Direct Inkjet Printing of Dental Prostheses Made of Zirconia

Abstract

CAD/CAM milling systems provide a rapid and individual method for the manufacturing of zirconia dental restorations. However, the disadvantages of these systems include limited accuracy, possible introduction of microscopic cracks, and a waste of material due to the principle of the 'subtractive process'. The hypothesis of this study was that these issues can be overcome by a novel generative manufacturing technique, direct inkjet printing. A tailored zirconia-based ceramic suspension with 27 vol% solid content was synthesized. The suspension was printed on a conventional, but modified, drop-on-demand inkjet printer. A cleaning unit and a drying device allowed for the build-up of dense components of the size of a posterior crown. A characteristic strength of 763 MPa and a mean fracture toughness of 6.7 MPam(0.5) were determined on 3D-printed and subsequently sintered specimens. The novel technique has great potential to produce, cost-efficiently, all-ceramic dental restorations at high accuracy and with a minimum of materials consumption.

Full text

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RESEARCH REPORTS
Biomaterials & Bioengineering
DOI: 10.1177/0022034509339988
Received May 7, 2008; Last revision February 25, 2009;
Accepted March 9, 2009
J. Ebert1, E. Özkol1, A. Zeichner1,
K. Uibel1,2, Ö. Weiss3, U. Koops3,4,
R. Telle1, and H. Fischer5*
1Department of Ceramics and Refractory Materials, RWTH
Aachen University, Mauerstrasse 5, 52064 Aachen,
Germany; 2ESK, Max-Schaidhauff-Strasse 25, 87437
Kempten, Germany; 3Heraeus Kulzer, Quarzstrasse 8, 63450
Hanau, Germany; 4W.C. Heraeus, Heraeusstrasse 12-14,
63450 Hanau, Germany; and 5Dental Materials and
Biomaterials Research, University Hospital Aachen,
Pauwelsstrasse 30, D-52074 Aachen, Germany; *corre-
sponding author, hfischer@ukaachen.de
J Dent Res 88(7):673-676, 2009
ABSTRACT
CAD/CAM milling systems provide a rapid and
individual method for the manufacturing of zirco-
nia dental restorations. However, the disadvan-
tages of these systems include limited accuracy,
possible introduction of microscopic cracks, and a
waste of material due to the principle of the ‘sub-
tractive process’. The hypothesis of this study was
that these issues can be overcome by a novel gen-
erative manufacturing technique, direct inkjet
printing. A tailored zirconia-based ceramic suspen-
sion with 27 vol% solid content was synthesized.
The suspension was printed on a conventional, but
modified, drop-on-demand inkjet printer. A clean-
ing unit and a drying device allowed for the
build-up of dense components of the size of a pos-
terior crown. A characteristic strength of 763 MPa
and a mean fracture toughness of 6.7 MPam0.5
were determined on 3D-printed and subsequently
sintered specimens. The novel technique has great
potential to produce, cost-efficiently, all-ceramic
dental restorations at high accuracy and with a
minimum of materials consumption.
KEY WORDS: rapid prototyping, direct inkjet
printing, zirconia, microstructure, mechanical
properties.
Direct Inkjet Printing of Dental
Prostheses Made of Zirconia
INTRODUCTION
The introduction of CAD/CAM milling systems in the dental field enabled
zirconia ceramics to be used as a standard material for dental prosthetic
restorations (Luthardt et al., 1999; McLaren and Terry, 2002). In the mean-
time, more than 20 milling systems have been introduced into the market.
Among CAD/CAM milling systems, two types can be differentiated. For
‘hard machining’, a restoration is milled out of a sintered monoblock, whereas
a white monoblock is milled for ‘soft machining’, with subsequent sintering.
The disadvantage of both systems is the considerable amount of waste of raw
material, because the unused portions of the monoblocks must be discarded
after milling, and recycling of the excess ceramic material is not feasible.
Advantages of restorations produced by ‘hard machining’ are accurate shape
and precise dimensions (Bindl and Mörmann, 2007). However, the tooling of
sintered high-strength ceramics is costly and time-consuming. The tools are
exposed to heavy abrasion and therefore withstand only short running cycles.
Moreover, there is a considerable risk of microscopic cracks that can be intro-
duced into the ceramic surface due to the tooling process of the brittle material
(Wang et al., 2008). Surface damage does not occur during ‘soft machining’,
because the shaping is performed prior to sintering. Furthermore, the mill-
ing of white monoblocks results in shorter machining times and longer ser-
vice life cycles of the tools. However, the accuracy of contour and shape of
‘soft-machined’ restorations is more critical compared with that of the ‘hard-
machined’ components because of the shrinkage during subsequent sintering,
which must be considered and controlled. Additionally, quality assurance of
the white monoblocks is difficult relative to storage and shipping, and with
respect to the sintering process, which is performed not in an (controlled)
industrial, but in a dental laboratory environment.
So-called generative manufacturing techniques exhibit the potential to over-
come the described deficiencies. With these techniques, a three-dimensional
component can be built up layer by layer. While the generative manufacturing
of metallic- and polymer-based materials is state-of-the-art and commercially
available, generative production with ceramic materials, worldwide, is still in
development (Tay et al., 2003). For ceramic materials, 5 generative manufac-
turing techniques are of special interest: (i) stereo-lithography (Doreau et al.,
2000); (ii) 3D-P, i.e., printing of a polymeric or inorganic binder into a ceramic
powder bed (Uhland et al., 1999); (iii) selective laser sintering (Bourell et al.,
1992); (iv) selective laser melting (Hollander et al., 2003); and (v) direct inkjet
printing (Zhao et al., 2002). Only porous structures, however, can be created
by the first 4 technologies (i-iv) mentioned above. In contrast, direct inkjet
printing of a ceramic suspension provides the possibility of generating dense
green bodies at a high resolution and complex shape (Ebert et al., 2008; Özkol
et al., 2009). Besides, only thin walls of some 100 µm of thickness and a height
of 1 mm at most, or small pillar-shaped arrays of less than 100 µm of diameter

