I like to read about research innovations even if most of them don’t represent in an advance that can be commercialized or even leveraged to boost technology in general. Sometimes, the work that the team has done demonstrates innovation, discusses ways they addressed and hopefully resolved problems, and even gives insight into areas where there are remaining barriers to overcome.
That’s the case with a research paper I came across on use of paper as a substrate of passive electronic components and interconnects. In the work, “High-Density Papertronics via Laser-Written Hydrophilicity on Hydrophobic Parchment Paper” published at ACS Applied Materials & Interfaces, researchers discuss the long path they have taken to develop their latest effort in paper electronics—dubbed “papertronics”—with the use of standard parchment paper as a substrate for passive electronic elements.
This project builds heavily on other work the team has done under Professor Seokheun “Sean” Choi of the Department of Electrical and Computer Engineering at Binghamton University’s Thomas J. Watson College of Engineering and Applied Science (New York State). They have investigated the use of paper to create fully biodegradable circuit elements such as batteries, sensors, and other components.
The papertronic devices are intended to be low cost, environmentally friendly, and suitable for applications such as single-use medical patches that power using sweat, water, or even saliva as their working fluid. While paper-based electronics of various types are being investigated in many labs, this group has done a significant amount of leading-edge work in this area with a lengthy track record.
Their previous attempts to build passive circuit elements used chromatography paper as a substrate; this paper is a specialized, high-purity cellulose material widely used as the stationary phase to separate mixtures of chemical substances, inks, or plant pigments via capillary action. Unlike everyday normal paper, which contains fillers and sizing agents that interfere with fluid movement, chromatography paper has a consistent texture, controlled thickness, and predictable capillary action, vital for accurate laboratory results.
That approach proved the concept but revealed a stubborn limitation: the wax barriers used to define circuit features blurred and spread when heated, limiting the smallest achievable features to about a millimeter. That kept circuits large—on the scale of tens of centimeters—and prevented the kind of compact, tightly packed designs needed for practical applications.
Rethinking the problem
Their new approach required a radical shift. Instead of hydrophilic chromatography paper, they switched to hydrophobic parchment paper, and using a laser to selectively create hydrophilic channels rather than wax to create hydrophobic barriers, the team inverted the entire fabrication paradigm (Figure 1).
Figure 1 By targeting the parchment paper with a laser, Professor Choi and his team removed the silicone coating to expose water-absorbing cellulose fibers. The channels then act as microscopic highways that guide water-based conductive inks to form electronic circuits. Source: Binghamton University
Note that parchment paper—also called “baking paper”—is a heat-resistant, silicone-coated paper used in cooking and baking as a non-stick, greaseproof surface; you can buy it at almost any supermarket as well as from cooking-supply sources. But there’s some potential for verbal confusion here; the research team is using silicone-based paper as the substrate rather than more-familiar “silicon.” Silicone, of course, is a synthetic rubber-like substance made by combining silicon with oxygen, carbon, and hydrogen.
They developed hybrid paperfluidic-electronic techniques that exploit parchment paper’s capillary action to wick functional materials into predefined regions, yielding components such as resistors, capacitors, inductors, and interconnects (Figure 2).
Figure 2 Conceptual illustration of wax-printed versus laser-induced papertronics. In row (a), wax-printed papertronics on hydrophilic cellulose paper highlights the multiple fabrication steps required for hydrophilic pattern formation via single-sided wax printing and thermal penetration (i) and hydrophilic patterning via asymmetric double-sided wax printing and thermal penetration (ii). In row (b), molten-wax spreading during thermal penetration inherently blurs feature boundaries and limits the achievable resolution. Laser-induced papertronics on hydrophobic parchment paper demonstrates a simplified, low-temperature fabrication workflow that enables direct hydrophilic patterning without wax spreading or high-temperature processing (i) and the realization of high-density papertronic circuits with narrow hydrophilic channels and tightly spaced insulating regions (ii). Source: ACS Publications
The result is circuit features as small as 250 micrometers wide with 300-micrometer spacing—a twofold to threefold improvement over the best wax-based methods, and small enough to fit complete filter circuits into a footprint measured in millimeters rather than centimeters.
The differences due to the inverted thinking are clear: with chromatography paper, they were fighting against molten wax spreading through the paper. With their laser approach and parchment paper, the pattern is defined by the laser spot size and stays exactly where they put it. There is no spreading, no blurring, and no uncertainty.
