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\begin{document}

\title{Introduction of Photopolymerization Technology and Application}



\author[1]{Xianglong He}%
\author[2]{Liheng Zang}%
\author[3]{Yangyang Xin}%
\author[1]{Yingquan Zou}%
\affil[1]{Beijing Normal University College of Chemistry}%
\affil[2]{Baoding Lucky Innovative Materials Co Ltd}%
\affil[3]{Hubei Gurun Technology Co Ltd}%


\vspace{-1em}



  
  \date{May 25, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Photopolymerization as an energy-saving and environment-friendly
technology has been applied in many fields and developed continuously
since the middle of the 20th century. Today, photopolymerization
technology is ubiquitous in every aspect of our lives. This review
starts from the principle of photopolymerization reaction, introduces
the process of photopolymerization, initiation mechanism of
photoinitiators and three kinds of polymerization methods according to
the wavelength of irradiation source. Subsequently, review focuses on
the application of photopolymerization technology in thiol-based click
reaction, 3D printing, photoresist, hydrogels and other fields, so as to
demonstrate its irreplaceable role in the present.%
\end{abstract}%



\sloppy


\textbf{Introduction of Photopolymerization Technology and Application}

Xianglong He \textsuperscript{a}, Liheng Zang \textsuperscript{b},
Yangyang Xin \textsuperscript{c,} *, Yingquan Zou \textsuperscript{a,} *

\emph{\textsuperscript{a} College of Chemistry, Beijing Normal
University, Beijing, 100875, People's Republic of China}

\emph{\textsuperscript{b} Baoding Lucky Innovative Materials Co., Ltd,
Baoding, 071000, People's Republic of China}

\emph{\textsuperscript{c} Hubei Gurun Technology Co., Ltd, Jingmen,
448000, People's Republic of China}

\textbf{Abstract}

Photopolymerization as an energy-saving and environment-friendly
technology has been applied in many fields and developed continuously
since the middle of the 20th century. Today, photopolymerization
technology is ubiquitous in every aspect of our lives. This review
starts from the principle of photopolymerization reaction, introduces
the process of photopolymerization, initiation mechanism of
photoinitiators and three kinds of polymerization methods according to
the wavelength of irradiation source. Subsequently, review focuses on
the application of photopolymerization technology in thiol-based click
reaction, 3D printing, photoresist, hydrogels and other fields, so as to
demonstrate its irreplaceable role in the present.

\section*{1. Introduction to
photopolymerization.}

{\label{introduction-to-photopolymerization.}}

As an environmentally friendly technology, photopolymerization in which
liquid monomers or oligomers are transformed into solid materials under
irradiation of light (ultraviolet, visible or infrared light) holds
these advantages of low volatile organic compounds (VOC), fast curing,
energy saving, environmental protection, and low temperature compared
with traditional thermal polymerization.\textsuperscript{{[}1-6{]}} In
addition, photopolymerization is widely used in many fields due to its
unique ``5E'' properties (efficiency, enabling, economical, energy
saving and environmental friendly).\textsuperscript{{[}3, 7-12{]}} The
history of photopolymerization can be traced back to the 19th century
when people observed that styrene polymerized into glassy resin under
light. Ostromislenski, the first researcher of photopolymerization,
discovered that the number of constitutional units of product far
exceeded the number of photons absorbed in photopolymerization reaction
of vinyl bromide, and concluded that the reaction was chain
reaction.\textsuperscript{{[}13{]}} The 1960s was the heyday of basic
research on photochemistry, its theory gradually improved, and began to
be widely applied to actual industrial production.

A complete photopolymerization system generally consists of monomer,
oligomer and photoinitiator (PI), all components are
indispensable.\textsuperscript{{[}14{]}} Monomers, also known as active
diluents, can reduce the viscosity of polymerization system and improve
solubility, more importantly, they contain one or more functional groups
that can participate in the polymerization reaction such as double
bonds, epoxy, oxetane, thiol, etc. Oligomers, whose molecular weights
range from several hundred to tens of thousands, have high viscosity and
also contain some polymerizable functional groups to participate
polymerization. The proportion of monomers and oligomers is large in
whole polymerization system, and the properties of polymerization
materials are largely determined by them. PI, as an essential component
accounting for 1-10 wt \% of photopolymerization system, can produce
active species under irradiation of light to initiate polymerization
reaction, which has great influence on the rate of photopolymerization
and the performance of polymerization
material.\textsuperscript{{[}15-16{]}}

The procedure of the photopolymerization reaction is exhibited in Figure
1, firstly, PI of ground state undergoes electron transition to excited
state under irradiation of light, then generate active species, which
are free radicals or cations generally. Active species can interact with
functional groups of monomers or oligomers to initiate chain reactions,
resulting in the formation of polymer.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Figure 1 The process of photopolymerization reaction.

\section*{2. Photoinitiator of
photopolymerization.}

{\label{photoinitiator-of-photopolymerization.}}

PI plays an extremely important role in photopolymerization reaction, it
undergoes a series of complex physical or chemical changes in initiating
process.\textsuperscript{{[}17-19{]}} As is shown in Figure 2, PI
absorbs light, then is excited from ground state (S\textsubscript{0}) to
singlet state (S\textsubscript{1}) which could be returned to ground
state by fluorescence emission or nonradiative deactivation, also may
reach to triplet state (T\textsubscript{1}) by intersystem crossing
(ISC). Similarly, PI of triplet state can return to ground state by
phosphorescence or nonradiative deactivation as well. When active
species are generated by cleavage or hydrogen abstraction of PI in
triplet state, photopolymerization can be initiated.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Figure 2 Initiation mechanism of photoinitiator.

According to the different absorbance wavelength, PI can be divided into
ultraviolet (UV, mainly below 400 nm), visible light (generally in the
range of 400 nm to 700 nm) and near-infrared (NIR, usually greater than
700 nm) PI.\textsuperscript{{[}20-22{]}} Besides, from the perspective
of generated active species, PI can be divided into radical type and
cationic type, in which radical type composed of cleavage type (Norrish
I) and hydrogen abstraction type (Norrish II).\textsuperscript{{[}15,
23-24{]}} The classification of PI is displayed in Figure 3.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Figure 3 Classification of photoinitiator.

\subsection*{2.1. UV photoinitiator}

{\label{uv-photoinitiator}}

The earliest species of PIs, mainly absorb UV light to achieve
initiation, have been widely employed in numerous fields of UV
photopolymerization for many years. Firstly, the cleavage type (Norrish
I), refers to formation of radicals by homolytic reaction of weak
covalent bond in molecular, includes these species of benzils, benzoins,
oxime esters and so on.\textsuperscript{{[}4-5, 10, 25-26{]}} Figure 4
presents several structures of commonly-used cleavable UV PIs such as
2-hydroxy-2-methylpropiophenone (1173), 1-hydroxycyclohexyl phenylketone
(184), 2,2-dimethoxy-2-phenylacetophenone (DMPA),
2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (2959), methyl
benzoylformate (MBF), benzil, diphenyliodonium (Iod),
2-benzyl-2-(dimethylamino)-4'-morpholinobutyrophenone (369),
phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO),
diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), ethyl
(2,4,6-trimethylbenzoyl) phenylphosphinate (TPO-L), oxime esters OXE-10
and OXE-02, and cleavage mechanism of PI
1173.\textsuperscript{{[}27{]}}Most UV PIs of cleavage type undergo
one-step cleavage to polymerize the monomers except oxime esters which
requires two-step cleavage.

Figure 4 Structures of commonly-used cleavable UV PIs, and cleavage
mechanism of 1173.

