Photosensitive naturally derived resins toward optical 3-D printing

Abstract

Recent advances in material engineering have shown that renewable raw materials, such as plant oils or glycerol, can be applied for synthesis of polymers due to ready availability, inherent biodegradability, limited toxicity, and existence of modifiable functional groups and eventually resulting to a potentially lower cost. After additional chemical modifications (epoxidation, acrylation, double bonds metathesis, etc.), they can be applied in such high-tech areas as stereolithography, which allows fabrication of three-dimensional (3-D) objects.“Autodesk’s”3-D optical printer “Ember” using 405-nm light was implemented for dynamic projection lithography.It enabled straightforward spatio-selective photopolymerization on demand, which allows development of vari-ous photosensitive materials. The bio-based resins’photosensitivity was compared to standard“Autodesk”“PR48”and“Formlabs”“Clear”materials. It turned out that the bioresins need a higher energy dose to be cured (a least16J·cm−2for a single layer varying from 100 to130μm). Despite this, submillimeter range2.5-D structural features were formed, and their morphology was assessed by optical profilometer and scanning electron microscope. It was revealed that a higher exposition dose (up to26J·cm−2) results in a linear increase in the formed structures height, proving controllability of the undergoing process. Overall, the provided results show that naturally derived resins are suitable candidates for tabletop gray-tone lithography.

Full text

Photosensitive naturally derived resins
toward optical 3-D printing
Edvinas Skliutas
Sigita Kasetaite
Linas Jonušauskas
Jolita Ostrauskaite
Mangirdas Malinauskas
Edvinas Skliutas, Sigita Kasetaite, Linas Jonušauskas, Jolita Ostrauskaite, Mangirdas Malinauskas,
“Photosensitive naturally derived resins toward optical 3-D printing,”Opt. Eng. 57(4),
041412 (2018), doi: 10.1117/1.OE.57.4.041412.

Photosensitive naturally derived resins toward optical
3-D printing
Edvinas Skliutas,a,*Sigita Kasetaite,bLinas Jonušauskas,a,c Jolita Ostrauskaite,b,*and Mangirdas Malinauskasa,*
aVilnius University, Laser Research Center, Vilnius, Lithuania
bKaunas University of Technology, Department of Polymer Chemistry and Technology, Kaunas, Lithuania
cFemtika Ltd., Vilnius, Lithuania
Abstract. Recent advances in material engineering have shown that renewable raw materials, such as plant oils
or glycerol, can be applied for synthesis of polymers due to ready availability, inherent biodegradability, limited
toxicity, and existence of modifiable functional groups and eventually resulting to a potentially lower cost. After
additional chemical modifications (epoxidation, acrylation, double bonds metathesis, etc.), they can be applied in
such high-tech areas as stereolithography, which allows fabrication of three-dimensional (3-D) objects.
“Autodesk’s”3-D optical printer “Ember”using 405-nm light was implemented for dynamic projection lithography.
It enabled straightforward spatio-selective photopolymerization on demand, which allows development of vari-
ous photosensitive materials. The bio-based resins’photosensitivity was compared to standard “Autodesk”
“PR48”and “Formlabs”“Clear”materials. It turned out that the bioresins need a higher energy dose to be
cured (a least 16 J·cm
−2for a single layer varying from 100 to 130 μm). Despite this, submillimeter range
2.5-D structural features were formed, and their morphology was assessed by optical profilometer and scanning
electron microscope. It was revealed that a higher exposition dose (up to 26 J·cm
−2) results in a linear increase
in the formed structures height, proving controllability of the undergoing process. Overall, the provided results
show that naturally derived resins are suitable candidates for tabletop gray-tone lithography. ©2018 Society of Photo-
Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.57.4.041412]
Keywords: photostructurization; dynamic projection lithography; bio-based resin; stereolithography; renewable materials; 3-D printing.
Paper 171569SSP received Oct. 3, 2017; accepted for publication Mar. 13, 2018; published online Apr. 4, 2018.
