Indentation mechanics of healthy and fibrotic murine lung reveals homogeneous surface stiffness

• Regional heterogeneity of the murine lung surface investigated via microindentation. • Stress-strain trends indicate increased surface stiffness of basal lung regions. • Minimal progression of disease presentation at surface observed. Pulmonary fibrosis is a serious disease that heterogeneously stiffens lung tissue and restricts whole organ expansion. To determine if fibrotic stiffening is detectable at the lung surface within the microindentation range, spherical indentation is conducted to directly measure regional surface mechanics of healthy and fibrotic murine samples. Indentation is performed at three regions along the cranial-caudal axis of the murine left lobe to collect force-displacement data for two separate fibrotic groups: early- and late-stage fibrosis. Hertz was used to extract the Young’s modulus from force-displacement data and stress-strain characteristics. Curves were further analyzed to assess the effects of cyclic preconditioning and for energy loss in the form of hysteresis. The Young’s modulus across the surface was consistent between both early- and late-stage control (1.19–1.24±0.36–0.38 kPa) and fibrotic samples (1.34–1.52 ± 0.32–0.61 kPa) despite regional analysis indicating slightly elevated peak stress in the basal region of fibrotic samples. Across the entire surface, the averaged energy loss was higher in fibrotic samples (19.2–20.3 ± 3–4.9 %) compared to controls groups (23.1–25.2 ± 4.1–5.9 %). Additionally, preconditioning elevated Young’s modulus and decreased peak force and energy loss in all treatment groups. The results indicate no detectable regional stiffening in control or fibrotic samples. This data is valuable for computation models and emerging surgical palpation devices and further indicates that fibrosis will likely not affect the ability of such devices to detect pulmonary tumors.