Different from cleavage type, the hydrogen abstraction type PIs produce
radicals by interacting with hydrogen donors, then triggers
photopolymerization reaction.\textsuperscript{{[}23, 28{]}} In some
cases, hydrogen abstraction type PIs with outstanding light absorption
can also sensitize other PIs with weak light absorption, they can be
called photosensitizer. Figure 5 records some structures of
commonly-used hydrogen abstraction type UV PIs and photosensitizers such
as benzophenone (BP), 4,4'-bis(diethylamino) benzophenone (EMK),
isopropyl thioxanthone (ITX), 9,10-dibutoxyanthracene (DBA) , and
initiation mechanism of BP and donor photoinitiating
system.\textsuperscript{{[}27{]}} In addition, several commonly-used
hydrogen donors are also exhibited in Figure 5, such as N-phenylglycine
(NPG), ethyl 4-dimethylaminobenzoate (EDAB), triethanolamine (TEOA),
N-methyldiethanolamine (MDEOA), 2-mercaptobenzothiazole (2-MBTA),
2-mercaptobenzimidazole (2-MBMA) and 2-mercaptobenzoxazole (2-MBXA).

Figure 5 Structures of commonly-used UV PIs of hydrogen abstraction type
and several hydrogen donors, initiation mechanism of BP/doner system.

Both types of PIs possess their own characteristics, for cleavage type,
rapid initiation and one-component are advantages in practical
application,\textsuperscript{{[}29{]}} for hydrogen abstraction type,
two-component PIS can effectively relieve the problem of oxygen
inhibition, and is generally multifunctional.

\subsection*{2.2. LED photoinitiator}

{\label{led-photoinitiator}}

The traditional high-pressure mercury lamp, as the light source of UV
photopolymerization, has some shortcomings such as generation of ozone,
toxic mercury, short service life and large consumption of energy, which
lead to its limitations in practical application. Recently, there has a
tendency that the light source of photopolymerization has been moving
from UV toward near-UV or visible light using light-emitting-diodes
(LEDs).\textsuperscript{{[}5{]}} LEDs possess many merits such as
higher-operating efficiency, long service life, low cost, safety, and
environmentally friendly,\textsuperscript{{[}3, 6-7, 11, 30-43{]}} as a
result, LED photopolymerization has boomed in recent years. Whereas the
absorption wavelengths of most traditional UV PI usually are shorter
than 365 nm, and do not match with the emission wavelengths (usually
higher than 365 nm) of LEDs, so most of the commercially available UV PI
cannot be used for LED-induced
photopolymerization.\textsuperscript{{[}1, 30{]}} Therefore, the
development of PI suitable for LED light sources is necessary.

At present, the LED PI of cleavage type reported mainly include TPO,
BAPO, ITX, coumarin-based oxime esters,\textsuperscript{{[}1, 16,
43{]}}naphthalimide sulfur ether,\textsuperscript{{[}23,
29{]}}glyoxylates,\textsuperscript{{[}3, 31, 44-45{]}} and
acylgermanes,\textsuperscript{{[}46-48{]}} and so on, the relevant
structural formulas are shown in Figure 6 -- Figure 9. LED PI of
hydrogen abstraction or photosensitizers reported mainly include
anthraquinone derivatives,\textsuperscript{{[}35-36, 49{]}}cyclohexanone
derivatives,\textsuperscript{{[}37, 50{]}}cinnamoylformate
derivatives,\textsuperscript{{[}7{]}} porphyrins
derivatives,\textsuperscript{{[}51-52{]}} and so on, the relevant
structural formulas are shown in Figure 10 -- Figure 13.

Figure 6 Several chemical structures of coumarin-based oxime esters.

Figure 7 Several chemical structures of naphthalimide sulfur ether.

Figure 8 Several chemical structures of glyoxylates derivatives.

Figure 9 Several chemical structures of acylgermane derivatives.

Figure 10 Several chemical structures of anthraquinone derivatives.

Figure 11 Several chemical structures of cyclohexanone derivatives.

Figure 12 Several chemical structures of cinnamoylformate derivatives.

Figure 13 Several chemical structures of porphyrins derivatives.

\subsection*{2.3. NIR photopolymerization}

{\label{nir-photopolymerization}}

Compared with Ultraviolet (UV) light, Near-Infrared (NIR) light
possesses a lower scattering coefficient which is beneficial for a
deeper penetration,\textsuperscript{{[}53{]}} and the release of heat is
useful for the photopolymerization.\textsuperscript{{[}54{]}} Cyanines
are good near infrared light absorbers, can be tailor made by changing
the length of methane chain and substitution pattern which can cover a
large absorption wavelength region between 700 nm and 1000
nm.\textsuperscript{{[}55{]}} Cyanines can react with iodonium salts to
generate free radicals and conjugate acid which can efficiently initiate
polymerization of acrylate monomers, epoxides and vinyl ether
monomers.\textsuperscript{{[}54, 56-57{]}} Oxime esters can interact
with cyanines to initiate photopolymerizaiton of radical
monomers.\textsuperscript{{[}58{]}} Alternatively, up-conversion
nanoparticles can absorb the NIR laser to emit visible light and UV
light which can be absorbed by traditional photoinitiators to trigger
photopolymerization.\textsuperscript{{[}59-60{]}} NIR
photopolymerization can be applied in digital imaging in Computer to
Plate (CtP),\textsuperscript{{[}61{]}} powder
coating,\textsuperscript{{[}62{]}} 3D
printing,\textsuperscript{{[}63{]}} etc.

\section*{3. Application of photopolymerization
technology}

{\label{application-of-photopolymerization-technology}}

As a widely-used technology, photopolymerization technology has been
used to prepare a variety of products, including coatings, inks,
adhesives, printing materials and other applications since Inmont
corporation obtained the first UV-curable ink in 1946. In addition to
these traditional applications mentioned above, today
photopolymerization has been given new applications in thiol-ene click
reaction, 3D printing, photoresist, hydrogels and other fields, which
will be described in detail below.

\subsection*{3.1. Thiol-ene click
reaction}

{\label{thiol-ene-click-reaction}}

The researches on the reaction between thiols and carbon-carbon double
bond has a history of one hundred years.\textsuperscript{{[}64{]}} In
1905, Posner et al. first carried out the researches on the chemical
reaction between thiols and enes,\textsuperscript{{[}65-66{]}} then,
Kharasch et al. proposed the reaction mechanism of thiols and enes
monomers in 1938,\textsuperscript{{[}67{]}} which is still widely
accepted today. The photopolymerization mechanism of thiols and enes is
demonstrated in Figure 14,\textsuperscript{{[}67-68{]}} first step, the
sulfur radicals are generated by the hydrogen abstraction reaction with
the free radicals which originate from the cleavage of PIs under
irradiation at light sources. The second step, sulfur radicals attack
the carbon-carbon double bond and produces alkyl radicals,
simultaneously, alkyl radicals can also react with thiols to form sulfur
radicals, thus continuously cycling. The third step, radicals combine
with each other to terminate the polymerization reaction.

Figure 14 Photopolymerization mechanism between thiols and enes.

The field of click chemistry has been rapid development since Sharpless
first proposed the new concept in 2001.\textsuperscript{{[}64,
69{]}}Reaction of click chemistry has many advantages including high
yield, short reaction time, solvent tolerance, a wide variety of
functional groups, regional and chemical selectivity, insensitivity to
oxygen, easy purification and atomic economy up to 100
\%.\textsuperscript{{[}64, 70-72{]}} The photopolymerization of thiols
and enes carry many of the attributes of click chemistry, and has been
applied in many fields such as hydrogels, dental prosthetics, optical
materials, bonding, coatings and so on.

