Given the numerous challenges associated with existing soft materials in the ocean exploration process of soft robots, we have developed a soft material with both self-healing and adhesive capabilities for soft robots. The material achieves self-healing through tandem repeat polypeptides with n repetitions (TRn) derived from squid ring teeth proteins and adhesion through mussel foot proteins 5 (Mfp5). Our material contributes to more durable ocean explorations by soft robots.

Fig. 1 | Self-healing adhesive material for soft robots.

To endow the soft material with self-healing capabilities, we chose multiple tandem repeat polypeptides derived from squid ring teeth proteins, which fold into structures rich in β-sheets. These β-sheets interact through hydrogen bonds, exhibiting strong cohesive forces, thereby endowing the material with self-healing abilities.

Fig. 2 | The tandem repeat polypeptides in squid ring teeth proteins and their self-assembled supramolecular β-sheet-stabilized networks.

Given the positive correlation between number of repeat units and magnitude of cohesive force, we achieved the production of higher repetition of specific sequences in squid ring teeth proteins employing the RNA cyclase ribozyme mechanism, which was a feasible method to express TRn in E. coli.

The genetic sequence of tandem repeat polypeptides with 5 repetitions (TRn5) is put into the intron of the T4 phage td gene. It belongs to Group I introns, which possesses the ability to self-catalyze its splicing and can remove itself from precursor RNA without the assistance of proteins or other cofactors. During transcription, the intron spontaneously connects the 5' and 3' splice sites through the catalytic action of guanosine, forming a back-splice junction (BSJ) that links the exons into a closed circular structure. This circular mRNA provides an infinite translation template for the ribosome, allowing it to continuously cycle during translation and synthesize TRn, enhancing the material's self-healing capabilities.

Fig. 3 | The gene circuit for TRn5 forming circular mRNA and its mechanism.

We designed a fusion protein TRn4-Mfp5, served as the adhesive material. For adhesion to the soft robot, the Mfp5 protein is rich in tyrosine residues, and is catalyzed by the tyrosinase TyrVs from Verrucomicrobium spinosum to form L-DOPA，which can bind to the surface of the soft robot through non-covalent bonds and intermolecular forces. For adhesion to the self-healing material, TRn4 binds to the TRn through hydrogen bonds from β-sheets. Ultimately, the fusion protein TRn4-Mfp5 makes the layer that processes self-healing materials adhere to the soft robot.

Fig. 4 | a. The gene circuit of TRn4-mfp5 and its adhesion mechanism. b. The adhesion mechanism of TRn4-mfp5.

In our project, TyrVs can catalyze the tyrosine residues in the TRn4-mfp5 protein, converting them into L-DOPA. L-DOPA exhibits excellent adhesion capabilities, particularly in moist environments. However, TyrVs exhibits bifunctional catalytic activity, which easily leads to the over-oxidation of tyrosine into Dopaquinone, significantly reducing the adhesive performance of the material. To address this issue, we co-expressed TyrVs with the Mfp6 protein from the mussel foot protein family. Mfp-6, through its abundant cysteine residues, reduces the oxidized Dopaquinone back to L-DOPA, thereby maintaining adhesion ability.

Fig. 4 | a. The gene circuit of Tyrosinase Catalysis System. b. Synthesis scheme of L-DOPA and further oxidized product L-dopaquinone.