Co-authors​ :

 Charles DE KERGARIOU (UNITED KINGDOM), Graham J. DAY , Adam PERRIMAN , James AMSTRONG (UNITED KINGDOM), Fabrizio SCARPA  

Hydrogel materials can be implemented in biomedical applications. For instance, they could be used to grow cells, to repair bone and to heal wounds. These applications require the control of the mechanical properties and the deformation capability of the material. One of the techniques used to achieve this is to add reinforcement to the material. In the past years, flax fibres have been raised as one of the most promising reinforcement biomaterials [1], it has also shown great potential to create actuation out of 3D printed structures [2]. Consequently, the objective of this study was to highlight the potential of the combination of alginate-based hydrogel and flax fibre. The first set of the study was to create a process to 3D print this material. After this new composite was created the mechanical properties were tested. Figure 1 shows the influence of the amount of flax fibre reinforcement on the stiffness and strength of the composite hydrogel. The optimal amount of fibre was determined at a 1% fibre weight fraction, leading to a 252% and 234% increase for E1 and σ, respectively. Mooney Rivlin coefficient was also calculated to model the mechanical behaviour over a longer range of strain than E1 (C10=-2.06e5±3.22e4Pa; C01=2.88e5±3.38e4Pa). A fractography inspection was conducted on the failed tensile specimens to better understand the fracture process. During this investigation crack deflect and crack branching mechanisms were spotted. These mechanisms explain why the highest amount of energy dissipated (4774.9J/m3) during fracture is obtained at 1% fibre weight fraction. Then the printability of the material was tested. Collapse tests showed that the addition of flax fibre allowed over gaps 10mm long. The rheology test conducted showed an increase of 39% and 129% of the viscosity thanks to the addition of the fibres at a strain rate of e0 and 5e0, respectively. Finally, the 4D printing capability was tested via hygro expansion test (e.g. [2]) with the hydrogel dried at room humidity. The addition of fibres increases the difference of expansion between transverse and longitudinal directions. The ratio between the transverse and longitudinal coefficients of moisture expansion rises from 1.86 to 4.67 when fibres are added to the hydrogel. Therefore, the fibres increase the 4D printing actuation potential of the material. Following this result, a 4D printing actuator in the shape of a rosette was printed and is displayed in Figure 2. Drying this specimen showed interesting actuation capability as the highest point of the structure raised to 8mm.