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When humidified at different moisture conditions, restrained spider silkfibers can exhibit a very high supercontraction phenomenon. The hydration watermolecules induce a Hydrogen-bonds disruption process that, due to entropiceffects, decreases the natural -- zero force -- end-to-end chains length. Byconsidering a bundle of macromolecules, we describe supercontraction as apossible actuation system and determine the maximum actuation force dependingon the silk properties at the molecular scale and on the constraining systemrepresenting other silk threads or the actuated device. The comparison withexperimental results of Argiope trifasciata silk fibers show the effectivenessof the proposed model in quantitatively predicting the experimental actuationproperties. The considered historical case study of obelisk rescue in SaintPeter's Square (Rome) through ropes hydration is discussed evidencing theoptimal performances of this natural material adopted as moisture poweredactuator: we obtain a work density of 2.19 kJ/m3 making spider silk the mostperformant hydration driven active material. Moreover we obtain a power densityof the order of 730 W/kg about three times the most performant carbon nanotubeactuators making such material very competitive as compared with all types ofactuator. The analytic description of the macroscopic actuation parameters frommicroscale properties shows the possibility of adopting our approach also inthe field of bioinspired artificial silks design, possibly considering alsoimportant non-linear effects in the actuated system.

"Water to the ropes": a predictive model for the supercontraction stress of spider silks