Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Upcycling end-of-life vehicle waste plastic into flash graphene

Abstract

Responsible disposal of vehicles at the end of life is a pressing environmental concern. In particular, waste plastic forms the largest proportion of non-recycled waste material from light-duty vehicles, and often ends up in a landfill. Here we report the upcycling of depolluted, dismantled and shredded end-of-life waste plastic into flash graphene using flash Joule heating. The synthetic process requires no separation or sorting of plastics and uses no solvents or water. We demonstrate the practical value of the graphene as a re-inforcing agent in automotive polyurethane foam composite, where its introduction leads to improved tensile strength and low frequency noise absorption properties. We demonstrate process continuity by upcycling the resulting foam composite back into equal-quality flash graphene. A prospective cradle-to-gate life cycle assessment suggests that our method may afford lower cumulative energy demand and water use, and a decrease in global warming potential compared to traditional graphene synthesis methods.

Introduction

Due to a decreasing cost of entry and an increasing global standard of living, automobile access has expanded ownership to record highs, with an estimated 1.4 billion passenger cars in use worldwide1,2. Inevitably, these vehicles come to the end of their useful life and must be managed3,4. End-of-life vehicles (ELV) present a complex environmental problem due to their heterogenous construction and ever-advancing robustness5. Processing standards vary broadly worldwide, but in the US as of 2020, depollution (removal of fluids and batteries) and dismantling remove 10–30% of the raw vehicle weight while the remainder is shredded6,7. These percentages vary by vehicle identity and construction. The metallic content is largely recovered, but the remainder of the ELV (12 to 32% of the raw vehicle weight) is typically landfilled8,9,10. The amount of plastic used in vehicles has increased an estimated 75% in the past 6 years to 350 kg per vehicle for weight reduction to improve fuel economy11. ELV waste plastic (ELV-WP) is the largest non-recycled material in vehicles, and the increased use of next-generation polymer composites exacerbates recycling of ELV-WP through traditional methods which generally focus on singular plastic sources12,13. Recycling or upcycling of ELV-WP is economically unfavorable due to the high cost of feedstock segregation12,13. Some nations have set mandatory recycling/recovery goals in attempts to minimize environmental impact while maximizing resource reclamation. For instance, the European Union implemented the ELV Directive to ensure that recovery of ELV raw materials achieves a minimum of 95% of vehicle weight by 2015; however, almost all member states failed to meet these guidelines14. Even with governmental policy incentives, ELV-WP management remains a dilemma. As ELV-WP draws more global attention, a number of strategies have been proposed by academic as well as automotive sectors. More wholistic understanding of the problem has recently been made possible by bottom-up analysis of the ELV waste management industry and life cycle assessments on a region by region basis, with many studies and reviews being recently published15,16,17. Generally, the ELV waste management strategy depends on the socioeconomic status of the region or country, as well as the systematic techniques required by the governing body18. Polypropylene often receives the most academic interest, as it is the most used plastic in automotive applications; many bumpers are made of polypropylene making for easier isolation of a single type of plastic for recycling. Recent lab-scale remediation strategies revolve around pyrolysis methods; however, these often require complex catalysts, inert atmospheres, and they struggle to recycle dirty or mixed streams of ELV-WP19,20.

The automotive sector produces an estimated 5% of the global industrial waste in the form of ELV, however given low virgin polymer costs there is little attraction to pursue ELV-WP recycling. While the automotive sector follows regional or government directives, it has been slower to confront the plastic waste problem21. Unlike valuable metal or electronic automotive components, there is little economic incentive to recover and recycle ELV-WP22. Many automotive manufacturers are studying novel ‘green’ polymers or composites and the use of more sustainable polymer reinforcements such as cellulose, waste textiles, agave fibers, or polymer waste23,24,25,26. These goals and products may markedly decrease the burden of producing virgin materials for new automobiles, but they do not combat the 1.4 billion passenger vehicles that contain >1012 kg of ELV-WP. For ELV-WP remediation to prosper outside of highly variable regional regulations, a high value upcycled or recycled product should be accompanied with minimal separation requirements, low-cost infrastructure, and a low energy and material overhead.

