Printed Recyclable and Flexible Thermocouple Temperature Sensors

Abstract

Temperature sensors play a pivotal role in modern electronics, finding use across a broad spectrum of applications. Nonetheless, traditional manufacturing methods for these devices consume substantial energy and materials, and their widespread utilization often contributes to substantial electronic waste, presenting significant environmental concerns. In this research, recyclable printed thermocouple temperature sensors are developed that emphasize both cost‐efficiency and ecological responsibility. The sensors utilize readily available fillers (i.e., nickel flakes and carbon black powders), paving the way for scalable production. By incorporating re‐dissolvable polymers as binders, the end‐of‐life sensors can be easily disassembled, eliminating the need for harsh treatment or hazardous chemicals. The use of ferromagnetic nickel flakes enhances the straightforward separation of different filler components, streamlining the recycling workflow. Importantly, the gentle recycling conditions preserve the functional fillers, preventing degradation or oxidation and thus enabling the reprocessed sensors to retain their original performance. In addition, the sensors boast high mechanical flexibility, making them suitable for seamless integration into various practical scenarios. All these innovations not only reduce economic costs but also align with the goals of sustainable development, demonstrating a promising pathway for the future of temperature sensing technology.

Introduction

Temperature sensors are essential components in both daily life and industrial production, with widespread applications in industrial machinery, home appliances, environmental monitoring systems, and medical devices. They not only form the foundation for equipment operation and process control but also play a crucial role in environmental protection and safety monitoring.^[1–5] However, traditional temperature sensors face several challenges, such as being bulky and rigid.^[6] More critically, their manufacturing processes often rely on energy-intensive and costly techniques, such as precision machining and assembly processes. These production methods not only significantly increase manufacturing costs but also lead to substantial resource and energy consumption, exacerbating environmental pressures. As the concept of sustainable development gains increasing importance, the limitations of traditional temperature sensors have become more evident, creating an urgent need for innovative materials and processes that can address these issues and meet the growing demand for environmentally friendly and flexible technologies in modern industries and daily life.^[7–11]

Printed flexible temperature sensors have garnered significant attention for their ability to overcome the limitations of traditional temperature sensors through simple and adaptable manufacturing processes.^[12–16] Utilizing printing techniques such as inkjet printing and screen printing, combined with roll-to-roll processing,^[17–19] these sensors can be produced rapidly and cost-effectively on a variety of substrates, including plastics,^[20,21] paper,^[22–24] and textiles,^[25,26] imparting mechanical flexibility to the printed sensors.^[27–29] Functional materials have also been extended to include metals,^[30,31] metal oxides,^[32,33] carbon,^[34,35] conductive polymers,^[36,37] and composites.^[38,39] Thanks to additive manufacturing methods, material efficiency has been greatly improved, avoiding the significant material waste typically associated with traditional subtractive manufacturing processes and thereby reducing production costs.^[40,41] These innovations, offering a combination of mechanical flexibility, cost efficiency, and an expanded range of materials, have significantly broadened the application scope of temperature sensors, laying the groundwork for their widespread adoption in the era of the Internet of Things.^[42–45] However, the large-scale deployment of temperature sensors inevitably leads to considerable electronic waste at the end of their lifecycle, posing severe environmental challenges.^[46–48] Recyclable electronics present an effective solution to this issue, but traditional recycling methods often rely on harmful chemicals or harsh treatments, causing secondary environmental damage. Additionally, temperature sensors typically incorporate multiple functional materials (e.g., for fabricating the sensing element or conductive electrodes), raising the challenge of efficiently recovering and separating these materials for future reprocessing.^[49–52] Therefore, it is imperative to develop an effective method to address the above challenges faced in the field of printed temperature sensors, aimed at minimizing their environmental footprint.