1 Introduction
In recent decades, new raw materials derived from non-
petrochemical feedstock have become an interest for the
production of bio-based polymers.1–3For example, many
biorenewable materials, such as thermosetting resins, ther-
moplastics, and biocomposites, can be prepared from plant
oil-based monomers and their derivatives.4–6Recently, glyc-
erol, the byproduct of biodiesel refining, became an impor-
tant feedstock chemical, which can be used as a monomer
in the synthesis of polymers as it is or after chemical
modification.7–9Heat, pressure,10 or irradiation by light
can be employed to cure prepolymers, including natural oils,
by cationic or free radical polymerization.11 Nowadays, the
photoinitiating systems have ever-increasing importance in
the industry as these have special significance in many wide-
spread fields, such as optoelectronics and laser imaging, or
technologies, such as stereolithography (SLA)12 and nano-
technology.13–15 Therefore, there are lots of reports investigat-
ing photocuring of various photosensitive systems, mostly of
cationic polymerization of epoxides.16–19
SLA is relatively straightforward technology practically
allowing three-dimensional (3-D) objects formation with
low raw material usage.20 Its precision can reach tens of
micrometers in X-, Y-, and Z-axes and speed tens of milli-
meters per hour in Z-axis (we do not emphasize speed in X,
Yplane, because Z-axis movement represents total speed of
the technology). The technology is based on a layer-by-layer
fashion and photopolymerization reaction—a process by
which light affects chains of low-molecular mass molecules
(monomers or oligomers) and makes link together forming
polymers in solid state. In such way, cured micrometer thick
range layers then make up the 3-D solid object. The attrac-
tiveness and usefulness of potential SLA application (rapid
prototyping or low-scale manufacturing of mechanical,
medical, optical, or microfluidical devices)21 allowed this
technology to evolve and become commercialized in the
form of tabletop devices. Currently, the prices of SLA 3D
printing (3DP) can reach a few thousands of Euros, which
is acceptable for advanced customers, yet the material
price dramatically influences the cost per printed piece.
Thus, the wide spread usage of this technology is still
limited. To remedy that a lot of attention is directed to
improving SLA technology in terms of hardware, firmware,
and applicable materials, initial assortment of resins was
limited to either only so-called clear resin or its colorful
derivatives. They were formulated to deliver the highest-
quality output and capture sufficient detail without sacrific-
ing strength. At the same time, in cases of functional print-
ing, objects must have specific mechanical,22 optical,23 or
biological24,25 properties. To achieve it, engineering resins,
or in other words, functional prototyping materials, were
designed. Also, they can simulate and replace a wide range
of injection-molded plastics. Objects made out of such mate-
rials are characterized by flexibility or durability, resistance
for high temperature and even biocompatibility. That highly
widens 3DP possibilities and applicability.
One of the most welcome current trends in the field of
SLA is open-source manner. It means that device hardware,
*Address all correspondence to: Edvinas Skliutas, E-mail: edvinas.skliutas@ff.
vu.lt; Jolita Ostrauskaite, E-mail: jolita.ostrauskaite@ktu.lt; Mangirdas
Malinauskas, E-mail: mangirdas.malinauskas@ff.vu.lt 0091-3286/2018/$25.00 © 2018 SPIE
Optical Engineering 041412-1 April 2018 •Vol. 57(4)
Optical Engineering 57(4), 041412 (April 2018)

firmware, software, electronics, and formulations of the
materials are revealed for the public. Users are able to modify
listed things by their own requirements as well as share it for
the community. It provides a great way to investigate and
develop new resins and, therefore, furthering the widening
appeal of SLA 3DP.
In this work, we investigate a feasibility for using open-
source dynamic light projection (DLP)-based SLA 3DP
“Ember”to structure naturally derived resins of glycerol
diglycidyl ether (GDGE) and epoxidized linseed oil (ELO).
The selective cationic polymerization of such materials
achieved via UV lithography and exposition with Ember
is shown, and light processing peculiarities are revealed.
Achieved results are compared to what is currently available
with commercial resin designed for SLA. Overall, we show
that photoresins based on renewable biosources are suitable
candidates for usage in tabletop SLA 3DP.
2 Materials and Methods
2.1 Equipment for UV Lithography
2.1.1 Thorlabs UV curing LED system CS2010
“Thorlabs”UV curing LED system “CS2010”(Fig. 1) was
used to investigate curing time of bio-based resins. The UV
LED power was P¼270 mW (corresponding to data sheet),
irradiation wavelength λ¼365 nm, and divergence αfrom
the optical axis 60 deg. The distance Dbetween a sample and
LED chip was fixed at 4 cm. An exposed area was calculated
by Eq. (1) and was equal to 150 cm2. Consequently, light
intensity Iwas evaluated by Eq. (2) and was 1.8 mW cm−2
EQ-TARGET;temp:intralink-;e001;326;719S¼πðDtan αÞ2;(1)
EQ-TARGET;temp:intralink-;e002;326;689I¼P
S:(2)
2.1.2 Autodesk 3-D printer Ember
“Autodesk’s”open-source 3-D optical printer Ember [Fig. 2
(a)] was implemented for a dynamic projection lithography
(DPL). The light source (projector “Wintech PRO4500”)
uses 5-W power and 405-nm wavelength, an LED diode,
and a digital micromirrors device, which consists of 1280 ×
800 (total 1.024 million) micromirrors spaced in the compact
9855 ×6161.4 μm2footprint. One micromirror pitch size is
7.6 μm, which creates a 50 ×50 μm2projection of a single
image pixel, which defines device resolution in X,Yplane.
