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Figure 4. Soft sensors for biomedical applications within the human body. (A) Force and pressure sensors for surgical procedures [156] ;
(B) Composition and application of an implantable and biodegradable tactile sensor to detect intracranial pressure [158] ; (C) Silicone-
composed crack sensor cantilever to measure the cardiac contractility [161] . [Images (A-C) are licensed under CC BY 4.0. http://
creativecommons.org/licenses/by/4.0/.]

terrains . Hydrodynamic perturbations in underwater environments create challenges when traveling to
[165]
the pelagic and benthic regions of the ocean . In addition to swimming, marine robots must also be
[21]
capable of capturing and investigating delicate materials and objects without damaging them and the
surrounding environments (i.e., coral reefs) . Examples of traditional underwater robotic systems include
[21]
[166]
autonomous underwater vehicles and remotely operated underwater vehicles . To offer new utility to
these vehicles and address some shortcomings of current end effectors, soft robots have been developed for
[21]
applications in marine environments . These efforts aim to offer greater mobility, resistance to high
pressure, impact-bearing capabilities, and reduced disruptions to marine life.

Soft actuators for marine environments
Since terrain in marine environments can be unstructured and delicate, soft robots need to deform
according to the environment, bear impact loads, and absorb energy in collision cases. Actuation
mechanisms for marine environments allow for different motion speeds underwater. Pneumatic actuation
mechanisms for underwater soft robot applications have been widely implemented as pneumatic actuators
enable close-to-neutral buoyancy and large deformations . Hydraulic actuators usually have faster
[167]
response speeds than other actuators for underwater applications as the environment in which they operate
is a liquid medium. This allows hydraulic underwater soft actuators to generate a large thrust force. They
also have better incompressibility than pneumatic actuators as they involve high-pressurized fluid. This
[168]
increases impact durability and actuation efficiency . Chemical reaction mechanisms have also been used
in underwater soft robots. For example, decomposition, redox, catalytic, combustion, and hypergolic
chemical reactions can be implemented to propel soft robots by generating gas or heat. However, chemical
actuation usually has long reaction times and cannot operate for long periods due to the need for replacing
reaction materials . The speeds of underwater soft robots also depend on the pressure level within the
[167]
region of water the soft robot is traveling in, the material composition, and the structure of the soft robot.