Stress-memory materials for artificial muscles

Abstract

Shape memory polymeric materials is one group of the most prevalent smart materials, which have attracted the extensive attention of both academic and industrial scientists. The shape variation to external stimuli and corresponding applications have been investigated maturely. However, the exploration and application of stress responses of shape memory polymers (SMPs) are still in preliminary stage and waiting to be excavated. It is believed that the memory of shape is not the only valuable variate with the stimulation, stress analysis would doubtlessly deepen the understanding and broaden the application of a shape memory material, especially for application of artificial muscles (AMs). AMs have high demand in many fields like robotics, medical devices, sensors and actuators as well as prosthesis. To achieve what a natural muscle can do for humans in AMs, an ideal material should be cheap, soft and have both isotonic and isometric contractions, high-energy-density, sensibility and trainability. However, only a single function of AM, isotonic contraction, was considered in most existing studies, while other critical functions are always neglected. Moreover, current AMs are normally made of either hard metals or polymer composites, or special structures typically twisted coils of thermal expansion polymers. Accordingly, this project is focused on: 1. To design and fabricate stress-memory polymeric materials for realistic and multifunctional artificial muscles with thermal and non-thermal responsiveness 2. To investigate the relationship between structure, thermomechanical properties, rheological responses and stress-memory behavior of the above materials and establish a fundamental mechanism through thermodynamics. 3. To realize a pure thermal responsive actin-inspired polymer artificial muscle (SPAM) with abilities to sense temperature and perform both isotonic and isometric contractions (dual-mode) muscle. 4. To explore a polyurethane with non-crystal titin-inspired disulfide bond switch, namely, structurally dynamic bonds for artificial muscles. 5. To further verify the stress-memory function and switch properties of animal hairs to support the analysis on enthalpy roles in performing muscle functions with an emphasis on non-thermal stimuli such as water and redox sensitivity. The principle of materials design, structural and mechanical analysis, function evaluation and application exploration were systematically introduced in this thesis. Firstly, polymeric artificial muscle materials mimicking real muscle structures (titin and actin in real muscles) were designed and fabricated. Then the thermomechanical properties, energy dissipation and memory properties of thermal-stimulated stress-memory polyurethanes were investigated through isothermal tension, stress relaxation, cyclic tensile loading-unloading and stress memory tests. The stress memory behavior and its mechanisms were investigated using Tm type stress memory polyurethane (Tm-SMPU) and Tg-SMPU while Polyamide 6 (PA6) and rubber were used for deliberation convenience, where a unique enthalpy-switch was discovered. The investigation of stress memory for Tm-SMPU specimen in different state discovered the important role of rubbery chains in soft segments, which control the memory-stress from entropic elasticity. Thus, a model for stress memory needs with three basic elements was proposed to provide fundamental understanding of stress memory polymers. Based on this, the synthesized single polymer artificial muscle (SPAM) mimicking real muscle structures was proved to have abilities in dual-mode contractions through enthalpy-battery, where spring structure is used as titin and semi-crystal structure as actin. Due to the high sensitivity of crystal phase to temperature, sensing ability to external conditions are reported and energy source from enthalpy change are unprecedentedly explored. Trainability of SPAM was proved by structure evolution of crystal phase through XRD and DSC analysis and the contraction ability were demonstrated to result from selective preloaded and stimulated conditions. Furthermore, the second series of polymer mimicking the structure of titin in muscles with different contents of disulfide bond (DB) were proved to possess the ability in isometric contraction, reaching 47% of programmed stress. As a covalent bond widely available in biosystems, DB provides plentiful enthalpy contribution to proteins both structurally and thermodynamically. This bond can be broken/repaired under Redox conditions like a switch to release the energy through the enthalpy change. Accordingly, a muscle model was proposed where a protein DB with enthalpy change is responsible for stress storage and release. Using the same method, the presence of memory stress in SMPU-HB (hydrogen bond) and camel hairs was verified. In summary, it could be concluded that: (1) the conventional SMP molecular network model with two elements are improved and the function of rubbery chains are explored, the definition of entropy switch was proposed; (2) the fundamental mechanism was established for stress-memory behavior and a basic model was proposed, where the division of enthalpy modulation and entropy domination were presented that can guide the design of smart polymers for artificial muscles; (3) dual muscle contractions, sensing ability and trainability are achieved using one single polymer through enthalpy-battery, which skip the complexity of structure design and analysis or making composites with various fillers, thus has great potential in AM applications; (4) a muscle-like stress-memory material with controllable DBs was fabricated, and isometric contraction was achieved on exposure to Redox with enthalpy change, of which the discovery will inspire exploration of the DBs in protein-based materials as well as biological systems and more fascinating smart function of artificial muscles. This study forms a fundamental platform for stress-memory materials and the strategy of exploiting enthalpy as energy source to produce muscle contraction is believed to establish a new direction in designing novel artificial muscles and sensors, being endowed with large potential in multiple applications at multiple scales. It is also believed that this reflects the smart behavior of biologic materials including natural fibers and facilitates their broader applications in energy storage areas.