Water absorption mechanisms and structural features in leather by a novel cryo-scanning electron microscopy approach

Water has a profound impact on the mechanical properties and durability of leather, leading to cracking, finish delamination, and aesthetic degradation. However, despite its importance, the mechanisms by which water penetrates and interacts with the internal structure of leather remain poorly understood—particularly before and after the tanning process. Traditional imaging methods, such as optical microscopy, provide limited insight due to their low resolution and inability to probe the inner structure of leather, while electron microscopy, on the other hand, is restricted to dried samples, offering only surface information and introducing structural artifacts. In this experimental study, we use for the first time Cryo-Scanning Electron Microscopy (Cryo-SEM) as a novel and versatile approach to leather water absorption research, by investigating water absorption mechanisms and microstructural features in leather under near-real hydration conditions. This technique preserves the native microstructure, enabling visualization of water interaction within the collagen network at an unprecedented resolution. Furthermore, by coupling Cryo-SEM observations with macroscopic characterization methods, a more in-depth understanding of water-induced absorption processes can be achieved. Comparative analyses on alternative leather materials further demonstrate the broader applicability of this approach to bio-derived materials. Results reveal that water interacts primarily with the porous network of collagen fibers, filling void spaces without causing significant swelling or deformation of the fibers themselves. This behavior is attributed to the cross-linking effects of tanning treatments, which stabilize the collagen matrix and limit water-induced structural modifications. Leather microstructural features, such as the D-spacing of collagen fibers, can also be quantitatively extracted from Cryo-SEM images and are in good agreement with results from the literature. These findings provide valuable insights into the physicochemical behavior of leather under hydration, establishing Cryo-SEM as a powerful tool for elucidating hydration-related phenomena in leather, paving the way for improved tanning methods and the development of more durable, application-specific leather materials.