Ronen Dolev | October 2026

For decades, industrial printing success has been judged primarily by the quality of the image it reproduces. Optical density, color accuracy, resolution, uniformity, color-to-color registration, and mechanical durability have been central measures of print performance. Yet a parallel technological field is expanding what a printed layer can do. Instead of depositing information intended only for human vision, functional materials can be printed to create electrically conductive paths, electrodes, sensors, and elements that actively participate in an electronic system.

This field, commonly called printed electronics, embraces several manufacturing methods for placing functional materials in precisely defined patterns. Some applications have already been demonstrated experimentally and in industrial contexts, while others remain at various stages of development. An especially interesting question is whether liquid electrophotography (LEP), an established digital industrial printing process, can be extended beyond color graphics to manufacture electrically functional structures.

The question is not merely speculative. Published European patent documents describe conductive liquid-electrophotographic formulations and printing methods. However, a substantial engineering gap remains between a printed trace with measurable electrical resistance and a dependable pressure sensor produced on a commercial press. Bridging that gap requires close examination of the underlying physics, ink chemistry, printing system, substrate, and finished device. [5, 6]

What Makes Functional Printing Different from Conventional Printing?

In graphic printing, an ink layer is designed primarily to produce an optical response. Pigments absorb and reflect light, while binders contribute film formation, adhesion, and mechanical properties. Even in processes that rely on complex electrostatic or thermal mechanisms, the final product is generally evaluated for its visual and physical performance.

Functional printing imposes an additional requirement. A conductive ink must form a path through which electrical charge can move. A pressure-responsive layer must produce a measurable change under mechanical loading. A dielectric layer must provide defined insulating behavior, and a multilayer construction must continue to function after bending, temperature changes, humidity exposure, and repeated use.

A visually flawless ink film is therefore not necessarily electrically useful. A smooth, uniform black line may be entirely insulating, while a less visually uniform pattern may provide stable conduction. Optical quality-control methods remain important, but they cannot qualify a printed electronic component on their own.

How Does a Printed Ink Become Electrically Conductive?

Most graphic inks combine pigments, polymers, a liquid vehicle, and functional additives. Conductive formulations include materials that can carry electrical charge. Candidates studied in printed electronics include silver and copper particles, graphite, graphene, carbon black, and carbon nanotubes (CNTs).

Adding conductive particles, however, does not automatically create a conductive film. The particles must establish a connected or electrically coupled network. This leads to a key concept in materials science: the percolation threshold. Below the relevant threshold, conducting particles may remain separated by an insulating polymer. As filler concentration rises, additional electrical pathways can emerge, and overall resistance can drop markedly.

The threshold depends on more than concentration. Particle geometry, aspect ratio, dispersion, orientation, polymer properties, and the way the printed layer is formed all matter. High-aspect-ratio materials such as CNTs may establish conducting pathways at lower loadings than nearly spherical fillers. Yet improved post-printing conductivity can introduce a new problem for LEP: a highly conductive formulation may be difficult to control during electrostatic development. [5, 6]

The Unique LEP Challenge: Conductive Materials in an Electrostatic Process

Liquid electrophotography forms a latent electrostatic image on a photoconductive surface. Charged ink particles are selectively deposited according to the charge pattern. The developed ink layer is then transferred to a heated intermediate surface and subsequently released onto the substrate. During development, particle charging, ink concentration, carrier-liquid characteristics, and electrical-potential differences influence where and how much material is deposited.

This creates an engineering conflict. The final printed structure should have a stable, sufficiently conductive network. During image development, however, the formulation must behave in a controlled electrostatic manner. A conductive additive that improves the finished trace may also affect the electric fields essential to pattern formation.

European patent publication EP3411756A1 explicitly addresses that interaction. It describes conductive liquid-electrophotographic inks and explains that higher ink conductivity can disrupt development electrostatics, impairing pattern fidelity. The disclosure discusses managing the electrical properties of components within the development system to mitigate that effect. The significance is fundamental: changing a graphic ink into a conductive material is not simply a pigment substitution. Ink formulation, particle charging, development hardware, and transfer behavior may have to be engineered together. [6]

Accordingly, a suitable web-handling architecture does not make an ordinary commercial LEP press suitable for experimental conductive inks. Any such work would require specific equipment and materials authorization, process development, safety review, and subsequent validation.

What Has Actually Been Demonstrated in Conductive LEP Research?

