Method for 3D printing of cubic microbubbles: fully enclosed thin-walled microcavities with ultra-high aspect ratios

arxiv: 2604.23618 · v1 · submitted 2026-04-26 · ⚛️ physics.optics

Method for 3D printing of cubic microbubbles: fully enclosed thin-walled microcavities with ultra-high aspect ratios

Sohail Khan , Zengbo Wang , Qingshan Yang , Liyang Yue This is my paper

Pith reviewed 2026-05-08 05:37 UTC · model grok-4.3

classification ⚛️ physics.optics

keywords two-photon polymerization3D printingcubic microbubblesmicrocavitiesthin wallshigh aspect ratioSU-8 photoresistoptical resonators

0 comments p. Extension

The pith

Two-photon polymerisation enables 3D printing of cubic microbubbles with ultra-thin walls around fully enclosed cavities.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper demonstrates a technique for creating cubic microbubbles using 3D printing that overcomes the difficulties of making thin-walled enclosed structures. Standard fabrication methods fail because solvents cannot reach inside to clear out unused material without breaking the fragile walls. By using a specific cubic shape and careful control of the laser printing process with a thick photoresist, the authors achieve walls with an aspect ratio of 340 to 1 and confirm the cavities are hollow through physical tests. The approach also speeds up production for many such structures at once. These microbubbles could form the basis for devices like pressure sensors and optical components.

Core claim

The authors establish that a two-photon polymerisation process with optimised parameters and a cubic design in high-viscosity SU-8 2050 photoresist can produce fully enclosed cubic microbubbles featuring thin walls at an aspect ratio of roughly 340:1, with complete removal of internal unexposed material and preserved structural integrity as verified by micromanipulator probing.

What carries the argument

The central mechanism is the combination of cubic structural design and optimised two-photon polymerisation parameters that permit efficient extraction of unexposed photoresist from the fully enclosed interior.

If this is right

The process significantly reduces printing time for large-scale and high-count builds while maintaining dimensional accuracy.
These cubic microbubbles can serve as fundamental building blocks for optical resonators and MEMS pressure sensors.
Applications extend to microfluidic reaction chambers and emerging metamaterials that rely on precise hollow microcavities.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

This fabrication approach could inspire similar solutions for other complex enclosed microstructures in photonics or microfluidics.
Scaling the method might lead to arrays of such microbubbles for advanced metamaterial designs where shape precision matters.
Further testing could explore the optical properties of these cubic resonators compared to spherical ones.

Load-bearing premise

The chosen cubic geometry combined with the optimised printing parameters permits complete extraction of unexposed photoresist from the fully enclosed interior without damaging or collapsing the ultra-thin walls.

What would settle it

A direct observation of residual photoresist inside the cavity or structural failure of the walls during micromanipulator probing or imaging would disprove the successful fabrication claim.

read the original abstract

A microbubble is, in essence, a fully enclosed thin-walled microcavity. Unlike spherical microbubbles formed by expansions, 3D printing enables the free definition of their geometry, allowing precise control over shape and dimensions during fabrication. However, the geometric nature of microbubbles poses significant challenges for conventional photoresist-based lithographic microfabrication due to their fragile thin-walls, enclosed hollow volumes, and high sensitivity to mechanical stresses. These characteristics prevent developer solvents from accessing the internal cavities to remove unexposed photoresist. Two-photon polymerisation (2PP) is a laser-based 3D microprinting technique capable of sub-diffraction-limited resolution, offering exceptional design freedom for fabricating complex micro-architectures in photoresists. In this study, we demonstrate a 2PP-based method that overcomes these limitations and, for the first time, enables the successful fabrication of cubic microbubbles with ultra-high-aspect-ratio thin walls and fully enclosed microcavities using high-viscosity SU-8 2050 photoresist. The optimised process parameters and structural design facilitate efficient extraction of unexposed photoresist from the cavity interior while achieving a thin-wall ultra-high aspect ratio of approximately 340:1. The hollow nature and mechanical integrity of the printed structures were experimentally confirmed using micromanipulator-based probing. The proposed method maintains excellent dimensional accuracy and significantly reduces printing time for large-scale and high-count builds in 2PP processes. Such microbubbles are fundamental building blocks for optical resonators, microelectromechanical systems (MEMS) pressure sensors, microfluidic reaction chambers, and emerging metamaterials.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

