Advances in soft robotics, materials science, and stretchable electronics have enabled rapid progress in soft grippers. Here, a critical overview of soft robotic grippers is presented, covering different material sets, physical principles, and device architectures. Soft gripping can be categorized into three technologies, enabling grasping by: a) actuation, b) controlled stiffness, and c) controlled adhesion. A comprehensive review of each type is presented. Compared to rigid grippers, end-effectors fabricated from flexible and soft components can often grasp or manipulate a larger variety of objects. Such grippers are an example of morphological computation, where control complexity is greatly reduced by material softness and mechanical compliance. Advanced materials and soft components, in particular silicone elastomers, shape memory materials, and active polymers and gels, are increasingly investigated for the design of lighter, simpler, and more universal grippers, using the inherent functionality of the materials. Embedding stretchable distributed sensors in or on soft grippers greatly enhances the ways in which the grippers interact with objects. Challenges for soft grippers include miniaturization, robustness, speed, integration of sensing, and control. Improved materials, processing methods, and sensing play an important role in future research.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201707035

Soft Robotic Grippers.

An overview on 3D printing technology: Technological, materials, and applications

We describe an approach to print composite polymers in high-resolution three-dimensional (3D) architectures that can be rapidly transformed to a new permanent configuration directly by heating. The permanent shape of a component results from the programmed time evolution of the printed shape upon heating via the design of the architecture and process parameters of a composite consisting of a glassy shape memory polymer and an elastomer that is programmed with a built-in compressive strain during photopolymerization. Upon heating, the shape memory polymer softens, releases the constraint on the strained elastomer, and allows the object to transform into a new permanent shape, which can then be reprogrammed into multiple subsequent shapes. Our key advance, the markedly simplified creation of high-resolution complex 3D reprogrammable structures, promises to enable myriad applications across domains, including medical technology, aerospace, and consumer products, and even suggests a new paradigm in product design, where components are simultaneously designed to inhabit multiple configurations during service.

Direct 4D printing via active composite materials

Four-dimensional (4D) active shape-changing structures based on shape memory polymers (SMPs) and shape memory nanocomposites (SMNCs) are able to be controlled in both space and time and have attracted increasing attention worldwide. However, conventional processing approaches have restricted the design space of such smart structures. Herein, 4D active shape-changing architectures in custom-defined geometries exhibiting thermally and remotely actuated behaviors are achieved by direct-write printing of ultraviolet (UV) cross-linking poly(lactic acid)-based inks. The results reveal that, by the introduction of a UV cross-linking agent, the printed objects present excellent shape memory behavior, which enables three-dimensional (3D)–one-dimensional (1D)–3D, 3D–two-dimensional (2D)–3D, and 3D–3D–3D configuration transformations. More importantly, the addition of iron oxide successfully integrates 4D shape-changing objects with fast remotely actuated and magnetically guidable properties. This research realizes ...

Direct-Write Fabrication of 4D Active Shape-Changing Structures Based on a Shape Memory Polymer and Its Nanocomposite

Soft robots have the appealing advantages of being highly flexible and adaptive to complex environments. However, the low-stiffness nature of the constituent materials makes soft robotic systems incompetent in tasks requiring relatively high load capacity. Despite recent attempts to develop stiffness-tunable soft actuators by employing variable stiffness materials and structures, the reported stiffness-tunable actuators generally suffer from limitations including slow responses, small deformations, and difficulties in fabrication with microfeatures. This work presents a paradigm to design and manufacture fast-response, stiffness-tunable (FRST) soft actuators via hybrid multimaterial 3D printing. The integration of a shape memory polymer layer into the fully printed actuator body enhances its stiffness by up to 120 times without sacrificing flexibility and adaptivity. The printed Joule-heating circuit and fluidic cooling microchannel enable fast heating and cooling rates and allow the FRST actuator to complete a softening–stiffening cycle within 32 s. Numerical simulations are used to optimize the load capacity and thermal rates. The high load capacity and shape adaptivity of the FRST actuator are finally demonstrated by a robotic gripper with three FRST actuators that can grasp and lift objects with arbitrary shapes and various weights spanning from less than 10 g to up to 1.5 kg.

