Upcycling Compact Discs for Flexible and Stretchable Bioelectronic Applications

Electronic waste is a global issue brought about by the short lifespan of electronics. Viable methods to relieve the inundated disposal system by repurposing the enormous amount of electronic waste remain elusive. Inspired by the need for sustainable solutions, this study resulted in a multifaceted approach to upcycling compact discs. The once-ubiquitous plates can be transformed into stretchable and flexible biosensors. Our experiments and advanced prototypes show that effective, innovative biosensors can be developed at a low-cost. An affordable craft-based mechanical cutter allows pre-determined patterns to be scored on the recycled metal, an essential first step for producing stretchable, wearable electronics. The active metal harvested from the compact discs was inert, cytocompatible, and capable of vital biopotential measurements. Additional studies examined the material’s resistive emittance, temperature sensing, real-time metabolite monitoring performance, and moisture-triggered transience. This sustainable approach for upcycling electronic waste provides an advantageous research-based waste stream that does not require cutting-edge microfabrication facilities, expensive materials, and high-caliber engineering skills.

It is true that "One-time-use, disposable sensors are in growing demand for reliable, accessible, and fast measurements, and that can be used anywhere or any time without recalibration or the worry of contamination" , but the realization requires a huge amount of work. It is not clear how long is the time of preparation of these electrodes respect to the usual procedures.
Acetone is not a real green solvent and several acids to. How much will be consumed in the fabrication of these electrodes?
The work is well done and really sounds scientifically. The approach and strategy are correct and data are well analysed, apart some questions in the electrochemical part. However, the idea of using the gold "lines" present in CD and DVD to fabricate electrodes is not new. The first application was published 20 years ago by L. Angnes et al. Gold Electrodes from Recordable CDs, Anal. Chem. 2000, 72, 5503-5506. The electrodes produces were not wearable, but the precious material was recycled and found application is many ways, as indicated in a recent review by G.Moro et al. "Disposable electrodes from waste materials and renewable sources for (bio) electroanalytical applications" Biosensors and Bioelectronics 146 (2019) 111758, https://doi.org/10.1016/j.bios.2019.111758 . The present manuscript is a good work and idea, but itrepresents an improvement of old ideas, and I think that it could be mentioned..
The quality of the data is good. The level of support for the conclusions for the electroanalytical part needs to be improved . Some analytical study should be complemented. The use of e-waste, particularly from CD/DVD is not a novelty, but the technology proposed for the fabrication of wearable sensors is a good novelty.