Two published European patent families offer a concrete basis for discussing conductive LEP. EP3198342A1, entitled "Liquid toner containing a low symmetry electrically conducting material for printing conductive traces," describes liquid toner using elongated or otherwise low-symmetry conductive materials, including carbon nanotubes, to establish a connected conductive structure after deposition. [5]

The document includes experimental examples exploring conductive loading, multiple printed separations, and thermal treatment. In one example, a formulation with CNT material printed in 16 separations showed a resistance reduction from approximately 5,000 to 300 ohms per square after a short thermal treatment at about 300°C, under the patent's stated test conditions. These are illustrative results for the disclosed experimental construction—not a universal process recipe or a validated property of standard LEP ink. [5]

The temperature matters. An experimental post-treatment near 300°C cannot automatically be applied to ordinary thin polymer films used for labels or flexible packaging. A practical PET-based device would need a conductive formulation and a thermal history compatible with the selected substrate and layer stack. Any proposed alternative curing, consolidation, or transfer process would have to be tested independently.

EP3411756A1 addresses another part of the problem: controlling the electrophotographic development of electrically conductive materials. It describes approaches to maintaining pattern formation while handling the altered electrical behavior of conductive ink formulations. Taken together, the disclosures support the technical feasibility of printing conductive patterns by LEP. They do not show that standard label presses running standard process inks can produce qualified pressure-sensor circuits at commercial yield. [6]

The evidentiary distinction is important. A patent may disclose experimental data and an engineering solution, but it is not equivalent to an independently replicated, peer-reviewed production qualification. Claims about reproducibility, mechanical lifetime, throughput, and end-use performance must be tested on the actual intended construction.

From Printed Pressure-Sensitive Material to a Measurable Signal

One promising application of printed electronics is the piezoresistive pressure sensor. In this type of sensor, mechanical loading changes the electrical resistance of a sensitive structure. In a polymer composite containing conductive particles, pressure can change particle contacts, spacing, charge-transport mechanisms, and the network's effective conductivity.

The relative resistance change is often represented by ΔR/R₀ = (R − R₀)/R₀, where R₀ is the reference resistance, and R is the measured resistance under the selected loading condition. In practice, the relationship between pressure and resistance may be nonlinear, dependent on loading history, or influenced by temperature and time.

A measured change in resistance does not, by itself, establish sensor accuracy. A useful sensor requires a defined operating range, sensitivity, calibration method, loading and unloading characterization, stability limits, and evaluation of measurement uncertainty. The 2024 review by Shu and colleagues examines materials, operating mechanisms, and performance limitations of flexible resistive tactile sensors, including the trade-offs among sensitivity, repeatability, durability, and device structure. [2]

Such sensors could be used for grip-pressure monitoring, load mapping, motion monitoring, or human-machine interfaces. However, the printed element is only part of the system. The acquisition circuitry, power supply, signal processing, communication functions, and any visual or vibration feedback may be provided by conventional electronics. Printing a working sensing layer does not mean the entire electronic system has been printed.

Successful Experimental Evidence from Other Printing Processes

The strongest peer-reviewed experimental evidence for flexible printed pressure sensors currently comes from several printing techniques, particularly screen printing and hybrid manufacturing. A 2024 review by Wu and colleagues documents advances in screen-printed pressure sensors, including the printing of electrodes, pressure-sensitive composites, and patterned functional structures. Screen printing can deposit useful volumes of functional material and suits specific material systems, but performance depends on the chemistry, mesh, substrate, and post-processing conditions. [1]

A 2026 study published in ACS Omega demonstrated screen-printed pressure sensors based on graphite-loaded epoxy composites toughened with carboxyl-terminated butadiene acrylonitrile (CTBN). The authors reported measurable piezoresistive response, low hysteresis, and resistance-baseline drift of less than approximately five percent after 1,000 loading cycles in their defined experimental setup. These results support the feasibility of tuning the conducting network and polymer matrix together. They do not establish the lifetime of a commercial product, nor were the samples produced by LEP. [3]

A second instructive example appeared in ACS Applied Materials & Interfaces in 2021. The researchers combined roll-to-roll screen-printed silver electrodes on stretchable polydimethylsiloxane (PDMS) with inkjet-deposited single-walled CNT micropatterns. The resulting devices demonstrated strain-sensitive electrical behavior and responses to bending and pressure. This shows that different deposition methods can complement one another within a functional device; it does not show identical results in an LEP process. [4]

These examples point to a practical development principle: no functional layer of a sensor must be manufactured with the same printing technology. A hybrid architecture may let each process perform the task it is best suited for.

