Combining Unique Planar Biaxial Testing with Full‐Field Thickness and Displacement Measurement for Spatial Characterization of Soft Tissues

Abstract

Soft tissues rely on the incredible complexity of their microstructure for proper function. Local variations in material properties arise as tissues develop and adapt, often in response to changes in loading. A barrier to investigating the heterogeneous nature of soft tissues is the difficulty of developing experimental protocols and analysis tools that can accurately capture spatial variations in mechanical behavior. In this article, we detail protocols enabling mechanical characterizations of anisotropic, heterogeneous soft tissues or tissue analogs. We present a series of mechanical tests designed to maximize inhomogeneous strain fields and in-plane shear forces. A customized, 3D-printable gripping system reduces tissue handling and enhances shear. High-resolution imaging and laser micrometry capture full-field displacement and thickness, respectively. As the equipment necessary to conduct these protocols is commercially available, the experimental methods presented offer an accessible route toward addressing heterogeneity.

Data-Driven Anisotropic Biomembrane Simulation Based on the Laplace Stretch

“…This is essential for data-driven modelling as well. To design our virtual experiments, we analyzed the protocols [32] proposed for the soft tissue study, in terms of the sufficiency of experimental data for data-driven modelling. The sample geometry and the sizes for the virtual sample correspond to realworld analogs [32]: we stretch the cruciform sample with even arms and collect data only in a small central region, rf.…”

“…Since we use the linear (P 1 ) finite elements, (ξ, ∂ψ/∂ξ) are constant on each triangle. To analyze sufficiency of experimental data, we studied protocols (Table 1 in [32]) represented by combinations of four values w1 : w2 : w3 : w4 = 1 : 1 : 1 : 1; 1 : 1 : 0 : 0; 1 : 0 : 0 : 0; 0 : 1 : 0 : 1; 1 : 0.67 : 0 : 1 with all possible permutations. In this case, we lack experimental points in the region ξ 2 < ξ 1 , rf.…”

“…To this end, we use synthetic data based on virtual experiments for Holzapfel-Gasser-Ogden model [30] with parameters corresponding to porcine skin [31]. For virtual experiments we use experimental protocols proposed for soft tissue [32] which are enriched for the sake of data completeness. The obtained experimental synthetic data is used explicitly for the finite element forward simulation of the inflation of a square membrane defined by the data-driven constitutive equation without any prescription about material anisotropy.…”

“…It is important to highlight that the utilization of the actual protocols established for cruciform specimens [32] serves multiple purposes. First and foremost, the authors [32] have proposed a "series of mechanical tests designed to maximize inhomogeneous strain fields and in-plane shear forces", which means that it is not necessary to introduce holes in order to achieve a heterogeneous deformation field as it was done previously for isotropic case by [13,29]. Furthermore, through the implementation of high resolution imaging and digital image correlation techniques, it becomes possible to capture accurately the complete displacement field.…”

Mechanical Characterization of Soft Membranes with One-Shot Projection Moiré and Metaheuristic Optimization

“…The anisotropic response of the tested material was confirmed by planar equi-biaxial tests [40] carried out on five specimens of different sizes, and the corresponding 2D displacement fields were measured with intrinsic moiré. The inflation test and the planar equi-biaxial test were selected for this study because they are the most commonly used experimental protocols for characterizing soft membranes made of hyperelastic material (e.g., [41][42][43][44][45][46] and the studies cited therein).…”