674 Ebert et al. J Dent Res 88(7) 2009
and a few 100 µm of height have been generated up to now
(Zhao et al., 2002; Noguera et al., 2005; Lewis et al., 2006).
The objective of the present study was to develop a tailored
direct inkjet printing process that can be used to build up dental
prosthetic restorations made of high-strength zirconia ceramics.
A tailored additive system was developed that allows for the
printing of a suspension with a high solid content of zirconia
powder, with the use of direct inkjet printing technology with
conventional drop-on-demand inkjet print heads (Uibel et al.,
2006). A well-balanced drying device was developed for the
creation of three-dimensional structures of the size of dental
prosthetic restorations at high accuracy with respect to its
dimensions. Additionally, a tailored cleaning unit, based on a
modified ultrasonic bath, was integrated into the printer, which
allows for long printing periods without nozzle clogging. With
our study, we tested the hypothesis that it is possible to over-
come the major issues of CAD/CAM milling sytems—limited
accuracy, possible introduction of microscopic cracks, and a
waste of raw material—by the direct inkjet printing technique.
MATERIALS & METHODS
Synthesis of the Ceramic Suspension
The ceramic suspension consisted of approximately 27 vol% of
zirconia powder, 55% distilled water, a boehmite sol, and dis-
persants (Uibel et al., 2006). The boehmite sol was used to
prevent agglomeration of the ceramic particles and to increase
the green body strength. We synthesized the boehmite sol by
adjusting distilled water to a pH value of 2.0. Boehmite (Disperal
P2, Sasol, Hamburg, Germany) was added at a temperature of
80°C. A 3 mol% quantity of yttria partially stabilized zirconia
powder (TZ-3YS-E, Tosoh, Tokyo, Japan) was added to the sol.
The mean ceramic particle size was 90 nm, and the specific
surface area was 7 m2/g. The bulk density of the powder was
6.05 g/cm3. The pH value of the ceramic ink was recorded by
potentiometry (In-Lab 417, Mettler Toledo, Gießen, Germany).
The viscosity of the slurry was determined with the use of a
rotational rheometer (Viscolab LC 10, Physica, Stuttgart,
Germany).
Set-up of the Printing Station and Printing Procedure
The ceramic suspension was injected into an empty standard HP
cartridge (HP 51645a, 42 mL, Hewlett Packard, Palo Alto, CA,
USA) by means of a syringe. The printing device, based on a
modified drop-on-demand deskjet printer (HP DeskJet 930c,
Hewlett Packard), consisted of the following units. The original
printing carriage held the cartridge, driven by the original servo-
motor. The cleaning system was comprised of an ultrasonic bath
(Carrera 2309, 50 W/50 Hz, Lutter & Partner, München, Germany)
and stripping rollers. The printhead was automatically soaked and
cleaned in the ultrasonic bath after each printing cycle, when the
cartridge returned to the starting position in the x-direction. A mix-
ture of water and ethanol was used as cleaning fluid. To prevent
jams or error messages such as ‘printer out of paper’ after the sub-
stitution of paper feed by a z-axis, we developed a paper-simulating
unit. As in commercial two-dimensional printers, printing occurred
line by line in the x-direction, and the maximum line width
(y-direction) was determined as the maximum width for the printed
component. A z-drive (Servo motor, Isel Elektronik, Eichenzell,
Germany) was implemented to allow for the printing of specific
Figure 1. Weibull plot with strength distribution of fired zirconia
specimens (N = 21) that were built up by the direct inkjet printing
technique.
Figure 2. Fracture surface of a flexural strength test specimen. (a) The
cross-section of the printed and sintered sample appears quite homoge-
neous and dense. No layering structure can be detected. Only one row
of process-related defects, due to nozzle clogging during printing, can
be seen at the right side of the specimen. One additional large single
process-related defect is located close to the bottom of the sample. (b)
Detailed view of the micrograph showing the large single defect and the
lower part of the string of defects at the side. The different perspective
in the detail view additionally shows the lower surface of the specimen,
which reveals that the string of defects (resulting from one single clogged
nozzle) was located over the complete length of the specimen.

