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Page 12 of 35 Kulkarni et al. Soft Sci. 2025, 5, 12 https://dx.doi.org/10.20517/ss.2023.51

The biomimetic structural designs of soft robots have been extensively explored to allow for higher-
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performance locomotion underwater . This includes soft robotic biomimetic fish , jellyfish , and
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octopus . These soft systems can also travel underwater easily and prevent disruptions to biological life
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while performing ecological monitoring and data collection .
Galloway et al. developed a bellows-type soft gripper that can deform and retrieve delicate samples of
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benthic fauna . The gripper has two elastomeric compartments creating bidirectional bending by
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pressurized fluid or vacuum . Another robot can achieve jet propulsion motion by expanding and
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contracting its origami structure with fluid [Figure 5, origami jellyfish (jet propulsion)] . Other traditional
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jet actuators composed of rigid parts are reported to be susceptible to impact damage . Therefore,
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developing jet actuators composed of soft materials allows for increased impact-bearing capabilities.
Smart, low-modulus materials that respond to external stimuli by deforming, changing elasticity, and
propelling have also been used for underwater applications. SMAs , known for their high power-to-
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weight ratio and requiring low voltage levels for actuation, have been incorporated as actuators into soft
robots. They also have low noise and high force-to-weight ratios. However, they may have lower response
speeds and poor power consumption performance . Cruz Ulloa et al. describe the development of a
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swimming robot that can travel underwater by SMA actuation when enclosed within a layer of silicone
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[Figure 5, soft robotic jellyfish (SMAs)]. This robot achieved a performance repeatability of 94% for lateral
motions. An octopus-inspired arm, developed using SMA coils, achieved a bending motion in response to
fluid temperature changes . Because the heat transfer of SMA occurs at higher rates underwater, the
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authors suggest these robots can leverage environmental conditions to grip and swim .
DEAs, consisting of a complaint capacitor structure, deform when an external electric field is applied .
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They have been explored for underwater soft robotic devices due to their density similar to that of water .
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Shintake et al. propose a soft robotic fish that achieves forward motion by the actuation of DEAs . This
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robot has compliant structures, rapid actuation response, and low water absorption [Figure 5, soft robotic
fish (DEAs)] . Christianson et al. propose fluid electrode DEAs that display increased flexibility by
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achieving a maximum curvature of 12.5 ± 0.4 m of a 73 mm bimorph that was actuated with an electric
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field of 20 mV·m . A Froude efficiency of 52% and a swimming speed of 1.9 mm/s was also achieved . In
another study, a soft robot manta ray composed of an ionic polymer metal composites (IPMC) wing
achieved complex 3D deformation underwater. The IPMC-based wing displayed a maximum twist angle of
15.5º and had free-swimming capabilities [Figure 5, soft robotic manta ray (IPMC)].
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Soft sensors for underwater environments
Sensors monitor environmental changes and external pressure signals in marine environments . Liquid
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metal sensors have a metallic conductivity of about 3.4 × 10 S/m and low modulus liquidity which make
them candidates for deformable elastomer-based building materials in soft robots. Their use can also
prevent damage such as cracks from underwater deformation movements due to elasticity and self-healing
characteristics . Lin et al. also describe using liquid metal-based sensors for the control of a soft robotic
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fish [Figure 6A]. The sensors comprise elastomeric fluidic channels with filled conductive liquid metal,
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eutectic gallium-indium (eGaIn), and can detect applied strain through changes in electrical resistance.

IPMC materials have been implemented for underwater sensing applications due to their soft structures and
ability to generate output voltages with no applied power. They can be used as sensors to detect changes in
deformation, pressure, velocity, and humidity [Figure 6B]. In addition, IPMC devices can perform tasks
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without the need for protective waterproofing . They were implemented as sensors for a biomimetic