This paper gives a workable 2PP process for printing cubic microbubbles with 340:1 aspect-ratio walls, but the evidence that the interior is fully cleaned rests on external probing that leaves some room for doubt.

read the letter

The main thing to know is that the authors have printed cubic microbubbles with fully enclosed cavities and walls at roughly 340:1 aspect ratio using two-photon polymerization on SU-8 2050. They claim this is the first time non-spherical enclosed microcavities with these dimensions have been made this way, and they show that optimized exposure parameters plus the cubic shape let the developer solvent reach and remove the unexposed resist without the structure collapsing. The structures were then tested with a micromanipulator to confirm they are hollow and mechanically intact. That demonstration is the concrete advance here, and it directly addresses a practical barrier in microfabrication where enclosed thin shells are hard to clear. For readers who need custom-shaped microcavities for resonators, sensors, or metamaterials, the process details on printing time reduction and dimensional control will be the useful part. The soft spot is exactly what the stress-test note flags: micromanipulator probing can show the outside is intact and that some volume exists inside, but it does not directly confirm that the interior surfaces are free of residual resist films or that wall thickness is uniform everywhere. Without cross-section imaging or quantitative metrology after development, the claim of “efficient extraction” and “fully enclosed microcavities” still carries an assumption about the development step. The paper is a methods report rather than a theoretical one, so the citation pattern is light and the central claim is experimental. It is aimed at experimentalists in optics, MEMS, and microfabrication who want to try the geometry themselves. I would send it to peer review because the fabrication route is new enough and the potential applications are clear, even though it would benefit from additional interior verification data. A referee could ask for those checks without rejecting the core result.

Referee Report

2 major / 2 minor

Summary. The manuscript presents a two-photon polymerization (2PP) method for fabricating cubic microbubbles—fully enclosed thin-walled microcavities—with ultra-high aspect ratios (~340:1) using high-viscosity SU-8 2050 photoresist. It claims that optimized process parameters and cubic geometry enable complete extraction of unexposed resist from the enclosed interior without collapse, with the hollow nature and mechanical integrity verified experimentally via micromanipulator probing. The work emphasizes reduced printing time, dimensional accuracy, and potential applications in optical resonators, MEMS sensors, microfluidics, and metamaterials.

Significance. If the fabrication claims are substantiated with quantitative evidence, the result would represent a meaningful advance in 3D microfabrication by enabling enclosed high-aspect-ratio geometries that are inaccessible to conventional lithography or spherical expansion methods. Successful use of viscous SU-8 in 2PP for such structures could broaden design options for optical and mechanical microdevices. The absence of detailed process parameters, metrology data, and direct interior characterization, however, prevents a full assessment of reproducibility and impact at this stage.

major comments (2)

[Abstract / experimental confirmation] Abstract and experimental confirmation paragraph: the central claim that the method achieves 'efficient extraction of unexposed photoresist from the cavity interior' and 'fully enclosed microcavities' rests on micromanipulator-based probing alone. This provides only indirect evidence of an internal volume and external mechanical integrity; it does not directly confirm the absence of residual resist films on interior surfaces or uniform wall thickness, leaving the 'complete removal' assertion unverified by quantitative metrology or cross-sectional imaging.
[Abstract] Abstract: the manuscript states that 'optimised process parameters and structural design facilitate efficient extraction' yet provides no numerical values for laser power, scan speed, hatch distance, development time, or specific cubic geometry dimensions (wall thickness, side length) that would allow reproduction or assessment of the 340:1 aspect ratio.

minor comments (2)

[Abstract] The abstract mentions 'significantly reduces printing time for large-scale and high-count builds' but offers no comparative timing data or scaling metrics to support this statement.
[Abstract] No error bars, standard deviations, or sample sizes are reported for the aspect-ratio value or probing results, which weakens the quantitative claims.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for the constructive and detailed comments. We address each major point below and will revise the manuscript to improve reproducibility and strengthen the experimental evidence where feasible.

read point-by-point responses

Referee: [Abstract / experimental confirmation] Abstract and experimental confirmation paragraph: the central claim that the method achieves 'efficient extraction of unexposed photoresist from the cavity interior' and 'fully enclosed microcavities' rests on micromanipulator-based probing alone. This provides only indirect evidence of an internal volume and external mechanical integrity; it does not directly confirm the absence of residual resist films on interior surfaces or uniform wall thickness, leaving the 'complete removal' assertion unverified by quantitative metrology or cross-sectional imaging.

