Determining the viscoelastic properties of murine placenta via micro-indentation

Introduction: : The placenta plays a central role in supporting fetal development, and disruptions in its structure or function are associated with serious pregnancy complications such as preeclampsia, intrauterine growth restriction (IUGR), and stillbirth. However, direct mechanical testing of human placental tissue prior to labor is limited by ethical and practical challenges. Mouse models offer a valuable alternative, providing access to tissues throughout pregnancy and enabling studies of pathological states in controlled environments.

Despite its importance, the biomechanical behavior of the placenta, particularly its viscoelastic properties, remains poorly defined. These properties are especially relevant given the complex vascular structure of placenta and its dynamic remodeling. Previous studies have shown that both human and murine placentas exhibit viscoelastic responses to mechanical loading, but a comprehensive evaluation of gestational changes and their potential clinical implications have yet to be fully explored.

In this study, we characterized the viscoelastic properties of murine placental tissue at mid- and late gestation using micro-indentation. We also examined how deviations from normative mechanical profiles might be linked to complications such as small-for-gestational-age (SGA) fetuses or anatomical malformations. By establishing a baseline for mechanical function and identifying abnormal responses, in this work we aimed to improve understanding of placental development and inform future approaches to assessing placental health.

Materials and

Methods: : Placental samples were harvested from C57BL6 mice at gestational day 15 (mid-gestation, N=24) and day 19 (late-gestation, N=27). Samples were stored in phosphate buffer saline and tested within 48 hours. Each sample was affixed to a platform and tested using a spherical indenter (0.5 mm diameter) at a controlled ramp speed (0.125 mm/s). The deformation was held at 0.25 mm for 120 seconds, and force-time data were recorded.

Indentation was performed at nine distinct sites per sample to assess regional differences in mechanical response. The force-time curves were used as inputs to an inverse finite element model, developed in ABAQUS, to extract viscoelastic parameters. For this model, we assumed isotropic behavior and used a two-term Prony series within a Maxwell viscoelastic framework. Initial material estimates were generated using a neural network trained on synthetic data. These estimates were refined through a least-squares optimization approach to achieve best fit between simulated and experimental curves.

Parameters of interest included the instantaneous modulus, steady-state modulus, and effective viscosity. Statistical analysis were used to compare mid- and late-gestation groups, and to

examine deviations in samples from pregnancies with known complications, such as SGA or fetal malformation. A p-value of less than 0.05 was considered significant.

Results, Conclusions, and Discussions:: Normative tissue analysis revealed a small but statistically significant decrease in the instantaneous elastic modulus from mid- to late-gestation (6.23 ± 0.10 kPa vs. 5.98 ± 0.096 kPa, p < 0.005). Steady-state modulus showed a slight increase in the late-gestation cohort, but the difference was not statistically significant. Effective viscosity declined between gestational stages but also did not reach statistical significance.

Several cases of complicated pregnancies were also examined. Four SGA placentas exhibited a 24% decrease in instantaneous modulus compared to normative age-matched controls (p < 0.05). A separate case involving a malformed fetus showed a 110% increase in instantaneous modulus and a 172% increase in steady-state modulus, indicating dramatically altered mechanical behavior.

These results suggest that deviations in viscoelastic properties can accompany or reflect adverse pregnancy outcomes. Establishing normative mechanical profiles and identifying outliers may offer new avenues for assessing placental health. While the current approach is limited to ex vivo analysis, the modeling framework and mechanical insights could support future development of non-invasive diagnostic tools, such as ultrasound elastography.

This study contributes to a growing body of work on placental biomechanics and highlights the value of integrating mechanical testing with computational modeling to understand pregnancy-related disorders. Further studies are needed to evaluate earlier gestational stages and extend this approach to human tissue.

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