Synthesis of ELV-WP-FG

A custom-made LC/HC FJH reactor was built for the conversion of ELV-WP into graphene. A circuit schematic and further details are shown in Supplementary Fig. 1. A LC rectified AC, longer duration heating (LC-FJH) is used to carbonize the ELV-WP (1–25 A, over 15–20 s, yielding 30% mass recovery). Then, shortly after, in the same reactor, a DC HC, short duration heating (HC-FJH) is used to convert the carbonized ELV-WP to graphene (200 A from 104 mF capacitors charged to 150 V, discharged in 500 ms). ELV-WP (2.6 g) was mixed with 5 wt% metcoke (0.13 g) during the hammer milling process to increase the conductivity of the sample. The ground material was compressed in a quartz tube and sandwiched between copper wool and graphite electrodes (graphite in contact with the sample) to conduct current through the sample that had an initial resistance of ~500 Ω. The loaded quartz tube was enclosed in a plastic vacuum desiccator (50 Torr) to trap and remove sublimated impurities and outgases. A final resistance of 1 Ω resulted after the sequential LC-FJH and HC-FJH processes, recovering 19–24% of the original plastic weight as FG powder. After the FJH, the newly formed graphene was removed from the quartz tube and used without further purification or treatment.

Fabrication of FG-polyurethane foam (ELV-WP-FG-PUF)

PUF was made from a reaction of petroleum polyol and diisocyanate using FG powder as a filler. The formulation begins with polyol combined with a cell opener, surfactant, crosslinker, catalysts, and a blowing agent. The chemicals used are outlined in Supplementary Table 1. Graphene powder was added to the mixture and mixed for 3 min at 1500 RPM to generate a homogenous-looking mixture of the filler. After mixing, the diisocyanate was added to the mixture and mixed for 12 s. The mixture was moved to a 30.5 × 30.5 × 5.1 cm3 mold and heated to 65 °C for 7 min. Chem-Rend PU-11331 was used as a mold-release. The foam sample was then post-cured in an oven at 65 °C for 30 min and then rested at room temperature for a minimum of 24 h. PUF samples were prepared and tested with a four different graphene loading levels, 0.01, 0.025, 0.05, and 0.1%, to determine the effect of the graphene on the mechanical, thermal and other physical properties.

Life cycle assessment

A prospective LCA was conducted for FG produced from FJH and compared with graphene produced from graphite exfoliated by sonication or by oxidation to graphene oxide followed by chemical reduction. The study goal was to compare the cradle-to-gate impacts among these three alternative graphene synthesis pathways and thus graphene use and disposal was excluded. A functional unit of 1 kg of graphene powder was considered. Powdered graphene rather than solution phase dispersed graphene was modeled since it is of wider utility industrially and lighter to ship. The CED, GWP over a 100-year timescale, and CWU were evaluated. Process input and output data for sonication and chemical pathways were based on literature60,61. Material transportation or waste stream disposal/remediation were outside the scope of this limited study. Background data was principally sourced from Argonne National Laboratory’s GREET model including both GREET.Net software and spreadsheet models56,57,62. A detailed spreadsheet of process inputs and outputs as well as inventory and impact calculations can be found in the attachment Supplementary Data 1.

