Paper-Based Green Laser-Induced Graphene for Disposable and Wearable Sensing Applications

Laser-induced graphene (LIG) has established itself as a very attractive material for electrode fabrication, within several applications in bioelectronics. The straightforward, high throughput graphitization of several precursor materials using this laser conversion process allows for the simultaneous synthesis and patterning of this 3D graphitic material with diverse electrode architectures, to target several biosensing applications, from biophysical to biochemical monitoring.<br/>Recently, paper has appeared as a viable alternative to conventional petroleum-based plastic polymer precursors, such as polyimide. This is due to the possibility of photothermally converting aliphatic cellulose monomers into graphene lattices, using several cellulose substrate treatment strategies. Application of fire-retardant chemical modifications and external aromatic moieties improves the graphitization potential of cellulose, to reach LIG film with 5 ohm.sq^-1sheet resistance and conductivities as high as 67 S.cm^-1.<br/>With these improved conductive properties of paper-derived LIG, this precursor material can be easily employed for the fabrication of disposable biosensing units, where paper acts as both the support substrate and precursor material for conductive electrode fabrication, without the need for more intricate printing techniques. Alternatively, strategies can be employed to separate converted and unconverted phases, through transfer methods, to make this material compatible with wearable applications.<br/>In this presentation, we report the use of cellulose as a material in the toolbox of LIG precursors, aimed at the development of both disposable and wearable biosensing applications. Paper-based electrochemical sensors using this material are presented, aiming at disposable sensor development for different analytes. Glucose and pH electrochemical sensors were fabricated, showing the compatibility of the material with several sensing strategies, such as enzymatic and non-enzymatic sensing. To translate patterned electrodes for wearable applications, a straightforward transfer method is presented, using a water-induced peel-off method. This method is capable of efficiently separating unconverted cellulose and converted LIG phases, allowing for the transfer of LIG patterns onto flexible, conformable, and elastomeric substrates with adhesive properties, for example, medical grade adhesives. Using this method, electrochemical biosensors, strain sensors for biophysical monitoring and electrodes for electrophysiological signal monitoring are presented.<br/>In conclusion, the concepts explored herein show the applicability of LIG towards the fabrication of robust point-of-care, disposable analytical devices, but also its potential for integration in wearable sensing systems, aiming at more sustainable, accessible bioelectronic applications.