Some particular comments:
Line 30:… "from the CD was 30.35 ± 1.92 µm, consisting of a protective, polymethylmethacrylate"… In the paper mentioned above Angnes indicated 50-100 nm for this thickness. How was it measured?
Lines 42-47: . "Energy dispersive X-ray spectroscopy (EDS) analysis of the metal layer after the solvent treatments are shown in Figure 1F and S9. After the soaking in acetone, Ag and Au could be seen within the spectrum at 70.95 and 29.05 wt.%, respectively (Supplementary Fig. 9A-B). Their presence confirmed the archival composition of the layer as predominantly Ag. Additional methods to treat the CD are discussed in the Supporting Information. The CD metal layer can be stripped down to nearly pure gold by soaking in a bath of nitric acid." However, on lines 296-297, the authors mention that for the construction of the reference electrode it is possible to use the TRACE amounts of Ag present within the electroactive material of the CD. There is some incoherence. Could the authors clarify this point? Lines 69-73: "A fully fabricated UCDE device consisted of two biopotential electrodes, a heater or temperature sensor, a reference electrode, a counter electrode, a pH electrode, an oxygen electrode, a lactate electrode, and a glucose electrode (Fig. 1K). The full, end-to-end fabrication and manufacturing required resources that can be found easily at conventional craft stores, negating the need for high-end instrumentation. " The authors say that the fully UCDE is composed by 2 biopotential electrodes , but immediately describe that there are electrode to measure: pH, oxygen, lactate and glucose (4). Please clarify this spoint.
Lines 178-180: "A Clark type oxygen sensor was based on the interaction of Nafion and a diluted PDMS layer (oxygen selective membrane) coating the UCDE's electrode following electrochemical cleaning". How is the oxygen sensor calibrated? Usually Clark electrode requires frequent calibration.
Lines 196-201: "The UCDE's glucose sensor produced a dynamic range between 0.15 mM to 0.75 mM at a sensitivity of -0.94 µA/cm2mM (R2 = 0.98) and limit of detection, 0.75 mM, with physiologically relevant concentrations for sweat glucose levels, 0.2 to 0.6 mM . The UCDE's lactate sensor demonstrated a dynamic range from 3 to 9 mM with a sensitivity of -21.5 nA/cm2 mM (R2 = 0.98) and limit of detection, 12 mM,…" Here there is a misunderstanding on the concept of "limit of detection ". According to IUPAC: The limit of detection, expressed as the concentration, cL, or the quantity, qL, is derived from the smallest measure, xL, that can be detected with reasonable certainty for a given analytical procedure. The value of xL is given by the equation xL= xbi+k sbi where xbi is the mean of the blank measures, sbi is the standard deviation of the blank measures, and k is a numerical factor chosen according to the confidence level desired. Probably the authors interpreted this values as the linear range or the dynamic range . (IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook.). Another aspect not clear is how do the authors intend to do the quantification of the analyte, since it is not possible to calculate the concentration from a calibration plot obtained from standard additions of the analytes, as presented in the work. This is an tricky point common to almost all electrochemical sensors. The authors did not gave information about the reproducibility of the fabrication and well as of the measurements. It is necessary to have a statistical study on the performance of the sensors.
Lines 253-255: "We hypothesize that in vivo, multinucleated macrophages, multinucleated giant cells, or foreign body giant cells would be able to clear out these flakes through phagocytosis at the expense of an elevated inflammatory response ". This hypothesis should be confirmed .
Lines 294-295: "Electrochemical Cleaning: All electrodes (except the reference and pH electrode) were cleaned in 0.1 M H2SO4 from -0.4 V to 1.4 V (vs. Ag/AgCl (1M KCl)) at 25 mV/s for 1 cycle." The cleaning of gold electrode surface is very tricky and important. Usually just one CV cycle is not enough to clean a gold electrode. See: Campos et al, https://doi.org/10.1016/j.electacta.2013.07.083 for cleaning a gold electrode.
Lines 296-297: "Reference Electrode: The reference electrode was fabricated by utilizing the trace amount of silver within the active electrode material from the CD." Here the authors indicate that the material of the CD useful as active electrode contains trace amount of silver. See a previous comment.
Probably many American researchers use hr and hrs to represent hour and hours. However the international abbreviation is h. I consider that it is important to respect international rules.
Lines 313-315: "The solution of Prussian Blue consisted of 100 mM KCl, 2.5 mM K3Fe(CN)6, 2.5 mM FeCl3, and 100 mM HCl. For the lactate sensor, the Prussian Blue mediator layer was electrochemically deposited through cyclic voltammetry from -0.5 V to 0.6 V (vs. Ag/AgCl)." It is well known that Prussian Blue is not soluble in water, so you cannot prepare a "solution of Prussian Blue". What is indicated in this text is the solution present in the cell for the electrochemical deposition of PB. Please correct.
Reviewer #4 (Remarks to the Author): The title of your manuscript is interesting given the importance of upcycling the waste resources within our technosphere. However, I find some bigger challenges not addressed by this manuscript.
First of all, as you have mentioned yourself in line 36 that the compact discs are dated technologies, then why spend precious resources on figuring out a waste treatment option for them. An estimated 5.5. million CDs are discarded currently, but what about coming years when they wouldn't appear anymore in our waste flows. Already, we don't see them in noticeable number/amount in e-waste pre-processing facilities in many countries. In such a case, are you looking to dig up the urban mines (landfills) to get the CDs for your upcycling process? and how environment friendly and sustainable that's going to be? Secondly, you mentioned in line 33 that 15-20% of e-waste gets recycled despite its valuable materials. Let me clarify for you that valuable materials are the first resources recovered from ewaste irrespective of developed or developing nations. So, this 15-20% could be a representative of valuable materials found in e-waste, where the recycling companies are simply not interested in recovering the remaining bulk for economic reasons. So, rather than focusing on obsolete technologies, it would be good to devote all our energies to figure out ways for the remaining bulk materials, e.g., plastics and glass.
It would be good to add a table of input and outputs for the upcycling process to visualize how many resources are used to little benefit. Also, what about a constructive discussion on demand for bioelectronic applications? 1

