Abstract: The invention relates to an apparatus for providing and maintaining access to a surgical site, particularly a surgical site that needs to be accessed through delicate tissue such as brain tissue. The invention relates particularly to an apparatus that uses an expandable member or trochar. In one arrangement the expandable member is positioned proximally relative to the distal end of an obturator when the apparatus is in an insertion configuration for insertion to the surgical site. The expandable member comprises a frame and a membrane attached to the frame. The frame and membrane are configured to allow bending of at least a portion of the surface of the expandable member about an axis perpendicular to the longitudinal axis without breakage of the expandable member.