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.

  • Review Article
  • Published:

3D metamaterials

Abstract

Metamaterials are rationally designed composites aiming at effective material parameters that go beyond those of the ingredient materials. For example, negative metamaterial properties, such as the refractive index, thermal expansion coefficient or Hall coefficient, can be engineered from constituents with positive parameters. Likewise, large metamaterial parameter values can arise from all-zero constituents, such as magnetic from non-magnetic, chiral from achiral and anisotropic from isotropic. The field of metamaterials emerged from linear electromagnetism two decades ago and today addresses nearly all conceivable aspects of solids, ranging from electromagnetic and optical, and mechanical and acoustic to transport properties — linear and nonlinear, reciprocal and non-reciprocal, monostable and multistable (programmable), active and passive, and static and dynamic. In this Review, we focus on the general case of 3D periodic metamaterials, with electromagnetic or optical, acoustic or mechanical, transport or stimuli-responsive properties. We outline the fundamental bounds of these composites and summarize the state of the art in theoretical design and experimental realization.

Key points

  • Metamaterials are rationally designed composites made of tailored building blocks, which are composed of one or more constituent bulk materials, leading to effective medium properties beyond those of their ingredients.

  • Metamaterials thereby fulfil a long-standing dream of condensed matter physics to design materials on the computer to avoid tedious trial-and-error procedures and excessive experimentation.

  • Although many 1D and 2D model architectures have been considered because of their ease of fabrication and reduced design complexity, the full potential of the metamaterial concept is opened up for 3D microstructures and nanostructures.

  • In electromagnetism and optics, examples are effective diamagnetism and paramagnetism up to optical frequencies, impedance matching and duality, negative refractive indices, maximum electromagnetic chirality, perfect optical absorption and non-reciprocal propagation of electromagnetic waves without static magnetic fields.

  • In acoustics and mechanics, examples are the complete flexibility in tailoring elastic parameters, chiral mechanical behaviour, sign reversal of the static effective compressibility, negative dynamic mass density, non-reciprocal sound propagation, broadband perfect sound absorption at the fundamental limit imposed by causality and highly nonlinear, multistable and programmable properties from linear elastic constituents.

  • In transport, examples are highly anisotropic thermal conductance, sign reversal of the absolute mobility and the Hall coefficient, highly anisotropic Hall tensors, giant magnetoresistances and thermoelectric power factors that are enhanced by orders of magnitude.

  • Future 3D material printers may achieve thousands of different effective metamaterial properties from only a small number of input material cartridges — in analogy to today’s 2D graphical printers that mix thousands of colours from just three colour cartridges.

This is a preview of subscription content, access via your institution

Access options

Buy this article

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: From atoms via 3D materials to designed unit cells and 3D metamaterials.
Fig. 2: Gallery of designed 3D optical metamaterial unit cells and corresponding experimental realizations.
Fig. 3: Gallery of designed 3D acoustical and mechanical metamaterial unit cells and corresponding experimental realizations.
Fig. 4: Gallery of designed transport metamaterial unit cells and corresponding experimental realizations.

Similar content being viewed by others

References

  1. Walser, R. M. Electromagnetic metamaterials. Proc. SPIE, Complex Mediums II: Beyond Linear Isotropic Dielectr. (eds Lakhtakia, A., Weiglhofer, W. S. & Hodgkinson, I. J.) 4467 (2001).

    ADS  Google Scholar 

  2. Bendsøe, M. & Sigmund, O. Topology Optimization: Theory, Methods and Applications (Springer, 2004).

  3. Browning, V. DARPATech 2002 7, 791–795 (2002).

    Google Scholar 

  4. Kittel, C. Introduction to Solid State Physics (Wiley, 2004).

  5. Milton, G. W. The Theory of Composites (Cambridge Univ. Press, 2002). This textbook provides a comprehensive theoretical introduction to man-made composite materials in electromagnetism and optics, acoustics and mechanics, and transport.

  6. Golden, K., Grimmett, G., James, R., Milton, G. & Sen, P. Mathematics of Multiscale Materials (Springer, New York, 2012).

  7. Chen, H.-T., Taylor, A. J. & Yu, N. A review of metasurfaces: physics and applications. Rep. Prog. Phys. 79, 076401 (2016).

    Article  ADS  Google Scholar 

  8. McCall, M. et al. Roadmap on transformation optics. J. Opt. 20, 063001 (2018).

    Article  ADS  Google Scholar 

  9. Craster, R. V., Kaplunov, J. & Pichugin, A. V. High-frequency homogenization for periodic media. Proc. R. Soc. Lond. Ser. A 466, 2341–2362 (2010).

    Article  ADS  MathSciNet  MATH  Google Scholar 

  10. Bensoussan, A., Lions, J. & Papanicolaou, G. Asymptotic Analysis for Periodic Structures (American Mathematical Society, 2011).

  11. Jikov, V., Yosifian, G., Kozlov, S. & Oleinik, O. Homogenization of Differential Operators and Integral Functionals (Springer, Berlin Heidelberg, 2012).

  12. Bakhvalov, N. & Panasenko, G. Homogenisation: Averaging Processes in Periodic Media: Mathematical Problems in the Mechanics of Composite Materials (Springer, Netherlands, 2012).

  13. Pham, K., Maurel, A. & Marigo, J.-J. Two scale homogenization of a row of locally resonant inclusions — the case of anti-plane shear waves. J. Mech. Phy. Solids 106, 80–94 (2017).

    Article  ADS  MathSciNet  Google Scholar 

  14. Brassart, M. & Lenczner, M. A two-scale model for the periodic homogenization of the wave equation. J. Math. Pures Appl. 93, 474–517 (2010).

    Article  MathSciNet  MATH  Google Scholar 

  15. Harutyunyan, D., Milton, G. W. & Craster, R. V. High-frequency homogenization for travelling waves in periodic media. Proc. R. Soc. Lond. Ser. A 472, 20160066 (2016).

    Article  ADS  MathSciNet  MATH  Google Scholar 

  16. Krushlov, E. Y. The asymptotic behavior of solutions of the second boundary value problem under fragmentation of the boundary of the domain. Math. USSR Sb. 35, 266–282 (1979).

    Article  Google Scholar 

  17. Auriault, J. L. & Boutin, C. Deformable porous media with double porosity III: acoustics. Transp. Porous Med. 14, 143–162 (1994).