In this study, we realized recyclable printed thermocouple temperature sensors that offer distinct advantages in cost-effectiveness and environmental sustainability. A comparison of similar sensors is presented in Supplementary Table S1 (Supporting information). To form printable inks, polymethyl methacrylate (PMMA) was selected as the polymer binder for its ease of processing and biocompatibility properties.^[53,54] Nickel flakes and carbon black, were incorporated as functional fillers to form the active elements of the thermocouple. The fillers are of low cost and commercially available, fostering large-scale fabrication. With the mixed inks, we fabricated a 6×6 temperature sensor array. Our sensors showed high flexibility, enabling operational stability in diverse application scenarios where deformation is required. Even after 1000 bending cycles, the sensors maintained their temperature-sensing performance with minimal degradation. At the end of its life cycle, the re-dissolvable nature of PMMA facilitates the easy decomposition of the sensors in acetone, enabling the release of functional fillers from the printed composite.^[55] Due to the responsiveness of the ferromagnetic materials to magnetic fields, the Ni flakes are easily extracted from the dissolved solutions, providing a simple and reliable method for separating different fillers. Because this recycling process is performed at room temperatures, which effectively mitigates the surface oxidation of metallic fillers, the reprocessed sensor reveals minimal degradation in sensing performance. The low-cost environment-friendly flexible temperature sensors exhibit promising potential in a wide spectrum of applications, e.g., for real-time monitoring of electric vehicle batteries and energy management of smart buildings.

Results and Discussion

Fabrication of Printed Temperature Sensors

The printing process of the temperature sensors is illustrated in Figure 1a_I–a_V. To prepare the printable inks, polymethyl methacrylate (PMMA) is used as the binder. Other polymers, such as polyepichlorohydrin (PECH), were also explored; however, their thermal stability was inferior to that of PMMA (Figure S1, Supporting information). After completely dissolving PMMA in acetone, carbon black, and Ni flakes are separately added to the PMMA solution, formulating two types of functional inks. Carbon black exhibits an irregular granular morphology (Figure S2a, Supporting information), with most particle diameters below 10 µm (Figure S2b, Supporting information), while the Ni flakes have a flat, platelet-like structure (Figure S2c, Supporting information), with diameters predominantly ranging from 20 to 60 µm (Figure S2d, Supporting information). Proper mixing of the inks is critical to achieve uniform filler distribution, thereby enhancing pattern quality and ensuring consistent sensor performance during printing. The PET film, sized equivalent to an A4 sheet (scalable to larger dimensions as required) was chosen as the hosting substrate (Figure 1a_I). The fabrication begins with printing the pattern using the PMMA-Ni flake ink (Figure 1a_II), followed by printing another functional layer with the PMMA-carbon black ink (Figure 1a_III). After printing, the solvent evaporates, causing a gradual volume shrinkage of the printed composite. This shrinkage promotes the formation of conductive pathways within the composite, thanks to the rearrangement of the fillers (Figure 1a_IV). The extensive compatibility of the ink formulation and the printing process highlights the tunability of the sensor in the printing processes. Scanning electron microscope (SEM) images of the printed composites indicate that the fillers are uniformly distributed in both functional segments, forming continuous conductive networks (Figure 1c,d). These robust filler networks are instrumental in improving the operational stability of the sensor. Figure 1e shows that the sensor maintains its structural integrity even under bending, with no delamination from the substrate or damage to the printed traces, demonstrating exceptional mechanical flexibility. This robustness is critical for flexible application scenarios where frequent bending may be required. Figure 1f highlights the temperature-sensing capability, as its output voltage is easily measurable with a multimeter, confirming reliable functional performance for practical use. The output voltage variation after multiple bending cycles at different temperatures is summarized in Figure 1g. Even after 1000 bending cycles (Figure S3, Supporting information), the output voltage of the sensor, measured at different temperatures, remains stable, which underscores the sensor's operational durability and reliability in flexible applications. The temperature sensor developed in this study offers significant cost advantages, as its fabrication process avoids complex procedures and expensive equipment. The use of low-cost fillers and binders further enhances the economic feasibility of the technology. Moreover, the printing process supports customizable patterned designs (Figure S4, Supporting information), enabling the sensors to meet diverse application needs.^[56,57]