In recent years, preparation of organosilicon polymers by UV
photopolymerization obtained more and more attention. However, the
polymerization reactions between organosilicon monomers modified with
acrylate are sensitive to oxygen, resulting in sticky surface and even
failure of curing process. On the other hand, the shrinkage rate of the
material after curing is very large, which may lead to the deformation
of material surface.\textsuperscript{{[}73{]}} Cationic curing is not
sensitive to oxygen,\textsuperscript{{[}74{]}} however, its photocuring
rate is lower than radical curing. Based on these limitations, Wang et
al.\textsuperscript{{[}68{]}} synthesized the novel sulfhydryl
functionalized fluorosilicone polymer (PTFPMS-SH) and a series of
fluorosilicone polymers (PTFPMS-Vi) with different vinyl contents, then
the curing coatings are obtained by thiol-ene click reaction, as
displayed in the Figure 15. The results show that the conversion of C=C
bond can reach more than 95 \% within 5 s, and the obtained
fluorosilicone coatings show a transparency greater than 90 \%. The
contact angle of the coatings ranges from 100\selectlanguage{ngerman}° to 110°, which showing
excellent hydrophobicity. In addition, the fluorosilicone coatings show
potential application as a protective coating for electronic devices due
to excellent corrosion resistance.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image15/image15}

\end{center}
\end{figure}

Figure 15 The schematic diagram of photocuring reaction of
fluorosilicone coatings.\textsuperscript{{[}68{]}}

About reactive diluents, there are some drawbacks of petroleum-based
reactive diluents such as non-renewable feedstock, environmental
pollution, skin irritation and poor adhesion in practical
application.\textsuperscript{{[}75-76{]}} Therefore, it is a great trend
in field of photopolymerization to develop more environmentally friendly
and healthy bio-based reactive diluents to substitute petroleum-based
one. Yang et al.\textsuperscript{{[}77{]}} synthesized a polyfunctional
and renewable reactive diluent tung oil-based methacrylate (TDMM) by
amidation, thiol-ene click reaction and esterification, TDMM and
acrylated epoxidized soybean oil (AESO) at different proportions to
formulate a ``green + green'' UV-LED curable systems with high
bio-content, the chemical structures of AESO and PIs, synthesis route of
TDMM are exhibited in Figure 16 and Figure 17, respectively. TDMM/AESO
systems show higher \emph{Tg} but lower thermal stability, better
modulus and tensile strength than neat AESO. In addition, the UV-LED
cured coatings exhibited excellent physical and chemical properties. In
a word, TDMM shows to be an effective reactive diluent, and considered
to have good potential in the UV-LED curable coatings.

Figure 16 Chemical structures of AESO and PIs.\textsuperscript{{[}77{]}}

Figure 17 Synthesis route of tung-oil-based acrylate
(TDMM).\textsuperscript{{[}77{]}}

As plant phenols from natural origin, cardanol and eugenol are
considered renewable raw material for alternatives to fossil
feedstock,\textsuperscript{{[}78{]}} and have attracted a lot of
attention because their structural versatility, such as C=C bonds,
hydroxyl groups, and benzene rings in their structure. Feng et
al.\textsuperscript{{[}70{]}} synthesized two allyl monomers (F-CA and
F-EU) by cardanol and eugenol underwent esterification reaction with
tetrafluoroterephthalic acid, next, a series of biobased polymer
networks were obtained via photoclick thiol--ene reaction and thermal
treatment between allyl monomers and thiols, the schematic diagram of
the preparation of the cardanol-based and eugenol-based thiol-ene
polymer networks is displayed in Figure 18. All the polymer networks
exhibited high thermal stability (up to 283 \selectlanguage{ngerman}°C), excellent solvent
resistance and gel content of over 91.36\%. Furthermore, the biobased
polymer networks displayed a good degradability in alkaline solution on
account of the presence of ester bonds and electron-deficient aromatic
ring system. The multifunctional biobased thiol--ene polymer networks
may have practical applications in many areas such as coating,
composites, adhesives, etc.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image18/image18}

\end{center}
\end{figure}

Figure 18 Schematic diagram of the preparation of the cardanol-based and
eugenol-based thiol-ene polymer networks.\textsuperscript{{[}70{]}}

Liu et al.\textsuperscript{{[}1{]}} synthesized a series of novel
coumarin-based oxime ester PIs which could effectively induce thiol-ene
click photopolymerization under irradiation at 450 nm LED source, the
chemical structures of the oxime ester (O-3, O-3F, O-3O and O-4),
pentaerythritol tetra (3-mercaptopropionate) (PETMP), 1,7-octadiyne
(ODY), triallyl isocyanurate (TAIC), triallyl cyanurate (TAC) and
pentaerythritol triallyl ether (APE) used in the photopolymerization
researches are displayed in Figure 19. The photopolymerization kinetics
results show that the conversions of thiols and C=C bonds can reach more
than 80\% initiated by O-3, and fast rate and high conversion of the
thiol (70\%) and alkynyl (80\%) indicate that O-3 can be used for the
efficient thiol--yne photopolymerization. Besides, O-3/thiol--ene
systems exhibit excellent photobleaching properties, which is favorable
for light penetration. Therefore, the novel coumarin-based oxime ester
O-3 can be used as visible light PI with enhanced performance in various
thiol-based click photopolymerization.

Figure 19 Structures of the oxime ester PIs, mercaptan and monomers used
in the photopolymerization.\textsuperscript{{[}1{]}}

He et al,\textsuperscript{{[}79{]}} designed a novel click lithography
strategy based on the rapid thiol-ene click reaction to realize
ultraefficient nanofabrication, the schematic illustration of thiol-ene
reaction and click lithography are displayed in Figure 20. First of all,
this work prepared a multialkene-functionalized zirconium
(Zr)-containing MOC with a molecular size of ~1.6 nm, and the material
exhibits ultrahigh alkene density and extremely small component size,
which is conducive to achieving high sensitivity and high resolution of
the thiol-ene reaction. Next, this novel method enables high-contrast
patterning of metal-containing nanoclusters at low deep-ultraviolet
exposure dose(e.g., 7.5 mJ cm\textsuperscript{--2}), which is 10--20
times lower than the dose used in the photoacid generator-based
photoresist system. At the same time, 45 nm dense patterns were obtained
at low dose using electron beam lithography, revealing the great
potential of this method for high-resolution patterning.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image20/image20}

\end{center}
\end{figure}

Figure 20 Schematic illustration of thiol-ene reaction and click
lithography. (a) Photoinitiated thiol-ene reaction. (b) Representation
of the click lithography approach. (c) Resist film composed of ZrO2-MAA
MOCs and thiol compounds. (d) Crosslinked and non-crosslinked structures
formed by radiation-induced thiol-ene click
reaction.\textsuperscript{{[}79{]}}

\textbf{3.2. 3D Printing}

Different from traditional manufacturing methods, the three-dimensional
(3D) printing techniques were introduced during the 1980s with the aim
to fabricate customized/complex objects without the need for molds or
machining, also known as additive manufacturing (AM)
technology.\textsuperscript{{[}80-82{]}} 3D printing method based on the
photopolymerization techniques has been a hot topic in recent years due
to many advantages such as highest accuracy, high production efficiency,
material utilization rate of nearly 100 \% and mild
facilities.\textsuperscript{{[}83-84{]}} Therefore, photopolymerization
3D printing have received revitalized attentions in a variety of 3D
printing such as stereolithography
(SLA),\textsuperscript{{[}85{]}}direct laser writing
(DLW),\textsuperscript{{[}86{]}} digital light processing
(DLP),\textsuperscript{{[}87{]}} ink jetting,\textsuperscript{{[}88{]}}
and real-time curing of DIW.\textsuperscript{{[}63, 89{]}}

Several commercial UV-light-sensitive PIS such as TPO, BAPO, Irgacure
184, and Irgacure 369 were used for the 3D printing processes of
photopolymerization under irradiation at UV
light.\textsuperscript{{[}35{]}} Whereas, as explained in section 2.2,
visible light source has obvious advantages over traditional UV light
source, hence the development for novel PIs or PISs for
photopolymerization 3D printing induced by visible light is essential.
Xiao et al.\textsuperscript{{[}35{]}} investigated a series of
disubstituted aminoanthraquinone derivatives which are capable of
initiating the radical photopolymerization under irradiation at blue to
red LEDs. The chemical structures of the studied disubstituted
aminoanthraquinone derivatives (AHAQ, 14-DAAQ, and 15-DAAQ) and
additives (Iod, NVK, TEAOH, and R-Br) are shown in Figure
21.15-DAAQ/TEAOH/R-Br PISs was used for the 3D printing due to the
highest capacity to initiate the radical polymerization of acrylates and
the 3D resin. It took less than 8 min to print the keyring when
using15-DAAQ/TEAOH/R-Br system under irradiation at polychromatic
visible light (400--730) nm with light intensity 3.6 mW
cm\textsuperscript{-2}, as a contrast, TPO took 35 min. Markedly, the
printing speed was significantly enhanced when using the
15-DAAQ/TEAOH/R-Br as the PISs, which indicates that 15-DAAQ/TEAOH/R-Br
system can be used as a promising PIS for fast 3D printing. The
experimental 3D printing product using 15-DAAQ/TEAOH/R-Br PISs is shown
in the Figure 22.