J Dent Res 88(7) 2009 Inkjet Printing of Zirconia Prostheses 675
cross-sections, layer on layer, to build up the three-dimensional
components. We used special software (CEC TestPoint, Capital
Equipment Corp., Billerica, MA, USA) to control the vertical
motion. A step size, i.e., a resolution in the z-direction of $z = 5
µm, was achieved. The drying unit consisted of 3 components: 2
narrow-spot spotlights (1000 W, PAR 64 can, Showtec, Köln,
Germany), magnifying glasses to focus the light, and a fan
(Minebea Co. Ltd., NMB 2408NL-04W-B40, Ayutthaya, Thailand)
to decrease the humidity in the printed area. The temperature in the
printing zone was approximately 90°C. Graphite plates (Ringsdorff
Werke, Bad Godesberg, Germany) of 4 mm thickness were used as
substrates. The three-dimensional components were printed page-
by-page from Microsoft Word files that contained the black-colored
cross-sections. Each cross-section represented a slice with a thick-
ness of 5 µm of the respective section in the z-direction (height) of
the component.
Heat Treatment and Characterization
of the Printed Components
The printed 3D components were first dried in a chamber dryer
(T 5022, Heraeus Kulzer, Hanau, Germany) at 80°C for 12 hrs.
The organic additives were then removed in a ceramic furnace at
550°C, and the parts were subsequently fired at 1450°C for 2.5
hrs. The density of the as-fired specimens was determined
according to the principle of Archimedes. SEM micrographs (Leo
440i, Carl Zeiss, Jena, Germany) were taken from the cross-
sections of cut specimens to analyze the microstructure. Printed
and subsequently sintered specimens (1.5 x 3.0 x 30.0 mm3, n =
21) were ground on a precision surface grinding machine (PS
R300, G&N, Erlangen, Germany) to determine the Weibull
parameters, i.e., characteristic strength S0 and Weibull modulus m
(Munz and Fett, 1999). The grid size of the final diamond charged
grinding wheel was 46 µm. Additionally, printed and sintered
specimens (3.0 x 6.0 x 30.0 mm3, n = 4) were ground to determine
the fracture toughness KIc. These KIc-specimens were notched by
means of a diamond-charged cut-off wheel (thickness: approx.
200 Mm). The notches (depth: 20% of specimen thickness) were
sharpened by the razor-blade method (Kübler, 1997) (SEVNB,
i.e., single-edge V-notched beam). Note that SEVNB specimens
were used only for fracture toughness measurements, and speci-
mens for strength measurements had inherent flaws without any
pre-cracking. The specimens were mechanically tested in a uni-
versal testing machine (model 1186H0425, Instron, Darmstadt,
Germany) in four-point bending mode. The inner and outer roller
spans were 12 and 24 mm, respectively. The stressing rate was set
at 100 MPa-1 to avoid subcritical crack growth during testing.
RESULTS
The pH value of the suspension was set at 8.5. The relative den-
sity of the fired specimens was 96.9%. An isotropic shrinkage of
20 vol% was determined. SEM micrographs of the printed and
sintered specimens (cross-section) revealed a rather homoge-
neous microstructure, with some submicron-sized pores. The
characteristic strength of the ground bars was S0 = 763 MPa,
with a 90% confidence interval of [678;859]. The Weibull
modulus was m = 3.5 [2.4;4.4] (Fig. 1). The fracture toughness
of the SEVNB specimens was KIc = 6.7 ± 1.6 MPam0.5. The
SEM analysis of the fracture surfaces of the flexural strength
specimens revealed homogeneous cross-sections. Only single
larger defects were detected on a few specimens (Fig. 2). These
process-related defects were a result of single clogged nozzles
that were either dried up or blocked by agglomerates during the
printing process (Fig. 3). It was possible for crack-free compo-
nents to be built up on the centimeter scale after optimization of
the drying and cleaning process. Based on a CAD file, three-
dimensional components of the size of a crown, with its charac-
teristic occlusal surface topography, were built up (Fig. 4).
DISCUSSION
In contrast to results of studies published previously (Zhao
et al., 2002; Noguera et al., 2005; Lewis et al., 2006), not only
thin structures of some 100-µm thickness, but also components
of high shape accuracy can be produced by direct inkjet print-
ing. It was demonstrated that it is possible, by this technology,
Figure 3. Single nozzle blocked by agglomerated particles.
Figure 4. SEM micrograph showing the 3D-printed occlusal surface of
a dental crown.