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Thermoelectric Performance of Printed Temperature Sensors

The thermoelectric (TE) performance of the thermocouple temperature sensors is systematically analyzed in Figure 2, which illustrates the variation of key TE parameters as the temperature gradient (ΔT) increases from 0 to 80 K. In Figure 2a, carbon black exhibits a linear increase in V_out with the rising ΔT, reaching ≈3 mV at ΔT = 80 K. In contrast, Ni flakes show an opposite trend, with V_out reaching ≈−2.5 mV at the same temperature gradient. Their linear response behaviors facilitate straightforward characterization, making it advantageous for sensor applications. The Seebeck coefficient of carbon black remains stable at ≈40 µV K⁻¹ across the entire ΔT range, and Ni flake maintains a negative coefficient of ≈−20 µV K⁻¹ (Figure 2b). The stability of carbon black's coefficient ensures a consistent sensitivity, making it well-suited for applications requiring reliable performance over varying temperatures. Both carbon black and Ni flakes demonstrate nearly constant electrical conductivities (Figure 2c), further contributing to a stable device performance in a wide temperature range. Intriguingly, the Ni-flake composite shows lower conductivity as compared to the carbon black composite, which might arise from the presence of insulating surface-oxide layers of Ni flakes. Collectively, these parameters offer valuable insights into the thermoelectric behavior of each material, serving as a foundation for the rational design of thermocouple temperature sensors.

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Simulation of the Thermocouple Temperature Sensor Arrays

For guiding sensor design, this study employed the Finite Element Method (FEM) to evaluate the performance of printed temperature sensors. Numerical simulations were conducted in the ANSYS Workbench environment, with thermoelectric module to simulate the heat conduction and thermal transfer processes within the sensor. In the simulation, the thermocouple's hot junction was set at a temperature of 102 °C, while the cold junction was maintained at 22 °C, creating a temperature difference of 80 K. This gradient was chosen to represent the sensor's potential operating conditions in real-world applications. To enhance the simulation accuracy, the key parameters of the thermoelectric materials, such as thermal conductivity, Seebeck coefficient, and electrical conductivity, were imported from experimental data. The simulation results demonstrate that the temperature sensor exhibits good thermoelectric conversion capability. Specifically, Figure 3a illustrates the temperature distribution on the sensor's surface, with a localized hotspot identified at the temperature sensor (4,4). The temperature gradient causes free electrons at the hot junction to diffuse toward the cold junction, where they accumulate on the surface, leading to the generation of thermoelectric voltage. Figure 3b shows the thermoelectric voltage generated at the hotspot pixel, validating the feasibility of the sensor design. The variation of V_out with ΔT appeared linear within a temperature range up to 80 K (Figure 3c). The calculated sensitivity was ≈52 µV K⁻¹. It is slightly higher than their experimental counterparts due to the thermal resistance across insulating substrates and suboptimal thermal contact.

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Large-Area Performance of the Printed Thermocouple Temperature Sensors

To evaluate the functional uniformity of the printed sensors, they were used to measure distinct temperature distributions over large areas. The sensors were placed on a heated surface with a uniform outward heat flux, while their measurement ends were exposed to ambient air. As the hot surface are 25 K above the room temperature (Figure 4a), the temperatures were recorded within ±1 K, indicating minimal spatial variation. At higher temperatures, broader temperature distributions were observed (Figure 4b,c). A 3D visualization of temperature deviations (Figure 4d) summarizes these discrepancies across the target temperatures. Such deviations are likely caused by incomplete local thermal contact. Material defects within the sensors may also affect the voltage output of individual sensors, contributing to measurement inaccuracies. To address this issue, enhancing the uniformity of the ink and the precision of the printing process can improve consistency among individual sensors, thereby increasing the accuracy of large-area temperature measurements. Notably, the output voltages of the sensor are determined solely by the temperature difference between the hot and cold junctions of the thermocouple, without being affected by heat conduction effects (Figure S5, Supporting information). This confirms that the sensor maintains high measurement accuracy and functional stability, even in the presence of a large temperature gradient.

