A collapsible medical device for occluding an aperture in a body, the medical device having a first configuration with a reduced profile and a second configuration with an expanded profile, the medical device being adapted to be delivered through a delivery system into a desired delivery location, the medical device comprising: an occluder having a proximal side having a proximal end and a distal side having a distal end that are configured to cooperate to close the defect, the occluder having a central portion disposed between the proximal side and the distal side, the proximal and distal sides (40, 20) of the occluder include slits (74, 84) axially formed in a tube to form struts; wherein the struts form petals (72, 82) which rotate in an orthogonal plane relative to the axis of the occluder when the axial length of the tube is shortened into a deployed state; and a catch system for holding the medical device in the second configuration, including a catch member with a flap at its proximal end; wherein said catch member comprises a catch body; characterised in that a first portion of the catch body extends distal from the flap, and a second portion of the catch body extends proximal from the flap; wherein the flap has an outer portion having a thickness t1 at outer ends_and a base portion of thickness t2 at the base where the flap joins the main body of the catch member, wherein t1 is less than t2, and has a radius of curvature at a junction between the flap and the portion of the catch member extending proximal from the flap; wherein the flap is configured to bend distally when the flap is pulled through the proximal end of the occluder during delivery of the device; wherein the proximal end of the occluder is configured to rest against a distal surface of the flap when the device is in the second configuration and the flap holds the medical device in the second configuration; wherein the flap is configured to fit inside an interior passage of the occluder during delivery and retrieval; wherein the flap is configured to have a first interference with the interior passage of the occluder during delivery and a second interference with the interior passage of the occluder during retrieval, and the second interference is greater than the first interference; and wherein a first force required to deform the flap for retrieval is greater than a second force required to deform the flap for deployment. an occluder having a proximal side having a proximal end and a distal side having a distal end that are configured to cooperate to close the defect, the occluder having a central portion disposed between the proximal side and the distal side, the proximal and distal sides (40, 20) of the occluder include slits (74, 84) axially formed in a tube to form struts; wherein the struts form petals (72, 82) which rotate in an orthogonal plane relative to the axis of the occluder when the axial length of the tube is shortened into a deployed state; and a catch system for holding the medical device in the second configuration, including a catch member with a flap at its proximal end; wherein said catch member comprises a catch body; characterised in that a first portion of the catch body extends distal from the flap, and a second portion of the catch body extends proximal from the flap; wherein the flap has an outer portion having a thickness t1 at outer ends_and a base portion of thickness t2 at the base where the flap joins the main body of the catch member, wherein t1 is less than t2, and has a radius of curvature at a junction between the flap and the portion of the catch member extending proximal from the flap; wherein the flap is configured to bend distally when the flap is pulled through the proximal end of the occluder during delivery of the device; wherein the proximal end of the occluder is configured to rest against a distal surface of the flap when the device is in the second configuration and the flap holds the medical device in the second configuration; wherein the flap is configured to fit inside an interior passage of the occluder during delivery and retrieval; wherein the flap is configured to have a first interference with the interior passage of the occluder during delivery and a second interference with the interior passage of the occluder during retrieval, and the second interference is greater than the first interference; and wherein a first force required to deform the flap for retrieval is greater than a second force required to deform the flap for deployment.