Figure 21 Chemical structures of the studied disubstituted
aminoanthraquinone derivatives and additives.\textsuperscript{{[}35{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image22/image22}

\end{center}
\end{figure}

Figure 22 (a) 3D model of keyring and (b) 3D printed keyring (64.0 mm \selectlanguage{ngerman}×
30.0 mm × 3.6 mm) using 15-DAAQ/TEAOH/R-Br (0.5\%/2\%/2\%, wt \%) as
PISs.\textsuperscript{{[}35{]}}

In addition to this, Xiao et al.\textsuperscript{{[}90{]}} also
investigated the application of indigo carmine (IDGCM) as a
photosensitizer with for 3D printing of photopolymerization. IDGCM
possesses excellent good water solubility, visible light sensitivity and
biocompatibility, hence it can give the corresponding properties of
photopolymerization and 3D printing. The chemical structures of IDGCM,
additive, and monomer/oligomers used in corresponding experiments are
shown in Figure 23. The indigo carmine-based photoinitiating system
induced a rapid photochemical reaction and photobleaching under blue and
green LEDs with low light intensity. The photopolymerization system
composed of IDGCM/Iod (0.5 \%/2 \%, wt) and PEGDA 700/deionized water
(80 \%/20 \%, wt) in the presence of 25\selectlanguage{english} mM NaOH can achieve rapid and
high-fidelity 3D printing, as shown in Figure 24.

Figure 23 Chemical structures of IDGCM, additive, and
monomer/oligomers.\textsuperscript{{[}90{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image24/image24}

\end{center}
\end{figure}

Figure 24. The top view of STL model file and the topography print cured
using the PEGDA 700/water (80 \%/20 \%, wt) in the presence of IDGCM/Iod
(0.5 \%/2 \%, wt) and 25 mM NaOH. (30 \selectlanguage{ngerman}× 30 × 15 mm, L × W ×
H).\textsuperscript{{[}90{]}}

In the process of photopolymerization 3D printing, the light transmitted
through the solidified polymer layers is highly likely to trigger
undesired lateral photopolymerization during the printing of subsequent
layers and therefore leads to deteriorated print
resolution.\textsuperscript{{[}91{]}} Nonreactive light absorbers can
improve print resolution by limiting light penetration, however, they
also reduce printing speed.\textsuperscript{{[}92{]}} It is a
challenging to develop a 3D printing technique with both high print
speed and resolution. Xie et al.\textsuperscript{{[}93{]}} designed an
efficient 3D printing method by using one ketocoumarin compound, i.e.,
3,3\selectlanguage{english}'-carbonylbis(7-diethylaminocoumarin) (KCD), as the photosensitizer
(also a reactive light absorber) because of its high photoinitiation
efficiency, low bleaching capacity and high molar extinction coefficient
of photolysis products (limiting light penetration and improving
resolution). The processes of generating radicals through photooxidation
or photoreduction are shown in Figure 25 a, it is difficult to limit the
light penetration due to the obvious blue shift (61nm) of the product
KCD ketyl radical in the photoreduction process, which resulting in low
resolution and the printed feature could not be identifiable (Figure 25
c). In comparison, the photopolymerization 3D printing based on the
photooxidation of KCD that functions as the photosensitizer can
simultaneously deliver high print speed (5.1\selectlanguage{english} cm h\selectlanguage{english}\textsuperscript{-1})
and high print resolution (23 \selectlanguage{greek}\emph{μ} \selectlanguage{english}m). The method provides a viable
solution towards efficient AM by controlling the photoreaction of
photosensitizers during photopolymerization.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image25/image25}

\end{center}
\end{figure}

Figure 25 (a) Schematic illustration of the photooxidation of KCD/TA and
photoreduction of KCD/NPG upon visible light irradiation, respectively.
(b) Schematic illustration of the bottom-up DLP 3D printing. (c)
3D-printed objects photomediated by KCD/TA and KCD/NPG,
respectively.\textsuperscript{{[}93{]}}

At present, UV and blue lights are the main irradiation sources in
photopolymerization 3D printing.\textsuperscript{{[}94{]}} Despite the
convenience offered by UV and blue lights, intensity of short wavelength
lights is a trade-off for better mechano-properties by uniformed
polymerization and health concern of UV
exposure.\textsuperscript{{[}63{]}} Among the photopolymerization
methodologies, NIR has a more salient role in rapid deep-curing for its
remarkable penetration in various media by employing up-conversion
strategies.\textsuperscript{{[}60, 95-96{]}} Liu et
al.\textsuperscript{{[}63{]}} reported a NIR photopolymerization 3D
printing strategy, and the fusion of NIR photocurable material and DIW
3D printing technology could achieve in-situ curing of thick filament
with high penetration. The structure of NIR-induced DIW setup, chemical
structures and photoreaction mechanism used in NIR DIW printing, and
monitoring of NIR photopolymerization are exhibited in Figure 26. This
enables the photocuring of deposited filaments up to 4 mm in diameter,
which far exceeding any existing UV-assisted DIW, in addition, the
strategy also possesses the capability of parallel fabrication to
multi-color filaments and freestanding objects simultaneously.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image26/image26}

\end{center}
\end{figure}

Figure 26 Schematic of NIR-DIW and real-time FTIR photorheology
analysis. a Scheme of NIR-induced DIW setup; b structures and reactions
applied in NIR DIW printing; c monitoring of NIR-induced
photopolymerization by real-time FTIR rheological
analysis.\textsuperscript{{[}63{]}}

\textbf{3.3. Photoresist}

As the key of photolithography, photoresist refers to thin film
materials whose solubility and adhesion change significantly after
exposure by light sources, then the micro-patterns can be obtained
through development, etching and other
processes.\textsuperscript{{[}97{]}} It is widely employed in the fields
of printed circuit boards (PCB),\textsuperscript{{[}98-100{]}} display
device,\textsuperscript{{[}101-103{]}} integrated
circuit,\textsuperscript{{[}104{]}} and other fine graphics processing.
Photoresist can be divided into positive photoresist and negative
photoresist on the basis of the area retained after development, in
which negative photoresist is photocurable materials. The schematic
diagram of photolithography is shown in Figure
27.\textsuperscript{{[}97{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image27/image27}

\end{center}
\end{figure}

Figure 27 Schematic diagram of photolithography.

Negative photoresist can be divided into photoinitiated radical
polymerization (mainly acrylic ester system) and photoinitiated cationic
polymerization (mainly epoxy system). The typical negative photoresist
consists of PISs, acrylate monomer, resin, additive and solvent, it is
mainly used in the field of PCB and display devices.

\subsubsection*{3.3.1. PCB dry film
photoresist}

{\label{pcb-dry-film-photoresist}}

With the rapid development of numerous industries of electronic
information such as automotive electronics, artificial intelligence,
commercialization of 5G and wearable devices, the Printed Circuit Board
(PCB) industry has witnessed a fast growth in recent years. The
photoresist used in the PCB industry mainly includes liquid photoresist
and dry film photoresist (DFRs). Due to its unique advantages of uniform
thickness, controllable, stability, high reliability, easy operation,
low energy of exposure and high resolution, DFRs has developed rapidly,
and has been widely utilized in PCB manufacturing
process.\textsuperscript{{[}105-108{]}} As displayed in Figure 28, the
polyester (PET) film, photoresist layer and polyethylene (PE) film from
bottom to top together constitute a complete
DFRs.\textsuperscript{{[}109{]}} The PET film plays the role of bearing
photoresist layer, and PE film is used to avoid the deterioration caused
by the contact between photoresist layer and dusts or oxygen, in
addition, prevents adhesion between multilayer film.

