Integrated iontophoresis and sweat sensing via paper-derived laser-induced graphene soft conductors

The development of mechanically compliant, functional materials toward skin-integrated electronic systems has been a rapidly evolving field, aiming at flexible and even stretchable architectures that can be mounted into curved skin surfaces and adapt to their changing topography and biomechanics. One of the components required for functional bioelectronic systems is bioelectrodes that can robustly interact with the skin to perform sensing or stimulation tasks. In this work, paper-based laser-induced graphene is studied as a compatible material for skin-integrated systems for iontophoretic sweat stimulation and sweat metabolite sensing. Based on a water-induced peel-off strategy, robust transferred LIG soft conductors using polyurethane film substrates were fabricated and characterized, showing a compatible electrical conductivity of 29.3 S cm^-2 and a high graphitization yield, confirmed by scanning transmission electron microscopy and Raman spectroscopy. Based on these results, paper-based, transferred LIG/PU bioelectrode architectures were used to develop iontophoresis stimulation systems for the delivery of cholinergic agents and sweat secretion activation. The delivery of carbachol as a model cholinergic agent was studied, demonstrating the capability of stimulating up to 60 μL of sweat with a 10-minute stimulation period, over a period of 1 hour. The iontophoresis system was also integrated with a flexible LIG-based enzymatic glucose sensor, showing a high sensitivity of 29.73 μA mM^-1 cm^-2 and a limit of detection of 5.45 μM, capable of distinguishing glucose levels at fasted and post-prandial states, using more sustainable materials and resource-efficient fabrication, towards non-invasive sweat metabolite quantification.