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Additive Manufacturing of Bone Scaffolds
which was used for obtaining the load-displacement were assumed to represent porous regions, whereas those
curves, is determined from this step. above were assumed to indicate bulk regions. Finally,
To determine the mechanical behavior of the adopted the porosity was obtained by calculating the number
polymers, compression tests have been performed on of voxels representing pores and their distribution was
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3D-printed samples made of two different polylactic calculated using a customized MATLAB code.
acid (PLA) materials, PLA1 and PLA2; for this purpose,
specimens with dimensions 40×20×20 mm have been 2.5. Mechanical tests
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used. The compression test provided the bulk material Several mechanical tests were performed to investigate
properties used in numerical simulations; in particular, different parameters’ effects on the mechanical response
Young’s modulus and Poisson’s ratio of the PLA1 and of additively manufactured scaffolds. Moreover, all
PLA2 materials have been obtained to be E =930 MPa, compression tests were performed with speed of 1 mm/
1
ν =0.4 and E =550 MPa, ν =0.4, respectively. min at room temperature (23°C). As described previously,
2
1
2
two different filaments made of pure poly lactic acid (PLA,
2.4. µCT labeled PLA1) and a PLA reinforced with carbon nano
It is difficult when the simulation of an object with a tube (CNT-PLA, labeled PLA2) were used (Figure 5).
complex geometry is required, especially when their In addition, scaffolds were printed in both vertical
CAD model cannot be obtained from exact mathematical and horizontal directions to investigate the effect of the
equations. For instance, natural structures, including bone printing direction on the mechanical response (Figure 6).
tissues, rock specimens, and leaves, are challenging to Slicing the specimens with a normal vector parallel to the
model. µCT facilitates modeling these natural structures largest dimension results in vertical printing, while using
by allowing to capture their microscale details. Another a normal perpendicular to the largest dimension results in
application of this technology is to evaluate the printing horizontal printing.
accuracy of AM objects. In this context, two 3D-printed Furthermore, TPMS gyroid structures were
scaffold structures were scanned to study the printing combined with both I-WP and diamond structures with
accuracy achieved. The scan files provided details about different degrees of TZ sharpness. When scaffolds made
the micropores between the deposited layers and extra of a combination of gyroid and diamond structures are
fused materials at the edges of the scaffolds. Figure 4 concerned, different sizes of the TZ have been used.
illustrates one scanned specimen and its comparison Figure 7A-D shows the nominal geometries for samples
with its nominal (i.e., mathematically exact) geometry. with the variation in TZ (specimens 4 – 7 in Table 2),
According to Figure 4, the printed samples have many Figure 7E and F illustrates the corresponding printed
anomalies which can affect the mechanical behavior scaffolds.
of the whole scaffold. For example, the unintentional Table 2 describes the details of each considered
accumulation of the materials on the sharp edges, which specimen; in particular, the parameter K defines the
is inevitable when using FDM, can increase the strength
of the scaffold and lower the stress concentration in the
mentioned regions. Hereafter, the scanned specimens will
be labeled as real specimens while the designed ones will
be termed as nominal specimens.
A desktop µCT scanner, equipped with a cone-beam
micro-focus X-ray source and a flat panel detector, was
used in this study (LOTUS-NDT, Behin Negareh Co.).
In scanning the samples, the X-ray tube voltage and its
current were set to 60 kV and 50 µA, respectively. The
total scan time for each sample was ~ 2.5 h, and the Figure 4. Comparison of nominal (left) and real (right) designed
obtained resolution was 70 microns. bone scaffolds.
Micro datasets were analyzed as follows: The
images were first converted to BMP format and the so- A B
called median filtering was applied to smooth the 3-D
images using a 3×3×3 kernel (in open-source ImageJ
software). Then, the image stack was set to a specific
threshold using a customized MATLAB code. The
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threshold level used for the CT data was defined to give a
99.5% porous structure as calculated from experimental Figure 5. Printed scaffolds obtained using different polymer
measurements. Voxels of intensity below the threshold filaments. (A) PLA (PLA1) and (B) CNT-PLA (PLA2).

44 International Journal of Bioprinting (2022)–Volume 8, Issue 3