Vla Sea 2015

Accepted Manuscript

Title: Control of Structural and Mechanical Properties inBioceramic Bone Substitutes via Additive ManufacturingLayer Stacking Orientation

Author: Mihaela Vlasea Robert Pilliar Ehsan Toyserkani

PII: S2214-8604(15)00014-7DOI: http://dx.doi.org/doi:10.1016/j.addma.2015.03.001Reference: ADDMA 31

To appear in:

Received date: 21-9-2014Revised date: 2-2-2015Accepted date: 5-3-2015

Please cite this article as: Vlasea M, Pilliar R, Toyserkani E, Controlof Structural and Mechanical Properties in Bioceramic Bone Substitutesvia Additive Manufacturing Layer Stacking Orientation, Addit Manuf (2015),http://dx.doi.org/10.1016/j.addma.2015.03.001

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Control of Structural and Mechanical Properties in Bioceramic Bone

Substitutes via Additive Manufacturing Layer Stacking Orientation

Mihaela Vlasea1 Robert Pilliar2,3 Ehsan Toyserkani1

1-University of Waterloo, Department of Mechanical and Mechatronics Engineering, Waterloo, Ontario, N2L 3G1

2-Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, Ontario, Canada M5S 3G9

3-Faculty of Dentistry, University of Toronto, Toronto, Ontario, M5G 1G6, Canada

Submitted to: Journal of Additive Manufacturing

Submission Date: September 21, 2014

Number of Pages: 21

Number of Figures: 8

Number of Tables: 3

Contact Author: Mihaela Vlasea, PhD

Address: Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Ontario, N2L 3G1 Canada

Email: [email protected]

Phone: 1-519-722-1368

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Control of Structural and Mechanical Properties in Bioceramic Bone

Substitutes via Additive Manufacturing Layer Stacking Orientation

Abstract: Additive manufacturing (AM) is a promising approach for fabricating structures to

serve as bone substitutes, or as biomaterial components in biphasic implants for repair of

osteochondral defects. In this study, the three dimensional printing (3DP) AM process was

investigated to determine the effect of powder layer orientation on mechanical and structural

properties of fabricated parts. Five types of standard cylindrical parts were manufactured via AM

with 0, 30, 45, 60 and 90stacking layer orientations relative to the vertical z-axis of the print

bed, using amorphous calcium polyphosphate (CPP) powder of irregular particle shape, average

aspect ratio 1.70 and particle size between 75-150 m. It was concluded that layer orientation

had an effect on porosity and compressive strength, based on induced powder particle orientation

in the green part during powder layering. The resulting bulk porosity values ranged between 30.0

2.4% to 38.2 2.7%, while the compressive strength ranged between 13.50 1.95 MPa to

45.13 6.82 MPa. The orientation with the highest compressive strength was 90, while

orientations with the weakest compressive strength were 0 and 45. Based on these results, it

was established that AM-made parts are structurally and mechanically anisotropic. The stacking

layer orientation which results in the highest strength performance along a preferred loading

orientation can be implemented to further optimize mechanical strength of constructs along the

maximum loading direction.

Keywords: Additive manufacturing, 3D printing, oriented layering, calcium polyphosphate,

bioceramic bone substitutes

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1 Introduction

Porous bone substitutes serve as an artificial matrix providing the mechanical and structural

template for new bone formation. Such porous structures must be biocompatible and ideally

should promote osteogenesis by being osteoconductive and osteoinductive while degrading in-

vivo at an appropriate rate to allow their replacement by newly-formed bone [1,2]. The porous

structure must also be designed to have an anatomically accurate three dimensional (3D) shape

in order to maintain a natural contact load distribution post implantation [3] and initial internal

structure in terms of micro- and macro-interconnected porosity to promote cell proliferation,

metabolic exchange and vascularisation [4]. Furthermore, the mechanical strength and porous

architecture of the bone substitute should ideally be designed to match the load bearing

requirements at the substitution site and to promote the appropriate bone growth cues as dictated

by mechanostat theory [5]. Ideally, this would suggest anisotropic structural and mechanical

characteristics throughout the construct, depending on the implantation site.

Powder-based additive manufacturing (AM) utilizing three dimensional printing (3DP) is a very

promising fabrication method for making scaffolds or porous constructs in the field of tissue

engineering and regenerative medicine, and specifically for bone substitute fabrication [2,610].