Authors: We acknowledge that micromanipulator probing constitutes indirect evidence of the hollow interior and mechanical integrity. The observed deformation behavior under applied force is consistent with an empty cavity, as residual resist would alter the mechanical response. In the revised manuscript we will expand the experimental section with quantitative force-displacement curves from the probing tests and additional post-probing SEM images. We note, however, that cross-sectional imaging of these ultra-thin-walled structures risks collapse and is not straightforward without introducing artifacts; we will explicitly discuss this limitation. revision: partial
Referee: [Abstract] Abstract: the manuscript states that 'optimised process parameters and structural design facilitate efficient extraction' yet provides no numerical values for laser power, scan speed, hatch distance, development time, or specific cubic geometry dimensions (wall thickness, side length) that would allow reproduction or assessment of the 340:1 aspect ratio.

Authors: We agree that explicit numerical values are required for reproducibility. Although the optimized parameters and dimensions are described in the methods and results sections, they were not restated in the abstract. In the revised manuscript we will add the specific values for laser power, scan speed, hatch distance, development time, wall thickness, and side length to both the abstract and main text, together with the metrology-based calculation of the 340:1 aspect ratio. revision: yes

standing simulated objections not resolved

Direct cross-sectional imaging or quantitative metrology of interior cavity surfaces without risking collapse of the ultra-thin-walled structures.

Circularity Check

0 steps flagged

No circularity: experimental methods paper with no derivations or fitted predictions

full rationale

The paper is a purely experimental report on a 2PP fabrication process for cubic microbubbles using SU-8 2050. It describes process parameters, structural design choices, and direct physical confirmation via micromanipulator probing. No equations, models, or predictions appear that could reduce to inputs by construction. No self-citations function as load-bearing uniqueness theorems or ansatzes. The central claim rests on fabrication outcomes and probing results rather than any logical or statistical reduction to prior fitted values, rendering the work self-contained.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The paper is a pure experimental methods demonstration. No free parameters are fitted to data in a mathematical sense; process parameters are stated as optimised but not quantified here. No new physical entities are postulated. The work relies on standard domain knowledge of two-photon polymerization.

axioms (1)

domain assumption Two-photon polymerisation enables sub-diffraction-limited resolution and complex 3D micro-architectures in photoresists.
Invoked in the abstract to justify the choice of 2PP for overcoming lithographic limitations.

pith-pipeline@v0.9.0 · 5606 in / 1357 out tokens · 61611 ms · 2026-05-08T05:37:06.814883+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

35 extracted references · 33 canonical work pages

[1]

Maruo S, Nakamura O and Kawata S 1997 Three-dimensional microfabrication with two-photon-absorbed photopolymerization Opt. Lett. 22 132-134 (doi: 10.1364/OL.22.000132 )

work page doi:10.1364/ol.22.000132 1997
[2]

Jaiswal A, Rastogi C K, Rani S, Singh G P, Saxena S and Shukla S 2023 Two decades of two-photon lithography: Materials science perspective for additive manufacturing of 2D/3D nano-microstructures iScience 26 106374 (doi: 10.1016/j.isci.2023.106374 )

work page doi:10.1016/j.isci.2023.106374 2023
[3]

Bougdid Y and Sekkat Z 2020 Voxels Optimization in 3D Laser Nanoprinting Sci. Rep. 10 10409 (doi: 10.1038/s41598-020-67184-2)

work page doi:10.1038/s41598-020-67184-2 2020
[4]

Pingali R, Kim H and Saha S K 2024 A computational evaluation of minimum feature size in projection two-photon lithography for rapid sub-100 nm additive manufacturing Micromachines 15 158 (doi: 10.3390/mi15010158)

work page doi:10.3390/mi15010158 2024
[5]

Micromech

Feng R and Farris R J 2003 Influence of processing conditions on the thermal and mechanical properties of SU8 negative photoresist coatings J. Micromech. Microeng. 13 80– 88 (doi: 10.1088/0960-1317/13/1/312)

work page doi:10.1088/0960-1317/13/1/312 2003
[6]

Laser Technol

Harinarayana V and Shin Y C 2021 Two-photon lithography for three -dimensional fabrication in micro/nanoscale regime: A comprehensive review Opt. Laser Technol. 142 107180 (doi: 10.1016/j.optlastec..107180 )

work page doi:10.1016/j.optlastec..107180 2021
[7]

