Fast Thermal Actuators for Soft Robotics

Robotic flytrap with an ultra-sensitive 'trichome' and fast-response 'lobes'

Nature abounds with examples of ultra-sensitive perception and agile body transformation for highly efficient predation as well as extraordinary adaptation to complex environments. Flytraps, as a representative example, could effectively detect the most minute physical stimulation of insects and respond instantly, inspiring numerous robotic designs and applications. However, current robotic flytraps face challenges in reproducing the ultra-sensitive insect-touch perception. In addition, fast and fully-covered capture of live insects with robotic flytraps remains elusive. Here we report a novel design of a robotic flytrap with an ultra-sensitive 'trichome' and bistable fast-response 'lobes'. Our results show that the 'trichome' of the proposed robotic flytrap could detect and respond to both the external stimulation of 0.45 mN and a tiny touch of a flying bee with a weight of 0.12 g. Besides, once the 'trichome' is triggered, the bistable 'lobes' could instantly close themselves in 0.2 s to form a fully-covered cage to trap the bees, and reopen to set them free after the tests. We introduce the design, modeling, optimization, and verification of the robotic flytrap, and envision broader applications of this technology in ultra-sensitive perception, fast-response grasping, and biomedical engineering studies.

Encapsulated stretchable amphibious strain sensors

Soft and stretchable strain sensors have found wide applications in health monitoring, motion tracking, and robotic sensing. There is a growing demand for strain sensors in amphibious environments, such as implantable sensors, wearable sensors for swimmers/divers, and underwater robotic sensors. However, developing a sensitive, stretchable, and robust amphibious strain sensor remains challenging. This work presents an encapsulated stretchable amphibious strain sensor. The conductive layer, made of silver nanowires embedded below the surface of polydimethylsiloxane, was sandwiched by two layers of thermoplastic polyurethane. Periodic sharp cuts were introduced to change the direction of flow from across the sensor to along the conductive path defined by the opening cracks. The crack advancing and opening is controlled by a unique combination of weak/strong interfaces within the sandwich structure. The cut design and the interfacial interactions between the layers were investigated. The strain sensor exhibited a high gauge factor up to 289, a linear sensing response, a fast response time (53 ms), excellent robustness against over-strain, and stability after 16 000 loading cycles and 20 days in an aqueous saline solution. The functionality of this amphibious strain sensor was demonstrated by tracking the motion of a robotic fish, undertaking language recognition underwater, and monitoring the blood pressure of a porcine aorta. This illustrates the promising potential for this strain sensor for both underwater use and surgically implantable applications.

Fast Responsive and High-Strain Electro-Ionic Soft Actuator Based on the 3D-Structure MXene-EGaIn/MXene Bilayer Composite Electrode

Electro-ionic soft actuators have garnered significant attention owing to their promising applications in flexible electronics, wearable devices, and soft robotics. However, achieving high actuation performance (large bending strain and fast response time) of these soft actuators under low voltage has been challenging due to issues related to ion diffusion and accumulation. In this study, an electro-ionic soft actuator is fabricated using Ti3C2Tx MXene and eutectic gallium-indium (EGaIn) composite material as the bilayer electrode and methylammonium formate/1-ethyl-3-methylimidazolium tetrafluoroborate/poly(vinylidene fluoride) (MAF-EMIMBF4/PVDF) as the ionic liquid-type electrolyte. The research results indicate that the prepared soft actuator exhibits excellent actuation performance with a peak-to-peak displacement of 35 mm and a bending strain of 0.69% (a peak-to-peak strain of 1.38%) under a low voltage (3 V). The electro-ionic soft actuator shows a wide frequency range (0.1-10 Hz), fast response time (0.35 s), and a rise time of 7.5 s. Furthermore, it demonstrates good cyclic durability, with a retention rate of 92.5% of its performance for 10 000 cycles. These excellent performances are attributed to the 3D structure of the Ti3C2Tx-EGaIn/Ti3C2Tx bilayer composite electrode, as well as the characteristics of the low viscosity, high conductivity, small ion volume, and larger volume difference between cations and anions in MAF ionic liquid. The high-performance electro-ionic soft actuator can be used in various fields such as artificial muscles, tactile devices, and soft robots.

A Graphene-Mica-Based Photo-Thermal Actuator for Small-Scale Soft Robots

Small-scale soft robots demonstrate intricate life-like behavior and allow navigation through arduous terrains and confined spaces. However, the primary challenges in soft robotics are 1) creating actuators capable of quick, reversible 22D-to-3D shape morphing with adjustable stiffness, 2) improving actuation force and robustness for wider applications, and 3) designing holistic systems for untethered manipulation and flexible multimodality in practical scenarios. Here, mechanically compliant paper-like robots are presented with multiple functionalities. The robots are based on photothermally activated polymer bimorph actuators that incorporate graphene for the photo-thermal conversion of energy and muscovite mica, with its high Young's modulus, providing the required stiffness. Conversion of light into heat leads to thermal expansion and bending of the stress-mismatched structures. The actuators are designed on the basis of a modified Timoshenko model, and numerical simulations are employed to evaluate their actuation performance. The membranes can be utilized for light-driven programmable shape-morphing. Localized control allows the implementation of active hinges at arbitrary positions within the membrane. Integrated into small-scale soft robots in mass production, the membrane facilitates locomotion, rolling, and flipping of the robots. Further, grasping and kicking mechanisms are demonstrated, highlighting the potential of such actuators for future applications.