The photoresist layer of DFRs generally composed of resins, monomers,
oligomers, photoinitiators, dyes and other
additives.\textsuperscript{{[}109-111{]}} Resin possesses the largest
mass ratio in all components of DFRs, and directly determines the
resolution, adhesiveness, developing time and flexibility, etc. the most
commonly-used PIs for DFRs are hexarylbiimidazole (HABIs)
compounds,\textsuperscript{{[}43{]}} among which
2,2-bis(2-chlorophenyl)-4,4,5,5-tetraphenyl-1,2-biimidazole (BCIM) has
been broadly employed due to its merits of good efficiency and
discoloration property. However, it always cooperates with
photosensitizers because the light absorption capacity of BCIM in
near-UV and visible light region is weak.\textsuperscript{{[}112{]}} The
structures and photoinitiation mechanism of BCIM/N-phenylglycine (NPG)
systems are shown in Figure 29, firstly of all, BCIM will generate
imidazole radicals at exposure to light source, which reacts further
with NPG to generate carbon-centered radical that can initiate
polymerization reaction rapidly.\textsuperscript{{[}112{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image28/image28}

\end{center}
\end{figure}

Figure 28 Structure composition of PCB dry film photoresist.

Figure 29 Structures and photoinitiation mechanism of BCIM/NPG systems.

EMK and PSS303 are used as photosensitizer in DFRs formula for many
years, however, it is essential to develop more efficient
photosensitizers due to the requirements of higher sensitivity and
resolution in DFRs. Jiang, et al.\textsuperscript{{[}113-114{]}}reported
several novel species of photosensitizers that can be utilized for DFRs,
respectively distyrylpyridine derivatives (DSPs) and anthracenes (ANs)
derivatives, and their structures are exhibited in Figure 30 and Figure
31. Firstly, among all DSPs, the 24DMOP-DSP has the highest
photosensitivity, which was reached 14 mJ.cm-2 and increased by 150\%
compared to contrastive commercial coumarin-based DAMC with 35 mJ.cm-2.
The singlet state of DSPs is easily quenched by BCIM, which proves that
it is easy to sensitize BCIM then initiate polymerization reaction.
Next, for ANs, they possess strong light absorption from 350 nm to 450
nm with high molar extinction coefficients, which can be matched with
multifarious light sources. BCIM can be photosensitized by ANs to
initiate the polymerization reaction of Hexamethylene diacrylate (HDDA)
effectively. The DFRs formula containing ANs/BCIM can obtain fine
high-resolution micro-patterns by 405 nm laser direct-writing
photolithography.

Figure 30 Chemical structures and synthesis methods of
DSPs.\textsuperscript{{[}113{]}}

Figure 31 Chemical structures of ANs.\textsuperscript{{[}114{]}}

Ho, et al.\textsuperscript{{[}112{]}} synthesizes a series of
p-substituted NPG derivatives OMe-NPG, Cl-NPG and NO2-NPG as shown in
Figure 32. The OMe-NPG, Cl-NPG and NO2-NPG have good thermal stability
and red-shifted absorption compared with NPG, the photopolymerization
performance of trimethylolpropane trimethacrylate (TMPTMA) initiated by
BCIM/NPGs systems was tested, the measured conversions sequence is
BCIM/NPG (51.7 \%) \textgreater{} BCIM/OMe-NPG (41.8 \%) \textgreater{}
BCIM/Cl-NPG (35.5 \%) \textgreater{} BCIM/NO2-NPG (0 \%), which
concluded that introduction of substituents may weaken the performance
of initiating for NPG.

Figure 32 Structures of N-phenyl glycine (NPG) and NPG derivatives
(OMeNPG, Cl-NPG and NO2-NPG).\textsuperscript{{[}112{]}}

As we know, radicals photopolymerization system possess some defects in
practical productions such as apparent volume shrinkage, poor precision
and low adhesion.\textsuperscript{{[}115{]}} Contrastively, the cationic
photopolymerization system has lower shrinkage, stronger adhesion and
hardness, making them particularly suitable for laser photolithography
technology which requires high precision and strong
adhesion.\textsuperscript{{[}116-118{]}} Based on this, as shown in
Figure 33, our group designs and synthesizes a series of cationic
photocurable resins which consists of methyl methacrylate (MMA),
methacrylic acid (MAA), ethyl methacrylate (EMA) and
3-ethyl-3-(methacryloyloxy) methyloxetane (EMO) by radical
copolymerization.\textsuperscript{{[}109{]}} Next, we used these resins
to prepare DFRs formulations with radical/cationic dual-curing. The DFR
based on the resin which containing 10 wt \% of EMO has the highest
photosensitivity (11 mJ[?]cm-2, ST = 20/41) and 20 \selectlanguage{greek}μ\selectlanguage{english}m (ST = 20/41) can be
obtained by using this resin. In conclusion, the DFRs that contain
cationic photocurable resin may have potential application in PCB field
for creating high resolution patterns.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image33/image33}

\end{center}
\end{figure}

Figure 33 A proposed photolithography based on radical and cationic
dual-curing photoresists.\textsuperscript{{[}109{]}}

The most commonly-used photoresist of cationic photopolymerization is
SU-8, which is a UV photoresist product specially designed for
Micro-Electro-Mechanical System applications with high aspect
ratio.\textsuperscript{{[}119-120{]}} The main component of SU-8 is a
multi-function group and multi-branch epoxy resin, which is synthesized
by the condensation reaction of bisphenol and glycerol ether, ideal
structural formula of SU-8 is shown in Figure
34.\textsuperscript{{[}121{]}} The PIs used in SU-8 series photoresist
is generally an onium salt, mainly sulfonium salt, which can produce a
strong acid under irradiation to polymerize the epoxy
groups.\textsuperscript{{[}122{]}}

Figure 34 Ideal structural formula of SU-8.

\subsubsection*{3.3.2. UV nanoimprint lithography
photoresist}

{\label{uv-nanoimprint-lithography-photoresist}}

As an emerging high-resolution patterning method, nanoimprint
lithography (NIL) provides a technology that could replicate the
nanoscale patterns below 10 nm, and is a promising solution to the
restriction of exposure wavelength in
photolithography.\textsuperscript{{[}123{]}} NIL has overcome tremendous
challenges over the past 20 years to become a realistic method for
commercial semiconductor
production.\textsuperscript{{[}124{]}}UV-nanoimprinting lithography
(UV-NIL) is regarded as the new next-generation lithography technique
due to the advantages of high throughput, good resolution, and low
manufacturing cost, room-temperature operation,\textsuperscript{{[}123,
125{]}} and has been receiving increasing attention in many fields such
as electronic, photonic, LED, magnetic and semiconductor
devices.\textsuperscript{{[}125-127{]}}

The cross-linker plays a very important role in the component of
traditional UV-NIL photoresist, which determines the mechanical and
chemical resistance to a certain extent. However, the high cross-linked
structure is difficult to strip from the mold of UV-NIL, conversely, the
linear structure displays better solubleness in
solvent.\textsuperscript{{[}128{]}} Yin et
al.\textsuperscript{{[}129{]}} designed a UV-NIL resist in which the
structure contains photoreversible coumarin derivative as degradable
cross-linker. The chemical structures of degradable cross-linker
(AHAMC), monomer phenoxy ethyleneglycol acrylate (AMP-10G), and the
mechanism of photodimerization and photocleavage of the
dimer-coumarin-bridged polymer are exhibited in Figure 35. Through
photodimerization of coumarin moieties under irradiation at 365 nm UV
light, and photocleaved by 254 nm UV light, the crosslinking and
uncrosslinking can be implemented to protect the mold.

