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1. Multiscale analysis of Benjamin Franklin’s innovations in American paper money

   https://doi.org/10.1073/pnas.2301856120  

   Manukyan, Khachatur; Yeghishyan, Armenuhi; Aprahamian, Ani; Jordan, Louis; Kurkowski, Michael; Raddell, Mark; Richter Le, Laura; Schultz, Zachary D.; Spillane, Liam; Wiescher, Michael (July 2023, Proceedings of the National Academy of Sciences)

   Benjamin Franklin was a preeminent proponent of the new colonial and Continental paper monetary system in 18th-century America. He established a network of printers, designing and printing money notes at the same time. Franklin recognized the necessity of paper money in breaking American dependence on the British trading system, and he helped print Continental money to finance the American War of Independence. We use a unique combination of nondistractive, microdestructive, and advanced atomic-level imaging methods, including Raman, Infrared, electron energy loss spectroscopy, X-ray diffraction, X-ray fluorescence, and aberration-corrected scanning transmission electron microscopy, to analyze pre-Federal American paper money from the Rare Books and Special Collections of the Hesburgh Library at the University of Notre Dame. We investigate and compare the chemical compositions of the paper fibers, the inks, and fillers made of special crystals in the bills printed by Franklin’s printing network, other colonial printers, and counterfeit money. Our results reveal previously unknown ways that Franklin developed to safeguard printed money notes against counterfeiting. Franklin used natural graphite pigments to print money and developed durable “money paper” with colored fibers and translucent muscovite fillers, along with his own unique designs of “nature-printed” patterns and paper watermarks. These features and inventions made pre-Federal American paper currency an archetype for developing paper money for centuries to come. Our multiscale analysis also provides essential information for the preservation of historical paper money. 

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2. Papertronics: Fully paper-integrated resistor, capacitor, and transistor circuits

   Landers, Mya; Elhadad, Anwar; Choi, Seokheun (June 2022, Technical digest SolidState Sensor Actuator and Microsystems Workshop)

   This work presents fabrication techniques for achieving individual electronic components both on the surface and within the fibers of a paper substrate, attaining full integration of paper and functional electronics materials. A process of hydrophobic wax patterning coupled with conductive and semiconductive poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT: PSS)-based ink injection and screen-printing has allowed for the implementation of all-paper-based, tunable resistors, capacitors, and transistors. The characteristics of the paper resistors can be adjusted as desired through finetuning of the PEDOT: PSS- based ink recipe, and the components can be combined in various arrangements to attain paper-based printed circuit boards (PCBs) for a wide range of practical applications. As a first step towards multiple component integration, a simple example circuit design is demonstrated that incorporates the three different components. Furthermore, through the strategic organization of the resistors, transistors, and capacitors and stacking of paper layers, more complex and diverse paper PCBs can be attained while minimizing the perceived surface area of the circuitry, allowing for a compact, pliable, and highly customizable means of fabricating paper-based electronic systems. 

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3. Micropatternable Janus Paper as a Wearable Skin Patch for Sweat Collection and Analysis

   https://doi.org/10.1002/admt.202300396  

   Gao, Yang; Choi, Seokheun (May 2023, Advanced Materials Technologies)

   Abstract Single‐use paper‐based wearable devices are receiving increasing attention as a novel platform for disposable, inexpensive, noninvasive, and real‐time sweat monitoring. The bidirectional liquid transport nature of paper is the most critical barrier to effectively controlling sweat samples for reliable and accurate sweat analysis. Excessive or additionally released sweat significantly interferes with analysis when mixed with old sweat. Moreover, bio‐receptors pre‐loaded in the sensing areas can backflow and move to another sensing region generating a cross‐talk issue. This work enables effective sweat sampling and delivery in paper by facilitating unidirectional sweat transport from the skin to the sensing reservoir. The design and fabrication of a single‐layered paper membrane to achieve Janus‐type properties, which only allow moisture to flow in one direction is introduced. When the hydrophobic side of the Janus paper is placed on the skin, sweat is unidirectionally self‐pumped from the hydrophobic side to the hydrophilic sensing areas, but not the reverse. The fabrication takes two steps including easy automatic and scalable printing of hydrophobic micropatterns on paper and simple heating of the printed paper for the wax penetration. Quantitative colorimetric assessment of pH, chloride, sodium, and glucose in sweat is simultaneously performed without cross‐talk between the sensing regions. 

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4. SOFT ROBOTICS: FLUID-DRIVEN SELF-FOLDING PAPERS

   Chun, H.; Mohammadifar, M.; Choi, S. (June 2018, Technical digest SolidState Sensor Actuator and Microsystems Workshop)

   We demonstrate a self-folding paper robot with capillary force driven fluid. When water is sprayed on fluidic channels patterned on paper, the 2-D sheet of paper can be controllably self-folded into various 3-D structures; half-oval, circle, round-edge square, triangle, half-circle, and table. The self-folding paper sheet can be readily fabricated via a double-sided wax printing method, forming a bilayer structure of the fluidic channel and the hydrophobic wax, in which these two layers have different swelling/shrinking properties. The patterned paper performs folding actuation with water and unfolding behavior with evaporation without being mechanically manipulated by external forces or moments. Finally, we create a paper gripper based on this self-folding actuation, conveying a low-weight object. This report demonstrates the possibility of paper microfluidics for self-folding actuation and soft robotics. 

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5. A Fully-Papertronic Biosensing Array for High-Throughput Characterization of Microbial Electrogenicity

   https://doi.org/10.1109/EMBC.2018.8513677  

   Tahernia, Mehdi; Mohammadifar, Maedeh; Hassett, Daniel J.; Choi, Seokheun (July 2018, Conf Proc IEEE Eng Med Biol Soc. 2018)

   For the first time, we report a low-cost, disposable fully-papertronic screening platform for rapid screening and identification of electroactive microorganisms. This novel papertronic device is capable of simultaneous characterizing the electrogenicity of 10’s of the newly discovered, genetically engineered, bacteria. This work explored an exciting range of possibilities with the goal of fusing microbial fuel cell technology with ‘papertronics,’ the emerging field of paper-based electronics. Spatially distinct 64 sensing units of the array were constructed by patterning hydrophilic anodic reservoirs in paper with hydrophobic wax boundaries and utilizing 3-D multi-laminate paper structures. Full integration of a high-performance microbial sensor on paper can be achieved by improving the microbial electron exchange with the electrodes in an engineered conductive paper reservoir and reducing cathodic overpotential by using a solid electron acceptor on paper. Furthermore, the intrinsic capillary force of the paper and the increased capacity from the engineered reservoir allowed for rapid adsorption of the bacterial sample and promote immediate microbial cell attachment to the electrode, leading to instant power generation with even a small amount of the liquid. 

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