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Soft Robotics for ocean exploration and reconnaissance missions of Navy

Most of the ocean is unknown. Yet we know that the most challenging environments on the planet reside in it. Understanding the ocean in its totality is a key component for the sustainable development of human activities and for the mitigation of climate change, as proclaimed by the United Nations. Emerging marine robotic developments will afford scientists advanced tools to explore and exploit the oceans at an unprecedented scale, in a sustainable manner.


Marine robots have various uses. The oil and gas industries use unmanned vehicles for tasks such as installing under-water structures, constructing pipelines, and drilling. Scientists, meanwhile, are increasingly using the technology for environmental sensing, biological research, and finding ways to protect marine resources.


Recently, Marine Robotics has grown from nascent navigation and control algorithms for underwater and surface vehicles, to powered autonomous underwater vehicles routinely able to dive beyond 6000 meters. We have seen underwater gliders cross the Atlantic Ocean and unmanned surface platforms (Wave Gliders) cross the Pacific. Autonomous Underwater Vehicles (AUVs)—gliders—are ideal for long-distance travel at mid-depth—up to 1500 m—and Remotely Operated underwater Vehicles (ROVs) are designed for low-speed maneuvering far from the disturbance of the sea surface.


ROVs commonly used for seabed exploration and industrial surveys are tethered and need to be constantly remotely operated. Traditional devices, often tethered, rigid and with limited activity range, cannot tackle the environmental conditions where ocean exploration is needed the most. The autonomous devices, for example, AUVs, gliders, drifters, and Argo floats, have limited battery life (up to 180 days). In addition to this Argo floats, drifters, and gliders are limited to the surface or the first 2 km of the water column. When the bathymetry is not known well enough to allow the operator to program the AUV for a safe mission, or the basin is swept by strong currents, or the survey needs real-time data or physical samples, AUVs are not suitable.

FIG. 1.

Hence, despite the growth in the employment of underwater robots, technological constraints prevent their regular usage in the extreme oceanic environment, such as the two ends of the water column: in deep waters and in superficial, highly perturbed waters.


The development of soft robotics technologies represents a unique opportunity to address these challenges, offering new perspectives in navigation, manipulation, propulsion  and sensing. Soft sensors and soft gripping techniques are optimum candidates to push ocean exploration into fragile and unknown areas that require delicate sampling and gentle navigation. The inherent compliance of soft robots also protects the environment and the payload from damage. More importantly, recent studies have highlighted the potential for the manufacturing of soft robots to be entirely biocompatible and, hence, to minimise the impact on the natural environment in case of loss or mass deployment.


Navies are also interested in soft robots for their Unmanned Underwater Vehicles. US military has already developed GhostSwimmer, a reconnaissance robot with an exterior shell built to look a lot like a shark cutting through the ocean depths. The robot uses its tail for propulsion and control, like a real fish. It can operate in water as shallow as 10 inches or dive down to 300 feet. It can be controlled remotely via a 500-foot tether, or swim independently, periodically returning to the surface to communicate. Complete with dorsal and pectoral fins, the robofish is stealthy too: It looks like a fish and moves like a fish, and, like other underwater vehicles, is difficult to spot even if you know to look for it. “GhostSwimmer will allow the Navy to have success during more types of missions while keeping divers and sailors safe,” Michael Rufo, director of Boston Engineering’s Advanced System Group, which developed the UUV for the Navy, said in a Navy press release. In the future, it could be tasked with swimming into hostile waters for reconnaissance missions, Navy officials say.


Some of the engineering challenges of underwater robots include increased pressure at ocean depths, corrosion from salt water, buoyancy needs, underwater communication challenges, and power needs.


University of California, San Diego (UCSD) reported in 2018

Caleb Christianson, a doctoral student at the University of California, San Diego (UCSD),  has developed a transparent prototype  soft robot resembles an eel larva—and for good reason: this marine robot was designed with a leptocephalus, the name of a curious stage in the lifecycle of several eel species, in mind. Replicating the larva’s transparency would help the team’s creation blend into the ocean environment.


