Abstract: An introducer for providing access to a surgical site in a patient includes a sheath having an interior diameter that is expandable from a reduced diameter configuration to an increased diameter configuration. The introducer may be inserted into and removed from the patient while the sheath is in the reduced diameter configuration to minimize trauma to the patient's vasculature. Once inserted into the patient's vasculature, the sheath may be expanded to the expanded diameter configuration to allow for secondary devices, such as delivery devices, to be passed through the interior of the sheath. The sheath may include a frame having a mechanism to enable it to expand or collapse. The frame mechanisms may include coiled wires, flexible rings, spine structures with deflectable ribs, non-auxetic expandable and collapsible cellular patterns, and auxetic expandable and collapsible cellular patterns.