    Article  Google Scholar 

  18. Zhikov, V. V. On an extension of the method of two-scale convergence and its applications. Sb. Math. 191, 31–72 (2010).

    Article  Google Scholar 

  19. Pendry, J. B., Holden, A. J., Stewart, W. J. & Youngs, I. Extremely low frequency plasmons in metallic mesostructures. Phys. Rev. Lett. 76, 4773–4776 (1996).

    Article  ADS  Google Scholar 

  20. Belov, P. A. et al. Strong spatial dispersion in wire media in the very large wavelength limit. Phys. Rev. B 67, 113103 (2003).

    Article  ADS  Google Scholar 

  21. Menzel, C. et al. Validity of effective material parameters for optical fishnet metamaterials. Phys. Rev. B 81, 035320 (2010).

    Article  ADS  Google Scholar 

  22. Soukoulis, C. & Wegener, M. Past achievements and future challenges in the development of three-dimensional photonic metamaterials. Nat. Photonics 5, 523–530 (2011). This reference is an extensive review on 3D optical metamaterials, with emphasis on negative refractive indices and chirality; it also provides a more detailed history of the field.

    Article  ADS  Google Scholar 

  23. Laude, V. Phononic Crystals: Artificial Crystals for Sonic, Acoustic, and Elastic Waves (De Gruyter, 2015). This is a textbook introduction to acoustic and elastic metamaterials.

  24. Kane, C. L. & Lubensky, T. C. Topological boundary modes in isostatic lattices. Nat. Phys. 10, 39–45 (2014).

    Article  Google Scholar 

  25. Süsstrunk, R. & Huber, S. D. Observation of phononic helical edge states in a mechanical topological insulator. Science 349, 47–50 (2015).

    Article  ADS  Google Scholar 

  26. Süsstrunk, R. & Huber, S. D. Classification of topological phonons in linear mechanical metamaterials. Proc. Natl Acad. Sci. USA 113, E4767–E4775 (2016).

    Article  ADS  Google Scholar 

  27. Landau, L. D. & Lifshitz, E. M. Electrodynamics of Continuous Media (Pergamon Press, 1960).

  28. Schelkunoff, S. A. & Friis, H. T. Antennas: the Theory and Practice (Wiley, 1952).

  29. Pendry, J. B., Holden, A. J., Robbins, D. J. & Stewart, W. J. Magnetism from conductors and enhanced nonlinear phenomena. IEEE Trans. Microwave Theory Tech. 47, 2075–2084 (1999).

    Article  ADS  Google Scholar 

  30. Schneider, H. J. & Dullenkopf, P. Slotted tube resonator: a new NMR probe head at high observing frequencies. Rev. Sci. Instrum. 48, 68–73 (1977).

    Article  ADS  Google Scholar 

  31. Ghim, B. T., Rinard, G. A., Quine, R. W., Eaton, S. S. & Eaton, G. R. Design and fabrication of copper-film loop–gap resonators. J. Magn. Reson. A 120, 72–76 (1996).

    Article  ADS  Google Scholar 

  32. Lagarkov, A. N. & Sarychev, A. K. Electromagnetic properties of composites containing elongated conducting inclusions. Phys. Rev. B 53, 6318–6336 (1996).

    Article  ADS  Google Scholar 

  33. Rose-Innes, A. & Rhoderick, E. Introduction to Superconductivity (Pergamon Press, 1978).

  34. Meade, R. & Diffenderfer, R. Foundations of Electronics: Circuits and Devices (Delmar Cengage Learning, 2002).

  35. Dolling, G., Enkrich, C., Wegener, M., Soukoulis, C. M. & Linden, S. Simultaneous negative phase and group velocity of light in a metamaterial. Science 312, 892–894 (2006).

    Article  ADS  Google Scholar 

  36. Veselago, V. The electrodnamics of substances with simultaneously negative values of and μ. Sov. Phys. Usp. 10, 509–514 (1968). This is an inspiring ‘what if’ paper, showing some of the enhanced possibilities with negative magnetic permeability and negative refractive index in optics; it stimulates much of the early work on electromagnetic and optical metamaterials.

    Article  ADS  Google Scholar 

  37. Pendry, J. B. Negative refraction makes a perfect lens. Phys. Rev. Lett. 85, 3966–3969 (2000).

    Article  ADS  Google Scholar 

  38. Shelby, R. A., Smith, D. R. & Schultz, S. Experimental verification of a negative index of refraction. Science 292, 77–79 (2001). This experimental paper on negative refractive indices at microwave frequencies is a catalyser for the metamaterial field.

    Article  ADS  Google Scholar 

  39. Zhang, S. et al. Experimental demonstration of near-infrared negative-index metamaterials. Phys. Rev. Lett. 95, 137404 (2005).

    Article  ADS  Google Scholar 

  40. Soukoulis, C. M., Linden, S. & Wegener, M. Negative refractive index at optical wavelengths. Science 315, 47–49 (2007).

    Article  Google Scholar 

  41. García-Meca, C. et al. Low-loss multilayered metamaterial exhibiting a negative index of refraction at visible wavelengths. Phys. Rev. Lett. 106, 067402 (2011).

    Article  ADS  Google Scholar 

  42. Kinsler, P. & McCall, M. W. Causality-based criteria for a negative refractive index must be used with care. Phys. Rev. Lett. 101, 167401 (2008).

    Article  ADS  Google Scholar 

  43. Zheludev, N. I., Prosvirnin, S. L., Papasimakis, N. & Fedotov, V. A. Lasing spaser. Nat. Photonics 2, 351–354 (2008).

    Article  ADS  Google Scholar 

  44. Noginov, M. A. et al. Demonstration of a spaser-based nanolaser. Nature 460, 1110–1112 (2009).

    Article  ADS  Google Scholar 

  45. Fang, A., Koschny, T., Wegener, M. & Soukoulis, C. M. Self-consistent calculation of metamaterials with gain. Phys. Rev. B 79, 241104 (2009).

    Article  ADS  Google Scholar 

  46. Wuestner, S., Pusch, A., Tsakmakidis, K. L., Hamm, J. M. & Hess, O. Overcoming losses with gain in a negative refractive index metamaterial. Phys. Rev. Lett. 105, 127401 (2010).

