How Water Can Affect Keratin: Hydration‐Driven Recovery of Bighorn Sheep (Ovis Canadensis) Horns

Abstract

Keratin is one of the most common structural biopolymers exhibiting high strength, toughness, and low density. It is found in various tissues such as hairs, feathers, horns, and hooves with various functionalities. For instance, horn keratin absorbs a large amount of energy during intraspecific fights. Keratinized tissues are permanent tissues because of their basic composition consisting of dead keratinized cells that are not able to remodel or regrow once broken or damaged. The lack of a self-healing mechanism presents a problem for horns as they are under continued high risk from mechanical damage. In the present work, we show for the first time that a combination of material architecture and a water-assisted recovery mechanism, in the horn of bighorn sheep, endows them with shape and mechanical property recoverability after being subjected to severe compressive loading. Moreover, we unravel the effect of hydration, on the material molecular structure and mechanical behavior, by means of synchrotron wide angle X-ray diffraction, Fourier transform infrared spectroscopy, nanoindentation, and in-situ and ex-situ tensile tests. The recovery and remodeling mechanism is anisotropic and quite distinct to the self-healing of living tissue such as bones.

Influence of water content on mechanical behaviour of gastropod taenioglossan radulae

“…insect cuticle [49][50][51][52][53][54][55][56][57][58][59][60]), but also vertebrates (e.g. mammal horn tissue [61,62]; bones, e.g. [63,64]).…”

Multiscale Toughening Mechanisms in Biological Materials and Bioinspired Designs

“…Under a tensile strain rate as high as 1000 s −1 , the strain can reach 70% in wet horns compared with less than 5% strain in dry horns . Thus, water as a second phase in keratin can increase the plasticity of the amorphous matrix, thus making the whole structure more energy absorbent …”

Engineering with keratin: A functional material and a source of bioinspiration

“…Keratin systems often function as protective layers which undergo significant deformation. Many of these systems are permanent and cannot remodel or self-heal through biological processes after experiencing considerable deformation, such as in the bighorn sheep horn ( Huang et al., 2019b ), feathers ( Liu et al., 2015b ; Sullivan et al., 2018 ), and pangolin scales ( Liu et al., 2016b ). A solution to this lack of regenerative capacity is keratin's ability to undergo hydration-assisted shape recovery.…”

“…(2015b) , who observed 98% shape recovery in compressed peacock tail feathers after seven cycles of deformation to over 90% strain ( Liu et al., 2015b ). After the keratin is deformed plastically, the recovery process involves water infiltrating the amorphous keratin matrix, causing swelling, which forces the deformed crystalline regions of the IFs to regain their initial shape by breaking and reforming hydrogen bonds ( Huang et al., 2019b ). Also, the feather shaft was shown to have hydration-assisted shape and strength recovery.…”

“…(2018) for the feather is shown in Figure 7 ( Sullivan et al., 2018 ). The Bighorn sheep horn was also shown to recover its shape by soaking in water after severe compression of 50% strain which was further assisted by the hollow tubules ( Huang et al., 2019b ). In a similar study by Liu et al.…”