Both stents are stretched to the same axial length by the balloon catheter. Each ring's radius follows from the geometry of its own strut unit cell: a vertical member of length h joined to inclined struts of length l at angle θ. Unit-cell height and width are H = h + 2l·sinθ and W = 2l·cosθ, so stretching (increasing H) forces θ to change, and W changes with it.
ν = −(ΔW/W₀) / (ΔH/H₀)
- Ordinary lattice — struts start splayed outward (convex cell). Stretching rotates them toward vertical, so W shrinks as H grows: ν > 0. The ends, which see the least axial constraint from neighbouring rings, shrink the most — the classic "pinch" that leaves gaps against the vessel wall.
- Auxetic lattice — struts start folded inward (concave, re-entrant "bowtie" cell). Stretching unfolds them, and W grows right alongside H: ν < 0. Because the whole cell unfolds uniformly, the radius grows evenly along the entire length, pressing the stent against the vessel wall with no end gaps.
- Drag to orbit, scroll to zoom. Strut color shows local wall contact: green struts touch the vessel wall, red struts leave a gap.
This is the mechanism exploited by re-entrant honeycomb (auxetic) metamaterial stents: pushing the deployment balloon expands the stent lengthwise, and the negative-Poisson's-ratio microstructure converts that same push into uniform radial opening, instead of the end-pinching that limits ordinary expandable stent designs.