    Article  ADS  Google Scholar 

  47. Valentine, J. et al. Three-dimensional optical metamaterial with a negative refractive index. Nature 455, 376–379 (2008).

    Article  ADS  Google Scholar 

  48. Nguyen, V. C., Chen, L. & Halterman, K. Total transmission and total reflection by zero index metamaterials with defects. Phys. Rev. Lett. 105, 233908 (2010).

    Article  ADS  Google Scholar 

  49. Moitra, P. et al. Realization of an all-dielectric zero-index optical metamaterial. Nat. Photonics 7, 791–795 (2013).

    Article  ADS  Google Scholar 

  50. Javani, M. H. & Stockman, M. I. Real and imaginary properties of epsilon-near-zero materials. Phys. Rev. Lett. 117, 107404 (2016).

    Article  ADS  Google Scholar 

  51. Wegener, M., Dolling, G. & Linden, S. Plasmonics: backward waves moving forward. Nat. Mater. 6, 475–476 (2007).

    Article  ADS  Google Scholar 

  52. Yao, J. et al. Optical negative refraction in bulk metamaterials of nanowires. Science 321, 930 (2008).

    Article  ADS  Google Scholar 

  53. Cui, T., Smith, D. & Liu, R. Metamaterials: Theory, Design, and Applications (Springer, 2009).

  54. Krishnamoorthy, H. N., Jacob, Z., Narimanov, E., Kretzschmar, I. & Menon, V. M. Topological transitions in metamaterials. Science 336, 205–209 (2012).

    Article  ADS  MathSciNet  MATH  Google Scholar 

  55. García-Chocano, V. M., Christensen, J. & Sánchez-Dehesa, J. Negative refraction and energy funneling by hyperbolic materials: an experimental demonstration in acoustics. Phys. Rev. Lett. 112, 144301 (2014).

    Article  ADS  Google Scholar 

  56. Ferrari, L., Wu, C., Lepage, D., Zhang, X. & Liu, Z. Hyperbolic metamaterials and their applications. Prog. Quantum Electron. 40, 1–40 (2015).

    Article  Google Scholar 

  57. Landy, N. I., Sajuyigbe, S., Mock, J. J., Smith, D. R. & Padilla, W. J. Perfect metamaterial absorber. Phys. Rev. Lett. 100, 207402 (2008).

    Article  ADS  Google Scholar 

  58. Liu, N. et al. Plasmonic analogue of electromagnetically induced transparency at the drude damping limit. Nat. Mater. 8, 758–762 (2009).

    Article  ADS  Google Scholar 

  59. Watts, C. M., Xianliang, L. & Padilla, W. J. Metamaterial electromagnetic wave absorbers. Adv. Mater. 24, 98–120 (2012).

    Google Scholar 

  60. Lee, Y., Rhee, J., Yoo, Y. & Kim, K. Metamaterials for Perfect Absorption (Springer, Singapore, 2016).

  61. Pfeiffer, C. & Grbic, A. Metamaterial huygens surfaces: tailoring wave fronts with reflectionless sheets. Phys. Rev. Lett. 110, 197401 (2013).

    Article  ADS  Google Scholar 

  62. Liu, S. et al. Optical magnetic mirrors without metals. Optica 1, 250–256 (2014).

    Article  Google Scholar 

  63. Lindell, I. Electromagnetic Waves in Chiral and Bi-isotropic Media (Artech House, 1994).

  64. Tretyakov, S., Sihvola, A., Sochava, A. & Simovski, C. Magnetoelectric interactions in bi-anisotropic media. J. Electro. Wave. Appl. 12, 481–497 (1998).

    Article  Google Scholar 

  65. Eringen, A. Elastodynamics Vol. 2 (Academic Press, 1974).

  66. Wegener, M. & Linden, S. in Tutorials in Metamaterials Ch. 8 (eds Noginov, M. A. & Podolskiy, V. A.) (Taylor and Francis, 2012).

  67. Gansel, J. et al. Gold helix photonic metamaterial as broadband circular polarizer. Science 325, 1513–1515 (2009).

    Article  ADS  Google Scholar 

  68. Gansel, J. et al. Tapered gold-helix metamaterials as improved circular polarizers. Appl. Phys. Lett. 100, 101109 (2012).

    Article  ADS  Google Scholar 

  69. Johannes, K. et al. A helical metamaterial for broadband circular polarization conversion. Adv. Opt. Mater. 3, 1411–1417 (2015).

    Article  Google Scholar 

  70. Kaschke, J. & Wegener, M. Gold triple-helix mid-infrared metamaterial by sted-inspired laser lithography. Opt. Lett. 40, 3986–3989 (2015).

    Article  ADS  Google Scholar 

  71. Fernandez-Corbaton, I., Fruhnert, M. & Rockstuhl, C. Objects of maximum electromagnetic chirality. Phys. Rev. X 6, 031013 (2016).

    Google Scholar 

  72. Lefier, Y., Salut, R., Suarez, M. A. & Grosjean, T. Directing nanoscale optical flows by coupling photon spin to plasmon extrinsic angular momentum. Nano. Lett. 18, 38–42 (2018).

    Article  ADS  Google Scholar 

  73. Chin, J. Y. et al. Nonreciprocal plasmonics enables giant enhancement of thin-film faraday rotation. Nat. Commun. 4, 1599 (2013).

    Article  Google Scholar 

  74. Fedotov, V. A. et al. Asymmetric propagation of electromagnetic waves through a planar chiral structure. Phys. Rev. Lett. 97, 167401 (2006).

    Article  ADS  Google Scholar 

  75. Kaelberer, T., Fedotov, V., Papasimakis, N., Tsai, D. & Zheludev, N. Toroidal dipolar response in a metamaterial. Science 330, 1510–1512 (2010).

    Article  ADS  Google Scholar 

  76. Papasimakis, N., Fedotov, V. A., Savinov, V., Raybould, T. A. & Zheludev, N. I. Electromagnetic toroidal excitations in matter and free space. Nat. Mater. 15, 263–271 (2016).

    Article  ADS  Google Scholar 

  77. Fernandez-Corbaton, I., Nanz, S. & Rockstuhl, C. On the dynamic toroidal multipoles from localized electric current distributions. Sci. Rep. 7, 7527 (2017).

