Mr. David Shepherd Profile

David Shepherd

at Univ of Wollongong

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Proceedings Article | 29 March 2019 Presentation

Double helix artificial muscles (Conference Presentation)

Geoffrey Spinks, David Shepherd

Proceedings Volume 10966, 109660T (2019) https://doi.org/10.1117/12.2513573

KEYWORDS: Artificial muscles, Polymers, Absorption, Process modeling, Actuators, Composites, Molecules, Molecular photonics, Polyurethane

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High performance artificial muscles based on twisted and coiled polymer fibers have attracted considerable attention since their discovery in 2014. These artificial muscles generate tensile strokes as a result of a torsional actuation occurring within the twisted fiber. The torsion is due to a volume expansion and is related to the helical topology of the twisted polymer fiber or fiber composite. The volume expansion can be induced thermally, electrochemically, photonically or by absorption of small molecules, such as water. The magnitude of the torsional stroke and/or the torque generated has been successfully modeled using a single helix approximation. This paper presents a new type of tensile artificial muscle that exploits the properties of the double helix. Two fibers are plied to form the double helix structure and diameter expansion of the fibers generates a large lengthwise contraction in the plied structure. The process is successfully modeled using the single helix approach. The example plied double helix actuators were fabricated from cotton yarn impregnated with hydrogel. The cotton was pultruded through a solution containing the polyurethane based hydrogel. Actuators were made by air drying followed by co-twisting two lengths of the hydrogel-cotton and heat-setting at 60 degrees C. The samples were tested in isotonic mode by tensioning and fully immersing in water. The samples contracted in length when wet and the process was reversed on drying. The effect of hydrogel content and twist density have been investigated. Single hydrogel-cotton yarn samples showed negligible change in length when immersed in water, but two-ply double helix samples showed contractions of more than 10% in length. The length contraction of the double helices was attributed to the increase in fiber diameter during water absorption. The geometry changes were successfully modeled using a single helix approach.

Proceedings Article | 27 March 2018 Presentation

The development and understanding of composite yarn hydrogel actuators (Conference Presentation)

David Shepherd, Geoffrey Spinks

Proceedings Volume 10594, 1059421 (2018) https://doi.org/10.1117/12.2300755

KEYWORDS: Composites, Actuators, Artificial muscles, Polymers, Niobium

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Artificial muscles have the potential to fill inherent gaps left behind by large, hard, complex traditional actuators. One such emerging category of artificial muscles which is showing potential in filling these gaps is composite yarn hydrogel actuators. Hydrogels are soft, potentially biocompatible, polymer materials which can be tuned to have varying swelling ratios to produce the desired mechanical response. Composite yarn hydrogel actuators function by an embedded hydrogel swelling within the confined fibrous structure of a twisted yarn. This hydrogel swelling exerts pressure on the yarn which, in turn, drives either torsional or linear actuation, depending on the twisted structure of the composite yarn. The focus of this research is to develop techniques to produce, test and model this new class of actuator. We will endeavor to garner a deeper understanding of the effects of hydrogel swelling ratio, applied yarn-twist, and the complex structure of the composite actuator. Composite yarn hydrogel actuators comprising of niobium nanowire/hydrogel twisted composites have been produced that can generate large and fast torsional stroke as high as 300 deg/mm over 15 seconds when stimulated by water. Simple cotton/hydrogel coiled composites have demonstrated large repeatable linear stroke lengths of 30% contraction upon hydration. A new class of composite actuator (hydrogel composite tube actuators) can show combined linear and torsional actuation within the same device.

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