Artificial phototropism for omnidirectional tracking and harvesting of light

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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.


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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.

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) doi:10.1038/s41565-019-0562-3

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