    Article  ADS  Google Scholar 

  78. Wegener, M. Extreme Nonlinear Optics (Springer-Verlag, 2005).

  79. Kauranen, M. & Zayats, A. V. Nonlinear plasmonics. Nat. Photonics 6, 737–748 (2012).

    Article  ADS  Google Scholar 

  80. Lee, J. et al. Giant nonlinear response from plasmonic metasurfaces coupled to intersubband transitions. Nature 511, 65–69 (2014).

    Article  ADS  Google Scholar 

  81. Samson, Z. L. et al. Metamaterial electro-optic switch of nanoscale thickness. Appl. Phys. Lett. 96, 143105 (2010).

    Article  ADS  Google Scholar 

  82. Buchnev, O., Ou, J. Y., Kaczmarek, M., Zheludev, N. I. & Fedotov, V. A. Electro-optical control in a plasmonic metamaterial hybridised with a liquid-crystal cell. Opt. Express 21, 1633–1638 (2013).

    Article  ADS  Google Scholar 

  83. Khurgin, J. B. & Sun, G. Plasmonic enhancement of the third order nonlinear optical phenomena: figures of merit. Opt. Express 21, 27460–27480 (2013).

    Article  ADS  Google Scholar 

  84. Jahani, S. & Jacob, Z. All-dielectric metamaterials. Nat. Nanotechnol. 11, 23–36 (2016).

    Article  ADS  Google Scholar 

  85. Staude, I. & Schilling, J. Metamaterial-inspired silicon nanophotonics. Nat. Photonics 11, 274–284 (2017).

    Article  ADS  Google Scholar 

  86. Hermans, A. et al. On the determination of χ (2) in thin films: a comparison of one-beam second-harmonic generation measurement methodologies. Sci. Rep. 7, 44581 (2017).

    Article  ADS  Google Scholar 

  87. Kadic, M., Bückmann, T., Schittny, R. & Wegener, M. Metamaterials beyond electromagnetism. Rep. Prog. Phys. 76, 126501 (2013).

    Article  ADS  Google Scholar 

  88. Ding, Y., Liu, Z., Qiu, C. & Shi, J. Metamaterial with simultaneously negative bulk modulus and mass density. Phys. Rev. Lett. 99, 093904 (2007).

    Article  ADS  Google Scholar 

  89. Lee, S. H., Park, C. M., Seo, Y. M., Wang, Z. G. & Kim, C. K. Composite acoustic medium with simultaneously negative density and modulus. Phys. Rev. Lett. 104, 054301 (2010).

    Article  ADS  Google Scholar 

  90. Wu, Y., Lai, Y. & Zhang, Z. Q. Elastic metamaterials with simultaneously negative effective shear modulus and mass density. Phys. Rev. Lett. 107, 105506 (2011).

    Article  ADS  Google Scholar 

  91. Cummer, S. A., Christensen, J. & Alu, A. Controlling sound with acoustic metamaterials. Nat. Rev. Mater. 1, 16001 (2016).

    Article  ADS  Google Scholar 

  92. Liu, Z. et al. Locally resonant sonic materials. Science 289, 1734–1736 (2000).

    Article  ADS  Google Scholar 

  93. Schoenberg, M. & Sen, P. N. Properties of a periodically stratified acoustic half-space and its relation to a Biot fluid. J. Acoust. Soc. Am. 73, 61–67 (1983).

    Article  ADS  MATH  Google Scholar 

  94. Milton, G. W., Birane, M. & Willis, J. R. On cloaking for elasticity and physical equations with a transformation invariant form. New J. Phys. 8, 248 (2006).

    Article  ADS  Google Scholar 

  95. Willis, J. R. Effective constitutive relations for waves in composites and metamaterials. Proc. Roy. Soc. Lond. A 467, 1865–1879 (2011).

    Article  ADS  MathSciNet  MATH  Google Scholar 

  96. Muhlestein, M. B., Sieck, C. F., Wilson, P. S. & Haberman, M. R. Experimental evidence of Willis coupling in a one-dimensional effective material element. Nat. Commun. 8, 15625 (2017).

    Article  ADS  Google Scholar 

  97. Bueckmann, T., Kadic, M., Schittny, R. & Wegener, M. Mechanical metamaterials with anisotropic and negative effective mass-density tensor made from one constituent material. Phys. Status Solidi B 252, 1671–1674 (2015).

    Article  ADS  Google Scholar 

  98. Yang, M., Chen, S., Fu, C. & Sheng, P. Optimal sound-absorbing structures. Mater. Horiz. 4, 673–680 (2017).

    Article  Google Scholar 

  99. Ma, G. & Sheng, P. Acoustic metamaterials: from local resonances to broad horizons. Sci. Adv. 2, e1501595 (2015).

    Article  ADS  Google Scholar 

  100. Liang, Z. & Li, J. Extreme acoustic metamaterial by coiling up space. Phys. Rev. Lett. 108, 114301 (2012).

    Article  ADS  Google Scholar 

  101. Frenzel, T. et al. Three-dimensional labyrinthine acoustic metamaterials. Appl. Phys. Lett. 103, 061907 (2013).

    Article  ADS  Google Scholar 

  102. Xie, Y., Konneker, A., Popa, B.-I. & Cummer, S. A. Tapered labyrinthine acoustic metamaterials for broadband impedance matching. Appl. Phys. Lett. 103, 201906 (2013).

    Article  ADS  Google Scholar 

  103. Krushynska, A. O., Bosia, F., Miniaci, M. & Pugno, N. M. Spider web-structured labyrinthine acoustic metamaterials for low-frequency sound control. New J. Phys. 19, 105001 (2017).

    Article  ADS  Google Scholar 

  104. Maurya, S. K., Pandey, A., Shukla, S. & Saxena, S. Double negativity in 3D space coiling metamaterials. Sci. Rep. 6, 33683 (2016).

    Article  ADS  Google Scholar 

  105. Fleury, R., Sounas, D. L., Sieck, C. F., Haberman, M. R. & Alu, A. Sound isolation and giant linear nonreciprocity in a compact acoustic circulator. Science 343, 516–519 (2014).

