A new sensor for measuring oxygen levels around tumors has been developed by researchers from the laboratory of KI faculty member and David H. Koch Professor in Engineering Michael Cima. The sensor, described in the Proceedings of the National Academy of Sciences, is an injectable device made of silicone, which is picked up in MRI scans. The sensor is the first MRI contrast agent that can be left in the body for long periods of time, allowing for the collection of oxygen tension over several weeks. Given that cancer cells thrive without oxygen and tumors in low-oxygen environments are generally more aggressive and resistant to treatment, long-term monitoring of oxygen tension will provide new insights into tumor growth and could aid therapeutic choices and tracking of treatment response.