But most ocean-going robots come with a catch: they are noisy. The decibel levels and vibrations of the propellers and jets scare away fish and other animals. What’s more, most robots are built with metallic frames, which can bump into and damage fragile ocean life. Christianson decided to take a different approach, with a design incorporating “muscles that don’t have any rigid materials to them,” he says. “They’re just completely soft.”


To create their eel-inspired robot, the researchers used transparent dielectric elastomer actuators (DEAs)—“pieces of rubber that respond to an electrical stimulation,” Christianson explains. While the rubber itself doesn’t conduct electricity, a voltage applied to the upper and lower surfaces of the elastomer causes it to flatten, making the material both thinner and longer. To achieve movement, each individual DEA, or artificial muscle, required two electrodes: one, a small pouch of water inside the rubber, and the other, the water surrounding the robot in a tank in the lab.


Initially, the researchers started with two actuators, which, placed back-to-back, had the ability to bend to the right or to the left. To achieve more-fluid motion and improve propulsion, the team next placed three of these actuator pairs end-to-end, creating a 22-cm-long, 5-cm-high, and 1.5-mm-thick robot. “We arranged several of these in such a way that we can get an undulating motion to mimic the motion of an eel swimming,” says Christianson. By applying voltage to the six actuators in diagonal pairs from the front to the back of the eelbot via external cables, Christianson’s group was able to prod the robot into eel-like motion around the tank.


The researchers also found that they could reproduce another aspect of eel biology in their robot—fluorescence—by adding a dye to the fluid pouch electrode. Certain species of eel naturally fluoresce, possibly as a form of communication during spawning events, or as a way to attract prey. But fluorescence might be useful for under-water robots, too, where other communication channels such as radio waves are impractical, the researchers suggest in their paper.


The eel-inspired device isn’t quite ready for use in the oceans, though. The current prototype moves forward at 1.9 mm per second while the electronics package that applies the voltage floats on the surface of the water of the tank. “The speed with which it’s moving is quite slow,” says Stefan Williams, who works on marine robotics at the University of Sydney in Australia and was not involved in the research. “I think there’d be a few challenges around power and building these things at a scale.”


Power’s not the only obstacle stopping the eelbot from competing with existing models being used in marine research. Currently available underwater devices can be remotely operated, launched from ship or shore, and can stay in the water for weeks or months. They also possess sensors that allow scientists to record several parameters such as water salinity, pH, and temperature. Christianson says the team plans to make its robot “untethered,” thus freeing it to travel farther. In the future, the soft robot could also be modified to have sensors and cameras, although this may alter the eelbot’s weight and translucency.


Chinese Soft robot swims in the Mariana Trench, reported in Feb 2021

A team of researchers affiliated with multiple institutions in China has developed a soft robot that can successfully swim in the Mariana Trench. In their paper published in the journal Nature,, the group describes their soft robot and its capabilities. Cecilia Laschi and Marcello Calisti with the National University of Singapore and the University of Lincoln, respectively, have published a News & Views piece in the same journal issue outlining the work by the team in China.


One of the problems with deep sea submersible craft, whether manned or operated remotely, is dealing with the immense pressure involved. Hulls have to be designed and constructed that are capable of withstanding the crushing pressure found in the deepest parts of the sea—a very expensive way to go. In this new effort, the researchers have taken a new approach to the problem: emulating soft fish.


Prior research has shown that it is possible to make soft robots out of pliable materials such as silicone and other polymers. Soft robots have been constructed from such materials to emulate squid and other soft-body sea creatures. In this new effort, the researchers built their robot using soft polymers in the shape of a flying-type fish—one that swims by flapping its “wings” like a ray. The wings flap via a well-known assembly of fabricated muscle in a silicone body. An applied electrical current forces the muscle to contract, pulling the wing up. Relaxing the current allowed the wing to relax to its natural state.