    Article  ADS  Google Scholar 

  106. Aurégan, Y. & Pagneux, V. 𝒫𝒯-symmetric scattering in flow duct acoustics. Phys. Rev. Lett. 118, 174301 (2017).

    Article  ADS  Google Scholar 

  107. Banerjee, B. An Introduction to Metamaterials and Waves in Composites (Taylor and Francis, 2011).

  108. Walpole, L. On bounds for the overall elastic moduli of inhomogeneous systems — I. J. Mech. Phy. Solids 14, 151–162 (1966).

    Article  ADS  MATH  Google Scholar 

  109. Milton, G. W. Complete characterization of the macroscopic deformations of periodic unimode metamaterials of rigid bars and pivots. J. Mech. Phy. Solids 61, 1543–1560 (2013).

    Article  ADS  MathSciNet  Google Scholar 

  110. Milton, G. W. & Cherkaev, A. V. Which elasticity tensors are realizable? J. Eng. Mater. Technol. 117, 483–493 (1995).

    Article  Google Scholar 

  111. Kadic, M., Bückmann, T., Stenger, N., Thiel, M. & Wegener, M. On the practicability of pentamode mechanical metamaterials. Appl. Phys. Lett. 100, 191901 (2012).

    Article  ADS  Google Scholar 

  112. Kadic, M., Schittny, R., Bückmann, T. & Wegener, M. On anisotropic versions of three-dimensional pentamode metamaterials. New J. Phys. 15, 023029 (2013).

    Article  ADS  Google Scholar 

  113. Bueckmann, T. et al. On three-dimensional dilational elastic metamaterials. New J. Phys. 16, 033032 (2014).

    Article  ADS  Google Scholar 

  114. Biot, M. Theory of propagation of elastic waves in a fluid-saturated porous solid. I. Low-frequency range. J. Acoust. Soc. Am. 28, 168–178 (1956).

    Article  ADS  MathSciNet  Google Scholar 

  115. Biot, M. & Willis, D. The elastic coefficients of the theory of consolidation. J. Appl. Mech. 24, 594–601 (1957).

    MathSciNet  Google Scholar 

  116. Gatt, R. & Grima, J. N. Negative compressibility. Phys. Status Solidi RRL 2, 236–238 (2008).

    Article  Google Scholar 

  117. Qu, J., Kadic, M. & Wegener, M. Poroelastic metamaterials with negative effective static compressibility. Appl. Phys. Lett. 110, 171901 (2017).

    Article  ADS  Google Scholar 

  118. Qu, J., Gerber, A., Mayer, F., Kadic, M. & Wegener, M. Experiments on metamaterials with negative effective static compressibility. Phys. Rev. X 7, 041060 (2017).

    Google Scholar 

  119. Sommerfeld, A. Mechanics of Deformable Bodies (Academic Press, 1950).

  120. Eringen, A. Microcontinuum Field Theories I: Foundations and Solids. (Springer, New York, 1999). This is a textbook on theoretical paths towards generalizing linear Cauchy elasticity.

  121. Frenzel, T., Kadic, M. & Wegener, M. Three-dimensional mechanical metamaterials with a twist. Science 358, 1072–1074 (2017).

    Article  ADS  Google Scholar 

  122. Rueger, Z. & Lakes, R. S. Strong cosserat elasticity in a transversely isotropic polymer lattice. Phys. Rev. Lett. 120, 065501 (2018).

    Article  ADS  Google Scholar 

  123. Zhu, H. et al. Observation of chiral phonons. Science 359, 579–582 (2018).

    Article  ADS  MathSciNet  Google Scholar 

  124. Coulais, C., Kettenis, C. & van Hecke, M. A characteristic length scale causes anomalous size effects and boundary programmability in mechanical metamaterials. Nat. Phys. 14, 40–44 (2017).

    Article  Google Scholar 

  125. Kadic, M., Frenzel, T. & Wegener, M. Mechanical metamaterials: when size matters. Nat. Phys. 14, 8–9 (2018).

    Article  Google Scholar 

  126. Nash, L. M. et al. Topological mechanics of gyroscopic metamaterials. Proc. Natl Acad. Sci. USA 112, 14495–14500 (2015).

    Article  ADS  Google Scholar 

  127. Hassanpour, S. & Heppler, G. R. Theory of micropolar gyroelastic continua. Acta Mech. 227, 1469–1491 (2016).

    Article  MathSciNet  MATH  Google Scholar 

  128. Carta, G., Jones, I. S., Movchan, N. V., Movchan, A. B. & Nieves, M. J. Deflecting elastic prism and unidirectional localisation for waves in chiral elastic systems. Sci. Rep. 7, 26 (2017).

    Article  ADS  Google Scholar 

  129. Abdoul-Anziz, H. & Seppecher, P. Strain gradient and generalized continua obtained by homogenizing frame lattices. Math. Mech. Complex Syst. 6, 213–250 (2018).

    MathSciNet  MATH  Google Scholar 

  130. Gudmundson, P. A unified treatment of strain gradient plasticity. J. Mech. Phys. Solids 52, 1379–1406 (2004).

    Article  ADS  MathSciNet  MATH  Google Scholar 

  131. Olive, M. & Auffray, N. Symmetry classes for odd-order tensors. J. Appl. Math. Mech. 94, 421–447 (2014).

    MathSciNet  MATH  Google Scholar 

  132. Cordero, N. M., Forest, S. & Busso, E. P. Second strain gradient elasticity of nano-objects. J. Mech. Phys. Solids 97, 92–124 (2016).

    Article  ADS  MathSciNet  Google Scholar 

  133. Liebold, C. & Mueller, W. H. Comparison of gradient elasticity models for the bending of micromaterials. Comput. Mater. Sci. 116, 52–61 (2016).

    Article  Google Scholar 

  134. Lecoutre, G., Daher, N., Devel, M. & Hirsinger, L. Principle of virtual power applied to deformable semiconductors with strain, polarization, and magnetization gradients. Acta Mech. 228, 1681–1710 (2017).

    Article  MathSciNet  MATH  Google Scholar 

  135. Bertram, A. Compendium on gradient materials. Redaktion http://www.redaktion.tu-berlin.de/fileadmin/fg49/publikationen/bertram/Compendium_on_Gradient_Materials_Dec_2017.pdf (2017).

  136. Wang, P., Casadei, F., Shan, S., Weaver, J. C. & Bertoldi, K. Harnessing buckling to design tunable locally resonant acoustic metamaterials. Phys. Rev. Lett. 113, 014301 (2014).