Critically, all of the functional inks used in the work are water-based and free of toxic metals or organic solvents. The circuits are biodegradable, breaking down in soil within weeks, and can be incinerated to ash in seconds if rapid disposal is needed. For applications that require longer operational lifetimes, a thin silicone encapsulation layer protects the devices from humidity and mechanical damage without affecting their electrical performance.
Fabrication technique
As a first step, they used Fourier-transform infrared (FTIR) spectroscopy to determine the chemical composition of the pristine parchment paper, the surface modifications induced by laser treatment, and the effect of “ink” deposition. Laser irradiation induces pronounced photothermal and photochemical modifications to the parchment paper surface, which they assessed as well.
They then used laser modification to convert selected regions into ink-guiding hydrophilic microchannels. The localized laser-induced heating modifies the surface chemistry and microstructure without requiring additional chemical coatings, fillers, or high-temperature treatments.
Using a 50-watt CO2 laser, the laser power and scan speed were systematically tuned to optimize local heating and patterning of silicone-coated parchment paper. It took a significant amount of trial and error to find the optimum power and speed pairing. With a high-power and low-scan speed, the laser overheated the substrate, burning through the hydrophilic cellulose layer and cutting the paper. At low power and high speed, the silicone coating was only partially removed or not removed at all, thus preventing proper penetration of the functional materials.
Critically, these laser-defined hydrophilic features exhibit no lateral spreading, enabling minimum feature sizes and spacings that approach the theoretical resolution limit of the optical system. This strategy enables unprecedented high-density patterning, robust pattern reproducibility, and device-scale miniaturization for a significant leap beyond what is achievable with wax printing or traditional paperfluidic techniques.
For ink used to define conductive traces and electronic components, they used poly (3,4-ethylenedioxythiophene)-poly (styrenesulfonate), known as PEDOT:PSS, with controllable viscosity and density. For many reasons, PEDOT:PSS is widely used in research projects. Using these formulations resulted in limited lateral spreading on the order of micrometers within 3D cellulose fiber network.
Then there’s performance
They constructed printed resistors, interdigitated capacitors, low-loss interconnects, and integrated low- and high-pass RC filters within a single paper layer, exhibiting predictable, tunable electrical behavior consistent with circuit theory. The conductive lines have a width of ∼250 μm and a line spacing of ∼300 μm, corresponding to a PCB pitch of ∼550 μm.
The team demonstrated the platform’s versatility by fabricating a complete set of electronic components on paper: resistors whose resistance can be tuned over three orders of magnitude simply by adjusting the ink formulation; interconnects with sheet resistance as low as approximately one ohm per square, rivaling rigid electronic systems; capacitors tunable from microfarads to millifarads; and fully functional low-pass and high-pass RC filters whose frequency response closely matches theoretical predictions.
For example, the resistor values were independently controlled through three parameters: ink concentration, dimethyl sulfoxide (DMSO) content, and resistive line length. The laser-defined hydrophilic channels acted as deterministic templates that confined conductive inks, while the surrounding silicone-coated parchment remained hydrophobic and electrically insulating. This spatial confinement decoupled electrical tuning from uncontrolled ink spreading and penetration, enabling predictable and reproducible resistor geometries.
Note that DMSO is a poor electrical conductor on its own because it’s a molecular liquid without free ions or electrons. However, when ionic compounds like salts or acids are dissolved in it, it acts as a polar aprotic solvent that readily dissociates those ions, allowing the resulting solution to conduct electricity.
The laser-defined hydrophilic channels act as templates that confine conductive inks, while the surrounding silicone-coated parchment remains hydrophobic and electrically insulating. This confinement decouples electrical tuning from uncontrolled ink spreading and penetration, enabling predictable and reproducible resistor geometries.
Overall, the resistance of the paper-based resistors can be continuously tuned over a broad range from approximately tens of ohms to several kilohms (Figure 3).
