Energetics and mechanics of humans running on surfaces of different compliance

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1999.Includes bibliographical references.by Amy Elizabeth Kerdok.S.M

Characterizing the Nonlinear Mechanical Response of Liver to Surgical Manipulation

Computer-aided medical technologies such as simulators for surgical training and planning require accurate representation of soft tissue behavior under large deformations. Limited datasets, and unrealistic models for soft tissues currently hinder the advancement of surgical simulation. This work identifies the nonlinear mechanical response of liver through the development of a physically-based constitutive model. The effects of perfusion on the viscoelastic response of liver are identified, and a perfusion apparatus is created that approximates the in vivo condition. Indentation tests measuring the response of whole, perfused, porcine livers under finite deformations (~30 % nominal strain) are conducted. Results indicate a time dependent, nonlinear, viscoelastic, force-displacement behavior. A constitutive model describes the time varying response through the combined contributions of three subsystems: collagenous capsule, parenchyma, and fluid filled vessels. Solving the inverse problem through iterative finite element modeling identifies the seven independent material parameters. The model is capable of capturing the salient features of the data. Modifications to th

A nonlinear finite element model of soft tissue indentation

Abstract. Mathematically describing the mechanical behavior of soft tissues under large deformations is of paramount interest to the medical simulation community. Most of the data available in the literature apply small strains (<10%) to the tissue of interest to assume a linearly elastic behavior. This paper applies a nonlinear hyperelastic 8-chain network constitutive law to model soft tissues undergoing large indentations. The model requires 2 material parameters (initial modulus, locking stretch) to reflect the underlying physics of deformation over a wide range of stretches. A finite element model of soft tissue indentation was developed and validated employing this constitutive law. Ranges of the initial shear modulus and locking stretches were explored based on values found for breast tissue [17, 25]. Results of the model are shown with a lookup table containing third order polynomial coefficient fits. This work serves as an initial method to determine the unique material parameters of breast tissue from indentation experiments. 1

Identifying a Minimal Rheological Configuration: A Tool for Effective and Efficient Constitutive Modeling of Soft Tissues

We describe a modeling methodology intended as a preliminary step in the identification of appropriate constitutive frameworks for the time-dependent response of biological tissues. The modeling approach comprises a customizable rheological network of viscous and elastic elements governed by user-defined 1D constitutive relationships. The model parameters are identified by iterative nonlinear optimization, minimizing the error between experimental and model-predicted structural (load-displacement) tissue response under a specific mode of deformation. We demonstrate the use of this methodology by determining the minimal rheological arrangement, constitutive relationships, and model parameters for the structural response of various soft tissues, including ex vivo perfused porcine liver in indentation, ex vivo porcine brain cortical tissue in indentation, and ex vivo human cervical tissue in unconfined compression. Our results indicate that the identified rheological configurations provide good agreement with experimental data, including multiple constant strain rate load/unload tests and stress relaxation tests. Our experience suggests that the described modeling framework is an efficient tool for exploring a wide array of constitutive relationships and rheological arrangements, which can subsequently serve as a basis for 3D constitutive model development and finite-element implementations. The proposed approach can also be employed as a self-contained tool to obtain simplified 1D phenomenological models of the structural response of biological tissue to single-axis manipulations for applications in haptic technologies.Engineering and Applied Science

The effects of testing environment on the viscoelastic properties of soft tissues

Abstract. Mechanical properties of biological tissues are needed for accurate surgical simulation and diagnostic purposes. These properties change postmortem due to alterations in both the environmental and physical conditions of the tissue. Despite these known changes, the majority of existing data have been acquired ex vivo due to ease of testing. This study seeks to quantify the effects of testing conditions on the measurements obtained when testing the same tissue in the same locations with two different instruments over time. We will discuss measurements made with indentation probes on whole porcine livers in vivo, ex vivo with a perfusion system that maintains temperature, hydration, and physiologic pressure, ex vivo unperfused, and untreated excised lobes. The data show>50 % differences in steady state stiffness between tissues in vivo and unperfused, but only 17 % differences between in vivo and perfused tests. Variations also exist in the time-domain and frequency domain responses between all test conditions. 1. Introduction/Motivation