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Page 6 of 35 Kulkarni et al. Soft Sci. 2025, 5, 12 https://dx.doi.org/10.20517/ss.2023.51

inspired by biological organisms including worms, aquatic creatures, and animals that can perform these
motions.

New soft robot design developments that increase sustainability have also been explored. For example, using
sustainable materials and resources such as biomaterial elastomers and solar power may help reduce the
ecological footprint of soft robots . Biodegradable materials can be used in implantable devices or seawater
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applications. These materials include biodegradable polyurethanes, polyesters, hydrogels, and gelatin-based
gels . Fabrication techniques including additive manufacturing also reduce waste. Three-dimensional (3D)
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printing methods including fused deposition modeling, direct ink writing, selective laser sintering, inkjet,
and digital light processing have been explored. These techniques enable the design of complex multi-
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material structures with limited material waste .

Sensing mechanisms have recently been demonstrated to monitor and detect the shape and position of soft
robots and the surrounding environment . These methods range from adding resistive, capacitive, and
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[99]
[98]
optical sensors to triboelectric nanogenerators . Hegde et al. describe multimodal sensor systems created
for soft robotic applications such as temperature sensing and tactile force sensing networks . The modeling
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and control of soft robots have also seen new developments. Several models including continuum
mechanics models, using finite element method (FEM) techniques for 3D continuum models, geometric
models, and discrete models, have been implemented to mathematically represent the structure and
motions of soft robots . Other soft robot control system implementations include bistable systems, which
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create soft logic modules eliminating the need for external rigid components . These new advances in
design, sensing, and control allow soft robots to become more efficient for use in extreme environments.

Each environment discussed in this paper presents unique design challenges and opportunities. For
example, the human body is a complex mechanochemical environment as tissues and organs have physical
characteristics such as viscoelastic properties, stiffnesses, and structures that vary widely . Even the
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development of wearable devices, worn outside the body, presents challenges such as comfort and safety,
which are discussed at length in another review paper . The benefit of using soft materials in implantable
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devices is that mechanical properties can be tuned to compliance-match human tissues. This can prevent
immune responses and rejection internally with the potential to biodegrade after use . Engineering in
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marine environments is challenging because of the need to withstand high pressure and salinized fluids
during deep ocean exploration . Soft robots are beneficial in underwater environments due to their soft
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structures which reduce disruptions to marine life. Hydraulic actuation mechanisms can reduce the
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pressure differential in devices and aid in underwater locomotion . Space is a challenging ultrahigh
vacuum environment with high-energy particles and radiation. Significant pressure differences in space
compared to the atmospheric pressure on Earth can influence the structural integrity of robots . Soft
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robots may be useful for space exploration due to their durability in extreme temperatures and their ability
to deploy from a small size. Finally, confined spaces that necessitate navigation through narrow areas and
obstructions such as debris, and collapsed structures pose a danger to humans conducting search, rescue,
and inspections . Soft robots allow for increased mobility and adaptability on obstructed surfaces and
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enclosed spaces due to their size, flexibility, and unique locomotion techniques . For these reasons, the
use of soft robots in these environments is explored.

SOFT ROBOTS INSIDE THE HUMAN BODY
The human body is a dynamic environment of systems responsible for coordinated biological functions.
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About 55%-60% of human body weight consists of fluid including water blood, interstitial fluids, and
gastric acid . These fluids have viscosities, densities, and pH levels that must be maintained for
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