Two-Way and Multiple-Way Shape Memory Polymers for Soft Robotics: An Overview
Abstract
:1. Introduction
2. Materials
2.1. One-Way SMPs
2.2. Multiple-Way SMPs
2.3. Two-Way SMPs
2.4. Experimental Testing
3. Manufacturing
4. Working Mechanisms and Applications
4.1. Single-Material Mechanism
4.2. Multi-Material Mechanism
4.3. Multi-Functional Mechanisms
5. Modeling and Simulation
6. Conclusions
- Materials: (i) Several routes for the synthesis of multiple-way and two-way SMPs are available from the literature, and they differ in terms of preparation method, reprocessibility, achieved shape memory, and mechanical properties. (ii) SMP properties influence the overall robustness and performance of soft robots. Accordingly, SMPs with tunable transition temperatures, high thermal stability, and good mechanical properties in the operational temperature range are highly desired. For example, soft robots for biomedical applications require a switching transition temperature close to the body temperature, while those used for aerospace applications require high transition temperatures. (iii) Appropriate characterization methods on both macroscopic and molecular/morphological levels should be performed for a comprehensive knowledge of the polymer under investigation. In general, shape memory behavior characterization at the macroscopic level must be chosen and tailored to the specific SMP category and application under investigation. (iv) Two-way SMPs under constant stress or stress-free conditions are very promising for achieving reversible actuation in soft robots and require extensive research to improve actuation strains/forces and their mechanical performances. In particular, material behavior under cyclic loading should be investigated.
- Manufacturing: (i) Most of SMP-based components are fabricated through conventional techniques rather than through 3D printing, due to the lack in the variety of SMPs that are usable in 3D printing and the limited applicability of existing 3D printing methods to new SMPs. In fact, polyjet printing and extrusion printing are the most used 3D printing techniques for SMP-based soft robotics: polyjet printing allows for the use of materials with tunable mechanical properties, but has, e.g., high equipment costs, several resin properties’ requirements, and limited material choices; extrusion-based printing is versatile, but has, e.g., slow printing speed and relatively low resolution. Extensive research should be dedicated to the development of two-way and multiple-way SMPs for 3D printing and to the analysis of suitable 3D printing methods. (ii) Composite structures present several advantages to enhance the actuation complexity. However, some 3D printing techniques (e.g., stereolithography) cannot enable multi-material printing. Therefore, modifications to current techniques should be investigated. (iii) Novel inks should be studied to enable 3D printed multi-functional SMPs.
- Working mechanisms and applications: (i) Few examples of real-world programmable soft robots, based on both single-material and multi-material working mechanisms, have been proposed in the literature to be used, mainly, for biomedical (e.g., drug delivery systems) and aerospace (e.g., deployable or exploration components) applications. Further efforts should be made to increase the range of application fields. As an example, two-way SMP-based actuators are promising for dynamic building facades and energy savings [289,290]. However, extensive research should focus on material properties, e.g., extension rate, transparency, recovery stress, operational temperatures, and long-term stability. (ii) Several examples of components (e.g., in the form of trusses, periodic structures, compliant mechanisms), capable of programmable motion, have been proposed in the literature. All these components can be potentially integrated into more complex soft robotics systems to achieve advanced capabilities. (iii) Both shape-change speed and response time are key factors for actuation and depend on materials properties, geometrical design, and actuation stimulus. More efforts should be done to improve these two features. (iv) Complex and controllable movements are preferred in advanced robotics applications. Localized heating provides a simple and efficient method to this purpose, and should be investigated in two-way and multiple-way SMPs. (v) More studies should be dedicated to the combination of two or more stimuli into one single polymer to achieve the two-way or multiple-way SME. In this way, SMPs may adapt better to the overall environmental conditions. Moreover, function or property-shifting features, in addition to shape-shifting, should be investigated in order to increase the autonomy of soft robots. To this end, integrated design and fabrication strategies should be developed, as proposed, e.g., by Wehner et al. [291]. (vi) The application potential for two-way and multiple-way SMPs appears unlimited. However, real examples are still limited due to the lack of standards, especially related to 3D printed SMPs, and of manufacturing techniques that allow the realization of complex components.
- Modeling and simulation: (i) Theoretical models and design methodologies are still limited for 4D printed components and are needed to accurately predict and optimize programmable soft robots. (ii) Constitutive models for multiple-way and two-way SMPs are fundamental for the simulation analysis of parts. More efforts should be done in this regard for both viscoelastic and phase transition approaches, especially in the three-dimensional finite strain framework and for two-way LCEs and two-way SMPs under stress-free conditions.
Funding
Conflicts of Interest
References
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Scalet, G. Two-Way and Multiple-Way Shape Memory Polymers for Soft Robotics: An Overview. Actuators 2020, 9, 10. https://doi.org/10.3390/act9010010
Scalet G. Two-Way and Multiple-Way Shape Memory Polymers for Soft Robotics: An Overview. Actuators. 2020; 9(1):10. https://doi.org/10.3390/act9010010
Chicago/Turabian StyleScalet, Giulia. 2020. "Two-Way and Multiple-Way Shape Memory Polymers for Soft Robotics: An Overview" Actuators 9, no. 1: 10. https://doi.org/10.3390/act9010010