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Recycling for Printed Temperature Sensors

The recycling of valuable materials in the printed temperature sensors (e.g., Ni flakes and carbon black) provides a sustainable solution to reduce electronic waste generation. Figure 5 proposes a recycling and reprocessing cycle. The practical step-by-step recycling process is realized in Figure 5a. Discarded sensors are first immersed in acetone (Figure 5aI), which dissolves the PMMA binder. As shown in Figure S6 (Supporting information), after 10 min, the sensor began to dissolve noticeably, with sediment forming at the bottom of the bottle. By gently shaking the bottle and periodically recording the decomposition process, we found that the sensor completely disintegrated within 4 h. The sensor structure releases the functional fillers, including Ni flakes and carbon black, into the acetone solution (Figure 5a_III). Due to the magnetic responsiveness of nickel flakes, they can be attracted by a magnet and effectively separated from the solution (Figure 5a_IV). To assess the material recovery rate, we printed 10 sensors and reclaimed their sensing materials during two recycling cycles. Supplementary Table S2 (Supporting information) records the amount of Ni flakes recovered from each recycling trial. To reduce material loss and maximize recovery efficiency, multiple acetone rinses were performed during the recycling process. Experimental results confirmed that ≈95% of Ni flakes can be recycled after two recycling cycles (Supplementary Table S2, Supporting information), thanks to the magnetic field assisted material collection. Notably, compared to ferromagnetic metals like iron, nickel exhibits superior oxidation resistance. As a result, the recovered nickel flakes can be directly repurposed for PMMA-Ni flake ink preparation without requiring additional treatments. Meanwhile, the remaining carbon black solution can be used to formulate PMMA-carbon black ink for subsequent thermocouple sensor printing. This recycling method is both efficient and straightforward, preventing the prolonged exposure of functional fillers to air and thereby minimizing potential oxidation issues. Experimental results confirm that the recycled functional fillers and the reprocessed sensors retain similar properties comparable to those of the original counterparts, demonstrating their stability and reusability (Figure 5b). To further evaluate the reliability of our recycling technique, we conducted two full recycling cycles. The reprocessed sensors were then systematically compared with the original ones in terms of their sensing performance (Figure 5c). The output voltages exhibited minimal variation after the recycling process, indicating that material degradation during recycling is negligible. It ensures the long-term stability and reusability of the recovered materials. These findings highlight the robustness of our recycling strategy and its potential for sustainable sensor fabrication without compromising performance. We note that the recycled Ni flakes and carbon black can be repurposed for the fabrication of various electronic devices, including but not limited to thermocouple sensors, thereby broadening their potential applications (Figure 5c). The implementation of recycling not only reduces the consumption of raw materials but also minimizes electronic waste emissions, thereby cutting costs and aligning with sustainable development principles.

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Conclusion

In this work, we have successfully developed printed recyclable thermocouple temperature sensors that offer significant advantages in terms of low cost, ease of processing, and environmental friendliness. By employing re-dissolvable polymers as binders, the sensors can be readily decomposed at the end of their life cycle, enabling recyclability without the need for harsh treatments or harmful chemicals. The incorporation of ferromagnetic Ni flakes facilitates the efficient separation of various fillers, thereby simplifying the recycling process. Thanks to the gentle treatment applied during recycling, the functional fillers were neither degraded nor oxidized, as confirmed by the consistent performance observed between the original and reprocessed sensors. Moreover, the performance of the sensors can be further enhanced, given that the developed method is highly compatible with diverse printing parameters, such as optimizing functional fillers, polymeric binders, and even solvents. With their high flexibility and potential for large-scale, cost-effective manufacturing, these sensors can be seamlessly integrated into a wide range of applications, minimizing both economic and environmental burdens and aligning with the principles of sustainable development.