Wang H, Pan C, Li C, Menghrajani K S, Schmidt M A, Li A, Fan F, Zhou Y, Zhang W, Wang H, Nair P N S, Chan J Y E, Mori T, Hu Y, Hu G, Maier S A, Ren H, Duan H and Yang J K W 2024 Two-photon polymerization lithography for imaging optics Int. J. Extrem. Manuf. 6 042002. (doi: 10.1088/2631-7990/ad35fe) 12

work page doi:10.1088/2631-7990/ad35fe 2024
[8]

El-Tamer A, Hinze U and Chichkov B N 2021 Two-photon polymerization in optics, microfluidics, and biomedicine Handbook of Laser Micro- and Nano-Engineering (Springer International Publishing) (doi: 10.1007/978-3-319-69537-2_35-1) p 1691–1735

work page doi:10.1007/978-3-319-69537-2_35-1 2021
[9]

Otuka A J G, Tomazio N B, Paula K T and Mendonça C R 2021 Two-photon polymerization: functionalized microstructures, micro-resonators, and bio-scaffolds Polymers 13 1994 (doi: 10.3390/polym13121994 )

work page doi:10.3390/polym13121994 2021
[10]

Faraji Rad Z, Prewett P D and Davies G J 2021 High-resolution two-photon polymerization: the most versatile technique for the fabrication of microneedle arrays Microsyst. Nanoeng. 7 71 (doi: 10.1038/s41378-021-00298-3)

work page doi:10.1038/s41378-021-00298-3 2021
[11]

Nanotechnol

Liu T, Tao P, Wang X, Wang H, He M, Wang Q, Cui H, Wang J, Tang Y, Tang J, Huang N, Kuang C, Xu H and He X 2024 Ultrahigh-printing-speed photoresists for additive manufacturing Nat. Nanotechnol. 19 51–57 (doi: 10.1038/s41565-023-01517-w)

work page doi:10.1038/s41565-023-01517-w 2024
[12]

Kamranikia K, Dominici S, Keller M, Kube N, Mougin K and Spangenberg A 2023 Very high-aspect-ratio polymeric micropillars made by two -photon polymerization Micromachines 14 1602 (doi: 10.3390/mi14081602)

work page doi:10.3390/mi14081602 2023
[13]

Marschner D E, Pagliano S, Huang P H and Niklaus F 2023 A methodology for two- photon polymerization micro 3D printing of objects with long overhanging structures Addit. Manuf. 66 103474 (doi: 10.1016/j.addma.2023.103474 )

work page doi:10.1016/j.addma.2023.103474 2023
[14]

Chansoria P, Rizzo R, Rütsche D, Liu H, Delrot P and Zenobi-Wong M 2024 Light from afield: fast, high-resolution, and layer-free deep vat 3D printing Chem. Rev. 124 8787–8822 (doi: 10.1021/acs.chemrev.4c00134 )

work page doi:10.1021/acs.chemrev.4c00134 2024
[15]

Park S H, Kim K H, Lim T W, Yang D Y and Lee K S 2008 Investigation of three- dimensional pattern collapse owing to surface tension using an imperfection finite element model Microelectron. Eng. 85 432–439 (doi: 10.1016/j.mee.2007.08.003 )

work page doi:10.1016/j.mee.2007.08.003 2008
[16]

Liu Y, Nolte D D and Pyrak-Nolte L J 2010 Large-format fabrication by two-photon polymerization in SU-8 Appl. Phys. A 100 181–191 (doi: 10.1007/s00339-010-5735-8)

work page doi:10.1007/s00339-010-5735-8 2010
[17]

LaFratta C N and Baldacchini T 2017 Two-photon polymerization metrology: characterization methods of mechanisms and microstructures Micromachines 8 101 (doi: 10.3390/mi8040101 )

work page doi:10.3390/mi8040101 2017
[18]

Chromatogr

Matheuse F, Vanmol K, Van Erps J, De Malsche W, Ottevaere H and Desmet G 2022 On the potential use of two-photon polymerization to 3D print chromatographic packed bed supports J. Chromatogr. A 1663 462763 (doi: 10.1016/j.chroma.2021.462763)

work page doi:10.1016/j.chroma.2021.462763 2022
[19]

Bhanvadia A A, Farley R T, Noh Y and Nishida T 2023 3D printing of hollow geometries using blocking liquid substitution stereolithography Sci. Rep. 13 434 (doi: 10.1038/s41598-022-26684-z)

work page doi:10.1038/s41598-022-26684-z 2023
[20]

