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Self-regulated underwater phototaxis of a photoresponsive hydrogel-based phototactic vehicle

Nature Nanotechnology volume 19, pages 77–84 (2024)Cite this article

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Abstract

Incorporating a negative feedback loop in a synthetic material to enable complex self-regulative behaviours akin to living organisms remains a design challenge. Here we show that a hydrogel-based vehicle can follow the directions of photonic illumination with directional regulation inside a constraint-free, fluidic space. By manipulating the customized photothermal nanoparticles and the microscale pores in the polymeric matrix, we achieved strong chemomechanical deformation of the soft material. The vehicle swiftly assumes an optimal pose and creates directional flow around itself, which it follows to achieve robust full-space phototaxis. In addition, this phototaxis enables a series of complex underwater locomotions. We demonstrate that this versatility is generated by the synergy of photothermofluidic interactions resulting in closed-loop self-control and fast reconfigurability. The untethered, electronics-free, ambient-powered hydrogel vehicle manoeuvres through obstacles agilely, following illumination cues of moderate intensities, similar to that of natural sunlight.

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Fig. 1: Schematics of full-space phototaxis enabled by spontaneously induced asymmetry with regulation.
Fig. 2: Photothermotactic flow-guided movement.
Fig. 3: Phototaxis of the PTV enabled by self-regulated induced asymmetry.
Fig. 4: Underwater steering of the PTV.

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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. Source data are provided with this paper.

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Acknowledgements

This work was supported by the National Key R&D Program of China (2020YFA0711500), the National Natural Science Foundation of China (52076127) and the Natural Science Foundation of Shanghai (20ZR1471700, 22JC1401800). X.Q. is grateful for support from the State Key Laboratory of Mechanical System and Vibration (grant number MSVZD202211, number GKZD020039/001), the Oceanic Interdisciplinary Program of Shanghai Jiao Tong University (project number SL2020MS009), the Prospective Research Program at Shanghai Jiao Tong University (19X160010008), the Thousand Talent Young Scholar Program, the Student Innovation Center and the Instrumental Analysis Center at Shanghai Jiao Tong University. N.X.F. acknowledges the start-up funding provided by the Global STEM professorship scheme sponsored by the Government of the Hong Kong Special Administrative Region. X.Q. thanks T. Li for supporting the commercial solar simulator and S. Lin for supporting the commericial simulation tools.

Author information

Authors and Affiliations

  1. State Key Laboratory of Mechanical System and Vibration, Shanghai Jiao Tong University, Shanghai, China

    Guodong Hou, Xu Zhang, Feihong Du, Xing Zhang, Zhijie Lei, Wei Lu, Feiyu Zhang, Huamiao Wang, Jiangping Chen, Guang Meng & Xiaoshi Qian

  2. Interdisciplinary Research Centre for Engineering Science, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China

    Guodong Hou, Feihong Du, Zhijie Lei, Wei Lu, Feiyu Zhang, Guang Meng & Xiaoshi Qian

  3. Institute of Refrigeration and Cryogenics, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China

    Guodong Hou, Feihong Du, Zhijie Lei, Wei Lu, Feiyu Zhang, Guang Yang, Jiangping Chen & Xiaoshi Qian

  4. Institute of Engineering Thermophysics, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China

    Guodong Hou & Zhenyu Liu

  5. Institute of Aerospace Propulsion, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China

    Yadong Wu

  6. Department of Mechanical and Energy Engineering, Southern University of Science and Technology, Shenzhen, China

    Rong Wang & Qi Ge

  7. Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China

    Nicholas X. Fang

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  1. Guodong Hou
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  2. Xu Zhang
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Contributions

X.Q. conceived the concept, designed the experiment and wrote the manuscript. X.Q., G.H., Xu Zhang, Zhijie Lei, F.D., W.L. and F.Z. carried out the material synthesis, characterization and systematic demonstration. G.H., Y.W., G.Y, X.Q. and N.X.F. conducted the fluid dynamic experiment and analysis. G.H., Xing Zhang, H.W., X.Q. and Zhenyu Liu carried out the numerical simulation. J.C. and G.M. supervised the device modelling. R.W. and Q.G carried out the 3D printing fabrication. X.Q. and N.X.F supervised the project. All authors analysed and interpreted the data.

Corresponding authors

Correspondence to Nicholas X. Fang or Xiaoshi Qian.

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Competing interests

X.Q. and G.H. are inventors on a provisional patent application related to the described work. The other authors declare no competing interests.