Characterization

All Raman spectra of FG were collected from samples, ground by mortar and pestle and not exposed to solvent. A Renishaw inVia Raman microscope outfitted with a 5 mW 532 nm laser was used, with ×50 optical objective lenses to collect high magnification spectra. All XRD spectra were collected of samples ground by mortar and pestle and not exposed to solvent. A Rigaku D/Max Ultima II Powder XRD 6 s were used to collect XRD patterns. A scan width of 0.05° per step and scan rate of 0.5° min−1 was used from 3° to 90°. Zero background sample holders were used. TGA thermograms were collected of samples ground by mortar and pestle and not exposed to solvent. A Q-600 Simultaneous TGA/DSC from TA Instruments was used. Alumina pans were used at a heating rate of 10 °C min−1 up to 780 °C. Atmospheric air at a flow rate of 80 ml min−1 was used to continuously purge the sample chamber. To determine the concentration of FG present in dispersed aqueous solutions, varying amounts of finely ground FG were added to a 1 wt% Pluronic-F127 aqueous solution. The solutions were cup horn ultrasonicated for 10 min at 25 °C, then centrifuged at 1000 RCF for 20 min. The supernatant was then diluted 200x and the absorbance measured at 660 nm. An extinction coefficient of 6600 l g−1 m−1 was used. An identical procedure was used to make commercial graphene dispersions. SEM images were taken with a FEI Helios Nanolab 660 Dual Beam SEM System. Low-voltage (1 keV) scans were taken of the PUF composites to minimize charging. XPS data were collected with a PHI Quantera SXM Scanning X-ray Microprobe with a base pressure of 5 × 10–9 Torr. Survey spectra were recorded using 0.5 eV step sizes with a pass energy of 140 eV. Elemental spectra were recorded using 0.1 eV step sizes with a pass energy of 26 eV. All of the XPS spectra were corrected using the C 1 s peaks (284.8 eV) as reference. TEM images were collected using a JEOL 2100F TEM system using samples of ELV-WP-FG briefly sonicated in ethanol, then drop cast on lacy carbon grids and allowed to air dry. An accelerating voltage of 200 kV was used. ASTM 3574-08 (Test A), ASTM 3574-08 (Test L), ASTM 3574-08 (Test C), ASTM 3574-08 (Test E) and ASTM D 624 (Die C) were used for apparent density, wet compression set, compression force deflection, tensile testing, and tear strength, respectively. At least six replicates were used for each measurement. The Tg was measured on a TA DSC 2500 DSC, using a sealed Tzero aluminum pan and lid. The foam samples of ~5 mg were investigated in nitrogen atmosphere from −90 to 100 °C, at a heating rate of 10 °C min−1 and a nitrogen flow rate of 50 ml min−1. The acoustic properties were found using a B&K Type 4206 two-microphone impedance tube (100 mm samples from 50–1600 Hz and 29 mm samples from 500–5000 Hz) to determine the plane wave absorption. The Transfer Function Method, ISO 10534-2 is used to calculate the Reflection factor and from that the Absorption and the Normal Specific Acoustic Impedance are calculated.

Data availability

The data used during this study are available in the manuscript and Supplementary Information. We have uploaded the source data to the Zenodo database, accessible at: https://doi.org/10.5281/zenodo.6335713. The source data in excel format is also uploaded as Supplementary Data 2.

References

1. Gross, M. A planet with two billion cars. Curr. Biol. 26, R307–R310 (2016).

2. Laborda, J. & Moral, M. J. Automotive aftermarket forecast in a changing world: the stakeholders’ perceptions boost! Sustainability 12, 7817 (2020).

3. Jody, B. J. & Daniels, E. J. End-of-Life Vehicle Recycling: The State of the Art of Resource Recovery from Shredder Residue (Argonne National Laboratory, Energy Systems Division: Du Page County, IL, USA, 2006; ANL/ESD/07–8).

4. Miller, L., Soulliere, K., Sawyer-Beaulieu, S., Tseng, S. & Tam, E. Challenges and alternatives to plastics recycling in the automotive sector. Materials 7, 5883–5902 (2014).

5. Zhao, Q. & Chen, M. A comparison of ELV recycling system in China and Japan and China’s strategies. Resour., Conserv. Recycl. 57, 15–21 (2011).

6. Sakai, S. et al. An international comparative study of end-of-life vehicle (ELV) recycling systems. J. Mater. Cycles. Waste Manag. 16, 1–20 (2014).

7. He, M., Lin, T., Wu, X., Luo, J. & Peng, Y. A systematic literature review of reverse logistics of end-of-life vehicles: Bibliometric analysis and research trend. Energies 13, 5586 (2020).

8. Vermeulen, I., Van Caneghem, J., Block, C., Baeyens, J. & Vandecasteele, C. Automotive shredder residue (ASR): Reviewing its production from end-of-life vehicles (ELVs) and its recycling, energy or chemicals’ valorisation. J. Hazard. Mater. 190, 8–27 (2011).