Using this approach, the anatomical shape and internal porous configuration of the implant is

first designed in a computer-aided design (CAD) environment. Subsequently, the CAD model is

converted into image slices and the scaffold is manufactured in a layer-by-layer fashion by

repeated stacking powder layers and subsequently injecting a binder solution at locations dictated

by the cross-sectional image of the part layer to be formed as shown in Figure 1. The resulting

product is referred to as a green part. For ceramic structures, such as the calcium polyphosphate

(CPP) used in the present study, further post-green part processing, usually involving thermal

annealing, is necessary to achieve required strength properties and structural characteristics (i.e.

% porosity, pore size and interconnectedness) of the final part [4,8].

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Figure 1 Process description for conventional AM via powder-based 3DP

Prior investigations by a number of groups have focused on fine-tuning 3DP process parameters

to examine their effect on mechanical and structural properties of the final product. The

appropriate binder-powder material systems compatible with both the additive manufacturing

process and the biological requirements of the bone substitute were investigated [2,11]. Other

studies have examined the effect of the liquid binder used during powder lay-up and green part

formation with regard to its chemical composition [12,13], concentration [10,12,13] and

saturation levels [1416] to determine appropriate binder-powder interactions that would

produce samples with a desired compressive and flexural strength and porosity [10,12,13,15].

Other studies have focused on defining powder composition and blends to yield better powder

flow characteristics [17,18] and improved mechanical performance of the final structure [12].

The effect of powder particle size and its effect on physical, structural and mechanical properties

of AM-formed constructs has also been studied [19]. Layer thickness is another parameter that

can be controlled during the 3DP process, with a range of layer thicknesses having been used for

preparation of samples in order to study the effect on mechanical properties [15]. These studies

concluded that, in general, the flexural and compressive strength performance is inversely

proportional to layer thickness [15,20]. The effect of including open or closed macro-channels

within the porous structures during the 3DP processing and its effect on mechanical strength and

biological response of porous constructs has also been studied [12,19,2124]. In this context, a

new type of 3DP platform has been investigated, capable of creating interconnected macro-

channels with a feature size below 500 m within the part while avoiding the risk of having

particles trapped within the macro-channels [25,26].

In 3DP, due to the nature of the layer-by-layer manufacturing process, the effect of layer

stacking orientation within the part may influence the physical, structural and mechanical

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properties of constructs so formed. Shanjani et al. [27] and Zhang et al. [20] studied the effect of

layer orientation along the direction of the printing axes and concluded that mechanical strength

characteristics were related to orientations used in forming parts. This effect has not been

explored in detail, as the two previously reported studies focused only on orientations along the

printing axes z, y, z, without considering intermediate orientations. In the present study, to better

understand the correlation between layer orientation and mechanical properties, standard

cylindrical parts with 0, 30, 45, 60, and 90 layer stacking orientations with respect to the vertical

axis (z-axis) in the build chamber were fabricated and characterized in terms of porosity, bulk

density, and compressive strength. It is proposed that the stacking layer orientation within a part

which results in the highest strength can be aligned during part fabrication in the direction of

anticipated maximum loading, if this is known during the design stage. Or, contrarily, the

orientation resulting in the lowest strength can be avoided from coinciding with expected high

load-carrying directions.

2 Materials and Methods

2.1 Materials

In this study, the powder material used in the AM process was amorphous calcium

polyphosphate (CPP) powder formed as reported in earlier studies [28]. The powder is

characterized by an irregular particle shape, an aspect ratio 1.70 and particle size range

between 75-150 m. This powder was mixed with polyvinyl alcohol (PVA) powder (Alfa Aesar,

Ward Hill, MA) of particle size < 63 m at a composition ratio of 90 wt% CPP and 10 wt%

PVA. To ensure a homogeneous blending, the CPP and PVA powders were mixed for 4 hours

using a rotating jar mill (US Stoneware, OH). The PVA powder served as an additional binding

agent in combination with the liquid binder (ZbTM58) (3D Systems, Burlington, MA) which also

served as an aqueous solvent for the PVA particles to give acceptable green strength to samples

formed by 3DP.

