Bending behavior of sandwich composite structures with tunable 3D-printed core materials

In this work, we advance printed active composites by combining 3D printing, printed electronics, and liquid crystal elastomers (LCEs) to achieve soft actuators with free-standing two-way shape changing behaviors. Incorporated LCE strips are activated by Joule heating produced by printed conductive wires, while uniaxial deformation of the LCE strip is utilized as a driving force to achieve bending in the printed composite. The bending behavior of laminated hinges is first characterized in order to obtain a precise control of actuation, which is then exploited to actuate four demonstrative designs: a morphing airplane, a miura-ori structure, a cubic box, and a soft crawler. The soft morphing airplane and miura-ori structure are designed and fabricated with multiple laminated hinges to demonstrate the synergistic actions during actuation. The cubic box is constructed to show the capability of sequential folding by implementing multiple groups of conductive wires to achieve accurately addressable heating with temporal control. Finally, the two-way transformation is utilized as a driving force for the locomotion of a soft crawler stimulated by a periodic rectangular wave current. These examples show the great potential of using the hybrid 3D printing and pick-and-place method and using LCEs to achieve controllable shape change structures for a variety of potential practical applications.

3D printed reversible shape changing soft actuators assisted by liquid crystal elastomers

A variety of natural biological tissues (e.g., skin, ligaments, spider silk, blood vessel) exhibit 'J-shaped' stress-strain behavior, thereby combining soft, compliant mechanics and large levels of stretchability, with a natural 'strain-limiting' mechanism to prevent damage from excessive strain. Synthetic materials with similar stress-strain behaviors have potential utility in many promising applications, such as tissue engineering (to reproduce the nonlinear mechanical properties of real biological tissues) and biomedical devices (to enable natural, comfortable integration of stretchable electronics with biological tissues/organs). Recent advances in this field encompass developments of novel material/structure concepts, fabrication approaches, and unique device applications. This review highlights five representative strategies, including designs that involve open network, wavy and wrinkled morphologies, helical layouts, kirigami and origami constructs, and textile formats. Discussions focus on the underlying ideas, the fabrication/assembly routes, and the microstructure-property relationships that are essential for optimization of the desired 'J-shaped' stress-strain responses. Demonstration applications provide examples of the use of these designs in deformable electronics and biomedical devices that offer soft, compliant mechanics but with inherent robustness against damage from excessive deformation. We conclude with some perspectives on challenges and opportunities for future research.

Design and application of ‘J-shaped’ stress–strain behavior in stretchable electronics: a review

DOI: 10.1002/admt.201900837 high-cost, and unsuitable for interaction with humans as well as intricate environments.[1–3] To overcome these challenges, great efforts have been taken to develop soft robots that are primarily made of compliant elastomers and polymers (silicone rubber, dielectric elastomers, liquidcrystalline elastomers and hydrogels, etc.) and possess unique properties such as lightweight, mechanical compliance, infinite degrees of freedom, continuous deformation, low cost, and easy fabrication.[2–6] Among these soft robotics, the class of soft crawling robots inspired from biological creatures has attracted increasing attention owing to their anticipated effective interaction with humans and uncertain environments, as well as potential capabilities of completing a variety of tasks, like search and rescue, infrastructure inspection, surveillance, drug delivery, and human assistance.[2,5] Various novel actuations and locomotion designs have been explored to drive soft crawling robots, with some of them even equipped with capabilities of the autonomous motion,[7] environment accommodation[4,8] and decision making.[9,10] For instance, soft crawling robots inspired by octopuses, worms, and jellyfishes have been designed to achieve complex motions with multiple gaits at low cost.[10,11] While there have been several excellent review papers on the topic of soft robotics,[3,4,12,13] there has not been a review paper that covers recent advances in soft crawling robots in depth. This review paper aims to fill that void. The locomotion mode, crawling speed, and working efficiency of soft crawling robots are mainly determined by the soft actuators they employed.[14] Generally, the external stimuli drive the actuators to generate the desired strains and/or deformations, and the induced strain and/or deformation will then supply the necessary propulsion for the robots to crawl. Up to now, a few actuation approaches have been successfully employed to drive the soft crawling robots, mainly consisting of pneumatic/hydraulic pressure,[15] chemical reaction,[16,18] and stimuli responses of soft active materials, such as dielectric elastomers (DEs),[19] shape memory alloys (SMAs),[20] magnetoactive elastomers (MAEs),[21] liquid-crystalline elastomers (LCEs),[22] piezoelectric materials (PEMs),[23] ionic polymer– metal composites (IPMCs),[24] and twisted and coiled polymers (TCPs).[25] Despite the inherent drawbacks of these actuation approaches, continuous efforts have been devoted to designing soft crawling robots capable of performing vivid, multigait, effective, and intelligent locomotion. Over the past decade, Soft crawling robots have attracted great attention due to their anticipated effective interactions with humans and uncertain environments, as well as their potential capabilities of completing a variety of tasks encompassing search and rescue, infrastructure inspection, surveillance, drug delivery, and human assistance. Herein, a comprehensive survey on recent advances of soft crawling robots categorized by their major actuation mechanisms is provided, including pneumatic/hydraulic pressure, chemical reaction, and soft active material-based actuations, which include dielectric elastomers, shape memory alloys, magnetoactive elastomers, liquid crystalline elastomers, piezoelectric materials, ionic polymer–metal composites, and twisted and coiled polymers. For each type of actuation, the prevalent modes of locomotion adopted in representative robots, the design, working principle and performance of their soft actuators, and the performance of each locomotion approach, as well as the advantages and drawbacks of each design are discussed. This review summarizes the state-of-the-art progresses and the critical knowledge in designing soft crawling robots and offers a guidance and insightful outlook for the future development of soft robots.