Experimental Section

Printing Temperature Sensors

The chemicals used for the ink synthesis, including Ni flakes (325 mesh, ≥99.5% trace metals basis, Sigma-Aldrich), polymethyl methacrylate (PMMA, assay 98%), and acetone (assay ≥99.9%), were sourced from Sigma-Aldrich Co. LLC. Carbon black powder was from Cabot Corporation. Ni flakes and carbon black powders were employed as thermocouple fillers. The morphology and composition of these fillers were examined via scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis using the Phenom XL Desktop Scanning Electron Microscope (Thermo Fisher Scientific, USA). The polymeric binder solution was prepared by dissolving PMMA in acetone at a mass ratio of 1:9. This mixture was stirred on a magnetic stirrer hotplate at 60 °C for 24 h. PMMA, a flexible material with a tensile modulus of 3100 MPa and tensile strength of 48–76 MPa, maintains a wide operational temperature range (glass transition temperature of ≈140 °C) and exhibits low toxicity. In addition, PMMA is soluble in acetone and other organic solvents. These properties make it suitable for applications requiring flexibility, durability, and recyclability.

To create printable composites, Ni flakes were incorporated into the elastomeric binder at a volume fraction of 20%, and carbon black was added at a concentration of 1.5 mg mL⁻¹. The mixtures were stirred using a digital vortex mixer (VWR) at 2500 rpm for 60 s to ensure homogeneous dispersion of the functional fillers. After defining the temperature sensor patterns on the substrate using suitable shadow masks, the prepared inks were applied to the substrate. For large-area flexible applications, PET films were employed as substrates. The printed composites were cured at room temperature for 180 min.

Characterization of Temperature Sensors

The real-time temperature of the sensor was measured using a KEYSIGHT U5855A TrueIR Thermal Imager, and the corresponding real-time output voltage was recorded using a KEYSIGHT TECHNOLOGIES 34460A Digital Multimeter. For large-area temperature measurements, the printed sensors were heated using an MRC hotplate. The thermocouple-based temperature sensor operates on the principle of the thermoelectric effect, where a voltage (V_out) is generated at one end when the other junction of the thermocouple is heated (Figure S7, Supporting information). The V_out is directly proportional to the temperature difference at the junction. The sensitivity of a thermocouple sensor is determined by the voltage produced per unit temperature difference (ΔT), which corresponds to the difference in Seebeck coefficients (𝛼) between two junction materials (𝛼₁ − 𝛼₂). Their relationship can be expressed as: S = 𝛼₁ − 𝛼₂ = V_out/ΔT.^[]

Bending Stability and Large-area Measurement for Printed Temperature Sensors

V_out values were continuously monitored throughout the bending process, with changes recorded after 1000 bending cycles. This enabled a thorough evaluation of the effects of repeated bending on the performance and reliability of the temperature sensors. By analyzing these V_out variations, the sensors' durability under mechanical deformation was assessed, providing valuable insights into their robustness for flexible applications. For large-area temperature measurements, the sensors were securely mounted on a hot plate to ensure intimate contact. Once the hot plate reached the specified temperatures, the output voltage of each sensor was measured to evaluate their temperature response. The assessment was conducted systematically at three target temperature differences (i.e., ΔT = 25, 50, and 75 K), providing an understanding of the sensors' accuracy across a range of operating conditions. This approach was specifically designed to explore the effectiveness of the sensors in practical applications requiring uniform temperature monitoring over extended surfaces.

Acknowledgements

The authors appreciate fruitful discussions and experimental support from Mr. Xilai Bao (Ningbo Institute of Materials Technology and Engineering). X.W. and L.G. are grateful to the China Scholarship Council (CSC) for supporting their PhD projects. This work is supported in part via the ERC grant 3DmultiFerro (Project number: 101141331) and European Commission (project REGO; ID: 101070066).

Conflict of Interest

The authors declare no conflict of interest.

Author contributions

R.X. conceived the concept. X.W. and L.G. conducted the experiments. X.W., R.X., Q.Z., Y.Z. and D.M. analyzed data and assembled figures. X.W. and R. X. wrote the manuscript with comments from Y.Z., Q.Z. and D.M.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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