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Page 16 of 35 Villeda-Hernandez et al. Soft Sci 2024;4:14 https://dx.doi.org/10.20517/ss.2023.52

temperatures and, therefore, pressures reached during the combustion process. These reactions require O
2
to occur, and, as previously mentioned, the amount of gas (O ) consumed is higher than the quantity of gas
2
(CO ) produced due to the formation of H O as a side product [Table 6]. Combustion reactions rely on heat
2
2
rather than gas production to generate pressure, which is the reason that, regardless of their use and
application in pneumatic soft actuators, these reactions are not considered GERs.
Thermal decomposition
Contrary to combustion reactions, thermal decomposition reactions deliver a gas surplus by the end of the
reaction. Take, for example, the use of baking powder in cooking. When subjected to elevated temperatures,
NaHCO in the baking powder undergoes decomposition, releasing CO , which contributes to the cake
2
3
rising. Notably, the decomposition of NaHCO commences at around 80 °C, a temperature already
3
considered high for biological systems.
Other more industrially focused instances of thermal decomposition reactions, such as the generation of N
2
[107]
from sodium azide or the production of O from mercury oxide, are known . However, these reactions
2
occur at even higher temperatures of 275 and 500 °C, respectively.
One important aspect to highlight is the endothermic nature of thermal decomposition reactions. This
necessitates considering supplemental energy sources to sustain the reaction. Furthermore, the requirement
for a contained environment, especially at elevated temperatures, poses challenges in controlling these
reactions. The need for precise temperature control and containment makes practically applying thermal
decomposition reactions in certain settings particularly difficult [Table 7].

Vaporization
For lower temperature ranges, the evaporation of solvents such as methanol or ethanol has already been
utilized in soft pneumatic actuators. Miriyev et al. proposed encapsulating these solvents in gas pockets
[109]
using a polydimethylsiloxane (PDMS)-based silicon elastomer . The inner volume of each pocket or
chamber increases due to the pressurization caused by the expansion of gases in the system when the
solvent reaches its boiling point. Although these systems produce effective actuation, no chemical
interaction occurs with the solvent and a secondary reactant. These vaporization processes are a physical
phenomenon driven by external stimuli (temperature increases) resulting in a liquid-to-gas phase change,
and thus not a chemical GER [Figure 5].

Lee et al. have recently reported the development of vaporization-based actuators using thermoelectric
materials to achieve fast actuation responses via quick heating/cooling cycles . Even though a chemical
[75]
reaction is not involved in vaporization, it can be categorized as a gas evolution process of interest for soft
robotics. Chemical reactions can offer a higher volume:reactant ratio compared with simple evaporation.
This means that a smaller quantity of reactants can generate a larger volume of gas, resulting in increased
actuation capabilities. Additionally, chemical reactions can be controlled and triggered with precision,
allowing for on-demand actuation and modulation of force without needing external power sources. The
advantages of one method over the other can vary depending on factors such as the desired actuation force,
response time, environmental considerations, and available resources.

NEGATIVE PRESSURE
Within the pneumatic actuation field, only a few systems have exploited using negative pressures to achieve
actuation compared to positive-pressure-driven actuators [39,64,111-113] . Similarly to pneumatic actuators, many
negative-pressure-driven soft actuators rely on mechanical vacuum pumps to operate successfully .
[114]