The researchers tested their robot first in the laboratory, then at a nearby lake, and following that, in the South China Sea. Finding success at all of the depth levels they tested, the team then hooked the robot up to a traditional submersible and sent it down into the depths of the Marina Trench and found it worked just as well down there.


The video shows the deep-sea free swimming of soft robot in the South China Sea. The soft robot was grasped by a robotic arm on ‘HAIMA’ ROV and reached the bottom of the South China Sea (depth of 3,224 m). After the releasing, the soft robot was actuated with an on-board AC voltage of 8 kV at 1 Hz and demonstrated free swimming locomotion with its flapping fins. The front view and side view of swimming process were recorded by the cameras and LED lights on the ROV. This video shows the potential of soft robots in deep-


The free swimming tests of soft robot. This video shows free swimming experiment in of the soft robot under hydrostatic pressures of 0 MPa (in a pool) and 110 MPa (in a chamber). The soft was attached on a time-controlled electromagnet and released. After the releasing, the soft robot was actuated with an on-board AC voltage of 7 kV at 1Hz and demonstrated a free swimming locomotion.


The researchers had to overcome a problem others had experienced in the past: adding electronics to control of the robot. They found that simulating the architecture of snailfish bones worked very well—instead of trying to pack the electronics into as small of a packet as possible, they spaced them out and embedded them in silicone. This resulted in greatly reduced stress on the components.



Soft robotics technologies

While robotics prototypes are progressively getting closer to their biological counterparts, these remain for the most part laboratory-scale experiments. Hence, if on one hand there is evidence that bioinspired soft robots are not simply an academic exercise, rather offering a clear advantage in terms of performance, on the other hand a major effort is still needed to drive the transition of these systems from prototypes to actual vehicles fit for operation at sea.


Some important issues that have to be addressed are energy consumption, autonomy, efficiency, sensing capabilities, memory, and pollution from polymers. Ubiquitous plastic pollution, ocean acidification, and chemical contamination are already heavily affecting ocean wildlife and coastal communities. Therefore, it is imperative to plan the future of marine soft robots minimizing their impact on the ocean.


The ratio of the task-oriented output energy from the robot to the total energy input is known as the efficiency of the robot, and it is a key figure of merit for all machines. The energy input to soft robots is usually sourced from batteries, pressurized gas or liquid, or chemicals, and it is converted into useful work by the robot to actuate, locomote, crawl, climb, grasp, pick-up objects, jump, or sense. Energy efficiency is, thus, an important indicator for guiding the design and optimization of enhanced soft robotic systems. The energy efficiency can influence the choice of actuator, energy source, materials, structural properties, and locomotion mode and ultimately justify the use of soft mechatronics systems rather than a conventional machine.


Recent development in soft robotics unveiled novel, bio-inspired maneuvering techniques, fluidic logic data logging capabilities and unprecedented dexterity. These characteristics would enable data collection, maintenance and repair interventions, unfeasible with rigid robots. The use of soft materials to constitute or protect core electronics could reduce the chance of a collision with unknown bottom or floating features to result fatal for an underwater mission.


Energy and Propulsion

When it comes to propulsive efficiency in the aquatic environment, experimental and theoretical evidence suggests that compliant bioinspired systems may yield better performances than standard propeller-driven robots, and biological studies show that soft organisms indeed benefit from an unprecedented degree of efficiency. Energy recovery techniques and energy harvesting techniques have been developed for fluidic soft robots to reduce the power consumption, which makes the robot more power efficient.


On one hand, compliant bioinspired design promises to enable soft vehicles to achieve higher propulsive efficiency, making them able to navigate over long distances at close proximity with the seabed. On the other hand, nature-inspired propulsive strategy will provide unprecedented maneuvering skills, which, coupled with soft adhesion systems, will enable operation in highly perturbed superficial environments where most of the industrial offshore activities are concentrated.