Modular multi-degree-of-freedom soft origami robots with reprogrammable electrothermal actuation

Soft robots often draw inspiration from nature to navigate different environments. Although the inching motion and crawling motion of caterpillars have been widely studied in the design of soft robots, the steering motion with local bending control remains challenging. To address this challenge, we explore modular origami units which constitute building blocks for mimicking the segmented caterpillar body. Based on this concept, we report a modular soft Kresling origami crawling robot enabled by electrothermal actuation. A compact and lightweight Kresling structure is designed, fabricated, and characterized with integrated thermal bimorph actuators consisting of liquid crystal elastomer and polyimide layers. With the modular design and reprogrammable actuation, a multiunit caterpillar-inspired soft robot composed of both active units and passive units is developed for bidirectional locomotion and steering locomotion with precise curvature control. We demonstrate the modular design of the Kresling origami robot with an active robotic module picking up cargo and assembling with another robotic module to achieve a steering function. The concept of modular soft robots can provide insight into future soft robots that can grow, repair, and enhance functionality.

Quasi-Static Modeling Framework for Soft Bellow-Based Biomimetic Actuators

Soft robots that incorporate elastomeric matrices and flexible materials have gained attention for their unique capabilities, surpassing those of rigid robots, with increased degrees of freedom and movement. Research has highlighted the adaptability, agility, and sensitivity of soft robotic actuators in various applications, including industrial grippers, locomotive robots, wearable assistive devices, and more. It has been demonstrated that bellow-shaped actuators exhibit greater efficiency compared to uniformly shaped fiber-reinforced actuators as they require less input pressure to achieve a comparable range of motion (ROM). Nevertheless, the mathematical quantification of the performance of bellow-based soft fluidic actuators is not well established due to their inherent non-uniform and complex structure, particularly when compared to fiber-reinforced actuators. Furthermore, the design of bellow dimensions is mostly based on intuition without standardized guidance and criteria. This article presents a comprehensive description of the quasi-static analytical modeling process used to analyze bellow-based soft actuators with linear extension. The results of the models are validated through finite element method (FEM) simulations and experimental testing, considering elongation in free space under fluidic pressurization. This study facilitates the determination of optimal geometrical parameters for bellow-based actuators, allowing for effective biomimetic robot design optimization and performance prediction.

Snapping for 4D-Printed Insect-Scale Metal-Jumper

The replication of jumping motions observed in small organisms poses a significant challenge due to size-related effects. Shape memory alloys (SMAs) exhibit a superior work-to-weight ratio, making them suitable for jumping actuators. However, the SMAs advantages are hindered by the limitations imposed by their single actuator configuration and slow response speed. This study proposes a novel design approach for an insect-scale shape memory alloy jumper (net-shell) using 4D printing technology and the bistable power amplification mechanism. The energy variations of the SMA net-shell under different states and loads are qualitatively elucidated through a spring-mass model. To optimize the performance of the SMA net-shell, a non-contact photo-driven technique is employed to induce its shape transition. Experimental investigations explore the deformation response, energy release of the net-shell, and the relationship between the light power density. The results demonstrate that the SMA net-shell exhibits remarkable jumping capabilities, achieving a jump height of 60 body lengths and takeoff speeds of up to 300 body lengths per second. Furthermore, two illustrative cases highlight the potential of net-shells for applications in unstructured terrains. This research contributes to miniaturized jumping mechanisms by providing a new design approach integrating smart materials and advanced structures.

Toward a Unified Naming Scheme for Thermo-Active Soft Actuators: A Review of Materials, Working Principles, and Applications

Soft robotics is a rapidly growing field that spans the fields of chemistry, materials science, and engineering. Due to the diverse background of the field, there have been contrasting naming schemes such as "intelligent," "smart," and "adaptive" materials, which add vagueness to the broad innovation among literature. Therefore, a clear, functional, and descriptive naming scheme is proposed in which a previously vague name-Soft Material for Soft Actuators-can remain clear and concise-Phase-Change Elastomers for Artificial Muscles. By synthesizing the working principle, material, and application into a naming scheme, the searchability of soft robotics can be enhanced and applied to other fields. The field of thermo-active soft actuators spans multiple domains and requires added clarity. Thermo-active actuators have potential for a variety of applications spanning virtual reality haptics to assistive devices. This review offers a comprehensive guide to selecting the type of thermo-active actuator when one has an application in mind. In addition, it discusses future directions and improvements that are necessary for implementation.

