Upcycling Compact Discs for Flexible and Stretchable Bioelectronic Applications

The disposal of electronic waste is a growing global concern due to short product life cycles and inefficient recycling processes, resulting in landfill accumulation and toxic pollution. Despite valuable materials in e-waste, only a small fraction is recycled owing to cost and lack of infrastructure. CDs represent a significant portion of dated technologies entering the waste stream and can release toxic monomers such as BPA upon degradation. In parallel, biointegrated, soft electronics are in demand for real-time health monitoring but often require costly, complex microfabrication using expensive materials and hazardous chemicals. This study addresses these gaps by proposing an inexpensive, rapid, and environmentally friendly approach to upcycle CDs into stretchable, flexible, and transient bioelectronics. The research question is whether thin metal layers harvested from CDs can be patterned into durable, biocompatible, and high-performing biosensors for biopotential, thermal, and electrochemical applications, providing a sustainable alternative to conventional fabrication.

Prior work in stretchable and flexible bioelectronics has focused on transforming rigid materials into soft, skin-conformal devices using thin polymer substrates and deterministic architectures. Traditional microfabrication and printing approaches, while high-performance, are expensive, time-consuming, and rely on hazardous chemicals, rendering them less suitable for rapid prototyping or disposable sensors. Upcycling of CDs has been explored to create gold and silver electrodes, detect metal ions (Pb, Hg, Cu), analyze organic compounds (DNA, cysteine, dopamine), and quantify oxidants (H2O2, Cl, iodine). However, earlier approaches have not demonstrated mechanically robust, wearable biosensor platforms or comprehensive biosensing functionalities needed for practical on-skin applications. This work builds on these foundations by presenting a mechanically durable, low-cost, and versatile upcycling route for biosensing modalities including biopotential, potentiometric, amperometric, enzymatic sensing, and transient electronics.

Upcycling and patterning: Archival gold CDs were soaked in acetone (~40 mL, 1.5 min) to release the thin metal layer by polycarbonate breakdown. The metal layer was harvested onto polyimide (PI) tape, serving as a robust substrate, then transferred to tattoo paper for handling. Deterministic stretchable patterns and insulation layers were cut using an affordable craft mechanical cutter (Cricut Maker). Excess material was removed and PI insulation aligned via marks and laminated. Alternative patterning via photolithography and laser engraving was also explored. Fabrication time was 20–30 min per device at ~US$1.50/device. Device structure and characterization: The CD metal stack included a PMMA protective layer and an archival metal layer (~70 nm). After lamination, device thicknesses were ~54 µm (PI-metal) and ~82 µm with insulation. FTIR verified PMMA remained after acetone; EDS showed Ag and Au composition (Ag ~70.95 wt%, Au ~29.05 wt%). UCDEs exhibited base four-probe resistance ~0.03 Ω/cm². Mechanical/electrical testing: Stretchable lattice patterns were characterized with tensile testing (Mark10 tensometer; n=3; strain rate 5.1 mm/min) and cyclic bending/stretching. Resistance changes were recorded (Keysight 34460A). Biopotential sensing: EMG and ECG signals were recorded using UCDE electrodes laminated to skin with liquid bandage. EMG and ECG signals were acquired with a PowerLab Quad Bio Amp at 1 kHz, with commercial gel electrodes as controls. A fully wireless ECG device integrated a low-power MCU with BLE, battery, and smartphone app. Thermal devices: Joule heating was characterized by IR thermography (ETS320) under DC bias 1–7 V, including deformation at 0–20% strain and on-palm tests. RTD calibration used four-probe resistance versus thermocouple temperature to extract TCR. Electrochemical sensors: Electrodes were electrochemically cleaned in 0.1 M H2SO4 (−0.4 to 1.4 V vs Ag/AgCl). UCDE Ag/AgCl reference electrodes were formed by Ag chlorination using LSV/CV in 0.1 M KCl/0.01 M HCl, then benchmarked against commercial Ag/AgCl (1 M KCl) via CV/EIS. pH ISE membranes (H+ ionophore in PVC matrix) were drop-cast on UCDE Ag/AgCl for potentiometry over pH 4–12. Oxygen sensing employed Nafion and a diluted PDMS membrane; CV identified the oxygen reduction potential; chronoamperometry quantified response versus O2 saturation. Enzymatic sensors: Prussian Blue mediators were electrodeposited; SWCNT-chitosan immobilized glucose oxidase or lactate oxidase were drop-cast to yield amperometric glucose and lactate sensors; chronoamperometry yielded calibration. Transient devices: For biodegradable resistors, the PMMA was fully removed via nitric acid to yield an ultrathin Au layer (~19 nm) transferred onto PVA (~50 µm, water-soluble) or PCL (~50 µm, hydrolytically degradable). Electrical stability was assessed in water, ethanol, acetone, and across pH values; dissolution was monitored in PBS (pH 7.4, 37 °C) with SEM to observe metal–polymer interfaces. Biocompatibility: HaCaT keratinocytes were cultured on UCDEs prepared via acetone, HCl, or HNO3 processing, and with gold flakes from transient devices; viability was assessed after 7 days using live/dead staining and fluorescence quantification (n=3). Statistics: Data presented as mean ± SEM (n=3); no a priori power calculation reported.