Figure 3 For paper-based resistors, schematic illustrates how laser-defined paper resistors are tuned by three independent parameters: ink concentration, DMSO addition, and resistor geometry (a). Optical microscopy image (i) and SEM image of the A–A′ cross-section of a representative resistor (ii) show that PEDOT:PSS is precisely confined within the laser-treated hydrophilic region and surrounded by untreated hydrophobic parchment paper (b). Measured resistance of resistors is fabricated with varying ink volumes, where PEDOT:PSS was systematically diluted with deionized water to concentrations of 100%, 75%, and 50%, plotted as a function of resistor length (c). Resistance as a function of DMSO concentration in the PEDOT:PSS matrix is shown with weight percentages ranging from 2% to 20%, plotted as a function of resistor length (d). Data in (c) and (d) represent mean ± standard error from at least 10 independent measurements per data point. Solid lines are least-squares linear fits. Source: ACS Publications
For capacitors, conductive PEDOT:PSS electrodes were precisely patterned into interdigitated fingers using laser-induced hydrophilic channels, while the surrounding silicone-coated parchment remains hydrophobic and electrically insulating. The capacitance increases monotonically with the number of interdigitated fingers, reflecting the enlarged effective electrode surface area and reduced ionic diffusion distance inherent to the interdigitated geometry.
They achieved capacitance values spanning from the low microfarad regime to over 1.2 millifarads (Figure 4). The ability to modulate capacitance over nearly three orders of magnitude using only electrolyte composition and electrode geometry demonstrated the exceptional tunability of this capacitor platform.
Figure 4 For paper-based capacitors, schematic illustrates a laser-defined interdigitated capacitor fabricated on hydrophobic parchment paper (a). Optical microscopy image (i) and SEM image of the A–A′ cross-section of a representative capacitor (ii) show that conductive PEDOT:PSS ink is precisely confined within the laser-treated hydrophilic regions to form interdigitated electrode fingers, while the surrounding parchment paper remains hydrophobic and electrically insulating (b). Measured capacitance of capacitors fabricated with varying H3PO4 concentrations in the PVA gel electrolyte is plotted as a function of electrode finger number (c). Galvanostatic charge–discharge (GCD) curves are measured at a constant current of 2 μA after different numbers of charge–discharge cycles (d). Capacitance retention as a function of cycling number demonstrates the cycling stability of the paper-based capacitor (e). Source: ACS Publications
Interconnects and filters
The capillary scheme was also used to create interconnects, leading to what can be considered as “integrated circuits”. To demonstrate circuit-level integration, they fabricated first-order low-pass and high-pass RC filters.
All of the hydrophilic regions were defined in a single laser step, and components were sequentially deposited using paper stencils. The first-order low-pass RC filter composed of a series resistor and a shunt capacitor (Figure 5).
Figure 5 For paper-based RC filters fabricated on parchment paper, part A shows a low-pass RC filter: schematic illustration (a), circuit diagram (b), and photograph of the low-pass filter fabricated on parchment paper (c). Frequency-dependent gain plots for low-pass filters with different resistance values (46 Ω, 95 Ω, and 150 Ω) demonstrate low-pass cutoff behavior (d). Part B shows a high-pass RC filter: schematic illustration (a), circuit diagram (b), and photograph of the high-pass filter fabricated on parchment paper (c). Frequency-dependent gain plots for high-pass filters with different resistance values (15 Ω, 46 Ω, and 95 Ω) demonstrate high-pass cutoff behavior (d). Source: ACS Publications
Since the resistor, capacitor electrodes, and interconnects required different ink formulations and additives, each printing step is performed using a dedicated paper stencil. Despite this, the overall process remains low-temperature and highly reproducible.
They also assessed environmental aspects and biodegradability. Although parchment paper incorporates a thin silicone coating to give it hydrophobicity, the substrate is predominantly cellulose-based with the silicone layer accounting for only a minor fraction of the total material volume (typically <2 μm in thickness).
As a result, degradation is governed primarily by the underlying cellulose fiber network. The silicone coating, while more persistent in natural environments, is widely recognized as biocompatible and chemically inert, and its limited mass minimizes its environmental burden.
But no transistors, at least not yet
The team’s paper is quite detailed with respect to the underlying materials science, elemental physics and chemistry, and other considerations. They also acknowledge a significant shortcoming; thus far, no active devices such as transistors.
I applaud their willingness to switch from one type of paper and fabrication to a very different scheme when the first approach reached a dead end. It takes an open mind to start over and re-assess what is working and what is not.
Based on their track record with various embodiments of papertronic devices, I wouldn’t be surprised if they get the needed funding to further pursue their work toward papertronic transistors using what they have learned thus far. That would certainly be impressive.
Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors, signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing.
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