INTRODUCTION: The mechanical properties of the cervix are believed to be related to preterm birth. In this study, we mechanically tested human cervical tissue using both indentation and uniaxial tensile tests to formulate an entropy-driven-fibrous reactive viscoelastic model to capture the nonlinear, time-dependent, anisotropic material response of the cervix.  
 

METHODS: 4 nonpregnant (NP) and 2 pregnant (PG) human cervices were collected with an IRB-approved protocol. Ramp-hold mechanical loading profiles were conducted on the samples (in PBS solution) in both spherical indentation and uniaxial tension. For both tests, the force-time-deformation response was recorded and the strain was calculated by a digital image correlation system (Vic2D v6) and a custom U-Net based deep learning code. Sample-specific FEA models were created using a custom Matlab code and were calculated using FEBio (V2.9, url: febio.org) to simulate both experiments. A custom genetic algorithm based inverse finite element analysis routine was conducted to fit the experimental force responses.
 

RESULTS: The elastic parameters E=1.4¬±1.0kPa, Œæ=170¬±98kPa for nonpregnant samples, are found significantly different (p<0.05, one way ANOVA) from pregnant samples (E=0.06¬±0.02kPa, Œæ=7.5¬±2,2kPa), illustrating the cervix remodeled to be softer during pregnancy. Other elastic parameters are ŒΩ=0.41¬±0.2, b=1.3¬±0.8, N=2.3¬±0.3. The viscous parameters are Ev=6.1¬±4.5kPa, Œæv=42¬±35kPa, œÑ0=41¬±32s, œÑ1=20¬±15s for nonpregnant samples and Ev=1.8¬±0.7kPa, Œæv=10.9¬±5.5kPa, œÑ0=58¬±15s, œÑ1=18¬±5s for pregnant samples.
 

CONCLUSIONS: Representative results show that both the equilibrium compressive and tensile mechanical behavior of the cervical tissue could be captured well by the proposed model. The strain under the indenter spot is predicted lower (30~50% off) using this model. But the shape deformation of the uniaxial test was predicted well. The nonlinear viscoelastic model could approximately describe the indentation experiment well. Our future studies will address these time-dependent mechanisms by including poroelastic and other viscoelastic effects.
 

REFERENCES:
1. Shi, L et al, 2019, J. BioMech. Eng. 141.9.

Tensile and Compressive Anisotropic and Time Dependent Material Properties of Human Cervical Tissue

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