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Page 26 of 35 Villeda-Hernandez et al. Soft Sci 2024;4:14 https://dx.doi.org/10.20517/ss.2023.52

and pressurizing the system. Although not yet proven with GER-GCR coupling systems, the working
principle based on fluid pressure seems compatible and suitable for achieving oscillating and cycling
actuation via the coupling of GER and GCR [Figure 9A]. The working principle of bistable flexible
membranes in rubber “poppers” allows these valves to harvest power only while transitioning between open
and closed configurations [Figure 9B and C]. Carefully selecting and tuning the mechanical properties of
membranes ensure that the switch remains in its selected position without any applied force and will not
switch its configuration until the necessary force to “snap” the membrane back is achieved.

CONCLUSIONS AND OUTLOOK
Efficiency and control
One of the most attractive features of chemo-driven pneumatic actuation is that it can be used to create
autonomous actuators, with the long-term goal of finding self-sustaining (oscillating) coupled chemical
reactions to negate the requirements for an external power source . This makes chemo-driven pneumatic
[70]
actuation well-suited for applications where power is limited or unavailable. However, achieving fully
oscillating pneumatic systems presents a significant challenge due to thermodynamic constraints . For
[150]
instance, certain fully oscillating chemical reactions, such as the Belousov-Zhabotinsky (BZ) reaction, are
known but are very slow and complex . Additionally, advantages and disadvantages of chemical-based
[151]
pneumatic actuation are generally related to the type of GER and GCR used, such as the actuation speed
and the achievable amplitude motion. It is noteworthy that other factors related to the materials selection
for fabricating the embedded reaction chambers and pneumatic actuators must be considered to avoid
issues related to material fatigue or degradation.

As this review focuses on using various chemical reactions for actuation in soft robotic systems, it is
important to highlight that the “ideal” chemical reaction highly depends on the specific application. In
medical applications, biocompatibility and precision are paramount, necessitating safe and controllable
[152]
reactions . For underwater robotics, reactions efficient in low-oxygen environments are preferred . In
[153]
industrial settings, factors such as speed, scalability, and cost take precedence . This diversity in
[154]
requirements underscores the need for tailored approaches in selecting chemical reactions for soft robotics,
a theme that has recurred throughout our discussions on GERs and GCRs.

Looking ahead, there is an unexplored opportunity in exploiting bio-based reactions such as enzyme-based
reactions for chemo-driven pneumatic actuation . The use of bio-catalysts could potentially offer more
[155]
sustainable and environmentally friendly alternatives while maintaining high efficiency at low temperatures
and pressures.

Furthermore, chemo-driven pneumatic actuation can also benefit from external agents or specific
conditions to boost its efficiency and control. In this context, materials science has made significant strides
by developing diverse materials that can include embedded or encapsulated catalysts for powering reactions
that drive actuation [156-158] . These materials not only serve as catalysts to augment GERs and GCRs, thereby
reducing the energy requirements for reactions to proceed, but also offer gas selectivity and affinity for
superior control over production and consumption ratios.

However, as mentioned, one of the primary challenges when coupling GERs and GCRs lies in exerting
control over gas capture and storage . To address this issue, a range of smart materials showing promising
[10]
potential for integration into chemo-driven pneumatic soft actuators has been introduced . One
[159]
promising strategy for controlling gas capture and storage involves employing porous materials [160-163] .
Characterized by an intricate network of interconnected pores, these materials can be tailored to selectively
trap and release gases. This intrinsic and hierarchical porosity positions them as highly effective candidates
for gas storage and separation.