Express 26 33245-33252 (doi: 10.1364/OE.26.033245 )

Ortiz-Huerta F, Chen L, Taverne M, Hadden J P, Johnson M, Ho Y L D and Rarity J G 2018 Fabrication of hybrid Fabry‑Pérot microcavity using two -photon lithography for single - photon sources Opt. Express 26 33245-33252 (doi: 10.1364/OE.26.033245 )

work page doi:10.1364/oe.26.033245 2018
[21]

Photochem

Bieda M, Bouchard F and Lasagni A F 2016 Two-photon polymerization of a branched hollow fiber structure with predefined circular pores J. Photochem. Photobiol. A Chem. 319– 320 1-7 (doi: 10.1016/j.jphotochem.2015.12.012 )

work page doi:10.1016/j.jphotochem.2015.12.012 2016
[22]

Saetchnikov A V, Tcherniavskaia E A, Saetchnikov V A and Ostendorf A 2024 Two- photon polymerization of optical microresonators for precise pH sensing Light Adv. Manuf. 5 624-636 (doi: 10.37188/lam.2024.054)

work page doi:10.37188/lam.2024.054 2024
[23]

Rohbeck N, Ramachandramoorthy R, Casari D, Schürch P, Edwards T E J, Schilinsky L, Philippe L, Schwiedrzik J and Michler J 2020 Effect of high strain rates and temperature on the micromechanical properties of 3D -printed polymer structures made by two -photon lithography Mater. Des. 195 108977 (doi: 10.1016/j.matdes.2020.108977 )

work page doi:10.1016/j.matdes.2020.108977 2020
[24]

Micromech

Lorenz H, Despont M, Fahrni N, LaBianca N, Renaud P and Vettiger P 1997 SU-8: a low-cost negative resist for MEMS J. Micromech. Microeng. 7 121–124 13 (doi: 10.1088/0960-1317/7/3/010)

work page doi:10.1088/0960-1317/7/3/010 1997
[25]

Micromech

Del Campo A and Greiner C 2007 SU-8: a photoresist for high-aspect-ratio and 3D submicron lithography J. Micromech. Microeng. 17 R81–R95 (doi: 10.1088/0960-1317/17/6/R01)

work page doi:10.1088/0960-1317/17/6/r01 2007
[26]

Maibohm C, Silvestre O F, Borme J, Sinou M, Heggarty K and Nieder J B 2020 Multi- beam two-photon polymerization for fast large area 3D periodic structure fabrication for bioapplications Sci. Rep. 10 8740 (doi: 10.1038/s41598-020-64955-9)

work page doi:10.1038/s41598-020-64955-9 2020
[27]

Hahn V, Rietz P, Hermann F, Müller P, Barner-Kowollik C, Schlöder T, Wenzel W, Blasco E and Wegener M 2022 Light-sheet 3D microprinting via two -colour two-step absorption Nat. Photon. 16 784–791 (doi: 10.1038/s41566-022-01081-0)

work page doi:10.1038/s41566-022-01081-0 2022
[28]

24 2671–2679 (doi: 10.1021/acs.nanolett.4c00505 )

Zhang L, Wang C, Zhang C, Xue Y, Ye Z, Xu L, Hu Y, Li J, Chu J and Wu D 2024 High-throughput two-photon 3D printing enabled by holographic multi -foci high-speed scanning Nano Lett. 24 2671–2679 (doi: 10.1021/acs.nanolett.4c00505 )

work page doi:10.1021/acs.nanolett.4c00505 2024
[29]

Ha C W 2023 Overcoming delamination in two -photon lithography for improving fabrication of 3D microstructures Micro Nano Syst. Lett. 11 8 (doi: 10.1186/s40486-023-00173-y)

work page doi:10.1186/s40486-023-00173-y 2023
[30]

Lee C J, Kim J, Lee G H, Hyeon J, Choi Y and Cho N 2024 Swelling behavior of acrylate-based photoresist polymers containing cycloaliphatic groups of various sizes Materials 17 5465 (doi: 10.3390/ma17225465)

work page doi:10.3390/ma17225465 2024
[31]

Denk W, Strickler J H and Webb W W 1990 Two-photon laser scanning fluorescence microscopy Science 248 73–76 (doi: 10.1126/science.2321027 )

work page doi:10.1126/science.2321027 1990
[32]