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Nature Nanotechnology thanks the anonymous reviewers for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Fabrication and assembly of the PTV.

a, Polymerization of the head part and leg part by moulding manufacturing. b, Assembly of the head and leg. c, Physical view of AuNPs-PNIPAAM based PTV. d, PTVs fabricated by 3D printing and moulding manufacturing.

Source data

Extended Data Fig. 2 Photothermal response characteristics of the PTV.

a, Local temperature evolution of PNIPAAM-AuNPs hydrogel illuminated by green light at different optical power densities. b, Local temperature evolution of PNIPAAM-r-GO hydrogel illuminated by white light at different optical power densities. c, Infrared thermal image of the PTV under green light irradiation (532 nm, 11.94 mW·mm−2). d, Infrared thermal image of the PTV under white light irradiation (6.63 mW·mm−2). d, Image of bell-shaped head illuminated by green light. f–h, Finite-element analysis of the bell-shaped head subjected to green light, simulated temperature distribution (f), strain field (g) and stress distribution (h). i, Image of the tentacle illuminated by green light. j–l, Finite-element analysis of the tentacle subjected to green light, simulated temperature distribution (j), strain field (k) and stress distribution (l). m, Image of PTV illuminated by white light. n–p, Finite-element analysis of the PTV subjected to white light: simulated temperature distribution (n), strain field (o) and stress distribution (p).

Source data

Extended Data Fig. 3 Comparison of the horizontal phototactic movement of two PTVs made of different hydrogels.

a, b, Force analysis and images of the PTV (a) and a control sample (b) showing a self-leaning effect of 18° due to symmetric breaking induced by the relocation of the bubble, where FB and G refer to buoyancy and gravity and B, O, and M the buoyancy centre, gravity centre and metacentre, respectively. c, Snapshot of the motion of a jump cycle of the conventional PTV (made of PAAM hydrogel) and the phototropic PTV (made of PNIPAAM hydrogel).

Source data

Extended Data Fig. 4 Force analysis of the locomotion of PTVs under illumination.

a, b, CFD results of G0 (a) and G1 (b) versus different attack angles, as analytically fitted by \({G}_{0}\approx 0.435\sin \alpha +1.02{\sin }^{2}\alpha\) and \({G}_{1}\approx 7.38+3.5{\sin }^{2}\alpha\), respectively. c, Drag coefficient of the PTV at different Reynolds numbers. CFD results are fitted by \({C}_{D}\approx {G}_{0}+{G}_{1}/{R}_{e}\). d, Force analysis of the PTV under vertical illumination and the corresponding PIV analysis. When the flow-induced lift force FL and buoyancy FB exceeds the gravity G and drag forces FD under illumination, upwards locomotion is initiated. Flow speed plots on the right show that the thermally induced flow moves vertically and carries the PTV towards the light source. e, Force analysis of the PTV under horizontal illumination and the corresponding PIV analysis. As the embedded bubble is pinched away from the centre position, the PTV tilts towards the light source. Flow speed plots on the right show that the thermally induced flow moves up and to the right and carries the PTV towards the light source at an attack angle of 50°.

Source data

Extended Data Fig. 5 Microscopic images and motions of 3D-printed PTVs.

a, b, Microscopic images of 3D-printed PTVs with a diameter of 0.5 mm (a) and 2mm (b). c, d, Snapshots (c), evolution curve of displacement and velocity with time (d) of upward locomotion. e, f, Snapshots (e), evolution curve of displacement and velocity with time (f) of horizontal jumping.

Source data

Extended Data Fig. 6 Phototactic performance and the speed-temperature correlation of the PTV.

a, Snapshots of the time evolution of the PTV phototaxis under horizontal illumination with a photonic power density lower than 1 Sun (1 mW·mm-2). b, Time-dependent displacement of the PTV in the x and z directions. c, d, Time evolution of the local temperature on the illuminated surface and speed of the PTV as directly measured (c) and simulated (d) through the FEM-based CFD tool. The experiment and the simulation exhibit a correlation between speed and temperature and are in good agreement with each other.

Source data

Extended Data Fig. 7 Omnidirectional phototaxis of the PTV.

a, The PTV exhibits a collective movement, swimming along a light incidence illuminated with a 30° angle to the bottom, without any external support. b, Corresponding time-dependent displacement of the PTV in the x and z directions.

Source data

Extended Data Fig. 8 Gravity modulation of the kinetic performance of the PTV.

a–f, Snapshots and time evolution of the displacement under different excess gravity conditions. The PTV moves vertically to the water surface, indicating a zero-frequency of hopping, as no cycle is accomplished (a, b, Gex = 4 × 10−6 N), hops towards the light with a finite frequency (c, d, Gex = 6 × 10−6 N) and slides to the right, essentially exhibiting an infinite frequency (e, f, Gex = 12 × 10−6 N). g, h, Measured feedback frequencies (g) and the displacement in the z direction (h) and of the PTV under different excess gravity.

