A method of operating a volume-filling structure, comprising sensing an impact event and expanding the volume-filling structure from a non-expanded state to an expanded state upon the sensed impact event. The volume-filling structure comprises an open celled material, a first end cap connected to one end of the open celled material and a second end cap connected to a second end of the open celled material. The volume-filling structure further includes an activation mechanism regulating expansion of the open celled material from the non-expanded state to expanded state in response to an activation signal, and the activation mechanism includes an actuator interfaced with the end caps. Embodiments may include triggering a pneumatic device to force the second end cap from the first end cap. Additionally, embodiments may include triggering a mechanical event to force the second end cap from the first end cap.