Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia StrategyP-29

The effect of fibre length on cement/fibre integration and mechanical propertiesof a DCPD/PLCL injectable composite biomaterialJohn Duckworth1, Mitsugu Todo2
1

Interdisciplinary Graduate School of Engineering, Kyushu University
2

Research Institute for Applied Mechanics, Kyushu UniversityAbstract
The effect of fibre length in a cement/fibre composite biomaterial on cement/fibre integration, ultimate
compressive strength and compressive elastic modulus was investigated. Dicalcium phosphate dihydrate/poly(lactide-co-ε-caprolactone) 75:25 was the composite material investigated, at a 25%w/w ratio.
A direct relationship between increased fibre length and increasing void volume in the material was
determined using micro-CT imaging and Finite Element modelling, attributed to entanglement of the fibres
causing poor cement/fibre integration. This in turn was used to explain a measured decrease in the compressive
strength of composites with longer fibres, from 32±2 MPa to 24±3 MPa for composites containing 400±100μm to 1280±350 μm length fibres respectively. It was also found that the compressive elastic modulus of all
cement/fibre composites was far lower than that of blank cement alone, from 1.3±0.2 GPa to 270±70 MPa.
However, no correlation could be drawn between compressive elastic modulus and void volume or fibre
length, as any fibre presence at the set weight ratio had a similar effect.
than that of either cortical or cancellous bone ataround 100 MPa and 50 MPa respectively [2,5].
This stiffness differential along with low
compressive strength causes significant failure of
the cement in any in vivo situation.

1. Introduction
Dicalcium phosphate dihydrate (DCPD) is apromising material for use as a bioresorbable,
injectable bone cement in a wide range oforthopedic surgeries. An anhydrous mix ofprecursor reactants, such as β-tri calcium
phosphate (β-TCP) and monocalcium phosphatemonohydrate (MCPM) in glycerol can be storedindefinitely in liquid form. Upon exposure toaqueous, physiological conditions, as upon
injection, a reaction occurs to form the hard, bonelike DCPD cement.

In an attempt to improve these properties,
researchers have incorporated fibres into thecement matrix [6]. One promising material, which
has been underinvestigated in this capacity, ispoly(lactide-co-ε-caprolactone) 75:25 (PLCL75:25). The material is known to be bioresorbable,
non-toxic and promoting of cell proliferation, andis already in use in other bioresorbable implanttechnologies available commercially. Its low
compressive elastic modulus of 14.2 MPa also
allows a more flexible composite, more closelyaligned with that of bone [7].

Whilst bone-like in mineral content, themechanical properties of DCPD cements are verysensitive to preparatory conditions, and in general
have yet to be made rugged enough to allow
medical application. DCPD has a compressive
strength of 10-20 MPa depending on powder/liquidratios and other environmental effects [1,2]. This isat the low end of known cancellous bone strengths
and far below cortical bone strengths [3,4].
Importantly, the compressive elastic modulus of
DCPD cement, at around 1-10 GPa, is far higher

Mean fibre length and cement/fibre integration areknown to be key factors in determining thematerial properties of any such composite material,as they can determine how the cement transfers
shear loads and resists fracturing [6,8]. For PLCL
containing composites, this is even more77

Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-29

important, as the hydrophobic nature of thematerial often prevents strong cement/fibreintegration. Micro-scale, x-ray computer
tomography (micro-CT) has previously been used
as a tool to map fibre distribution and cement/fibre
integration within a given sample in medical
engineering [9].

incubated in phosphate buffer solution (PBS)(Wako Chem., Japan) under 5% CO2 at 37˚C for
48 hours to simulate physiological conditions postinjection.
2.2 X-ray Diffractometry
X-ray diffractometry (XRD) was performed using
filtered CuK-α radiation generated at 30kV and
15mA using a Rigaku MiniFlex II Tabletop XRD(Rigaku, Japan) x-ray diffractometer. Each of thefive samples as well as samples of each reactant
and the product of the reaction were detected. Thiswas done in order to confirm the completion of thereaction, and determine whether the fibre length
present in samples prevented completion.

2. Methods
This study uses PLCL fibres in a DCPD cement
matrix to investigate these relationships on amacro scale.

2.1 DCPD cement/PLCL fibre composite
2.3 Micro-CT imaging

Monocalcium phosphate monohydrate (SigmaAldrich, #BCBP5940V, USA) was combined with
β-tricalcium phosphate (Taihei Chemicals Ltd.,#09090301, Japan) in a 1:1 molar ratio to form thedry powder as suggested by Han et al. [10]. Thiswas in turn combined with glycerol (SigmaAldrich, #SHBG8251V, USA) in a solid/liquidmass ratio of 3.7, to form the anhydrous slurry.

