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Over the past two decades, the development of hydrogels with high mechanical strength and toughness shows both fundamental and practical significances. It is challenging yet highly desired to process these tough hydrogels into structural elements with desired architectures, which are important for real applications. Three-dimensional (3D) printing, as an emerging additive manufacturing strategy, demonstrates great potentials for engineering tough hydrogels with versatile applications. This paper aims to review the recent progress on 3D printing of tough hydrogels and their relevant applications. We give an overview on the advantages and requirements of tough hydrogel systems for specific 3D printing technologies, followed by detailed summary of the network structures, mechanical performances, toughening mechanisms, and representative applications of the printed tough hydrogels. Finally, recent advances of 4D printing of tough hydrogels are discussed as a prospective direction to devise soft actuators and robots. [ABSTRACT FROM AUTHOR]

Soft actuators with large deformation and high stability in response to multi-stimuli are highly demanded in biomimetic applications. However, most of the present actuators driven by a single stimulus often exhibit low performance and high energy consumption. In this work, a sandwich structural soft actuator (MPDMS/MXene/PTFE) which possesses electrothermal/magnetic coupling actuation is fabricated by combining magnetic NdFeB/Polydimethylsiloxane (MPDMS) composite layer, MXene film, and PTFE tape together. Due to the large difference of thermal expansion coefficient between MPDMS film and PTFE tape, the soft actuator shows a wonderful electrothermal actuation and the bending deformation can reach to as high as 353° under a 3 V power supply. The excellent electrical conductivity of MXene component endows the sandwich film with good thermal management performance, thus the soft MPDMS/MXene/PTFE sandwich films can be applied in the thermal treatment of body and thermal imaging. Most importantly, the soft actuator also exhibits a typical magnetic biomimetic actuation and the programmable actuation performance of the dragonfly actuator can be demonstrated by both experiments and finite element simulations. Eventually, the electrothermal/magnetic coupling actuation is successfully applied in an intelligent crawling robot to overcome the traditional inchworm movement simulation. A dual-mode biomimetic soft actuator based on MPDMS/MXene/PTFE with excellent electrothermal and magnetic drive performance is developed. The soft actuator enables to prepare grippers for transferring objects and assemble the wings of a biomimetic dragonfly in a magnetic field. Finally, a crawling robot is developed to demonstrate the electrothermal/magnetic coupling actuation. [Display omitted] • The highlight of this work is: • The MPDMS/MXene/PTFE soft actuator presents both electrothermal and magneto-actuated deformation behaviors. • The soft actuator shows a wonderful electrothermal actuation and the bending deformation can reach to as high as 353° under a 3 V power supply. • The finite element analysis model can provide design guidance for the deformation process of bionic actuator with complex structure. • The soft actuator strategy presented in this paper is expected to be useful for the development of bionic intelligent actuators with multi-stimulus response. [ABSTRACT FROM AUTHOR]

Abstract: Actuation of electrochemical, electro-magnetic, and electro-active actuators composed of CNF and/or Ni nanowire/polymer nanocomposites was evaluated with different materials and preparation processes. The actuated strain was compared with frequency, applied voltage, and wave type. The hysteresis of the actuated strain was continuously delayed in electrochemical actuators, whereas the strain was uniformly actuated in the electro-magnetic actuators. The actuated strain decreased with increasing frequency in both electrochemical and electro-magnetic actuators. In magnetic fields the actuated strain increased with increasing Ni nanowire content whereas the current increased with applied voltage. Ni nanowire/cellulose actuators in a magnetic field responded better at high frequencies, compared to the other actuators studied. Actuated strain of cellulose or Ni nanowire/cellulose nanocomposite in air was larger than either PVDF or PVDF/cellulose nanocomposite. In Ni nanowire/cellulose nanocomposite, actuated strain also decreased with increasing frequency and increased with increasing voltage. Electro-active actuators responded well in air when low voltages and high frequencies were applied compared to the other two actuators. Electro-active actuators in this paper have unique advantages for many practical applications, including easy fabrication, lightweight and low application voltages. [Copyright &y& Elsevier]

Abstract: Piezoelectric fiber composites were developed to overcome drawbacks of typical monolithic piezoceramic (PZT) actuators. Although piezoelectric fiber composites had many improvements over the monolithic PZT, there are still improvements. Thus, the single crystal piezoelectric fiber composite actuator is proposed. Single crystal piezoelectric materials such as PMN-PT have larger piezoelectric strain constants, higher bandwidth and higher energy density than polycrystalline counterparts. Piezoelectric fiber composites can improve the performance of various structures, and can be subject to wide temperature range where the thermoelastic behavior is important. Therefore, this paper studies the coefficients of thermal expansion (CTE) for single crystal piezoelectric fiber composites. The Macro Fiber Composite (MFC) as the piezoelectric fiber composite is considered. To calculate the effective properties of two orthotropic layers of the MFC, PMN-PT(or PZT)/epoxy and copper/epoxy layers, the rule of mixture is adopted. With the effective properties known for each layers, the two CTE of the MFC actuator are obtained from the classical lamination theory considering thermal effects. The difference of the CTE between the single crystal MFC and the standard MFC is studied. [Copyright &y& Elsevier]