A fluid channel cannot be made as narrow as you like
Why the bonding process rather than the cutter sets the minimum width.
A fluid channel's practical minimum width is set by how much solid, undisturbed plastic the bonding process needs beside it to seal properly without the channel collapsing or deforming, and that requirement is almost always stricter than anything the cutting tool imposes.
A wax seal with two close lines
Two engraved lines cut close together on a wax-seal stamp can look perfectly crisp on the metal die, fine grooves separated by an equally fine ridge. Press that stamp into real wax with the force needed for a legible impression, and the thin ridge of wax between the lines can bend, squash sideways or vanish, leaving one blurred groove where two were engraved. The die was capable of the detail; the wax under pressure was the limit.
A bonded manifold's channel wall is in the same position. Cutting a groove into flat plastic is forgiving, and a sharp cutter or fine laser can produce a channel narrower than a human hair. The trouble starts when the cut sheet is bonded to its matching half, because bonding, by solvent or by heat, makes the plastic around the channel soft and mobile exactly where clamping pressure is highest. A channel wall, or the strip of solid plastic between two adjacent channels, called the land, has to survive that pressure while softened without being crushed or closed over, and a wall the cutter makes easily can be far too thin for the clamping step that follows.
Solvent and heat reach the same limit
Solvent bonding softens the plastic chemically at the surface, and a thin wall offers too little undisturbed material beneath that layer to resist clamp pressure, especially if solvent has crept into the wall from both adjacent channels at once, the risk the previous article described. Diffusion bonding softens it thermally, and a thin rib heated near its softening point loses much of its stiffness just as pressure bears down hardest. The routes differ and the outcome is the same: the land beside a channel needs enough undisturbed thickness to carry real pressure.
The limit belongs to channels formed by bonding two separately cut halves. A channel drilled or machined into a solid block from one side has no second piece clamped against a softened wall. A review of thermoplastic microfluidic fabrication and bonding methods shows how many routes exist to a finished fluidic part, each with its own relationship between what forming can achieve and what joining can tolerate, and a designer chasing minimal channel spacing sometimes finds it cheaper to change manufacturing route than to push a bonded two-piece design past its land-width limit.
Designing the layout around the bond
Minimum spacing between adjacent channels therefore has to be set by whichever bonding method the manifold will use. The expensive way to learn this is to draw a tight, ambitious layout and discover in the first bonded prototype that half the walls between channels have sagged or partly closed. A land width that looks conservative on paper, wider than the drawing seems to need, is often the detail separating a manifold that bonds cleanly first time from one that needs a redesign after the first batch comes out of the press with channels that no longer match the drawing.