Based on the photoreversible coumarin derivatives, Wei et
al.\textsuperscript{{[}130{]}} reported a pH-UV dual-responsive
photoresist for UV-NIL that improves mold release, the chemical
structures of the photoreversible cross-linker
5,7-diacryloyloxy-4-methylcoumarin (DAMC), acrylic anhydride (ALA),
3,6-dioxa-1,8-dithiooctane (EGDT) and the PI DMPA for the
dual-responsive resist are displayed in Figure 36. The mechanism of
degradation for dual-responsive resist is displayed in Figure 37, the
cross-linked networks can be photocleaved by 254 nm UV light and
degraded in alkaline aqueous solution, which contributes to protect
UV-NIL mold and reduce damages in process of imprinting patterns.

Figure 35 Mechanism of photodimerization and photocleavage of the
dimer-coumarin-bridged polymer.\textsuperscript{{[}129{]}}

Figure 36 Chemical structures of each component for the dual-responsive
resist.\textsuperscript{{[}130{]}}

Figure 37 Mechanism of degradation for dual-responsive
resist.\textsuperscript{{[}130{]}}

Volume shrinkage is inevitable due to the van der Waals distance before
polymerization becomes the covalent distance after polymerization,
consequently, volume shrinkage has a negative impact on the patterns
transfer in UV-NIL.\textsuperscript{{[}131{]}} It is necessary and
meaningful to reduce volume shrinkage in UV-NIL field. Based on the
disulfide bond of reducing volume shrinkage and endowing materials with
the degradability. Sun et al.\textsuperscript{{[}125, 132{]}}synthesized
two disulfide bond-containing acrylate monomers 2,2-dithiodiethanol
diacrylate (DTDA) and disulfanediyl bis (1,4-phenylene) diacrylate
(ADSDA) used for UV-NIL. The chemical structures of DTDA and ADSDA are
exhibited in Figure 38. For the photoresist containing DTDA, it
underwent a repeated ``contraction--expansion--contraction''
volume-modulatory process during the polymerization because of the
dynamic reversible property of homolysis and recombination of disulfide
bonds, which was conducive to releasing stress and reducing volumetric
shrinkage. Such as the DTDA/MMA/IOBA system, the minimum rate of volume
shrinkage can drop to 0.93 \%. The mechanism of reducing volume
shrinkage is displayed in Figure 38.

Figure 38 Chemical structures of DTDA and ADSDA.\textsuperscript{{[}125,
132{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image39/image39}

\end{center}
\end{figure}

Figure 39 Mechanism of reducing volume shrinkage based on the reversible
disulfide-bond.\textsuperscript{{[}125{]}}

For the photoresist system containing ADSDA, it does not contain any
additional PI due to the arylthiyl radical generated by ADSDA possesses
a good capability of initiating photopolymerization. The photoresists
with ADSDA exhibited a capability of photopolymerization in the absence
of PI, and their double bond conversion can reach up to 86 \%, at the
same time, the volume shrinkage of the polymerized photoresists with
ADSDA could even drop to 0.56 \% owing to the
``breakage--recombination'' reversible reaction of disulfide bonds.

Oxygen inhibition of acrylate-based resists limits the potential of
UV-NIL for high throughput, however, the photopolymerization system of
thiol--ene leads to extremely little oxygen inhibition, simultaneously,
it also has the properties of high monomer conversions, low shrinkage,
homogeneous mechanical properties. Besides, the etch resistance and
thermal stability of cured resist are important because these properties
allow good pattern transfer. The inorganic polyhedral oligomeric
silsesquioxane (POSS) combines many desirable properties as an NIL
photoresist such as optical clarity, high thermal stability, high
density, high Young's modulus, and a low dielectric constant, meanwhile,
it does not affect the resolution of the NIL
pattern.\textsuperscript{{[}133-135{]}} Based on this, Yin et
al.\textsuperscript{{[}136{]}} developed a novel POSS containing
mercaptopropyl (POSS-SH), the structures of each component of the hybrid
UV-NIL resist are displayed in Figure 40. POSS-SH can enhance the
thermal properties, dimensional stability and etching resistance of
UV-NIL resist, and reduce the volume shrinkage. The hybrid resist can
obtain the high aspect ratio patterns as showed in Figure 41, which is
beneficial for graphics transfer onto Si.

Figure 40 Chemical structures of each component of the hybrid UV-NIL
resist.\textsuperscript{{[}136{]}}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image41/image41}

\end{center}
\end{figure}

Figure 41 The perspective AFM and sectional-profile images of hybrid
resist (a) imprinted, (b) after direct etching without transfer film and
(c) after bilayer etching with PMMA as transfer
film.\textsuperscript{{[}136{]}}

\subsection*{3.4. Hydrogel}

{\label{hydrogel}}

Hydrogel, as a class of hydrophilic polymer with three-dimensional
network, possesses the properties of absorbing and retaining a large
amount of water due to many hydrophilic functional groups such as
hydroxyl (-OH), carboxyl (-COOH), amino (-NH2), sulfonic acid (-SO3H)
and so on, at the same time, maintaining their three-dimensional network
structure.\textsuperscript{{[}116, 137-141{]}} Hydrogel materials are
widely utilized in biomedical engineering and pharmaceutical industry
due to biocompatibility, biodegradability, easy to synthesize and other
merits.\textsuperscript{{[}140-143{]}}

At present, the formation methods of hydrogel mainly include physical
and chemical crosslinking. The chemical crosslinking by means of forming
chemical bond among monomer or oligomer is more stable than physical
crosslinking through weak interactions such as hydrogen
bond.\textsuperscript{{[}144{]}} Chemical crosslinking includes Michael
addition, Schiff base, enzymatic reaction, click chemistry, and
photopolymerization.\textsuperscript{{[}144{]}} Among them,
photopolymerization is probably the most effective and commonly
crosslinking route. To form hydrogel by photopolymerization has many
merits, firstly, the photopolymerization process is extremely rapid
which could be completed in a few minutes or even seconds, secondly, the
photopolymerization allows spatial and temporal control over the
cross-linking process, this feature is particularly exploited by
stereolithography and 3D-(bio)-printing, thirdly, there is no necessity
for high temperature or extreme pH value because the photopolymerization
process only requires lower energy and
temperature.\textsuperscript{{[}144-145{]}} The mentioned-above
technology has been specifically applied in the biological field.

For the last few years, as a 3D-(bio)-printing technology, Digital Light
Processing (DLP) getting more and more attention because it can create
more complex structure of tissue and
organ.\textsuperscript{{[}146-148{]}} Hong et
al.\textsuperscript{{[}149{]}} used the silk fibroin (SF) which modified
by glycidyl methacrylate (GMA) to construct the chondrocyte-laden
hydrogel scaffold using DLP 3D-printing technology by UV light source at
365 nm. The schematic diagram for modification of SF with GMA and
bioprinting of chondrocyte with Silk-GMA DLP are illustrated in Figure
42. The Silk-GMA shows strong effectiveness for chondrogenesis in vitro
and in vivo transplantation, meanwhile, suggests the good
biocompatibilities and mechanical advantages for defected tissue
regeneration.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image42/image42}

\end{center}
\end{figure}

Figure 42 Schematic diagram for modification of SF with GMA (Silk-GMA)
and bioprinting of chondrocyte with Silk-GMA DLP 3D-printing
technology.\textsuperscript{{[}149{]}}

Gelatin methacryloyl (GelMA) has been utilized as precursor for
3D-printed hydrogels because its biological activity, fast
photocrosslinking ability and temperature adjustable
viscosity,\textsuperscript{{[}150{]}} whereas the poor mechanical
strength, low stability and non-self-healing behavior have hampered its
practical application in field of biomaterials. To deal with this
limitation, Mao et al.\textsuperscript{{[}151{]}} synthesized a novel
host-guest supramolecular compound HGSM which based on the inclusion
interaction between cyclodextrin (CD) and adamantane (Ad), as exhibited
in Figure 43. The HGSM that has three ``arms'' of acrylates and GelMA
can photopolymerize to construct the hydrogel HGGelMA which possesses
the excellent mechanical and rapid self-healing properties due to the
non-covalent bonds. Compared with pure GelMA hydrogel, the compression
modulus of HGGelMA hydrogel increased by 525\% which reached the level
of most human soft tissues. In addition, the HGGelMA hydrogel displayed
an excellent printability, The scaffold of 3D-printed HGGelMA hydrogel
has the exquisite and homogeneous porous structures, good
biocompatibility and histocompatibility with potential biomedical
applications.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image43/image43}