Such an optical system ensures I¼18.61 mW cm−2
light intensity projected through the UV light transparent
polydimethylsiloxane (PDMS) window where the printing
process occurs. The intensity is high enough to cure standard
resins rapidly. Moreover, it can be modified by changing the
electric current flowing through the UV LED. First of all,
CAD model is sliced into cross-sectional layers that are
transferred to the printer. Then, printing process starts and
the images are projected on the PDMS window. The exposed
area makes resin to cure one layer at a time and adhere to the
build platform or previously formed layers. Single layer
height can be from 10 to 100 μmand is controlled by
Z-axis stepper and exposure time. The build platform dimen-
sions are 64 mm ×40 mm ×134 mm in X-, Y-, and Z-axes.
The print preparation software is called “Print Studio,”which
allows to fix and prepare 3-D files and then delivers them
wirelessly to the device. Device working principle is
explained in Fig. 2(b).
2.2 Commercial Resin PR48 and Bio-Based Resins
Commercial resin Autodesk “PR48”was used as a standard
printing material. The resin is acrylate based and mainly con-
sists of methacrylate oligomers and monomers.26 Exact com-
ponents are EBECRYL 8210 (Allnex, 39.776% w/w) and SR
Fig. 1 Thorlabs UV curing LED system CS2010. LED is turned on
and exposes 150-cm2area (in blue color). There is a sample at
the center of the area.
Fig. 2 (a) An open-source 3-D optical printer Ember and (b) its principle working scheme. (I) Irradiation
from the projector reflects to the mirror and exposes resin trough the PDMS window, (II) after exposure,
the tray slides (1) and the build head rises up, and (III) the tray comes back in it primary position (1) and
the build head lowers (2). (I) side view and (II and III) front views.
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494 (Sartomer, 39.776% w/w). Being enriched with double
bonds, they are suitable for polymerization reaction.
Photoinitiator (PI) diphenyl(2,4,6- btrimethylbenzoyl)phos-
phine oxide (TPO, Esstech, 0.4% w/w) is added to increase
photoreactivity. Usually PI used for tabletop SLA makes the
resin sensitive to 420-nm wavelength light and lower, with a
peak absorption around 365 nm. Also, reactive diluent (RD)
plays important role in resin’s consist. For example,
Genomer 1122 (Rahn, 19.888% w/w) is used to reduce vis-
cosity of the base resin. What is more, RD can react with
curing agents to become a part of the polymerization
reaction and optimize cured resin’s properties, such as
impact strength, adhesion, and flexibility.27 UV blocker 2,2-
(2,5-thiophenediyl)bis(5-tertbutylbenzoxazole) (OB, Mayzo,
0.16% w/w) is used in PR48 resin with the purpose to control
the light penetration depth, which is needed to confine cured
layer thickness. Additionally, various pigments can be mixed
up to make resin colorful. However, dye concentration is
important, because too much of the pigment makes it to settle
out faster. This results in longer exposure time. PR48 initiating
system is free radical polymerization proper for a UV
lithography.
GDGE (technical grade, Sigma-Aldrich) and ELO
(Chemical Point, Germany) were chosen as monomer
sources for bio-based resins. Their absorption spectrum is
shown in Fig. 3. Both GDGE and ELO can be obtained
from some of the cheapest and most abundant nontoxic
annually renewable natural resources available in large quan-
tities (in accordance with statistical data, the total production
of vegetable oils worldwide amounted to about 177.73 mil-
lion tons in 2015/2016 year28 and about 2 million tons of
crude glycerol consistently reached the market every
year29). GDGE and ELO have a high content of reactive
functional groups making them suitable components for
the preparation of bioresins.19,30 However, these monomers
exhibit low photoreactiveness, and thus efficient PI is man-
datory. To cure bio-based resins employing Thorlabs UV
Curing LED System CS2010, the following composition
was GDGE (or ELO), 3,4-epoxycyclohexylmethyl-3,4-
epoxycyclohexane carboxylate (Sigma-Aldrich) as RD,
and triarylsulfonium hexafluoroantimonate salts (mixed
50 wt. % in propylene carbonate, Sigma-Aldrich) as PI.
