Tissue engineers are working on ways to grow skin, cartilage, and bone in the lab so that injury victims don't have to rely on replacement tissues extracted from donors or from their own bodies. One obstacle researchers face is that while they can easily examine the health of cells on a tissue's surface, checking whether the cells deep inside are thriving or dying remains tricky. Chemical engineer Zhanfeng Cui of Oxford University has developed a small polymer probe that determines the cells' health instantaneously. A fraction of a millimeter in diameter, the needlelike probe can be inserted into growing tissue and measures the levels of certain key substances, such as nutrients or cellular waste products. The probe is made of a porous membrane that the target molecules can pass through; the number of molecules that enter correlates with their concentration in the tissue. Cui hopes to license the technology for commercial development or find investors for a startup to manufacture the probes. As commercial products, the probes could speed the development of tissue engineering-delivering new hope to desperate patients.