Cartridges

The architectural keystone. A cartridge is a sealed, hot-swappable, flow-through microbial fuel cell stack with a standardized fluid + electrical interface. Same physical interface across the family, different biology inside.

This is the project’s mechanical thesis: the swimming undulation peristaltically pumps water through the gut. The fish’s locomotion is also the cartridge’s pump.

Anatomy

                inflow port (head end)
                      │
                      ▼
              ┌───────────────┐
              │ pre-filter    │  mesh, traps macro-debris
              │ (mesh)        │
              ├───────────────┤
              │  MFC STACK:   │  50–100 micro-cells in series-parallel
              │  N-doped      │  each ~0.5–1 mL chamber
              │  graphene     │  anode: N-GA on flexible carbon substrate
              │  aerogel      │  cathode: air-cathode (gas exchange via
              │  anode array  │           hydrophobic membrane to
              │               │           swim-bladder air reservoir)
              │               │  membrane: sulfonated PEEK or bacterial
              │               │            cellulose composite
              ├───────────────┤
              │ post-filter   │  captures microplastics / precipitates
              │ (collection)  │  offloaded at dock
              └───────┬───────┘
                      │
                      ▼
              outflow port (tail end / gill flaps)

Cells produce 3.3 V regulated output via a low-Vin boost converter, cold-starting from ~200 mV.

The family

Cartridge	Target	Biology	Notes
gut-hc-v1	Hydrocarbons (oil, BTX, PAH)	Pseudomonas / Paenalcaligenes / Providencia	Highest energy yield
gut-org-v1	Dissolved organic load	Geobacter / Shewanella	Generalist
gut-n-v1	Nitrogen / ammonia	Anammox + nitrifiers	Slow but specific
gut-metal-v1	Heavy metal precipitation	Desulfovibrio	Energy-negative; subsidized
gut-plastic-v1	Microplastics	None — passive adsorptive matrix	No MFC, just filtration
gut-pfas-v1	PFAS (research-grade)	Acidimicrobium + activated carbon	Active research target

Lifecycle

1. Manufactured at a partner lab or future shoal facility, sterile-sealed
2. Shipped to a deployment region’s dock-side biology reservoir
3. Inoculated at the dock from local culture stocks
4. Installed in a fish during the next dock visit
5. Deployed for an estimated 30–90 days, depending on pollutant load
6. Returned to the dock when degraded
7. Reseeded in the dock biology reservoir, or flagged for full refurbishment

Cartridges never enter the open environment. Bacteria stay inside.

Deeper

→ Architecture §2.2: The gut cartridge → Architecture §1.2: Microbial fuel cell prior art