multi-cell

Multi-cell simulations with 2D physics, built on process-bigraph and pymunk.

Demos

Goal

Provide a composable framework for simulating populations of growing, dividing cells with realistic 2D physics and shared chemical environments. Cells are modeled as capsule-shaped rigid bodies (pymunk segments) — or as multi-segment compound bodies that can flex and bend — that grow, divide, secrete particles, attach to surfaces, and interact physically through collisions and confinement. Concentration fields are modeled with a finite-difference diffusion process and coupled to cells through a per-tick exchange step. The framework uses the bigraph process architecture, making it easy to compose new cell behaviors, environmental structures, and analysis pipelines.

Default parameters are calibrated to E. coli proportions (~1 μm wide, ~2 μm at birth, ~4 μm at division, ~30–40 min doubling time).

Quick Start

git clone https://github.com/vivarium-collective/Viva-munk.git
cd Viva-munk
pip install -e .

# Run all experiments and open the HTML report
python -m multi_cell.experiments.test_suite

# Run without auto-opening the browser
python -m multi_cell.experiments.test_suite --no-open

Architecture

multi_cell/
  pymunk_agent_type.py       # Custom PymunkAgent type with optimized dispatch
  types/
    positive.py              # PositiveFloat / PositiveArray / Concentration / SetFloat
  processes/
    multibody.py             # PymunkProcess — 2D physics (rigid + bending cells)
    grow_divide.py           # GrowDivide — exponential growth, division, mutation
    pressure.py              # Pressure — vectorized per-cell mechanical pressure
    diffusion_advection.py   # DiffusionAdvection — 2D field diffusion + advection
    cell_field_exchange.py   # CellFieldExchange — cell ↔ field uptake/secretion
    secrete_eps.py           # SecreteEPS — EPS particle secretion
    remove_crossing.py       # RemoveCrossing — remove cells past a boundary
  plots/
    multibody_plots.py       # GIF rendering with phylogeny / pressure coloring
                             # and concentration-field heatmap overlays
  experiments/
    test_suite.py            # Experiment registry, runner, HTML report generator

Processes

• PymunkProcess (Process): Manages the pymunk 2D physics space. Handles three kinds of agents — rigid segment cells, multi-segment bending cells (compound bodies linked by pivot joints + damped rotary springs), and circular particles. Syncs state to bodies, steps the physics with sub-stepping, applies damping/jitter, optionally enforces adhesion to a wall, and emits position/velocity/polyline deltas.
• GrowDivide (Process): Per-cell exponential mass growth. Optionally gated by a Monod factor on a local nutrient (local[nutrient_key]) and inhibited by mechanical pressure (exp(-pressure / pressure_k)). When mass exceeds a threshold, the cell divides into two daughters along its axis; daughters can inherit mutated parameters and (for bending cells) a fresh straight polyline so they don't inherit the mother's pose.
• Pressure (Step): Computes a per-cell mechanical pressure proxy from cell-cell and cell-wall overlap depths (vectorized via numpy). Written back to each cell's pressure field for downstream consumers.
• DiffusionAdvection (Process): Explicit FTCS diffusion + upwind advection on a 2D map[mol_id → array] field, with ghost-layer boundary conditions (periodic / neumann / dirichlet / dirichlet_ghost).
• CellFieldExchange (Process): Couples pymunk_agent cells to a 2D field map. Each tick it samples each cell's local field concentration into cell.local, and applies each cell's cell.exchange amounts back into the corresponding field bin (Δconcentration = Δamount / bin_volume).
• SecreteEPS (Process): Per-cell EPS particle secretion at a rate proportional to cell mass. Particles are placed on the cell surface and added to the environment. Optionally gated to attached cells only.
• RemoveCrossing (Step): Removes any cell whose position exceeds a configurable x/y boundary. Used to model the flow channel in mother-machine experiments.

Custom types

pymunk_agent is a Node-subclass type with hand-optimized apply/reconcile/realize/check dispatch (no per-field plum dispatch on the hot path). It carries the geometric and physical state of one cell (mass, radius, length, location, velocity, …) plus optional fields used by downstream processes (polyline, attached, pressure, local, exchange).

The multi_cell.types.positive module provides minimal PositiveFloat, PositiveArray, Concentration, and SetFloat types for the field state, accumulator-with-clamp semantics adapted from spatio-flux.

Experiments

The current registry (multi_cell/experiments/test_suite.py):

• daughter_machine — single cell grows in an open chamber with an absorbing right wall.
• mother_machine — narrow dead-end channels (~1.5 μm wide); cells grow vertically and are removed when they reach the flow channel.
• with_particles — cells grow in a chamber seeded with passive particles; cells push and rearrange them.
• attachment — adhesin-bearing cells attach to the bottom surface via PivotJoints; adhesins split between daughters at division.
• glucose_growth — cells on a 2D glucose field. DiffusionAdvection spreads glucose, CellFieldExchange applies cell consumption every tick, and GrowDivide gates rate by Monod kinetics. Cells stop dividing once their local patch is depleted.
• bending_pressure — multi-segment bending capsules grow into a colony. Pressure computes mechanical pressure from neighbor / wall contacts and GrowDivide applies exp(-pressure / pressure_k) inhibition. Cells visibly bend AND slow.
• chemotaxis — twelve non-growing cells run/tumble up a static exponential ligand gradient in a long chamber. Each cell maintains a memory of its local concentration and modulates tumble rate as λ = λ₀ · exp(-k · dc/dt).
• inclusion_bodies — a colony grows while each cell accumulates an inclusion-body aggregate (size in nm, logistic growth toward an 800 nm plateau). Aggregation inhibits growth; at division the IB is transferred entirely to one daughter so the IB-free sibling out-grows the laden one. Cells are soft bending capsules and a Pressure step adds a second slowdown as the colony packs. Colored by IB size.

test_suite.py runs each one, captures a GIF, a bigraph composition viz, and a serialized state JSON, and generates an HTML report (out/report.html) with timing, cell counts, descriptions, and embedded media.

Dependencies

• process-bigraph — bigraph process framework
• bigraph-schema — typed schemas
• bigraph-viz — bigraph visualization
• pymunk — 2D physics engine
• numpy, scipy, matplotlib, pillow