RESEARCH ARTICLE
www.advmat.de
Approaching Standardization: Mechanical Material Testing
of Macroscopic Two-Photon Polymerized Specimens
Thomas Koch,* Wenxin Zhang, Thomas T. Tran, Yingjin Wang, Adrian Mikitisin,
Jakob Puchhammer, Julia R. Greer, Aleksandr Ovsianikov,* Franziska Chalupa-Gantner,*
and Markus Lunzer*
Two-photon polymerization (2PP) is becoming increasingly established as
additive manufacturing technology for microfabrication due to its
high-resolution and the feasibility of generating complex parts. Until now, the
high resolution of 2PP is also its bottleneck, as it limited throughput and
therefore restricted the application to the production of microparts. Thus,
mechanical properties of 2PP materials can only be characterized using
nonstandardized specialized microtesting methods. Due to recent advances
in 2PP technology, it is now possible to produce parts in the size of several
millimeters to even centimeters, finally permitting the fabrication of
macrosized testing specimens. Besides suitable hardware systems, 2PP
materials exhibiting favorable mechanical properties that allow printing of
up-scaled parts are strongly demanded. In this work, the up-scalability of
three different photopolymers is investigated using a high-throughput 2PP
system and low numerical aperture optics. Testing specimens in the cm-range
are produced and tested with common or even standardized material testing
methods available in conventionally equipped polymer testing labs. Examples
of the characterization of mechanical, thermo-mechanical, and fracture
properties of 2PP processed materials are shown. Additionally, aspects such
as postprocessing and aging are investigated. This lays a foundation for
future expansion of the 2PP technology to broader industrial application.
1. Introduction
Two-photon polymerization (2PP) is a powerful high-resolution
additive manufacturing technology that covers a broad range of
dimensions, including the nano-, micro-, and mesoscale. 2PP
T. Koch, J. Puchhammer, A. Ovsianikov, F. Chalupa-Gantner
Institute of Materials Science and Technology
TU Wien, Vienna 1060, Austria
E-mail:
thomas.koch@tuwien.ac.at
;
aleksandr.ovsianikov@tuwien.ac.at
;
franziska.gantner@tuwien.ac.at
The ORCID identification number(s) for the author(s) of this article
can be found under
https://doi.org/10.1002/adma.202308497
© 2024 The Authors. Advanced Materials published by Wiley-VCH
GmbH. This is an open access article under the terms of the
Creative
Commons Attribution
License, which permits use, distribution and
reproduction in any medium, provided the original work is properly cited.
DOI: 10.1002/adma.202308497
takes advantage of the nonlinear nature
of two-photon absorption to reach high-
resolution features.
[
1–3
]
By focusing a
pulsed laser with a microscope objective
and scanning the focal point, solid poly-
mer is cured from photopolymer resin
only within the laser focus.
[
3
]
This distinct
volume is called the polymerization voxel.
The dimensions of the voxel determine
the achievable feature resolution and
the throughput, which is defined as the
polymerized volume per time. Those two
characteristics show a reciprocal correlation
and need to be balanced in accordance with
the targeted fabrication quality. The size of
the polymerization voxel depends on the
numerical aperture (NA) of the objective,
the processing parameters and the reactiv-
ity and nature of the 2PP resin.
[
4,5
]
Further,
the field number and magnification of
the objective define the field of view (FoV),
which can be scanned at a time.
[
6
]
By choos-
ing an adequate objective, the focal spot
size and FoV can be adapted to match the
targeted part dimension as well as feature
resolution. In general, the throughput can
be increased in reducing the amount of
scanoperationsbyincreasingthevoxelspacingorscanningfewer
FoVs.
Since the introduction of 2PP in the 1990s,
[
2,5
]
mainly,
three process development and research trends can be ob-
served. While some studies focus on advancing resolution and
W. Zhang, T. T. Tran, Y. Wang, J. R. Greer
Division of Engineering and Applied Sciences
California Institute of Technology
Pasadena, CA 91125, USA
A. Mikitisin
Central Facility for Electron Microscopy
RWTH Aachen, 52074 Aachen, Germany
J. R. Greer
Kavli Nanoscience Institute
California Institute of Technology
Pasadena, CA 91125, USA
M. Lunzer
UpNano GmbH
Vienna 1030, Austria
E-mail:
markus.lunzer@upnano.com
Adv. Mater.
2024
, 2308497
2308497 (1 of 19)
© 2024 The Authors. Advanced Materials published by Wiley-VCH GmbH
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www.advmat.de
precision,
[
7–11
]
others aim to improve the reactivity of the printing
materials
[
5,12–14
]
or increase the writing speed and throughput of
the technology.
[
15–26
]
Especially the latter is a substantial prereq-
uisite for industrial relevance and promotes the persistent mar-
ket presence of this relatively young technology. Initially, most
academic research was conducted on experimental setups. Now,
several commercial systems are available that operate either at
a wavelength of
≈
520 nm (green light, Microlight3D,
[
11
]
Multi-
photon Optics,
[
27
]
Femtika
[
28
]
)or
≈
780 nm (near infrared light,
UpNano,
[
29
]
Nanoscribe
[
30
]
).
With the increasing availability of commercial 2PP systems
and with regard to industrial applications, the mechanical char-
acterization of 2PP materials becomes crucial. So far, in liter-
ature, experimental approaches to determine the mechanical
properties of 2PP produced parts have almost always been re-
ported on the microscale due to the low achievable throughput
of 2PP in the past. For example, compliant microbeams were
bent by calibrated AFM cantilevers
[
31,32
]
or optical tweezers.