Components were mixed accordingly to the weight content
of 1 mol. %:30 mol. %:3 mol. %. Polymerization mechanism
of such system was mainly cationic since exposure to the UV
light of the systems containing no cationic PI did not lead to
the appreciable hardening.27 To cure the resins using Ember,
the initiation mechanism was changed to free radical pro-
moted cationic polymerization (FRPCP) following Lalevée
protocol,31 due to the knowledge that the previous PI system
was not sensitive to 405-nm wavelength. Chemical com-
pounds were phenylbis(2,4,6-trimethylbenzoyl) phosphine
oxide (BAPO, Sigma-Aldrich), diphenyl(2,4,6-trimethylben-
zoyl)phosphine oxide (TPO, Rahn) as radical PIs, diphenyl
iodonium hexafluorophosphate (Ph2Iþ, Sigma-Aldrich) as a
cationic PI, and N-vinylcarbazole (NVK, Sigma-Aldrich) as
a promoter. The investigated photoinitiating systems were
based on PI∕NVK∕Ph2Iþwith weight contents of 3% and
2% for NVK and iodonium salt, respectively.
All composites were mixed by these steps: weighted
ingredients, poured into stirring bowl without addition of
organic solvent, and stirred with magnetic mixer in the
dark for at least 12 h (overnight) until the homogeneous sol-
ution was obtained. After it, the composites were ready
to use.
2.3 Other Equipment and Materials
Thermal power laser measurement sensor “OPHIR 3A-FS”
was employed to measure irradiance intensity on the Ember
printing area. To evaluate custom-made resins, absorption
spectra spectrophotometer “SHIMADZU UVProbe”was
implemented. “Q150R”rotary-pumped sputter coater was
used to metallize photopolymerized samples as the addi-
tional metal layer provides higher conductivity and optical
reflectiveness making optical profilometer (OP) and scan-
ning electron microscope (SEM) measurements easier. For
characterization, SEM “HITACHI TM-1000”was employed.
An OP “SENSOFAR PLμ2300”was used to obtain formed
features profiles and determine their height dependence on
absorbed irradiance energy dose.
Isopropyl alcohol was used to leach out noncross-linked
monomers from the objects printed out of standard resin
PR48. Approximate rinsing time varied from 10 to 15 min.
Acetone as a more chemically reactive solvent was
implemented to dissolve uncured bio-based resins (rinsing
duration 1 to 2 min).
Autodesk “AutoCAD 2017”student version software was
employed for CAD models designing.
3 Results
3.1 Photostructuring Employing CS2010
Estimated exposure time for commercial resins Autodesk
PR48 is a couple of seconds. Ember software Print Studio
provides possibility to modify exposure time tuning it to
the requirements dictated by any particular material. By
default settings, it is set up 1.8 s. However, the first layer
is very important for successful fabrication of the whole
3-D object. It must firmly adhere to the building plate,
thus longer exposure usually is used (8 s). Naturally derived
resins are less sensitive because of long alkyl chains of
monomer molecules. For this reason, UV dosages required
to polymerize commercial and bio-based resins were com-
pared. Thus, a UV lithography experiment was employed
Fig. 3 GDGE and ELO absorption spectrum (λrange from 250 to
1500 nm). ELO absorbs several times more irradiance than GDGE
almost in the whole measured range.
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[Fig. 4(a)]. 10-μldroplets of the resins were cast on the
150-μmglass. The hydrophilic interaction between the glass
surface and droplets was strong enough to keep the samples
upside-down. It ensured adhesion of the formed structure
after exposition to the substrate. After exposing the material
to UV radiation of 365 nm, the samples were immersed into
the solvent (acetone) for 1 min. If the droplets were polym-
erized, it stayed on the substrate, and if not, it was dissolved.
Also, selective UV lithography was performed [Fig. 4(b)].
For this, a micropatterned amplitude mask was used. The
mask was the same 150-μmglass slide, coated with 200-
nm gold layer. The gold was selectively removed with a
femtosecond pulsed laser to form a binary mask (method
is described elsewhere).32 The width of abraded lines was
150 to 400 μm, and the opaque slits between them were
150 μm. After exposing the material to UV radiation
through, the mask selective photopolymerization was
observed. Acetone dissolved the UV unaffected area, leaving
only the formed microstructures.
First, commercial resin PR48 was tested. “Formlabs”
“Clear”as additional reference was also investigated. The
results showed that both resins are completely polymerized
after 2 s of exposure, what corresponds to energy dose
E¼3.6 mJ cm−2, calculated by Eq. (2) times exposure
time. The formed structures are demonstrated in Fig. 5.
