It has a delrin hoop frame. From the ULA website...
Q: What is the suspension hoop? How does it work?
A: The suspension hoop used in the Circuit is our minimal solution to transferring load between shoulder and hip. If you imagine an upside 'U' you get a sense of how the hoop is positioned and how it functions. The upper arch of the 'U' ties into the shoulder straps, while each leg of the 'U' ties into the hipbelt. These points of contact allow for a range of suspension flexibility while being rigid enough to support loads up to 35 lbs. In addition, because the suspension is located on the periphery of the pack body, and the backpanel is stiffened slightly with dual density foam, the result is a pack that can handle the load but does not dictate your body's movements --- but rather responds to how you move. Personally, when the loads are kept below 35 lbs, I find this pack to be the most comfortable in the ULA line-up.