High cycle performance of twisted and coiled polymer actuators

Twisted and coiled polymer actuators (TCPA), also known as coiled artificial muscles, are gaining popularity in soft robotics due to their large contractile actuation and work capacity. However, while it has been previously claimed that the stroke of TCPA remains stable after thousands of cycles, their absolute length change has not been rigorously studied. Here, we constructed an isobaric cycling setup that relies on fast heating and cooling by water immersion. This enables testing for 10k cycles in a duration of 56 hours, where the muscle temperature is varied between 15 °C and 75 °C at a rate of 20 seconds per cycle. Surprisingly, while the stroke usually remains unchanged for the entire 10k cycles as previously claimed, the final muscle loaded length exhibits all the geometrical possibilities of creep behavior as it can remain unchanged, elongate (creep), or contract (reverse creep) at the end of the test. Based on a wide range of experiments, we derived an empirical law which captures the observed relationship between the final muscle length change $\Delta L$, the stroke $\alpha$, and the passive strain ${\epsilon }_0$: ${\epsilon }_0+\alpha =\Delta L$. Using this relation, the final length change of the muscle can be predicted from the first 100 cycles only. We show that polyvinylidene fluoride (PVDF), which does not swell in water, and nylon, which swells, follow this empirical law by testing in water with and without a protective coating, respectively. These results offer practical design guidelines for predictive actuation over thousands of cycles.