The short lifespan of electronics contributes significantly to the global e-waste problem. Traditional e-waste recycling processes are often inefficient and environmentally damaging. The 12th Sustainable Development Goal (SDG) highlights the need for responsible consumption and production to address this challenge. Compact discs (CDs), once ubiquitous but now largely obsolete, represent a substantial portion of e-waste. Their polycarbonate substrate can depolymerize into Bisphenol A (BPA), a potentially harmful xenoestrogen. This necessitates exploring sustainable methods for CD upcycling. Biointegrated electronics offer new possibilities for real-time health monitoring through various biomarkers. However, conventional microfabrication techniques used to create stretchable electronics are expensive and time-consuming. This research addresses both challenges by exploring the upcycling of CDs into low-cost, sustainable, and flexible bioelectronics for various sensing applications, including biopotential measurements, temperature sensing, and electrochemical detection of various metabolites. The study focuses on using readily available and inexpensive tools for fabrication, making the process accessible and scalable.

Previous research has explored using CDs to create gold and silver electrodes for various electrochemical applications, including detecting metal ions and organic compounds. However, these methods often lack the mechanical flexibility and biocompatibility necessary for wearable biosensors. The high cost and complexity of conventional microfabrication techniques for creating stretchable electronics also limits their widespread application. Existing methods often rely on expensive materials like evaporated gold and time-consuming processes such as lithography. The need for disposable, low-cost sensors, especially in point-of-care diagnostics, further motivates the search for alternative fabrication techniques. This work builds upon existing research by developing a novel approach to upcycle CDs into fully functional, biocompatible, and sustainable biosensors for various applications in healthcare and beyond.

The researchers developed a novel method for upcycling CDs into stretchable bioelectronics. The process begins by soaking the CD in acetone to separate the metal layer from the polycarbonate substrate. The metal layer is then harvested using polyimide (PI) tape, which serves as a substrate for the device. A Cricut Maker, an affordable craft-based mechanical cutter, is used to pattern the PI-metal layer with precision down to 25 µm feature sizes, creating stretchable architectures. An insulation layer of PI tape is then added. The entire fabrication process takes approximately 20–30 minutes and costs approximately $1.50 per device. The resulting Upcycled CD Electronics (UCDEs) were characterized using various techniques including scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and energy-dispersive X-ray spectroscopy (EDS). Mechanical testing was performed using a Mark10 tensometer to evaluate the stretchability and durability of the UCDEs. Biopotential measurements (EMG and ECG) were conducted using a PowerLab data acquisition unit, comparing UCDE performance to commercial gel electrodes. Temperature sensing and heating capabilities of UCDEs were assessed using an infrared (IR) camera. Electrochemical characterization (cyclic voltammetry, electrochemical impedance spectroscopy) evaluated the performance of UCDEs as various electrochemical sensors (pH, oxygen, glucose, and lactate). In vitro biocompatibility was tested using HaCaT cells, assessing cell viability after 7 days of culture on UCDE substrates. Finally, a biodegradable version of the UCDEs was created using polyvinyl alcohol (PVA) and polycaprolactone (PCL) substrates, evaluating their transient behavior in different environments.

The UCDEs demonstrated excellent stretchability, achieving up to 62.35 ± 1.81% elongation at yield with an elastic modulus of 5.59 ± 0.16 MPa. The devices exhibited negligible resistance changes under cyclic bending and stretching, demonstrating their robustness for wearable applications. Biopotential measurements showed that the UCDEs performed comparably to commercial gel electrodes for both EMG and ECG, even producing higher amplitude EMG signals potentially due to the larger electrode surface area. As a resistive temperature detector (RTD), the UCDEs showed a temperature coefficient of 9.21 x 10⁻⁴ °C⁻¹ at 20 °C and a linear relationship (R² = 0.99) between resistance and temperature, performing similarly to an IR camera. As heaters, the UCDEs generated temperatures comparable to commercial hand warmers, reaching an average of 35.6 °C at 5 V. Electrochemical testing showed that the UCDEs, after electrochemical cleaning, exhibited low impedance, demonstrating their suitability for various sensor applications. The UCDEs were successfully used to fabricate a pH sensor (−36.5 mV/decade sensitivity), an oxygen sensor (65 nA/(cm²O2%) sensitivity), a glucose sensor (−0.94 μA/cm²mM sensitivity), and a lactate sensor (−21.5 nA/cm²mM sensitivity), all within physiologically relevant ranges. The biodegradable UCDEs, fabricated using PVA and PCL substrates, exhibited moisture-triggered transience, with PVA showing rapid dissolution in water and PCL demonstrating slower, hydrolytic degradation, suitable for different applications such as rapid wound assessments or long-term implantable sensors. Finally, in vitro biocompatibility studies demonstrated high cell viability of HaCaT cells cultured on UCDE substrates (Acetone soak ~96.7%, Hydrochloric acid soak ~94.7%, Nitric acid soak ~93.0%), indicating good biocompatibility.

This study successfully demonstrates a sustainable and cost-effective approach to upcycling CDs into versatile, high-performing bioelectronic sensors. The use of an inexpensive mechanical cutter for patterning simplifies the fabrication process, making it accessible to researchers and industries with limited resources. The key findings highlight the multi-functionality of the UCDEs, demonstrating their potential for various applications in healthcare and beyond. The biocompatibility and stretchability of the UCDEs make them well-suited for wearable applications, offering a significant advantage over rigid electronic sensors. The development of biodegradable versions adds further value, opening possibilities for transient electronics and resorbable implants. The results of this study contribute to the growing field of sustainable electronics and point-of-care diagnostics.

This research presents a novel, sustainable, and cost-effective method for upcycling CDs into flexible, stretchable, and biocompatible biosensors. The multi-functional UCDEs demonstrate promising performance in various sensing applications, including biopotential monitoring, temperature sensing, and electrochemical detection. The development of biodegradable versions further enhances their versatility. This approach offers a valuable solution to e-waste management while advancing the field of bioelectronics. Future research should focus on long-term stability of electrochemical sensors, fully integrated wireless systems for continuous monitoring, and further exploration of biodegradable devices for implantable bioelectronics.

The current study primarily focuses on in vitro biocompatibility assessment. Further in vivo studies are needed to confirm the long-term biocompatibility and efficacy of the UCDEs. The sensitivity and dynamic range of the electrochemical sensors could be further optimized by exploring different enzyme immobilization techniques and modifying electrode materials. The mechanical properties and performance of the UCDEs under various environmental conditions (e.g., prolonged exposure to sweat, extreme temperatures) warrant further investigation. While the biodegradable sensors show promise, further research is necessary to fully understand and optimize their degradation kinetics for various clinical applications.