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Lopez de Armentia, et al.
thermal polymerization energy (E total ) and polymerization times. Shore D hardness tester (Bareiss GmbH, Berlin,
temperature was studied. The E total expressed in J/g Germany) was used for these measurements, applying 50
was determined as the area under the heat flow versus N, following the standard UNE-EN ISO 868. At least four
polymerization time curve. measurements were taken for each sample.

(2) UV polymerization 3.6. UV-visible spectroscopy
To study the UV polymerization process and the effect Absorbance of the different samples at wavelength of
that nanofillers could have on the UV polymerization 405 nm was measured by UV-visible spectroscopy using
degree, three samples of the pristine and the reinforced a Cary 4000 UV-visible spectrophotometer (Agilent
resin were cured inside a Form Cure chamber (Formlabs, Technologies, Santa Clara, CA, USA). R sample was used
Somerville, MA, USA), during different exposure times. as reference to study the absorbance of the nanofillers.
The wavelength used for the UV cure was 405 nm, the
same value during the printing of the samples. After UV 3.7. Printability and dimensional stability
exposition, DSC analysis was carried out to the samples
with a heating ramp from 20°C to 250°C at 10°C/min to The effect of the nanofillers on printability and
complete the cure of the samples. By knowing the E total dimensional stability was studied by 3D printing cube
and the energy needed to complete polymerization after samples containing holes of different geometric shapes
UV (E ), polymerization degree due to UV radiation may (i.e., circular and square holes) on their faces in order to
tc
be calculated (Eq. 3.1). determine if the addition of nanoparticles influences the
quality and accuracy of the 3D printed samples. To obtain
E − E micrographs, an Olympus DSX1000 digital microscope
( ) =
Curing degree % total tc 1 00× (Olympus, Shinjuku, Tokyo, Japan) was used. Image
E total (3.1) measurement was completed with GIMP 2.10.12
software. The background of the image was removed, and
it was transformed into a binary image. The difference
(3) Glass transition temperature image with the reference (CAD file) was obtained and
The glass transition temperature (T ) of each 3D printed black and white pixels were counted. Accuracy was
g
[51]
sample was also determined. The heating ramp used in this obtained using Eq. 3.2 :
case was from 20 to 200°C with a heating rate of 20°C/min.
T was measured using the midpoint value. All DSC tests % Accuracy = Black pixels × 100
g
were conducted using aluminum crucibles with a capacity Black pixels White pixels+ (3.2)
of 40 µL and 50 µm hole in the lid; the amount of sample
tested was between 10 and 15 mg. Nitrogen was used as
the purge gas and was delivered at a rate of 80 mL/min. 3.8. Dispersibility

3.4. Fourier-transformed infrared spectroscopy Dispersibility of the GBN was observed by opto-digital
microscopy on printed samples surface. Olympus
Infrared spectra were obtained with a Tensor27 FTIR DSX1000 (Olympus, Shinjuku, Tokyo, Japan) was used
spectrometer from Bruker (Bruker Optik GmbH, Madrid, with polarized light to observe GBN agglomerates.
Spain), with attenuated total reflectance (ATR) technique.
Golden Gate ATR accessory (Specac, Orpington, UK) 3.9. Thermal conductivity
formed by a 0.5 mm diameter diamond embedded in
a ZnSe crystal was used. The ratio signal-to-noise is Thermal conductivity was measured using DSC following
[52]
better than 8000:1 (5.4 × 10 noise absorbance). Spectra the procedure proposed by Hakvoort and van Reijen As
−5
were recorded with a resolution of 4 cm from 4000 to pure metal, gallium, was used and silicone oil was applied
−1
400 cm across 32 scans. to improve the contact with the sample, which was cut
−1
FTIR spectra at different UV polymerization times, into cylinders with a height between 1 and 2 mm, and a
prepared as previously explained, were normalized from diameter of 6 mm. The scan was set from 28 to 38°C at
the peak of –CH symmetric stretching (1375 cm ), 0.5°C/min. At least 5 measurements were completed for
-1
3
and the polymerization process was followed by observing each sample and outliers were detected by Grubbs test
the reduction of the C=C peak (1,636 cm ) . with α = 0.01.
−1 [50]
3.5. Hardness 3.10. Wettability
Furthermore, Shore D hardness was measured for 3D Cell adhesion is influenced by surface hydrophilicity,
[53]
printed samples as a function of different UV exposure and an improvement in this parameter could improve
International Journal of Bioprinting (2022)–Volume 8, Issue 1 185