Since then, the UK and most other countries have banned compressed gas cylinders and other liquid or gel products on aircraft.

BBC: Short debate: The right to breathe

On 10 August 2006, authorities in the United Kingdom uncovered an alleged plot to sabotage as many as 10 US airliners travelling from the UK to the USA, reportedly by using liquid and gel based explosives.

BBC: Short debate: The right to breathe

Silica gel is put through a carefully controlled drying process that turns the liquid in the gel into gas.

FORBES: Technology

Instead of slabs of gel, it uses capillaries filled with a liquid polymer, so that continuous flow is possible without the need to change slabs.

ECONOMIST: Genetic warfare