Sheppard C J R and Gu M 1990 Image formation in two-photon fluorescence microscopy Optik 86 104–106

1990
[33]

Microchem SU-8 2000 Data Sheet, link: https://refractiveindex.info/download/data/2011/Microchem%20SU - 82000DataSheet2000_5thru2015Ver4.pdf , 2011

2000
[34]

Metze F K, Sant S, Meng Z, Klok H A and Kaur K 2023 Swelling-activated, soft mechanochemistry in polymer materials Langmuir 39 3546–3557 (doi: 10.1021/acs.langmuir.2c02801 )

work page doi:10.1021/acs.langmuir.2c02801 2023
[35]

Hu Q, Rance G A, Trindade G F, Pervan D, Jiang L, Foerster A, Turyanska L, Tuck C, Irvine D J, Hague R and Wildman R D 2022 The influence of printing parameters on multi - material two-photon polymerisation based micro additive manufacturing Addit. Manuf. 51 102575 (doi: 10.1016/j.addma.2021.102575 )

work page doi:10.1016/j.addma.2021.102575 2022

[1] [1]

Maruo S, Nakamura O and Kawata S 1997 Three-dimensional microfabrication with two-photon-absorbed photopolymerization Opt. Lett. 22 132-134 (doi: 10.1364/OL.22.000132 )

work page doi:10.1364/ol.22.000132 1997

[2] [2]

Jaiswal A, Rastogi C K, Rani S, Singh G P, Saxena S and Shukla S 2023 Two decades of two-photon lithography: Materials science perspective for additive manufacturing of 2D/3D nano-microstructures iScience 26 106374 (doi: 10.1016/j.isci.2023.106374 )

work page doi:10.1016/j.isci.2023.106374 2023

[3] [3]

Bougdid Y and Sekkat Z 2020 Voxels Optimization in 3D Laser Nanoprinting Sci. Rep. 10 10409 (doi: 10.1038/s41598-020-67184-2)

work page doi:10.1038/s41598-020-67184-2 2020

[4] [4]

Pingali R, Kim H and Saha S K 2024 A computational evaluation of minimum feature size in projection two-photon lithography for rapid sub-100 nm additive manufacturing Micromachines 15 158 (doi: 10.3390/mi15010158)

work page doi:10.3390/mi15010158 2024

[5] [5]

Micromech

Feng R and Farris R J 2003 Influence of processing conditions on the thermal and mechanical properties of SU8 negative photoresist coatings J. Micromech. Microeng. 13 80– 88 (doi: 10.1088/0960-1317/13/1/312)

work page doi:10.1088/0960-1317/13/1/312 2003

[6] [6]

Laser Technol

Harinarayana V and Shin Y C 2021 Two-photon lithography for three -dimensional fabrication in micro/nanoscale regime: A comprehensive review Opt. Laser Technol. 142 107180 (doi: 10.1016/j.optlastec..107180 )

work page doi:10.1016/j.optlastec..107180 2021

[7] [7]

Wang H, Pan C, Li C, Menghrajani K S, Schmidt M A, Li A, Fan F, Zhou Y, Zhang W, Wang H, Nair P N S, Chan J Y E, Mori T, Hu Y, Hu G, Maier S A, Ren H, Duan H and Yang J K W 2024 Two-photon polymerization lithography for imaging optics Int. J. Extrem. Manuf. 6 042002. (doi: 10.1088/2631-7990/ad35fe) 12

work page doi:10.1088/2631-7990/ad35fe 2024

[8] [8]

El-Tamer A, Hinze U and Chichkov B N 2021 Two-photon polymerization in optics, microfluidics, and biomedicine Handbook of Laser Micro- and Nano-Engineering (Springer International Publishing) (doi: 10.1007/978-3-319-69537-2_35-1) p 1691–1735

work page doi:10.1007/978-3-319-69537-2_35-1 2021

[9] [9]

Otuka A J G, Tomazio N B, Paula K T and Mendonça C R 2021 Two-photon polymerization: functionalized microstructures, micro-resonators, and bio-scaffolds Polymers 13 1994 (doi: 10.3390/polym13121994 )

work page doi:10.3390/polym13121994 2021

[10] [10]

