Sensors You Can Print: What "solution-processed" actually means
Author: Kimia Shahbazi, Digital Chemist Lead, Sentia Technologies Inc.
Most of the electronics around you were built the hard way. A silicon chip starts as a polished wafer that goes into a vacuum chamber, gets coated, baked, masked, and etched — layer after layer, at high temperatures, in a cleanroom that costs more than most factories. It works beautifully. It's also slow, expensive, rigid, and wasteful.
There's another way to make a sensor: start with a liquid.
That's solution processing in one sentence. You take the material that does the sensing, mix it into a liquid until it becomes a kind of functional ink, spread that ink onto a surface, and let it dry. What's left behind is a thin solid film — and that film is your sensor. Figure 1 shows the spin coating process.
Figure 1 — Spin coating, one of the simplest solution-processing methods: ink is dispensed onto the substrate, which spins to spread it into a uniform thin film that is then dried.
Printing techniques
Screen printing pushes a thick, paste-like ink through a stencil mesh with a squeegee (Figure 2). It lays down robust layers and is currently the most widely used process in printed-electronics production.
Figure 2 — Screen printing: a squeegee forces ink through a patterned mesh stencil onto the substrate below.
And here's what actually comes out of it (Figure 3)— a complete, working circuit printed straight onto a thin sheet of flexible plastic. The durability number here is the one that surprises people: after 10,000 bending cycles, the resistance of those printed lines rises by only about 1.8%.
Figure 3 — A printed flexible circuit: fine conductive tracks laid straight onto a thin, transparent, bendable plastic film.
Inkjet printing fires tiny droplets from a nozzle — your office printer, but with functional ink. Nothing touches the surface, and the pattern is fully digital, so you can change the design without cutting a new stencil. The process is shown in Figure 4.
Figure 4 — Inkjet printing: a piezoelectric actuator or heater ejects tiny droplets of functional ink on demand, with the nozzle never touching the substrate.
Gravure and flexographic printing are the industrial heavyweights — the techniques that print magazines and food packaging, borrowed for electronics. They run on rollers, which means they run fast: flexographic presses move material at up to 100 metres per minute. That's what "roll-to-roll" means. The substrate unwinds off a spool, gets printed as it travels, and winds back up at the other end (Figure 5). This is the step from making sensors one at a time to making them by the kilometre.
Figure 5 — Roll-to-roll manufacturing: the substrate runs off a giant spool, gets printed as it travels, and winds back up at the far end.
Real sensors, real measurements
The clearest way to show what solution processing can do is to look at published devices measuring the things that matter most in industrial and environmental monitoring.
Temperature
Temperature sensing is the most familiar kind of all — the same job a household thermometer does, now printed onto a flexible film about the size of a postage stamp (Figure 6).
Figure 6 — A printed temperature sensor: a fine resistive trace laid onto a thin, flexible film about the size of a postage stamp.
It works across −60 °C to 180 °C, responds in under 10 seconds, and stays stable and repeatable across that whole range.
Pressure
A touch-and-force sensor works a bit like the screen you may be reading this on — but printed as a thin sandwich: a pressure-sensitive polymer layer between two printed electrodes (Figure 7).
Figure 7 — A printed pressure sensor is a thin sandwich: a pressure-sensitive polymer layer between two printed electrodes on a flexible substrate.
The PET version produces 1.2 volts per newton of applied force; the paper version 0.3 V/N. Same design, different substrate, four times the sensitivity — a good illustration of how much the surface underneath matters.
Humidity
The humidity sensor in Figure 8 shows the whole idea in miniature: silver interdigitated electrodes are printed onto a plastic film, then a sensing ink is coated over the top and dried in an ordinary oven at 60 °C. Like every printed sensor, it begins life as a liquid.
Figure 8 — The printed humidity sensor: silver comb-shaped electrodes on a plastic film, with a carbon-nanotube sensing ink coated over the top.
The result is a flexible sensor whose resistance climbs by more than 500% between dry and near-saturated air: No vacuum equipment. No cleanroom. Water and alcohol as the main solvents.
Why go to the trouble?
It's cheap and it scales. Printing is additive — material goes only where you want it, instead of coating everything and etching most of it away.
It works on soft, cheap surfaces. Everything happens at low temperature, so you can build directly onto plastic, paper, or textiles. The humidity sensor in Figure 8 cures at 60 °C — an oven temperature, not a furnace temperature. It's the difference between a sensor that must sit on a rigid ceramic chip and one that can wrap around a pipe or curve along a greenhouse gutter.
It covers big areas that would be prohibitively expensive to fill with conventional silicon.
It's tunable. Change the ink, change the sensor. The same line can produce humidity, temperature, or pressure sensing depending on what you dissolve into the liquid.
The honest part: it isn't easy
Getting a liquid to dry into a perfectly even film is genuinely hard. Spill coffee on a table and you'll see the problem — as it dries, particles pile up at the edge and leave a ring instead of a uniform disc. The same physics fights every printed film (Figure 9). The bigger challenge is consistency — getting every drop to dry down the same way, every time.
Figure 9 — The coffee-ring effect: as a drop dries, particles drift to the rim and pile up there instead of spreading evenly. The same physics fights every printed film.
The same uneven piling-up happens at microscopic scale inside a printed film. When the sensing particles don't spread evenly through the polymer, sensors made from the same batch behave differently from one another. Across the field as a whole, device-to-device and batch-to-batch variation remains the main obstacle to reliable printed electronics.
This is why solution processing isn't really a printing problem. It's a materials problem, a chemistry problem, and a process-control problem at the same time — and the companies that solve it are the ones treating the ink, the surface it lands on, and the finished device as a single design.
Where this leaves us
Solution processing makes sensing cheap enough and flexible enough to put everywhere — on curved surfaces, across large areas, in places where a rigid sensor doesn't fit or doesn't pay for itself. At Sentia, this is the foundation of how we build. Our printed and solution-processed sensing platforms are designed alongside the inks that go into them, so what comes off the line is not just a working device but a consistent one — producing data steady enough to trust, and to feed straight into automated monitoring and control.
References
Maddipatla, D., Narakathu, B. B., & Atashbar, M. (2020). Recent Progress in Manufacturing Techniques of Printed and Flexible Sensors: A Review. Biosensors, 10(12), 199. https://doi.org/10.3390/bios10120199
Santos, A. R., & Viana, J. C. (2024). The Development of a Flexible Humidity Sensor Using MWCNT/PVA Thin Films. Nanomaterials, 14(20), 1653. https://doi.org/10.3390/nano14201653
Naderi, P., & Grau, G. (2024). Inkjet-printed transistors with coffee ring aligned carbon nanotubes. Flexible and Printed Electronics, 9(1), 015001. https://doi.org/10.1088/2058-8585/ad1ccf
Naderi, P., Sheuten, B. R., Amirfazli, A., & Grau, G. (2023). Inkjet printing on hydrophobic surfaces: Controlled pattern formation using sequential drying. The Journal of Chemical Physics, 159(2). https://doi.org/10.1063/5.0149663
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