An Innovative Self-Adhesive Wearable Sensor Based on Fiber Bragg Grating Technology for Plant Growth Monitoring

In a time of rapid global population growth, ensuring both food security and optimal crop yields is crucial. The effective monitoring of plant health is essential to address these challenges. Wearable sensing technology offers advantages over traditional methods, which often suffer from limitation in spatial and temporal resolution or susceptibility to environmental conditions. Recent advances have led to miniaturized sensors that can be affixed to plant organs. Fiber Bragg grating (FBG) sensors embedded in flexible matrices are particularly suitable for plant wearables (PWs) due to their lightweight, transparency, and superior metrological properties. However, these flexible sensors need extra adhesive solutions, such as tapes or strings, to adhere to plants. This necessity could potentially impact plant physiology and compromise measurements accuracy. This study introduces an innovative PW sensor that integrates an FBG sensor within a self-adhesive, transparent, hydrogel-based matrix composed of natural polymers (sodium alginate and xanthan gum). The adhesiveness and flexibility of the material, combined with FBG advantages, make it an ideal sensing solution for stable monitoring of plant growth by conformally adhering to plant surfaces without additional adhesives. This work details the hydrogel matrix preparation and characterization, the sensor design, fabrication, and metrological assessment of the sensor that showed a linear response to strain (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {R}^{{2}} \gt 0.99$ </tex-math></inline-formula>; <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text {S}_{\varepsilon } =0.147$ </tex-math></inline-formula> nm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula> m<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon ^{-{1}}$ </tex-math></inline-formula>). Finally, the results of in vivo tests on plants demonstrated that the proposed sensor could autonomously adhere to plant surface without any adhesion loss and effectively monitor stem elongation without significantly affecting plant physiology.