9. Li, W., Bai, H., Yin, J. & Xu, H. Life cycle assessment of end-of-life vehicle recycling processes in China—take Corolla taxis for example. J. Cleaner Prod. 117, 176–187 (2016).

10. Sakai, S., Noma, Y. & Kida, A. End-of-life vehicle recycling and automobile shredder residue management in Japan. J. Mater. Cycles. Waste Manag. 9, 151–158 (2007).

11. Lyu, M.-Y. & Choi, T. G. Research trends in polymer materials for use in lightweight vehicles. Int. J. Precis. Eng. Manuf. 16, 213–220 (2015).

12. Santini, A. et al. Auto shredder residue recycling: Mechanical separation and pyrolysis. Waste Manag. 32, 852–858 (2012).

13. Bhadra, J., Al-Thani, N. & Abdulkareem, A. 11—Recycling of polymer-polymer composites. In Micro and Nano Fibrillar Composites (MFCs and NFCs) from Polymer Blends (eds Mishra, R. K., Thomas, S. & Kalarikkal, N.) 263–277 (Woodhead Publishing, 2017).

14. Despeisse, M., Kishita, Y., Nakano, M. & Barwood, M. Towards a circular economy for end-of-life vehicles: a comparative study UK—Japan. Procedia CIRP 29, 668–673 (2015).

15. Othman, N., Razali, A., Chelliapan, S., Mohammad, R. & Kamyab, H. Chapter 18—a design framework for an integrated end-of-life vehicle waste management system in malaysia. in Soft Computing Techniques in Solid Waste and Wastewater Management (eds Karri, R. R., Ravindran, G. & Dehghani, M. H.) 305–319 (Elsevier, 2021).

16. Cardamone, G. F., Ardolino, F. & Arena, U. Can plastics from end-of-life vehicles be managed in a sustainable way? Sustainable Prod. Consump. 29, 115–127 (2022).

17. Karagoz, S., Aydin, N. & Simic, V. End-of-life vehicle management: a comprehensive review. J. Mater. Cycles. Waste Manag. 22, 416–442 (2020).

18. Khodier, A., Williams, K. & Dallison, N. Challenges around automotive shredder residue production and disposal. Waste Manage. 73, 566–573 (2018).

19. Miskolczi, N., Juzsakova, T. & Sója, J. Preparation and application of metal loaded ZSM-5 and y-zeolite catalysts for thermo-catalytic pyrolysis of real end of life vehicle plastics waste. J. Energy Inst. 92, 118–127 (2019).

20. Jung, S. et al. Catalytic pyrolysis of plastics derived from end-of-life-vehicles (ELVs) under the CO2 environment. Int. J. Energy Res. 45, 16781–16793 (2021).

21. Russo, S., Valero, A., Valero, A. & Iglesias-Émbil, M. Exergy-based assessment of polymers production and recycling: an application to the automotive sector. Energies 14, 363 (2021).

22. Gallone, T. & Zeni-Guido, A. Closed-loop polypropylene, an opportunity for the automotive sector. Field Actions Sci. Rep. 19, 48–53 (2019).

23. Reale Batista, M. D., Drzal, L. T., Kiziltas, A. & Mielewski, D. Hybrid cellulose-inorganic reinforcement polypropylene composites: lightweight materials for automotive applications. Poly. Comp. 41, 1074–1089 (2020).

24. Al Faruque, M. A. et al. Graphene oxide incorporated waste wool/PAN hybrid fibres. Sci. Rep. 11, 12068 (2021).

25. Langhorst, A. E. et al. Blue-agave fiber-reinforced polypropylene composites for automotive applications. BioResources 13, 820–835 (2018).

26. Kiziltas, A., Dowling, Z., Bedell, M. L. & Mielewski, D. Polyurethane foams containing additive manufacturing waste as filler for automotive applications and processes for manufacturing the same. 2021-11-25, US20210363317A1 (2021).

