tech

Adhesion is one of the important capabilities of living organisms, such as human grasping and octopus suction.

Vacuum suction is a very special way of adhesion. Compared with the grasping of hands, it does not need to completely envelop the object, only a small contact surface can achieve adhesion. At the same time, its energy consumption is extremely low, and the load capacity is extremely high.

Therefore, many organisms in nature have evolved organs with suckers, which are commonly found in mollusks, such as cephalopods (octopuses), gastropods (snails, conchs), and some parasites.

With its unique advantages, artificial vacuum suckers have also become important tools in human society, serving daily life and industrial production.

However, the current artificial vacuum suckers are still far from the biological suckers possessed by octopuses and snails.Firstly, current artificial suction cups are prone to leakage on irregular surfaces. In contrast, the suckers of creatures such as octopuses can firmly adhere to the surfaces of rocks, shells, and other irregular objects.

Secondly, current artificial suction cups can only statically attach to the surface of objects, while snails can maintain a high vacuum suction force while sliding.

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Therefore, is it possible to improve artificial vacuum suction cups through technical means to break through the existing bottleneck and achieve the special abilities of biological suckers?

In recent years, Dr. Yue Tianqi, a doctoral graduate from the University of Bristol, and his team have conducted a series of studies on the sucker organs of octopuses and snails.

They have successively developed vacuum suction cups with high adaptability to irregular surfaces (hereinafter referred to as "adaptive suckers") and sliding suction cups that simulate the sliding of snails (hereinafter referred to as "sliding suckers").These achievements have significant potential for application in the terminal manipulation and attachment mobility of robots.

Specifically, the adaptive suction cups developed by the research team can grasp many rough and unevenly surfaced objects (such as rocks, wood, etc.) without the assistance of a vacuum pump, which will greatly enhance the universal terminal execution capability of robots.

Compared with the traditional scheme of using auxiliary vacuum pumps for continuous suction to compensate for leaks on irregular surfaces, their scheme can greatly reduce energy consumption, lower equipment costs, and reduce noise.

At the same time, the sliding suction cups they have developed can provide a new type of robot attachment and mobility method.

Compared with previous robot attachment methods (such as magnetic adsorption, gecko adhesion, electrostatic adsorption, etc.), sliding suction cups have the following unique advantages:Firstly, it has a strong load-bearing capacity and constant adhesion, eliminating the problem of sudden changes in adhesion when legged climbing robots move;

Secondly, the energy utilization rate is extremely high, with all load forces passively borne by the negative pressure of the suction cups, without the need for additional energy supply.

Previously, the energy consumption of climbing wall robots was high, and they had to be continuously powered by cables, which limited the range of motion and application scenarios of the robots.

In contrast, for the inspection and maintenance of surfaces that are difficult for humans to cover, sliding suction cup technology can provide a solution.

At the same time, the energy consumption of sliding suction cups is extremely low, and they can work for a long time with only a battery pack, possessing the foundation for remote operation in the field, over a large area, and for a long time.The sliding suction cup, with its high load-bearing capacity, also allows it to carry a variety of working tools, achieving online operations during surface inspection.

Its application scenarios cover the exterior walls of high-rise buildings, the outer shells of large chemical and storage equipment, wind turbine blades, ship hulls, etc.

It is reported that this series of research began in 2019. At that time, Yue Tianqi joined the University of Bristol's Robotics Laboratory to pursue a doctoral degree, with his supervisor being Professor Jonathan Rossiter, a renowned scholar in soft robotics.

Professor Jonathan Rossiter encourages students to freely choose research directions, so Yue Tianqi almost did not conduct any specific research in his first year of the doctoral program, but instead gained a broad understanding of the current development level of soft robots.

After a year of research and trial, Yue Tianqi became interested in the snail, this ordinary but not simple soft-bodied animal, and began to try to replicate its powerful load-bearing gliding ability into robots.Many people believe that snails adhere to the surface of objects by using mucus, but he realized that mere stickiness would hardly achieve such a strong adhesive force.

He speculated that snails, like abalones which belong to the same class of Gastropoda, would utilize vacuum suction. To verify this idea, he placed snails on a non-breathable surface and found that they adhered very firmly.

When he poked several extremely small breathable holes in the surface with a needle (without affecting the adhesive force), the snails could be easily removed. This indicates that the adhesion of snails on smooth surfaces is largely due to the vacuum suction force.

He realized that drawing inspiration from this could provide a new way of moving for robots. However, how snails achieve stable and continuous gliding while maintaining extremely high vacuum sealing is still an unsolved mystery.

