Artificial Spider Silk is Smart like Natural One: Having Humidity-Sensitive Shape Memory with Superior Recovery Stress

Spider dragline silk features a supercontraction with its great sensitivity to humidity/water. Enlightened by this phenomenon, we here put forward an understanding of spider silk supercontration as a shape memory behaviour and first time realised this behaviour with our genetically engineered major ampullate spidroin 2 (eMaSp2) fiber, which has abundant polyalanine and proline motif. Interestingly, at 75% relative humidity (RH), eMaSp2 fibre demonstrated a humidity-triggered shape memory behavior with good shape fixity and recovery ratio of 82.1 ± 2.1% and 98.5 ± 0.4%, respectively. Apart from that, eMaSp2 fibre displayed a recovery stress of 18.5 ± 0.5 MPa at 90% RH, outperforming almost all shape memory polymers or even most shape memory composites reported to date. Evidenced by the results, β-sheets within the artificial silk fibre played as netpoints and hydrogen bonds (HB) designated as switch in the amorphous region (consisting of α-helix, β-turn and random coil), collectively defines a typical shape memory model. To further support this shape memory model, eMaSp2 was evaluated by using equilibrated molecular dynamics (EQMD) simulation for 200 ns. Under various humidities, the change of HB in crystalline and amorphous regions not only demonstrated the contraction mechanism but also proved the shape memory theory. This study shows that the eMaSp2 fibre is a humidity-sensitive shape memory material, providing a new insight into smart behaviours presented by artificial spider silks.