Summary Recommendation
The article is interesting. The results are very well presented and also well discussed. The text is also complete in terms of images and diagrams that make the reading fluent and effective. The sensor preparation method is certainly useful and interesting also from the point of view of the circular economy. The main problem with this article, however, is originality. In the literature there are numerous examples of the reuse of CDs/DVDs for the production of sensors with methods similar, if not in some cases the same as the one proposed here. In particular, the authors are advised to read the following articles, to include them in the introduction and to underline their novelty and the advantages / disadvantages with respect to their method. Our response: We appreciate the time and effort taken to review our publication. We hope that given the additional changes we made to our manuscript, demonstrate our originality and significance, to reconsider your concerns.
We agree that there are previous examples of reusing CDs to produce electrochemical sensors. However, our paper is significant because it demonstrates transfer printing and harvesting active components from CDs, enabling electrochemical sensors by introducing enzymatic, Clark-type, and pH (potentiometric) sensing for biointegrated biomedical applications, which has yet to be demonstrated in literature. The previously reported research is unable to be use for on-skin biosensing applications and is limited to lab-based analysis due to rigid substrates. We highlight the development of a full 3-electrode system on ultrathin flexible and stretchable electronics. Our work exists on a stretchable and flexible platform, upcycled from CDs, and describes a completely integrated full 3-electrode system. These articles only reference electrochemical sensors which, as described in the manuscript, accounts for only 20% of our study. We have yet to see published work that demonstrates our other sensing mechanisms (e.g., biopotential, resistive, and biodegradable sensors) on CDs.
We incorporated the previous studies that you recommended with a few additional studies.
The last paper 10.1016/j.cirpj.2015.01.005 used low powered laser cleaning to recover the polycarbonate, which we do not explore within our study because of the costly equipment required. Additionally, these researchers didn't upcycle any components following the cleaning process; therefore, we feel that this paper would be an irrelevant source to cite considering the method remains unproven for applicational uses.

Summary Recommendation
The authors reported an upcycling process that enables sustainable solutions for CDs and other e-waste recycling. These CDs can be transformed into soft bioelectronics for noninvasive monitoring, while fully integrating with human skin and that can communicate with a smartphone via Bluetooth. This paper is a very interesting, well written, complete with all necessary information in supplementary section and suitable for publication in Nature Communications.
Our response: We thank the reviewer for the time they took to assess our manuscript and we value their response. 5

Summary Recommendation
In this manuscript the authors describe a procedure to prepare wearable gold electrodes with the material from CD-DVD . As pointed by the authors, this is very important from the point of view of environmental issue and e-waste .
Our response: We appreciate the reviewer's detailed comments to improve our manuscript. We hope that the changes we implemented in our revision are deemed acceptable. It is true that "One-time-use, disposable sensors are in growing demand for reliable, accessible, and fast measurements, and that can be used anywhere or any time without recalibration or the worry of contamination" , but the realization requires a huge amount of work. It is not clear how long is the time of preparation of these electrodes respect to the usual procedures.
Our response: We agree that not many proof-of-concepts remain applicable from a commercial point of view, however, we believe that our study demonstrates an impactful approach at a first step towards upcycling e-waste. From a feasibility approach, two full devices can be created from one CD (illustrated in Figure 1J) on orders of magnitude cheaper and quicker than microfabrication techniques, costing around $1.50 and taking between 20 to 30 mins per device.
The procedure and feasibility are demonstrated in paragraph 1 of the main text.
We agree that the adoption of these processes would require a large scale of work and require more development to be fully implemented. We are presenting a first of its kind, case study. We will continue modifying and improving our upcycled devices to move towards the implementation of these types of sensors. But even so, stretchable bioelectronics are not currently available for real-world uses thus far, let alone one-time-use sensors. As a field, a lot of advancements are required to translate these developments to commercial markets.
In paragraph 2 of the Introduction, we added associated processing cost and lead time with microfabrication. We describe the time it takes to make these devices, around 20 to 30 minutes. In comparison to microfabrication, lithography-based techniques, ignoring the requirement of all the expensive equipment required, similar device fabrication can range from a few hours to days. Additionally, the increased cost associated with microfabrication wouldn't make sense for development of one-time-use sensor, since the production costs are significant.
With the advantages of our study, we believe the novel approach reported here can be readily used in small-scale applications, biomedical and other disciplines (e.g., anthropology, nursing, and psychology) for evaluating physiological information with on-body systems. • Comment (2) Acetone is not a real green solvent and several acids to. How much will be consumed in the fabrication of these electrodes?
Our response: Yes, we agree. Though acetone isn't necessarily a green solvent, it is one of the least hazardous industrial solvents when compared to photoresist, developer, and chemical etchants commonly used through lithography patterning. Additionally, multiple samples can be treated with an acetone bath which significantly reduces the required solvent volume per device.
To develop the main, upcycled electronics, acetone is the only solvent required. Therefore, our significance here is such that, for fabricating these devices we only require the use of acetone which is inexpensive and less toxic than photoresist, developer, and chemical etchants. For the transient device, diluted nitric acid is required. This part of the manuscript is purely a proof-ofconcept, and we don't regard it as the main significance of our proposed study.
We added to the manuscript the volume of acetone utilized for soaking the CD.
Modifications to the manuscript: (Main Text, line 10-12) "The CD was soaked in 100 mL of acetone for 1.5 mins, releasing the metal layer by breaking down the polycarbonate substrate and dissolving the BPA (Fig. 1A1; Supplementary Fig. 4 and 5)."