They consist of single lines, which are strict, defined,
thin, and according to the shape of the mask. Keeping the
same order, experiments with bio-based resins followed.
The exact exposure parameters required to induce photopo-
lymerization reaction in the custom-made resins were
unknown. Consequently, the experiment was started with
UV lithography to deduce the required dose of irradiation.
It was determined that both compositions need to be
exposed at least 150 s to UV light (which equals to
E¼270 mJ cm−2). Exposed droplets of the resins solidified,
did not dissolve in acetone, and were easily removable from
the substrate. Then, selective UV mask lithography was
used. As the mask added additional glass layer, more
light was absorbed before affecting the droplets resulting
in even longer exposition. For composition with GDGE,
time increased to 260 s (468 mJ cm−2), and for composition
with ELO, time increased to 220 s (396 mJ cm−2). Despite
this, it was possible so obtain selectively polymerized struc-
tures (Fig. 6).33 Compared with objects made from commer-
cial resins, these also had single lines yet with the tendency
of merging into the one object. Also, the whole structure was
noticeably thicker.
3.2 Determination of Ember’s Spatial Resolution
Resolution is a very important specification for determining
printer’s quality. It is defined as a distance among features
yet commonly used as a feature size as well. The goal of
determining the spatial resolution was to deduce the thinnest
separate lines that can be printed using Ember. Thus, a spe-
cial CAD model was designed (Fig. 7). It consisted of thin
vertical walls attached to the base. To determine if there is a
Fig. 4 Resin photocuring scheme: (a) UV lithography: (1) sample droplet exposed to UV (λ¼365 nm),
(2) polymerized droplet, and (3) adhered sample to the substrate after rinsing in acetone.(b) Selective UV
lithography: (1) exposition through mask, (2) irradiation affected areas, and (3) dissolved sample, only
formed microstructures.
Fig. 5 SEM images: (a) micropatterned amplitude mask. Microstructures after 2-s exposure to UV:
(b) Formlabs Clear and (c) Autodesk PR48.
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resolution dependence on the feature aspect ratio, the walls
were made of various altitude a, which starts from 0.3 mm
(counting from the top of the base) and increases up to
1.5 mm ×0.3 mm step. The highest one was 2 mm. The
range of thickness dof the walls was from 50 to 250 μm
every 50 μm(total five different models). The slit lbetween
walls varied from 50 to 300 μmevery 50 μm. Fixed thick
walls with fixed width slits were arranged in arrays. Such
arrays were separated from each other with 1-mm gaps.
The CAD models were printed with Ember out of standard
resin PR48. After manufacturing, the objects were immersed
into a solvent. The printed samples were characterized
using SEM. Figure 8shows that the thinnest printed walls
were approximately d¼110 μm(image a). l¼200-μm
slit between them was enough to have fully separated
walls (image c). Features with a narrower slit between
them used to merge to the one object (image b).
Compared (a) and (b), it is noticeable that lower features
tended to separate from each other better than higher ones
with the same slit. It is visible that walls of 0.6-mm height
stood apart better than 2-mm ones. It can be explained by
overexposure, which occurs during a new layer formation.
Light transmits deeper into the previous layers and addition-
ally exposes it. For this reason, the polymerization reaction
lasts longer, and the feature spreads wider and merges with
the other closest feature. Also, in a similar way, it was
deduced that Ember enables printing of 200-μm-diameter
circle holes and allows the creation of 250-μm-width rectan-
gular holes.
3.3 Formation of Woodpile Scaffold Structures
A lot of results were acquired in collaboration with colleagues
from the Institute of Biochemistry, Department of Biological
Models (Vilnius University). One of the most frequent tasks
was to print 3-D microporous woodpile scaffolds for cell
proliferation.34 Accordingly, it was necessary to investigate
if it is possible to manufacture required structures with an
available DLP 3DP. The scaffolding microarchitecture models
were designed (Fig. 9). Scaffolds’features sizes were set up
according to printers’resolution capabilities: d¼100 μm,
l¼200 μm;d¼200 μm,l¼150 and 200 μm. After fabri-
cation, the objects were postprocessed according the standard
protocol. From their SEM images (Fig. 10), it was assessed
that structures with 150 μmand narrower slits between
Fig. 6 (a) Polymerized linseed oil-based resin droplets (after 260-s exposure to UV radiation), (b) selec-
tively polymerized droplets (after 220-s exposure to UV radiation), and (c) enlarged view of the droplet
(image obtained with SEM).
Fig. 7 3-D model of resolution test in “AutoCAD”software.