A Bruker SKYSCAN 1176 – High Resolution XRay Microtomograph (Bruker, Belgium) was used
to image samples of cement containing each fibre
type, alongside a blank control. Images were takenusing a 0.5mm aluminium filter at 50kV and
500μA current. The resulting images were accurate
to a scale of 8.8μm. The software MechanicalFinder v7.0 was used to stitch individual images
into three-dimensional Finite Element (FE) models
for analysis.

Raw PLCL fibres were formed using a commercial
melt-spinner (EAST, EA-WA2805, Japan) by
heating PLCL 75:25 pellets (Gunze Ltd., Japan) to155˚C under high rotation. The resulting fibre matswere further processed to create four distinct loose
fibres of varying length and length distribution.
Fibre Type A was fresh PLCL, manually shredded.
Fibre Type B was fresh PLCL shredded into loose
fibres using a mechanical rotary cutter. Type C and
D fibres were formed from PLCL aged in a humid
atmosphere for 3 months to degrade the floss byhydrolysis. Type C was then manually shredded,whilst Type D used the mechanical rotary cutter.
In all instances, n=10. Fibre lengths weredetermined visually, using a field-emission
scanning electron microscope (FE-SEM) (HitachiLtd., S-4100, Japan).

2.4 Determining Cement/Fibre Integration
From the FE models, the external volume, VE wasdetermined. This is the volume assuming the sample
has no voids or fibre content within. Then, the true
volume, VT was determined. The difference
between the two, minus the fibre volume, VF, gives
the volume of voids, VS. The ratio of VS/VE provides
a dimensionless parameter for cement/fibreintegration. Equation 1 summarizes this calculation.
S / VE = ( E − ( T +Anhydrous cement slurries were combined with
PLCL fibres at 25%w/w ratio and mechanically
mixed with a spatula for >5 minutes. This finalslurry was compressed into a Teflon mold using aspatula and Teflon rod. Samples were thenF)) / VE(Eq.1)

2.5 Mechanical Testing
The compressive elastic modulus of each sample
was determined via compression tests using aShimadzu EZ-S Compact Tabletop Testing78

Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-29Machine (Shimadzu, Japan). A flat crosshead andbaseplate were used with a crosshead speed of
1mm/min and a 500N load cell. The region undermeasurement was 5mm in radius and of thickness
measured in situ. Samples were tested wet, directly
after removal from PBS.
Compressive strength testing was performed using
the same Shimadzu EZ-S Compact Tabletop
Testing Machine (Shimadzu, Japan) with a steel
45˚ wedge crosshead against a flat baseplate at
1mm/min crosshead speed with a 100N load cell.
Samples were tested wet, directly after removalfrom PBS.

3. Results
3.1 Mean Fibre Length
The results from the FE-SEM imaging aresummarized in Table 1, establishing the variationin fibre length from the four processing methods.
Table 1: Variation in mean fibre length anddistribution of length for each processing method.
LabelType A
Type B
Type CType D

Mean FibreLength / µm
1280
416
744
445

σ / µmn350
107
238
127

1020
1620Figure 1: XRD data showing conversion of MCPM toDCPD in all four cement/fibre samples.

3.2 X-Ray Diffractometry
Figure 1 gives the XRD summary of each sample.
All cement/fibre composites showed 2θ peaks at
11.8 and 21.1, indicating DCPD present. All also
showed no 2θ peaks at 15.2 and 46.6, indicating no
significant MCPM present, hence a completedreaction. 2θ peaks at 27.9, 31.1 and 34.4 are
attributed to excess β-TCP.

3.3 Micro-CT Images and Void Calculation
VE and VT of each sample was determined using
Mechanical Finder v7.0. Figure 2 visualizes eachvolume. Figure 3 uses the software data andEquation 1 to calculate the percentage volume ofFigure 2: Fig 1: FE models from micro CT showing
VE and VT of a) blank DCPD b) 1280, c) 744, d) 445and e) 416 μm length fibre containing DCPD cement.

79

Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-29

void space in cement with each fibre length, S /VE. As expected, the percentage volume of voids inblank DCPD cement approached zero, indicatingthat fibre presence was the major cause of voidformation.
Decreasing fibre length correlated linearly todecreasing percentage volume of voids within thecomposite. Thus, it is accurate to use VS / VE, thepercentage volume of voids, as a parameter for
fibre/cement integration.

Figure 4: Variation in compressive elastic modulus
for DCPD/PLCL cement/fibre composites withincreasing percentage volume of voids in the cement,
n=10.

Figure 3: S / VE, for various length fibre containing
composites.

3.4 Mechanical Testing
The compressive elastic modulus of DCPD cement
was significantly reduced by fibre content of any
length, from 1.3±0.2 GPa to 270±70 MPa. Figure 4
shows the reduction in compressive elasticmodulus across varying percentage volume ofvoids, S / VE. It seems that no relationshipstatistically exists between percentage empty spaceand compressive elastic modulus, indicating thefibre material itself, or some other variable, is themost important factor for determining compressiveelastic modulus.