\end{center}
\end{figure}

Figure 43 Schematic of the HGSM synthesis (left) and HGGelMA
construction (right).\textsuperscript{{[}151{]}}

The water-soluble ultraviolet PI Irgacure 2959 has been widely used in
cell encapsulation and tissue
engineering,\textsuperscript{{[}152-154{]}} however, there are
significant limitations to its application. Firstly, UV light has been
known to be harmful for delivered cells and host tissues, secondly, its
solubility in water is relatively poor.\textsuperscript{{[}152,
155-156{]}} Thus, the limitations of UV sources can be avoided by
employing PI which could match to visible light. Lithium acylphosphinate
salt (LAP) as a visible light PI possesses fine water solubility and
allows cell encapsulation at lower PI concentrations, enabling efficient
photopolymerization compared to Irgacure 2959. Monteiro et
al.\textsuperscript{{[}157{]}} prepared the gelatin methacryloyl (GelMA)
hydrogels encapsulating odontoblast-like cells by using dental curing
devices (DL) with emission wavelength in visible light range, as shown
in Figure 44. The results show that short-time exposure to DL did not
significantly affect cell survival, simultaneously, preparation of GelMA
hydrogels by DL source is faster than UV source, and showed higher
cellular activity with larger pore sizes.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image44/image44}

\end{center}
\end{figure}

Figure 44 Synthesis of GelMA macromolecular monomers (A), cell
encapsulation (B), process of photopolymerization (C) and cell-laden
hydrogel materials (D).\textsuperscript{{[}157{]}}

In view of these advantages including high efficiency, safety,
environmentally friendly and energy saving for visible light source,
Shao et al.\textsuperscript{{[}158{]}} investigated the
photopolymerization kinetics of the PIS which is composed of
camphorquinone (CQ) and diphenyl iodonium hexafluorophosphate (DPI) that
have been widely used as a blue light PIS. Then a hydrogel was
synthesized by N, N-dimethylacrylamide (DMAA) and sodium acrylate (SA)
according to the kinetic results. The CQ/DPI system resulted in fast
polymerization and high double bond conversion (approximately 90 \%)
when maximum polymerization rate was approximately 6.53 \selectlanguage{ngerman}×
10\textsuperscript{--5} s\textsuperscript{--1} with light intensity of
32.3 mW/cm\textsuperscript{2}, 0.5 wt \% CQ, and mass ratio of monomers
was 4 (DMAA):6 (SA). The hydrogel synthesized by poly (DMAA/SA) under
irradiation blue light displayed excellent water absorbability (about
750 g/g), robust mechanical properties (about 2500\% strain and 0.6 MPa
stress) and fine dimensional stability. The kinetic mechanisms proposed
in this research can provide some reference for the synthesis of
hydrogels in biomedical field.

Naphthalimide derivatives have been widely employed as PI in the field
of visible light photopolymerization due to their excellent light
absorption properties in visible light region. Lalevée et
al.\textsuperscript{{[}39, 159{]}} synthesized a series of naphthalimide
derivatives PIs that can be matched with LED sources with emission
wavelengths of 385 nm, 405 nm, 455 nm and 470 nm, chemical structures of
them are shown in Figure 45. The naphthalimide derivatives/additives
systems can effectively initiate cationic radical polymerization, among
them, NDP2 and 2,6-di-O-Me-\selectlanguage{greek}β-\selectlanguage{english}cyclodextrin (DM-\selectlanguage{greek}β-\selectlanguage{english}CD) can form
supramolecular complex NPD2-CD together, which exhibited excellent
solubility in water and good capacity of initiating polymerization of
hydroxyethyl acrylate (HEA) to obtain hydrogel at exposure to 405 nm LED
source, as shown in Figure 46.

Figure 45 Chemical structures of naphthalimide derivatives.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image46/image46}

\end{center}
\end{figure}

Figure 46 Preparation of poly (hydroxyethyl acrylate) in water by using
NPD2-CD/MDEA system upon LED at 405nm irradiation.

The supramolecular complex DNND4/SBE-\selectlanguage{greek}β-\selectlanguage{english}CD which was composed of DNND4
and sulfobutylether-\selectlanguage{greek}β-\selectlanguage{english}cyclodextrin can also initiate polymerization
reaction of HEA or hydroxyethyl methacrylate (HEMA) to obtain hydrogel
under irradiation at 405 nm LED source.

The development of macromolecular water-soluble PIs with low-migration
is extremely significant for achieving biosafety of photopolymerization.
In terms of the preparation of hydrogels, Zhao et
al.\textsuperscript{{[}160{]}} designed and synthesized a new
lignin-based water-soluble macromolecular PI L-PEG-2959 based on the
Irgacure 2959, in addition, the function of PEG chain in L-PEG-2959 is
to improve the water solubility of lignin. Transparent hydrogel can be
prepared by using glycidyl methacrylate modified gelatin as monomer and
L-PEG-2959 as PI. The synthesis of L-PEG-2959 and the preparation method
of the hydrogel are displayed in Figure 47. Compared to the hydrogel
containing Irgacure 2959, the extraction amount of L-PEG-2959 from the
hydrogels was tremendously reduced, which indicates L-PEG-2959 is a
promising macromolecular PI in fabricating biosafety hydrogel.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image47/image47}

\end{center}
\end{figure}

Figure 47 Fabrication process of lignin-gelatin hybrid hydrogel.

\subsection*{3.5. Applications of in photopolymerization other
fields}

{\label{applications-of-in-photopolymerization-other-fields}}

In addition to the above fields, the application of photopolymerization
is gradually expanding to other fields, such as frontal polymerization,
photochromic materials, self-healing materials and so on.

Frontal polymerization is a self-propagating reaction where monomers are
crosslinked into polymers by the propagation from localized reaction
zone to entire system.\textsuperscript{{[}161-162{]}} Generally
speaking, no extra energy is required except in initial
stage.\textsuperscript{{[}162{]}}According to the initiation sources,
frontal polymerization can be divided into the thermal-induced frontal
polymerization (TFP) and photo-induced frontal polymerization
(PFP).\textsuperscript{{[}163{]}}PFP is initiated by the irradiation
usually UV light,\textsuperscript{{[}164-166{]}} however, as mentioned
above, UV light has many disadvantages and limitations in practical
application, it is a new strategy to use more secure NIR light as
irradiation source of frontal polymerization. Based on this, our group
employed a two-component system including cyanine (NIR sensitizer) and
iodonium salt to initiate the frontal polymerization of cationic
polymerizable monomers upon 808 nm NIR laser
exposure.\textsuperscript{{[}57{]}} The heat released by NIR sensitizer
and the polymerization reaction can dissociate AIBN or TPED to achieve
frontal polymerization of vinyl ethers and epoxides. The frontal
velocity and starting time can be modulate by changing the different
kinds of monomers, thermal initiators and their concentration.

Solid-state photochromic materials especially photochromic polymers have
potential application prospect in the fields of photoswitching devices,
chemosensors, solid-state optical devices, imaging and 3D optical-data
storage devices, and so on owing to their advantages of lightweight and
good processability.\textsuperscript{{[}167-171{]}} At present, the main
preparation method of photochromic polymers is thermopolymerization,
which is complicated and energy consuming. Therefore, it is extremely
meaningful to develop a simple, efficient and environmentally friendly
preparation method of photochromic polymers.

