The pace of technological obsolescence in electronic devices is staggering, giving rise to an increasingly severe e-waste problem. Conventional environmental monitoring sensors, particularly temperature and humidity sensors, typically rely on non-biodegradable 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, leading to environmental pollution.

Is it possible to create sensors that are both high-performance and environmentally friendly—perhaps even nearly “invisible”? A groundbreaking study recently published in Small Methods offers an exciting answer: by leveraging aerosol jet printing (AJP), researchers 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 the next generation of 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 release microplastic pollution.

Electrodes/sensitive materials: Often contain 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 accuracy are limited by the interplay between droplet volume and surface energy, making it equally challenging to overcome the miniaturization bottleneck. Moreover, excessively large feature sizes compromise device transparency.

Single Functionality and Complexity: Traditional temperature and humidity sensors often require diverse materials and intricate multilayer structures, which increase manufacturing complexity and recycling challenges.

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: This approach substantially reduces the environmental impact of sensor waste at the source, thereby addressing the long-term pollution caused by plastic substrates.

2. Core Material: Multifunctional PEDOT:PSS

PEDOT:PSS is a biocompatible, water-soluble, and semi-transparent 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)) component in PEDOT:PSS is hydrophilic and can absorb water vapor from the air. As ambient humidity changes, the material’s intrinsic electrical conductivity varies, leading to a corresponding change in its resistance. This resistance variation can be detected by an external circuit, enabling real-time humidity monitoring.

Temperature sensing: By incorporating crosslinking agents (such as GOPS) into PEDOT:PSS, the material’s temperature sensitivity can be enhanced. Temperature-induced subtle structural adjustments in the material result in resistance changes, thereby 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 sensor using only one “raw material.” The key difference lies in:

Humidity sensing: Uncrosslinked PEDOT:PSS can absorb moisture and respond to changes in humidity;

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

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

3. Aerosol Jet Printing (AJP)

Conventional fabrication techniques, such as printing and lithography, struggle to produce microstructures with feature sizes below 20 micrometers. The advent of aerosol jet printing (AJP) has fundamentally overcome this limitation.

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

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

Core advantages of aerosol jet printing (AJP) technology:

• Ultra-high resolution: capable of printing fine lines with a width of 13 μm—far superior to screen printing and inkjet printing. This is key to achieving device miniaturization and high transparency.
• Dramatic reduction in material consumption: Thanks to the capability for high-resolution printing, the volume of the sensor electrode is extremely small—approximately 5.84 × 10⁻⁷ mm³. Compared with conventional printing methods (screen printing and inkjet), material consumption is reduced by an astonishing 99.75% (see Figure 1b).
• Excellent material compatibility: It enables the printing of a wide range of functional materials—such as conductive, semiconductive, and dielectric materials—on diverse substrates, including flexible and fragile cellulose-based materials.
• 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 high-PSS-content AI4083—we identified the optimal formulation (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). The addition of 15% v/v ethylene glycol (EG) as a low-volatility co-solvent effectively maintains droplet wetting, significantly reduces overspray, and yields lines with smooth edges (see Figure 3).

Conductivity Optimization: The addition of EG also helps to enhance the conductivity of the PEDOT:PSS film, reaching approximately 88,548 S/m.

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

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

Finding the optimal FR: Too low an FR results in excessively wide lines, while too high an FR leads to blurred, indistinct lines. Under the system conditions of this study—ink type, nozzle, and printing speed—FR = 5 is the optimal setting, producing 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-responsive—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 the realization of a single ink formulation for two distinct sensor types.

Technical Advantages

1. Ultra-high transparency:

Thanks to the ultra-fine feature sizes achievable with aerosol jet printing (AJP)—with line widths <15 μm and film thicknesses of ~0.45 μm—and the inherently high optical transmittance of PEDOT:PSS (~85%), the entire sensor exhibits an optical transmittance exceeding 91% across the visible spectrum (400–800 nm).

Performance: The sensor is virtually “invisible” and difficult to detect with the naked eye (see Fig. 1e). This is crucial for integration into smart windows, transparent displays, or wearable devices where aesthetic appeal is essential.

2. Excellent humidity-sensing performance (GOPS-free):

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

Fast response: Response time (humidity increase) of 1.66 seconds, recovery time (humidity decrease) of 1.55 seconds (for respiratory monitoring).

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

Excellent cycling performance: After 6 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) over 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 Figures 5e and 5f).

This 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 sensors to N95 and FFP2 masks, respiratory rate and airflow changes can be monitored in real time.

Skin patches: Miniature, invisible sensors can be integrated into skin-adhesive patches to measure surface body temperature and humidity, thereby supporting health diagnostics.

2. Smart Wearable Devices

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

Fatigue Alert: By analyzing temperature and humidity data, this system can detect physiological abnormalities at an early stage to help prevent exercise-related injuries.

3. Smart Home and Environmental Monitoring

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

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

4. Green and environmentally friendly

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

Avoid the accumulation of electronic waste: By deploying “use-and-throw-away” microsensors, electronic waste can be significantly reduced.

Illustrated Guide

Figure 1. a) Schematic diagram 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 then deposited onto the substrate through the nozzle, with automated stages generating the desired pattern. b) Relationship between electrode volume and sensor area for a 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 and the cross-linking 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: the PSS formulation at 45% RH for PH 100 and ai4083. d) The variation of relative resistance with humidity for PEDOT:PSS aerosol jet–printed (AJP) sensors on cellulose membranes at different GOPS concentrations.

Figure 3. Effect of EG on Printed Lines in Ink Formulation 2

Figure 4. Optical micrographs 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 focus 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 for six cycles of a PEDOT:PSS humidity sensor printed on a cellulose substrate over the relative humidity range of 10–50%. c) Temperature dependence of the relative resistance of PEDOT:PSS-printed sensors on cellulose films at different concentrations of GOPS. d) Cyclic response of a temperature sensor fabricated by printing 5% GOPS on a cellulose substrate. e) Responses of a P sensor printed on a cellulose substrate during human exhalation and inhalation at different respiratory rates. f) An enlarged view of the highlighted curve in Figure 5e, showing the response and recovery times at a normal respiratory 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, and high‑performance temperature–humidity sensors. This breakthrough not only addresses the environmental challenges associated with materials and manufacturing in conventional sensors but, thanks to its miniaturization, high transparency, and multifunctional integration, also delivers revolutionary solutions for smart packaging, precision agriculture, wearable health monitoring, and intelligent building applications.

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