Protein-based Shape-Memory Fibers for Soft Actuators

Abstract

Soft actuators can contribute to the flexible and adaptive movement of soft robots by generating mechanical force. The different movements of the soft actuators are driven by external stimuli or pressure, which can take effect individually or synergically. Among them, shape-memory materials are outstanding soft actuators owing to their lightweight, powerful, and untethered characteristics. As the emerging applications in biomedical engineering and the concern of environmental benefits, biocompatibility, biodegradability, mechanical robustness, and a green fabrication approach are expected to be merged into the shape-memory materials design. Therefore, it is believed that biopolymers are excellent candidates for shape-memory materials fabrication because the shape-memory effect has been discovered in many biological substrates such as animal hairs, spider silk, horns, etc. However, the exploration and utilization of natural resources for shape-memory material design are limited, and current biobased shape-memory materials have shortcomings such as insufficient mechanical properties and low shape recovery rate. Therefore, this thesis aims to develop water-responsive shape-memory protein fibers from natural resources and unveil the underlying mechanisms for the responsiveness. In addition, their applications in artificial muscles and smart textiles are also explored:

Chapter 1 systematically reviews related literature on shape-memory polymers and the shape-memory effect, structure and responsiveness of proteins, shape-memory effects in natural biological substrates and their responsive mechanisms, methods of fabricating proteins for smart materials design, and the potential applications of smart protein materials. It can be concluded that protein materials are biopolymers of great potential in the fabrication of shape-memory materials and the application of smart textiles, biomedical devices, and soft actuators. The mechanisms of the water-responsiveness of the protein materials can be attributed to the entropy change of the molecular alignment change in the amorphous region and the energy generated by the destruction and reformation of hydrogen bonds in the amorphous regions. This can guide the design of the shape-memory regenerated protein materials.

In Chapter 2, the possibility of fabricating regenerated water-triggered shape-memory keratin fibers from wool and cellulose is investigated in a more effective and environmentally friendly manner. The regenerated keratin fibers exhibit comparable shape-memory performance to other hydration-responsive materials, with a shape fixity ratio of 94.8 ± 2.15% and a shape recovery rate of 81.4 ± 3.84%. Owing to their well-preserved secondary structure and crosslinking network, keratin fibers exhibit outstanding water-stability and wet stretchability, with a maximum tensile strain of 362 ± 15.9%. In this system, the reconfiguration of the protein secondary structure between α-helix and β-sheet is assigned as the fundamental actuation mechanism in response to hydration. This responsiveness is studied under force loading and unloading along the fiber axis. Hydrogen bonds act as the “switches” clicked by water molecules to trigger the shape-memory effect, while disulfide bonds and cellulose nanocrystals play the role of “net-points” to maintain the permanent shape of the material. Keratin fibers are manipulable and exhibit potential in fabricating textile actuators, which may be applied to smart apparel and programmable biomedical devices.

Chapter 3, to further enhance the shape-memory performance, the gold coordination is adopted for enhancement in the keratin system, and a well-aligned anisotropic structure with α-helix to the fiber axis is established in the regenerated keratin fibers by wet spinning. The strong crosslinking effect from Au-S bonding endows the regenerated keratin fibers with outstanding shape-memory performance, exhibiting a shape fixity ratio of ~92% and a near-full shape recovery rate of ~96%. The regenerated keratin fibers exhibit superior water stability and wet extensibility due to the well-restored secondary structure and crosslinked network. In this system, we assign the transition between α and β conformation under external force and water stimulation as the fundamental mechanism in response to hydration. In hydration and dehydration, the hydrogen bonds act as ‘switches’ that enable the deformation, while the strong Au-S bonding plays the role of ‘net points’ that maintain the original and programmed shapes. The well-established shape-memory keratin fibers are processible and show great potential as fiber actuators and smart fabrics in smart systems like textiles and biomedical devices.

Chapter 4, a further study is carried out to explore the possibility of other fibrous proteins in developing regenerated fibers with a shape-memory effect, such as cocoon silk. Hierarchically structured silk fibers with anisotropic long-range molecular organization and water-responsive effects resembling natural spider silk. The regenerated silk fibers exhibit the water-triggered shape-memory effect and a water-driven cyclic response. The reversible hydrogen bonds and transformation in the metastable secondary structure from α-helices/random coils to β-sheets are assigned as the mechanisms responsible for the water responsiveness. The silk fibers possess a tensile strength higher than 104 MPa at a fracture strain of ~100%, showing noticeable toughness. The water-responsive silk fibers exhibit a shape recovery rate of ~83% and generate a maximum actuation stress of up to 18 MPa during the water-driven cyclic contraction that outperforms most traditional natural textile fibers. The regenerated silk fibers show potential for use in water-driven actuators, artificial muscle, and smart fabrics based on integrating suitable mechanical properties and water responsiveness.

Hence, this thesis investigates the possibility of fabricating shape-memory fibers from natural resources. The mechanisms are explored, and the potential in artificial muscles, soft actuators, and smart fabrics is investigated. This thesis may provide examples and inspiration for designing shape-memory materials from bio-resources.

Date of Award	9 Jun 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jinlian HU (Supervisor) & Hong Hu (External Co-Supervisor)