    Article  ADS  Google Scholar 

  137. Florijn, B., Coulais, C. & van Hecke, M. Programmable mechanical metamaterials. Phys. Rev. Lett. 113, 175503 (2014).

    Article  ADS  Google Scholar 

  138. Kang, S. H. et al. Complex ordered patterns in mechanical instability induced geometrically frustrated triangular cellular structures. Phys. Rev. Lett. 112, 098701 (2014).

    Article  ADS  Google Scholar 

  139. Bauer, J. et al. Nanolattices: an emerging class of mechanical metamaterials. Adv. Mater. 29, 1701850 (2017).

    Article  Google Scholar 

  140. Bertoldi, K., Vitelli, V., Christensen, J. & van Hecke, M. Flexible mechanical metamaterials. Nat. Rev. Mater. 2, 17066 (2017). This is a further reading on recent progress in 2D and 3D elastic metamaterials.

    Article  ADS  Google Scholar 

  141. Tobias, F., Claudio, F., Muamer, K., Peter, G. & Martin, W. Tailored buckling microlattices as reusable light-weight shock absorbers. Adv. Mater. 28, 5865–5870 (2016).

    Article  Google Scholar 

  142. Coulais, C., Teomy, E., de Reus, K., Shokef, Y. & van Hecke, M. Combinatorial design of textured mechanical metamaterials. Nature 535, 529–532 (2016).

    Article  ADS  Google Scholar 

  143. Schenk, M. & Guest, S. D. Geometry of miura-folded metamaterials. Proc. Natl Acad. Sci. USA 110, 3276–3281 (2013).

    Article  ADS  Google Scholar 

  144. Wei, Z. Y., Guo, Z. V., Dudte, L., Liang, H. Y. & Mahadevan, L. Geometric mechanics of periodic pleated origami. Phys. Rev. Lett. 110, 215501 (2013).

    Article  ADS  Google Scholar 

  145. Waitukaitis, S., Menaut, R., Chen, B. G.-G. & van Hecke, M. Origami multistability: from single vertices to metasheets. Phys. Rev. Lett. 114, 055503 (2015).

    Article  ADS  Google Scholar 

  146. Silverberg, J. L. et al. Origami structures with a critical transition to bistability arising from hidden degrees of freedom. Nat. Mater. 14, 389–393 (2015).

    Article  ADS  Google Scholar 

  147. Coulais, C., Sounas, D. & Alu, A. Static non-reciprocity in mechanical metamaterials. Nature 542, 461–464 (2017).

    Article  ADS  Google Scholar 

  148. Gao, H., Ji, B., Jaeger, I. L., Arzt, E. & Fratzl, P. Materials become insensitive to flaws at nanoscale: lessons from nature. Proc. Natl Acad. Sci. USA 100, 5597–5600 (2003).

    Article  ADS  Google Scholar 

  149. Schaedler, T. A. et al. Ultralight metallic microlattices. Science 334, 962–965 (2011).

    Article  ADS  Google Scholar 

  150. Zheng, X. et al. Ultralight, ultrastiff, mechanical metamaterials. Science 344, 1373–1377 (2014).

    Article  ADS  Google Scholar 

  151. Meza, L. R., Das, S. & Greer, J. R. Strong, lightweight, and recoverable three-dimensional ceramic nanolattices. Science 345, 1322–1326 (2014).

    Article  ADS  Google Scholar 

  152. Bauer, J., Hengsbach, S., Tesari, I., Schwaiger, R. & Kraft, O. High-strength cellular ceramic composites with 3D microarchitecture. Proc. Natl Acad. Sci. USA 111, 2453–2458 (2014).

    Article  ADS  Google Scholar 

  153. Bauer, J. et al. Nanolattices: an emerging class of mechanical metamaterials. Adv. Mater. 29, 1701850 (2002).

    Article  Google Scholar 

  154. Popovic, R. S. Hall Effect Devices (Institute of Physics Publishing, Philadelphia, 2004).

    Book  Google Scholar 

  155. Briane, M., Milton, G. W. & Nesi, V. Change of sign of the corrector’s determinant for homogenization in three-dimensional conductivity. Arch. Ration. Mech. Anal. 173, 133–150 (2004).

    Article  MathSciNet  MATH  Google Scholar 

  156. Briane, M. & Milton, G. W. An antisymmetric effective Hall matrix. SIAM J. Appl. Math. 70, 1810–1820 (2010).

    Article  MathSciNet  MATH  Google Scholar 

  157. Tornow, M. et al. Anisotropic magnetoresistance of a classical antidot array. Phys. Rev. Lett. 77, 147–150 (1996).

    Article  ADS  Google Scholar 

  158. Kadic, M., Schittny, R., Bückmann, T., Kern, C. & Wegener, M. Hall-effect sign inversion in a realizable 3D metamaterial. Phys. Rev. X 5, 021030 (2015).

    Google Scholar 

  159. Kern, C., Kadic, M. & Wegener, M. Experimental evidence for sign reversal of the Hall coefficient in three-dimensional metamaterials. Phys. Rev. Lett. 118, 016601 (2017).

    Article  ADS  Google Scholar 

  160. Kern, C., Graeme, W. M., Kadic, M. & Wegener, M. Theory of the Hall effect in three-dimensional metamaterials. New J. Phys. 20, 083034 (2018).

    Article  ADS  Google Scholar 

  161. Bergman, D. J. & Strelniker, Y. M. Calculation of strong-field magnetoresistance in some periodic composites. Phys. Rev. B 49, 16256–16268 (1994).

    Article  ADS  Google Scholar 

  162. Strelniker, Y. M. & Bergman, D. J. Thermoelectric response of a periodic composite medium in the presence of a magnetic field: angular anisotropy. Phys. Rev. B 96, 235308 (2017).

    Article  ADS  Google Scholar 

  163. Liu, L. Feasibility of large-scale power plants based on thermoelectric effects. New J. Phys. 16, 123019 (2014).

    Article  ADS  Google Scholar 

  164. Schittny, R., Kadic, M., Guenneau, S. & Wegener, M. Experiments on transformation thermodynamics: molding the flow of heat. Phys. Rev. Lett. 110, 195901 (2013).

