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Saltwater-responsive bubble artificial muscles using superabsorbent polymers

Gosden, Daniel; Diteesawat, Richard Suphapol; Studley, Matthew; Rossiter, Jonathan

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Daniel Gosden

Richard Suphapol Diteesawat

Jonathan Rossiter


Robots operating in changing underwater environments may be required to adapt to these varying conditions. In tidal estuaries, for example, where the degree of salinity cycles in step with the level of the water, a robot may need to adapt its behaviour depending on the position of the tide. In freshwater bodies, the unexpected presence of a pollutant may also require the robot to respond by altering its behaviour. Embodying this sensing and response in the body of the robot means that adaptivity to the environment can be achieved without resorting to centralised control. This can also allow direct responsivity using ‘free’ environmental energy, actuating without requiring stored onboard energy. In this work we present a soft artificial muscle, the contraction of which varies in response to the salinity the water surrounding it. The novel actuator uses a super-absorbent polymer gel encapsulated within a series of discrete cells. This gel readily absorbs water through the membrane wall of the actuator, and can swell to over 300 times its initial volume. This swelling generates significant pressure, changing the shape of the cells and driving the contraction of the muscle. The degree of swelling is significantly reduced by the presence of salts and pollutants in the surrounding water, so transitioning from a freshwater to a saltwater environment causes the muscle to relax. In this paper, we discuss the design and fabrication of these superabsorbent polymer-based Bubble Artificial Muscle (SAP-BAM) actuators. The tensile properties of the muscle under actuated (fresh water) and relaxed (salt water) conditions are characterised, showing a maximum generated force of 10.96N. The length response under constant load for a full actuation cycle is given, showing a maximum contraction of 27.5% of the initial length at 1N load, and the performance over repeated actuation and relaxation cycles is shown. The SAP-BAM muscles are straightforward to fabricate and are composed of low-cost, freely-available materials. Many existing pneumatically-actuated muscles can be modified to use the approach taken for this muscle. The muscle presented in this work represents the first example of a new class of super-absorbent polymer-driven environmental soft artificial muscles.


Gosden, D., Diteesawat, R. S., Studley, M., & Rossiter, J. (2022). Saltwater-responsive bubble artificial muscles using superabsorbent polymers. Frontiers in Robotics and AI, 9, 960372.

Journal Article Type Article
Acceptance Date Jul 27, 2022
Online Publication Date Aug 29, 2022
Publication Date Aug 29, 2022
Deposit Date Sep 8, 2022
Publicly Available Date Sep 8, 2022
Journal Frontiers in Robotics and AI
Electronic ISSN 2296-9144
Publisher Frontiers Media
Peer Reviewed Peer Reviewed
Volume 9
Pages 960372
Keywords Robotics and AI, artificial muscle, superabsorbent polymer, hydrogel, sodium polyacrylate, soft robotics
Public URL
Publisher URL
Related Public URLs The datasets presented in this study can be found in online repositories. Data is available at the University of Bristol data repository, data.bris, at


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