Soft Crawling Robots: Design, Actuation, and Locomotion

Windshield is one of the most important components in automobile to protect pedestrian and passenger, as well as enhance vehicle crashworthiness. The most widely used material for windshield is polyvinyl butyral (PVB) laminated glass. This paper uses both quasi-static compression and dynamic Split Hopkinson Pressure Bar (SHPB) compression experiments under different strain/loading rates to investigate the mechanical behavior of PVB laminated glass. Experimental results show that PVB laminated glass is a strong rate-dependent material with nonlinearity in its constitutive behavior under both quasi-static and dynamic loading circumstances. In quasi-static cases, major failure onset (MFO) strain increases with the loading rate while MFO stress remains the same; however MFO strain and stress will both increase in higher strain rates in dynamic loading. A constitutive model covering all strain/loading rates is then established based on Johnson–Cook model mathematically. Further, crack branching based on crack fractal theory is investigated and an explicit expression describing the crack velocity and number of crack branching is proposed. In addition, dynamic stress intensify factor is calculated to be β ≈ 1.5 and an “impact caused brittleness” effect in PVB laminated material is discussed. Results can provide important experiment data and useful model to further research on vehicle crash safety.

Experimental study on mechanical behavior of PVB laminated glass under quasi-static and dynamic loadings

Characteristics of windshield cracking upon low-speed impact: Numerical simulation based on the extended finite element method

When a stiff nanowire is deposited on a compliant soft substrate, it may buckle into a helical coil form when the system is compressed Using theoretical and finite element method (FEM) analyses, the detailed three-dimensional coil buckling mechanism for a silicon nanowire (SiNW) on a polydimethylsiloxane (PDMS) substrate is studied A continuum mechanics approach based on the minimization of the strain energy in the SiNW and elastomeric substrate is developed Due to the helical buckling, the bending strain in SiNW is significantly reduced and the maximum local strain is almost uniformly distributed along SiNW Based on the theoretical model, the energy landscape for different buckling modes of SiNW on PDMS substrate is given, which shows that both the in-plane and out-of-plane buckling modes have the local minimum potential energy, whereas the helical buckling model has the global minimum potential energy Furthermore, the helical buckling spacing and amplitudes are deduced, taking into account the influences of the elastic properties and dimensions of SiNWs These features are verified by systematic FEM simulations and parallel experiments As the effective compressive strain in elastomeric substrate increases, the buckling profile evolves from a vertical ellipse to a lateral ellipse, and then approaches to a circle when the effective compressive strain is larger than 30% The study may shed useful insights on the design and optimization of high-performance stretchable electronics and 3D complex nano-structures

Helical coil buckling mechanism for a stiff nanowire on an elastomeric substrate

Experimental and macroscopic investigation of dynamic crack patterns in PVB laminated glass sheets subject to light-weight impact

Superelasticity and reversible energy absorption of polyurethane cellular structures with sand filler

A novel mechanism for slithering locomotion of a snake-like soft robot is presented. A rectangular beam with an isotropic coefficient of friction of its contact surface with the flat ground can move forward or backward when actuated by a periodic traveling sinusoidal wave. The Poisson's ratio of the beam plays an important role in the slithering locomotion speed and direction, particularly when it is negative. A theoretical model is proposed to elucidate the slithering locomotion mechanism, which is analogous to the rolling of a wheel on ground. There are two key factors of slithering locomotion: a rotational velocity field and a corresponding local contact region between the beam and ground. During wriggling motion of the rectangular beam, a rotational velocity field is observed near the maximum curvature point of the beam. If the beam has a negative Poisson's ratio, the axial tension will cause a lateral expansion so that the contact region between the beam and ground is located at the outer edge of the maximum curvature (the largest lateral expansion point). The direction of the beam's velocity at this outer edge is usually opposite to the traveling wave direction, so the friction force propels the beam in the direction of the traveling wave. A similar scenario is found for the relatively large amplitude of wriggling motion when the beam's Poisson's ratio is positive. Finite element method (FEM) simulation was conducted to verify the slithering locomotion mechanism, and good agreement was found between the FEM simulation results and theoretical predictions. The insights obtained here present a simple, novel and straightforward mechanism for slithering locomotion and are helpful for future designs of snake-like soft robots.

Yuan Yan

Papers

Characteristics of windshield cracking upon low-speed impact: Numerical simulation based on the extended finite element method

Helical coil buckling mechanism for a stiff nanowire on an elastomeric substrate

Experimental and macroscopic investigation of dynamic crack patterns in PVB laminated glass sheets subject to light-weight impact

Superelasticity and reversible energy absorption of polyurethane cellular structures with sand filler

A novel slithering locomotion mechanism for a snake-like soft robot