CADFamily.com: FIDAP-Drug Delivery

Here is an example to demonstrate how to setup up a problem in FIDAP and to solve it. The example deals with the analysis of a birth control patch. The contraceptive patch is a very effective method for birth control. The patch can be placed on multiple parts of the skin and the drugs are then transferred through the tissue by diffusion to the blood stream. The blood stream in turn transports these drugs to various parts of the body. Norelgestromin and ethinyl estradiol are the two drugs used in the patch. In this example, we model the movement of Norelgestromin through the tissue. The patch is considered to be circular with a radius of 2.4 cm. We model 0.6 cm of the skin around it to see how the drug spreads out from the edge of the patch. The thickness of the tissue is assumed to be 1.2 cm and the drugs diffuse into the blood stream after that. The schematic is shown in Fig. 1. It is assumed that the entire drug is taken away by the blood stream. The diffusivity of Norelgestromin is 1.11 x 10^-11 m²/s and the patch provides a constant flux of 8.849 x 10^-7 g/m²s.

(a) (b)

Figure 1. (a) Three dimensional view of the skin-patch system, (b) Axisymmetric geometry

The geometry is modeled as shown in Fig. 1(b) in FIDAP. The transport of the drug is symmetric about the axis and so we can use the axisymmetric geometry to simplify the problem. The problem is non-dimensionalized since we are dealing with a very low value of diffusivity. The non-dimensional values of the parameters are given below:

1. Diffusivity: 1

2. Thickness of the tissue: 1

3. Radius of the patch: 20

4. Length of the skin surrounding the patch: 5

5. Flux = 95.665

6. 1 week is equivalent to 4.662 time units in the non-dimensional form.

We now go ahead and solve the problem in FIDAP.

Step 1: Run the software GAMBIT to create the geometry and to mesh it

In the Command Prompt, type: gambit –id drugd

drugd is the filename

Remember: The filename should be at the most 7 characters long.

Now Gambit is launched. Click on Solver menu at the top of the Gambit window and choose FIDAP.

Step 2: Create Geometry and Mesh in GAMBIT

The geometry in this case is a cylinder. However, since this is an axi-symmetric problem we will solve it with the help of a 2D geometry. We will first create the vertices and then connect these vertices in pairs to form edges and then create the faces by selecting the appropriate edges.

Create the vertices

This step specifies the coordinates of the vertices to be created. The coordinates of the vertices of the rectangle for our problem are (0, 0, 0), (1,0,0), (0,20,0), (0, 25, 0) and (1, 25,0)

1) Under the Operations panel, click on the Geometry command button

2) Under the Geometry panel, click on the Vertex command button

3) Under the Vertex panel, click on Create Vertex. Create Real Vertex Window pops up.

4) In the Create Real Vertex Window under Global, type in the coordinates of the vertex (0, 0, 0) in the text boxes

5) Click on Apply

6) Repeat for the other vertices: (1, 0, 0), (0, 20, 0), (0, 25, 0) and (1, 25, 0)

1) Click on Fit to Window to see the zoomed view

Create the edges

We now create the edges by selecting pairs of vertices.

1) Under the Geometry panel, click on the Edge command button

2) Under the Edge panel, right click and hold on the Create Edge drop-down menu and choose Straight

3) In the Graphics window, Shift-left-click on the vertices A, B, C, D, E (See Figure) (in the given order) to select these vertices. You will see that the vertices turn red on selection.

4) In the Create Straight Edge window, click on Apply

5) In the Graphics window, Shift-left-click on the vertices A and E

6) In the Create Straight Edge window, click on Apply