2.2 Sample Fabrication

Previous studies reported on preparation of porous CPP samples and the effect of CPP powder

size and method of porous construct preparation have been reported for conventional gravity

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sintering and AM using 3DP [11,27]. In vivo studies of samples so-made have been reported

[29,30].The focus of the present study is the determination of how the layer orientation affects

mechanical strength characteristics of samples. To achieve this, cylindrical test samples of 4 mm

diameter and 6 mm height were manufactured with layer orientations at 0, 30, 45, 60, and 90

with respect to the vertical axis in the build compartment as seen in Figure 2. A 3DP machine

(ZPrint 310 Plus, 3D Systems, Burlington, MA) was used to manufacture the parts in a layer-by-

layer fashion. The layer thickness was selected to be 175 m with the AM layer-by-layer powder

spread process being undertaken at a 38C. The cylindrical test parts were designed using

computer-aided design (CAD) software (SolidWorks Corp., Concord, MA) and imported into the

3D printing software (ZPrintTM) in stereolithography (STL) file format. The cylindrical test parts

were then oriented within the build bed, as seen in Figure 2, with n=10 parts prepared for each

orientation. The green parts were then air-annealed (Lindberg/Blue M, ThermoScientific) with a

50% R.H. in-furnace environment using a pre-established heat treatment protocol [31]. The

annealing cycle used has been reported elsewhere [11,31], with a heat-uprate of 10C/min from

room temperature to 400C, 2 h dwell at 400C to burn off organic binder constituents, continued

heat-up at 10C/min to 500C and then5C/min to 630C, hold for 1 h (this is the so-called Step-

1 sinter [31]),and then increasing the temperature to 950C at 10C/min and holding at 950C for

1 h (to achieve complete CPP crystallization and final microstructural development [31]).

Samples were then allowed to furnace cool to room temperature.

Figure 2Parts printed with 0, 30, 45, 60, 90 layer orientation, arranged in the build compartment.

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2.3 Porosity Characterization

The bulk porosity of the sintered cylindrical samples was determined as described previously

[11] by ethanol displacement using Archimedes principle (ASTM C373 standard). An ethanol

bath kit (Sartorius YDK01 Density Determination Kit, Sartorius AG, Goettingen, Germany) and

a precision micro-scale balance (APX-203, Denver Instrument, Bohemia, NY, US) were used to

first determine the dry weight of each specimen. Each specimen was then immersed in

ethanol and sonicated (VWR Ultrasonics Cleaner B2500A-DTH, VWR International, West

Chester, PA, US) for one hour at 30C and soaked for another hour. Subsequently, the weight of

the specimen suspended in ethanol was measured. Each specimen was then removed from

ethanol, dabbed with a lint free cloth to remove excess ethanol and weighed immediately after in

order to determine the wet weight, . The bulk porosity and bulk density were

determined based on the formulae below, (Equations 1 and 2), where the density of ethanol

at room temperature equals 0.785 g/cm3 and the theoretical density of non-porous CPP

equals 2.850 g/cm3[28]. A population of n=10 samples was used for this data.

(1)

(2)

2.4 Structural Characterization

The microstructure of final sinter-annealed cylindrical CPP samples with different layer

orientations was examined using secondary electron emission scanning electron microscopy

(SEM, JSM-6460, Jeol, Akishima, Tokyo) operating at 20 kV accelerating voltage. In

preparation for SEM examination, the samples were sputter-coated with a 10 nm thick gold layer

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to make them electrically conductive (Desk II, Denton Vacuum, LCC, Moorestown, NJ, USA).

One representative sample from each category was considered for SEM analysis.

2.5 Powder Size Characterization

The powder size and aspect ratio were determined by placing a thin layer of powder on a

conductive substrate and using secondary electron emission scanning electron microscopy (SEM,

JSM-6460, Jeol, Akishima, Tokyo) at 20 kV accelerating voltage to view particle images. Five

images were captured at 100x magnification and three at 200x magnification. The particle

dimension, across the longest orientation (length) and along the dimension perpendicular to this

direction (width), were recorded using the SEM AnalySIS tool in order to estimate particle

aspect ratios.

2.6 Uniaxial Compression Characterization

Eight samples (n=8) for each different layer orientation were tested in uniaxial compression.