27. Algozeeb, W. A. et al. Flash graphene from plastic waste. ACS Nano 14, 15595–15604 (2020).

28. Luong, D. X. et al. Gram-scale bottom-up flash graphene synthesis. Nature 577, 647–651 (2020).

29. Ye, R. & Tour, J. M. Graphene at fifteen. ACS Nano 13, 10872–10878 (2019).

30. Allen, M. J., Tung, V. C. & Kaner, R. B. Honeycomb carbon: a review of graphene. Chem. Rev. 110, 132–145 (2010).

31. Universal Matter Inc Website. https://www.universalmatter.com/ (2022).

32. Wyss, K. M. et al. Converting plastic waste pyrolysis ash into flash graphene. Carbon 174, 430–438 (2021).

33. Wyss, K. M., Wang, Z., Alemany, L. B., Kittrell, C. & Tour, J. M. Bulk production of any ratio 12C:13C turbostratic flash graphene and its unusual spectroscopic characteristics. ACS Nano 15, 10542–10552 (2021).

34. Wyss, K. M., Luong, D. X. & Tour, J. M. Large-scale syntheses of 2D materials: flash joule heating and other methods. Adv. Mater. 34, 2106970 (2022).

35. Advincula, P. A. et al. Flash graphene from rubber waste. Carbon 178, 649–656 (2021).

36. Ferrari, A. C. & Basko, D. M. Raman spectroscopy as a versatile tool for studying the properties of graphene. Nat. Nanotech. 8, 235–246 (2013).

37. Carozo, V. et al. Raman signature of graphene superlattices. Nano Lett. 11, 4527–4534 (2011).

38. Merlen, A., Buijnsters, J. G. & Pardanaud, C. A guide to and review of the use of multiwavelength Raman spectroscopy for characterizing defective aromatic carbon solids: from graphene to amorphous carbons. Coatings 7, 153 (2017).

39. Wu, J.-B., Lin, M.-L., Cong, X., Liu, H.-N. & Tan, P.-H. Raman spectroscopy of graphene-based materials and its applications in related devices. Chem. Soc. Rev. 47, 1822–1873 (2018).

40. Poncharal, P., Ayari, A., Michel, T. & Sauvajol, J.-L. Raman spectra of misoriented bilayer graphene. Phys. Rev. B 78, 113407 (2008).

41. Jorio, A. & Cançado, L. G. Raman spectroscopy of twisted bilayer graphene. Solid State Commun. 175–176, 3–12 (2013).

42. Stanford, M. G. et al. Flash graphene morphologies. ACS Nano 14, 13691–13699 (2020).

43. Beckham, J. L. et al. Machine learning guided synthesis of flash graphene. Adv. Mater. 34, 2106506 (2022). 1-11.

44. Gama, N. V., Ferreira, A. & Barros-Timmons, A. Polyurethane foams: past, present, and future. Materials 11, 1841 (2018).

45. Miller, L. J., Sawyer-Beaulieu, S. & Tam, E. Impacts of non-traditional uses of polyurethane foam in automotive applications at end of life. SAE Int. J. Mater. Manufac. 7, 711–718 (2014).

46. Kiziltas, A., Mielewski, D. F. & Lee, E. C. United States Patent: 9868835—Bio-based polyurethane foam materials including graphite materials. (2018).

47. Lu, B. et al. High performance broadband acoustic absorption and sound sensing of a bubbled graphene monolith. J. Mater. Chem. A 7, 11423–11429 (2019).

48. Jin, H., Zhang, T., Bing, W., Dong, S. & Tian, L. Antifouling performance and mechanism of elastic graphene–silicone rubber composite membranes. J. Mater. Chem. B 7, 488–497 (2019).

49. Young, R. J., Kinloch, I. A., Gong, L. & Novoselov, K. S. The mechanics of graphene nanocomposites: a review. Compos. Sci. Tech. 72, 1459–1476 (2012).

50. Rafiee, M. A. et al. Enhanced mechanical properties of nanocomposites at low graphene content. ACS Nano 3, 3884–3890 (2009).

51. Reignier, J., Alcouffe, P., Méchin, F. & Fenouillot, F. The morphology of rigid polyurethane foam matrix and its evolution with time during foaming—new insight by cryogenic scanning electron microscopy. J. Colloid Interface Sci. 552, 153–165 (2019).