Fig. 8 Ember printed feature size test sample SEM images. dand arepresent wall thickness and
height, respectively, and lis the distance between walls. (a) d¼100 μm, a¼0.6mm, l¼150 μm;
(b) d¼100 μm, a¼2mm, l¼150 μm; (c) d¼150 μm, a¼2mm, l¼200 μm. Compared (a) and
(b), higher features tend to merge more than smaller ones. Image (c) shows that features are well sep-
arated using more than 150-μm gaps.
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features were fabricated as one uniform object [Fig. 10(a)].
When lwas increased to 200 μm, it was possible to manufac-
ture scaffolds with separated walls [Figs. 10(b) and 10(c)].
Fabricated microporous structures consisted of even
100-μmsize features, which is finer to the ones achieved
by Jonušauskas et al.35 The results of cell growing on the
3-D printed scaffolds will be published in a forthcoming
publication.36
3.4 Photostructuring of Naturally Derived Resins
Employing Ember
As mentioned before, 405-nm wavelength light source is
used in Ember. Using spectrophotometer SHIMADZU
UVProbe, it was measured that custom-made resins do
not absorb above 375-nm wavelength. Thus, photoinitiating
systems were changed from cationic to FRPCP. The system
was modified using two different PIs (BAPO and TPO) and
varying their weight content from 1% to 5%. The other
part of the mixture was GDGE (or ELO) monomers plus
30 mol. % of RD. The concept of the experiment remained
the same as photostructuring with CS2010. Glass substrates
were used to cast 10-μldroplets on them. The substrates with
samples were placed on the PDMS window. Then, selective
exposition was turned on. It was projections of special
designed CAD model. The model was created corresponding
to Ember resolution test results and consisted of 50- to
400-μm-width lines with 50- to 500-μm(both increasing
Fig. 9 3-D microporous woodpile scaffold model in AutoCAD software. dis the wall width, lis the gap
width, and Tis the period (lþd).
Fig. 10 SEM images of microporous woodpile scaffolds fabricated using Ember. (a) d¼200 μm,
l¼150 μm; (b) d¼200 μm, l¼200 μm; (c) d¼100 μm, l¼200 μm.
Fig. 11 Selectively polymerized bio-based resin samples and their SEM images: given line width
(a) d¼50 μm and (b) d¼400 μm. Both samples were cured after E≈18 Jcm
−2. Yellow dotted rec-
tangles show not fully polymerized lines. (c) d¼100 μm and (d) d¼400 μm. Cured after E≈24 Jcm
−2.
Lines were formed more precisely than after shorter exposure. Green rectangles show features, formed
perpendicularly to the main lines. In the images (c) and (d), red nets represent pixel arrangement on the
PDMS window. Their orientation may cause additional feature formation in the gaps.
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every 50 μm)-width slits in between. The light intensity
through the PDMS substrate was increased to the maximum
and reached 26.54 mW cm−2value. Changing exposure dose
selectively, photopolymerized structures were obtained. It
was observed that to cure custom-made resins with Ember
increased energy dose was required. The best results were
obtained when monomers source was GDGE and the initiat-
ing system was BAPO∕NVK∕Ph2Iþ(3%∕3%∕2% w∕w). In
this case, samples were cured after E≈16 J cm−2. Using less
PI (1% and 2%), resulted in dose exceeding 24 J cm−2.
Adding more PI (4% and 5%) did not make any significant
changes in curing time, showing that inhibition of polymeri-
zation started using such high concentrations of PI.37 When
TPO was used instead of BAPO, no samples were photo-
structurized in 16- to 32-J cm−2energy dose range. After
exposure, affected areas looked more transparent than unaf-
fected. However, the samples used to dissolve in acetone
leave nothing on the substrate. The same phenomenon
was observed with both PIs when ELO was used as a mono-
mer source instead of GDGE.
For further investigations, all obtained samples were sput-
tered 20-nm gold layer. From SEM images (Fig. 11), it was
evaluated that after E≈16-to18-J cm−2lines used to be less
ordered, discontinued, prone to be affected by the solvent,
shifted, or tilted. 20- to 24-J cm−2exposure ensures well-
defined, continuous, and straight lines. Results are summa-
rized in Table 1. Also, wider ones (>200 μmand more) were
better adhered to the substrate because of a bigger contact
surface area. In all cases, fully separated lines were obtained
when a slit in between them was >200 μm. When slit was
100 to 200 μm, it was filled with the leftover of the uncured
resin still attached to the produced lines. This made it diffi-
cult to distinguish boundaries of the lines. Narrower than
100-μmslits were totally filled with uncured material or
clogged up. What is more, small circular “craters”can be
noticeable on the formed features (see Figs. 11 and 12).
