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Page 24 of 35 Villeda-Hernandez et al. Soft Sci 2024;4:14 https://dx.doi.org/10.20517/ss.2023.52

[139]
[138]
times to the millisecond range and offer tunable force output depending on the intended application .
In contrast, the response time of chemically powered pneumatic actuators is typically within the seconds to
minutes range [31,70] , and the force generated directly depends on the amount of gas produced and the
internal volume of the soft actuators. It is important to note that while chemical reaction-driven actuators
might not match the rapid response or high-force generation of conventional systems, they offer unique
advantages in terms of design flexibility, miniaturization, and suitability for specific environments or
applications.

[31]
In investigations where the coupling of GER and GCR has been evaluated for soft actuators , the reaction
chambers comprise over 95% of the total system volume (including the actuator). The volume of reactants,
for both GER and GCR coupling, required only about 5% of the total volume of the system. As mentioned
earlier, surface interactions are crucial for the effective completion of the GER and GCR involved, with
interfacial areas of the systems and gas solubility playing a crucial role on the reaction kinetics for this type
of reaction [140-142] . Challenges, including small gas-liquid interface areas and poor gas miscibility, can be
addressed by reducing the volume allocated for the power source and carefully designing reaction chambers
with larger surface area for optimal chemical interactions. Furthermore, scaling these systems to larger
robots introduces additional challenges, such as maintaining effective reaction kinetics at increased scales.
Such challenges will necessitate innovative design and material selection approaches for larger-scale soft
actuators.

To achieve independence from electrical systems, the soft and compliant properties of materials used in soft
system fabrication have been leveraged to create integrated and non-electric fluidic circuits with logic-
operating principles similar to those found in electronic circuits [143,144] . Analogous to electronic circuits, a
fluidic “switch” can be considered a basic form of control, allowing, or blocking the signal propagation.
Microfluidic boards composed of check-valves and switch-valves, which are autoregulated by geometric
design, can function as charging capacitors as pressure builds up and bends the soft deformable layer
between microchannels [Figure 8A and B]. In this manner, system pressure dynamically autoregulates
[145]
the soft controller, allowing for frequent oscillations under specific conditions. Constraints in channel and
component sizes are the main limitations for optimal performance in these systems. Soft valves have been
developed by exploiting the instability of elastomeric membranes, enabling them to switch, in a binary
fashion, between pressures.

By selectively adjusting the printing or manufacturing density of materials, it becomes possible to create
reaction chambers within the actuator, provided the reactants are safe for interaction . In addition to
[146]
reaction chambers, a flexible power chamber, such as an expanding bladder made from non-extensible
materials, can also be employed. However, it is important to ensure that the volume change under pressure
does not influence the robot actions . Developing suitable containment systems is crucial for enhancing
[147]
efficiency and reducing the number of reactants needed per reaction. Furthermore, miniaturization and
portability of power sources are essential for creating optimal, efficient, and fully autonomous soft systems.

The pursuit of autonomous soft systems promotes developing systems capable of executing routines with
minimal human intervention. Depending on the desired routine, power input in soft pneumatic systems is
typically regulated by the passive operation of electrical switches, regulators, or valves that periodically
supply the actuators.

Wehner et al. demonstrated control methods with GERs . Oscillating pneumatic actuation has been
[70]
achieved by transporting H O through microfluidic control boards into catalytic chambers, generating O
2
2
2