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Thread: In Biphasic tissue swelling

  1. #1
    Join Date
    Mar 2015
    Location
    Brest, France
    Posts
    48

    Default In Biphasic tissue swelling

    Dear Febio developpers,
    another question considering my model using the biphasic module is about the simulation of swelling effects. It is possible to swell the tissue by adding a fluid supply source?

    In my case extra interstitial fluid is accumulating in the prostate gland.
    If I use the starling equation for fluid supply i will have this swelling effect?
    Is there any other fluid source model available?
    And finally could I combine it with a load curve to study different time evolution patterns?

    Thank you in advance,
    Konstantinos.

  2. #2
    Join Date
    Dec 2007
    Posts
    639

    Default

    Hi Konstantinos,

    It is possible to swell the tissue by adding a fluid supply source?
    Yes.
    If I use the starling equation for fluid supply i will have this swelling effect?
    Yes.
    Is there any other fluid source model available?
    You can prescribe a desired fluid pressure on a boundary. Or you can prescribe a desired fluid flux on a boundary. Both of these options only work as boundary conditions, whereas the fluid supply term applies to every point within the material.
    And finally could I combine it with a load curve to study different time evolution patterns?
    Yes, every material parameter in FEBio may have its own loadcurve.

    Best,

    Gerard

  3. #3
    Join Date
    Mar 2015
    Location
    Brest, France
    Posts
    48

    Default

    Dear Gerard,
    thank you for your replies in my questions. I have already applied the starling equation and i visualise swelling effects however I am not sure for the results.
    What external pressure i need to give to have a desired volume increase?

    Lets say i need to have a 50% volume increase. I have prescribed a P = 5 Pa fluid pressure in my model. I think that this P participates in the starling equation as biphasic material fluid pressure. (Correct me if i am wrong)

    I calculate the external source pressure pv as Pv = P + Vol.Inc / (kp*t) and I expect with this Pv to observe the desired volume increase in the analysis.

    However when I check the relative volume chage in the postprocessing the average value is not the expected and it decreases if I increase the time analysis where I guess it should be steady since the total fluid volume increase should be constant. I guess I miss many points in the implementation of the fluid supply. Could you help me please with this issue?

    Best regards,
    Konstantinos.

  4. #4
    Join Date
    Dec 2007
    Posts
    639

    Default

    Hi Konstantinos,

    The amount of swelling induced by the Starling equation in a biphasic material is dependent on the prescribed pressure, the permeability of the biphasic material, the elasticity of the solid matrix, the dimensions and geometry of the tissue, and the boundary conditions. The only way to obtain a formula for the amount of swelling for a given set of these parameters is to (a) solve the problem analytically under certain simplifying assumptions, or (b) perform the numerical analysis in FEBio. Normally, you would measure the permeability and elasticity of the solid matrix experimentally and plug those values into the model. The geometry and boundary conditions can be set by your choice of model. This would leave only the prescribed pressure as the unknown parameter to achieve a desired level of swelling. So, it would be relatively straightforward, at that point, to perform a few iterations until you converge to the desired value.

    You also need to understand that the swelling response will be initially transient, until reaching steady state. The time to reach steady state also depends on the same parameters listed above, so you have to develop a basic understanding of the underlying physics for this problem to help you estimate the time needed to achieve steady state. (You can also run the biphasic analysis under the steady-state setting, if you don't care about the transient response.)

    Best,

    Gerard

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