Although some biological literature has confirmed that the gliding of snails is achieved through the synergistic action of mucus with non-Newtonian fluid properties and muscles that generate transmission waves.However, at that time, Yue Tianqi did not have a feasible robot design scheme to simulate this behavior. Therefore, the study of imitating the snail's sliding sucker was temporarily shelved.

He turned to the study of octopus suckers. At that time, the academic community believed that the strong adaptability of octopus suckers to irregular surfaces came from their flexible muscle movements and the soft skin's seal to the shape of the object.

However, the seal between solids always leaves tiny gaps, and even micrometer-level tiny gaps can lead to the failure of the entire sucker.

Obviously, solid sealing alone is not enough to achieve the strong adaptability similar to that of octopus suckers.

Some scholars realized that liquid sealing provided another way to improve the adaptability of suckers, because creatures with sucker organs always live in a liquid environment.The principle of liquid sealing is to use liquid to fill the residual micro-gaps, utilizing the viscosity of the liquid, which is much higher than that of gas, to slow down the leakage rate and extend the adsorption time.

However, past research has not delved deeper into the principle of liquid sealing in biological suckers.

By reading relevant biological literature, Yue Tianqi found that mucus glands are widely present in biological suckers (including fish, snails, octopuses, and other creatures).

Moreover, the secretion rate of the sucker mucus glands has been proven to be actively controlled by the nervous system. This indicates that the adaptability of biological suckers comes from the solid-liquid coupling sealing effect produced by the muscle-epidermis-mucus gland.

This conjecture enlightened him, and based on this, he designed a biomimetic adaptive sucker.Later, Yue Tianqi's team collaborated with Dr. Siweiyong from the Bristol Robotics Laboratory and Professor Yang Chenguang to complete a challenging experiment on the coordinated adaptive suction of mechanical arms for handling extremely irregular objects.

Ultimately, the related paper was published in PNAS[1] with the title "Bioinspired multiscale adaptive suction on complex dry surfaces enhanced by regulated water secretion."

Yue Tianqi is the first author, and Professor Jonathan Rossiter serves as the corresponding author[1].

In the design of the adaptive suction cup, they used hydrophilic silicone as the base of the suction cup to allow water (simulating mucus) to evenly diffuse in tiny gaps.

They found that water can not only greatly enhance the adaptability of the suction cup but also significantly reduce the friction between the suction cup and the surface.The suction cup moistened by water can glide almost frictionlessly on the surface while still maintaining a strong vacuum suction. This is almost identical to the adhesive sliding behavior of a snail.

This plan also allowed the previously shelved research on the imitation snail sliding suction cup to be restarted. Subsequent research was also very smooth, only needing to design the robot's mechatronics system and control algorithm, and then prove its effectiveness through experiments.

After a brief design, manufacturing, and experiment, Yue Tianqi and others proved that the sliding suction cup robot has a strong load capacity (tested load 11 times its own weight, theoretical load 55 times its own weight) and extremely low energy consumption (theoretical energy consumption is more than 90% lower than the existing wall-climbing robot schemes).

During this period, in order to let Yue Tianqi better observe the physiological structure and movement of the snail, Professor Jonathan Rossiter personally went to the grass to catch snails. "His enthusiasm and purity for scientific research left a deep impression on me," said Yue Tianqi.

Finally, the relevant paper was published in Nature Communications with the title "Snail-inspired water-enhanced soft sliding suction for climbing robots" [2].Yue Tianqi is the first author, and Professor Jonathan Rossiter serves as the corresponding author.

Despite the research team's profound understanding of the principles of biomimetic vacuum adhesion and the implementation of some robotic applications, the current artificial suction cups still have a certain gap compared to biological suction cups.

For example, the suckers of mollusks are not merely organs for attachment; they also perform various functions such as perceiving the external environment and participating in the regulation of neural behavior.

Therefore, Yue Tianqi and his team's next plan is to develop more intelligent robotic suction cups, making artificial suction cups an intelligent robotic component that integrates attachment, perception, control, and actuation.

In the research they are currently conducting, they are very interested in the multimodal perception and adaptive control involving the nerves within the suckers of octopuses.The discovery shows that a simple multi-layer neural network, combined with certain physical characteristics of artificial suction cups, can replicate the intelligent perception and control capabilities of an octopus's suction cup with extremely low computational power and a highly simplified system structure.

In this way, a simple artificial suction cup can become a multifunctional robotic component that integrates attachment, multimodal perception, adaptive control, and driving. "For more on this, please follow our progress in subsequent work," said Yue Tianqi.