Femtosecond infra-red laser carbonization and ablation of polyimide for fabrication of Kirigami inspired strain sensor

Microfabrication of polyimide (PI) with femtosecond laser of wavelength 1030 nm is studied in two process conditions. Firstly, the low power-low scan speed regime is investigated for laser carbonization producing piezoresistive laser induced graphene (LIG). The heat accumulation model is modelled to find the temporal evolution of temperature at the laser focus for a single laser scan. Secondly, the high power-high scan speed regime is studied for laser ablation where clean ablation was observed due to multiphoton absorption. To demonstrate the application of this process, a two-dimensional (2D) LIG based strain sensor is drawn on a Kapton PI sheet using laser carbonization and transformed into a three-dimensional (3D) conformal sensor by cutting into a Kirigami design using laser ablation. The strain in the sensor is calculated from finite element analysis and the gauge factor is 88.58 +/- 0.16. This laser process enables the transformation of any 2D PI sheet into a 3D conformal sensor using femtosecond laser, which is useful for wearable sensors and health-monitoring applications. The fabricated sensor is demonstrated used on a knee-joint to monitor real-time tracking of bending and twisting knee movements.