Shell
Programmable stiffness, self-healing, colour, and the three-layer division
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The shell is not one material. Quantifying it forces a division into three layers that cannot substitute for one another, and all three compete for the same mass budget.
4.1 Programmable stiffness
| Mechanism | Modulus range | Speed | Maturity |
|---|---|---|---|
| Magnetorheological — fluid or elastomer | 2–30× at ~1000 mT | milliseconds | LAB/TM |
| Electrorheological | ~16× (stiffness 64 → 1065 mN·mm⁻¹, >1500% variation) | tens of ms | LAB |
| Jamming (granular or layer) | large, up to ~two orders | tens to hundreds of ms | LAB |
| Shape-memory polymer (thermal) | large | seconds to minutes | TM |
| Piezoelectric / magnetostrictive | small amplitude | sub-millisecond | TM |
Sourced.
This is the difference in kind from programmable matter: a real material changes stiffness within a finite range. It does not change state of matter.
A constraint that comes attached: magnetorheological operation needs ~1000 mT. Generating that across a large volume requires heavy coils, draws power and makes heat. Therefore variable stiffness is a local capability, not a whole-body one. Where to place it is
⟦CTRL⟧; placing it everywhere puts the mass loop of 02 outside convergence.
4.2 Self-healing
Graphene–PEDOT:PSS polymer: heals in seconds, stretches to ~600%, and senses pressure, temperature and pH. LAB, published 2025. Sourced.
This layer is both skin and sensor — it feeds directly into 05 rather than merely covering the body.
4.3 Colour and texture
| Axis | Reality | Maturity |
|---|---|---|
| Colour — electrochromic | ~1–5 seconds typical; best large-area ~0.8 s to colour, 4.2 s to bleach. Slows as area grows | LAB/TM (sourced) |
| Texture | Limited — surface microstructure only, no rearrangement of matter | LAB in part |
Three response classes, and they must not be merged. Stiffness changes in milliseconds; colour changes in seconds; shape changes through mechanical folding. A specification that folds all three into "morphing" promises a speed only one of them delivers.
Volume-preserving morphing is reduced to predefined folding and jointed mechanisms; solid matter does not rearrange on demand. Mass-changing morphing is not achievable and is not specified.
4.4 Fire resistance
A civil-fire-resistant outer layer, for limited duration. The specific temperature and duration are ⟦IMPL⟧ — they must anchor to an existing firefighting-garment standard rather than being derived here.
The architectural constraint is firm: the fire layer is outermost and sacrificial. It must not be the layer carrying sensors, because sensors do not survive the temperatures this layer exists to take.
4.5 The three-layer division
| Layer | Function | Cannot also |
|---|---|---|
| Sense-and-heal | E-skin plus self-healing polymer; soft, 600% stretch | carry structural load, or survive high heat |
| Variable stiffness | MR/ER/jamming, local | self-heal; requires a magnetic or electric field |
| Load and fire | Group 5 protection (02.4) plus a sacrificial fire layer | sense, or change stiffness |
This connects straight back to the mass loop. All three layers are non-scaling mass — they enter
m_extand are multiplied byγ. They compete directly with armour inside the same 15–25 kg budget declared in 02.5.Areal densities for the sense-and-heal and variable-stiffness layers are
⟦IMPL⟧— no reliable figure was found. But the constraint is already firm: every kilogram of smart skin is a kilogram of armour given up, multiplied byγ. Full-body e-skin and full-body armour and full-body variable stiffness are three things that do not coexist.
Operating point: ⟦CTRL⟧. This chapter supplies amplitudes, speeds, and three layers that cannot cover for each other. Allocating area between them is the controller's decision.