Fast-Response, Stiffness-Tunable Soft Actuator by Hybrid Multimaterial 3D Printing

This article presents a novel method of shape memory polymer (SMP) processing for additive manufacturing, in particular, fused-deposition modeling (FDM). Critical extrusion process parameters have been experimented to determine an appropriate set of parameter values so that good-quality SMP filament could be made for FDM. In the FDM process, effects of different printing parameters such as extruder temperature and scanning speed on object printing quality are also studied. In all the process studies, we aim to achieve good-quality parts by evaluating part density, tensile strength, dimensional accuracy, and surface roughness. Based on these studies, sample SMP models have been successfully built. Due to the thermal sensitive nature of the printed SMP parts, they can potentially be used as fasteners in active assembly/disassembly, smart actuators, deployable structures for aero-space applications, etc.

3D printing of shape memory polymer for functional part fabrication

The compliance of soft grippers contributes to their great superiority over rigid grippers in grasping irregularly shaped objects and forming soft contact with environments. Due to a relatively small pressure, soft grippers lack the stiffness required for wider applications. Particle jamming has been frequently reported as a means of stiffness control. Unlike previous research using vacuum for particle jamming, this paper proposes a novel passive particle jamming principle that does not need any vacuum power or other control means. The proposed method is by simply patching a silicone rubber soft actuator and a pack (made of strain-limiting membrane) of particles to form an integral gripping finger. The inflation of the soft actuator applies a pressure to the particle pack causing particles inside it to jam. A larger squeezing pressure will result in tighter particle jamming, thus increasing the stiffness of the finger. The stiffness of the finger is controllable as it is proportional to the actuator's air pressure, which has been verified by experiments in this research. The stiffness can increase more than six fold when air pressure changes from 20 to 80 kPa in the experimental studies. The reported discovery may enhance the capabilities of soft robotic grippers so that more robotic picking operations could be performed by soft grippers.

Passive Particle Jamming and Its Stiffening of Soft Robotic Grippers

In this article, we have proposed a novel robotic finger design principle aimed to address two challenges in soft pneumatic grippers-the controllability of the stiffness and the controllability of the bending position. The proposed finger design is composed of a 3D printed multimaterial substrate and a soft pneumatic actuator. The substrate has four polylactic acid (PLA) segments interlocked with three shape memory polymer (SMP) joints, inspired by bones and joints in human fingers. By controlling the thermal energy of an SMP joint, the stiffness of the joints is modulated due to the dramatic change in SMP elastic modulus around its glass transition temperature (Tg). When SMP joints are heated above Tg, they exhibit very small stiffness, allowing the finger to easily bend around the SMP joints if the attached soft actuator is actuated. When there is no force from the soft actuator, shape recovery stress in SMP contributes to the finger's shape restoration. Since each joint's rotation can be individually controlled, the position control of the finger is made possible. Experimental analysis has been conducted to show the finger's variable stiffness and the result is compared with the analytical values. It is found that the stiffness ratio can be 24.9 times for a joint at room temperature (20°C) and at an elevated temperature of 60°C when air pressure p of the soft actuator is turned off. Finally, a gripper composed of two fingers is fabricated for demonstration.

Bioinspired Robotic Fingers Based on Pneumatic Actuator and 3D Printing of Smart Material

Compared to traditional rigid robots, soft robots, primarily made of deformable, or less rigid materials, have good adaptability, conformability and safety in interacting with the environment. Although soft robots have shown great potentials for extended applications and possibilities that are impossible or difficult for rigid body robots, it is of great importance for them to have the capability of controllable stiffness modulation. Stiffness modulation allows soft robots to have reversible change between the compliant, or flexible state and the rigid state. In this paper, we summarize existing principles and methods for stiffness modulation in soft robotic development and divide them into four groups based on their working principles. Acoustic-based methods have been proposed as the potential fifth group in stiffness modulation of soft robots. Initial design proposals based on the proposed acoustic method are presented, and challenges in further development are highlighted.

Yingtian Li

Papers

3D printing of shape memory polymer for functional part fabrication

Passive Particle Jamming and Its Stiffening of Soft Robotic Grippers

Bioinspired Robotic Fingers Based on Pneumatic Actuator and 3D Printing of Smart Material

Principles and methods for stiffness modulation in soft robot design and development

Novel Design and Three-Dimensional Printing of Variable Stiffness Robotic Grippers