The pace of technological obsolescence in electronic devices is staggering, and the resulting e-waste problem is becoming increasingly severe. Conventional environmental monitoring sensors, particularly those for temperature and humidity, typically rely on non‑degradable plastic substrates—such as PET and PE—and toxic metal materials. These components are not only difficult to recycle but also release harmful microplastic particles during degradation, contaminating the environment.

Is it possible to create sensors that are both high‑performance and environmentally friendly—perhaps even nearly “invisible”? A recent study published in Small Methods offers an exciting answer: by leveraging aerosol jet printing (AJP), scientists have successfully fabricated a highly transparent, ultra‑compact, and ultra‑low‑material‑consumption temperature–humidity sensor on a biodegradable cellulose substrate. This could herald the dawn of next‑generation green electronics.

Background

1. The materials are not environmentally friendly:

Substrates: Commonly used plastic substrates such as polyimide (PI), polypropylene (PP), and polyethylene terephthalate (PET) can take hundreds of years to degrade in soil and may contribute to microplastic pollution.

Electrode/sensing material: Often contains precious or toxic metals, making recycling and disposal complex and increasing the environmental burden.

2. Manufacturing Process Limitations:

Screen printing: Limited resolution (typically above 50–100 μm), making device miniaturization challenging.

Inkjet printing: Resolution (20–100 μm) and precision are constrained by the interplay between droplet volume and surface energy, making it equally difficult to overcome the miniaturization bottleneck. Moreover, excessively large feature sizes compromise device transparency.

Single-functionality and Complexity: Conventional temperature and humidity sensors typically require diverse materials and intricate multilayer architectures, which increase manufacturing complexity and pose challenges for recycling.

Working principle

1. Green substrate: cellulose-based material

Cellulose diacetate film is used as the sensor substrate.

Advantages: Biodegradable (much faster than conventional plastics), derived from renewable resources, and environmentally sustainable.

Significance: It substantially reduces the environmental impact of discarded sensors at the source, addressing the long-term pollution issues associated with plastic substrates.

2. Core material: multifunctional PEDOT:PSS

PEDOT:PSS is a biocompatible, water-soluble, semitransparent conductive polymer composite. It consists of the conductive poly(3,4-ethylenedioxythiophene) (PEDOT) and the insulating polystyrene sulfonate (PSS).

Humidity sensing: The PSS (poly(styrene sulfonate)) segment in PEDOT:PSS is hydrophilic and can absorb water vapor from the air. When ambient humidity changes, the material’s internal electrical conductivity varies, leading to a corresponding change in its resistance. This variation can be detected by an external circuit, enabling humidity monitoring.

Temperature sensing: By incorporating a crosslinking agent (such as GOPS) into PEDOT:PSS, the material’s temperature sensitivity can be enhanced. Temperature-induced subtle structural adjustments in the material lead to changes in electrical resistance, enabling temperature monitoring.

Single-material, multifunctional sensing:

By leveraging the tunability of PEDOT:PSS, the research team was able to integrate both humidity and temperature sensing into a single device using only one “raw material.” The key difference lies in:

Humidity sensing: Un-crosslinked PEDOT:PSS can absorb moisture and respond to changes in humidity.

Temperature sensing: Cross-linked PEDOT:PSS (with the addition of GOPS) is resistant to moisture interference and primarily responds to temperature changes.

In this way, not only is the material system simplified, but manufacturing costs and complexity are also reduced.

3. Aerosol Jet Printing (AJP)

Traditional manufacturing processes, such as printing and photolithography, struggle to fabricate microstructures smaller than 20 micrometers. The advent of aerosol jet printing (AJP) technology has fundamentally overcome this limitation.

Aerosol Jet Printing (AJP) workflow (see Figure 1a):

• Atomization: The PEDOT:PSS ink—typically water-based—is atomized into micron-sized liquid droplet aerosols using either ultrasonic or pneumatic methods (in this study, ultrasonic atomization was chosen to achieve higher resolution).
• Transport: An inert carrier gas, such as nitrogen, is used to convey the aerosol to the print head.
• Focusing: At the print head, a high‑velocity sheath gas—typically an inert gas such as nitrogen—is introduced to compress the aerosol stream into an extremely fine jet, much like a focusing lens.
• Deposition: The focused aerosol jet is ejected from a nozzle—100 μm in diameter in this study—and deposited onto a substrate (glass or cellulose membrane) in a non-contact manner, at a nozzle–substrate distance of 2–5 mm, thereby forming a fine pattern. The substrate is mounted on a programmable motion stage.

