Microstructure-dependent biomechanical heterogeneity in irises using atomic force microscopy

This study aimed to characterize regional microstructure and biomechanical heterogeneity in rabbit iris and to determine associations between structural features and mechanical parameters. Surface morphology and mechanical properties of the right eye iris of six rabbits were analyzed using atomic force microscopy (AFM). Morphological parameters (Ra, Rq, Skew, Kurtosis, Anisotropy Index, AI; Principal Direction Angle, PDA; Average Fiber Width, AFW) and mechanical metrics (Adhesion Force, AF; mechanical heterogeneity, CV; Young's modulus; hardness; Residual Deformation Index, RDI) were evaluated. Regional differences were assessed using a mixed-effects model, and associations were examined with Spearman correlation analysis. Kurtosis (r = -0.226) and RDI (r = -0.408) showed negative correlations with hardness (P = 0.057, P = 0.048), whereas AFW (r = 0.352) and CV (r = 0.234) were positively correlated with hardness (P = 0.002, P = 0.000). PDA was negatively correlated with Young's modulus (r = -0.254, P = 0.031). Among mechanical parameters, AF exhibited significant regional variation, peaking in the mid zone and decreasing towards the root (104.2 ± 5.95 nN > 92.79 ± 5.95 nN > 90.50 ± 5.95 nN). Young's modulus followed a similar regional trend of pupillary zone > mid zone > root (40.8 ± 3.61 kPa > 36.99 ± 3.61 kPa > 32.62 ± 3.61 kPa), although this difference was not statistically significant (P = 0.094). These findings establish a microscale structure-mechanics relationship in the rabbit iris, showing that local topography and collagen fiber architecture are closely associated with spatial biomechanical heterogeneity.