HP made inkjet cheap by boiling the ink on purpose

Drop-on-demand inkjet needed a tiny pump at every nozzle. Piezo actuators worked, but they kept the printhead mechanically delicate and expensive.

HP's clever move was to make the disturbance useful. A resistor, made with integrated-circuit-like processing, heated ink for only microseconds until a vapor bubble expanded and pushed one drop through the orifice.

The same mechanism could be repeated across nozzles. IEEE Spectrum notes that the April 1984 ThinkJet used a 12-nozzle cartridge, and IBM's SID paper describes thin-film heaters thermally generating gas bubbles for HP and Canon drop-on-demand heads.

The invention turned desktop printing from hammering characters into firing controlled liquid pixels.

Use a bubble as the moving part

The actuator was made by thin-film processing, so many nozzles could be built in parallel.

Each nozzle fired only when needed, avoiding the waste of continuous inkjet streams.

The bubble appeared and collapsed fast enough for repeated droplet generation.

The disposable cartridge could contain the fragile fluidic system instead of plumbing ink through the printer.

Sometimes the elegant actuator is not a part at all. If physics can move the load repeatably, manufacturing can shift from precision mechanics to arrays on a chip.

Thermal inkjet became a dominant desktop printing mechanism and later escaped paper entirely. IEEE Spectrum describes its use in DNA arrays, printed electronics, 3D printing and other droplet-based manufacturing.