The core advantages of aerosol jet printing (AJP) technology are:

• Ultra-high resolution: capable of printing fine lines as narrow as 13 μm—far surpassing both screen printing and inkjet printing. This is key to achieving device miniaturization and high transparency.
• Significant reduction in material consumption: Thanks to its high‑resolution printing capability, the sensor’s electrode volume is extremely small—approximately 5.84 × 10⁻⁷ mm³. Compared with conventional printing methods (screen printing and inkjet), material usage is reduced by an astonishing 99.75%! (See Figure 1b)
• Excellent material compatibility: It enables the printing of a wide range of functional materials—including conductive, semiconductive, and dielectric materials—on various substrates, including flexible and fragile cellulose-based ones.
• Non-contact: Avoids damage to flexible or fragile substrates.
• Scalability: Supports multi-nozzle parallel printing, with the potential for large-scale production.

Technological breakthrough

To fully leverage the advantages of aerosol jet printing (AJP) and PEDOT:PSS, the research team addressed several key challenges:

1. Optimize the ink formulation:

Enhanced Humidity Sensitivity: By blending different commercial PEDOT:PSS dispersions—high-conductivity PH1000 and AI4083 with a high PSS content—we identified the optimal ratio (PH1000:AI4083 = 90:10 v/v), achieving high sensitivity (12.16%/RH) across a relative humidity range of 10–80%.

Suppression of Overspray: Pure water-based PEDOT:PSS ink tends to dry and form overspray during aerosol jet printing (AJP). Adding 15% v/v ethylene glycol (EG) as a low‑volatility co‑solvent effectively maintains droplet wetness, significantly reducing overspray and yielding lines with smooth edges (see Figure 3).

Conductivity Optimization: The incorporation of EG also enhances the conductivity of PEDOT:PSS films, reaching approximately 88,548 S/m.

2. Precise control of aerosol jet printing (AJP) parameters:

The focusing ratio (FR) is critical: FR = sheath gas flow rate / carrier gas flow rate. Studies have shown that increasing the FR can effectively reduce the line width (see Figure 4).

Finding the optimal setting: An FR that is too low results in excessively wide lines, while an FR that is too high leads to blurred, indistinct lines. Under the system conditions of this study—ink type, nozzle, and printing speed—the optimal FR value is 5, yielding uniform, fine lines with minimal splattering.

Gas flow control: The carrier gas flow rate must be sufficient (≥10 SCCM) to ensure continuous deposition, but excessive flow (>20 SCCM) can cause line bulging. The sheath gas flow rate should be optimized in conjunction with the FR.

3. “One-Click Switch” Temperature and Humidity Function

Sensors printed with PEDOT:PSS ink doped with GOPS exhibit resistance changes that are primarily temperature‑dependent—performing well in the 20–50 °C range, with a temperature coefficient of resistance (TCR) of −1.7 × 10⁻³ /°C)—and are virtually insensitive to humidity. In contrast, sensors printed with undoped ink respond mainly to humidity. This approach enables a single ink formulation to yield two distinct types of sensors.

Technological Advantages

1. Ultra-high transparency:

Thanks to the ultra-fine lines produced by aerosol jet printing (AJP)—with widths less than 15 μm and thicknesses around 0.45 μm—and the inherently high transmittance of PEDOT:PSS (~85%), the entire sensor achieves a light transmittance exceeding 91% in the visible spectrum (400–800 nm).

Effect: The sensor is virtually “invisible,” barely perceptible to the naked eye (see Fig. 1e). This is crucial for integration into smart windows, transparent displays, or wearable devices where aesthetics are paramount.

2. Excellent humidity-sensing performance (without GOPS):

High sensitivity: Achieves 12.16%/RH on a cellulose substrate (10–80% RH).

Fast response: Response time (when humidity rises) is 1.66 seconds, and recovery time (when humidity falls) is 1.55 seconds (for respiratory monitoring).

Good stability: Performance remains stable for up to 8 days; after 6 months, although a slight drift is observed—likely due to silver electrode corrosion—the device still maintains a linear response.

Good cycling performance: After six cycles between 10% and 50% RH, the response remains stable and reliable.

3. Practical temperature-sensing performance (including GOPS):

It exhibits a stable resistance–temperature relationship (negative temperature coefficient) within the range of 20–50°C.

Good cycle repeatability.

4. Real-world Application Demonstration: Respiratory Monitoring

Attach the humidity sensor to the inner surface of the FFP2 mask.

Successful real-time monitoring of the periodic humidity variations induced by the subject’s normal breathing and rapid breathing (simulating an exercise state) was achieved (see Fig. 5e, f).

It demonstrates its potential for immediate application in wearable health monitoring, such as respiratory rate monitoring.

