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Artificial phototropism for omnidirectional tracking and harvesting of light


Many living organisms track light sources and halt their movement when alignment is achieved. This phenomenon, known as phototropism, occurs, for example, when plants self-orient to face the sun throughout the day. Although many artificial smart materials exhibit non-directional, nastic behaviour in response to an external stimulus, no synthetic material can intrinsically detect and accurately track the direction of the stimulus, that is, exhibit tropistic behaviour. Here we report an artificial phototropic system based on nanostructured stimuli-responsive polymers that can aim and align to the incident light direction in the three-dimensions over a broad temperature range. Such adaptive reconfiguration is realized through a built-in feedback loop rooted in the photothermal and mechanical properties of the material. This system is termed a sunflower-like biomimetic omnidirectional tracker (SunBOT). We show that an array of SunBOTs can, in principle, be used in solar vapour generation devices, as it achieves up to a 400% solar energy-harvesting enhancement over non-tropistic materials at oblique illumination angles. The principle behind our SunBOTs is universal and can be extended to many responsive materials and a broad range of stimuli.

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Fig. 1: The concept of the artificial phototropism and the SunBOTs.
Fig. 2: The design and mechanism of SunBOTs.
Fig. 3: Complex and robust phototropic behaviours of SunBOTs.
Fig. 4: The tracking kinetics of the SunBOT.
Fig. 5: A demonstration of the energy maximization function of phototropism: OLC in the SVG.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.


  1. 1.

    Delves, P. J. & Roitt, I. M. Ivan M. Encyclopedia of Immunology (Academic, 1998).

  2. 2.

    Poppinga, S. et al. Toward a new generation of smart biomimetic actuators for architecture. Adv. Mater. 30, 1703653 (2018).

    Article  Google Scholar 

  3. 3.

    White, T. J. & Broer, D. J. Programmable and adaptive mechanics with liquid crystal polymer networks and elastomers. Nat. Mater. 14, 1087–1098 (2015).

    CAS  Google Scholar 

  4. 4.

    Zhao, Y. et al. Soft phototactic swimmer based on self-sustained hydrogel oscillator. Sci. Robot. 4, eaax7112 (2019).

    Google Scholar 

  5. 5.

    Serak, S. et al. Liquid crystalline polymer cantilever oscillators fueled by light. Soft Matter 6, 779–783 (2010).

    CAS  Google Scholar 

  6. 6.

    Gelebart, A. H., Vantomme, G., Meijer, E. W. & Broer, D. J. Mastering the photothermal effect in liquid crystal networks: a general approach for self-sustained mechanical oscillators. Adv. Mater. 29, 1606712 (2017).

    Google Scholar 

  7. 7.

    Li, C., Liu, Y., Huang, X. & Jiang, H. Direct sun-driven artificial heliotropism for solar energy harvesting based on a photo-thermomechanical liquid–crystal elastomer nanocomposite. Adv. Funct. Mater. 22, 5166–5174 (2012).

    CAS  Google Scholar 

  8. 8.

    He, X. et al. Synthetic homeostatic materials with chemo-mechano-chemical self-regulation. Nature 487, 214–218 (2012).

    CAS  Google Scholar 

  9. 9.

    Atamian, H. S. et al. Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits. Science 353, 587–590 (2016).

    CAS  Google Scholar 

  10. 10.

    Greffet, J.-J. et al. Coherent emission of light by thermal sources. Nature 416, 61–64 (2002).

    CAS  Google Scholar 

  11. 11.

    Fink, Y. et al. A dielectric omnidirectional reflector. Science 282, 1679–82 (1998).

    CAS  Google Scholar 

  12. 12.

    Teperik, T. V. et al. Omnidirectional absorption in nanostructured metal surfaces. Nat. Photon. 2, 299–301 (2008).

    CAS  Google Scholar 

  13. 13.

    Huang, J. et al. Harnessing structural darkness in the visible and infrared wavelengths for a new source of light. Nat. Nanotechnol. 11, 60–66 (2015).

    Google Scholar 

  14. 14.

    Wang, E., Desai, M. S. & Lee, S.-W. Light-controlled graphene–elastin composite hydrogel actuators. Nano Lett. 13, 2826–2830 (2013).

    CAS  Google Scholar 

  15. 15.

    Ahir, S. V. & Terentjev, E. M. Photomechanical actuation in polymer–nanotube composites. Nat. Mater. 4, 491–495 (2005).

    CAS  Google Scholar 

  16. 16.

    Zhao, Y. L. & Fraser Stoddart, J. Azobenzene-based light-responsive hydrogel system. Langmuir 25, 8442–8446 (2009).

    CAS  Google Scholar 

  17. 17.

    Liu, X. et al. Reversible and rapid laser actuation of liquid crystalline elastomer micropillars with inclusion of gold nanoparticles. Adv. Funct. Mater. 25, 3022–3032 (2015).