• Comment (3)
The work is well done and really sounds scientifically. The approach and strategy are correct and data are well analysed, apart some questions in the electrochemical part. However, the idea of using the gold "lines" present in CD and DVD to fabricate electrodes is not new. The present manuscript is a good work and idea, but it represents an improvement of old ideas, and I think that it could be mentioned.
The quality of the data is good.
The level of support for the conclusions for the electroanalytical part needs to be improved . Some analytical study should be complemented.

The use of e-waste, particularly from CD/DVD is not a novelty, but the technology proposed for the fabrication of wearable sensors is a good novelty.
Our response: We agree that there are previous examples of reusing CDs. We addressed this per Reviewer 1's recommendations and made the appropriate changes to our manuscript by adding in additional references. Please look at our response to Reviewer 1. We have developed enzymatic, Clark-type, and potentiometric sensors, and a full 3-electrode system on a flexible and stretchable soft bioelectronics platform which have not been demonstrated with a CD thus far.
Our work isn't solely describing electrochemical sensors. We have yet to see published work that demonstrates our other sensing mechanisms (e.g., biopotential, resistive, and biodegradable sensors) on CDs.
Our supporting electroanalytical studies are presented within the supplementary text of the manuscript.
• Comment (4) Line 30:… "from the CD was 30.35 ± 1.92 µm, consisting of a protective, polymethylmethacrylate"… In the paper mentioned above Angnes indicated 50-100 nm for this thickness. How was it measured?
Our response: This was measured by SEM and presented in Supplementary Figure S7. As mentioned in the text, this thickness measurement includes the protective and metal layer from the CD. When the protective layer is removed, the thickness is decreased to ~70 nm which would match to what Angnes et al. has described. Figure 1F and S9. After the soaking in acetone, Ag and Au could be seen within the spectrum at 70.95 and 29.05 wt.%, respectively (Supplementary Fig. 9A-B). Their presence confirmed the archival composition of the layer as predominantly Ag. Additional methods to treat the CD are discussed in the Supporting Information. The CD metal layer can be stripped down to nearly pure gold by soaking in a bath of nitric acid."

• Comment (5) Lines 42-47: . "Energy dispersive X-ray spectroscopy (EDS) analysis of the metal layer after the solvent treatments are shown in
However, on lines 296-297, the authors mention that for the construction of the reference electrode it is possible to use the TRACE amounts of Ag present within the electroactive material of the CD. There is some incoherence. Could the authors clarify this point?
Our response: The nitric acid soak is only used for the transient device.
Therefore, for lines 296-297, we are discussing the CD treated following the acetone soak. The metal composition is Ag and Au and we can use the Ag present in the CD to create a reference electrode.
• Comment (6) Lines 69-73: "A fully fabricated UCDE device consisted of two biopotential electrodes, a heater or temperature sensor, a reference electrode, a counter electrode, a pH electrode, an oxygen electrode, a lactate electrode, and a glucose electrode (Fig. 1K). The full, end-to-end fabrication and manufacturing required resources that can be found easily at conventional craft stores, negating the need for high-end instrumentation. " The authors say that the fully UCDE is composed by 2 biopotential electrodes , but immediately describe that there are electrode to measure: pH, oxygen, lactate and glucose (4). Please clarify this point.
Our response: Yes, the full device consists of 2 biopotential electrodes, 6 electrochemical electrodes, and a heater/RTD. The configuration is presented in Figure 1J-L. This is the design of the device we engineered and proposed. Any configuration can be developed based on the user's design. If researchers wanted a separate biopotential, electrochemical, or heater/RTD device, that can be easily created. For simplicity, we proposed a full device consisting of all these components, rather than 3 separate devices.

