Miniaturized Wearable Biosensors for Continuous Health Monitoring Fabricated Using the Femtosecond Laser-Induced Graphene Surface and Encapsulated Traces and Electrodes

Wearable sensors are increasingly being used as biosensors for health monitoring. Current wearable devices are large, heavy, invasive, skin irritants, or not continuous. Miniaturization was chosen to address these issues, using a femtosecond laser-conversion technique to fabricate miniaturized laser-induced graphene (LIG) sensor arrays on and encapsulated within a polyimide substrate. The femtosecond laser-converted conductive traces can have a size of 20 to 2 μm compared to the traditionally larger CO₂ laser dimensions of around 300 to 100 μm. This marks a 93-98% decrease in trace size when using a femtosecond laser. This miniaturization allows for the ability to process temperature, electrocardiography (ECG), electromyography (EMG), and glucose data in the same space that would have been occupied by a single sensor. The femtosecond laser-converted graphene (FSLIG) electrodes were modified to function as glucose sensors, and comprehensive electrochemical analyses using cyclic voltammetry (CV) and chronoamperometry (CA) were performed. These tests confirmed the capability of the sensors to detect glucose levels, showing a stability of 96.14%. Encapsulation of FSLIG within polyimide was achieved for the first time, demonstrating the ability to nondestructively create FSLIG electrodes within existing materials, thereby protecting them from external environmental factors. The encapsulated FSLIG shows potential as a method to produce LIG-coated Cu traces for improved multilayered printed circuit boards or layered circuits with complex geometries in polyamide to reduce size and increase functionality. Even sterile probes for use inside the body or under dermis polyamide injections and subsequent FSLIG circuit tattoos are possible. This study demonstrates the novel miniaturization and encapsulation capabilities enabled by the femtosecond laser, developing next-generation wearable biosensors focusing on miniaturization, flexibility, continuous monitoring, multifunctionality, and comfort.