    CAS  Google Scholar 

  18. 18.

    Camacho-Lopez, M., Finkelmann, H., Palffy-Muhoray, P. & Shelley, M. Fast liquid–crystal elastomer swims into the dark. Nat. Mater. 3, 307–310 (2004).

    CAS  Google Scholar 

  19. 19.

    Shastri, A. et al. An aptamer-functionalized chemomechanically modulated biomolecule catch-and-release system. Nat. Chem. 7, 447–454 (2015).

    CAS  Google Scholar 

  20. 20.

    Qin, M. et al. Bioinspired hydrogel interferometer for adaptive coloration and chemical sensing. Adv. Mater. 30, 1800468 (2018).

    Google Scholar 

  21. 21.

    Sun, M. et al. Hydrogel interferometry for ultrasensitive and highly selective chemical detection. Adv. Mater. 30, 1804916 (2018).

    Google Scholar 

  22. 22.

    Qin, M., Sun, M., Hua, M. & He, X. Bioinspired structural color sensors based on responsive soft materials. Curr. Opin. Solid State Mater. Sci. 23, 13–27 (2019).

    CAS  Google Scholar 

  23. 23.

    Gelebart, A. H. et al. Making waves in a photoactive polymer film. Nature 546, 632–636 (2017).

    CAS  Google Scholar 

  24. 24.

    Wani, O. M., Zeng, H. & Priimagi, A. A light-driven artificial flytrap. Nat. Commun. 8, 15546 (2017).

    CAS  Google Scholar 

  25. 25.

    Klajn, R. Spiropyran-based dynamic materials. Chem. Soc. Rev. 43, 148–184 (2014).

    CAS  Google Scholar 

  26. 26.

    Schild, H. G. Poly(N-isopropylacrylamide): experiment, theory and application. Prog. Polym. Sci. 17, 163–249 (1992).

    CAS  Google Scholar 

  27. 27.

    Taylor, L. D. & Cerankowski, L. D. Preparation of films exhibiting a balanced temperature dependence to permeation by aqueous solutions—a study of lower consolute behavior. J. Polym. Sci. Polym. Chem. Ed. 13, 2551–2570 (1975).

    CAS  Google Scholar 

  28. 28.

    Zhou, L. et al. 3D self-assembly of aluminium nanoparticles for plasmon-enhanced solar desalination. Nat. Photonics 10, 393–398 (2016).

    CAS  Google Scholar 

  29. 29.

    Ghasemi, H. et al. Solar steam generation by heat localization. Nat. Commun. 5, 4449 (2014).

    CAS  Google Scholar 

  30. 30.

    Zhao, F. et al. Highly efficient solar vapour generation via hierarchically nanostructured gels. Nat. Nanotechnol. 13, 489–495 (2018).

    CAS  Google Scholar 

  31. 31.

    Li, X. et al. Graphene oxide-based efficient and scalable solar desalination under one Sun with a confined 2D water path. Proc. Natl Acad. Sci. USA 113, 13953–13958 (2016).

    CAS  Google Scholar 

  32. 32.

    Zhou, L. et al. Self-assembled spectrum selective plasmonic absorbers with tunable bandwidth for solar energy conversion. Nano Energy 32, 195–200 (2017).

    Google Scholar 

  33. 33.

    Chen, Q. et al. A durable monolithic polymer foam for efficient solar steam generation. Chem. Sci. 9, 623–628 (2018).

    CAS  Google Scholar 

  34. 34.

    Wang, Y., Zhang, L. & Wang, P. Self-floating carbon nanotube membrane on macroporous silica substrate for highly efficient solar-driven interfacial water evaporation. ACS Sustain. Chem. Eng. 4, 1223–1230 (2016).

    CAS  Google Scholar 

  35. 35.

    Wang, Z. et al. Bio-inspired evaporation through plasmonic film of nanoparticles at the air–water interface. Small 10, 3234–3239 (2014).

    CAS  Google Scholar 

  36. 36.

    Donald, B. R., Levey, C. G., McGray, C. D., Paprotny, I. & Rus, D. An untethered, electrostatic, globally controllable MEMS micro-robot. J. Microelectromech. Syst. 15, 1–15 (2006).

    Google Scholar 

  37. 37.

    Liu, Q., Nian, G., Yang, C., Qu, S. & Suo, Z. Bonding dissimilar polymer networks in various manufacturing processes. Nat. Commun. 9, 846 (2018).

    Google Scholar 

  38. 38.

    Le Floch, P. et al. Wearable and washable conductors for active textiles. ACS Appl. Mater. Interfaces 9, 25542–25552 (2017).

    Google Scholar 

  39. 39.

