The newest coffee related Kickstarter project is a portable espresso brewer called the Xbar. It employs either CO2 cartridges or a bike tire pump to create water pressure.
A pair of valves on the side of the Xbar allow for the two different pressures sources. One is a Shrader valve compatible with many kinds of bike pumps. The other valve, of course, takes one use 8-gram CO2 cartridges. Much like those in small seltzer makers.
If using a bike pump, the pump controls the upward ramp of pressure during brewing. If using a cartridge, a paddle on the top of the Xbar lets brewers control the upward ramp of pressure. In both cases a metal knob on the side is used to throttle the pressure down. A standard pressure gauge on the face of the espresso maker lets brewers monitor the pressure level.
The brew head is stainless steel. The pressure control system is aluminum, stainless steal and brass. The aluminum stand comes in two heights depending on what kind of cups brewers prefer.
Xbar says a user should get 4-8 double or triple shots of espresso from one CO2 cartridge—all dependent on water volume, extraction duration etc.