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Three commercially-available conductive filaments were evaluated for 3D printing flexible circuits on paper. While all three filaments were printed successfully, the resulting conductive traces were found to have significantly different impedances when characterized by electrochemical impedance spectroscopy. Using a graphite-doped polylactic acid filament, the flexibility of paper-based conductive traces was evaluated, methods of integrating common electrical and electronic components with the conductive traces were demonstrated, and the resistive heating of the traces was characterized. The ability to 3D print conductive traces on paper using commercially available materials opens many opportunities for rapid prototyping of flexible electronics and for integrating electronic circuits with paper-based microfluidic devices. 

Microfluidic paper-based analytical devices (microPADs) are emerging as cost-effective and portable platforms for point-of-care assays. A fundamental limitation of microPAD fabrication is the imprecise nature of most methods for patterning paper. The present work demonstrates that paper patterned via wax printing can be miniaturized by treating it with periodate to produce higher-resolution, high-fidelity microPADs. The optimal miniaturization parameters were determined by immersing microPADs in various concentrations of aqueous sodium periodate (NaIO₄) for varying lengths of time. This treatment miniaturized microPADs by up to 80% in surface area, depending on the concentration of periodate and length of the reaction time. By immersing microPADs in 0.5-M NaIO₄for 48 hours, devices were miniaturized by 78% in surface area, and this treatment allowed for the fabrication of functional channels with widths as small as 301 µm and hydrophobic barriers with widths as small as 387 µm. The miniaturized devices were shown to be compatible with redox-based colorimetric assays and enzymatic reactions. This miniaturization technique provides a new option for fabricating sub-millimeter-sized features in paper-based fluidic devices without requiring specialized equipment and could enable new capabilities and applications for microPADs.