An on-board oxygen concentrator is required during long duration manned space missions to supply medical oxygen. The commercial medical oxygen generators based on pressure swing adsorption (PSA) are large and highly power intensive. TDA Research, Inc. (TDA) proposes to develop a small, lightweight, portable oxygen generator based on a vacuum swing adsorption (VSA) to produce concentrated medical oxygen. The unit uses ambient vehicle cabin air as the feed and delivers high purity oxygen while meeting NASA's requirements for high flow capacity, closed-loop tissue oxygen control and operation in microgravity/partial gravity. TDA's VSA system uses a modified version of the lithium exchanged low silica X zeolite (Li-LSX), a state-of-the-art air separation sorbent extensively used in commercial Portable Oxygen Concentrators (POCs) to enhance the N2 adsorption capacity. In Phase I, we demonstrated the scientific, technical, and commercial feasibility of the oxygen concentrator module (OCM). In Phase II, we will develop a higher fidelity prototype with an adjustable pressure output to produce 2-15 lpm of O2 at 50% to +90% purity from ambient cabin air. The OCM will be capable of self-regulating the oxygenation of the patient using a closed loop feedback system that senses tissue oxygenation level. We will evaluate the sorbent performance in a breadboard bench-scale prototype under simulated microgravity/partial gravity exploration atmospheres and carry out a 1,500 hr longevity test (at a minimum) to determine its mechanical durability. Based on the experimental results, we will design a prototype unit that will meet all of NASA's requirements (e.g., low power draw over the range of flows and oxygen levels, lightweight and volume), while delivering the desired oxygen flow and purity.