  Although research studies in pneumatic soft robots develop rapidly, most
pneumatic actuators are still controlled by rigid valves and conventional
electronics. The existence of these rigid, electronic components sacrifices the
compliance and adaptability of soft robots.} Current electronics-free valve
designs based on soft materials are facing challenges in behaviour consistency,
design flexibility, and fabrication complexity. Taking advantages of soft
material 3D printing, this paper presents a new design of a bi-stable pneumatic
valve, which utilises two soft, pneumatically-driven, and
symmetrically-oriented conical shells with structural bistability to stabilise
and regulate the airflow. The critical pressure required to operate the valve
can be adjusted by changing the design features of the soft bi-stable
structure. Multi-material printing simplifies the valve fabrication, enhances
the flexibility in design feature optimisations, and improves the system
repeatability. In this work, both a theoretical model and physical experiments
are introduced to examine the relationships between the critical operating
pressure and the key design features. Results with valve characteristic tuning
via material stiffness changing show better effectiveness compared to the
change of geometry design features (demonstrated largest tunable critical
pressure range from 15.3 to 65.2 kPa and fastest response time $\leq$ 1.8 s.
