Soft-rigid Hybrid Research Articles

Stiffness-Tunable Soft Gripper with Soft-Rigid Hybrid Actuation for Versatile Manipulations.

Highly flexible and environmentally adaptive soft robots have received considerable attention. There remains a demand for soft robots to realize the stiffness modulation and variable workspace for robust and versatile manipulations. This article presents a compact soft gripper with a polylactic acid-based variable stiffness module (VSM) and a rigid retractable mechanism to achieve soft-rigid hybrid actuation. The soft gripper can enhance its stiffness by 18-fold without sacrificing flexibility due to the VSM. A heating circuit is designed to divide the VSM into three regions. Each region can be activated separately for varying flexible segments to amplify the dexterity. Meanwhile, the water-cooling system accelerates the heat exchange, thus reducing the cooling time from ∼400 to 39 s. The rigid retractable mechanism can adjust the initial layout of the gripper to expand the workspace and perform manipulation by opening and closing fingers. The soft finger combined with stiffness tunability can maintain its deformation after being stiffened to realize morphing. Therefore, it can efficiently perform a grasp with a high load and avoid repeated heating and cooling, especially for items with a similar shape. The performance of the gripper is further validated by measuring the grasping force and grasping demonstration with various objects, showing its robustness and dexterity in versatile tasks.

Development of modular multi-degree-of-freedom hybrid joints and robotic flexible legs via fluidic elastomer actuators

Soft actuators with high safety, adaptivity, and energy-to-weight ratio have the potential to be used in developing more adaptive legged robots. In this work, we incorporate soft actuators into rigid parallel mechanisms and develop multi-degree-of-freedom (multi-DOF) soft-rigid hybrid joints that can actively achieve 1, 2, and 3 DOFs actuated by 2, 4, and 8 bellows-type fluidic elastomer actuators (FEAs), respectively. The FEAs exhibit large axial strain (ϵ e max = 176%, ϵ c max = 25%), small radial expansion (ϵ r max = 12%) at 70 kPa, and are light weight, and the rigid parallel mechanisms constrain motions of the joints to the desired DOFs. We characterize the proposed joints’ kinematic and static performances by measuring their range of motion and blocked torque upon actuation. Results show that these joints successfully achieve all desired DOFs and are of high torque to weight ratio (4.07 N·m·kg−1). A bucking prediction model is established to evaluate the critical buckling pressure. As a demonstration for legged robots, we use the proposed joints and develop two types of multi-DOF legs based on inspirations from the DOF configuration of legged mammals’ musculoskeletal systems. Preliminary results demonstrate that FEAs-based multi-DOF legs can perform fundamental biomimetic movements (e.g. leg swing) through pressure adjustment, and high-speed tasks (e.g. ball kicking and jumping) through high-pressure and short-pulse actuation.

Enhancing the Universality of a Pneumatic Gripper via Continuously Adjustable Initial Grasp Postures

Recently, various soft universal grippers have been developed due to their reduced control complexity and satisfying grasping capability. The gripping range of a gripper plays a key role in its universality. This article presents a pneumatically actuated soft-rigid hybrid multifinger gripper that has a wide gripping range by adjusting its initial grasp postures. The gripper is compact (dimensions: <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{100} \times \text{60} \times \text{170}$</tex-math></inline-formula> mm), lightweight (weight: 380 g), and modular. It consists of four modules, with each module containing three pneumatic actuators (a distance-adjusting actuator, an angle-adjusting actuator, and a finger actuator) and rigid connectors. Through initial grasp posture adjustment, and fingers with tapered angles to generate nonconstant bending while grasping, the gripper can grasp objects of a wide variety of sizes and weights, thus increasing its universality. The gripper is tested to characterize its distance adjustment range, angle adjustment range, and stiffnesses in load-bearing directions, at continuously changing pneumatic pressures. The distance adjustment range and angle adjustment range of the gripper are 0–64.4 mm (64.4% of the initial gripper length) and 0 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ}$</tex-math></inline-formula> –140 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ}$</tex-math></inline-formula> , respectively. The maximum stiffnesses of distance-adjusting actuator and angle-adjusting actuator are 3331 and 1.15 N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\cdot$</tex-math></inline-formula> m/rad, respectively. Finally, grasping experiments show that our gripper can successfully grasp objects with diameters ranging from 0.5 to 180 mm, lengths ranging from 10 to 325 mm, and the heaviest object it can grasp is 2.1 kg (more than five folds of its own weight). The results demonstrated that our pneumatic gripper has an increased gripping range without adding other types of energy sources and its enhanced universality will expedite various applications in daily life and industry.

Transforming non-adhesive hydrogels to reversible tough adhesives via mixed-solvent-induced phase separation

Non-adhesive hydrogels are tailored to show tough adhesion to various solid surfaces by a universal phase-separation method. This method opens the possibility of utilizing non-adhesive hydrogels for emerging soft–rigid hybrid devices.

A bio-inspired soft-rigid hybrid actuator made of electroactive dielectric elastomers

In this article, inspired by biological structures, we develop a soft-rigid hybrid actuator (SRHA), based on fiber-reinforced dielectric elastomers (DEs) and a skeleton backbone system. The skeleton backbone is designed by mimicking the septa-muscle structure of earthworms. The fiber-reinforced DEs are conceived from the unidirectional and antagonistic deformation of natural muscles. Effect of distribution of skeletons on the SRHA is firstly studied through both experiments and FEM simulations. Besides, the fabricated SRHA is discovered to be with two actuation modes: (a) the voltage control mode, which is suitable for small deformation and accurate tuning; (b) the pressure control mode, which can be utilized for high output force and coarse tuning. Moreover, upon the critical pneumatic pressure (a prime trigger which highly relies on the distribution of skeletons), the pressure control mode can evolve to an enhancement mode by harnessing the buckling instability. Furthermore, in this enhanced mode, the electric power reduces to an ultra-low threshold for DEAs (even less than 2000 V), and hopefully degrades into a direction guiding disturbance. On the other hand, to ensure the stiffness maintaining capability, based on the fiber jamming mechanism, the variable stiffness unit (VSU) is designed for the SRHAs by absorbing the inspiration from the protective constraint mechanism of ligament-bone system. Remarkable stiffness maintaining capability of SRHA is experimentally proved to carry more than 20 times loads than its own weight. This proposed SRHA holds a great promise for application in continuum soft robotics explorations.

A Layered Manufacturing Approach for Soft and Soft-Rigid Hybrid Robots.

We present a manufacturing process for creating centimeter-scale multichambered inflatable robots and structures that can include both soft and rigid components. Our process uses a thermoplastic polyurethane (TPU) adhesive film to bond together layers of textiles, plastics, or other materials. The structures are heated and compressed a few layers at a time with a heat press machine or bonded in an oven all at once. We present two methods for arranging textiles and thermal adhesive film to achieve airtight structures and perform modeling and measurements on the resulting inflatable chambers. We characterize the set of textiles and rigid materials that will work with this process, measuring how strongly the TPU film bonds with them. We also describe how to include corners, where several pieces of material come together at a point, and determine which corner constructions are airtight. We characterize how different seam widths behave, determine the maximum pressure chambers fabricated with this process can support, and determine the cycle life of actuators built with this process. Finally, we present an actuator with an embedded sensor and three examples of robots constructed with textiles and TPU film, including a hybrid soft/rigid robotic arm, a soft robot that can roll along the ground, and a robot that can climb inside tubes or other confined spaces.

Geometric Confined Pneumatic Soft–Rigid Hybrid Actuators

In this work, we propose a new kind of soft-rigid hybrid actuator composed mainly of soft chambers and rigid frames. Compared with the well-known fiber-reinforced soft actuators, the hybrid actuators are able to ensure the design of noncircular cross-sectional shapes. It is demonstrated that rigid frames are capable of providing geometric constraints, reducing the ineffective deformation, and improving the energy utilization for the hybrid actuators with noncircular cross-sections. The essential characteristics of rigid constraints and flexible constraints are obtained by simulation and experiments on specimens with three different cross-sectional shapes. Furthermore, a spring-fluid film model is introduced to characterize the behavior of a representative hybrid linear actuator and a bending actuator with a rectangular cross-section, and it is also proved by the corresponding experiments. The change of the cross-sectional shape of fiber-reinforced soft actuators under pressurization is also explained theoretically as a contrast. Then, two application examples, namely, a robotic gripper and a caudal fin formed from linear actuators, are designed and demonstrated, showing the advantages and potential applications of the proposed geometric confined hybrid actuators. The proposed soft-rigid hybrid actuators combine the properties of soft and rigid materials, expand the design scope of the compliant actuators, and provide new solutions for robotics, especially for soft robots with specific requirements for their shapes or profiles.

Driving of microgripper having soft-rigid hybrid structures

Driving of microgripper having soft-rigid hybrid structures

Structurally isolated photoactuation of graphene-mixed temperature-responsive hydrogels in soft-rigid series structure

This paper presents fabrication and actuation methods for a soft microrobot with a hybrid structure composed of soft microactuators and a rigid supporting body. This hybrid structure enables actuation of the microrobot with independent driving of multiple actuators to provide complex movement like that of living microorganisms. We use the temperature-responsive hydrogel poly(N-isopropylacrylamide) (PNIPAAm) as a soft microactuator. PNIPAAm swells with water at low temperature but shrinks at high temperature. This volume change thus allows PNIPAAm to be used as an actuator by controlling its temperature. We successfully fabricated the microrobot with its soft-rigid hybrid structure composed of PNIPAAm and rigid photoresist using a multilayered microfabrication process. In addition, we used a sacrificial layer process to release the fabricated microrobot from the substrate to allow it to move freely. To actuate the microrobot, we mixed PNIPAAm with graphene, which has a high photothermal conversion efficiency. The temperature of the soft actuator when mixed with graphene can be increased by irradiating it with light. Therefore, actuation of the microrobot is achieved by sequentially irradiating the microactuators with focused light. We present the fabrication, release and partial actuation of the microrobot to demonstrate the feasibility of the proposed microrobot with the soft-rigid hybrid structure in this paper.