Furthermore, the inherent dexterity of soft materials empowered bioinspired propulsion, paving the way for novel navigation techniques achievable for soft robots. Even in case the propulsion would not entirely rely on the elongated body theory of fish locomotion, soft fins and bladders can aid stabilizing the robots’ navigation route and depth. As far as the composition of the used soft materials is concerned, recent progresses support the use of highly biodegradable blends, which would attenuate the environmental impact of those soft parts that will go lost or replaced.



At high depth, the impracticality of accurate manipulation control gives way to soft grippers, which can better deal with a larger variety of objects to be grasped and can account for fragile samples of complex shape. Coral reefs, for example, are one of the most delicate and important ecosystems of the planets.


Soft Sensors

Soft materials: incompressible, resistant, compliant, and versatile can alleviate the risk associated with explorative missions of traditional robots. In recent times, the interest in wearable devices has fostered the development of new flexible sensors and bioinspired technologies have further promoted the study of sensing technologies.


soft sensors, that is, sensors that adapt to the change in shape, tension, and extensibility of the body of the robot. Recent developments in sensory skins, including material advance (e.g., hydrogel employment), sensing technique, manufacturing progress, and communication, are promising also for marine applications. Moreover aquatic soft sensing can benefit from the development of biomedical soft sensing, as they share similar challenges, such as adhesion, resilience to environmental changes, adaptability, biocompatibility, and reliability.


Recent examples entail whisker-inspired sensors, devices which replicate the flow diagnostic capabilities of the lateral line of fish and sensor-embedded wearable skin for marine mammals. These sensors are designed to be distributed as dense arrays over the surface of a body travelling underwater, thus enabling a better spatial description of the parameters of interest, as well as accurate prognostic of the state of the robots, for the purpose of control and localization. Given the importance that turbulence measurements of microstructure hold in the understanding of the nature of energy dissipation in the ocean,118 these new breed of sensors may unveil an unprecedented degree of information.


Soft actuators

According to an analysis of energy efficiency of mobile soft robots, the efficiency of most mobile soft robots in literature is low, with most robots having an efficiency lower than 0.1%. In general, inflation-based elastomeric actuators for soft robots have a low efficiency, and they are not only affected by the reversible expansion of the elastomer but also are influenced by the strain, strain rate, and viscous losses in the flowing gas. Another class of soft actuators—Vacuum-Actuated Muscle-inspired Pneumatic structures—which use deflation rather than inflation and operate at low strain levels, achieves a relatively higher efficiency of ∼27%. This efficiency value is comparable to human muscle efficiency


Pneumatic soft actuators, particularly “Pneunets,” are very popular among soft robotic researchers for many different applications, despite their low efficiency. Analysis of PneuNet actuators with various wall thicknesses and different soft materials shows that the efficiency of these soft actuators lies in the range of 0.4–2.5%



Structural Materials

One of the challenges that soft robots face is the balance between softness and load bearing capacity of the robot, where the soft robot needs to be able to withstand its own weight. The size and weight of the soft robot, or parts of the robot, are two important factors that need to be carefully studied in the design stage.


The body of soft robots is often made of polymeric materials. Given the increasing concerns about the accumulation of plastic materials in marine and freshwater environments  and especially in light of the toxicity and persistence of many petroleum-based polymers, it is paramount that the massive deployment of man-made robots in the aquatic environment does not further exacerbate the widespread issue of plastic pollution. Therefore, natural and biodegradable materials should be always preferred over synthetic polymers, marking a compromise between environmental impact and technical performance.


