Mechanical Characterization of an Unusual Elastic Biomaterial from the Egg Capsules of Marine Snails (Busycon spp.)

Abstract

Egg capsule material serves as a putative protection mechanism for developing snail embryos facing the perils of the marine environment. We conducted the first quantitative study of this acellular structural protein with the goals of characterizing its chemical and mechanical properties and the relationship of these properties to its biological protective function. We have found that this protein polymer exhibits long-range elasticity with an interesting recoverable yield evidenced by an order of magnitude decrease in elastic modulus (apparent failure) that begins at 3%-5% strain. This material differs significantly from other common structural proteins such as collagen and elastin in mechanical response to strain. Qualitative similarities in stress/strain behavior to keratin, another common structural protein, are more than coincidental when composition and detailed mechanical quantification are considered. This suggests the possibility of alpha-helical structure and matrix organization that might be similar in these two proteins. Indeed, the egg capsule protein may be closely related to vertebrate keratins such as intermediate filaments. We conclude that while this material's bimodal tensile properties may serve as useful protection against the impact loading egg capsules encounter in the intertidal zone, the full biological importance of these capsules is not known.

Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration

“…Transmission electron microscopic investigations show that the bands of both large and small whelks (Buccinum undatum and Urosalpinx cinerea) have a regular lateral separation of about 4.2 nm (Fig. 46d and e) [222], and this hierarchical structure is also observed for other whelk capsule walls, such as B. canaliculatus [226].…”

“…The whelk egg capsules also show very unique mechanical properties as an effective shock/energy absorber: under extension it undergoes a-helix M b-sheet transition with large and reversible extensibility, and although this suggests of an elastomer that features low modulus, high extensibility, and entropy-driven elastic recovery, the capsule wall recovery is correlated to the internal energy arising from the facile and reversible a M b transition [116,219,226]. Fig.…”

Reversibly labile, sclerotization-induced elastic properties in a keratin analog from marine snails: whelk egg capsule biopolymer (WECB)

“…We have found WECB to posses a similar order of magnitude breaking strain of 95%, but all other parameters are greatly reduced: 5·MPa tensile strength, 3·MJ·m -3 toughness, and a 100·MPa maximum elastic modulus (Rapoport and Shadwick, 2002). Since hard alpha-keratin is based on hierarchical arrangements of IF structures, and WECB is putatively similarly organized, it is reasonable to consider differences in higher order structure, accessory proteins, matrix and stabilization as causative factors in resultant mechanical properties.…”

“…Whelk egg capsule biopolymer (WECB) possesses unusual tensile properties (Rapoport and Shadwick, 2002). Quasi-static mechanical tests revealed the following stress-strain features: an initial region of low strain (>3-5%) distinguished by relatively high modulus of elasticity and low hysteresis, followed by an apparent yield to a plateau-like region with relatively low modulus and high hysteresis that persists up to strains of at least 50% (see Fig.·1).…”

“…Quasi-static mechanical tests revealed the following stress-strain features: an initial region of low strain (>3-5%) distinguished by relatively high modulus of elasticity and low hysteresis, followed by an apparent yield to a plateau-like region with relatively low modulus and high hysteresis that persists up to strains of at least 50% (see Fig.·1). Although this transition has the appearance of tensile failure, it is transient only, and complete recovery occurs during unloading, as evidenced by stable stress-strain curves from repeated cycles to high strains (Rapoport and Shadwick, 2002). For simplicity in referring to these material properties, the initial stiff portion of the stress-strain curve is termed the 'Hookean' region, due to its linear elastic response.…”