676 Ebert et al. J Dent Res 88(7) 2009
to build up dense three-dimensional components of the size and
shape of a dental crown out of high-strength zirconia ceramics.
Although the microstructures of the printed and fired samples
were not completely free of process-related defects, the obtained
density was at 96.9% of the theoretical density—high enough to
provide mechanical properties (S0 = 763 MPa, KIc = 6.7 ± 1.6
MPam0.5) that can be compared with those of conventionally
produced 3Y-TZP via cold isostatic pressing (NN, 2006).
The strong scattering of the strength (m = 3.5) is attributed to
the clogging of single nozzles during printing, which was dem-
onstrated on some single specimens and was responsible for the
decreased strength of those samples. However, strengths of up
to 1200 MPa were achieved on those specimens that contained
no large defects due to clogged nozzles during printing. The
strengths of those specimens without process-related large
defects were determined by the inherent microscopic flaw dis-
tribution of the zirconia material itself.
It is remarkable that the developed ceramic suspension, with
27 vol% of solid content, was printable through original HP
inkjet nozzles (diameter: approx. 28 µm), although the system
had been developed originally for inks with a solid content of
less than 5 vol%. The successful printing arose from the nano-
scaled ceramic powder and the tailored additive system, which
has been described in more detail elsewhere (Özkol et al.,
2009). Moreover, the build-up of crack-free three-dimensional
parts of the size presented was obtained due to the well-designed
drying and cleaning system.
Regarding adjusted drying, pre-heating of the substrate ensured
the avoidance of temperature gradients leading to internal stresses
and bending of layers during drying. Concerning the cleaning
system, an ethanol content of < 10 wt% in aqueous solution was
determined as sufficient in terms of nozzle cleaning, without
intense evaporation and the formation of bubbles on the printed
surface during drying that would lead to later delamination.
In the next step of development, an advanced 3D printer will
be used. The most important new feature will be the control of
each single nozzle of the print heads by special software,
whereby, in case of sudden nozzle-clogging during printing, the
cartridge can be immediately moved to the cleaning unit, where
the clogged nozzle will be re-opened, and the printing process
can proceed. Moreover, this advanced printer will consist of
more than one cartridge, to print support material in parallel. This
will allow for the manufacture of not only a three-dimensional
occlusal surface of a restoration, but also complete crowns and
bridges with hollow spaces. It should be noted that shrinkage
due to drying or sintering can be a critical issue in individually
made dental ceramic prostheses. Isotropic shrinkage was deter-
mined on rectangular dense specimens. This may differ when
cap structures with various wall thicknesses are to be printed.
Therefore, the optimization of the drying process and a tailored
multiple-stage sintering process, as well as an advanced design
and scale of the three-dimensional data, are additional steps for
further development.
ACKNOWLEDGMENT
This work was supported by Heraeus Kulzer, Hanau, Germany.
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