The cell model

In HemoCell every cell is a closed triangulated surface — a mesh of vertices (material points) connected by edges into triangles. The mesh represents the cell membrane (and, for red blood cells, the combined membrane + cytoskeleton). The mechanical behaviour of the cell emerges from forces defined on this mesh, which are recomputed from its instantaneous shape at every material update.

Building the mesh

When you add a cell type with HemoCell::addCellType(), the second argument selects how the initial mesh is constructed:

Construction method	Description
RBC_FROM_SPHERE	Generate a biconcave red-blood-cell shape from the analytical equation in the HemoCell paper, refining a sphere to at least minNumTriangles triangles.
ELLIPSOID_FROM_SPHERE	Generate an ellipsoid, mainly used for the platelet (PLT) model.
WBC_SPHERE	Generate a sphere representing a (white-blood-cell-like) nucleated cell.
MESH_FROM_STL	Load the mesh vertices from an STL file referenced in the cell’s CELL.xml.
STRING_FROM_VERTEXES	Legacy, no longer used.

The construction method, the target resolution (minNumTriangles), and the reference geometry (radius, Volume) are read from the cell’s CELL.xml (see CELL.xml and CELL.pos).

The membrane forces

The constitutive (mechanical) model evaluates a set of force contributions over the mesh. Most HemoCell cell models share the same four core terms, scaled by coefficients you set per cell type in CELL.xml:

Coefficient	Force term	Resists / represents
kLink	Link force	Stretching of the membrane along mesh edges (the in-plane elastic response, derived from a worm-like-chain / persistence-length model).
kArea	Local area force	Change in the area of individual triangles (local area conservation).
kBend	Bending force	Bending between adjacent triangles, i.e. the membrane’s resistance to being curved (set in units of kBT).
kVolume	Volume force	Change of the enclosed cell volume (quasi-incompressibility).

The deformation modes are bounded by global constants (MaxCellVolumetricChange, MaxCellSurfaceAreaChange, MaxCellBendingAngle, MaxCellPersistenceLength) defined in constant_defaults.h (the cellgroup constants). These were tuned for validated RBC behaviour and improved stability under high shear; see [1] and [6].

The actual force law lives in each model’s ParticleMechanics method (in the mechanics/ directory). Different models add, remove, or modify terms — for example the white-blood-cell model adds an inner rigid-core term, while the platelet model uses a much higher maximum bending angle. See Mechanical (cell) models for the catalogue, and the FAQ for how to implement your own.

Optional membrane physics

Two optional effects can be enabled per cell type, at the cost of extra computation (both are compile-time features, see config/constant_defaults.h):

• Interior viscosity — gives the fluid inside the cell a different viscosity from the surrounding plasma, set via enableInteriorViscosity and viscosityRatio in CELL.xml. Requires the library to be built with INTERIOR_VISCOSITY. See Colliding cells with interior viscosity.

• Solidification mechanics — used to model thrombus (clot) formation by freezing cells in place under certain conditions, enabled with HemoCell::enableSolidifyMechanics() and the SOLIDIFY_MECHANICS build option.

Note

eta_m (membrane viscosity) appears in the cell models and CELL.xml but is currently not used by the standard models. It is retained for models that implement a viscous membrane response.

Cell–cell and cell–wall interaction

Because of the immersed-boundary coupling, two membranes already interact when they are about half an IBM kernel-width apart, before the meshes visually touch. On top of this, an explicit short-range repulsion force can be enabled between cells, and between cells and solid boundaries, to prevent overlap at high hematocrit. See Repulsive forces, HemoCell::setRepulsion(), and HemoCell::enableBoundaryParticles().