Morphostructural and Biomechanical Characterization of the Human Large Bowel: Toward Computational Modeling for Device Design in Colostomy

Colostomy, a procedure that redirects fecal matter to an external bag, often causes physical and psychological chal lenges [1]. Emerging biomedical technologies aim to develop continence devices that enable bag-free periods, improving patients’ quality of life. To this purpose, this study focused on morphostructural (19 patients)/mechanical (8 patients) characterization of human large bowel (LB) obtained from patients undergoing abdominal surgery for benign conditions at Padova University Hospital (Ethics Approval: CESC Code AOP3212). These analyses are required to support the devel opment of realistic computational models that are essential for accurately simulating tissue- device interaction, which is critical for optimizing device design. The mean thicknesses of different tissue layers were: 669.7 ± 331.5 μm, mucosa; 18.5 ± 8.5 μm, muscularis mucosae; 1173.1 ± 705.7 μm, submucosa; 1813.5 ± 433.4 μm, circular muscle layer; and 1209.2 ± 617.8 μm, longitudinal muscle layer. In parallel, collagen content in the submucosa was evaluated using semi- quantitative analy sis of Azan-Mallory–stained sections via ImageJ software. Col lagen quantification was expressed as the percentage of image area occupied by collagen relative to the total image area (% = collagen area / total area × 100), yielding an average of 51.16% ± 7.91. Mechanical testing was performed (Mach-1 system, ©Biomomentum) on samples oriented along three directions: longitudinal (L, n=8), circumferential (C, n=20), and radial (R, n=12). After preconditioning, L and C samples underwent tensile testing either to failure (strain rate: 100%/s) or under stress-relaxation (SR) conditions (six 30% strain ramps; strain rate: 100%/s; 600 s rest). R samples were subjected to compres sion protocols, including loading-unloading (LU: 60% strain, 1%/s, 10 cycles) and SR (six 10% strain ramps; 100%/s; 600 s rest). For tensile tests, key parameters were calculated from stress- strain curves: ultimate tensile strength (UTS), failure strain (FS), and both initial (Ei) and final elastic moduli (Ef), corresponding to the toe and quasi-linear regions, respec tively. From SR tests, equilibrium relative stiffness (γ∞) was determined as the final normalized stress. LU tests yielded values for Ei, Ef, and peak stress (PS). Results highlighted the mechanical behavior of LB tissue as anisotropic, non-linearly elastic, and time-dependent. Key parameters in each direction were: UTS [kPa]: 427 ± 173 (L), 588±290 (C) and – (R); FS [-]: 2.46 ± 0.72 (L), 3.30 ± 1.50 (C) and – (R); PS [kPa]: – (L), – (C) and 12.13±1.98; Ei [kPa]: 20±17 (L), 18±13 (C) and 0.85±0.34 (R); Ef [kPa]: 485±259 (L), 278±93 (C) and 147±74 (R) ; γ∞ [-]: – (L), 0.12±0.02 (C), 0.07±0.02 (R). The study provided the basis for a visco-hyperelastic constitutive model of fiberreinforced materials, enabling accurate simulation of LB tissue mechanics for the design of advanced continence devices.