Biosynthetic self-healing materials for soft machines

Abstract

Self-healing materials are indispensable for soft actuators and robots that operate in dynamic and real-world environments, as these machines are vulnerable to mechanical damage. However, current self-healing materials have shortcomings that limit their practical application, such as low healing strength (below a megapascal) and long healing times (hours). Here, we introduce high-strength synthetic proteins that self-heal micro- and macro-scale mechanical damage within a second by local heating. These materials are optimized systematically to improve their hydrogen-bonded nanostructure and network morphology, with programmable healing properties (2-23 MPa strength after 1 s of healing) that surpass by several orders of magnitude those of other natural and synthetic soft materials. Such healing performance creates new opportunities for bioinspired materials design, and addresses current limitations in self-healing materials for soft robotics and personal protective equipment.

Fig. 1. Cephalopod-inspired biosynthetic proteins.

a, The analysis of squid proteins, design of a squid-inspired master sequence, and biosynthesis of protein libraries yield protein-based functional self-healing materials for soft actuators and robotics applications. b, Protein sizes of native Loligo vulgaris protein complex and biosynthetic TRn4, TRn7, TRn11, and TRn25 polypeptides. (Source Data Fig. 1b). c, The nanostructure of biosynthetic tandem repeat polypeptides is composed of a β-sheet nanocrystal network (blue) connected by flexible chains (yellow), with molecular defects (dangling ends and loops). d, Self-healing properties of squid-inspired proteins (at room temperature) improved beyond those of native proteins owing to an optimized network morphology. Error bars, standard deviation (n = 5).

Fig. 2. Self-healing polypeptides.

a, The self-healing mechanism relies on β-sheet nanocrystals, which act as physical crosslinks, and diffusion across the damaged areas. b, The performance of squid-inspired self-healing proteins is compared to that of state-of-the-art self-healing materials (see Supplementary Data 1 for references) that use different chemistries. This provides a benchmark for self-healing materials.

Fig. 3. Self-healing of extreme mechanical damage.

a, Scratch damage on TRn11 protein-coated substrates was partially and totally healed, each healing requiring less than 2 s. b, Puncture damage of free-standing, flexible TRn11 protein films (by a needle through the film) was healed in less than 1 s. c, Total cut damage of TRn11 protein dog-bone-shaped samples (I and II) was healed in less than 1 s. Healed samples withstand large deformations of up to 200% stretching strain before failure from a random pristine location, and healed areas are at least as strong as pristine areas.

Fig. 4. Self-healing, protein-based soft actuator.

a,b, Schematic and images of a soft pneumatic actuator fabricated from TRn11 protein disc membranes. c, A single-chamber actuator achieves 400% strains and 5 N force output, with no distinguishable difference in performance between pristine and puncture-healed actuators. Error bars, standard deviation (n = 5). (Source Data Fig. 4c) d, Soft gripper fabricated from two opposed protein actuators, capable of gripping soft, delicate objects (for example, a cherry tomato). e, Protein-based artificial muscle, with performance exceeding that of biological muscle. f, On-demand degradation of protein actuators by induced pH stimulus, which causes them to vanish at the end of their functional life (photodye added for enhanced visualization).