This paper presents the melt spinning and characterisation of polymer actuator fibres; fibres that reversibly contract along the fibre axis in response to heat. Recently, Haines et al (1) showed that low-cost filaments, e.g. fishing lines, can be relevant precursors for artificial muscles. They demonstrated a reversible fibre-direction thermal contraction, which was significantly amplified when the fibres were twisted and coiled. The effect was explained to result from an increase in the conformational entropy of the amorphous phase. In earlier studies on negative thermal expansion in anisotropic polymer structures, it has been shown that the negative thermal expansion in oriented highly crystalline polymers approaches values typical of polymer crystals (2).

To further investigate the mechanisms behind these seemingly simple artificial muscles, we have melt spun fibres from poly(vinylidene fluoride) (PVDF) – Solef  1006 and 1008 kindly provided by Solvay (Milan, Italy) – and compared their properties to a commercially available PVDF-fishing line. The fibres were characterised with respect to their thermal actuation properties, crystal morphology and degree of orientation along the spin-line axis.

We have further done modelling on the molecular and macroscopic levels examining the possible mechanisms of negative thermal expansion in semi-crystalline PVDF. We believe that tie molecules (a polymer chain linking two crystalline regions) are the predominant factor influencing actuation. Two mechanisms are considered: an entropic effect and a conformational change effect. The entropic effect causes an increase in the elastic stiffness with an increase in temperature, effectively resulting in a contraction of a strained fibre. The conformational change effect is also expected to contribute to contraction as tie molecules, under strain, revert to their unloaded preferred conformation when heated.

1. C. S. Haines et al., Artificial Muscles from Fishing Line and Sewing Thread. Science 343, 868-872 (2014).
2. C. L. Choy et al., Negative Thermal Expansion in Oriented Crystalline Polymers. Journal of Polymer Science: Polymer Physics Edition 19, 335-352 (1981).

Textile sensor for human motion detection in healthcare applications This project aims to develop a wearable and comfortable sensor system useful for continuous monitoring of symptoms of epilepsy and Parkinson´s disease, and progress during rehabilitation after a stroke. The system is to monitor both physiological electrical signals and movement, providing an objective assessment tool for hospital personnel monitoring the wearer’s progress. This gives a possibility to improve diagnosis, monitor disease progression or improvement and tailor treatments. By integrating the wearable sensors into a garment, preferably into the fabric itself, we aim to develop a functional demonstrator that is comfortable enough to be accepted by the patients for daily use. [1] The use of textile sensors in healthcare applications is one step closer to a more comfortable wearable sensor system for continuous measurements. For heart rate detection textile based electrodes in garments have been investigated. e.g. [2] Our current focus is on developing textile sensors for human motion detection, connected to the specific motions regarding the neurological disorders. One approach is to integrate electromechanical properties in the textile structure, creating strain sensitive structures which give an electrical output when stretched mechanically due to movement. When investigating a suitable textile construction developing textile sensors often takes a trial and error approach, which is time consuming. In a recent study[3] we showed that the textile construction influenced the performance of a textile sensor. The study pointed to a need of a more controlled developing method, such as computer simulation, to make more accurate predictions of the sensors performance. By investigating the possibility to combine existing computer simulation programmes, such as Comsol Multiphysics and TexGen, for an assessment of the behaviour and performance of the electromechanical properties of textile structures a new design method for smart textile sensors could be achieved. In an ongoing interdisciplinary research project, wearITmed, partners from healthcare, electronics and textiles development (Sahlgrenska Academy, Acreo ICT, The Swedish School of Textiles, Swerea IVF