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a century, sensational engineering efforts have been performed to develop new synthetic materials, and intensive studies of forces needed to deform those soft materials have examined the use of pneumatic, thermal, electrical, and chemical energy. [11][12][13] The second research stream designs low-profile, sheet-like robots, named "robotic origamis" (or "robogamis") that offer softness and reconfigurability with multiple bending degrees of freedom. [8][9][10] In robogamis, the rigid sheets are connected through soft joints, where 2D patterns can be folded into 3D structures by low profile actuations such as electrostatic forces, shape memory alloys, and pneumatic pressures. [2] Such robogamis can autonomously transform themselves into programmed shapes and perform complex tasks in unpredictable environments. Substantial progress in soft material research has produced achievements that are critical in terms of robogami techniques [2] while soft robotics and origami folding concepts have opened new applications. [9] However, soft robotics and robogamis have historically developed separately, and studies on robogamis with soft matter were unavailable or quite limited even though their integration is expected to generate substantial achievements in terms of cost, fabrication, operation, and performance. Built-in folds made of soft materials have the considerable potential to yield continuum, compliant, and configurable properties for robots.In this work, we introduce a new approach to combine these two robotic techniques by using soft paper substrates and a waterresponsive origami technique. The capillary action of sprayed water molecules is constrained by hydrophilic channels on paper, whose number and size are carefully defined by double-sided printing and the asymmetrical penetration of wax. In particular, a bilayered single sheet of paper that uses wax and water can lead to programmed deformations from different swelling and shrinking properties of the layers. A wax-water pattern on paper develops a folding actuation with a specific degree and shape and unfolding with evaporation without being mechanically manipulated by external forces or moments. The 2D sheet of paper can be controllably self-folded into various 3D structures, demonstrating self-folding actuation, low-weight object manipulation, and biomimetic locomotion through bending and relaxation. Although a couple of paper-based robotic concepts were partly proposed before, [14,15] no technique has yet emerged as a simple print-and-fold actuator powered by environmental humidity or water for comprehensive robotic functions including self-folding, Soft robotics driven by origami can fundamentally advance robotic functionalities by significantly improving continuum, compliance, and configurability. Here, a new field is proposed, "paper robotics," which is based on moistureresponsive self-folding of paper substrates into functional 3D machines using origami-inspired techniques. By properly designing a hydrophobic wax layer and a hygro-expandable hydrophilic ...

Water quality monitoring is becoming an essential part of our lives as increasing human activities continue to spill unknown and unexpected contaminants into our water systems. To ensure the provision of safe and clean water to the public and the ecosystem, the development of rapid and sensitive in situ early warning systems for water toxicity monitoring is crucial. In this work, an entirely paper-based microbial fuel cell sensor utilizing freeze-dried bacteria is demonstrated as a portable and disposable water toxicity sensor. The bacterial cells were preinoculated on the anode reservoir of the device, and they were freeze-dried, making their on-site and on-demand applications possible. Upon rehydration of the bacteria with the water samples, current readings were obtained, and inhibition ratios (IRs) were calculated for different concentrations of formaldehyde as a model toxin. For 0.001, 0.01, and 0.02% of formaldehyde, IRs of 7.88, 16.08, and 23.14% were obtained, respectively. These IRs showed a very good linearity with the formaldehyde concentrations at R 2 = 0.995. Additionally, the shelf life of the freeze-dried microbial fuel cell sensor was investigated. Even after 14 days of storage in the desiccator, at 4, and at −20 °C, the performance outputs compared to the new device were all at 96%.

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