Artificial Muscles as Cardiac Assist Devices
Creators: name: Aloko, Sinmisola
Publisher: name: University of Wollongong
Year: 2025
Subjects: classificationCode: 3499
subject: Other chemical sciences
classificationCode: 4003
subject: Biomedical engineering
classificationCode: 4016
subject: Materials engineering
subject: FOS: Materials engineering
classificationCode: 320101
subject: Cardiology (incl. cardiovascular diseases)

description: Muscle-like actuators can function in various medical applications, such as heart failure, where the gold standard of treatment is currently heart transplantation with further research. With limited donors and strict criteria for suitable donors, the emergence of superior or similar-effect cardiac support devices is a welcome development, as currently available devices are large and cause undesired effects like thrombosis. Direct mechanical cardiac compression eases such burdens using miniature materials, such as coiled muscle-like actuators, which are noiseless and provide good actuation, similar to natural heart muscles.Twisted and coiled artificial muscles have twenty times the power-to-weight ratio of skeletal muscles and are made from everyday materials, making them quite affordable. However, a major limitation to their exploration is the low output to input power ratio which is lower than the ratio for human muscles and implies that they operate at higher than desired temperatures. In a bid to improve the output, this research work focuses firstly on the exploration of and the effect of coil geometry on their actuation properties. Secondly, to find ways to make them operate at lower temperatures by incorporating thermosensitive gels into the yarns to induce volume change actuation.Initially, twisted, and coiled polymer samples of varying indices were made using the mandrel wrapping technique. The tensile actuation parameters were obtained by stretching the samples at a certain preload force in the actuated (80<sup>o</sup>C) and unactuated state (30<sup>o</sup>C) to generate their load-unload force-extension curves in both states. Torsional actuation was evaluated by tethering a twisted but not coiled sample to a torsional sensor and inserting manual turns. Like linear actuation, twist-untwist curves of torque versus twist were obtained in both actuated and unactuated states.Samples were analysed for their tensile and torsional properties for samples prepared with coil indices from 1.8 to 2.9 and produced measured tensile strains from 15% to 85%. The tensile strains were reliably predicted as 20% to 96% using spring mechanics calculations. Stiffnesses of 0.04 N/mm to 0.22 N/mm was also measured in the samples with spring mechanics prediction of 0.03 N/mm to 0.20 N/mm.Next, yarn gel composites of thermosensitive poly (MAA-coOEGMA), iron chloride gel and polyester yarn were successfully developed. The gel and polyester yarn composites produced reversible and repeatable volume changes in the form of swelling and deswelling with small temperature changes, first as a gel and when incorporated into yarns to form a composite. The yarn composites were subjected to a 7.5% stretch at 50oC (unactuated) and 27oC (actuated) to obtain tensile actuation data from samples made using various quantities of the crosslinker- iron chloride (0-2 M). Isotonic strains up to 39% were measured in the gel-yarn composites with the 1 M iron chloride concentration selected as the optimum crosslinking concentration. The maximum operation temperature of gel-yarn twisted, and coiled composites was 50<sup>o</sup>C which is lower than the 80<sup>o</sup>C operating temperature for actuators made from twisted and coiled nylon 6 fibres.Lastly, an attempt was made to assess whether the low temperature activated actuators of yarn and gel could be successfully developed and applied to a prototype cardiac assist device. A simple model of a water-filled balloon with actuators wrapped around it and a transparent tube fixed to the balloon top to read the difference in water levels was used. The model did not show discernible differences in the water level when the actuators were stimulate. Another model using a pair of silicone tongs with actuators fixed to the handle and the balloon supported by the grips of the tong demonstrated pumping action when twisted and coiled nylon-6 fibres were used as the actuator. There was no discernible effect when the low-temperature composite yarn and poly (MAA-coOEGMA) gel actuators were used. In order to explain the lack of pumping in the tongs experimental prototype, the force generated by each set of actuators was calculated, and the lack of visible pumping action by the low-temperature composite actuators was attributed to the low force generation during the heating and cooling of the gel composite samples.Overall, this thesis has contributed to the understanding of how coil geometry influences the actuation of twisted and coiled polymer fibres and how to a reasonable extent, spring mechanics can be used to predict actuation properties. A low temperature activated actuator was also developed from poly (MAA-coOEGMA) iron chloride gel and yarn to produce low temperature actuation at 50<sup>o</sup>C.