A Viscosity Model for Viscoelastic Deformable Objects
Virtual Reality Lab
The CO-ME, Computer Aided and Image Guided Medical Interventions,
is an NCCR long-term project that involves many Swiss Institutes and
Universities. The aim of the project is to use the potential of information
technology for the optimization of medical interventions in order to
improve both the treatment of individual patients and overall health care
for the whole society.
Our lab is contributing to the project by creating a biomechanical
model of joints. In this model, joint postures are calculated based on
contact between different tissues, rather than traditional approaches
based on transformations. In this case, it’s important to have a correct
behavior of each comprised tissue in order to obtain realistic postures.
We are working on a physically based deformation model of soft
tissues to represent biological tissues like cartilage and ligament. Such
tissues have a complex behavior involving viscoelasticity and anisotropy
among other features.
A viscoelastic material is one which the elasticity coeficient is time
dependent, i.e., the strain on the tissue varies according to the duration
of the applied elongation. For example, if the tissue is submitted to a
certain elongation and imediately released, the deformation back to the
original elongation will be faster than if the elongation is kept for some
time before releasing.
In its current state, our model ignores viscous behavior, focusing only
on the linear-elastic part of the viscoelastic deformation. However, our project
issues require that viscous behavior is considered.
Thus, the problem is to include a vicscous function as a new component
of the existing elastic model to make it vicsoelastic.
Our approach to solve the problem will be based on the physiology
of joint tissues deformation. In tissues like cartilage, viscosity is caracterized
by the flow of liquid from the tissue to the joint capsule space and vice-versa
according to the applied stress; just like when you squish a sponge in the
water. We will control this fluid equilibrium in our model in order to obtain
a similar effect.
The student is asked to use and update the existent C++ classes and methods,
and implement a new set of methods to compute viscoelastic behavior of cartilage
and other bio-tissues. To evaluate results, we will compare data
measured on the model considering viscosity with data obtained regardless
viscosity and possibly with data measured on real tissue available on literature.
The first approach to compare may be the Stress x Time graph, like shown
in the image bellow (bottom-right):