Figure 5: Variation in compressive strength of DCPD
cement composites with increasing percentage empty
space, n=10. Trend shows the relationship between
fibre containing cements.

significantly, but increasing S / VE is correlated
linearly to decreasing compressive strength. Thisindicates that fibres that increase the volume ofvoids in a cement are likely to directly weaken theoverall compressive strength of the composite.

Figure 5 shows the variation in compressive
strength for composites with varying S / VE.
Cement containing the fewest voids, at 7.7%v/v,
showed the largest increase in compressive
strength from the blank cement, from 16±3 MPa to
32±2 MPa. All fibre content increased thecompressive strength of the composite

4. Discussion
Micro-CT imaging allows us to calculate thepercentage volume of voids, S, inside a composite
material in a completely non-destructive way. It80Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-29

was shown that shorter fibres are inversely,
linearly correlated to S in DCPD/PLCL
cement/fibre composites. The hydrophobic natureof PLCL, or natural entanglement could beencouraging longer fibres to resist integration. It
has been shown in Figure 5 that the increase invoid volume directly weakens the composite,
decreasing its compressive strength. The best
performing composite materials had void volumesof 7.7%v/v, but lower void volumes with fibre
containing composites were not investigated. As
the relationship appears to be linear, this suggests
even greater compressive strengths than 32±2 MPa
are possible, potentially entering ranges wheremedical applications become possible.

6. Acknowledgements
I would like to thank Mitsugu Todo sensei for his
support and guidance, as well as Nakamuta Sanand Aziza Intan San for their technical advice. Iwould also like to thank Kanji Tsuru sensei andTakaaki Arahira sensei at Fukuoka Dental Collegefor allowing me access to their x-ray laboratory,
which made this work possible. I would also liketo thank Green Asia for their financial andadministrative support.
References[1] Ginebra MP (2008) Calcium phosphatebone cements. Woodhead Publishing Ltd.,
Cambridge.
[2] Charriere E, Terrazzoni S, Pittet C,
Mordasini Ph, Dutoit M, et al. (2001)
Mechanical characterization of brushite
and hydroxyapatite cements. Biomaterials
22: 2937-2945.
[3] Carter DR, Hayes WC (1977) The
compressive behaviour of bone as a twophase porous structure. Clin Orthop Relat
Res 59A: 954-962.
[4] Burstein AH, Reilly DT, Martens M(1977) Aging of bone tissue: mechanicalproperties. J Bone Joint Surg 58A: 82-86.
[5] Misch CE, Qu Z, Bidez MW (1999)
Mechanical properties of trabecular bonein the human mandible: implications fordental implant treatment planning andsurgery placement. J Oral Maxillofac Surg
57(6): 700-706.
[6] Canal C, Ginebra MP (2011) Fibrereinforced calcium phosphate cements: Areview. J Mech Behav Biomed 4(8): 16581671.
[7] Kwon IK, Kidoaki S, Matsuda T (2005)
Electrospun nano- to microfiber fabrics
made of biodegradable copolyesters:
structural characteristics, mechanicalproperties and cell adhesion potential.
Biomaterials 26(18): 3929-3939.
[8] Lassila LV, Tezvergil A, Lahdenpera M,
Alander P, Shinya A, et al. (2005)
Evaluation of some properties of two
fibre-reinforced composite materials. Acta
Odontol Scand 63(4): 196-204.

It was also seen that the compressive elastic
modulus depends more on the presence of fibres,rather than baring any relationship to the volumeof void space within the composite. However,whilst showing no relationship with the volume ofvoids in the material, DCPD/PLCL composites all
showed a greatly reduced compressive elastic
modulus, bringing them within the range ofcortical and cancellous bone.

5. Conclusion
This study showed that the modern, nondestructive technique of micro-CT imaging can beused to analyse the internal structure of
DCPD/PLCL fibre composite materials. It wasshown through this method that fibre length
correlated linearly to increasing void space in thecomposite material, and that this has a direct effecton the compressive strength of the material. It wasshown that PLCL fibre presence reduced thecompressive elastic modulus of the material towithin the biological range of cortical andcancellous bone, but held no clear correlation tovoid volume.
Further reduction of void volume in DCPD/PLCL
cements promises to provide stronger composites,
whilst the confirmed reduction of compressive
elastic modulus gives hope of the material’s
application in the near future.

81

Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia StrategyP-29[9] Uzun IH, Malkoc MA, Keles A, Ogreten
AT (2016) 3D micro-CT analysis of void
formations and push-out strength of resin
cements used for fiber post cementation. JAdv Prosthodont 8(2): 101-109.
[10] Han B, Ma P, Zhang L, Yin Y, Tao K, etal. (2009) β-TCP/MCPM-based premixedcalcium phosphate cements. Acta
Biomater 5: 3165-3177.

82