A microfluidic structure is formed with one or more microfluidic channels for receiving fluid and passing the fluid through a channel in communication with a collection chamber integrated with that channel through a transition stage that allows the collection chamber to gather targets within the fluid, such as cells, including circulating tumor cells in blood. The transition stage may be formed for an asymmetrical configuration with an obstacle and chamfered face configuration. A gas permeable membrane provides perfusion control for directing gas to the collection chamber. Porous elements provide bubble venting from the fluid flow
