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Foaming photopolymers as a high-resolution biomimetic printing platform

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Acknowledgements

We thank J. Wu, S. Chen and L. Tang for assistance with sample preparation and image processing; H. Takahashi, S. Kishimura of Nanohub at Kyoto University for access to their facility and instruments; the Analysis Centre at iCeMS, KUIAS, Kyoto University for access to their SEM and confocal microscope; T. Fujiwara and F. Ishidate for their help with confocal microscope experiments; D. Yamaguchi, K. Nishida, Y. Nakanishi and H. Ogawa for their help with X-ray characterizations; H. Lu and L. Yang from H. Xu’s group at Kyoto Institute of Technology for their help with bending tests; Y. Nishioka of Otsuka Chemicals, and Higashiyama Films for kind donation of PET substrates; T. Kato and H. Cheong at the Venture Business Laboratory at Nagoya University for access to their clean room facilities and microLED; R. Kurita at the Tokyo Metropolitan University for the discussions; and the Louvre (Paris), the Mauritshuis (the Hague) and the Library of Congress for permission to use their artworks.

Funding

This work was supported by JSPS Kiban-S (23H05468) and the JST K-Program Grant (JPMJKP23H4), and the central facilities were supported by the World Premier International Research Initiative (WPI), MEXT, Japan.

Author information

Authors and Affiliations

  1. Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Kyoto, Japan

    Detao Qin, Xianghui Liu, Baian Kuang, Yuzhe Zhang, Masateru Ito, Yuan Liu, Jianduo Zhang, Claudia Rosa, Hiroshi Imahori, Ganesh N. Pandian & Easan Sivaniah

  2. Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto, Japan

    Detao Qin, Xianghui Liu, Baian Kuang, Yuzhe Zhang, Masateru Ito, Yuan Liu, Jianduo Zhang, Claudia Rosa, Hiroshi Imahori, Ganesh N. Pandian & Easan Sivaniah

  3. State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, China

    Xianghui Liu

  4. Center for Anatomical Studies, Graduate School of Medicine, Kyoto University, Kyoto, Japan

    Tatsuya Katsuno

  5. State Key Laboratory of Advanced Fiber Materials, College of Materials Science and Engineering, Donghua University, Shanghai, China

    Yuan Liu & Meifang Zhu

  6. Institute for Chemical Research, Kyoto University, Kyoto, Japan

    Mikihito Takenaka

  7. Institute for Liberal Arts and Sciences (ILAS), Kyoto University, Kyoto, Japan

    Hiroshi Imahori

  8. Department of Physics, Graduate School of Science, Kyoto University, Kyoto, Japan

    Taiki Yanagishima

  9. Department of Physics, Tokyo Metropolitan University, Tokyo, Japan

    Taiki Yanagishima

Authors

  1. Detao Qin
  2. Xianghui Liu
  3. Baian Kuang
  4. Yuzhe Zhang
  5. Masateru Ito
  6. Tatsuya Katsuno
  7. Yuan Liu
  8. Jianduo Zhang
  9. Claudia Rosa
  10. Meifang Zhu
  11. Mikihito Takenaka
  12. Hiroshi Imahori
  13. Ganesh N. Pandian
  14. Taiki Yanagishima
  15. Easan Sivaniah

Contributions

E.S. and D.Q. devised the initial concept. D.Q., E.S. and T.Y. managed the execution of the overall project and manuscript preparation. D.Q. performed most of the sample preparation, microscopy and data analysis. B.K., M.I., M.T., J.Z. and G.N.P. provided additional contributions to general thin film work. X.L., Y.Z. and H.I. conducted the optical studies. Y.L. and M.Z. conducted the fibre-related studies. T.K. performed specialized FIB microscopy measurements. C.R. managed the scale-up demonstration. All authors contributed to manuscript refinement.

Corresponding authors

Correspondence to Detao Qin or Easan Sivaniah.

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Competing interests

The authors declare no competing interests.

Peer review

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Nature thanks Seung Hwan Ko, Bodo Wilts and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Formation of Deepfoam Photolithography (DFP) structures in the absence and presence of photoinitiator.

a, A near-surface foam was formed in the absence of photoinitiator. A 192-kDa polystyrene (PS) film with a thickness of 6.1 μm was prepared by spin-casting a solution that does not contain any photoinitiator. The film was illuminated using UV 254 nm (CX-2000 UVP) for 2 h. Foaming is limited to a few microns in depth even with prolonged development in 32 °C acetic acid. b, DFP can generate foaming deep within the polymer film in the presence of photoinitiator. A 192-kDa PS film with a thickness of 120 μm was prepared by casting a solution that contains 5 wt% thioxanthone relative to the PS. The film was illuminated using a custom LED chamber (Thorlabs, λi: 385 nm) with an energy dosage of 3000 J cm−2. Then, the film was developed in 32 °C acetic acid for 2 h. The SEM cross-section of the developed film demonstrated foaming over hundreds of micrometres in depth for a film containing a photoinitiator. Scale bar is 10 μm in the primary panels and 1 μm in the zoom-in panels.

