A new concept for reusable eco‐friendly hydrogel electrolytes based on cellulose is introduced. The reported electrolytes are designed and engineered through a simple, fast, low‐cost, and eco‐friendly dissolution method of microcrystalline cellulose at low temperature using an aqueous LiOH/urea solvent system. The cellulose solution is combined with carboxymethyl cellulose, followed by the regeneration and simultaneous ion incorporation. The produced free standing cellulose‐based electrolyte films exhibit interesting properties for application in flexible electrochemical devices, such as biosensors or electrolyte‐gated transistors (EGTs), because of their high specific capacitances (4–5 µF cm−2), transparency, and flexibility. Indium–gallium–zinc‐oxide EGTs on glass with laminated cellulose‐based hydrogel electrolytes (CHEs) as the gate dielectric are produced presenting a low working voltage (<2 V), showing an on–off current ratio (Ion/off) of 106, a subthreshold swing lower than 0.2 V dec−1, and saturation mobility (μSat) reaching 26 cm2 V−1 s−1. The flexible CHE‐gated transistors on paper are also demonstrated, which operate at switching frequencies up to 100 Hz. Combining the flexibility of the EGTs on paper with the reusability of the developed CHEs is a breakthrough toward biodegradable advanced functional materials allied with disposable/recyclable and low‐cost electronic devices.

Conductive flexible hydrogel composites were printed on paper substrates using a functional ink, which was designed and formulated for screen-printing. The inks were prepared using abundant and eco-friendly materials by blending carbon fibers into the matrix of a water-soluble cellulose derivative, carboxymethyl cellulose. For an optimal concentration of carbon fibers (10 wt.%), the printed patterns exhibit a sheet resistance of around 300 Ω/sq without any post-printing annealing process. The resistance of the screen-printed hydrogel patterns is sensitive to variations of relative air humidity through moisture adsorption and swelling of the cellulosic matrix surrounding the carbon fibers. It was found that the sensitivity to temperature and humidity can be triggered by drying the printed patterns at 120 °C. A negative temperature coefficient thermistor with a sensitivity of 0.079 °C−1 at 25 °C and a hygristor, where a variation in the RH from 10% to 60% increases the resistance by 15 times, were screen-printed on paper using the formulated cellulose/carbon fibers based ink.

is the deposition of oxide semiconductors that already demonstrated to be suitable for low power electronics [8] and for the development of logic circuits on paper substrates. [9] Recent research pointed to easier, faster, and less expensive approach for film deposition by using commercially available rollerball pens for the transfer of functional inks onto a wide range of substrates. This trend paves the way for the introduction of the PoP technique, where simple applications, as protein inks deposition, [10] conductive tracks, [11] piezoelectric devices, [12] or 3D antennas have already been demonstrated. [13] Nevertheless, in order to produce active devices, it is crucial to have as well a highly reliable and consistent method to deposit semiconductor materials using the same approach. By doing so, we pull printing and particularly PoP to a revolutionary technology stage, permitting the creation of low-cost and eco-friendly paper electronics "on-the-fly" by simply using a pen, proper functional inks, and a sheet of paper.To the best of the authors' knowledge, the successful deposition of inorganic semiconducting materials by the PoP technique has not yet been explored. Here, we report for the first time a method for a reliable deposition of ZnO nanoparticles (NPs) based dual-phase layer on paper substrates at room temperature (RT) by the PoP approach. These layers were used to fabricate hybrid fully printed/hand-drawn UV sensors and field effect transistors, where paper is simultaneously used as the physical support and as dielectric. Although the use of conventional rollerball pens is very appealing as a portable patterning instrument for paper-based printed/written electronics, only narrow lines (typically between 250 µm and 1 mm) can be drawn and the ink throughput is not always continuous. [13] For more complex and wider patterns, various parallel lines need to be drawn, where each line has to be in contact with the neighboring one to achieve a bidimensional functional film. This approach turns rollerball pen deposition process unpractical and difficult to control. In order to overcome this bottleneck we used a parallel metal plate pen, well-known from calligraphy applications, capable of dispensing ink over a large area (see Figure 1). Figure 1a shows the complete head of the used parallel pen and the two insets in Figure 1c illustrate the parallel plate structure, seen as side-and top-view in SEM, respectively (nib size of 6.0 mm). Here, the concept of direct ink passage becomes visually clear: whereas a rollerball pen disperses the ink by means of a rotating sphere, the parallel pen permits a direct ink flow between the two parallel metal plates. Thus, the ink flow depends on the capillary forces between the two plates and the The present work reports on the handwriting of electronic circuits on paper based on the deposition of an inorganic oxide semiconductor, exploiting the pen-on-paper (PoP) approach. The method relies on the use of a parallel metal plate pen, well known from calligraphy applications...

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