A self-expanding stent is laser-cut from a Nitinol (Ni-Ti) tube in a large, austenite "remembered" diameter, then cold-worked into a thin, martensitic crimped profile inside the delivery sheath. Retracting the sheath removes the confining stress; each freed segment reverts toward austenite and pushes outward until it meets the vessel wall or its own free diameter.
The struts flex, not stretch — the diamond-cell lattice mechanically amplifies a modest material strain at each hinge into a large change in overall device diameter. That hinge strain ε follows an idealized flag-shaped superelastic law:
ε ≤ ε1 σ = E·ε (elastic austenite)
ε1 < ε ≤ ε2 σ = σ_plateau(dir, T) (stress-induced martensite)
ε > ε2 σ = σ_plateau + Ek·(ε−ε2) (full detwinned lock-up)
σ_plateau(loading) = σ_UP0 + C·max(0, T − Af)
σ_plateau(unloading) = [σ_LP0 + C·max(0, T − Af)] · recovery(T)
The loading plateau σ_UP (crimping, resisted by the sheath) sits well above the unloading plateau σ_LP (self-expansion) — that gap is the superelastic hysteresis loop. Both plateaus rise with temperature at roughly C ≈ 6.5 MPa/°C above the austenite-finish temperature Af, a Clausius–Clapeyron-like relation. If body temperature dips within ~8 °C of Af, recovery(T) collapses toward zero and the stent cannot fully self-expand — the reason device Af is deliberately engineered several degrees below 37 °C.
The chronic outward force pressing on the vessel wall is estimated as F ≈ σ(ε,T) · A_strut · N_struts · k, with A_strut the strut cross-section, N_struts the ring's circumferential strut count, and k a geometric transmission factor. Values here are an illustrative order-of-magnitude model, not a specific certified device.
- Sheath retraction — releases the stent from the tip backward; released segments spring toward the vessel wall.
- Vessel diameter — if it exceeds the stent's free diameter, the stent never touches the wall (strain → 0, force → 0): a real malapposition risk from undersizing.
- Body temperature — sliding below the Af margin shows the self-expansion stall directly.
- Recrimp — reloads the stent (upper plateau), illustrating that unloading and loading trace different paths — the essence of superelastic hysteresis.