Extended Data Fig. 2 DFP film growth as a function of time.

a, Normalized DFP film thickness versus development time, normalized by the as-UV-exposed thickness. Error bars denote standard deviation (n = 3). b, SEM cross-section of the UV exposed film. c–l, SEM cross-section of the DFP film after a development time of 5 s (c), 15 s (d), 25 s (e), 60 s (f), 2 min (g), 3 min (h), 6 min (i), 15 min (j), 2 h (k), and 24 h (l), respectively. Scale bar: 2 μm in the zoom-in panels. In (a)–(l), DFP films were prepared under standard conditions: 5-μm-thickness, thioxanthone-to-polystyrene ratio of 5 wt%, 3000 J cm−2 (Thorlabs, λi: 385 nm), developed in 32 °C acetic acid. This result corresponds to the red-circle plot in Fig. 3a of the main text.

Source data

Extended Data Fig. 3 Workflow of the FIB-SEM analysis.

Firstly, a series of cross-sectional SEM images were obtained via FIB milling. Secondly, the FIB-SEM image stack was reconstructed into a three-dimensional (3D) volume using the software Dragonfly 3D World ZEISS edition (Version 2025.1). Thirdly, spatial segmentation of the reconstructed 3D volumes was performed using the machine learning model (Pre-trained U-Net Depth=5) implemented in the software. The segmented bubbles were used to determine their centroid positions and volume-equivalent diameters. Scale bars, 2 μm.

Extended Data Fig. 4 Confocal laser scanning microscopy (CLSM) and spectroscopic analysis of in situ DFP development.

a, Schematic of the sample configuration on the microscope objective. b, 3D reconstruction of in situ DFP development by CLSM. The thickness of the remaining original polymer layer (h1) was tracked to evaluate diffusion kinetics. For visualization purposes, polystyrene (PS) film (fluorescence from Rhodamine B) and the developer (acetic acid, fluorescence from SeTau-647-NHS) are pseudo-coloured as turquoise and red respectively. Scale bar, 5 μm. c, h1 thickness versus development time by in situ CLSM measurement. Inset is h1 reduction rate versus energy dose (Error bars denote standard deviation, n = 2). d, Schematic of the setup of in-situ spectroscopic measurement. e, Schematic of the bilayer optical model used for fitting the in situ reflectance spectra. See more details in Supplementary Note 1. f, Example of measured reflectance spectra. g, Example of simulated reflectance spectra. h, Change in film thickness and acetic acid fraction over development time by in situ spectroscopic analysis. i, h1 thickness versus development time under different development temperatures by in situ spectroscopic analysis. For (c) and (f)–(i), energy dose and development temperature were varied while all other parameters were kept at their standard values. Development temperature is 22 °C for (c). Energy dose and development temperature are 200 J cm−2 and 32 °C for (f)–(h). Development temperature is 32 °C for (i).

Source data

Extended Data Fig. 5 Structural evolution of 35-kDa polystyrene (PS) DFP film under a higher energy dosage.

a, Normalized DFP film thickness versus development time, normalized by the as-UV-exposed thickness. Error bars denote standard deviation (n = 3). b–h, SEM cross-section of the DFP film after a development time of 5 s (b), 25 s (c), 1 min (d), 1.5 min (e), 6 min (f), 0.5 h (g), and 2 h (h), respectively. Scale bar: 5 μm in the primary panels and 1 μm in the zoom-in panels. In (a)–(h), 2.7-μm-thick PS (35 kDa) film was prepared by spin-casting a solution containing 5 wt% thioxanthone relative to PS. UV exposure was performed in a custom LED chamber (Thorlabs, λi: 385 nm) with an energy dosage of 2000 J cm−2. The UV-exposed films were developed in acetic acid at 22 °C. This result corresponds to the green triangles in Fig. 3f of the main text.

Source data

Extended Data Fig. 6 DFP in a wide variety of polymers using various photoinitiators.

Photos of DFP printing and cross-sectional SEM images of the foams are given for different polymer systems: polycarbonate (PC) (a, f), cellulose triacetate (CTA) (b, e), polyethylene terephthalate (PET) (c, g), polysulfone (PSU) (d, h), and poly(methyl methacrylate) (PMMA) (i). Scale bars, 5 mm for the macrolens photos and 1 μm for the SEM cross-sections. 2.5-μm-thick polymer film was prepared by spin-casting a polymer solution (solvent: trifluoroacetic acid for PET and chloroform for the others) on cover glass. The photoinitiator (thioxanthone) to polymer ratio was maintained at 9 wt% for all the polymers. The films were illuminated using a custom LED chamber (Thorlabs, λi: 385 nm) at an energy dosage of 600 J cm−2. Photomask was made using an OHP sheet. Development was performed at room temperature (22 ± 1 °C) for all polymers: 10 min for 1-proponal: toluene (v/v: 8/2) for the PET film, 130 s in acetic acid for the PC film, 120 s in acetone: acetic acid (v/v: 6/5) for the PSU film, and 60 s in 1-propanol: acetone (v/v: 7/3) for the CTA film, and 60 s in acetic acid: H2O (v/v, 5/3) for PMMA film. All films were dried using an air blower upon completion of development. Note we have also successfully realized formation of DFP structures via a diverse set of photoinitiators, which includes thioxanthone, benzophenone and its derivatives e.g., 2-methylbenzophenone, 4,4′-bis-(diethylamino)-benzophenone, etc., anthraquinone, and phenanthrenequinone. These photoinitiators have a broad absorption window in the wavelength range of 300–400 nm.

