Proceedings of International Forum for Green Asia 2019
5th International Exchange and Innovation Conference on Engineering & Sciences (IEICES 2019)

GA22 / P34
Identification of a Novel Fracture Mechanism in a Bioceramic/Biopolymer Composite
1

John Duckworth1, Mitsugu Todo2
Interdisciplinary Graduate School of Engineering Science, Kyushu University
2
Research Institute of Applied Mechanics, Kyushu University

Abstract: In order to bring on the next generation of composite biomaterials, the nature of their fracture mechanisms
needs to be understood. Hybrid materials of ceramic and polymer are considered particularly promising. As such,hydroxyapatite beads were formed and included in a PLLA matrix to form a novel biomaterial. The mechanical propertiesof the composite were measured by 3-point bending tests to determine fracture toughness and elasticity. The mechanismof fracture was investigated using optical microscopy, SEM and EPMA. It was seen that increasing bead concentrations
caused a reduction in fracture toughness from 34±7 Pa.m-1/2 for blank PLLA, to 18±1 Pa.m-1/2. A novel fracture mechanism
was identified to explain this change. Bead presence caused two distinct regions of fracture, one high energy and one low.
The exact nature of this mechanism is still under investigation, but SEM and EPMA analysis rules out obvious causessuch as void formation and calcium leakage into the polymer matrix.
Keywords: Hydroxyapatite; Tricalcium Phosphate; Biomaterial; Ceramics; PLLA; fracture mechanism.
1. INTRODUCTION
This investigation fabricated and then characterized
micrometer scale beads of hydroxyapatite (HA), thencombined them in a poly L-lactic acid (PLLA) matrix toform a composite biomaterial. This material is hoped tocombine the high strength and fracture toughness of
PLLA with the biointegrating properties of HA for use inbone plate implants [1]. Mechanical testing showed thatthe beads reduced the fracture toughness of pure PLLA,
and the mechanism for this decrease was investigated. 3point notched bending tests, electron probe microanalysis (EPMA), optical microscopy and scanningelectron microscopy (SEM) were all used to investigate
the fracture behaviour of the beaded PLLA material andexplain the observed behaviour.

Fig. 1. Geometry of single edge notched bending testspecimen, from ASTM-399.
Fracture surfaces were analyzed using optical and SEM
microscopy to locate the types of fracture which may
have occurred in each area, and where and how theenergy was dissipated during crack propagation.
EPMA was used to track the concentration of calcium
across the bead/polymer interface, as a measure ofpolymer inclusion into the semi-porous beads andcalcium leakage into the surrounding polymer.

2. METHODS
A two-step process was used to produce HA beads [2].
Firstly, α-tricalcium phosphate (α-TCP) powder and 1%sodium alginate solution were blended and subjected toultrasonic dispersion to create a smooth slurry, thendripped into a 4°C 1% calcium chloride solution througha 30G gauge needle to produce beads of fixed radius. The
preliminary beads of α-TCP confined in the crosslinked
calcium alginate were dried at room temperature, before
firing at 1300°C for 5 hours to produce the final HAbeads. X-ray diffraction and SEM showed the beads to infact be semi-porous biphasic calcium phosphates, amixture of α-TCP and HA, with a mean bead radius of
750±50 µm.
Inclusion into the PLLA matrix was done using a brassmold, square grid and hot press. After hot pressing thematerial was cut to shape, then notched using a 1mm
diamond tipped saw and razor blade, as outlined in theindustry standard, to produce a sharp crack tip, shown inFig. 1 [3]. Materials with varying concentrations of beads
were used, from 0% to 10% by volume.
Three-point bending tests were used to determine elastic
modulus for each material group.
Three-point notched bending tests were used to calculate
the critical stress intensity factor, KIC, for each group,
using the method outlined in ASTM E399.

3. RESULTS AND DISCUSSIONFirstly, the material was successfully fabricated. Thebeads showed good distributions in the x,y and z axes,
and seemed to be well incorporated into the matrix.
Figure 2 shows the surface under an optical microscopeat 2 times magnification.
Elastic modulus was found to be invariant within error
across all bead-containing groups, and only showed aslight increase compared with blank PLLA, from 690±70
MPa to 550±30 MPa on average, indicating that the strainenergy was transferring primarily through the polymermatrix.
KIC however, was seen to decrease linearly with beadconcentration as shown in Fig 3a. This indicated that thebeads were causing a fracture mechanism by means otherthan stiffening the entire material, whilst still causing amore brittle fracture, indicated by the sharp angle of theload/displacement curve in Fig 3b.

- 59 -Fig 5. a) SEM image of two regions of fracture surface,b) upper brittle, c) lower ductile fracture.

Fig. 2. HA beads evenly distributed in PLLA matrix afterfabrication.

It was noted that blank PLLA fracture surfaces showed
uniform hackle patterns across all scales, indicating
ductile deformation before fracture [4]. Beaded samples
however only showed this behaviour in a region below
the bead. The region above the bead showed instead a
sharp fracture plane, indicating fast, brittle fracture in thisregion, shown in Fig 5. The limit of this region was a
distinct slope away from the middle of the bead, towards
a plane at the base of the bead. It is therefore suggested
that the bead localizes a region of high stiffness in thesurrounding polymer, allowing strain energy density toincrease locally.
Upon fracture initiation, this energy is quickly released
through the stiffened region to form a brittle fracture,
which transitions to a ductile fracture away from the bead.
4. CONCLUSIONS
The discovery of this fracture mechanic suggests several
ways to improve the fracture toughness of the material
further, to make it better suited for bone plate
applications. These include; increasing bead porosity,
decreasing bead stiffness, decreasing the local stiffness
around the bead by altering the polymer bead interaction,increasing the radius of the bead to better integrate withthe polymer and reduce local stress concentrations.
It is also necessary to test the material for cell affinity andbiocompatibility, to see how osteoclasts will react to thenew material surface. Most importantly, these results
indicate that it is feasible to create a material with fracture
toughness comparable to blank PLLA, but with theincreased biocompatibility of HA, and also give us
directed avenues of approach for achieving this.

Fig. 3. a) KIC of material with bead conc. b) Sample
load/displacement curves of each group

5. REFERENCES[1] S. B. Sulaiman et al., TCP-HA bone scaffold as
potential candidate for the formation of tissue
engineered bone Indian J Med Res., 137 (6) (2013)
1093–1101.
[2] M. Vallet-Regi, L. M. Rodriguez-Lorenzo and A. J.
Salinas, Synthesis and characterization of calcium
deficient apatite, Solid State Ionics, 101 (1997) 12791285.
[3] ASTM E399-17, Standard test method for linearelastic plane-strain fracture toughness ASTM
International (2017).
[4] M. Todo, S. D. Park, T. Takayama and K. Arakawa,Fracture mechanics of bioabsorbable PLLA/PCL
polymer blends, Eng Fract Mech. 74 (12) (2007) 18721883.

Fig 4. a) SEM image of bead along crack plane and b)
EPMA Ca conc. of bead at fracture surface.
Imaging showed that the bead polymer interface was notseparating and creating voids for crack growth andpropagation as might be expected, but instead consistentshearing of the beads along the crack plane wereobserved, shown in Fig 4a. This indicated polymer
intrusion into the beads, confirmed by EPMA, shown inFig 4b. It also suggested another mechanism isresponsible.

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