[
33
]
Tensile
[
34-36
]
and compression tests
[
34-37
]
were performed using
special
×
or piezo microtesting equipment, often as optional up-
grades of nanoindentation devices. Further, nanoindentation de-
vices were used for the conventional determination of modu-
lus and hardness by indentation of compact specimens,
[
36,38-42
]
but also for compression testing of bulk
[
34,36,37,43
]
and lattice
structures,
[
43-48
]
or for bending of relatively stiff microbeams.
[
49
]
Even nanodynamic-mechanical analysis was performed at room
temperature at constant
[
50
]
or varying frequency.
[
51
]
Moreover,
AFM-based indentation was often applied to characterize 2PP
printed soft biomaterials.
[
52-55
]
Besides these rather common test-
ing methods for 2PP structures, very customized methods can
be found, ranging from tensile tests of nanosprings,
[
56
]
oscilla-
tion of drum-like structures,
[
57
]
and microbeams,
[
58
]
bending re-
covery of nanowires
[
59
]
to the determination of the critical length
of collapse under capillary drying forces.
[
60
]
Even a microparti-
cle impact test on lattice carbon structures
[
61
]
or tensile tests on
woven structures
[
62
]
were demonstrated. Recently, laser-induced
resonant acoustic spectroscopy (LIRAS) has been presented for
nondestructive measurement of dynamic material properties.
[
63
]
In all of those cases, suitable microtest equipment is needed, and
often the data analysis seems relatively complex. Moreover, mi-
cromechanical testing is quite sensitive to thermal fluctuations
and accessing thermo-mechanical properties is hardly possible.
Besides high-precision printing, the latest generation of com-
mercial 2PP systems also allows high throughput production and
thus permits the fabrication of large structures of up to several
cm.
[
64-67
]
This is achieved by the use of appropriate optics (10
×
or
even 5
×
magnification), high-power lasers, high-speed scanners,
adaptiveresolution,andoptimizedscanningstrategies.Scanning
speeds of up to meters per second, voxel rates of several million
per second, and building rates of several tens to more than hun-
dred mm
3
h
−
1
are now possible (Table
2). These substantial im-
provements of 2PP permit to close the gap to other additive man-
ufacturing or conventional fabrication technologies in the meso-
and macrorange.
The advances in 2PP technology demand materials suitable
for up-scaled 2PP fabrication. A literature overview of photopoly-
mers suitable for rapid 2PP processing was given by Kiefer et al.
in late 2020.
[
13
]
Additional developments have been shown in
the meantime.
[
14,12,68,69
]
All these studies target the improvement
of processability by examining and evaluating novel photoini-
tiator systems or resin systems that go beyond (meth)acrylate
chemistry, as Barner-Kowollik et al.
[
70
]
suggested. However, be-
sides the general processability, the mechanical performance of
2PP materials is a critical aspect for up-scaling. In particular, the
strength and fracture behavior should be in focus for meso- and
macroscale applications. Not all photopolymers that work well
for nano- and microscale 3D printing, which requires high cross-
linking density, can be printed at a larger scale. Due to substantial
shrinkage stress, such highly crosslinked material systems will
very likely exhibit warping or even cracks when being used for
realization of large structures.
Those challenges, along with some studies,
[
71,72
]
point to the
importance of performing mechanical testing of 2PP-processed
materials at larger scales. In ref. [
73], microtensile tests on
stitched log-pile specimens were used to create a base to further
optimize the mechanical behavior of lattice structures with larger
dimensions. Two publications deal with the up-scalability of mi-
crotension and microcompression tests by comparison to the be-
havior of bulk macroscopic material.
[
34,36
]
While the strain rate
and temperature dependence of the commercial acrylate-based
photopolymer IP-Dip (Nanoscribe) were compared to literature
data of PMMA by Rohbeck et al.,
[
36
]
Bauer et al. compared mi-
crotest results with casted and subsequently UV-cured bulk sam-
ples made from the same photopolymer.
[
34
]
The strain rate de-
pendency of the compressive yield strength of micropillars and of
PMMA at the macroscale was comparable. In ref. [
34], a decrease
in double bond conversion, Young’s modulus, and compressive
yield strength was observed when increasing the edge length of
the compression specimens from 20 to 50
μ
m. Still, the values
are in the range of conventionally UV-cured molded macroscopic
specimens. Bending tests on millimeter-sized 2PP specimens
(7 mm
×
0.65 mm
×
0.25 mm) from a shape-memory thiol-vinyl
material, fabricated with a 25
×
objective and 14 mm s
−
1
scan-
ning speed were shown,
[
74
]
building time
≈
17 h.
[
74
]
The obtained
mechanical values were in the range of microsized objects.
Nevertheless, to the best of our knowledge, the mechanical be-
havior of 2PP-fabricated bulk specimens in the mm- to cm-range
has not been thoroughly investigated so far. However, the appli-
cation of standardized test procedures for 2PP materials would
be especially important to advance the industrial use of the tech-
nology. Upscaling 2PP test specimens is crucial for developing
reliable 2PP materials for the manufacturing of high-resolution
parts in the mm- to cm regime. The mechanical behavior cannot
be presumed or extrapolated from miniaturized microscale test
experiments alone. It is unknown how the cross-linking density
and inhomogeneities within the polymer network influence the
mechanical properties of larger volume parts. For example, poly-
merization shrinkage and resulting intrinsic stresses can nega-
tively affect the behavior of thick-walled objects, leading to cracks
warpage (see ETA/TTA below).
Moreover, for the generation of larger parts that exceed the
size of a FoV, several FoV need to be stitched together. The
influence of these stitches on the mechanical behavior of a part
has to be considered as well. Furthermore, the stability of the
2PP system as well as the fabrication process can influence
the test results. Moreover, to assess the 2PP process stability,
the specimen strength under tensile and bending conditions
is particularly useful. Compared to results from compression
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2024
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2308497 (2 of 19)
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