A Triboelectric Nanogenerator as a Self‐Powered Sensor for a Soft–Rigid Hybrid Actuator

AbstractTriboelectric nanogenerators are a promising technology for use in self‐powered sensors, which take advantage of multiple choices of materials and high response to low‐frequency triggering. Here, a conductive sponge/porous silicone‐based triboelectric nanogenerator (SS‐TENG) is demonstrated as a tactile sensor for a hybrid actuator. To fabricate the SS‐TENGs, liquid silicone and ethanol are mixed well and then infiltrated inside a conductive sponge. When heated, the ethanol evaporates from the silicone, leaving plenty of empty pores. Every pore supplies the contact and separation space for the conductive sponge and silicone to form a micro TENG, and the whole bulk makes up the SS‐TENG. When deformed, the SS‐TENG outputs electric signals. The open‐circuit voltage linearly increases from 3.9 to 17 V as the force increases from 2.9 to 9.9 N. In addition, when touched, the electric signals generated by SS‐TENGs can differentiate the corresponding objects. Finally, the SS‐TENGs are integrated with a soft–rigid hybrid actuator to form a bionic skeleton–muscle–skin hybrid gripper. The gripper can catch different objects and feedback electric signals, which show great potential for use in bionic robots.

Fabrication of micro-gel robot having soft-rigid hybrid structures driven by light

Fabrication of micro-gel robot having soft-rigid hybrid structures driven by light

Chain-Like Granular Jamming: A Novel Stiffness-Programmable Mechanism for Soft Robotics

The ability to regulate the mechanical stiffness in a large range could be extremely important for soft robots to interact with the environment more effectively. In this article, we propose a novel chain-like granular jamming mechanism to achieve a large range of stiffness variation instantly, based on a method that is totally different from existing vacuum-based granular jamming systems. Theoretical modeling is introduced to find the best combination of granules to form the chain-like structure (CLS) and experiments are conducted to demonstrate it. The experimental results indicate that the novel jamming structure is able to achieve a stiffness variation range as large as 50.7 folds. To further validate the effectiveness of the CLS, a soft-rigid hybrid actuator based on the jamming structure is proposed and an integrated fabrication method is provided. Furthermore, an anthropomorphic hand based on the hybrid actuators is developed and the experimental results show that the hand is not only versatile enough to manipulate various objects with different weights, material properties, shapes, and surface characteristics at the soft state, like existing soft grippers, but also can lift heavy objects (1.5 kg in a cylindrical grasping gesture and 3.52 kg in a hook gesture) at the rigid state, which could be difficult for other soft grippers. Finally, the hand is integrated into our homemade service robot, significantly improving the practicability and safety of the robot when serving humans.

Soft Robots Manufacturing: A Review.

The growing interest in soft robots comes from the new possibilities offered by these systems to cope with problems that cannot be addressed by robots built from rigid bodies. Many innovative solutions have been developed in recent years to design soft components and systems. They all demonstrate how soft robotics development is closely dependent on advanced manufacturing processes. This review aims at giving an insight on the current state of the art in soft robotics manufacturing. It first puts in light the elementary components that can be used to develop soft actuators, whether they use fluids, shape memory alloys, electro-active polymers or stimuli-responsive materials. Other types of elementary components, such as soft smart structures or soft-rigid hybrid systems, are then presented. The second part of this review deals with the manufacturing methods used to build complete soft structures. It includes molding, with possibly reinforcements and inclusions, additive manufacturing, thin-film manufacturing, shape deposition manufacturing, and bonding. The paper conclusions sums up the pros and cons of the presented techniques, and open to developing topics such as design methods for soft robotics and sensing technologies.

Lithium Metal Anodes: Artificial Soft–Rigid Protective Layer for Dendrite‐Free Lithium Metal Anode (Adv. Funct. Mater. 8/2018)

In article number 1705838, Jia-Qi Huang and co-workers demonstrate a soft–rigid hybrid artificial protective layer for dendrite-free lithium metal anodes. The hybrid artificial protective layer can tolerate large volume fluctuations and suppress the growth of lithium dendrites during repeated lithium plating/stripping. This stratergy provides a feasible approach to improve Li metal anodes towards the application of high-energy-density lithium metal batteries.

Fabrication of micro gel robot having a soft-rigid hybrid structure

Fabrication of micro gel robot having a soft-rigid hybrid structure