They were caused by air microbubbles that appeared
when sample droplets were casted on the glass substrate.
However, this issue can be solved putting droplets in vac-
uum. After several iteration of vacuuming, no bubbles
were seen in hand-spread droplets. Microbubbles should
not be a problem for layer-by-layer SLA 3DP, due to trans-
lation of build head, which pushes out all the bubbles in
the resin as it is in standard use of commercial materials.
Also, small stripes were noticeable in the slits oriented
perpendicular to the main lines similar to repolymerization
at nanoscale as explained in other reports.38 They most likely
appeared due to diagonal pixel matrix orientation to the
PDMS window. Figure 12 demonstrates how perpendicular
stripes could be formed. Red-yellow squares net represents
pixel matrix with yellow ones symbolizing ON state pixels
(which expose the material) and red symbolizing OFF state.
As it is shown, only half of upper yellow squares overlap
with the line (marked with a green cursor). In such case,
allegedly, the pixels turn to ON state and illuminate addi-
tional area, which normally should not be exposed. The
same happens with pixels overlapping with other lines.
Stripes were periodically arranged and their locations
Table 1 Quality of photostructured various width lines after different
energy dose exposure.
Lines width (μm)
Energy dose (J cm−2)
16 to 18 20 24
50, 100 to 200 Poorly formed lines,
barely adhered to the
substrate
Well-defined,
continuous, and
straight lines
>200 Well-defined,
continuous, and straight lines
Typical height (μm) 100 to 130 190 to 220 250 to 280
Fig. 12 Enlarged part of the fabricated lines. It shows pixel matrix and
its positioning in respect to the lines. Yellow squares represent
switched ON pixels and green dashed cursor marks their overlap
with line. Blue arrows show scale of the perpendicular stripes.
Fig. 13 Measured formed lines cross sections. Exposure dose, measured (and given) width, and height:
(a) E≈16 Jcm
−2,d¼88ð100Þμm, a¼121 μm; (b) E≈20 Jcm
−2,d¼365ð400Þμm, a¼201 μm;
(c) E≈24 Jcm
−2,d¼219ð200Þμm, a¼252 μm. Red dashed cursors mark given width.
Optical Engineering 041412-7 April 2018 •Vol. 57(4)
Skliutas et al.: Photosensitive naturally derived resins toward optical 3-D printing

coincided with pixels positioning. Distance between those
stripes varied from 65 to 75 μm, which correlates with
square hypotenuse (≈70.7 μm). In our predictions, such a
device working principle explains perpendicular stripes ori-
gin and narrow slits clogging. Varying and achieving optimal
printable objects orientation in respect to the pixels align-
ment, it might be possible to achieve higher resolution.
Furthermore, the cross sections of formed lines were
obtained with an OP (Fig. 13). It allowed evaluation of
their height depending on absorbed energy dose. It was
assessed that after lower doses lines had less height, com-
pared to those, which absorbed more energy. Usually,
100- to 130-μmheight lines were photostructurized after E≈
16 to 18 J cm−2, 190 to 220 μmafter E≈20 J cm−2and 250
to 280 μmafter E≈24 J cm−2. It shows that more irradiance
is absorbed, and more material is polymerized. From sec-
tions picture, it is noticeable that thinner lines were formed
less accurately than wider ones.
4 Discussion
The prospect of bioresins that can be structured via light in a
spatio-selective manner is important for many reasons. First
of all, being made from renewable sources, they should be
cheap, easily obtainable, and biocompatible. Later, quality
makes them suitable for home use, medicine, and simplified
disposal of objects created out of them. Also, this correlates
well with the fast growth of 3DP technologies, especially
SLA-based ones. Open sourcing of the 3DP brings a great
value in development of new photocurable materials.39
Combining the aforementioned two should lead to creation
of structures that could be applicable in many different sci-
ence fields, such as microfluidics,40 optics, and biomedicine.
In our work, we showed that bioresins, based on naturally
derived monomers, can be selectively photostructurized
employing both UV lithography (365 nm) and DPL
(which uses practically visible light—405 nm). Depending
on photoinitiating system (cationic or FRPCP), it is possible
to adjust materials cross-linking rate and photosensitivity to
a certain wavelength. We investigated that the bioresins
are an appropriate medium to reach hundreds of micrometers
in spatial resolution. There were successfully structured
100-μm-width lines with 200-μmslits between them.