Extended Data Fig. 7 Change in thickness and surface morphologies of polycarbonate (PC) DFP films over development time.

a, SEM cross-sections of PC DFP films at different development times. Panel number in (a) denotes as-illuminated film (1), and development time of 90 s (2), 30 min (3), and 2 h (4), respectively. b, Surface SEM images of PC films at different development times. c, Surface SEM images of 35-kDa polystyrene (PS) films at different development times. Panel number in (b) and (c) denotes as-illuminated film (1), and development time of 5 min (2), 15 min (3), 30 min (4), 1 h (5), 1.5 h (6), and 2 h (7). The polystyrene (35 kDa) and polycarbonate DFP films are prepared in the same conditions as those tested for the water contact angles (Fig. 4g in the main text). Briefly, photoinitiator (thioxanthone) to polymer ratio was kept at 7 wt% for both polymers. The films were illuminated using the custom LED chamber (Thorlabs, λi: 385 nm). The energy dose and development temperature for 35-kDa polystyrene films were 1000 J cm−2 and 22 °C, respectively. The energy dosage and development temperature for polycarbonate films were 3000 J cm−2 and 32 °C, respectively. Scale bars, 2 μm (a); 20 μm (primary panels, b, c); 1 μm (zoom-in panels, b, c).

Extended Data Fig. 8 DFP microfluidics and printing on flexible substrates.

a, Schematic of capillary flow in DFP microfluidics. b, SEM image of a DFP microfluidic film. c, Liquid (ethanol/water: 9/1, v/v, fluorescence from ATTO 495) flow in a high-resolution DFP microfluidics. 2-μm-thick polystyrene (PS, 35 kDa) film was prepared by spin casting a solution with thioxanthone content of 12.5 wt% (relative to PS) on a glass cover slip. The film was illuminated using a microLED instrument with an energy dosage of 80 J cm−2 and developed in 22 °C acetic acid for 90 sec. d–g, Photographs show bending of DFP films (d, f) and preservation of the patterns after releasing the bending (e, g). For (d) and (e), 2.5-μm-thick 192-kDa polystyrene film was cast on a 0.25-mm-thick polypropylene sheet. Thioxanthone (photoinitiator) was added at 9 wt% relative to PS in the casting solution. The film was illuminated using a large LED array (Micro Square, λi: 405 nm) with an energy dosage of 300 J cm−2. Photomask is made using an OHP sheet. For (f) and (g), a 2.5-μm-thick 192-kDa polystyrene film was cast on a 0.25-mm-thick polypropylene sheet. 2-methylbenzophenone (photoinitiator) was added at 9 wt% relative to PS in the casting solution. The film was illuminated using a large LED array (Micro Square, λi: 365 nm) with an energy dosage of 100 J cm−2. Photomask is made using an OHP sheet. Development used 22 °C acetic acid for (d) and (e), and 32 °C acetic acid for (f) and (g), respectively. Scale bars, 50 μm (b); 100 μm (c); 1 cm (e, g).

Extended Data Fig. 9 DFP fibers.

a, Schematic of the DFP process in fibers. b–d, SEM cross-sections of foamed 192-kDa polystyrene (PS) fibers after different development times: 25 s (b), 110 s (c), and 2 h (d). 192-kDa PS was dissolved in N,N-dimethylformamide (DMF) at a concentration of 35 wt% with a thioxanthone (THX) content of 4 wt% relative to PS. The prepared solution was electrospun at 16 kV with a flow rate of 2.0 mL h−1. The fibrous mat was illuminated using a large LED array (Micro Square, λi: 385 nm) with an energy dosage of 1500 J cm−2. The illuminated mat was developed in 32 °C acetic acid. e, Printing via the DFP process using a PS/THX electrospun mat. Blue colour of the fibers is from a dye (Disperse Blue 14) added to the polymer solution. A lower detail photomask from Fig. 5l was made using a OHP sheet. Ruler for scale (cm). f and g, Microscope photos of the sample in Fig. 5l at progressively higher magnifications. For (e)–(g), the sample was made under the same conditions, developed for 60 sec. Scale bar, 2 μm (b–d); 500 μm (f); 200 μm (g). Substrates: a glass cover slip (b–d); 1-mm-thick glass plate (e–g).

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Qin, D., Liu, X., Kuang, B. et al. Foaming photopolymers as a high-resolution biomimetic printing platform. Nature (2026). https://doi.org/10.1038/s41586-026-10968-9

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