Light-Stable, Ultrastretchable Wearable Strain Sensors for Versatile Plant Growth Monitoring.

Wearable electronics have been applied to plants for various applications, including microclimate detection, health diagnosis, and growth rate measurement. However, previously reported plant growth strain sensors have limitations in the strain sensing range, optical transparency, and uncertain stability and reproducibility. Our recent work reported a transparent, conjugated polymer-based strain sensor that achieved above 400% operating strain in measurements of growth in a grass. In this work, we develop second-generation plant strain sensors to broaden their application scope in plant growth monitoring by (1) imparting photostability through device engineering and (2) boosting stretchability through direct ink writing. We first fabricate a strain sensor using room-temperature-cured Au-C-Al electrodes, which drastically improve sensor stability under direct light illumination. The sensors are successfully applied to leaves and stems of tomatoes as well as cotyledons and fruits of cucumbers to track the elongation or radial growth rate in day/night cycles. Notably, the cucumber cotyledon is so far the youngest plant organ (3 days after germination) on which a strain sensor has been used for growth monitoring. Moreover, we attain a significantly improved strain sensing range by patterning and encapsulating strain sensors using direct ink writing. An "elastic rope" model is proposed, which revealed that the shape and contour length of patterned sensors jointly determine the strain sensing range along with the intrinsic stretchability of the material. The most stretchable long-horseshoe pattern reaches a maximum apparent operating nominal strain of 1000% tested on grass leaves, a new record in strain sensors applied to plant growth monitoring.