This paper presents a novel pneumatic soft actuator for continuum robotic arms. In this actuator, by utilizing the instability of silicone rubber, the need for multiple air tubes to have multiple independent inflating chambers has been removed. To control which part to be inflated and to add more degrees of freedom to the actuator, shape memory alloy (SMA) springs have been used in the actuator structure. The resultant actuator can produce a high displacement and can maintain its final state without consuming energy. The goal for designing such an actuator is to decrease the size of current soft pneumatic continuum robotic arms by reducing the number of air supply tubes while preserving the number of degrees of freedom of the continuum robot arm. The current paper discussed the main principles behind the actuator design, the behavior of the first prototype of the actuator and the main aspects of controlling the actuator. In the end, the possible improvements and main issues regarding the enhancement of the actuator function have been discussed.