Source data

Extended Data Fig. 9 Accuracy of motion of the PTV with different drive modes and remote manipulation performance.

a, 3D and 2D schematics of tentacle-paddling and photo-fluidic phototaxis. The PTV quickly deviates from the laser beam due to misalignment when utilizing the tentacle-paddling mode, whereas the vehicle can travel for a long distance accurately using the regulated photothermal-fluidic interaction. b, Accurate long-distance motion of the PTV. c, d, Phototropic manoeuvring of the PTV underwater (c) and photophobic taxis at the water surface (d), drawing four letters (SJTU) by solely adjusting the direction of illumination.

Source data

Extended Data Fig. 10 Phototactic cycling of the microalgae-embedded PTV.

a, Photographs of the Tetraselmis subcordiformis-embedded PTV (left) and Chrysophyceae-embedded PTV (right). b, Merged snapshots exhibit the time evolution of the phototactic sliding of the microalgae-embedded PTV under horizontal illumination. The inset shows the recorded displacement in the x and z directions. c, Schematic diagram of diurnal vertical plankton migration. d, Automatic cyclic movement of the Chrysophyceae-embedded PTV under illumination from the left.

Source data

Supplementary information

Supplementary Information

Supplementary Figs. 1–37, Discussion and Tables 1–3.

Supplementary Video 1

Video 1. Continuous phototropic jumping of a PTV. Asymmetric illumination produces a horizontal phototropic jumping motion of a PTV, which is driven by horizontal forces and moves towards the light source while ascending. We recorded the continuous jumping motion of a PTV driven by different incoming light directions and light source types. Demo 1, continuous phototropic jumping driven by green light. When the light source is a 532 nm laser, we set the incident power to 200 mW and illuminate it from the right side and left side, corresponding to the generation of continuous rightwards and leftwards jumping motion. It is proven that there is no special requirement for the direction of illumination when the PTV jumps horizontally. In the x–y plane, no matter which direction of illumination is applied, the PTV can respond accordingly and achieve all-round phototropism. Movies are played at 30× speed for rightwards jumps and 15× speed for leftwards jumps. Scale bar, 1 cm. Demo 2, continuous phototropic jumping driven by white light. By changing the 532 nm laser to artificial white light, the PTV is able to produce a more intense phototropic motion. The white light illumination has a larger working area, and the PTV is illuminated by a larger and longer range of light on one side, allowing it to jump higher and farther. Movies are played at 15× speed for horizontal leftward jumps and 10× speed for leftward jumps with an incident angle of 30°. Scale bar, 1 cm.

Supplementary Video 2

Video 2. Air bubble in the cavity deviates from incident light. The hydrogel for the head of the PTV was moulded on a coverslip pretreated with TMSPMA, and we controlled the volume of the bubbles to be approximately 10 µl, keeping the same volume as the volume of gas in the cavity of the experimental PTV. Green light (input power, 200 mW) is irradiated asymmetrically on the sample, and the movement of the bubbles can be clearly observed through the coverslip microscope. In the movie, we use white circles to represent the volume contour of the bubble at the initial moment, and after the light is removed, the bubble moves to the backlight side. The movie is played in real-time. Scale bar, 50 μm.

Supplementary Video 3

Video 3. PIV measurements for upwards and horizontal motions. To investigate the surrounding fluid under different motions of the PTV and thus examine how the fluid affects the motion of the PTV, we carried out PIV tests. With the help of tracer particles, we observed changes in the fluid motion and the distribution of the flow field. For the upwards motion, we used a white light to illuminate the PTV vertically. With a CCD camera, we clearly observed and recorded the upwards motion of the PTV and the evolution of the fluid. In the case of asymmetric illumination with white light, we recorded the PTV’s light-tending jumping motion. The flow field in both cases clearly shows that the fluid plays an active role in the PTV motion. Movies are played in real-time. Scale bar, 1 cm.

