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Villeda-Hernandez et al. Soft Sci 2024;4:14 https://dx.doi.org/10.20517/ss.2023.52 Page 23 of 35

Having a good understanding of the kinetics, the rate of the reactions, the potential for side reactions, and
the stability of the products attained is essential for a successful and safe coupling of GERs and GCRs. Most
importantly, the safety considerations to mitigate the potential hazards of GERs, including pressure build-
up, fire and explosion, and toxic gas release, must be carefully discussed and assessed before designing and
carrying out safe GERs and GCRs [134-136] .

Once risk assessments are completed, and any safety concerns and potential hazardous outcomes of a GER
and GCR coupling have been addressed, additional variables such as temperature, volume, pressure and
stability of the reactants and products should be considered. As these variables are connected by the ideal
gas law, the size and construction of the materials in contact with the reagents and products are
fundamental factors that require a careful design process. By carefully selecting the volume of the container
and the amount of reactant used, it is possible to create conditions where the GERs and GCRs can proceed
efficiently without being significantly influenced by temperature changes. A good example of this size
[31]
consideration can be found in our previous work where a GER and GCR were successfully combined. The
latter was set by carefully studying the selected chemical reaction and tailoring the reaction chamber to it,
which allowed the use of low amounts of reactants to develop a safe and appropriate coupling of GERs and
GCRs without noticeable temperature variation.

Design and control
Working with matter in its separate phases simultaneously is common in the chemistry field. The
combination of and reaction between solids, liquids and gases are determined by surface interactions at two
or more phases. Since applying chemical reactions in soft pneumatic systems relies on the interaction of
gases with other reactants, a good understanding of such interactions is needed . The primary concern to
[11]
create coupled GERs-GCRs is to enable gas interaction (formed from the GER) with GCR reactants. To this
end, the focus is set on detailing actuator designs that could allow this type of interaction in various ways.
Since most of the produced pneumatic soft systems focus on the actuator design, the design of the power
sources is neglected (apart from miniaturization). When having two separate but compatible power sources,
e.g., GER and GCR, their design and correct coupling is crucial to obtaining the maximum and most
effective actuation response. As shown in Figure 7, the configuration, isolation, and integration of both GER
and GCR in a unified system offer initial variations and options to satisfy varied actuation mechanisms.

A parallel and isolated configuration [Figure 7] is desired when the actuation and functioning of a system
rely upon the simultaneous application of positive and negative pressures. As detailed by Fatahllilah et al.,
positive-negative pressure (PNP) actuators contain two separate pneumatic actuators; when actuated
simultaneously, via pressure increase or decrease, they achieve a larger bending force at lower bending
[115]
angles . Most of the soft pneumatic systems reported in literature work with individual pressure stimuli
(positive or negative) comprising single or interconnected fluid chambers. For this type of individual
pressure actuation, a serial and interconnected configuration of GER and GCR is desired. Both PNP and
individual pressure actuation systems, with the appropriate configuration, would need a cascade
arrangement of chemical reactions to allow an oscillating and cyclic performance akin to those found in
electronically or electromechanically controlled systems.

Typically, actuators driven by chemical reactions exhibit varying response times and actuation forces,
primarily influenced by the cycles of gas evolution and consumption reactions [31,84,88,137] . Compared to
conventional pneumatic systems, these chemical reaction-driven actuators may demonstrate slower
response times and reduced force generation. For example, standard pneumatic systems, comprising a
compressed air source, connection lines, flow-controlling valves, and regulated air receivers, can reduce rise