Scope of Application

1. Personal Health Monitoring

Respiratory monitoring: By attaching the sensor to N95 or FFP2 masks, it can continuously track respiratory rate and airflow changes in real time.

Skin patches: Miniature, invisible sensors can be integrated into adhesive patches to measure skin surface temperature and humidity, aiding in health diagnostics.

2. Smart Wearable Devices

Activity monitoring: Sensors are integrated into sportswear and wristbands to track ambient temperature and humidity, helping users adjust their exercise intensity.

Fatigue Alert: By analyzing temperature and humidity data, it detects early signs of physiological abnormalities to help prevent sports injuries.

3. Smart Home and Environmental Monitoring

Air Quality: Enable micro‑scale, distributed humidity and temperature monitoring to precisely enhance air‑conditioning efficiency.

Warehouse Monitoring: Tracks the storage conditions of perishable goods to ensure product quality and minimize waste.

4. Green and environmentally friendly

Low environmental footprint: Utilizing biodegradable cellulose-based substrates and generating virtually no hazardous waste during manufacturing, it advances the development of plastic-free, biodegradable sensors.

Avoid the accumulation of electronic waste: By using “use-and-throw-away” micro-sensors, electronic waste is significantly reduced.

Image and Text Guide

Figure 1. a) Schematic of the aerosol jet printing (AJP) process using an ultrasonic atomizer. The formulated ink is subjected to ultrasonic atomization under the pressure of an inert gas (N2). The resulting aerosol is conveyed by a carrier gas to the deposition head, where it is focused and accelerated by an additional inert sheath gas. The jet is deposited onto the substrate through a nozzle, with automated stages generating the desired pattern. b) Relationship between electrode volume and sensor area for the reported printed humidity sensor. c) Comparative plot of the number of years required for the degradation of petroleum-based and bio-based plastics under simulated landfill conditions (industrial composting). d) Conceptual schematic of the temperature–humidity monitoring sensor platform. e) Images of the sensor on glass and cellulose substrates, demonstrating the high transparency of the disposable sensor. Scale bar: 2000 μm.

Figure 2. a) The sensing mechanism of PEDOT, illustrating the swelling behavior of the humidity sensor during water absorption, as well as the crosslinking between PSS and GOPS that enhances the temperature sensitivity of the temperature sensor. b) The relationship between relative resistance and relative humidity for a PEDOT:PSS humidity sensor printed on a glass slide (n = 4). c) Electrical conductivity as a function of PEDOT: PH 100 and ai4083 in a PSS formulation at 45% RH. d) Relative resistance of PEDOT:PSS aerosol jet‑printed (AJP) sensors on cellulose membranes as a function of humidity, under varying GOPS concentrations.

Figure 3. Effect of EG on printed lines in Ink Formulation 2

Figure 4. Optical microscopy images of printed lines on a glass slide, illustrating the effects of sheath gas and carrier gas flow rates on line quality. Lines with the same focal ratio are grouped together. Scale bar: 50 μm.

Figure 5. a) Relative resistance of PEDOT:PSS humidity sensors (n = 4) on glass and cellulose substrates, plotted as a function of relative humidity at 25°C. b) Response–recovery curves of PEDOT:PSS humidity sensors printed on a cellulose substrate over six cycles within the 10–50% RH range. c) Temperature dependence of the relative resistance of PEDOT:PSS‑printed sensors on a cellulose membrane for different concentrations of GOPS. d) Cyclic response of a temperature sensor fabricated by printing 5% GOPS onto a cellulose substrate. e) Responses of a P‑type sensor printed on a cellulose substrate during human exhalation and inhalation at varying respiratory rates. f) An enlarged view of the highlighted curve in Figure 5e, showing the response and recovery times at a normal breathing rate.

Aerosol jet printing (AJP) technology, combined with biodegradable cellulose substrates and the remarkable polymer material PEDOT:PSS, has successfully enabled the fabrication of near‑“invisible,” environmentally friendly, high‑performance temperature and humidity sensors. This breakthrough not only addresses the environmental challenges associated with conventional sensors—both in materials and manufacturing—but, thanks to its miniaturization, high transparency, and multifunctional integration, it offers revolutionary solutions for smart packaging, precision agriculture, wearable health monitoring, and intelligent buildings.

It shows us a clear path: high-performance electronic devices can indeed coexist harmoniously with the planet’s sustainable development. When “green” and “smart” are seamlessly integrated, a future world built on environmentally friendly, invisible, and ubiquitous sensors is rapidly approaching. Aerosol Jet Printing (AJP) is undoubtedly the key that illuminates this vision of the future!