Supplementary Video 4

Video 4. Upwards phototropic locomotion of different PTVs. In our experiments, upwards motion serves as the basis and is a prerequisite for other complex motions. To examine the motion of the PTV under different conditions, we carried out a corresponding investigation. The video shows the effect of different incident powers and different heights of the PTV on motion performance. Demo 1, upwards phototropic locomotion at different input powers. For our photothermal responsive material, the higher the optical power we apply, the higher the temperature of the surface and cavity (Extended Data Fig. 2). To evaluate the effect of different light powers on the ascending motion of the PTV, we conducted experiments on the same sample (height, 12 mm; width, 8 mm). Demo 1 integrates the ascending videos of the PTV under four light power conditions; the PTV moves faster (and maintains this speed) as the light power increases. Supplementary Fig. 11 demonstrates the displacement and velocity changes of the PTV’s motion in these four conditions so that the ascending motion is positively correlated with the applied light stimulus within a certain range. The movie is played at 5× speed. Scale bar, 1 cm. Demo 2, upwards phototropic locomotion of PTVs with different heights. We also investigated the upwards motion of PTVs with different heights, making three kinds of PTVs with different heights (a, 12 mm; b, 18 mm; c, 27 mm) and applying 270 mW of green light to them, and there was a small difference in the time taken to rise to the water surface. From the histogram, the shorter PTV has a slight advantage in gaining faster speed, but the overall change is still small. Our PTV’s leg length plays a minimal role in the ascent motion and is therefore at a disadvantage when comparing the length speed. The movie is played at 5× speed. Scale bar, 1 cm. Demo 3, upwards phototropic locomotion of PTVs with different design structures. We assembled PTVs in different shapes (a, PTV; b, cylindrical robot) to investigate the impact of structure on PTV movement. Initially setting both in a neutral buoyancy state and applying the same illumination, PTV moves faster than the cylindrical soft robot. With the same driving force, our soft robot design has less resistance, and this structural design is better. The movie is played at 5× speed. Scale bar, 1 cm.

Supplementary Video 5

Video 5. Upwards phototropic locomotion of a PTV driven by white light. The r-GO-PNIPAAM hydrogel has excellent response properties to artificial white light, which is used to apply vertical illumination to the PTV, which moves vertically upwards. The video demonstrates the upwards light-driven motion of the PTV under different excess gravity conditions without presenting the position. Demo 1, upwards phototropic locomotion of a PTV at different initial positions. The intensity of light at different initial positions is very different (Supplementary Fig. 7), and the distinction of upwards movement of the PTV was recorded by setting its initial position at 0–4 cm from the centre of the light source. The results show that the closer to the centre of the light source, the faster the PTV moves. The movie is played in real-time. Scale bar, 2 cm. Demo 2, upwards motions of a PTV under different excess gravity. The motion of the PTV is also different under different force conditions. The direction of gravity is opposite to the upwards motion, which can be predicted to be detrimental to the upwards motion of the PTV. By adjusting the content of bubbles in the cavity of the PTV, the PTV can be in different states of excess gravity (a, Gex = 12 × 10−6 N; b, Gex = 10 × 10−6 N; c, Gex = 8 × 10−6 N; d, Gex = 6 × 10−6 N; e, Gex = 4 × 10−6 N; f, Gex = 2 × 10−6 N; g, Gex = 0 × 10−6 N). The movie is played in real-time. Scale bar, 2 cm.

Supplementary Video 6

Video 6. Phototropic forward jumping cycle of a PTV. Controlled by feedback loops, the PTV moves towards the light when subjected to asymmetric illumination. Notably, when the PTV is under different excess gravity (a, Gex = 12 × 10−6 N; b, Gex = 10 × 10−6 N; c, Gex = 8 × 10−6 N; d, Gex = 6 × 10−6 N; e, Gex = 4 × 10−6 N), the PTV generates different feedback and thus different motions. A jump cycle of the PTV was recorded, and the feedback frequency changed from infinity to 0. Movies are played at 2×, 5×, 10×, 10× and 10× speeds. Scale bar, 1 cm.

Supplementary Video 7

Video 7. Self-rotating on the water surface. By controlling the volume of the cavity bubbles, we can regulate the state of the PTV underwater, and when the bubble volume reaches a certain level, the PTV hovers on the water surface. The legs of the PTV are equivalent to a column of the hydrogel. When we shine a light on the legs of the PTV from the right side, the legs deform due to asymmetric irradiation. This deformation induces interaction with water, and the PTV thus gains the ability to rotate itself. The PTV is set up with six legs, and when leg 1 is deformed by light, the PTV rotates, and soon leg 2 enters the previous scene and interacts with water to drive the PTV to spin. The gap in the legs allows the PTV to obtain continuous intermittent driving, and the motion cascades together to form a rotational advance of the PTV. The driving light source for this motion is a 532 nm laser with an incident power of 150 mW. The movie is played at 30× speed. Scale bar, 1 cm.