Significant results are that lines height can be controlled
by exposed irradiation dosage. We evaluated that in the
16- to 26-J cm−2range it is possible to modify features height
in several hundreds micrometers range. The results show that
renewable biosources can be used in SLA 3DP as new resins
for rapid prototyping. However, it is still directly difficult to
compare the custom-made resins with commercially avail-
able ones. Comparative properties can be distinguished in
sense of printing properties (photoreactivity,19 viscosity,41
layer thickness, and resolution) and ones of printed objects
(mechanical42 and chemical characteristics43). The presented
photoinitiating systems require higher energy doses, result-
ing in prolonged exposure times. Thus, additional chemical
modifications, more powerful light sources, other photoini-
tiating systems, or monomers sources could be an option for
achieving more practical results. For example, Miao et al.44
showed photostructuring of photosensitized acrylated epoxi-
dized soybean oil based on free radical photopolymerization
(as usual in optical 3DP), and Voet et al.41 demonstrated suc-
cessful fabrication of complex shaped prototypes structures
from bio-based acrylate photopolymer resins, employing
tabletop SLA. Such resins would have a significant advan-
tage because of their easy production, environmental friend-
ship, and relatively low price. Furthermore, they can be
applied for 3-D optical structuring down to nanoscale by
employing ultrafast lasers.45
5 Conclusions
In this work, a tabletop open-source 3-D optical printer
Ember was evaluated and its spatial resolution was deter-
mined. It was assessed that the device is capable of forming
3-D structures having internal microarchitecture with feature
sizes in the hundreds micrometers range. Furthermore, nat-
urally derived monomer-based resins were presented in this
paper. Cationic and free radical promoted cationic photoini-
tiating systems employing UV lithography and DPL were
investigated. Custom-made resins photoreactivity was com-
pared with standard ones. It was evaluated that bio-based res-
ins are less photoreactive and require higher energy doses to
cure. Despite this, we showed that it is possible to implement
selective photopolymerization in it and to control formed
features sizes varying irradiation dosage.
The development of practical visible light PI systems for
the rapid photopolymerization of naturally derived mono-
mers makes it possible to consider these systems for wide
use in many applications. The results show their great per-
spectives to be applied in UV lithography field for 2.5-D
structure formation or even in the 3DP.
Acknowledgments
The financial support from the Research Council of
Lithuania (No. S-LAT-17-2) is gratefully acknowledged.
Mr. Mindaugas Motiejūnas (Biolabas) is acknowledged
for sharing initiative toward 3DP of renewable bioresins.
The authors declare no conflict of interest.
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Edvinas Skliutas currently is an engineer at the Laser Research
Center. He received his BS degree in applied physics from Vilnius
University in 2017 and continues his studies on MS, specialized in
laser technology. His research interests include optical three-dimen-
sional (3-D) printing, UV lithography, two-photon polymerization,
3-D microporous scaffolds fabrication, and photosensitive naturally
derived resin development. He is an officer of the SPIE Vilnius
University Chapter and the OSA Student Chapter of Vilnius University
Sigita Kasetaite received her bachelor’s degree in chemistry in 2012
and her master’s degree in chemical engineering from Kaunas
University of Technology in 2014. Currently, she is a PhD student of
chemical engineering at Kaunas University of Technology. Her main
scientific interests include modification of naturally occurring materi-
als, synthesis of bio-based polymers, and investigation of their
properties.
Linas Jonušauskas received his bachelor’s degree as a physicist
from Vilnius University, Faculty of Physics in 2014. Currently, he is
working toward his master’s degree. His main scientific interests
include fabrication of various functional microdevices using direct
laser writing lithography and its fabrication optimization.
Jolita Ostrauskaite received her master’s degree in polymer chem-
istry from Kaunas University of Technology in 1993. She obtained
her PhD from the Department of Organic Technology, Kaunas
University of Technology in 2002. Her main scientific interests include
synthesis, modification, and investigation of bio-based polymers, bio-
degradable polymers, and biocomposites, and synthesis and proper-
ties of organic electronically active low-molar-mass and polymeric
compounds.
Mangirdas Malinauskas received his bachelor’s and master’s
degree as a physicist from Vilnius University in 2006. He obtained his
PhD from the Department of Quantum Electronics in 2010. During his
career, he has made traineeships in LZH (Hannover) and IESL
FORTH (Heraklion). Currently, he continues investigation on ultrafast
laser 3-D additive and subtractive structuring of polymers and its com-
bination with alternative lithographic techniques for potential applica-
tions in micro-optics, photonics, cell studies, and biomedicine at the
Laser Research Center, Vilnius University.
Optical Engineering 041412-9 April 2018 •Vol. 57(4)
Skliutas et al.: Photosensitive naturally derived resins toward optical 3-D printing