    Hum, S. V. & Perruisseau-Carrier, J. Reconfigurable reflect arrays and array lenses for dynamic antenna beam control: a review. IEEE Trans. Antennas Propag. 62, 183–198 (2014).

    Google Scholar 

  40. 40.

    Zhu, M. et al. Transparent and haze wood composites for highly efficient broadband light management in solar cells. Nano Energy 26, 332–339 (2016).

    CAS  Google Scholar 

  41. 41.

    Li, T. et al. Wood composite as an energy efficient building material: guided sunlight transmittance and effective thermal insulation. Adv. Energy Mater. 6, 1601122 (2016).

    Google Scholar 

  42. 42.

    Zeng, H., Wasylczyk, P., Wiersma, D. S. & Priimagi, A. Light robots: bridging the gap between microrobotics and photomechanics in soft materials. Adv. Mater. 30, e1703554 (2018).

    Google Scholar 

  43. 43.

    Kim, Y., Yuk, H., Zhao, R., Chester, S. A. & Zhao, X. Printing ferromagnetic domains for untethered fast-transforming soft materials. Nature 558, 274–279 (2018).

    CAS  Google Scholar 

  44. 44.

    Costanza, G. & Tata, M. E. A novel methodology for solar sail opening employing shape memory alloy elements. J. Intell. Mater. Syst. Struct. 29, 1793–1798 (2018).

    CAS  Google Scholar 

  45. 45.

    Ullery, D. C. et al. Strong solar radiation forces from anomalously reflecting metasurfaces for solar sail attitude control. Sci. Rep. 8, 10026 (2018).

    Google Scholar 

  46. 46.

    Gelebart, A. H., Mc Bride, M., Schenning, A. P. H. J., Bowman, C. N. & Broer, D. J. Photoresponsive fiber array: toward mimicking the collective motion of cilia for transport applications. Adv. Funct. Mater. 26, 5322–5327 (2016).

    CAS  Google Scholar 

  47. 47.

    Yao, Y. et al. Electrically tunable metasurface perfect absorbers for ultrathin mid-infrared optical modulators. Nano Lett. 14, 6526–6532 (2014).

    CAS  Google Scholar 

  48. 48.

    Lee, E., Heng, R.-L. & Pilon, L. Spectral optical properties of selected photosynthetic microalgae producing biofuels. J. Quant. Spectrosc. Radiat. Transf. 114, 122–135 (2013).

    CAS  Google Scholar 

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This work was supported by Air Force Office of Scientific Research (AFOSR) Grant FA9550-17-1-0311, AFOSR award FA9550-18-1-0449, Office of Naval Research (ONR) Award N000141712117, ONR Award N00014-18-1-2314, the Hellman Fellows Funds and the UCLA Faculty Career Development Award from the University of California, Los Angeles. X.H. is a Canadian Institute for Advanced Research Azrieli Global Scholar in the Bio-inspired Solar Energy Program. We thank Derek Tseng from UCLA for fabricating the mould for the SunBOT arrays and control samples.

Author information




X.H. conceived the concept, planned the project and supervised the research. X.H., X.Q., Y.Z., Y.A. and H.G. designed and conducted the experiments and data analysis. X.Q., Y.Z., Y.A., M.H., Y.Y., H.G. and J.C. conducted the fabrication of various SunBOTs and SVG characterization. N.L. synthesized the AuNPs. X.W., H.J., T.G. and L.P. developed the model and numerical code and carried out the computational simulations. X.H., X.Q., Y.Z., Y.A., H.J., L.T. and M.M. wrote the manuscript. X.Q., Y.Z. and Y.A. contributed to the work equally.

Corresponding author

Correspondence to Ximin He.

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The authors declare no competing interests.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–36, Tables 1–3, Equations 1–23, methods, video captions 1–10 and refs. 1–37..

Supplementary Video 1.

Tracking Light of Variable Incident Angles.

Supplementary Video 2.

Light Tracking Experiment vs. Simulation

Supplementary Video 3.

Omnidirectional Phototropism: Zenith and Azimuth Angles.

Supplementary Video 4.

Omnidirectional Continuous Tracking.

Supplementary Video 5.

Area Light Tracking.

Supplementary Video 6.

Light-Matter Handshake: Auto-correction of Tracking Direction.

Supplementary Video 7.

Hydrogel Volume Change Behaviours.

Supplementary Video 8.

Phototropic Movement of LCE SunBOT in Air.

Supplementary Video 9.

Phototropic Movement of PANi-PDMAEMA SunBOT.

Supplementary Video 10.

Phototropic Movement at Various Temperatures under Laser and White Light of Different Spot Sizes.

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Qian, X., Zhao, Y., Alsaid, Y. et al. Artificial phototropism for omnidirectional tracking and harvesting of light. Nat. Nanotechnol. 14, 1048–1055 (2019).

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