Water peel-off transfer of electronically enhanced, paper-based laser-induced graphene for wearable electronics

Laser-induced graphene (LIG) has gained preponderance in recent years, as a very attractive material for the fabrication and patterning of graphitic structures and electrodes, for multiple applications in electronics. Typically, polymeric substrates, such as polyimide, have been used as precursor materials, but other organic, more sustainable, and accessible precursor materials have emerged as viable alternatives, including cellulose substrates. However, these substrates have lacked the conductive and chemical properties achieved by conventional LIG precursor substrates and have not been translated into fully flexible, wearable scenarios. In this work, we expand the conductive properties of paper-based LIG, by boosting the graphitization potential of paper, through the introduction of external aromatic moieties and meticulous control of laser fluence. Colored wax printing over the paper substrates introduces aromatic chemical structures, allowing for the synthesis of LIG chemical structures with sheet resistances as low as 5 Ω·sq–1, translating to an apparent conductivity as high as 28.2 S·cm–1. Regarding chemical properties, ID/IG ratios of 0.28 showcase low defect densities of LIG chemical structures and improve on previous reports on paper-based LIG, where sheet resistance has been limited to values around 30 Ω·sq–1, with more defect dense and less crystalline chemical structures. With these improved properties, a simple transfer methodology was developed, based on a water-induced peel-off process that efficiently separates patterned LIG structures from the native paper substrates to conformable, flexible substrates, harnessing the multifunctional capabilities of LIG toward multiple applications in wearable electronics. Proof-of concept electrodes for electrochemical sensors, strain sensors, and in-plane microsupercapacitors were patterned, transferred, and characterized, using paper as a high-value LIG precursor for multiples scenarios in wearable technologies, for improved sustainability and accessibility of such applications.This work is funded by National Funds through FCT I.P., under the scope of the project UIDB/50025/2020-2023. The authors acknowledge the ERC AdG project DIGISMART ref 787410, EC project SYNERGY H2020-WIDESPREAF-2020-5, CSA, proposal number 952169, EC project EMERGE, No. 101008701, and project BEST - ALT20-03-0247-FEDER-113469 | LISBOA-01-0247-FEDER-113469. T.P. and R.C. acknowledge funding from FCT I.P. through the Ph.D. Grants DFA/BD/8606/2020 and UI/BD/151295/2021. The authors also want to thank Jonas Deuermeier for the help with XPS measurements and analysis.