Mechanical (cell) models

A mechanical model defines the constitutive (force) behaviour of a cell type. You select one as the template argument to hemocell.addCellType<Model>(name, constructType). All models live in the mechanics/ directory, derive from CellMechanics, and implement a ParticleMechanics method that computes the membrane forces from the cell’s current shape (see The cell model).

Available models

Class (header)	Typical use	Description
RbcHighOrderModel (rbcHighOrderModel.h)	Red blood cells	The validated, optimised RBC model. Uses the link, area, bending, and volume force terms (kLink, kArea, kBend, kVolume). The recommended starting point for RBCs [1, 6].
PltSimpleModel (pltSimpleModel.h)	Platelets (PLT)	A simpler model for the small, stiff platelet, usually constructed with ELLIPSOID_FROM_SPHERE. Shares the same four force terms but is tuned (and allowed a much larger bending angle) for a near-rigid ellipsoid.
WbcHighOrderModel (wbcHighOrderModel.h)	White blood cells / nucleated cells	Extends the RBC-style membrane with an inner rigid core (k_inner_rigid, k_cytoskeleton, core_radius), modelling a generic single-nucleus cell with a stiff nucleus.
RbcMalariaModel (rbcMalariaModel.h)	Infected / stiffened RBCs	An RBC variant with an additional inner-link term (k_inner_link) used to study malaria-infected red blood cells [7].
NoOp (NoOp.h)	Rigid / passive particles	A null model: ParticleMechanics does nothing, so the cell experiences no internal forces. Useful for tracer-like or perfectly rigid particles.

Force coefficients

The coefficients shared by the deformable models are set per cell type in the CELL.xml file (see CELL.xml and CELL.pos):

kLink, kArea, kBend, kVolume, and (where used) k_inner_link / k_inner_rigid. The hard limits on each deformation mode are compiled in as the cellgroup constants in constant_defaults.h:

Constant	Meaning
MaxCellVolumetricChange	Maximum volumetric change allowed (quasi-incompressibility).
MaxCellSurfaceAreaChange	Maximum surface-area change allowed.
MaxCellBendingAngle	Maximum bending angle for red/white blood cells.
MaxPLTBendingAngle	Maximum bending angle for platelets (much larger, ~near rigid).
MaxCellPersistenceLength	Maximum persistence length, allowing up to ~300 % stretching.

Note

These constants were tuned for validated, stable behaviour and are stored as squared values for efficiency. Changing them affects stability; see the commentary in constant_defaults.h before adjusting.

Writing your own model

To create a new constitutive model, duplicate an existing model’s .h and .cpp (rbcHighOrderModel is the usual template), rename the class, and re-implement ParticleMechanics with your force law. Any response that can be expressed as a force as a function of deformation will work. Then use it via addCellType<YourModel>(...). See the FAQ entries on adding constitutive models and internal structural mechanics, and Custom cells and reading cells from STL.