Peer review of: 3D printing of multi-scalable structures via high penetration near-infrared photopolymerization (2020)

REVIEWER COMMENTS 

Reviewer #1 (Remarks to the Author): 

This is a very interesting manuscript. The authors disclose 3D printing of free standing objects 
based on an up-conversion protocol applying NIR laser radiation at 980 nm for excitation of 
UCNPs. They also applied different colors for printing for which regular UV printing would fail. 

This is from my point of view a difficult objective. I had for myself similar ideas many months ago 
but I failed. 

Authors designed a special ink that facilitates manufacture of the aforementioned targets. UCNPs 
generate only 1 % UV photons accessible to initiate UV processes applying NIR laser radiation at 
980 nm. The remaining part is heat which is from my point of view responsible for the observed 
reactions disclosed in Figure 1. Although the material started to polymerize, the overall elasticity of 
the material is high enough for processing while higher higher conversions provide the necessary 
mechanical properties as shown at later by increase of the respective modulus. 

This paper combines a clever combination of chemistry and engineering which results only in a 
combination well functioning 3D printing application described in this manuscript. It is from my 
point novel and presumable saves processing time compared to two-photon excitation. I am 
convinced this paper will have a deep impact to the community. I would also expect further 
industrial applications based on the knowledge disclosed I this manuscript. Typical applications 
may cover areas in medicine to fabricate prosthesis, mechanical engineering to make prototypes 
for lab testing - just to name o few of them. 

The authors also concluded well prospective developments for the future. 

There are only a few small changes necessary. Ref. 27 does not relate to up-conversion. An 
alternative would be 
D. Oprych, C. Schmitz, C. Ley, X. Allonas, E. Ermilov, R. Erdmann, B. Strehmel, "Photophysics of 
Up-Conversion Nanoparticles: Radical Photopolymerization of Multifunctional Methacrylates 
Comprising Blue- and UV-Sensitive Photoinitiators" ChemPhotoChem 2019, 3, 1119-1126. 

fitting much better since it also explains the right up-conversion process. Furthermore, the labels 
in the figures are too small, they should be enlarged and spelling errors in the figures must be 
corrected. Neevrtheless, this is a small thing which can be easily fixed. Again, it is from my point 
of view a very interesting paper and I enjoyed to read it. I learned many things from it. 

Authors should also better comment on the processing time to fabricate such objects. Desirable 
would would be to make a rough comparison with two-photon excitation since this represents a 
competitive technique. The advance of this manuscript is, although it also bases on multi-photon 
excitation, the fact that this happens in serial steps shall lead to shorter processing cycles. This 
method is from my point of view the more efficient way to fabricate the objects disclosed. 

Experiments were disclosed somehow that experts in the field shall be able to rework and 
reproduce the results disclosed by the authors. 

I strongly recommend publication of this ms. 



Reviewer #2 (Remarks to the Author): 

The authors in this paper suggest the use of NIR in 3D printing applications. The starting point is 
thir previous studies in the use of NIR light in free radical as well as cationic polymerization by 
utilizing up-conversion nanoparticle (UCNP), and successfully photocured resins for over 10 cm 
with relatively uniformed conversion. In this paper, the authors report a novel 3DP strategy with 
NIR induced 
photopolymerization, and the fusion of NIR photocurable material and DIW 3DPtechnology. By this 
strategy they could achieve in-situ curing of thick filament with high penetration. 
The paper is very well wittena and all the curing parameters well investigated. Besides designing 
the photocurable formulations and the irradiation conditions, 
The authors fabricate freestanding objects, and with NIR-DIW rapid curing the structure directly 
stacked to form freestanding spring with different colours. 
This is definitely very interesting in the field of 3D-printing and it is an important step forward in 
finding new technologies for 3D printing. For this reason I would suggest to accept this paper in 
Nature Communication in the present form without any modifications. 



Reviewer #3 (Remarks to the Author): 

Zhu et al. presented the fabrication of three-dimensional architecture enabled by up-conversion 
nanoparticles (UCNPs), which utilize near-infrared light for the generation of free radicals to initiate 
the radical polymerization of acrylate monomers and crosslinkers. The introduction of UCNPs 
greatly reduces the energy required for the crosslinking of a network and increases the light 
penetration length for a more holistic structure. The authors demonstrated preliminary results in 
the manuscript, showing interesting aspects of introducing UCNPs into 3D printing. However, early 
work has shown that up-conversion materials could be employed in stereolithography for building 
up three-dimensional objects (J. Me´ndez-Ramos, et al., J. Mater. Chem. C, 2016, 4, 801—806) 
and it would be advised for the authors to further clarify the design novelty compared with 
aforementioned work to distinguish a pure adaptation to DIW. Hence, this paper is not novel 
enough to warrant publication in Nat. Commun. And the reviewer would like to suggest the 
authors submit this paper somewhere else. 
1. The manuscript is entitled “3D Printing of Multi-scalable Structures via High Penetration Near-
Infrared Photopolymerization” but the concept of “multi-scale” was not addressed in the 
manuscript, nor did this reviewer trace experiments relating to multi-scale structure. It would be 
better if the authors would clarify or revise accordingly. This reviewer would like to suggest the 
authors listing the scales of existing UV-assisted DIW for the purpose to compare. 
2. The schematics in Figure 1 were incorrect topologically since acrylate is not monovalent and the 
authors should revise this. 
3. The conversion of the vinyl groups was calculated using ATR-FTIR. However, it has been known 
that IR spectra is more a qualitative rather than a quantitative analysis. This reviewer is concerned 
about how accurate the conversion obtained by the authors is since all the conversions are 
presented with three significant numbers. 
4. The UCNPs in the manuscript, even though promising in near-infrared polymerization, were not 
provided with any chemical structure, such as the chemical composition, doping elements, size, 
PDI, and so on. It would be better if the authors could further clarify all the essential information. 
5. The freestanding 3D-printed objects in Figure 4, in the opinions of this reviewer, are far from 
controllable if not intentionally designed that way. This reviewer is concerned about how precise 
this technique developed by the authors could be if the ink cannot be solidified. To demonstrate 
the 3D printability, more 3D printed monolith should be demonstrated other than spiral structure. 
6. There are numerous grammatical errors. The authors are suggested to proofread more carefully 
to polish their manuscript significantly. Some (incomplete) examples are shown below. 
In line 21, “facing with” should be “facing” 
In line 36, “progresses” should be “progress” 
In line 45, “has” should be “have” 
In line 78, “a series works of” should be “a series of” 
In line 92, “get” should be “gets” 
In line 99, “glass palate” should be “glass plate” 
In line 326, “nozzzle” should be “nozzle” 
7. From this paper, it is unclear to this reviewer how the pigments were integrated into the system 
and whether the printer colors were robust against different conditions. 
8. This reviewer suggests some mechanical measurements on the printed structures to exhibit the 
robustness of the printed objects.