A Model of Electromechanical Coupling in the Small Intestine - Multiscale Computer Modeling in Biomechanics and Biomedical Engineering - page 193

1

λ = 1.3

λ = 1.2
λ

0.9

= 1.1

0.8

λ = 1.0

0.7

λ = 0.9

0.6

λ = 0.8

0.5

0.4

0.3

0.2

0.1

0

0 0.5 1 1.5

[Ca 2 +] ( μ M)

Fig. 5 Steady-state Ca 2 þ -tension-extension relationship. Simulated isometric tension- ½ Ca 2 þ i
relationship at different extension-ratios (k)

Visualization of the electromechanical simulation using the SS-T-L-Ca 2 þ rela-
tionship is shown in Fig. 5 .

4.4 Boundary Conditions and Numerical Solutions

Boundary conditions were chosen to restrict torsion and whole body movements of
the intestine on one face of the model, but should allow contractions in the lon-
gitudinal and radial directions. In the anatomical model (Fig. 1 c), the geometric
elemental nodes on the left hand side face were fixed in the longitudinal direction.
In addition, two nodes on the inner circular layer were fixed in the vertical cir-
cumferential direction, and the two nodes orthogonal to the previous two nodes
were fixed in the horizontal direction. This is representative of physical constraints
applied in several experimental studies on whole segments of small intestine
[ 28 , 50 ], in which only one end was constricted in the axial direction, so that
longitudinal contractions could be measured. Boundary conditions also influence
the degrees of freedom of a deformation problem. The degrees of freedom for the
mechanical solution is an important indication of the computational time required
for solving the model. The degrees of freedom may be measured by the number of
dependent variables (deformed geometric coordinates) required at each geometric
node, omitting the variables that are fixed due to boundary constraints. For
example, the present mechanical model described in this chapter contained 1,240