Eco-friendly polymers are emerging as an alternative solution to the most common “traditional polymers.” Bio-based materials (i.e., produced from renewable resources), however, cannot always be classified as biodegradable. Several products marketed as compostable or biodegradable do not always achieve significant degradation rates when released into the environment


Ideally, selected polymers should meet international standards for biodegradability in the marine environment (e.g., ASTM). An example is the recent design and large-scale deployment of biodegradable oceanic drifters by the CARTHE Consortium. After careful considerations, polyhydroxyalkanoates (PHA)—a nontoxic bio-based thermoplastic—were chosen to build the drifter body by industrial injection molding, guaranteeing structural resistance in the marine environment for the duration of the experiment and full bacterial degradation of the drifter body after 5 years at sea with a rate of 0.1 mm/month.



Preferably, all accessories and electronic components need to be nontoxic, favoring the use of lithium batteries, which do not contain lead, mercury, or other hazardous substances. The use of metal should be encouraged, so that it will eventually oxidize in the ocean, as well as other less harmful components such as wood, plant-based materials, or natural rubber. All components should be compliant with the most stringent European and U.S. EPA regulations on hazardous substances, restricting as much as possible the use (and leaching) of toxic compounds such as phthalates, PCBs, PBDs, heavy metals, PAHs, and so on.


Besides the most common thermoplastics such as PVC, PET, PS, and PC, other polymers commonly used in the production of body and skin of aquatic soft robot prototypes are synthetic foams, such as Lycra, silicon rubber, elastomers, latex, acrylic, PDMS, and epoxy resins. Nontoxic bioplastics, manufactured from industrial food waste, are being tested as artificial robotic skins and for the development of biodegradable electronic circuits, to make the entire device biodegradable


Among innovative materials being tested for the construction of soft robots, there are fluidic elastomers, ionic polymer–metal composites, and piezoceramic materials, whose environmental impacts and biodegradation times are currently unknown.


Control Electronics

Another challenging aspect for soft robots is having soft onboard memory. Soft robots are still usually interfaced with hard electronic components that control and power the robot (e.g., batteries and microprocessors). However, soft memory, as soft sensors, would allow the employment of environmental friendly materials reducing the e-waste introduced in the ocean.


Exploiting the digital fluidic logic principle for the onboard memory would reduce the problem of energy support for recording data and reduce the fire hazard constituted by electronic devices around offshore assets, as suggested by recent trends in soft robotics. Developing memory using these fluidic logic gates can be quite complex and bulky. A fluidic S-R latch is the closest example to a soft memory device. A single S-R latch also requires multiple components (three logic gates and a monostable membrane).


According to Calais et al., chalcogenides is a potential source for providing soft robots with onboard memory capabilities. Chalcogenides, which are natural semiconductors, are also referred to as phase-change materials and are continuing to attract major attention for nonvolatile memory devices with high switching speeds and cycle endurance. Chalcogenides are good candidates for nonvolatile memory devices because of their phase-changing properties, where they can change from amorphous to polycrystalline structures through thermal annealing. This phase change significantly increases their electrical conductivity and results in an optical change, allowing them to be used as nonvolatile optical memory materials. Das Gupta et al. and Li et al. demonstrated the integration of chalcogenides on soft substrates—polydimethylsiloxane (PDMS), where this integration shows the potential of using such materials for onboard memory for soft robotic systems.


Based on the literature reviewed for this perspective piece, eight core aspects can bring soft robots to full operability: autonomy, communication, efficiency, bio-inspiration, maneuverability and control, memory, resilience, sensors. The development of new sensors, particularly soft sensors, and new biodegradable materials is prime both for deep-sea exploration and for industrial applications. Flexible sensors will transform these vehicles in nodes of a self-propelling sensor network, and the use of biodegradable materials will make them entirely disposable, minimizing their impact on the environment. In addition, abyssal expeditions would benefit especially from a design capable to optimize efficiency, communication, memory, and autonomy. Similarly, designs that focus on resilience, maneuverability, and control would strengthen surface operations, important on offshore assets situated in harsh areas, write Simona Aracri, and others in liebertpub.com.



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