aluminum VLA horn. cloth. soluble insulation
Suppose you want to make a metal sheet that has channels running through it, say for coolant. One possible way to do this is to make the shape of the channels out of wire of steel or aluminum, electroform copper onto them until you have a solid sheet (after a flash of some intermediate metal in the case of steel), and then dissolve the channels out of the copper, using alum solution for steel, or either hydrochloric acid or lye for aluminum.
This is potentially useful for many things, such as heat exchangers, cooling jackets, heated floors, and “labs on a chip” in which a sequence of processes is carried out in tubes within a monolithic device. Another similar application is making coils for induction heating: at high frequencies only the very surface of the coil’s metal can be used, and you normally want to run water through it to keep it coool.
In many cases like this, it’s useful to have a minimum separation between the channels to prevent fluid flow between them. For example, in a long serpentine coolant channel, a “short circuit” for the fluid could eliminate one or more loops, producing a hot spot due to inadequate coolant flow; and, in an induction coil, unwanted contact between successive coils can produce an electrical short circuit, which can produce a hot spot due to a narrow resistive connection.
To provide such a minimum separation, it’s potentially useful to form the original wire network using wires bearing a layer of flexible “spacer”, similar to the insulation used on electrical wires. Once the wires are all in place, you can remove the spacer, for example by burning it off with fire (if it is an organic chemical or, say, amorphous sulfur); dissolving it off with water (if it is a water-soluble material such as gelatin, albumin, pectin, starch, carboxymethylcellulose, poly(ethylene glycol), poly(acrylic acid), xanthan or guar gum, or polyvinyl alcohol, or held together with a water-soluble binder; mixing some baking powder in may speed this process); melting it off, if it has a low enough melting temperature; degelling it by changing the pH, if it’s a pH-dependent gel like poly(acrylic acid) or some carrageenans; etc. Once the spacer is gone, hopefully without the wires sagging much, the electroforming process can begin.
Ideally the wires would themselves be hollow, having a thin open tube through their centers, thus containing within themselves the seed of their own doom — the alum or lye or whatever can attack them enormously more rapidly if it can be run through this pipe, rather than having to diffuse in from the ends.
Of course, copper is not the only metal that can be electroformed, and alternatives to electroforming exist: you may be able to pot the channel wires in resin, for example. But these processes are perhaps less interesting.