Du et al.\textsuperscript{{[}172{]}} designed and synthesized two novel
UV-LED-induced fluorophenyl oxime ester PIs (E-FBOXEs) with the ability
to prepare photochromic polymers, and then photochromic polymers based
on E-FBOXEs were prepared by fast photopolymerization. Proposed
mechanism for the photochromism of the film initiated by E-FBOXEs is
exhibited in Figure 48, Residual E-FBOXEs in photopolymerized films will
generate colored iminyl radicals in the planar conformation under
irradiation at 395 nm, and the recombination of the radicals to form
colorless E-FBOXEs in the nonplanar conformation under heating, which
results in the photochromism of the polymeric films.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image48/image48}

\end{center}
\end{figure}

Figure 48 Proposed Photochromic Mechanism of E-FBOXEs.

The fast development of self-healing materials based on supramolecular
interactions has attracted considerable attention in recent
years,\textsuperscript{{[}173-179{]}} among the supramolecular
interactions, ionic interactions are very interesting because of their
high aggregation strength that results in low healing agent content for
self-healing materials.\textsuperscript{{[}180{]}} However, most of
self-healing materials based on ionic interactions are all thermocured
materials.

Based on the above background, Sun et
al.\textsuperscript{{[}181{]}}designed and synthesized a novel
imidazolium-containing photocurable monomer,
6-(3-(3(2-hydroxyethyl)-1H-imidazol-3-ium bromide) propanoyloxy) hexyl
acrylate (IM-A), based on 1,6-hexanediol diacrylate (HDDA) and
1H-imidazole and 2-bromoethanol, the synthesis route of IM-A is
displayed in Figure 49. Then, the self-healing flexible materials was
prepared by fast photocuring with IM-A, isobornyl acrylate (IBOA),
2-(2-ethoxyethoxy) ethyl acrylate (EOEOEA), and 2-hydroxyethyl acrylate
(HEA). Herein, IM-A-containing imidazolium plays a self-healing role in
the materials. The self-healing process of the photocured polymer
contained IM-A is exhibited in Figure 50. The as-prepared self-healing
polymer IB7-IM5 exhibited a tensile strength of 3.1 MPa, elongation at
break of 205\%, healing efficiency of 93\%, and a wide healing
temperature range from room temperature to 120 \selectlanguage{ngerman}°C. The self-healing
polymer was also employed as a flexible substrate to fabricate a
flexible electronic device, which could be healed and completely restore
its conductivity after the device was damaged.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image49/image49}

\end{center}
\end{figure}

Figure 49 Synthesis route of IM-A.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image50/image50}

\end{center}
\end{figure}

Figure 50 Illustration of the self-healing process of the photocured
polymer.

Graphene quantum dots (GQDs) have important applications in photovoltaic
devices, especially as emission layers in light-emitting diodes (leds)
due to some merits such as outstanding photoluminescence properties, low
toxicity, good biocompatibility, good electron transport capacity, and
excellent thermal conductivity.\textsuperscript{{[}182-186{]}} However,
GQDs usually suffer from serious fluorescence quenching in aggregates
and the solid state due to easy agglomeration and aggregation-induced
quenching, which seriously restrict their practical
applications.\textsuperscript{{[}187{]}} For this reason, our group
report a work which constructed the blue-emitting reduced graphene oxide
quantum dot (rGOQD)-based light emitting diodes (LEDs) with efficient
solid state emission by UV photolithography,\textsuperscript{{[}188{]}}
as shown in Figure 51. In this work, preparing rGOQDs by the in situ
photoreduction of graphene oxide quantum dot (GOQDs), preparation of the
rGOQD/photoresist patterns, and the in situ photoreduction of GO in the
aforementioned photoresist to rGO were achieved simultaneously. The PL
spectrum for rGOQDs prepared by UV photolithography is observed to be
blueshifted with a narrow FWHM compared to GOQDs, which leads to the
fabrication of monochromatic blue-emitting devices.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image51/image51}

\end{center}
\end{figure}

Figure 51 The preparation process of blue-emitting reduced graphene
oxide quantum dot (rGOQD)-based light emitting diodes
(LEDs).\textsuperscript{{[}188{]}}

Oxygen-containing functional groups of graphene oxide (GO), which
destroy the path of electron movement, result in the conductivity of GO
being far lower than that of graphene, which seriously restricts its
application. Our group designed a kind of conductive patterned film with
high resolution comprised of reduced graphene oxide (RGO) which prepared
by the ultraviolet photolithography
technique.\textsuperscript{{[}189{]}} This work developed a strategy of
introducing GO into the photocuring formula and used UV photolithography
technology to create conductive patterned polymer films in one step with
high efficiency. BAPO not only acts as a PI to promote the
photopolymerization of films under UV irradiation but also serves as a
reductant to reduce GO. The mechanism for reducing GO in the composites
by a simple and efficient photoreduction method is shown in Figure 52.
Consequently, the maximum conductivity of photoresist/RGO films is 9.90
S cm\textsuperscript{-1} when the GO content is 10.7 wt\%, BAPO content
is 3 times that of GO and the exposure time is 15 min. Furthermore,
conductive micropatterns can be obtained on diverse substrates such as
PET films, silicon wafers and glasses substrates by photolithography
techniques with a high conductivity of 0.98 S cm\textsuperscript{-1},
and the micropatterns demonstrate excellent flexibility and stability.
This study suggests an approach to fabricate conductive polymer films
and micropatterns with promising potential in the field of flexible
electronic devices.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image52/image52}

\end{center}
\end{figure}

Figure 52 Schematic illustration of the formation of photoresist/RGO
composites.\textsuperscript{{[}189{]}}

\section*{Summary and perspective}

{\label{summary-and-perspective}}

Due to the advantages of 5E (efficiency, enabling, economical, energy
saving and environmental-friendly), photopolymerization technology has
been applied in many fields, such as coatings, inks, adhesives, printing
materials, microelectronic products, 3D printing, biological materials,
etc., and plays an increasingly important role in our daily life. With
the continuous development of technology and enhancement of
environmental protection consciousness, photopolymerization will attract
new development opportunities, simultaneously, it also faces new
requirements. Firstly, it is a trend that LED photopolymerization
replaces traditional UV photopolymerization. A large number of
scientists are committed to the development of LED photopolymerization.
So there are a lot of research results on LED photopolymerization in
recent years. However, the color problem stemmed from the PIs also
limits the application of LED photopolymerization in colorless and deep
layer polymerization systems. In addition, the cost of producing LED
source is relatively expensive, and further improvement of the
manufacturing technology is needed. Secondly, the greenization of
production process for photopolymerization raw material is also worth
noting even though it as a green technology has been deeply rooted in
people's concepts. Furthermore, the toxicity of PIs has attracted much
attention, and many countries have introduced regulations or licensing
lists to restrict the use of it. Therefore, it is very important to
design PIs with low toxicity and high migration stability, or based on
natural materials. Thirdly, radical photopolymerization will encounter
some limitations in practical application due to its obviously
shortcomings such as serious volume shrinkage and oxygen inhibition. For
example, 3D printing based on radical photopolymerization system leads
to the decrease of dimensional accuracy of products, while cationic
photopolymerization can overcome the problem of volume shrinkage due to
epoxy ring-opening. At present, the research of cationic
photopolymerization needs further development to meet the application
requirements. Last but not least, the development of new applications
for photopolymerization technology is extremely necessary thing for its
continuous progress, as well as the key factor that keeps it from being
eliminated. For the moment, 3D printing, electronic information
industry, biomaterials field, aviation field, etc. are the focus and
research hotspots of the whole world scientific community. Researchers
on photopolymerization technology should develop new applications to
make photopolymerization continue to prosper.

\textbf{AUTHOR INFORMATION}

Corresponding Author

*E-mail: xinyangyang@mail.tsinghua.edu.cn (Yangyang Xin)

*E-mail: zouyq@bnu.edu.cn (Yingquan Zou)

ORCID

Xianglong He: 0000-0002-8352-463X

\textbf{CONFLICTS OF INTEREST}

The authors declare no competing financial interest.

\textbf{ACKNOWLEDGMENT}

The financial support from Hubei Gurun Technology Co., Ltd
(10200-230210362) and Baoding Lucky Innovative Materials Co., Ltd
(10200-230200095) is gratefully acknowledged.

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