Testing was conducted on dry samples at room temperature using a 1-kN load cell at a loading

rate of 0.2 mm/min (Instron 5548 Micro-Testing, MA).

2.7 Statistical Analysis

Volume % porosity (denoted hereafter as bulk porosity), bulk density of the porous structure, and

compression strength data are reported as means and standard deviation. A one-way ANOVA

single factor analysis of variance was used to evaluate the statistical significance of

measurements, followed by Tukey-Kramer post hoc pairwise comparisons to identify the so-

called honest significant differences (HSD) between classes of samples using STATISTICA

V12 (StatSoft, Tulsa, OK) with p

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where is the failure probability, is the central value or scale parameter showing the Weibull

characteristic strength and is the shape parameter, also known as the Weibull modulus. The

Weibull modulus is a measure of reproducibility of samples [33]. In this work, a population of

eight samples (n = 8) was used for all sample categories for compressive strength data. Equation

4 was used as a probability estimator for the Weibull linear regression, where is the

corresponding rank of the sample measurement.

(4)

3 Results

3.1 Structural Characterization Results

Figure 3 illustrates an SEM image of a sample prepared at a 60 layer stacking orientation. In

Figure 3a), the parallel layer orientations are highlighted by the dashed white lines. Figure 3b)

and Figure 3c) show examples of sinter necks resulting from the post-AM thermal sinter/anneal

treatments used for sample preparation. It can be seen that the particles are well bonded together,

forming an open-pored structure. The SEM images obtained for the other layer stacking

orientations were very similar in nature.

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Figure 3 Images of a CPP sample with 60 layer stacking orientation as viewed under SEM with a magnification of a)x22, b) x100, and c) x300

3.2 Powder Size Characterization

The SEM image analysis revealed the distribution of powder size along the length and width of

particles as shown in Figure 4. The powder had an irregular shape, with a mean aspect ratio

greater than one. The bulk of particles had the longest axis above 150 m (average particle size

of 177 42 m), while the smallest axis normal to this long dimension was on average below

120 m (average particle size of 104 28 m). The average particle aspect ratio was determined

to be 1.70.

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Figure 4 a) The particle size distribution for particle size sieved between 75-150 m, (n=105) b) Scanning electron microscope (SEM) view of powder particles at magnification x250

3.3 Bulk Porosity (i.e. volume % porosity) and Density

The bulk density and porosity results of the cylindrical samples with oriented layers at 0, 30, 45,

60, 90 were calculated using the Archimedes method for density determination. The results are

summarized in Table 1. Figure 5 illustrates the bulk porosity measurements for the different

layer orientation samples indicating statistically significant differences. The maximum bulk

porosity occurred at an orientation of 0 and 45, with values of 38.22.7% and 37.6 3.1%

respectively. The minimum bulk porosity corresponded to samples prepared at an orientation of

90, where the bulk porosity value was equal to 30.0 2.4%.

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Table 1 Bulk porosity and bulk density characteristics for cylindrical CPP samples printed with layer orientations of 0, 30, 45, 60, and 90 respectively, (n=10)

3DPLayer Orient.

(p < 0.05) (p < 0.05)0 38.2 2.7 1.76 0.08

30 32.2 2.8 1.93 0.0845 37.6 2.1 1.78 0.0960 34.0 3.2 1.88 0.0990 30.0 2.4 2.00 0.07

Figure 5 Bulk porosity characteristics of cylindrical samples with layer stacking orientations 0, 30, 45, 60, and 90 respectively (n=10). *(p

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90 45.13 6.82 7.02 (0.90)

Figure 6 illustrates the compressive strength measurements showing statistically significant

differences. Figure 7 shows the linear regression computed for the Weibull distribution used to

predict the probability of failure of ceramic parts.

Figure 6 Compression strength characteristics of cylindrical samples with layer stacking orientations 0, 30, 45, 60, and 90 respectively, (n=8), (p

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Figure 8 Fracture surface propagation of samples with orientations 0, 30, 45, 60, and 90 respectively. These images are representative of more than 50% of the samples in each category

The samples were analyzed qualitatively under the microscope to view the orientation of the

fracture path after compression testing to failure. From representative illustrations shown in

Figure 8, for samples with 30, 45, 60, and 90, the failure occurred, not surprisingly, along

planes parallel to the stacked powder layers. For the 0 orientation, fracture resulted in an

irregular v-shaped pattern.