Faraji Rad Z, Prewett P D and Davies G J 2021 High-resolution two-photon polymerization: the most versatile technique for the fabrication of microneedle arrays Microsyst. Nanoeng. 7 71 (doi: 10.1038/s41378-021-00298-3)

work page doi:10.1038/s41378-021-00298-3 2021

[11] [11]

Nanotechnol

Liu T, Tao P, Wang X, Wang H, He M, Wang Q, Cui H, Wang J, Tang Y, Tang J, Huang N, Kuang C, Xu H and He X 2024 Ultrahigh-printing-speed photoresists for additive manufacturing Nat. Nanotechnol. 19 51–57 (doi: 10.1038/s41565-023-01517-w)

work page doi:10.1038/s41565-023-01517-w 2024

[12] [12]

Kamranikia K, Dominici S, Keller M, Kube N, Mougin K and Spangenberg A 2023 Very high-aspect-ratio polymeric micropillars made by two -photon polymerization Micromachines 14 1602 (doi: 10.3390/mi14081602)

work page doi:10.3390/mi14081602 2023

[13] [13]

Marschner D E, Pagliano S, Huang P H and Niklaus F 2023 A methodology for two- photon polymerization micro 3D printing of objects with long overhanging structures Addit. Manuf. 66 103474 (doi: 10.1016/j.addma.2023.103474 )

work page doi:10.1016/j.addma.2023.103474 2023

[14] [14]

Chansoria P, Rizzo R, Rütsche D, Liu H, Delrot P and Zenobi-Wong M 2024 Light from afield: fast, high-resolution, and layer-free deep vat 3D printing Chem. Rev. 124 8787–8822 (doi: 10.1021/acs.chemrev.4c00134 )

work page doi:10.1021/acs.chemrev.4c00134 2024

[15] [15]

Park S H, Kim K H, Lim T W, Yang D Y and Lee K S 2008 Investigation of three- dimensional pattern collapse owing to surface tension using an imperfection finite element model Microelectron. Eng. 85 432–439 (doi: 10.1016/j.mee.2007.08.003 )

work page doi:10.1016/j.mee.2007.08.003 2008

[16] [16]

Liu Y, Nolte D D and Pyrak-Nolte L J 2010 Large-format fabrication by two-photon polymerization in SU-8 Appl. Phys. A 100 181–191 (doi: 10.1007/s00339-010-5735-8)

work page doi:10.1007/s00339-010-5735-8 2010

[17] [17]

LaFratta C N and Baldacchini T 2017 Two-photon polymerization metrology: characterization methods of mechanisms and microstructures Micromachines 8 101 (doi: 10.3390/mi8040101 )

work page doi:10.3390/mi8040101 2017

[18] [18]

Chromatogr

Matheuse F, Vanmol K, Van Erps J, De Malsche W, Ottevaere H and Desmet G 2022 On the potential use of two-photon polymerization to 3D print chromatographic packed bed supports J. Chromatogr. A 1663 462763 (doi: 10.1016/j.chroma.2021.462763)

work page doi:10.1016/j.chroma.2021.462763 2022

[19] [19]

Bhanvadia A A, Farley R T, Noh Y and Nishida T 2023 3D printing of hollow geometries using blocking liquid substitution stereolithography Sci. Rep. 13 434 (doi: 10.1038/s41598-022-26684-z)

work page doi:10.1038/s41598-022-26684-z 2023

[20] [20]

Express 26 33245-33252 (doi: 10.1364/OE.26.033245 )

Ortiz-Huerta F, Chen L, Taverne M, Hadden J P, Johnson M, Ho Y L D and Rarity J G 2018 Fabrication of hybrid Fabry‑Pérot microcavity using two -photon lithography for single - photon sources Opt. Express 26 33245-33252 (doi: 10.1364/OE.26.033245 )

work page doi:10.1364/oe.26.033245 2018

[21] [21]

Photochem

Bieda M, Bouchard F and Lasagni A F 2016 Two-photon polymerization of a branched hollow fiber structure with predefined circular pores J. Photochem. Photobiol. A Chem. 319– 320 1-7 (doi: 10.1016/j.jphotochem.2015.12.012 )

work page doi:10.1016/j.jphotochem.2015.12.012 2016

[22] [22]

Saetchnikov A V, Tcherniavskaia E A, Saetchnikov V A and Ostendorf A 2024 Two- photon polymerization of optical microresonators for precise pH sensing Light Adv. Manuf. 5 624-636 (doi: 10.37188/lam.2024.054)

work page doi:10.37188/lam.2024.054 2024

[23] [23]