52. Gunashekar, S. & Abu-Zahra, N. Characterization of functionalized polyurethane foam for lead ion removal from water. Int. J. Poly. Sci. 2014, e570309 (2014).

53. Aviv, O. et al. Controlled iodine release from polyurethane sponges for water decontamination. J. Control. Release 172, 634–640 (2013).

54. Sheldon, R. A. Metrics of green chemistry and sustainability: past, present, and future. ACS Sustainable Chem. Eng. 6, 32–48 (2018).

55. Kauling, A. P. et al. The worldwide graphene flake production. Adv. Mater. 30, 1803784 (2018).

56. Arvidsson, R., Kushnir, D., Sandén, B. A. & Molander, S. Prospective life cycle assessment of graphene production by ultrasonication and chemical reduction. Environ. Sci. Technol. 48, 4529–4536 (2014).

57. Cossutta, M., McKechnie, J. & Pickering, S. J. A comparative LCA of different graphene production routes. Green Chem. 19, 5874–5884 (2017).

58. Beloin-Saint-Pierre, D. & Hischier, R. Towards a more environmentally sustainable production of graphene-based materials. Int. J. Life Cycle Assess. 26, 327–343 (2021).

59. Cullen, J. M. Circular economy: theoretical benchmark or perpetual motion machine? J. Ind. Ecology 21, 483–486 (2017).

60. Xu, Y., Cao, H., Xue, Y., Li, B. & Cai, W. Liquid-phase exfoliation of graphene: an overview on exfoliation media, techniques, and challenges. Nanomaterials 8, 942 (2018).

61. Marcano, D. C. et al. Improved synthesis of graphene oxide. ACS Nano 4, 4806–4814 (2010).

62. Wang, M. et al. Summary of expansions and updates in GREET 2020, Lemont, IL, US, 2020 report. https://doi.org/10.2172/1483843 (2022).

Download references

Acknowledgements

K.M.W. acknowledges the NSF Graduate Research Fellowship program for generous funding. The Air Force Office of Scientific Research (FA9550-19-1-0296) and the DOE-NETL (DE-FE0031794) funded this work. The authors acknowledge Dr. Rachel A. Meidl for assistance and fruitful conversation regarding the prospective LCA and closed-loop discussion. The authors acknowledge Ross Stineman from Ferrous Processing and Trading for the sample of ELV-WP from Ford F-150s. The authors acknowledge the use of Electron Microscopy Center (EMC) at Rice University, and Dr. Bo Chen for help with the XPS data analysis.

Author information

Authors

Contributions

K.M.W. performed FJH station fabrication, all graphene synthesis and characterization, and undertook the LCA, as well as writing the manuscript. R.L.C. and A.K. performed PUF fabrication and characterization. R.D.D.K. contributed to the LCA. J.M.T. and D.F.M. assisted in managing, experimental design, and funding. All authors have given approval to the final version of the manuscript.

Corresponding author

Correspondence to James M. Tour.

Ethics declarations

Competing interests

Rice University owns intellectual property on the flash Joule heating process to make graphene. That intellectual property has been licensed to Universal Matter Inc. and Universal Matter LLC. J.M.T. is a stockholder in those licensee companies, but he is not an officer, director, or employee in those companies. Potential conflicts of interest are mitigated through regular disclosures to and compliance with Rice University’s Office of Sponsored Programs and Research Compliance. None of the other authors have competing interests to declare.

Peer review

Peer review information

Communications Engineering thanks Filomena Ardolino and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Danielle Densley Tingley and Rosamund Daw.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

Reprints and Permissions

About this article

Cite this article

Wyss, K.M., De Kleine, R.D., Couvreur, R.L. et al. Upcycling end-of-life vehicle waste plastic into flash graphene. Commun Eng 1, 3 (2022). https://doi.org/10.1038/s44172-022-00006-7

Download citation

• Received:

• Accepted:

• Published:

• DOI: https://doi.org/10.1038/s44172-022-00006-7

Further reading

• Graphene from plastic waste makes cars greener

• Giulia Pacchioni

Nature Reviews Materials (2022)

Search

Quick links

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing