Year

2020

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials, Mechatronic and Biomedical Engineering

Abstract

Artificial muscles are devices or materials which can contract, expand, or rotate when acted upon by an external stimulus (such as electricity, pH, pressure, magnetic field, or temperature). The applications of artificial muscles are versatile because they can exhibit linear actuation as in hydraulic rams, torsional actuation like electric motors, and bending actuation as seen in nature. Artificial muscles have also exhibited great potential in robot fabrication and in surgery tools due to their resemblance to biological muscles along with high actuation force per unit mass. One successful type of artificial muscle uses a braided bladder that generates muscle-like contractions when the bladder is pressurised with gas or liquid. These systems are used commercially in industry, but are not useful in portable applications, such as robotics, because of the need for an external pump or compressor. This thesis investigated the use of a volume-changing hydrogel material as a means for pressurizing and depressurizing a braid. The hydrogel has the potential to replace the pump/compressor and make possible a smaller, lighter artificial muscle. For further investigation of these artificial muscles as linear actuators and for the practical application of linear actuators, it was imperative to standardize methods for characterizing their performance.

This thesis introduces such an integrated characterisation method and demonstrates its use in the determination of the free stroke of a hydraulic artificial muscle (HAM); the stroke while operating against an externally applied force (isotonic); the blocked force of these muscles upon applying constant length (isometric); force and displacement change at constant pressure (isobaric); and actuation against a return spring (variable force, pressure). The linear mechanics approach has been verified and allows the prediction of fundamental characteristics of the actuator: free stroke, blocked force and stiffness while varying the preload condition. This thesis further demonstrates the method of fabricating temperature-sensitive, hydrogel filled, novel braided muscles with different hydrogel compositions and investigates the characteristics of these hydrogel filled muscles by employing a spring-test method.

The experimental investigation of the gel-filled muscles is carried out by heating and cooling the muscle at different temperatures. The results show that the blocked force and free stroke depend on the monomer concentration used to prepare the hydrogels and the spring stiffness against which the muscle actuates. The force and displacement are more significant in the muscle with higher monomer concentration. The hydrogel muscles are then further developed into a new type of thermally-actuated braided hydrogel muscle made by systematically integrating cotton fibre and hydrogel. The performance of these actuators is also studied experimentally based on the integrated test method. The actuation mechanism is examined by varying the monomer composition and braid angle. The degree of actuation is found to be greater for higher monomer concentration and higher braid angle due to the relationship of volume change to the applied temperature change. Hysteresis is shown to be affected by temperature change. The generated blocked force and displacement are examined at varied pre-load conditions when combined with the measured muscle stiffness. It has been found that the greater the stiffness and pre-load, the higher the blocked force and displacement. The effect of hydrogel fill on the linear actuation is also investigated, and it is observed that solid hydrogel filled braided hydrogel muscle is capable of generating almost double the force compared with a braided hydrogel muscle without hydrogel filling (shell-gel). This was the first time these porous braided hydrogel muscles are fabricated. These novel actuators are experimentally investigated using the integrated method to determine the influence of hydrogel thickness on the actuation mechanism. Finally, to show the applicability of braided hydrogel muscles in a closed system, a novel enclosed shell-gel braided hydrogel muscle is introduced in which the linear actuation of a shell-gel muscle occurs within an encapsulating rubber bladder. A hot air gun is used for heating while cooling occurred naturally to room temperature. There is no significant difference in actuation acquired for encapsulated shell-gel compared to the shell-gel muscle operated in an open water bath and this suggests the compatibility of these braided hydrogel muscles for both open and closed system.

Overall, this thesis establishes fabrication strategies for hydrogel based novel braided artificial muscles, determines their actuating response using a new efficient, integrated characterisation method and validates the experimental results by using graphical modelling approaches. The thesis concludes with suggestions for further work that include both theoretical and practical areas.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.