# Cell Motility. Applying force to cells.¶

In some CC3D simulations we need make cells move in certain direction. Sometimes we do it using chemotaxis energy term (if indeed in real system that chemotaxis is the reason for directed motion) and sometimes we simply apply energy term which simulates a force. In the CC3D manual we show how to apply constant force to all cells or on a type-by-type basis. Here let us concentrate on a situation where we apply force to individual cells and how change its value and the direction. You can check simulation code in Demos/CompuCellPythonTutorial/CellMotility. To be able to use force in our simulation ( we need to include ExternalPotential Plugin in the CC3DML:

<Plugin Name="ExternalPotential"/>


Let us look at the steppable code:

from random import uniform

class CellMotilitySteppable(SteppableBasePy):
def __init__(self, _simulator, _frequency=10):
SteppableBasePy.__init__(self, _simulator, _frequency)

def start(self):
print "This function is called once before simulation"

# iterating over all cells in simulation
for cell in self.cellList:
break
# Make sure ExternalPotential plugin is loaded
# negative lambdaVecX makes force point in the positive direction
cell.lambdaVecX = 10.1 * uniform(-0.5, 0.5)  # force component along X axis
cell.lambdaVecY = 10.1 * uniform(-0.5, 0.5)  # force component along Y axis
#         cell.lambdaVecZ=0.0 # force component along Z axis

def step(self, mcs):

for cell in self.cellList:
cell.lambdaVecX = 10.1 * uniform(-0.5, 0.5)  # force component along X axis
cell.lambdaVecY = 10.1 * uniform(-0.5, 0.5)  # force component along Y axis


Once ExternalPotential plugin has been loaded we assign a constant force in a given direction by initializing lambdaVecX, lambdaVecY, lambdaVecZ cell attributes.

Remark: when pushing cell along X axis toward higher X values (i.e. to the right) use lambdaVecX negative. When pushing to the left use positive values.

In the start function we assign random values of X and Y components of the force. The uniform(-0.5, 05) function from the Python random module picks a random number from a uniform distribution between -0.5 and 0.5.

In the step function we randomize forces applied to the cells in the same way we did it in start function.

As you can see the whole operation of applying force to any given cell in the CC3D is very simple.

The presented example is also very simple. But you can imagine more complex scenarios where the force depends on the velocity, of neighboring cels. This is however beyond the scope of this introductory