Fig. 6. Graphite-based conductive inks.

a Image of a roller pen filled with GOx ink (left), a drawing of the active enzyme layer created on a bare sensor strip using the GOx ink-filled pen and a template (middle), a close-up view (top right), and a microscopic image of the sensor surface with the active enzyme layer drawn by the pen (bottom right). Reprinted with permission from Wang et al.²³. Copyright 2015 WILEY-VCH. b Image of epidermal glucose sensor on human skin (left), coupled with a potentiostat (right). Reprinted with permission from Wang et al.²³. Copyright 2015 WILEY-VCH. c Recorded current level from administering increasing levels of glucose. Reprinted with permission from Wang et al.²³. Copyright 2015 WILEY-VCH. d Schematic of the pencil-on-paper electronics drawing process (left) and various sensors (right). Reprinted with permission from Yan et al.²⁴. Copyright 2020 National Academy of Sciences. e Schematic showing the concept of humidity energy harvesting, achieved by creating a gradient of oxygen-containing groups on cellulose paper between interdigital graphite electrodes drawn with a pencil, through the process of moisture-electric polarization. Reprinted with permission from Yan et al.²⁴. Copyright 2020 National Academy of Sciences.