• Comment (7) Lines 178-180: "A Clark type oxygen sensor was based on the interaction of Nafion and a diluted PDMS layer (oxygen selective membrane) coating the UCDE's electrode following electrochemical cleaning". How is the oxygen sensor calibrated?
Usually Clark electrode requires frequent calibration.

Our response:
The oxygen sensor is calibrated against a commercial dissolved oxygen probe. The procedure is described in the supplementary section entitled, "Electrochemical Performance Evaluation". Our response: We thank the reviewer for pointing out this typographical error. We completely agree that the term, "limit of detection" was misused. We meant to write limit of linearity and we made the changes in the manuscript.
Modifications to the manuscript: (Main Text, line 196-201) "The UCDE's glucose sensor produced a dynamic range between 0.15 mM to 0.75 mM at a sensitivity of -0.94 µA/cm 2 mM (R 2 = 0.98) and limit of linearity, 0.75 mM, with physiologically relevant concentrations for sweat glucose levels, 0.2 to 0.6 mM 56 . The UCDE's lactate sensor demonstrated a dynamic range from 3 to 9 mM with a sensitivity of -21.5 nA/cm 2 mM (R 2 = 0.98) and limit of linearity, 12 mM…" • Comment (9) Another aspect not clear is how do the authors intend to do the quantification of the analyte, since it is not possible to calculate the concentration from a calibration plot obtained from standard additions of the analytes, as presented in the work. This is an tricky point common to almost all electrochemical sensors. The authors did not gave information about the reproducibility of the fabrication and well as of the measurements. It is necessary to have a statistical study on the performance of the sensors.
Our response: We have demonstrated similar procedures to Wei Gao et al. and cited them within the electrochemistry section describing our results [1]. We present a calibration curve that can be used to estimate the concentration. Enzymatic sensors also suffer from being unstable and typically suffer from a short shelf-life. We hope that our development here, as a prototyping method could help researchers address these types of issues to improve sensor reliability moving forward. Our response: We agree, but this is a communications paper, and not the main focus of our study. We have presented previous literature that confirms our hypothesis [1]. In our future study, we will devote our efforts to the transient CD electronics to fully realize the potential of this device.  Our response: We agree, cleaning is an important and difficult step, usually 1 cleaning cycle is not enough. But, after cleaning with 1 cycle we can see a clean redox reaction within potassium ferrocyanide ( Figure S15). We suspect that since the material is very thin and delicate, only 1 cycle is required to maintain the robustness of the whole device. It is also important to note that we are using a very slow scan rate of 25 mV/s. When we performed additional cycles, we noticed the performance of the electrode degraded substantially.
• Comment (12) Lines 296-297: "Reference Electrode: The reference electrode was fabricated by utilizing the trace amount of silver within the active electrode material from the CD." Here the authors indicate that the material of the CD useful as active electrode contains trace amount of silver. See a previous comment.

Probably many American researchers use hr and hrs to represent hour and hours. However the international abbreviation is h. I consider that it is important to respect international rules.
Our response: We addressed this concern with the previous comment. Please see above response. To respect international rules, we have changed hr. and hrs. within our manuscript.