Could LEP Become Part of a Hybrid Pressure Sensor?

A realistic research opportunity is not necessarily an entirely LEP-printed sensor. One candidate architecture would use an appropriately developed LEP process for selected conductive traces and electrode geometries, while the pressure-responsive layer is deposited by screen printing, coating, or another validated method. The layers would then be assembled with suitable interconnections, protection, and conventional measurement electronics.

The possible benefit of LEP in this scenario is not necessarily superior conductivity. Rather, its digital pattern generation could enable rapid variation of conductor widths, electrode spacing, line routing, and contact-pad geometry without making a new physical screen or printing plate for every design. A single experimental roll could, in principle, contain multiple controlled test designs if the ink, press configuration, and film were independently qualified.

That operating flexibility is valuable only if the electrical and mechanical requirements are met. Fast artwork changes cannot compensate for unstable line resistance, poor adhesion, weak electrical contacts, or cracks introduced by repeated bending. LEP's proposed role must therefore be evaluated on an end-to-end engineering basis, not on digital flexibility alone.

Substrate Compatibility: Adhesion Is Not the Same as Electrical Performance

A crucial challenge in functional printing is the interface between the deposited material and the substrate. PET can serve as a practical reference film in controlled laboratory trials because of its relatively useful dimensional characteristics. Yet a sensor intended to stretch, conform, or flex repeatedly places substantially different demands on the assembly.

In industrial LEP, surface treatment such as corona discharge or a primer may be used to improve ink wetting or adhesion in a qualified configuration. But better adhesion does not automatically produce better electrical performance. A primer can alter the mechanical interface, local strain transfer, or layer contact. For electrically interconnected layers, interface design may also influence contact resistance.

A tape test can provide information about aspects of adhesion, but it cannot replace resistance measurements before and after mechanical loading. Likewise, surface-energy measurements commonly expressed in dyne/cm can help characterize surface conditions but do not directly qualify conductivity. A competent functional-printing study must examine surface condition, adhesion, line continuity, contact resistance, and mechanical durability together. Improvement in one metric does not prove improvement in all the others.

What Must Be Measured Before Industrial Production Is Credible?

Transitioning from a proof of concept to a functional product requires quality-control methods that extend beyond graphic print inspection. Electrical line resistance is a starting point. You must also measure and document trace width, length, and deposited thickness. For suitable uniform layers, sheet resistance in ohms per square may provide useful information, while narrow tracks need methods appropriate to their specific geometry.

Contact resistance must be separated, where practical, from the resistance of the printed material itself. Four-point probe or four-wire measurement methods may be appropriate, depending on sample geometry and the question being tested. A poorly controlled probe contact can create an apparently unstable trace even when the underlying film is relatively consistent.

A pressure-sensitive construction must also be tested through both loading and unloading. Hysteresis is the difference in response that depends on the load history. Creep describes a time-dependent response under sustained loading; baseline drift is a longer-term displacement of a reference reading. These terms are not interchangeable, and the source of the observed behavior may lie in the sensitive composite, contact interface, substrate, enclosure, or measurement fixture.

Bend testing should reflect the intended application. Radius, direction, number of cycles, rate, environmental conditions, and applied mechanical stress need to be defined. Electrical continuity after a few manual bends does not prove useful long-term durability. Humidity, temperature, protective-film effects, connection stability, and sample-to-sample reproducibility also matter.

Encapsulation illustrates why device-level assessment is necessary. A protective layer may reduce surface damage or environmental exposure but can also change how pressure reaches the sensing region. The protected assembly may require its own calibration and performance limits. Ultimately, the finished stack—not merely each isolated layer—must satisfy the defined acceptance criteria.

Standards: Good Graphic Printing Is Not Electronic Product Qualification

The IEC 62899 series provides an important technical framework for printed electronics. IEC 62899-202:2023 defines characterization and evaluation methods for conductive inks and layers. Its 2023 edition also addresses mechanical testing of conductive layers. IEC 62899-401:2025 describes the structure and scope of printability evaluation, including the quality and reproducibility of printed patterns and the role of interactions among printing process, ink, substrate, and environmental conditions. [7, 8]

These documents help define what should be measured, but performing one method from a standard is not the same as qualifying an entire device. Application-specific requirements will depend on the intended use, environment, manufacturing process, and regulatory obligations. Devices intended for medical applications may require additional risk management, biological evaluation, usability engineering, and other assessments appropriate to their defined purpose.

