"Mullins' effect" is always observed when mechanically testing tendons in the laboratory. The "Mullins' effect" is an irreversible softening of a structure after it is stretched. This effect also leaves a tendon longer than it was before it was stretched. All tendons stretched in a lab, however, have been previously loaded in life. If a standard Mullins' type of effect occurred, all laboratory loads at or below those already seen in life would not soften the structure. But, this is not observed. To explore whether living cell contraction (occurring in life) can explain this dichotomy in behaviors, fresh, cellular tendons will be tested and compared to tendons that were frozen to remove cell activity. Mechanical behavior and tissue morphology will be measured, focusing on tissue recovery. To further investigate whether the observed differences between in life versus in the lab behaviors are due to cell contraction, the contractile ability in tendon cells will be removed with a biochemical inhibitor. Then these tendons (with live cells) will be tested in the lab for Mullins' behaviors. Finally there will be a test to determine whether changes in tissue hydration and post-load rehydration affect recovery from Mullins' behaviors. For this objective, tendons in hypo- and hyper-osmotic solutions, thus changing hydrations levels in the tendon, will be tested. The findings of this reasearch will be disseminated on the web and incorporated into course work.
During in vitro testing, tendon displays a Mullins' effect softening, which is typically an irreversible phenomenon in inert materials. That irreversibility, however, is in conflict with observed behaviors of biological tissues. All harvested tendons have been loaded in vivo, so if a standard Mullins' effect occurred, all in vitro loads at or below those already seen physiologically would not produce Mullins' softening. This clearly is untrue. To explore this phenomenon in terms of active cell contributions, fresh, cellular tendons will be tested and compared to tendons frozen to remove cell activity. Metrics will include mechanical behaviors and morphology, focusing on recovery. To further investigate whether changes in crimp and/or in Mullins' effect recovery are due to cellular contraction, the physical contribution of cell contraction will be removed with the addition of cytochalasin D (an inhibitor of actin polymerization known to drive cell motility). Thus the mechanical aspect of cell activity will be removed while maintaining cell viability. Finally there will be a test to determine whether tissue rehydration affects recovery from Mullins' effect softening via testing in hypo- and hyper-osmotic solutions.
