Barbieri E and Gupta HS (2023), Is Stress Relaxation in Sea Cucumber Dermis Che...

ECB-ART-52770

Mar Drugs 2023 Nov 25;2112:. doi: 10.3390/md21120610.

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Is Stress Relaxation in Sea Cucumber Dermis Chemoelastic?

Barbieri E , Gupta HS .

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Echinoderms, such as sea cucumbers, have the remarkable property of changing the stiffness of their dermis according to the surrounding chemical environments. When sea cucumber dermal specimens are constantly strained, stress decays exponentially with time. Such stress relaxation is a hallmark of visco-elastic mechanical behavior. In this paper, in contrast, we attempted to interpret stress relaxation from the chemoelasticity viewpoint. We used a finite element model for the microstructure of the sea cucumber dermis. We varied stiffness over time and framed such changes against the first-order reactions of the interfibrillar matrix. Within this hypothetical scenario, we found that stress relaxation would then occur primarily due to fast crosslink splitting between the chains and a much slower macro-chain scission, with characteristic reaction times compatible with relaxation times measured experimentally. A byproduct of the model is that the concentration of undamaged macro-chains in the softened state is low, less than 10%, which tallies with physical intuition. Although this study is far from being conclusive, we believe it opens an alternative route worthy of further investigation.

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References [+] :

Benedetto, Production, characterization and biocompatibility of marine collagen matrices from an alternative and sustainable source: the sea urchin Paracentrotus lividus. 2014, Pubmed, Echinobase

	Figure 1. Finite element model of staggered fibrils of length LF, connected by an interfibrillar matrix. (a) Tissue specimen of length LT, with an applied constant strain ϵT and fibrillar microstructure; (b) fibril elements (cylinders), shear elements (blue lines) and node numbering (circled numbers); (c) The matrix cannot carry tensile loads, only shear stresses, τij.
	Figure 2. Chemoelastic relaxation for tissue stress; the continuous lines are the numerical results, the dots are the mean of the experimental measures, and the error bar plots are the corresponding standard deviations.
	Figure 3. Chemoelastic relaxation for the fibril strain; the continuous lines are the numerical results, the dots are the mean of the experimental measures, and the error bar plots are the corresponding standard deviations.
	Figure 4. Chemoelastic relaxation parameters for the interfibrillar matrix.
	Figure 5. Chemoelastic relaxation parameters for the fibrils.
	Figure 6. Elastic fibrils made by collagen macromolecules: hypothesized reversible reaction of chain scission and recombination; A are macro-chains before scission and B are those after scission.
	Figure 7. Interfibrillar matrix composed of crosslinked macro-chains (species A); the hypothesized reactions of chain scission, crosslink splitting, and recombination. Species B represents chains after chain scission and C is chains after chain scission and cross-link splitting.
	Figure 8. The surfaces in Equation (A47). The intersection of the surfaces (red lines) is all the possible reaction rates.
	Figure 9. Interfibrillar matrix reaction times for ASW, KASW, and CaFASW: in all three conditions, the fastest times are due to chain recombination, with crosslink splitting being around 10 times slower and chain scission being extremely slow.
	Figure 10. Initial conditions of the chemical species for all the permissible reaction times.