Supplementary Video 8

Video 8. Continuous climbing forward. To investigate the climbing performance of the PTV, we shine a light source along a 30° slope to the head of the PTV. After a short period of self-adjustment, the PTV starts an upwards climbing motion. During upwards climbing, we can observe the bending of the PTV legs towards the light. When the PTV jumps horizontally, the legs are bent by light; we discuss the role of leg deformation on the PTV motion elsewhere (Fig. 2f). When the PTV climbs and falls, the bent legs touch the slope surface first, providing a soft landing for the PTV. The light source irradiated during the climbing process is a 532 nm laser with an event light angle of 30° and an incident power of 200 mW. The movie is played at 30× speed. Scale bar, 1 cm.

Supplementary Video 9

Video 9. Phototropic steering of a PTV on an extremely complex path. PTV has a very positive response to horizontal asymmetric lighting. A complex path (meaning ‘nature’ in cursive style) is preset, and the direction of the light source is adjusted in real-time so that the PTV is guided by the light and moves flexibly underwater to achieve the specified trajectory of manipulation. The frequent reorientation of the incident light inevitably results in a slightly jittery shot, and adjusting the direction of the incident light is accompanied by a clear alternation of light and dark, presenting different backgrounds, which is a visual effect that does not contradict actual continuous photography. The movie is played at 20× speed. Scale bar, 1 cm.

Supplementary Video 10

Video 10. PTV crossing a barrier. Obstacle crossing is a common performance test in soft robotics research, and we designed two modes of obstacle crossing for the PTV, one for moving to the surface to cross the obstacle and the other for crossing the obstacle completely underwater. The video shows that PTV has a good ability to cross obstacles with green light and white light. Demo 1, crossing an obstacle driven by green light. To evaluate the ability of the PTV to cross the barrier from the water surface, two 532 nm lasers were used to generate vertical and horizontal illumination. Light 2 is directed horizontally to the left, driving the PTV to the left, where it stays on the surface for a period of time and then sinks into the water, achieving a rectangular barrier. Both incident light powers are set to 200 mW, and the movie is played at 30× speed. Scale bar, 1 cm. Demo 2, crossing an obstacle driven by white light. Since white light is a light source with a large irradiation area, the whole water is illuminated asymmetrically from one side. Therefore, when white light drives the PTV over the barrier, light is applied from one side, and the PTV rises to the water surface, moves to the backlight side using Marangoni convection, and finally lands on the other side of the barrier. The movie is played at 10× speed. Scale bar, 1 cm. Demo 3, PTV crosses an obstacle underwater. The PTV has the ability to cross obstacles while moving to the surface. To examine the ability of the PTV to cross obstacles underwater with continuous motion, we continuously reoriented the light source to make it jump continuously without reaching the water surface, using the climbing ability of the PTV to cross the wedge-shaped obstacles underwater. The PTV is irradiated at an angle, and it jumps cyclically in the direction of oblique incidence. The incident light power is set to 200 mW, and the movie is played at 20× speed. Scale bar, 1 cm.

Supplementary Video 11

Video 11. Automatic cycling between the water surface and bottom. Adjusting the state of the PTV, the white light is illuminated from one side, and the PTV produces an upwards motion. Similar to crossing the barrier from the water surface, after the PTV moves to the water surface, Marangoni convection caused by surface tension causes it to move to the other side and then land on the bottom. At this point, the PTV receives asymmetric illumination underwater and gradually tends to jump again, forming a complete and uninterrupted cycle. Movies are played at 20× speed. Scale bar, 1 cm.

Supplementary Video 12

Video 12. Horizontal and circular motions of a microalgae-embedded PTV. Our research on PTVs has focused on hydrogel materials with embedded nanoparticles (AuNPs and r-GO). Here, using the absorption of light by microalgae, we fabricated a microalgae-embedded PTV. The movie shows the phototropism of this PTV with embedded microalgae, which similarly achieves similar motor properties to PTVs made of artificial materials. The movie is played at 10× speed. Scale bar, 1 cm.

Supplementary Video 13

Video 13. Synthetic swarm of PTVs. Many microorganisms and life forms have the ability to coordinate with each other, and we have placed multiple PTVs under symmetric and asymmetric illumination in an attempt to investigate their performance when moving together. The movie shows PTVs reproducing the clustering effect of life-like organisms. Movies are played in real time and at 20× speed. Scale bar, 1 cm.

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Hou, G., Zhang, X., Du, F. et al. Self-regulated underwater phototaxis of a photoresponsive hydrogel-based phototactic vehicle. Nat. Nanotechnol. 19, 77–84 (2024). https://doi.org/10.1038/s41565-023-01490-4

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