    Article  ADS  Google Scholar 

  165. Ros, A. et al. Brownian motion: absolute negative particle mobility. Nature 436, 928–928 (2005).

    Article  ADS  Google Scholar 

  166. Kern, C., Schuster, V., Kadic, M. & Wegener, M. Experiments on the parallel Hall effect in three-dimensional metamaterials. Phys. Rev. Appl. 7, 044001 (2017).

    Article  ADS  Google Scholar 

  167. Onsager, L. Reciprocal relations in irreversible processes. I. Phys. Rev. 37, 405–426 (1931).

    Article  ADS  MATH  Google Scholar 

  168. Debord, J. D. & Lyon, L. A. Thermoresponsive photonic crystals. J. Phys. Chem. B 104, 6327–6331 (2000).

    Article  Google Scholar 

  169. Stuart, M. A. C. et al. Emerging applications of stimuli-responsive polymer materials. Nat. Mater. 9, 101–113 (2010).

    Article  ADS  Google Scholar 

  170. Schroden, R. C., Al-Daous, M., Blanford, C. F. & Stein, A. Optical properties of inverse opal photonic crystals. Chem. Mater. 14, 3305–3315 (2002).

    Article  Google Scholar 

  171. Theato, P., Sumerlin, B. S., O’Reilly, R. K. & Epps, T. H. III Stimuli responsive materials. Chem. Soc. Rev. 42, 7055–7056 (2013).

    Article  Google Scholar 

  172. Skylar, T. 4D printing: multi-material shape change. Archit. Des. 84, 116–121 (2014).

    Google Scholar 

  173. Hao, Z. et al. Light-fueled microscopic walkers. Adv. Mater. 27, 3883–3887 (2015).

    Article  Google Scholar 

  174. Martella, D. et al. Light activated non-reciprocal motion in liquid crystalline networks by designed microactuator architecture. RSC Adv. 7, 19940–19947 (2017).

    Article  Google Scholar 

  175. Momeni, F., Hassani, S. M. M., Liu, X. & Ni, J. A review of 4D printing. Mater. Des. 122, 42–79 (2017).

    Article  Google Scholar 

  176. Akihiro, N., Ahmed, M., Hang, Z. & Martin, M. In-gel direct laser writing for 3D-designed hydrogel composites that undergo complex self-shaping. Adv. Sci. 5, 1700038 (2017).

    Google Scholar 

  177. Park, H. et al. Mechanical metamaterials with thermoresponsive switching between positive and negative poisson’s ratios. Phys. Status Solidi RRL 12, 1800040 (2018).

    Article  Google Scholar 

  178. Hess, O. et al. Active nanoplasmonic metamaterials. Nat. Mater. 11, 573–584 (2012).

    Article  ADS  Google Scholar 

  179. Shadrivov, I., Lapine, M. & Kivshar, Y. Nonlinear, Tunable and Active Metamaterials (Springer, 2014).

  180. Fan, K. & Padilla, W. J. Dynamic electromagnetic metamaterials. Mater. Today 18, 39–50 (2015).

    Article  Google Scholar 

  181. Tong, X. Functional Metamaterials and Metadevices (Springer, 2017).

  182. Rout, S. & Sonkusale, S. Active Metamaterials: Terahertz Modulators and Detectors (Springer, 2017).

  183. Yu, K., Fang, N.-X., Huang, G. & Wang, Q. Magnetoactive acoustic metamaterials. Adv. Mater. 30, 1706348 (2018).

    Article  Google Scholar 

  184. Estep, N. A., S, J., Sounas, D. L. & Alu, A. Magnetic-free non-reciprocity and isolation based on parametrically modulated coupled-resonator loops. Nat. Phys. 10, 923–927 (2014).

    Article  Google Scholar 

  185. Bacot, V., Labousse, M., Eddi, A., Fink, M. & Fort, E. Time reversal and holography with spacetime transformations. Nat. Phys. 12, 972–977 (2016).

    Article  Google Scholar 

  186. Deck-Léger, Z.-L., Akbarzadeh, A. & Caloz, C. Wave deflection and shifted refocusing in a medium modulated by a superluminal rectangular pulse. Phys. Rev. B 97, 104305 (2018).

    Article  ADS  Google Scholar 

  187. Halimeh, J. C., Thompson, R. T. & Wegener, M. Invisibility cloaks in relativistic motion. Phys. Rev. A. 93, 013850 (2016).

    Article  ADS  Google Scholar 

  188. Fang, K., Yu, Z. & Fan, S. Realizing effective magnetic field for photons by controlling the phase of dynamic modulation. Nat. Photonics 6, 782–787 (2012).

    Article  ADS  Google Scholar 

  189. Nassar, H., Chen, H., Norris, A. N. & Huang, G. L. Quantization of band tilting in modulated phononic crystals. Phys. Rev. B 97, 014305 (2018).

    Article  ADS  Google Scholar 

  190. Milton, G. W. & Mattei, O. Field patterns: a new mathematical object. Proc. Roy. Soc. Lond. A 473, 20160819 (2017).

    Article  ADS  MathSciNet  MATH  Google Scholar 

  191. Xu, S. & Wu, C. Space-time crystal and space-time group. Phys. Rev. Lett. 120, 096401 (2018).

    Article  ADS  MathSciNet  Google Scholar 

  192. Burckel, D. B. et al. Micrometer-scale cubic unit cell 3D metamaterial layers. Adv. Mater. 22, 5053–5057 (2010).

    Article  Google Scholar 

  193. Sreekanth, K. V., Luca, A. D. & Strangi, G. Experimental demonstration of surface and bulk plasmon polaritons in hypergratings. Sci. Rep. 3, 3291 (2013).

    Article  Google Scholar 

  194. Correa, D. et al. Negative stiffness honeycombs for recoverable shock isolation. Rapid Prototyp. J. 21, 193–200 (2015).

    Article  Google Scholar 

  195. Wang, Q. et al. Lightweight mechanical metamaterials with tunable negative thermal expansion. Phys. Rev. Lett. 117, 175901 (2016).

    Article  ADS  Google Scholar 

  196. Ozaki, M., Shimoda, Y., Kasano, M. & Yoshino, K. Electric field tuning of the stop band in a liquid-crystal-infiltrated polymer inverse opal. Adv. Mater. 14, 514–518 (2002).

