Encapsulation is a widespread biological process particularly in the formation of protective egg cases of oviparous animals. The egg capsule wall of the channelled whelk Busycon canaliculum is an effective shock absorber with high reversible extensibility and a stiffness that changes significantly during extension. Here we show that post-stretch recovery in egg capsules is not driven by entropic forces as it is in rubber. Indeed, at fixed strain, force decreases linearly with increasing temperature, whereas in rubber elasticity the force increases. Instead, capsule wall recovery is associated with the internal energy arising from the facile and reversible structural α-helix -sheet transition of egg capsule proteins during extension. This behaviour is extraordinary in the magnitude of energy dissipated and speed of recovery and is reminiscent of strain-induced crystallization in some polymeric fibres and of superelastic deformations associated with diffusionless phase transitions in shape-memory alloys.
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We thank B. Shadwick (University of British Columbia, Vancouver, Canada) and F. Zok (Materials Department, UCSB) for helpful criticism. Y. Li (Materials Research Laboratory (MRL), UCSB) and H. Gupta (Max-Planck Institute for Colloids and Interfaces, Golm, Germany) provided critical guidance for WAXS and time-resolved synchrotron experiments, respectively. K. Field (Mechanical Engineering Department, UCSB) helped in developing the microtensiometer. S.W. was financially supported by a California Sea Grant # R/MP-97B and UC BREP GREAT traineeship #2007-02. This work made use of MRL Central Facilities supported by the MRSEC Program of the National Science Foundation under award No. DMR05-20415. C.F.C was supported by an internship from the ‘Research Internships in Science and Engineering’ (RISE) programme sponsored by the MRL and the California NanoSystems Institute (CNSI) at UCSB.
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Miserez, A., Wasko, S., Carpenter, C. et al. Non-entropic and reversible long-range deformation of an encapsulating bioelastomer. Nature Mater 8, 910–916 (2009). https://doi.org/10.1038/nmat2547
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