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The mucilaginous viscin tissue within mistletoe berries possesses an extraordinary ability to be rapidly processed under ambient conditions into stiff cellulosic fibers (>14 GPa) through simple mechanical drawing. This rapid and extreme transformation process is hydration-dependent and involves an astonishing >200-fold increase in length, providing a relevant role model for efforts to produce advanced composites from cellulose-based structures such as cellulose nanocrystals or cellulose nanofibrils. Using a combination of in situ polarized light microscopy, synchrotron X-ray scattering, and humidity-controlled mechanical analysis, we examine here the dynamic transition of a viscin cell bundle from hydrogel-like tissues to high-performance fibers. Our findings indicate a massive phase transition in which cellulose microfibrils containing high-aspect-ratio crystalline domains undergo dramatic reorganization, facilitated by a water-responsive noncellulosic matrix. Transition from an aligned, yet flowing state to a stiff fiber is likely triggered by rapid water loss below 45% relative humidity. These findings not only help understanding the adaptive success of mistletoe but may also be relevant for the development of new facile processing methods for next-generation cellulosic composites.

Silk fibers are well known for their mechanical properties such as strength and toughness and are lightweight, making them an interesting material for a variety of applications. Silk mechanics mainly rely on the secondary structure of the underlying proteins. Lacewing egg stalk silk proteins obtain a cross-β structure with individual β strands aligned perpendicular to the fiber axis. This structure is in contrast with that of silks of spiders or silkworms with β strands parallel to the fiber axis and to that of silks of honeybees with α helices arranged in coiled coils. On the basis of the cross-β structure the mechanical properties of egg stalks are different from those of other silks concerning extensibility, toughness, and bending stiffness. Here we show the influence of relative humidity on the mechanical behavior of lacewing egg stalks and propose a model based on secondary structure changes to explain the differences on a molecular level. At low relative humidity, the stalks rupture at an extension of 3%, whereas at high relative humidity the stalks rupture at 434%. This dramatic increase corresponds to breakage of hydrogen bonds between the β strands and a rearrangement thereof in a parallel-β structure.

Collagen is a versatile structural molecule in nature and is used as a building block in many highly organized tissues, such as bone, skin, and cornea. The functionality and performance of these tissues are controlled by their hierarchical organization ranging from the molecular up to macroscopic length scales. In the present study, polarized Raman microspectroscopic and imaging analyses were used to elucidate collagen fibril orientation at various levels of structure in native rat tail tendon under mechanical load. In situ humidity-controlled uniaxial tensile tests have been performed concurrently with Raman confocal microscopy to evaluate strain-induced chemical and structural changes of collagen in tendon. The methodology is based on the sensitivity of specific Raman scattering bands (associated with distinct molecular vibrations, such as the amide I) to the orientation and the polarization direction of the incident laser light. Our results, based on the changing intensity of Raman lines as a function of orientation and polarization, support a model where the crimp and gap regions of collagen hierarchical structure are straightened at the tissue and molecular level, respectively. However, the lack of measurable changes in Raman peak positions throughout the whole range of strains investigated indicates that no significant changes of the collagen backbone occurs with tensing and suggests that deformation is rather redistributed through other levels of the hierarchical structure.