Shape-memory polymers and their composites: Stimulus methods and applications

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.

Stimuli-responsive (active) materials undergo large-scale shape or property changes in response to an external stimulus such as stress, temperature, light or pH1,2. Technological uses range from durable, shape-recovery eye-glass frames, to temperature-sensitive switches, to the generation of stress to induce mechanical motion3,4,5,6,7,8,9. Here, we demonstrate that the uniform dispersion of 1–5 vol.% of carbon nanotubes in a thermoplastic elastomer yields nanocomposites that can store and subsequently release, through remote means, up to 50% more recovery stress than the pristine resin. The anisotropic nanotubes increase the rubbery modulus by a factor of 2 to 5 (for 1–5 vol.%) and improve shape fixity by enhancing strain-induced crystallization. Non-radiative decay of infrared photons absorbed by the nanotubes raises the internal temperature, melting strain-induced polymer crystallites (which act as physical crosslinks that secure the deformed shape) and remotely trigger the release of the stored strain energy. Comparable effects occur for electrically induced actuation associated with Joule heating of the matrix when a current is passed through the conductive percolative network of the nanotubes within the resin. This unique combination of properties, directly arising from the nanocomposite morphology, demonstrates new opportunities for the design and fabrication of stimuli-responsive polymers, which are otherwise not available in one material system.

Remotely actuated polymer nanocomposites--stress-recovery of carbon-nanotube-filled thermoplastic elastomers.

Shape memory polymers (SMPs) belong to a class of smart polymers, which have drawn considerable research interest in last few years because of their applications in microelectromechanical systems, actuators, for self healing and health monitoring purposes, and in biomedical devices. Like in other fields of applications, SMP materials have been proved to be suitable substitutes to metallic ones because of their flexibility, biocompatibility and wide scope of modifications. The shape memory properties of SMPs polymers might surpass those of shape memory metallic alloys (SMAs). In addition to block copolymers, polymers blends and interpenetrating network structured SMP systems have been developed. The present review mainly highlights the recent progress in synthesis, characterization, evaluation, and proposed applications of SMPs and related composites.

Recent advances in shape memory polymers and composites : a review

Remotely actuated polymer nanocomposites-stress-recovery of carbon-nanotube-filled thermoplastic elastomers

Advances in shape memory polymer actuation

The measurement of temperature change as an indication of a material's relative thermal conductivity has often been utilized as a means of tactile sensing. Unfortunately, the long time response of most thermal sensors makes such a technique too slow for normal industrial robotic uses. The authors consider human tactile performance with particular regard to temperature sensing and introduce a means by which a usable risetime may be achieved. Two methods, using devices hitherto not utilized for tactile sensing, are demonstrated. >

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Thermal tactile sensing

Closed reduction of the long bones is associated with the use of considerable force. This force must be maintained for the reduction maneuver and fixation process. At present, apart from the extension table or the large AO distractor, only rather inadequate reduction aids are available. A solution to this problem is being sought in the form of a robotic system with which precision can be improved and the holding effort reduced. In the research project presented here, a synthetic femur with integrated tensioned mainspring and a 32-A3 type fracture served as a bone reduction model. The fracture was stabilized with a standard AO fixator. A Staubli robot (model RX130) was converted by appropriate modification so that it could be used for the reduction of femoral shaft fractures in vitro. The robot was equipped with a pneumatic 2-fingered gripper, on which the fingers have been modified so that they can grip the AO fixator clamp. A Force-Feedback-Sensor was inserted between the gripper and the robot to obtain online recordings of the forces and moments in all three axes. With this setup it is possible to achieve precise reduction of the fracture in all planes under visual control.

Reduction of femoral shaft fractures in vitro by a new developed reduction robot system 'RepoRobo'.

This paper illustrates means by which the techniques of compliance and electroadhesion can, using electror-heological, and other fluids, be combined to provide very effective shape adaptive robot end-effectors and similar holding surface.

Compliant robotic devices, and electroadhesion

The destacking and assembly of textile fabric garment components is a process found throughout the clothing industry. The paper considers in particular the example of shirt collar assembly, in which three fabric collar panels have to be separated from their respective stacks, inspected for flaws and then placed one on top of the other before being fed into a fusing press which makes the bond between them. There is a split in the ply. It is explained why handling by contact, by air jet separation and by vacuum pick-up are inappropriate. It is then explained how robotic grippers using electrostatic adhesion are used to accomplish the task. Such gripping devices have many potential uses within the textile (and other) industries. >

G.J. Monkman

Papers

Advances in shape memory polymer actuation

Thermal tactile sensing

Reduction of femoral shaft fractures in vitro by a new developed reduction robot system 'RepoRobo'.

Compliant robotic devices, and electroadhesion

Electrostatic grippers for fabric handling