The MMDM membrane can be deflected only in the direction of the electrode structure because the electrostatic force can be only attractive. This means the deflected membrane can produce only concave optical shapes with a reference to a flat surface. To achieve bi-directional operation the membrane should be initially deflected towards the actuators and made perfectly spherical by adjusting the actuator voltages. From this state the membrane can be moved to both positive (from the electrode structure) and negative (to the actuator structure) directions by controlling the actuators voltage.
