Soft robots are known for their intelligence, flexibility, and adaptability, showing great potential in the field of ocean exploration. However, these robots are easily damaged in the complex and ever-changing marine environment. If soft robots are not repaired in time, it may greatly decrease their lifespan and increase repair costs.

Considering the existing problems, our project tries to develop a new type of self-healing material for underwater soft robots.

In our project, we connected mussel foot proteins with squid ring teeth proteins to create fusion proteins that acts as a "double-sided adhesive", enabling self-healing materials to adhere to the surface of soft robots. Additionally, we designed a circular mRNA employing the RNA cyclase ribozyme mechanism to produce highly repetitive squid ring teeth proteins. These proteins formed a network structure through β-sheet interactions, enable to self heal after damage.

Overall, our project developed a new type of material with outstanding self-healing capabilities, helping underwater soft robots self-heal after damage. This material can significantly expand the lifespan of soft robots, providing robust support for ocean exploration.

Soft robots are made from flexible materials, which can perform complex motions by imitating the movement mechanisms of natural soft-bodied organisms. Due to superior flexible bodies, biomimetic soft robots have a wide range of applications in industrial production, scientific research, and especially in ocean exploration.

Ocean exploration is significant and beneficial to discover ocean mysteries, such as exploring marine biodiversity, energy sources and mineral resources.

However, challenges like extreme hydrostatic pressure and rough terrains at those profound depths are prone to causing damages to rigid robots.

In contrast, soft robots, made from flexible materials that imitate the movement of natural soft-bodied organisms, offer significant advantages in pressure resilience, mobility, and bio-friendliness, adapting to complex geographical environments. Studies have shown that compared to rigid grippers, soft grippers can remarkably reduce the stress response of jellyfish during handling.

Table 1 | Stress gene transcription profiles in jellyfish using different handling methods.

Although soft robots have many advantages, they are faced with challenges in damage resistance. Especially in the uncontrolled and unpredictable underwater environment, soft robots, disturbed by unsteady ocean currents or groups of marine organisms, can be directly knocked into sharp objects and scratched, or they may incur scratches due to friction.

1. This highly limits their lifespan. For instance, after being damaged, the lifespan of soft robots has been reduced by 90%, with grabing about 10 times less than before.

2. Moreover, it may result in the loss of valuable researches and sampling opportunities. For example, when observing the behavior of marine organisms such as Acropora austera spawning, soft robots usually need to work continuously for over a month, which makes durability crucial for them.

3. Additionally, the cost of salvaging damaged soft robots is enormous, often exceeding $10,000 each time.

We discovered through mathematical modeling that minor disturbances in the ocean significantly increase the damage rate of soft robots.

Fig. 1 | Mathematical modeling results.

Meanwhile, we also found that the existing self-healing materials for soft robots have some problems, such as inability to achieve multiple repairs and the requirement of high temperatures for the healing process. Most importantly, soft robots need to be retrieved to the land for maintenance, which makes it difficult to apply these materials to soft robots for ocean exploration.

Table 2 | Comparison of self-healing materials.

To address these issues above, we have designed SAMUS, a kind of self-healing material using mussel foot proteins (providing strong adhesion) and squid ring teeth proteins (providing self-healing capability). This material can adhere to the surfaces of soft robots, quickly repairing stabs or scratches that soft robots encounter during ocean exploration, ensuring they can finish their ocean exploration tasks before returning to land for maintenance. This significantly improves the endurance of soft robots, reducing environmental pollution and resource waste caused by damage.

The self-healing materials consist of two components: the TRn4-mfp5 fusion proteins and the tandem repeat polypeptides with n repetitions (TRn) derived from squid ring teeth proteins.

In the TRn4-mfp5 fusion proteins, Mfp5, derived from mussel foot proteins, has tyrosine residues on their surface. In the presence of tyrosinase TyrVs, the tyrosine residues are catalyzed to form L-DOPA, which provides strong adhesion, thereby enabling Mfp5 to adhere to soft robots. TRn4, tandem repeat polypeptides with 4 repetitions, can connect with other squid ring proteins through their common β-sheet. As a result, we fused Mfp5 with TRn4, enabling TRn adhere to soft robots.

Given the positive correlation between the number of repeat units and the magnitude of cohesive force, we designed a circular mRNA to achieve a more efficient cohesive effect by expressing tandem repeat sequences of squid ring teeth proteins. A self-splicing Group I intron was incorporated to form circular mRNAs, providing a continuous translation template for ribosomes. This allows the production of proteins with a controlled number of repeats, thereby enabling us to obtain materials with enhanced self-healing properties.

Ultimately, the TRn4-Mfp5 fusion proteins are applied to the surface of soft robots, followed by the addition of TRn, forming protein networks with inherent self-healing capabilities.

Fig. 2 | Entire design of the project.

By applying fusion proteins and squid ring teeth proteins, we expect to develop self-healing materials that can adhere to the surfaces of soft robots. This enhances the durability and functionality of soft robots, reducing environmental pollution and resource waste caused by damage. The highly repetitive squid ring teeth proteins can form self-healing protein networks and it can adhere to the surface of soft robots through mussel foot proteins. This innovative material will allow soft robots to self heal in time, ensuring continuous operation in marine environments. Our project has developed a new type of self-healing material that will enhance the capability and durability of soft robots in ocean exploration.