Self-reinforced architecture design for simultaneously transparent and tough sheep horn sheath

Biological systems exhibit extraordinary abilities to synthesize multifunctional materials. The inner layer of the natural sheep horn sheath simultaneously demonstrates high transparency and high toughness, a combination that represents a trade-off difficult to achieve in artificial materials. Here, we investigate the relationship between the microstructure and optical/mechanical properties of the sheep horn sheath. The horn sheath possesses a multilayer self-reinforcing structure, where keratin fibers, acting as the reinforcing phase, match the refractive index of the surrounding keratin. This structural design enables the sheep horn sheath to maintain high transmittance (>85 %) while achieving a tensile toughness of 48.99 ± 2.88 MJ⋅m^-3, which is higher than values reported for biobased transparent materials to date. Compared to disordered fiber structures, the highly oriented fiber structure reduces transverse scattering, thereby improving light transmittance and imparting the ability to modulate light. Mechanically, multiscale toughening, from microscale fiber pull-out/bridging to nanoscale α-helix to β-sheet conformational transitions, acts synergistically to dissipate energy and impede crack propagation. We believe that the bioinspired strategies offered by this self-reinforced structure could provide new insights into the design of transparent materials with toughness. STATEMENT OF SIGNIFICANCE: This study identifies a protein-based, self-reinforced fibrous architecture in sheep horn sheaths that exhibits mechanical-optical synergy, overcoming a long-standing trade-off in synthetic materials. It further clarifies how nano fiber orientation influences transparency and reveals toughening mechanisms such as fiber bridging and protein conformational transitions. Practically, it provides inspiration for the design of bioinspired transparent structural materials with potential in flexible electronics, optical sensing, and transparent protective systems.