    Article  Google Scholar 

  197. Kamenjicki, M., & Lednev, I. & Asher, S. A. Photoswitchable spirobenzopyran-based photochemically controlled photonic crystals. Adv. Funct. Mater. 15, 1401–1406 (2005).

    Article  Google Scholar 

  198. Cui, T. J., Qi, M. Q., Wan, X., Zhao, J. & Cheng, Q. Coding metamaterials, digital metamaterials and programmable metamaterials. Light Sci. Appl. 3, e218 (2014).

    Article  ADS  Google Scholar 

  199. Zhao, J. et al. Controlling spectral energies of all harmonics in programmable way using time-domain digital coding metasurface. Preprint at arXiv https://arxiv.org/abs/1806.04414 (2018).

  200. Gracias, D. H. Stimuli responsive self-folding using thin polymer films. Curr. Opin. Chem. Eng. 2, 112–119 (2013).

    Article  Google Scholar 

  201. Laude, V. et al. Extraordinary nonlinear transmission modulation in a doubly resonant acousto-optical structure. Optica 4, 1245–1250 (2017).

    Article  Google Scholar 

  202. Roy, D., Cambre, J. N. & Sumerlin, B. S. Future perspectives and recent advances in stimuli-responsive materials. Prog. Polym. Sci. 35, 278–301 (2010).

    Article  Google Scholar 

  203. Courjal, N. et al. Acousto-optically tunable lithium niobate photonic crystal. Appl. Phys. Lett. 96, 131103 (2010).

    Article  ADS  Google Scholar 

  204. Shin, D. et al. Scalable variable-index elasto-optic metamaterials for macroscopic optical components and devices. Nat. Commun. 8, 16090 (2017).

    Article  ADS  Google Scholar 

  205. Babaee, S., Viard, N., Wang, P., Fang, N. X. & Bertoldi, K. Harnessing deformation to switch on and off the propagation of sound. Adv. Mater. 28, 1631–1635 (2016).

    Article  Google Scholar 

  206. Zhang, X., Liu, J., Chu, M. & Chu, B. Flexoelectric piezoelectric metamaterials based on the bending of ferroelectric ceramic wafers. Appl. Phys. Lett. 109, 072903 (2016).

    Article  ADS  Google Scholar 

  207. Weissman, J. M., Sunkara, H. B., Tse, A. S. & Asher, S. A. Thermally switchable periodicities and diffraction from mesoscopically ordered materials. Science 274, 959–963 (1996).

    Article  ADS  Google Scholar 

  208. Kubo, S. et al. Tunable photonic band gap crystals based on a liquid crystal-infiltrated inverse opal structure. J. Am. Chem. Soc. 126, 8314–8319 (2004).

    Article  Google Scholar 

  209. Nicolaou, Z. G. & Motter, A. E. Mechanical metamaterials with negative compressibility transitions. Nat. Mater. 11, 608–13 (2012).

    Article  ADS  Google Scholar 

  210. Qu, J., Kadic, M., Naber, A. & Wegener, M. Micro-structured two-component 3D metamaterials with negative thermal-expansion coefficient from positive constituents. Sci. Rep. 7, 40643 (2017).

    Article  ADS  Google Scholar 

  211. Zhang, H., Guo, X., Wu, J., Fang, D. & Zhang, Y. Soft mechanical metamaterials with unusual swelling behavior and tunable stress-strain curves. Sci. Adv. 4, eaar8535 (2018).

    Article  ADS  Google Scholar 

  212. Kamenjicki, M., Ladnev, I. K., Mikhonin, A., Kesavamoorthy, R. & Asher, S.-A. Photochemically controlled photonic crystals. Adv. Funct. Mater. 13, 774–780 (2003).

    Article  Google Scholar 

Download references

Acknowledgements

The authors acknowledge stimulating discussions with C. Rockstuhl and C. Kern (Karlsruhe Institute of Technology (KIT)). M.K. acknowledges support by the Engineering and Innovation through Physical Sciences, High-technologies and Cross-disciplinary Research (EIPHI) Graduate School (contract ANR-17-EURE-0002) and the French Investissements d’Avenir program, the I-SITE Bourgogne Franche-Comté (BFC) project (contract ANR-15-IDEX-03). G.W.M. thanks the National Science Foundation for support via grant DMS-1211359 and DMS-1814854. M.W. acknowledges support by the Excellence Cluster '3D Matter Made to Order', the Helmholtz program 'Science and Technology of Nanosystems' (STN), the Karlsruhe School of Optics & Photonics (KSOP) and the 'Virtual Materials Design' (VIRTMAT) project of KIT.

Author information

Authors and Affiliations

Authors

Contributions

All authors contributed to all aspects of manuscript preparation, revision and editing.

Corresponding author

Correspondence to Martin Wegener.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note

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

Glossary

Inverse design

A design approach that defines the desired properties and searches for a microstructure exhibiting them; the inverse of taking a microstructure and calculating its properties.

Topology optimization

A mathematical method that finds a microstructure that optimizes a set of parameters within a given design space and for given constraints.

Auxetic

An auxetic material shrinks laterally when being contracted axially. This behaviour corresponds to a negative Poisson ratio ν.

Pentamode

A pentamode material is a linear elastic 3D material for which five of the six possible modes of deformation cost no energy. Such materials behave approximately as a liquid.

2D Huygens metasurfaces

2D arrangements of electric and magnetic polarizable elements that modify the transmitted light yet lead to zero reflections.

Faraday active

Refers to media that rotate the polarization axis of linearly polarized light in the presence of a static magnetic field but that do not change the sense of rotation upon back-reflection of light.

Cauchy elasticity tensor

This tensor is the generalization of Hooke’s spring constant. It connects the stress and the strain tensors.

Lamé parameters

A possible pair of parameters that characterize the Cauchy elasticity tensor in an isotropic homogeneous medium. The second Lamé parameter is identical to the shear modulus.

Eringen micropolar continuum mechanics

A generalization of Cauchy continuum mechanics, in which four rank-4 tensors are required to describe the connection between strain and microrotation tensors and between stress and torque tensors.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Kadic, M., Milton, G.W., van Hecke, M. et al. 3D metamaterials. Nat Rev Phys 1, 198–210 (2019). https://doi.org/10.1038/s42254-018-0018-y

Download citation

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s42254-018-0018-y

This article is cited by

Search

Quick links

Nature Briefing

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