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Non-entropic and reversible long-range deformation of an encapsulating bioelastomer

Abstract

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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Figure 1: Busycon canaliculum photographs and SEM micrographs.
Figure 2: Thermomechanical properties of whelk egg capsule material.
Figure 3: WAXS diffraction patterns at various strains and after full unloading.
Figure 4: Tangent modulus versus strain at various temperatures.
Figure 5: Schematic diagram of α-helix -sheet transition during straining.
Figure 6: Transition stress and strain at various temperatures.

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Acknowledgements

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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A.M. and S.S.W. contributed equivalently to this work. A.M. designed and carried out experiments, analysed and interpreted data and wrote the manuscript. S.S.W. designed and carried out experiments, analysed data, contributed to writing the manuscript and wrote the proposal for the UC-BREP GREAT Grant that helped support the work. C.F.C. conducted experiments. J.H.W. wrote the manuscript and edited all portions, checked theory and calculations and was principal investigator of research grants supporting the investigation experiments.

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Correspondence to Ali Miserez or J. Herbert Waite.

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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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