Rohbeck N, Ramachandramoorthy R, Casari D, Schürch P, Edwards T E J, Schilinsky L, Philippe L, Schwiedrzik J and Michler J 2020 Effect of high strain rates and temperature on the micromechanical properties of 3D -printed polymer structures made by two -photon lithography Mater. Des. 195 108977 (doi: 10.1016/j.matdes.2020.108977 )

work page doi:10.1016/j.matdes.2020.108977 2020

[24] [24]

Micromech

Lorenz H, Despont M, Fahrni N, LaBianca N, Renaud P and Vettiger P 1997 SU-8: a low-cost negative resist for MEMS J. Micromech. Microeng. 7 121–124 13 (doi: 10.1088/0960-1317/7/3/010)

work page doi:10.1088/0960-1317/7/3/010 1997

[25] [25]

Micromech

Del Campo A and Greiner C 2007 SU-8: a photoresist for high-aspect-ratio and 3D submicron lithography J. Micromech. Microeng. 17 R81–R95 (doi: 10.1088/0960-1317/17/6/R01)

work page doi:10.1088/0960-1317/17/6/r01 2007

[26] [26]

Maibohm C, Silvestre O F, Borme J, Sinou M, Heggarty K and Nieder J B 2020 Multi- beam two-photon polymerization for fast large area 3D periodic structure fabrication for bioapplications Sci. Rep. 10 8740 (doi: 10.1038/s41598-020-64955-9)

work page doi:10.1038/s41598-020-64955-9 2020

[27] [27]

Hahn V, Rietz P, Hermann F, Müller P, Barner-Kowollik C, Schlöder T, Wenzel W, Blasco E and Wegener M 2022 Light-sheet 3D microprinting via two -colour two-step absorption Nat. Photon. 16 784–791 (doi: 10.1038/s41566-022-01081-0)

work page doi:10.1038/s41566-022-01081-0 2022

[28] [28]

24 2671–2679 (doi: 10.1021/acs.nanolett.4c00505 )

Zhang L, Wang C, Zhang C, Xue Y, Ye Z, Xu L, Hu Y, Li J, Chu J and Wu D 2024 High-throughput two-photon 3D printing enabled by holographic multi -foci high-speed scanning Nano Lett. 24 2671–2679 (doi: 10.1021/acs.nanolett.4c00505 )

work page doi:10.1021/acs.nanolett.4c00505 2024

[29] [29]

Ha C W 2023 Overcoming delamination in two -photon lithography for improving fabrication of 3D microstructures Micro Nano Syst. Lett. 11 8 (doi: 10.1186/s40486-023-00173-y)

work page doi:10.1186/s40486-023-00173-y 2023

[30] [30]

Lee C J, Kim J, Lee G H, Hyeon J, Choi Y and Cho N 2024 Swelling behavior of acrylate-based photoresist polymers containing cycloaliphatic groups of various sizes Materials 17 5465 (doi: 10.3390/ma17225465)

work page doi:10.3390/ma17225465 2024

[31] [31]

Denk W, Strickler J H and Webb W W 1990 Two-photon laser scanning fluorescence microscopy Science 248 73–76 (doi: 10.1126/science.2321027 )

work page doi:10.1126/science.2321027 1990

[32] [32]

Sheppard C J R and Gu M 1990 Image formation in two-photon fluorescence microscopy Optik 86 104–106

1990

[33] [33]

Microchem SU-8 2000 Data Sheet, link: https://refractiveindex.info/download/data/2011/Microchem%20SU - 82000DataSheet2000_5thru2015Ver4.pdf , 2011

2000

[34] [34]

Metze F K, Sant S, Meng Z, Klok H A and Kaur K 2023 Swelling-activated, soft mechanochemistry in polymer materials Langmuir 39 3546–3557 (doi: 10.1021/acs.langmuir.2c02801 )

work page doi:10.1021/acs.langmuir.2c02801 2023

[35] [35]

Hu Q, Rance G A, Trindade G F, Pervan D, Jiang L, Foerster A, Turyanska L, Tuck C, Irvine D J, Hague R and Wildman R D 2022 The influence of printing parameters on multi - material two-photon polymerisation based micro additive manufacturing Addit. Manuf. 51 102575 (doi: 10.1016/j.addma.2021.102575 )

work page doi:10.1016/j.addma.2021.102575 2022