Twisted and Coiled Polymer (TCP) muscles are recently introduced thermal actuators that offer unprecedented performance. Merits such as low cost, high load capacity, and light weight, make them suitable for robotics and biomechanics. Using these actuators for practical applications requires an understanding of their behavior in response to the electrical input. In this work, a physics-based model is proposed to predict the displacement of TCP muscles with respect to the input electrical energy and the applied load. We quantified effects of temperature on coefficients of thermal expansion and elastic modulus. It is found that the change of properties above the polymer glass transition temperature has a crucial effect on the actuator performance. A stiffness model is proposed, which include the effect of change in geometry and elastic modulus of TCP muscles. Finally, a set of equations for simulating the TCP muscles are formulated in discrete time representation. The model is generalized for the 2-ply case by considering the difference in geometry. The accuracy of the model is tested in comparison with experimental data, acquired at several actuation levels and applied loads, for both 1-ply and 2-ply configurations. The model showed a good accuracy in predicting the behavior of the actuators both in temperature and displacement. Our model was also compared with other representative models and revealed improvement in the prediction accuracy.

Nylon filament is a widely used thermoplastic material in extrusion-based 3D printing, favored for its strength, durability, and excellent printability. It enables the fabrication of parts with complex geometries, high design flexibility, and cost-effective production, making it ideal for both prototyping and functional components. However, one significant drawback of nylon is its hygroscopic nature—it readily absorbs moisture from the surrounding environment, often at a rapid rate. This moisture uptake can negatively impact the filament’s performance during printing, leading to poor surface finish, reduced mechanical strength, and altered thermal behavior in the final printed parts. To better understand the effects of moisture absorption, this study investigates the mechanical and thermal properties of nylon parts printed using filaments with varying levels of moisture content. The nylon filament was conditioned in a controlled humidity chamber for different durations to simulate moisture exposure over time. Specimens were then printed using these conditioned filaments, and a series of tests were performed to assess their mechanical integrity and thermal stability. By analyzing the test results, the study aims to establish a correlation between filament moisture content and part quality, offering valuable insights into the degradation mechanisms and guiding best practices for filament handling and storage in nylon 3D printing applications.

A soft actuator is an essential component in a soft robot that enables it to perform complex movements by combining different fundamental motion modes. One type of soft actuator that has received significant attention is the twisted and coiled polymer artificial muscle (TCP actuator). Despite many recent advancements in TCP actuator research, its use as an extensile actuator is less common in the literature. This works introduces the concept of using TCP actuators as thermal-driven extensile actuators for robotics applications. The low-profile actuator can be easily fabricated to offer two unique deformation capabilities. Results from the characterization indicate that extensile actuators, made with various rod diameters and under different load conditions, display remarkable elongation deformation. Additionally, a proof-of-concept soft-earthworm robot was developed to showcase the potential application of the extensile actuator and to demonstrate the benefits of combining different types of motion modes.