Biological tissue can be analyzed in a similar manner to nonliving materials, but it is important to preserve its viability as much as possible, so that the measurements carried out in a test-rig in a laboratory re£ect the behavior of the tissue in its native state. Invertebrate tissue is easier to test, because it remains viable for longer periods at laboratory temperatures. Most vertebrate tissues need to be maintained at 37°C. All tissue needs to be kept moist with a saline (Ringer’s) solution. Important parameters are stress (measured in pascal, Pa) and strain (which is dimensionless).