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 not repaired in time, this can greatly decrease their lifespan and increase salvage and 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 act as a "double-sided adhesive", enabling self-healing materials to adhere to the substrate. 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, enabling self-healing after damage.
Overall, our project aims to develop a new type of material with outstanding self-healing capabilities, helping underwater soft robots self-heal after damage. This material can significantly increase 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 discovering ocean mysteries, such as exploring marine biodiversity, energy sources and mineral resources.
However, challenges like extreme hydrostatic pressure and rough terrain at those profound depths are prone to causing damages to the 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 significantly reduce the stress response of jellyfish during handling.
Table 1. Stress gene transcription profiles in jellyfish using different handling methods
| Treatment (compared to no stress control) | Upregulated | Downregulated |
|---|---|---|
| Ultra-gentle soft robotic fingers treatment | 11 | 15 |
| Rigid claw-grab | 28 | 27 |
| Rigid claw-grab with agitation | 92 | 29 |
Although soft robots have many advantages, they do present challenges for damage resistance. Especially in the uncontrolled and unpredictable underwater environment, soft robots, disturbed by unsteady ocean currents or a group 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%, now only able to perform 5,000 grabs instead of the previous 50,000 times.
2. Moreover, it may result in the loss of valuable research 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.
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
| No. | Material Name | Healing Mechanism | Healing Condition | Recovery Degree | Can Heal Multiple Times | Recyclable | Healing Strength (Mpa) |
|---|---|---|---|---|---|---|---|
| 1 | Nano-microcapsule encapsulated liquid healant (e.g., dicyclopentadiene, epoxy resin) | Depends on chemical reagent reaction | 48 h (room temperature) | Can restore 70-75% of initial properties | No | No | <1.50 |
| 2 | Thermally reversible Diels-Alder self-healing polymers (e.g., furan and maleimide polymers) | Reversible covalent bond dissociation mechanism (based on Diels-Alder reaction) | 12-24 h (50 °C) | Can restore 80-100% of initial properties | Yes | Yes | 0.40-22.00 |
| 3 | Reversible disulfide bond self-healing polymers (thiol-disulfide oligomers) | Reversible covalent bond dissociation mechanism (based on disulfide exchange reaction) | Light exposure for 20 h (room temperature) | Can restore 100% of initial material properties | Yes | Yes | 0.40-22.00 |
| 4 | Light-induced self-healing polymers (e.g., coumarin and polyurethane) | Reversible covalent bond dissociation mechanism (based on Diels-Alder reaction and photodimerization reaction) | UV or visible light, slow healing | Can restore 80% of initial material properties | No | Yes | -------- |
| 5 | Elastic nanocomposite materials (e.g., graphene oxide and hydrogen-bonded polymers) | Multiple dynamic interactions of covalent and non-covalent bonds | 1 h (room temperature) | Can restore 50-90% of initial material properties | Yes | No | 0.16-22.00 |
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 squid ring proteins with various long tandem repeats.
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 dopamine, which provides strong adhesion, thereby enabling Mfp5 to adhere to soft robots. TRn4, composed of squid ring teeth proteins with four tandem repeats, can connect with other squid ring proteins through their common β-sheet. As a result, we fused Mfp5 with TRn4, enabling highly repetitive squid ring teeth proteins to adhere to soft robots.
Given the positive correlation between number of repeat units and magnitude of cohesive force, we designed a circular mRNA to achieve a more effective cohesive effect with short sequences. This strategy can use short sequences to express highly repetitive squid ring teeth proteins. A self-cleaving RNA cyclase ribozyme was incorporated to form the circular mRNAs, allowing ribosomes to repeatedly translate the sequence of interest and producing proteins with different repeat numbers, thus we could obtain proteins with exceptional self-healing properties.
Ultimately, the TRn4-Mfp5 fusion proteins are applied to soft robots, followed by the addition of proteins containing different long tandem repeats, forming protein networks with inherent self-healing capabilities.
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.
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