Methods and systems for measuring viability and function of islet cells or stem cell-derived beta cells in an implantable device featuring setting the temperature of the cells in the implantable device to a low temperature to reduce metabolic levels of the cells and reduce oxygen requirements of the cells, and measuring oxygen consumption rates. An oxygen sensor at the inlet of the implantable device and an oxygen sensor at the outlet of the implantable device are used to calculate oxygen consumption rates of the cells, which in turn are indicative of viability. The reduction in temperature can also be used for loading cells into the implantable devices to help reduce ischemic and/or physical injury. The present invention may be used with other cell types, e.g. hepatocytes, heart cells, muscle cells, etc.