4 Discussion

In this study, the optimization of the additive manufacturing process focused on establishing an

orientation that would yield improved mechanical strength characteristics under given loading

conditions, while providing the interconnected porosity required for implant stabilization by

bone ingrowth throughout a bone substitute. Five categories of test samples, 4 mm in diameter

and 6 mm in height, were fabricated, with different layer orientations at 0, 30, 45, 60, and 90

with respect to the vertical z -axis in the build compartment (Figure 2). Experimental results

showed that layer orientation had a statistically significant impact on porosity and compressive

strength of samples. The results showed that the 45-oriented samples had the lowest

compressive strength, (13.43 4.60 MPa), while the 90-oriented samples showed the highest

compressive strength, (45.13 6.82 MPa). This considerable difference in strength corresponded

to differences in porosity, whereby the 45-oriented samples had a high bulk porosity value (37.6

2.1%), while the 90 orientation samples, displaying the highest strength, had the lowest

porosity (30.0 2.4%). It is expected that higher porosity would reduce the overall mechanical

strength of a sample.

The difference in compressive strength can be attributed to the additive manufacturing process,

where the orientation of the irregular-shaped CPP particles within each powder layer is

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influenced by the action of the counter-rotating roller as described by Shanjani et al. [27]. The

results shown in Figure 4 illustrate that the CPP powder used in this work has an irregular shape,

with one distinct longer axis. The counter-rotating roller compacts the powder and aligns the

irregular-shaped particles having an aspect ratio larger than 1 with the longest axis generally

parallel with the build plane. As reported elsewhere [27], the resulting particle-particle packing

favors inter-particle contacts at particle ends within the build planes (i.e. at smaller radius of

curvature particle profile contacts, with sharper contact zone). The sinter necks are stronger

when the contact profile between particles has a sharper contact zone, where the contact angle

between particles is larger, therefore the sinter necks between particles within each build plane

(parallel to xy) are expected to be stronger than the sinter necks between build planes (along z

axis) [27,34].

For compressive loading of homogeneous samples displaying isotropic properties, maximum

shear stresses will act at 45 to the applied force direction resulting in shear fracture along this

direction characterized by an oblique fracture path of failed samples. The observed fracture path

for the 45-oriented samples follows this direction which also corresponds to the weakest sinter

neck direction (i.e. that corresponding to larger radius of curvature contact points between

particles in general). These are also the lowest fracture strength samples. The 30- and 60-

oriented samples approach this condition and in view of the irregular particle shapes, their

measured lower strengths may be similarly explained. Fracture path directions shown in Figure 8

correspond closely to these weakest sample plane directions. This is further expanded on as

discussed below using stress tensor equations summarized in Table 3. In contrast, the 90

orientation corresponds to the weakest direction being parallel to the applied force direction, so

that crack propagation and failure is likely along this direction. The 0-oriented samples develop

highest normal stresses along the weakest planes (Table 3) resulting in transverse fractures (or

more likely, complex fracture paths with both transverse and oblique fracture path segments).

This is reflected in the observed fractures displayed by these samples. In porous ceramic

structures such as CPP, sinter necks represent stress concentration sites, therefore the stronger

the sinter neck, the better the resistance to crack propagation. This means that structurally, the

parts fabricated via additive manufacturing are orthotropic parallel and normal to the z- build

directions.

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Stress tensor theory for uniaxial compression [35] allows shear stress and normal stress at an

orientation to be computed based on Equation 5 and Equation 6, where the only load acting on

the body in uniaxial compression is , also denoted by for compressive loading

.

