The Hierarchical Structure of Sheep Wool and Its Impact on Physical Properties—A Comprehensive Study

Sheep wool is the most abundant and ubiquitous α-keratinous fiber in the history of textiles, with evidence of wool fabrics dating from the fourth millennium BCE and hundreds of sheep breeds raised worldwide today. This genetic diversity provides access to a broad range of morphologies and textile-relevant properties, such as mechanical strength and hydrophilicity, offering a unique platform for the development of versatile, natural, and biodegradable fibers and fabrics. Although the science of sheep wool and other α-keratinous materials is a mature field, important hierarchical aspects of the structure and properties of wool remain unclear. We lack fundamental understanding of structure-property relationships across levels of organization, from chemical structure, to nanoscale self-assembly, and microstructural topology. Here, we leveraged the diversity of 16 different sheep wool fiber types to investigate the connections between morphological structure at various length scales and textile-relevant macroscopic properties, such as tensile strength, elastic modulus, thermal stability, and hydrophilicity. We quantified the microstructure, protein conformation, and chemical composition of the fibers, by leveraging electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis. This comprehensive study enabled us to uncover a correlation between protein secondary structure, polymer crystallinity, and microscale topology (e.g. fiber diameter, linear density of scales, and cortical cells), bridging the gap between the different length scales of hierarchical ordering. Moreover, we investigated the hydrophilicity and mechanical properties of the wool fibers through tensiometry and uniaxial tensile testing, correlating elastic properties and water contact angle to hierarchical ordering. To the best of our knowledge, this work represents the most comprehensive study of sheep wool structure and properties to date, contributing significant insight for the development and processing of α-keratinous fibers with tailored properties.