Selaginella lepidophylla-Inspired Multi-Stimulus Cooperative Control MXene-Based Flexible Actuator

Predictable bending deformation, high cycle stability, and multimode complex motion have always been the goals pursued in the field of flexible robots. In this study, inspired by the delicate structure and humidity response characteristics of Selaginella lepidophylla, a new multilevel assisted assembly strategy was developed to construct MXene-CoFe2O4 (MXCFO) flexible actuators with different concentration gradients, to achieve predictable bending deformation and multi-stimulus cooperative control of the actuators, revealing the intrinsic link between the gradient change and the bending deformation ability of the actuator. The thickness of the actuator shows uniformity compared with the common layer-by-layer assembly strategy. And, the bionic gradient structured actuator shows high cycle stability, and it maintains excellent interlayer bonding after bending 100 times. The flexible robots designed based on the predictable bending deformation and the multi-stimulus cooperative response characteristics of the actuator initially realize conceptual models of humidity monitoring, climbing, grasping, cargo transportation, and drug delivery. The designed bionic gradient structure and unbound multi-stimulus cooperative control strategy may show great potential in the design and development of robots in the future.

Caterpillar-inspired soft crawling robot with distributed programmable thermal actuation

Many inspirations for soft robotics are from the natural world, such as octopuses, snakes, and caterpillars. Here, we report a caterpillar-inspired, energy-efficient crawling robot with multiple crawling modes, enabled by joule heating of a patterned soft heater consisting of silver nanowire networks in a liquid crystal elastomer (LCE)-based thermal bimorph actuator. With patterned and distributed heaters and programmable heating, different temperature and hence curvature distribution along the body of the robot are achieved, enabling bidirectional locomotion as a result of the friction competition between the front and rear end with the ground. The thermal bimorph behavior is studied to predict and optimize the local curvature of the robot under thermal stimuli. The bidirectional actuation modes with the crawling speeds are investigated. The capability of passing through obstacles with limited spacing are demonstrated. The strategy of distributed and programmable heating and actuation with thermal responsive materials offers unprecedented capabilities for smart and multifunctional soft robots.

Highly Sensitive, Stretchable, and Robust Strain Sensor Based on Crack Propagation and Opening

Soft and stretchable strain sensors have been attracting significant attention. However, the trade-off between the sensitivity (gauge factor) and the sensing range has been a major challenge. In this work, we report a soft stretchable resistive strain sensor with an unusual combination of high sensitivity, large sensing range, and high robustness. The sensor is made of a silver nanowire network embedded below the surface of an elastomeric matrix (e.g., poly(dimethylsiloxane)). Periodic mechanical cuts are applied to the top surface of the sensor, changing the current flow from uniformly across the sensor to along the conducting path defined by the open cracks. Both experiment and finite element analysis are conducted to study the effect of the slit depth, slit length, and pitch between the slits. The stretchable strain sensor can be integrated into wearable systems for monitoring physiological functions and body motions associated with different levels of strain, such as blood pressure and lower back health. Finally, a soft three-dimensional (3D) touch sensor that tracks both normal and shear stresses is developed for human-machine interfaces and tactile sensing for robotics.

Snapping for high-speed and high-efficient butterfly stroke-like soft swimmer

Natural selection has tuned many flying and swimming animals to share the same narrow design space for high power efficiency, e.g., their dimensionless Strouhal numbers St that relate flapping frequency and amplitude and forward speed fall within the range of 0.2 < St < 0.4 for peak propulsive efficiency. It is rather challenging to achieve both comparably fast-speed and high-efficient soft swimmers to marine animals due to the naturally selected narrow design space and soft body compliance. Here, bioinspired by the flapping motion in swimming animals, we report leveraging snapping instabilities for soft flapping-wing swimmers with comparable high performance to biological counterparts. The lightweight, butterfly stroke-like soft swimmer (2.8 g) demonstrates a record-high speed of 3.74 body length/s (4.8 times faster than the reported fastest flapping soft swimmer), high power efficiency (0.2 < St = 0.25 < 0.4), low energy consumption cost, and high maneuverability (a high turning speed of 157°/s).

Stretchable Sensors and Electro-Thermal Actuators with Self-Sensing Capability Using the Laser-Induced Graphene Technology

Laser-induced graphene (LIG) represents a fast-speed and low-cost method to prepare the customizable graphene-based patterns in complex configurations with exceptional electrical performance. This paper presents the applications of LIG formed on the commercial polyimide (PI) film as the stretchable strain sensor and electrical-actuated actuators. First, the conductive performances of the LIG were systematically revealed under different fabrication conditions via investigating the effects of processing parameters, and the fluence of the laser was experimentally demonstrated as the only crucial parameter to evaluate the LIG formation, facilitating the selection of optimized manufacturing parameters to prepare the LIG with desired electrical performances. Then, the LIG-based strain sensor which can undergo over 50% tensile strain was fabricated by transfer of the LIG from the PI film to polydimethylsiloxane. The variety of LIG-based electro-thermal actuators to achieve pre-designed 3D architectures was presented, along with their parameter analysis. After incorporating the multimeter system, the actuator can even feedback its transformation from 2D precursor to 3D architecture by monitoring the resistance variation of LIG, revealing the integrated capability of our design in serving as sensors and actuators. Finally, the wearable glove with the LIG sensors was presented to demonstrate its ability to remotely control the soft robotic hand.