- Fabrication and materials: Craft cutting achieved ~25 µm feature sizes and ~20% strain capability; device cost ~US$1.50 and 20–30 min fabrication without toxic chemicals or expensive equipment. Metal stack thickness ~30 µm including PMMA; overall device thickness with insulation ~82 µm. EDS of acetone-treated metal showed Ag 70.95 wt% and Au 29.05 wt%; FTIR confirmed PMMA retention. Base resistance ~0.03 Ω/cm². - Mechanics and durability: Triangular lattice UCDEs (n=3) showed elastic modulus 5.59 ± 0.16 MPa and elongation at yield 62.35 ± 1.81%. Cyclic bending (100 cycles, 3.5 mm radius) increased resistance by only 0.29% for patterned UCDEs (vs 21.7% unpatterned). Cyclic stretching (10 cycles, 0–20% strain) increased resistance by 0.59%. - Biopotentials: EMG and ECG signals from UCDEs matched commercial gel electrodes; UCDEs showed clear P and T wave identification. A fully wireless ECG with BLE showed PQRST waves comparable to benchtop acquisition. - Heater and RTD: At 5 V, heater average temperature 35.6 °C (max 52.3 °C), comparable to commercial hand warmers. Under 20% strain at 5 V, average temperature decreased ~19% (35.6 °C to 28 °C), recoverable by increasing voltage (e.g., 7 V). RTD exhibited temperature coefficient 9.21 × 10^-4 °C^-1 at 20 °C with R² = 0.99 and response comparable to IR camera. - Electrochemistry: After H2SO4 cleaning, UCDE electrodes exhibited redox performance similar to bare gold; EIS indicated reduced resistance/reactance post-cleaning. UCDE Ag/AgCl reference electrodes showed slight negative potential drift with decreased chloride versus commercial references but overall comparable performance. - pH sensor: Potentiometric H+ ISE displayed near-Nernstian response with sensitivity −36.5 mV/decade (R² = 0.99) over pH 4–12. - Oxygen sensor: Chronoamperometric detection from 20.2–100% O2 saturation with sensitivity 65 nA/(cm²·%O2) (R² = 0.98) and t90% ~42 s. - Glucose sensor: Linear 0.15–0.75 mM, sensitivity −0.94 μA/cm²·mM (R² = 0.98), relevant to sweat glucose (0.2–0.6 mM). - Lactate sensor: Linear 3–9 mM, sensitivity −21.5 nA/cm²·mM (R² = 0.98), spanning healthy to elevated wound levels. - Transient electronics: PVA-based resistors lost conductivity in <1 s in water but were stable in organic solvents; PCL devices remained stable in water and pH variations but were perturbed by organic solvents. In PBS at 37 °C, PCL devices exhibited gradual resistance increase (36 to 426 Ω over 7 days) with uniform polymer dissolution and metal microcracking. - Biocompatibility: HaCaT viability remained high after 7 days for UCDEs processed by acetone (~96.7%), HCl (~94.7%), and HNO3 (~93.0%); gold flakes from transient devices reduced viability to ~77.8% (p < 0.05).

The study demonstrates that metal layers harvested from CDs can be patterned into soft, stretchable UCDEs that integrate with skin and deliver multi-modal biosensing at low cost and with rapid turnaround, addressing limitations of traditional microfabrication. Durable deterministic geometries enable high mechanical resilience with minimal resistance drift during bending and stretching, satisfying requirements for on-skin wear. Biopotential recordings are comparable to commercial electrodes and were validated in a wireless form factor, supporting practical deployment. Thermal devices provide clinically relevant heating profiles and accurate temperature sensing; electrochemical cleaning restores high-quality electrochemical interfaces for redox reactions. The UCDE platform supports potentiometric (pH), amperometric (oxygen), and enzymatic (glucose, lactate) sensing within physiological ranges, highlighting versatility. Transient device variants based on PVA and PCL introduce moisture- and solvent-triggered biodegradability for recyclable or resorbable use-cases. Biocompatibility results with keratinocytes indicate cytocompatibility of processed UCDEs. Collectively, the findings answer the research question by showing UCDEs can function as robust, low-cost biosensors derived from e-waste, advancing sustainable bioelectronics and enabling broader access to rapid prototyping and disposable sensors.

This work introduces an eco-friendly, low-cost upcycling route to fabricate stretchable and transient bioelectronics from CDs using an accessible craft cutting process. The platform yielded robust mechanical performance, high-quality biopotential signals, heaters and RTDs with clinically relevant outputs, and versatile electrochemical sensors for pH, oxygen, glucose, and lactate. Transient PVA and PCL devices demonstrated moisture- or solvent-triggered behavior suitable for recyclable or resorbable applications, and in vitro tests confirmed cytocompatibility. Future research directions include: (1) evaluating long-term stability and performance of electrochemical sensors under realistic wearable conditions, (2) expanding fully integrated wireless systems beyond ECG for continuous multimodal monitoring, and (3) further studies to realize transient devices for implantable bioelectronics.

- Long-term stability and drift of electrochemical sensors in real-world wearable environments were not comprehensively evaluated; most tests were short-term and in controlled solutions. - Wireless integration was demonstrated primarily for ECG; other sensing modalities were not shown in fully integrated, on-body wireless configurations. - Biocompatibility was assessed in vitro with HaCaT cells over 7 days; in vivo biocompatibility and immune responses, particularly to transient device byproducts (e.g., gold flakes), were not studied. - Mechanical and electrical characterizations used small sample sizes (typically n = 3); no a priori power analysis was reported. - Transient PCL devices were monitored up to weeks for morphology and 7 days for resistance under alternating temperature; complete in vivo degradation timelines and performance during degradation were not established.