying. After curing, the sacrificial fibers within the membrane were removed by water, supposedly leaving aligned cylindrical pores in the membrane. Both SEM and AFM illustrated aligned grooves on the surface of the resultant PGS membrane, indicating the successful removal of sacrificial fibers. The PGS membrane was validated to be mechanically anisotropic using uniaxial tensile testing along and perpendicular to the predominant pore direction. The in vitro cytocompatibility of the PGS membrane was confirmed. As a demonstration of its potential application in vascular tissue engineering, a tubular scaffold was constructed by wrapping a stack of two axisymmetric pieces of the anisotropic PGS membranes on a mandrel. The compliance of the scaffold was found to depend on the pitch angle of its double helical structure, imitating the anisotropic mechanical behavior of the arterial media. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 760-770, 2018.

Trimetallic magnesium complexes bearing amine-bis(benzotriazole phenolate) derivatives as bifunctional catalysts for ring-opening polymerization and CO2/epoxide coupling.

New trimetallic magnesium complexes were synthesized for bifunctional catalysis. Complex catalysed ε-caprolactone polymerizations in not only a 'living' manner but also in an 'immortal' fashion. Additionally, tri-Mg was active for the coupling of CO(2) with cyclohexene oxide (CHO) on addition of n-Bu(4)NBr co-catalysts to give cis-cyclohexene carbonate under mild conditions.