Twisted and coiled polymer actuators: design, mechanics, and biomimetics

In the past few decades, interest in novel soft actuators that can mimic the compliant nature of biological muscles has grown fast. Such actuation technologies have the potential to improve current human-machine interactions in applications such as smart exoskeletons and prosthetics, wearables, intelligent surgical tools, and even humanoids. To this end, it was recently shown that inexpensive drawn polymer monofilaments, such as nylon fishing lines, can be used to create thermally driven linear or torsional soft actuators. These actuators are called twisted polymer actuators (TPAs), and their actuation mechanism relies on the anisotropic microstructure of the virgin material used for fabrication, specifically, the axial thermal contraction and radial thermal expansion. In the past few years, researchers have attempted to model the actuation response of TPAs; however, it has been shown that such actuation is moisture-content- and time-dependent (viscoelastic), which have limited the accuracy of actuation models for TPAs. Furthermore, the thermal activation of TPAs is generally an inefficient and time-consuming (particularly during cooling) driver. This Ph.D. work first presents a literature review on soft actuators and a comparison to biological muscles, focusing on those properties that make biological muscles highly adaptable systems. This study helps better understand the current accomplishments of each soft actuation technology, the remaining challenges, and future directions that are required for soft actuation technologies to be used as successful muscle substitutes. Next, a study on the hygroscopic behavior and moisture content effects of TPAs actuation performance is presented. This study found that TPAs made from drawn monofilaments of nylon 66 are hygroscopic and moisture-content-dependent. Moreover, the hygroscopic properties of TPAs are strongly affected by temperature changes, and temperature changes are required for actuation, which makes TPAs challenging to model and therefore control in environments with different relative humidity levels. As a result of the previously mentioned shortcomings of thermally activated twisted polymer actuators, therest of this Ph.D. work focuses on the conception, characterization, modeling, and initial optimization of a new soft actuation technology inspired by TPAs named cavatappi artificial muscles. Cavatappi artificial muscles use an actuation mechanism that relies upon specific processing of inexpensive polymer tubes to develop microstructural anisotropy, which mimics the anisotropic mechanical properties of the precursor monofilaments used to fabricate TPAs. These tubes can be configured as torsional actuators when twisted or linear actuators when helically coiled, similar to TPAs. After drawing and twisting, hydraulic or pneumatic pressure applied inside the tube results in localized untwisting of the helical microstructure. This untwisting manifests as a contraction of the helical pitch for the coiled configuration. Given the hydraulic or pneumatic activation source and the constant material temperature, these new devices have been shown to outperform TPAs with regard to actuation bandwidth, efficiency, modeling, and practical implementation. Since cavatappi can generate high forces and power, this technology could be used in bioengineering and robotics applications. Cavatappi show contractions greater than 50% of their initial length, mechanical contractile efficiencies of about 45%, and specific work and power metrics ten and five times higher than human skeletal muscles, respectively. Finally, this dissertation also presents an actuation model that uses the material properties of the precursor structure to predict the actuation response of cavatappi artificial muscles and validation using experimental actuation dat