(5)

(6)

The shear stress and normal stress computed for each print stacking orientation are

summarized in Table 3. These theoretical results, along with the structural anisotropy introduced

by additive manufacturing, explain the experimental behavior of the oriented parts shown in

Figure 6 and Figure 7, and summarized in Table 2. The parts manufactured in the 0 and 45

orientations had the lowest compressive strength, 13.50 1.95 MPa and 13.43 4.60 MPa,

respectively. This occurs because the layer orientations within those parts coincide with the

direction perpendicular to the principal normal stress for , and along the direction of the

principal shear stress for , respectively as seen in Table 3. The 90 plane, based on the

stress tensor theory, does not experience shear or normal stress, therefore the loading is

distributed along the parallel stacked build planes, where the inter-particle contact results in the

strongest sinter neck formation, therefore these parts show the highest compression strength of

45.13 6.82 MPa. When comparing the compressive strength of the 30 orientation, 20.60

6.23 MPa, and 60 orientation, 28.19 2.46 MPa, it can be seen that the 30 orientation can

sustain a lower compressive strength, as the stacked build planes along the 30 orientation

experienced a higher normal stress than compared to the 60 orientation. The experimental

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results, supported by the stress tensor theory and the structural anisotropy hypothesis,

demonstrate that the 3DP process can significantly influence the mechanical strength of parts.

Table 3 Normal and shear stress distribution relative to layering planes for samples manufactured with stacked layers of orientation 0, 30, 45, 60, 90 deg respectively

Print orientation () 0 30 45 60 90

Stress plane angle () 90 120 135 150 180

Shear stress along orientation

0 0

Normal stress to orientation

0

In this study, there is an inversely proportional correlation between the measured porosity and

compressive strength, where the lowest porosity of 30% corresponded to the highest mechanical

strength of 45 MPa, and the highest porosity of 38% and 37% corresponded to the lowest

mechanical strength of 13.5 MPa and 13.4 MPa respectively. This finding is in accordance

with the literature, where an increased porosity is ideal for promoting bone ingrowth into bone

substitutes, however this comes at the cost of reducing mechanical properties [33,36,37]. The

reason for the observed volume % porosity difference between sample categories is not clear but

may be related to the level of binder deposited per volume of sample during the powder layer

build-up.

The porosity of all samples is between the measured porosity of trabecular bone (50-90%) and

cortical bone (3-12%) [33]. In addition, the compressive strength of each category of samples is

between the measured ultimate strength of trabecular bone (4-12 MPa) and cortical bone (130-

180 MPa) [7]. From a previous study [27], the pore size for CPP samples appears independent of

layer orientation, and ranges between 20-150 m in size, with a mean of ~56 m, which is in an

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acceptable range for bone substitutes [33,38,39]. The range of compressive strength obtained in

this study for CPP samples at 90 orientation (45MPa) is similar in value with values obtained

in other studies of 3D printing of CPP at the same orientation (50 MPa) [27]. Furthermore, the

values of Weibull modulus reported for the CPP samples fabricated in this work (3-12) are

comparable with reported Weibull moduli reported for calcium phosphate samples in the

literature (3-9)[40] and (5-10)[27], and indicate good reliability in measurements presented in

this work, as the Weibull modulus is inversely proportional with data scatter. A higher

correlates with a good distribution of porous distribution within the part and good repeatability

[41].

Based on the results shown in this study, the powder-based additive manufacturing process using

CPP as the raw material is a promising approach in manufacturing implants as bone substitutes

or as components of biphasic tissue-engineered constructs for osteochondral defect repair.

Furthermore, the AM-made structures are anisotropic in nature, offering the possibility of

aligning the layer stacking orientation during part fabrication perpendicular to the path of

maximum anticipated compressive loading based on kinetic and kinematic data. This benefit will

be explored in a future work. The layer orientation within the part is an important design

parameter in manufacturing bone substitutes, as the loading kinetics and kinematics on the part

can be estimated and layer orientation can be tuned to ensure greater probability of implant

survival under peak loading conditions.

5 Conclusion

In this work, the powder-based additive manufacturing process was studied to quantify the effect

of powder layer orientation on mechanical properties of the generated parts. It has been shown

that the layer orientation within the part has a significant influence on the compressive strength

of the resulting structure. Thus layer orientation is an important optimization parameter in the

additive manufacturing design cycle. Furthermore, the results shown in this study can be used to

tune the mechanical strength of an implant along the orientation of maximum loading, if this is

known during the implant design stage. It was concluded that samples made with the 90

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stacking orientation have the highest compressive strength (45.13 6.82 MPa), whereas those

made with the 0 and 45 stacking orientations exhibit the weakest compressive strength (13.50

1.95 MPa).

Acknowledgment

The authors appreciate the funding support received from The Natural Sciences and Engineering

Research Council of Canada (NSERC), grant # RGPIN312074 37063.

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