The Use of Phase Change Materials to Create Variable Stiffness Linkage-Fabrics

Animate materials that can respond to the input from their environment and dynamically change shape have seen increasing applications in recent years in the fields of soft robotics and assistive technology [1,2]. Their ability to morph from a flexible to a rigid state is one of their key attributes. However, current shape-changing structures that utilise actuators often require a constant energy supply to retain their rigid shape, while they return to the original structure when the energy is removed [3,4]. This limits the utility of the materials for applications such as assistive technology and deployable structures where energy use needs to be minimised to meet design requirements. This research aims to address this problem by developing a stiffness-changing structure that does not require constant energy input to maintain either the low or high stiffness state.<br/><br/>Phase change materials(PCMs) have traditionally been used as energy storage materials due to their high latent heat [5], but little attention has been paid to their potential for mechanical applications using the cyclic liquid to solid phase change of these materials. Phase change materials can be cooled below their solidification temperature and stay in their supercooled liquid phase for an indefinite period. Solidification can be triggered through the introduction of nucleation sites such as crystals by using electrical or mechanical triggers. By heating the material above its solidification temperature the phase change can be reversed.<br/><br/>In this work, we show that the mechanical properties of linkage-fabric structures can be controlled by the use of a phase change material incorporated into the fabric. We use sodium acetate trihydrate (CH3COONa×3H2O) as the phase change material with the nucleation from the liquid to the solid-state controlled by an electric impulse [6]. The linkage-fabric structures were designed using CAD software to create a flexible interlocking structure that when actuated modulates the stiffness of the fabric. The structures were 3D printed in nylon using Selective Laser Sintering (SLS). For each sample, supercooled sodium acetate and the conducting paths for the electrical signals were manually incorporated. The phase change response of the sodium acetate was triggered by silver electrodes with microcrystals embedded onto their surface, through which a DC voltage has been supplied. We present data on the effect of sodium acetate solution composition on the mechanical stiffness of the actuated fabrics, the reversibility of this change and the impact of fabric design on its property. The work shows that phase change materials can be used for the construction of linkage-fabrics with variable stiffness without requiring constant energy input. The work opens up myriad opportunities to exploit these structures in applications for deployable structures and assistive technology.<br/><br/>Refs:<br/>1. Animate Materials, Royal Society Perspective, Feb 2021, ISBN: 978-1-78252-515-8. https://royalsociety.org/-/media/policy/projects/animate-materials/animate-materials-report.pdf<br/>2. Ball, P. Animate Materials, MRS Bulletin volume 46, pages553–559 (2021)<br/>3. M. Ransley, P. Smitham, and M. Miodownik, Active Chainmail fabrics for soft robotic applications, Smart Mater. Struct., 26, 08LT02 (2017).<br/>4. Ploszajski, A. R., Jackson, R., Ransley, M., & Miodownik, M. (2019). 4D Printing of Magnetically Functionalized Chainmail for Exoskeletal Biomedical Applications. MRS Advances, 4 (23), 1361-1366. doi:10.1557/adv.2019.154<br/>5. Lane, G. A., & Ph, D. (1983). Lane, G. A - Solar Heat Storage_ Latent Heat Materials vol II Technology-CRC Press (1983).<br/>6. Dong, C., Qi, R., Yu, H., & Zhang, L. (2022). Electrically-controlled crystallization of supercooled sodium acetate trihydrate solution. Energy and Buildings, 260, 111948. https://doi.org/10.1016/j.enbuild.2022.111948