The design of fast pneummatic networks was first outlined in the Harvard University paper: Pneumatic Networks for Soft Robotics that Actuate Rapidly. Its fabrication process is described in the Soft Robotics Toolkit website.
As opposed to Slow Pneunets that are composed of uniformly interconnected pressurized cells, the fast pneunet inserts a gap between cells in order to minimize the volume that is pressurized. This results in a more efficient design that requires less pressure in order to actuate.
These molds are designed as Fast Pneumatic Networks: