J. Biomedical Science and Engineering, 2010, 3, 138-147 JBiSE
doi:10.4236/jbise.2010.32019 Published Online February 2010 (http://www.SciRP.org/journal/jbise/).
Published Online Febru ary 2010 in SciRes. http://www.scirp.org/journal/jbise
A method to fabricate small features on scaffolds for tissue
engineering via selective laser sintering
S. Lohfeld1, M . A. Tyndyk1, S. Cahill1,2, N. Flaherty1, V. Barron1, P. E. McHugh1,2
1National Centre for Biomedical Engineering Science, National University of Ireland, Galway, Ireland
2Department of Mechanical and Biomedical Engineering, College of Engineering and Informatics, National University of Ireland,
Galway, Ireland
Email: Stefan.Lohfeld@nuigalway.ie
Received 24 November 2009; revised 18 December 2009; accepted 21 December 2009.
Purpose: Selective laser sintering (SLS) is a rapid pro-
totyping technique applied to produce tissue-engineer-
ing scaffolds from powder materials. The standard
scanning technique, however, often produces struts of
extensive thickness, which means fabrication of high-
ly porous scaffolds with small overall dimensions is
quite difficult. Nevertheless, this study aims to over-
come this shortfall. Design/methodology/approach: To
this end, three scanning methods were evaluated in
terms of minimum feature size and freedom of design,
using a test polyamide (PA) material. Polycaprolac-
tone (PCL) was then employed to create highly po-
rous 3D scaffolds using the preferred scanning me-
thod to produce thin struts. Findings: While in nor-
mal scanning mode some features were well above the
laser spot diameter, strut thicknesses below the laser
spot diameter were achieved when using the “outline
scan” function for PA material. Those achieved for
PCL were slightly higher and in the 500-800 m range,
with an average pore size of 400 µm. Investigations
on the properties of the scaffolds revealed an effective
compression modulus of the PCL scaffold of 6.5 MPa.
Furthermore, there was no change in physical or che-
mical properties when the scaffolds were stored in a
physiological environment for 7 weeks. Originality/
value: Though SLS is considered as a fabrication te-
chnique for tissue engineering scaffolds, actually pro-
duced scaffolds did not comply with porosity require-
ments and limitations of the SLS process in produ-
cing features at the size of the laser beam spot have
not been discussed. The present paper shows the ca-
pabilities of the SLS process based on two materials
and presents a method to minimize feature size in
Keywords: Selective Laser Sintering; Rapid Prototyping;
Scaffolds; Polycaprolactone; Tissue Engineering
Over the last ten years, there has been significant interest
in tissue engineering since it offers an alternative ap-
proach with great potential for reconstruction or re-
placement of damaged bone tissues [1-5]. Three main
elements are required to engineer tissue, namely (i) a
scaffold, (ii) cells and (iii) a dynamic environment in
which the cell-scaffold construct are conditioned [6-8].
Current bone tissue engineering techniques employ the
use of porous, 3D, biodegradable, biocompatible, and
bioresorbable scaffolds, which act as temporary plat-
forms for initial cell attachment and subsequent tissue
formation. However, to date there is no optimum scaf-
fold structure available, which matches the material
properties of native bone tissue, and at the same time
allows for sequential growth of the neotissue as the
scaffold degrades with time.
Recently, several novel materials and processing tech-
niques have been developed to address this shortfall
[9-12]. With recent advances in design and manufactur-
ing capabilities, there has been significant interest in
developing biomimetic scaffolds with similar microar-
chitectures, mechanical and biological properties that are
similar to those of native tissue. In recent years, the ad-
vanced manufacturing technique of rapid prototyping
(RP) has played an increasingly important role in scaf-
fold fabrication, with the potential to overcome the limi-
tations of conventional manual-based fabrications tech-
niques [12-15]. One such technique is selective laser
sintering (SLS), which has the advantage that complex
3D shapes with various pore sizes, shapes and inter-
connectivities can be produced from CAD files. Addi-
tionally, this technique is a fully automated system that
employs a CO2 laser to melt polymeric powders and
form scaffold geometries without the use of any toxic
chemicals, blowing agents or support structures [16].
Porosity has always been cited as an important re-
quirement for cell growth on tissue engineering scaffolds.
Using SLS fabrication methods, porosity can be ma-
nipulated by varying scaffold parameters such as pore
size and strut thickness. Yang et al. [17] reported that the
minimum feature size of the SLS process is limited to
S. Lohfeld et al. / J. Biomedical Science and Engineering 3 (2010) 138-147
Copyright © 2010 SciRes. JBiSE
400 µm, which basically is due to the laser spot diameter.
In this situation a requirement for high porosity, com-
bined with a small pore size, necessitates the generation
of a low scaffold strut thickness. However, specific de-
tails about scaffold strut thickness in the literature are
difficult to obtain. When strut thickness values are quoted,
as for example by Das et al. [18] and Partee et al. [19],
they are deemed to be multiples of the proposed laser
spot diameter; at least 700 µm. Some studies concentrate
on the size of the pores without giving details on the
achieved strut thickness [20,21], while Smith et al. [22]
gave no dimensions of the actual scaffold features at all.
Other studies discussed selective laser sintering of scaf-
folds, however, for the experiments simple flat disks
were manufactured to investigate sintering of the mate-
rial used. In these cases the pore size and shape were not
the specific focus, but the porosity was generated by
adjusting the process parameters [16,23]. A comparison
between intended design (CAD) and actual achieved
structure is often missing. Williams et al. [21] designed
pores of 1.75 to 2.5 mm and calculated porosities of
63.1% to 79%, while the porosity of the fabricated scaf-
folds actually achieved was consistently 27 percentage
points lower than the values aimed for. This was due to
particle size and growth of the struts by heat conduction
into adjacent powder particles.
Strut thickness is an important consideration in SLS
fabricated scaffolds to achieve high porosities with small
pores. Due to the importance of being able to generate
thin struts, the study presented here was focussed on an
examination of the different SLS scan options with a
view to generating a minimum strut thickness. With pro-
cess parameters for sintering of polycaprolactone (“PCL”),
determined as part of the study, scaffolds were fabricated
and characterised in terms of strut morphology and me-
chanical properties.
2.1. SLS Processing Methods
In this study, a Sinterstation 2500plus (DTM, USA) was
employed to fabricate the scaffolds. Initially, Duraform
PA powder (polyamide, “PA”) (3D Systems, UK) was
employed to optimise the build parameters. Thereafter,
PCL (CAPA6501, Solvay, UK) was used to fabricate the
scaffolds. The software “Sinter v3.3” (3D Systems) was
used to prepare the builds. The Sinterstation 2500plus is
actually optimised to build large models and the optics
focus the CO2 laser spot to a diameter of about 400 to
500 µm. To achieve feature sizes in the appropriate
range for tissue engineering scaffolds, as a first step, the
laser processing parameters have to be optimised.
The main sintering path of the laser is along the x-axis
of the machine, which is parallel to the front wall of the
build chamber. In general, the cross sections of the 3D
models are sintered with parallel line scans in the
x-direction, incrementally shifted in the y-direction (fill
scan). In the y-direction, thin walls or struts are fabri-
cated by single dot-like shots, building up to form a line
parallel to the y-axis, as pictured in Figure 1. Ideally, a
strut built this way has a thickness similar to the laser
spot diameter.
Surfaces that have not been parallel to the y-axis dur-
ing fabrication may appear stepped due to the scanning
technique. To reduce this, a so-called “outline scan” can
be added to improve the surface quality. After perform-
ing the fill scan, the laser scans along the edge of the
part in each cross section similar to a plotter. As laser
power and offset values of both the fill scan and the out-
line scan can be controlled separately, the latter can be
used to fabricate thin, smooth walls in any direction in
the x-y-plane by turning off the “fill scan” completely.
To compensate for the diameter of the laser spot when
scanning, the laser usually is offset from the edges of the
cross section. Depending on the offset value and the di-
mensions of the strut in the 3D design, one or two main
struts in the fabricated scaffold can be created. This will
be explained further below.
In order to find the most suitable method to fabricate
thin struts in scaffolds, the present paper investigated
three approaches:
a) using the fill scan and a laser offset of about half of
the strut thickness
b) using the outline scan, but no laser offset (laser
path along the edge of the strut)
c) using the outline scan and a laser offset of about
half of the strut thickness
Method a) is the common sintering technique used for
all models fabricated using the Sinterstation. Therefore,
Figure 1. Build preview showing the laser scan paths. Thin
lines parallel to the y direction of the build chamber consist
of single laser shots aligned along the y axis.
S. Lohfeld et al. / J. Biomedical Science and Engineering 3 (2010) 138-147
Copyright © 2010 SciRes. JBiSE
Figure 2. Laser spot over-
lap when the strut thickness
t is smaller than the laser
spot diameter D.
Figure 3. Designed 3D model (left) and actually built model
it was investigated to examine the accuracy of the scaf-
folds generated using the standard sintering technique.
Method b) fabricated a frame around the designed
strut with two short and two long edges. During the de-
sign stage it was taken into account that the long edges
were likely to be fused together if their distance (= strut
width) was less than the laser spot diameter of 410 µm.
This is due to an overlap of the laser spot in the centre of
the strut when scanning around the edge of the strut
(Figure 2). Therefore, the strut width was set to 800 µm,
and the distance between the struts was set to be the
same to ensure even distances between the sintered paths.
Each single strut in the 3D design actually resulted in
Figure 4. Laser scans along the centreline of each strut.
two struts connected at the ends in the fabricated scaf-
fold (Figure 3). As only one pass of the laser per strut
was used here, these struts were expected to have the
smallest achievable dimensions. For Method c) the offset
value was set to approximately half the thickness of a
designed strut to make the laser scan along the centre
line of each strut (Figure 4). As the outline has to be a
closed line, the laser scanned along the centre line twice.
It was assumed that this adds to the thickness of the final
fabricated strut. On the other hand, unlike Method b), the
fabricated scaffold resembled the 3D design as the actual
strut thickness achieved by this method has been antici-
pated in the design.
2.2. Scaffold Design Dependence of SLS
Parameter settings
All scaffolds were designed using Pro/Engineer Wildfire
(PTC, USA) and exported into the. STL file format.
Scaffolds designed for Method a) consisted of four struts
per layer, each strut 500 µm wide and with 2 mm dis-
tance between the centrelines of the struts. For Method b)
four struts per layer were necessary in the 3D design to
result in eight struts in the fabricated model. In this case,
the struts were designed with a width of 1 mm, a height
of 500 µm and a centreline separation distance of 2 mm.
Scaffolds designed for Method c) consisted of eight
struts per layer, each 500 µm wide, with a distance of 1
mm between the strut centrelines. In all cases the layers
were oriented alternating parallel to the x or to the y-axis.
Three layers with a height of 500 µm each and, to dis-
tinguish between top and bottom of the scaffold, a top
layer of 1000 µm height, were included. CAD images of
the models are illustrated in Figure 5.
The important sintering parameters can be seen in Ta-
ble 1. The laser power was reduced for Methods b) and c)
because the laser scan speed is slower in the “outline
scan” mode. When not reducing the laser power, the
energy density
S. Lohfeld et al. / J. Biomedical Science and Engineering 3 (2010) 138-147
Copyright © 2010 SciRes. JBiSE
(with ED = energy density in J/mm², LP = laser power in
Watt, SS = scan spacing in mm, and LS = laser scan
speed in mm/s), as described in Caulfield et al. [24],
would have been excessive over Method a), resulting in
thicker struts.
Using the laser scan method that was found to be most
suitable to fabricate thin struts, a parameter study was
performed to find process parameters most suitable for
sintering PCL scaffolds. The variable parameters were
laser power (3.5 up to 11.5 W with 0.5 W increments),
number of scans (one or two) and preheat temperature
for the powder (38 up to 50). Subsequently, cylin-
drical scaffolds with diameters of 5 and 8 mm (Figure 6)
were fabricated from PCL. Resulting from the parameter
study, the powder bed preheat temperature was set to 38
and sintering was performed with a single scan at a
laser power of 4 W. The scaffolds were sliced into 45
horizontal layers with a layer thickness of 0.11 mm each.
Each set of struts consisted of 5 layers.
Figure 5. CAD models for scan method experiments.
Table 1. Sintering parameters for the polyamide scaffolds.
Laser offset
Fabrication method
x-direction µmy-direction µm
Laser power
Scan spacing
a) 240 200 9 0.10
b) 0 0 5 n/a
c) 243 243 5 n/a
Figure 6. CAD models for PCL scaffolds.
S. Lohfeld et al. / J. Biomedical Science and Engineering 3 (2010) 138-147
Copyright © 2010 SciRes. JBiSE
Figure 7. Polyamide models fabricated via the three scan methods.
Table 2. Strut width in x and y directions for each SLS fabrication method.
Strut Width Method a) Method b) Method c)
x - direction [µm] 325 +/- 8 325 +/- 21 355 +/- 20
y - direction [µm] 955 +/- 49 330 +/- 20 375 +/- 8
2.3. Characterisation of Scaffold Properties
2.3.1. St rut Morphology
The scaffold structures were examined by scanning elec-
tron microscopy (SEM) (Hitachi, UK). All surfaces were
coated with gold using an Emitech K550 gold coater
(Emitech, UK) to avoid charging under the electron beam.
The pore size, pore area, and strut thickness values as
well as the surface topography were obtained using Im-
ageJ analysis software (NIH, Public Domain).
2.3.2. Compression Tests
To determine the effective modulus of the scaffolds, the
PCL scaffolds (diameter 5 mm) were tested in compres-
sion (n=6). The influence of cells and physiological con-
ditions on the effective modulus were investigated using
two additional scaffolds, one of which was seeded with
SaOs-2 cells in a well (3x104 cells/well) and left in the
physiological solution described below for seven weeks,
while the other was stored as is in the solution for seven
weeks. All compression tests were carried out on a
Zwick Z2.5 (Zwick Testing Machines Ltd., UK) using a
1 kN load cell and at a compression rate of 1 mm/min.
The modulus values were determined from subsequent
stress-strain graphs.
2.3.3. Stability of the SLS Fabricated Scaffold in a
Physiological E nvi ronment
To examine the physical integrity of the SLS fabricated
constructs in a physiological environment, scaffolds
were stored in the cell media solutions. Subsequently,
the scaffolds were covered with 1 ml of media contain-
ing 500 ml RPMI-1640, 5 ml penicillin-strep- tomycin, 5
ml L-glutamine, 50 ml and 10 % foetal bovine serum
and incubated at 37, 100 % humidity and 5 % CO2 for
7 weeks (n=6). Thereafter, the scaffolds were visually
examined using scanning electron microscopy; the po-
rosity and surface topography were also examined. Be-
cause of the non-conductive nature of the polymer, sam-
ples were gold-coated prior to imaging. Samples were
viewed and images captured over a range of magnifica-
tions. Furthermore, to determine if there were any ad-
verse affects on the chemical composition of the scaf-
folds after storage in the cell culture media over a 7-
week period, Fourier transform infra red spectroscopy
(FTIR-ATR Shimadzu, UK) was employed (n=6). Spec-
tra were recorded in the wavelength range of 4000 and
400 cm1 by 2 cm1 resolution in 16 scans.
3.1. Optimisation of Scaffold Structure
While for the struts in the x-direction the thickness was
satisfying when using the standard scanning method,
perpendicular struts had an excessive thickness, see Fig-
ure 7. As shown in Table 2, the struts in y-direction fab-
ricated using Method a) were found to be more than
twice the laser diameter, possibly caused by too much
energy applied to a small area in the fast shot of the laser,
or due to an inaccurate control of the laser beam move-
ment. Though the laser spot diameter is only 410 µm, the
struts were 955 ± 49 µm wide. There were no gaps found
between them. This was in contrast to the struts parallel
S. Lohfeld et al. / J. Biomedical Science and Engineering 3 (2010) 138-147
Copyright © 2010 SciRes. JBiSE
Figure 8. Curved struts following cutting a cylindrical section from the original design
approach not suited for cylindrical scaffolds.
Figure 9. Photograph of the 8 mm and 5 mm PCL scaffolds.
to the main sintering direction of the laser along the
x-axis of the build chamber, which tended to match the
size of the laser spot diameter. Hence, this method was
deemed unsuitable for scaffold fabrication. Methods b)
and c), on the other hand, resulted in scaffold struts that
were in both x- and y-direction even less thick than the
laser beam spot diameter. For Method b), struts in the
x-direction and y-direction were about the same widths
of 325 +/- 20 µm. This matched the width of the struts in
the x-direction achieved with Method a), because for this
direction the scanning technique is the same for these
two methods. With Method c) slightly higher widths as
Method b) were achieved in both directions. This was
attributed to the fact that with Method c) each strut was
scanned twice along its centre.
Method b) gave the best results in achieving minimal
strut thickness. However, it has a few general drawbacks.
Using this method it is quite difficult to foresee the
structure of the fabricated scaffold based on the designed
one, as each strut in the 3D design results in two struts in
the built model. In addition, this method is only suitable
when the edges of the design are parallel. Using a cylin-
drical design, some of the fabricated struts would be
curved (Figure 8). Therefore, Method b) is not suited for
scaffolds with curved surfaces, such as cylindrical scaf-
folds. Finally, the flow through the structure from the
sides is obstructed, because some of the struts are con-
nected at their ends, unless this part is cut away. Due to
the good results obtained using Method c), it was chosen
for further scaffold fabrication. The thickness achieved
was only slightly higher than for Method b), it allows for
more complex designs, and, when anticipating the fabri-
cated strut thickness in the 3D design, final outcome and
design do not differ. Non-parallel shapes can be built as
intended, and there is virtually no limitation in the mac-
roscopic shape of the scaffold, i.e. cylindrical scaffolds
can be fabricated.
It was observed that when processing PCL with me-
thod c), the achievable minimal strut thickness was
higher then when processing PA. In the x-direction it
was 450 µm, while it was approximately 700 to 800 µm
in y-direction. The increased thickness compared to the
PA samples is due to the different process parameters
used and different thermal properties of the material. The
difference between struts in x- and y-direction, however,
is surprising, as sintering conditions for both directions
were the same. In an additional test of all three methods
applied to PCL material, however, Method c still gave
the best results. In order to achieve pores of 300 to 400
µm diameter, the number of struts in the y-direction was
reduced to three compared to five struts parallel to the
x-axis in a 5 mm diameter cylindrical scaffold. In an 8
mm diameter scaffold there were six struts parallel to the
y-axis and eight parallel to the x-axis. At the bottom and
at the top of the scaffolds, two perpendicular struts were
added to increase the mechanical stability and prevent
inclining when loaded with compression forces. Based
on the CAD design, the scaffolds had a porosity of 50 %.
The fabricated scaffolds are shown in Figure 9.
S. Lohfeld et al. / J. Biomedical Science and Engineering 3 (2010) 138-147
Copyright © 2010 SciRes. JBiSE
3.2. Materials Properties of the Scaffolds
3.3.1. St rut Morphology
Visual inspection of the PCL constructs using SEM re-
vealed strut thickness values of approximately 500 µm
and 800 µm, respectively, depending on their orientation.
The average pore size was around 400 µm (Figure 10a).
On the microscale each strut within the scaffold con-
sisted of a large number of randomly oriented grains
(Figure 10 a, b, c). The grains had different shapes and
their surface area ranged from 500 to 5000 mm². The
surface of the single particles appeared smooth and each
grain had rounded, regular edges (Figure 10d). The
grains were usually connected by molten material
through sintering, however, many grains were connected
to each other by a system of thin struts, leaving an inter-
nal microporosity within each strut.
3.3.2. Compression Tests
The effective modulus for the untreated scaffolds was
determined to be 6.5 ± 0.7 MPa. The effective moduli
for both scaffolds stored in the physiological solution for
seven weeks, whether or not seeded with cells, are
within one standard deviation of this value.
3.3.3. Stability of the SLS Fabricated Scaffold in a
Physiological E nvi ronment
The surface topography of the scaffolds stored in the
physiological solution (Figure 11) appeared to be slightly
different to the as-fabricated PCL scaffolds. The number
of grains in the struts appeared to be reduced; there were
more areas where no separate grains were attached. The
grains appeared to have irregular shapes with uneven,
jagged surfaces (Figur e 11 d).
As-fabricated scaffolds
(a) (b)
(c) (d))
Figure 10. SEM analysis of strut morphology. Untreated scaffold at various magnifications, (a) X30, (c) X100, (e) X 250, (g) X
S. Lohfeld et al. / J. Biomedical Science and Engineering 3 (2010) 138-147
Copyright © 2010 SciRes. JBiSE
Scaffold after immersion in physiological solution
(a) (b)
(c) (d))
Figure 11. SEM images of scaffold after seven weeks in media at various magnifications, (b) X 30, (d) X100, (f) X 250, (h) X1000.
Figure 12. FTIR spectra showing chemical composition of SLS scaffolds. There is no
change in the functional groups after 7 weeks in media, suggesting the SLS fabricated
scaffold maintains its chemical integrity.
S. Lohfeld et al. / J. Biomedical Science and Engineering 3 (2010) 138-147
Copyright © 2010 SciRes. JBiSE
Based on the FTIR analysis, it was seen that the chemi-
cal composition of the scaffolds stored in the media did
not change over a 7-week period (Figure 12). The char-
acteristic C–H stretching of CH2 and CH3 groups of
saturated structures was observed in the range 2980–2850
cm1. Typical absorption frequencies associated with
ester moieties present in PCL were readily evidenced
mainly at 1740 cm1 (C=O stretching). The absorption
band at 1220 cm1 was attributable to the C(O)–O and
C–N stretching and N–H bending (amide III) vibrations.
Thereby, no alteration in the chemical composition of
the scaffolds was observed.
The selected approach to fabricate scaffolds using the
outline scan function of the Sinterstation proved to be
successful with respect to producing minimal strut thick-
nesses, which to date had not been seen for selective
laser sintered PCL scaffolds. The phenomenon of dif-
ferent strut thicknesses in x- and y-directions is more
obvious for the PCL scaffolds than for PA. This is pro-
bably due to the different heat conduction properties of
this material and will be further investigated. However,
as it occurs for both tested materials, an influence of the
prototyper’s non-accessible scan parameters is undeni-
able. Nonetheless, porous PCL scaffolds were fabricated
by selective laser sintering, using a Sinterstation, with a
pore size range of 300–400 m in diameter and a mini-
mum wall thickness of 450 m in the x-direction and 700
m in the y-direction.
In addition, there was no dramatic change in the
physical properties or chemical composition of the SLS
fabricated scaffolds stored in a cell culture environment
for 7 weeks. This was echoed in the mechanical proper-
ties. Though signs of degradation of the material were
obvious, the mechanical performance of the scaffolds
remained unchanged after 7 weeks under the influence
of the physiological solution and cells. Degradation ap-
peared to be limited to surface particles that did not con-
tribute to the mechanical properties as they did not have
a strong mechanical bond to the main structure. More-
over, the mechanical property values were at the lower
end of the compressive modulus values quoted for can-
cellous bone [25].
Taken together, these results highlight the potential of
the described selective laser sintering method as an al-
ternative fabrication technique for highly porous tissue
engineering scaffolds with minimal strut thickness. Op-
timisation of process parameters to further reduce the
strut thickness, in particular for PCL, is currently in pro-
The authors acknowledge research funding from the European Union
through the STEPS FP6 project (contract number FP6-500465) and
also the partners in the STEPS project for their input. M.A. Tyndyk
acknowledges funding from the Irish Research Council for Science,
Engineering and Technology (IRCSET) through a Postdoctoral Fel-
lowship. The authors acknowledge support from the Programme for
Research in Third Level Institutions (PRTLI) administered by the
Higher Education Authority.
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