Characterization of in-vitro passive hyperelastic and damage properties of lamb esophagus treated with melatonin

A thorough understanding of esophageal biomechanics is essential for predicting tissue failure and preventing iatrogenic tearing during surgical procedures, such as esophageal atresia repair. This knowledge is also crucial for the engineering design of mechanically compatible tissue replacements. Melatonin, known for its regenerative properties, could improve the mechanical integrity of esophageal tissue and minimize post-surgical complications, including anastomotic leakage. This study characterizes the passive hyperelastic and damage behavior of the neonatal lamb esophagus and evaluates the effect of melatonin treatment. In-vitro monotonic uniaxial tensile tests were performed on esophageal samples from newborn lambs, which were divided into control (n=4) and melatonin-treated (n=4) groups. The samples were separated into internal (mucosa/submucosa), external (muscularis), or integrated (intact wall) layers, and tested in both longitudinal and circumferential directions. A hyperelastic-damage constitutive model was calibrated to the experimental data to quantify the anisotropic and softening response of the tissues. The esophageal tissue exhibited significant anisotropy, with greater stiffness in the longitudinal direction. No statistically significant differences were observed between the control and melatonin-treated groups, in the Cauchy stress versus stretch response or in an analysis of characteristic curve descriptors computed in nominal stress (q>0.05). However, the small sample sizes left the study underpowered to detect the moderate-to-large effect sizes (Hedges’ g = 0.5-0.9) observed in several comparisons, which therefore warrant confirmation in adequately powered cohorts. The proposed constitutive model accurately captured the non-linear hyperelastic and damage behavior, including the softening up to rupture, across all tissue layers (R²>0.86). This study provides experimentally calibrated material parameters for an anisotropic hyperelastic-damage model of the neonatal lamb esophagus, extending prior hyperelastic descriptions to include softening behavior. Although a 30-day low-dose melatonin treatment did not produce robust alterations in passive mechanical properties, the established constitutive framework provides valuable baseline data to inform computational simulations of surgical procedures and optimize tissue-engineered esophageal grafts.