A sensor that measures grip pressure and produces a visual or vibration cue does not, by that fact, restore damaged neural sensation or demonstrate clinical effectiveness. Laboratory electrical tests must not be presented as medical evidence.

Where Could LEP Development Go Next?

For LEP to become more relevant to industrial manufacture of functional electronics, several connected problems must be solved. Conductive formulations must be chargeable and developable without undermining electrostatic imaging, yet form a dependable conducting network after deposition and transfer. Film thickness and uniformity must be controlled without sacrificing flexibility. Thermal histories must be compatible with the chosen polymer substrate, and repeated mechanical deformation must not destroy the current-carrying pathways.

Web-fed experiments also require control of machine-direction and cross-direction behavior, web tension, registration, dimensional stability, and thermal deformation. Functional quality inspection would need to incorporate resistance and continuity measurements, potentially alongside conventional optical inspection. Batch-to-batch reproducibility, storage stability, operator safety, and contamination risks would have to be documented before production deployment.

A final question is economic and strategic: where would LEP outperform established alternatives? For simple conductors manufactured in very long runs, screen printing may remain the more straightforward choice. For short runs, complex geometries, or research programs that demand frequent pattern changes, LEP's digital flexibility might offer an advantage. Those opportunities are hypotheses to be tested—not inherent performance advantages established by the current literature.

Conclusion: The Printed Image Is No Longer the Only Product

Printed electronics is gradually expanding the definition of a printed product. A deposited layer can become a conductive path, an electrode, or a material structure whose electrical behavior responds to the environment. Peer-reviewed research has already shown that functional, flexible pressure-sensitive devices can be fabricated through printing and that their response and durability can be experimentally measured and improved.

Published work on LEP, particularly the two European patent disclosures discussed here, points to a credible technological route for producing conductive patterns with specialized liquid-electrophotographic materials. It does not yet establish that ordinary LEP presses can manufacture complete, qualified pressure sensors.

The nearer-term opportunity may lie in hybrid manufacturing: digitally generated conductor geometry where it provides measurable value, combined with other processes better suited to responsive layers and mechanical structures. That approach preserves each technology's strengths while subjecting the complete product to electrical, mechanical, environmental, and manufacturing validation.

The transition from graphic printing to functional printing is therefore much more than a change of ink. It changes how a product is designed, how quality is defined, what is measured, and what we expect a press to produce. When a printed layer can carry a stable electrical signal, the visual image is no longer the only possible outcome of printing. The printed structure can become part of a system that senses, measures, and responds.

Selected Scientific and Technical References

Wu, G. et al. (2024). "Research progress of screen-printed flexible pressure sensor." Sensors and Actuators A: Physical, 374, 115512. DOI: 10.1016/j.sna.2024.115512.
Shu, Q. et al. (2024). "Flexible resistive tactile pressure sensors." Journal of Materials Chemistry A, 12, 9296–9321. DOI: 10.1039/D3TA06976A.
Chen, H. et al. (2026). "Flexible and Tunable Piezoresistive Sensors Based on Graphite-Loaded CTBN-Toughened Epoxy Composites." ACS Omega, 11, 12687–12696. DOI: 10.1021/acsomega.5c13045.
Ervasti, H. et al. (2021). "Inkjet-Deposited Single-Wall Carbon Nanotube Micropatterns on Stretchable PDMS-Ag Substrate–Electrode Structures for Piezoresistive Strain Sensing." ACS Applied Materials & Interfaces, 13, 27284–27294. DOI: 10.1021/acsami.1c04397.
European patent publication EP3198342A1. "Liquid toner containing a low symmetry electrically conducting material for printing conductive traces."
European patent publication EP3411756A1. "Liquid electrophotographic inks."
IEC 62899-202:2023. "Printed electronics — Part 202: Materials — Conductive ink."
IEC 62899-401:2025. "Printed electronics — Part 401: Printability — Overview."
IEC 62899-202-4:2021. "Printed electronics — Materials — Conductive ink — Measurement methods for properties of stretchable printed layers."

Evidence note. Patent descriptions support feasibility under specified research conditions; they do not qualify production. Results from screen printing, inkjet, and hybrid sensors should not be attributed to LEP without direct testing.

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