Summary Recommendation
The title of your manuscript is interesting given the importance of upcycling the waste resources within our technosphere. However, I find some bigger challenges not addressed by this manuscript.
Our response: We thank Reviewer 4 for their time and effort to review our work. We hope our revision addresses some of their main concerns. Our response: We agree, CDs are dated technologies, however, they're still a large polluter within our waste stream, and they're still shipped at large quantities. In 2021, 40.6 million CDs were distributed in the US, a 1.1% increase from the previous year [1]. We couldn't track down any global numbers for this, however, we can assume that global shipments will be much higher. Additionally, this metric is only considering music CDs and excluding other types such as DVDs, video games, and software discs. This was surprising to us too! CDs are still being produced and shipped at larger values then you may think. Most of the e-waste seen today is from dated technologies, such as CDs [2].
We believe that Universities can install CD collection boxes, while Companies that provide CD collection methods, such as GreenDisk, may adopt or outsource the proposed fabrication techniques as an alternative to alleviate the accumulation of CDs in landfills.
We included a U.S. domestic policy that was recently approved for recycling. Although we would like to have a global prospective, we believe that since the U.S. is one of the leading producers of e-waste, these policies will have a bigger impact on the global scale.
Please see our detailed revision within the manuscript discussion possibilities.  1 . The 12th SDG, "Responsible Consumption and Production", seeks to address ewaste challenges by ensuring countries adopt a more responsible approach to the proliferating ewaste stream 2 . Inefficient recycling processes are a global concern for e-waste management as they contribute to an increase in landfill waste and produce toxic pollution 3 . Additionally, Stephan Sicars (Director of the Department of Environment UN Industrial Development Organization) described e-waste as "a serious threat to the environment and human health worldwide" 4 . In 2019, the United Nations documented 1.7 kg per capita of e-waste recycled out of 7.3 kg per capita generated. To ensure the recycling of all e-waste by 2030 the recycling rate will need to be roughly 10 times greater 2 . To reduce landfill and pollution accumulation, a more sustainable method is required to manage the flow of e-waste. Currently, only ~15-20% of ewaste is recycled despite its valuable materials-iron, steel, copper, silver, and gold 5,6,7 .
Whereas the remaining 80% of e-waste is not collected for recycling due to expense and lack of a global infrastructure 5,6,7,8 . Meanwhile, the toxic and hazardous components of e-wastemercury, lead, and synthetic resins-threaten the environment and are left to degrade in landfills or incinerated 5,6,7 . Today, e-waste primarily consists of dated technologies which accounts for the ever-growing trail 5 . Products from years past such as compact discs (CDs), old televisions, and computer monitors are the biggest contributors to e-waste 5  Modifications to our manuscript: (Conclusion, line 271-290) "Our paper illuminates the challenges that plague e-waste recycling and, subsequently, provides a remedy. Consumer confusion and a lack of infrastructure knowledge remain critical issues with regard to proper recycling. Emphasizing the development of novel upcycling approaches through scientific dissemination will increase awareness within this area. New programs incorporated into the Infrastructure Investment and Jobs Act, specifically the RECYCLE Act, aim to relieve the inundated recycling stream and provide new opportunities to support innovative recycling and upcycling ideas. Thus far, recycling and upcycling activities have accounted for 681,000 jobs, $37.8 billion in wages, and $5.5 billion in tax revenues in the United States 68 . Policy programs that fund new jobs and ideas will assist the United States attain global sustainability objectives. Upcycling is a sustainable practice as it "meets the needs of the present without compromising the ability of future generations to meet their own needs" 69 through the transformation of waste into secondary products. The proposed upcycling approach remains sustainable if the cost of microfabrication remains exuberant, rapid prototyping persists as an essential business and institutional practice, and healthcare increasingly demands one-time use sensors. As a simple and cost-effective method, this technology can be adopted on both an academic research and commercial scale. Institutes and Universities may install CD collection boxes, while companies that provide CD collection methods, such as GreenDisk, may adopt or outsource the proposed fabrication techniques as an alternative to alleviate the accumulation of CDs in landfills. Any effort towards increasing both recycling and upcycling will advance the establishment of environmentally sustainable practices." (Reference update):

68.
Recycling Economic Information. United States Environmental Protection Agency (2020 Our response: Some e-waste is recycled (approximately 15-20%). The valuable materials are recovered first, however, there is still 80% of e-waste that is unaccounted for. The 15-20% represents e-waste that is documented and recycled. Whereas the remaining 80% of e-waste is not collected for recycling due to expense and a lack of global infrastructure [1,2,3]. Our paper illuminates this challenge and provides a remedy.
See previous comment and associated revision explaining our rational for CDs as a valuable ewaste stream.