EFFECT OF ADDICTION OF FIBERS OBTAINED BY
“ELETROSPINNING” ON THE MECHANICAL PROPERTIES OF
COMPOSITES BASED ON α-TCP
Jader André Dal Sochio¹, Rafaela Silveira Vieira², Wilbur Trajano Coelho², Luís Alberto dos
Santos³ , Vânia Caldas de Sousa³.
1,2,3
Departamento de Engenharia de Materiais, Universidade Federal do Rio Grande do Sul,
Porto Alegre (RS), Brasil.
E-mail: j_ads21@hotmail.com
Resume: Calcium phosphate cements (CFC) are of great interest in the field of biomaterials
for bone repair due to its bioactivity and moldability "in vivo". However, a major problem in
using this type of cement is its poor mechanical properties, which limits its application. To
improve this properties was investigated addition of polymeric fibers on the properties of
CFC based on alpha-tricalcium phosphate. This work aim to study the effect of addition of
PLGA (poly (lactic-glycolic acid)) obtained by "electrospinning" on the mechanical
properties of composite. Fibers were added in the layers form and dispersed in the cement.
The samples were characterized by scanning electron microscopy, electrospinning, porosity,
density, diametral compressive and evaluation "in vitro" (simulated body fluid). The results
show the increasing of mechanical properties of cement. The objective of this work is the
study of fiber reinforcement in calcium phosphate cement for clinical applications, with
special focus on its mechanical properties.
Keyword: Calcium phosphate, polymeric fibers, PLGA, biomaterials, composite .
INTRODUCION
Calcium phosphate cements (CPC) are able to harden in vivo, through a lowtemperature setting reaction. The products formed in this setting reaction have many
similarities with the mineral phase that constitutes 70 wt% of the bone tissue. However, their
mechanical properties are far from those of the cortical or cancellous bone. Not only in
strength, but especially in toughness, ductility and fatigue resistance. The similarity of CPC
with the bone mineral arises from their origin. Both are obtained by precipitation in aqueous
solutions at physiological temperature. When set, CPC consist of a network of calcium
phosphate crystals, with a chemical composition and crystal size that can be tailored to closely
resemble the biological hydroxyapatite occurring in living bone (Morgan et al., 1997; Ginebra
et al., 2010).
The calcium phosphate ceramics, especially hydroxyapatite, are considered the best
material for the remodeling and reconstruction of bone defects. This preference is mainly due
to their excellent biocompatibility, bioactivity, osteoconductivity and hardening "in situ".
CFC systems studied only based on the -TCP complies with the requirement for the pH
(between 6.5 and 8.0) (Driessens et al., 1997).
The main disadvantage of calcium phosphate cements known is its low mechanical
strength, which can at best equal to the trabecular bone, or one fifth of the cortical bone. To
solve this problem fiber may be utilized. A fiber embedded in a matrix contributes to increase
the effort of supporting the body. The load is transferred from the matrix to the fiber by shear
strain in the fiber-matrix interface. The incorporation of fibers in cement matrices fragile
serves to increase the fracture toughness of the composite by breaking the process of cracking
and consequent increasing its tensile and bending strength (Kelly, 1970).
The cement compressive strength, when no pre-compaction is applied, ranges from 10
to 90 MPa (Ginebra, 2008), the apatitic cements being stronger than brushite cements. These
values overcome those of trabecular bone, which range between 1.5 and 45 MPa (Carter and
Hayes, 1977), or fall in the lower range of the compressive strength of cortical bone, that
varies between 90 and 209 MPa (Ontañón et al., 2000; Burstein et al., 1977). Nonetheless, the
major constraints of the mechanical performance of CPC arise from the intrinsic brittleness
derived from their composition and microstructure. CPC are in fact intrinsically porous
ceramics, with porosities that ranges between 20% and 50% depending on the liquid to
powder ratio used in their preparation (Espanol et al., 2009). Thus, the bending strength
values reported for CPC, typically in the range of 5–15 MPa (Martin and Brown, 1995;
Ginebra et al., 2001) are well below that of cortical bone, which is close to 200 MPa (Currey
and Butler, 1975). With respect to the fracture properties of CPC, Morgan et al. (1997)
reported a fracture toughness of 0.14 MPa for a carbonated apatite CPC, comparable to other
brittle cellular materials such as chalk or Portland cement (Maiti et al., 1984), and far from the
fracture toughness of human cortical bone, 2–5 MPa m (Nalla et al., 2003).
The mechanical limitations of CPC can be balanced with the effects of progressive
remodeling that eventually is expected to lead to the replacement of the CPC by new bone.
However, even if the material is completely transformed in newly formed tissue, at the initial
stages after implantation it would be desirable CPC with high mechanical properties. This has
led to the development of fiber-reinforced CPC. In fact, fiber reinforcement has been
extensively explored in the field of hydraulic cements and concretes for civil engineering and
building applications. The incorporation of fibers into a brittle cement matrix has been proven
to increase the fracture toughness of the composite by the resultant crack arresting processes
as well as the tensile and flexural strengths (Beaudoin, 1990). Fiber reinforcement has proven
also to be effective in other types of brittle cements, such as the acrylic bone cements used for
orthopaedic or dental applications (Schreiber, 1974; Pal and Saha, 1982; Puska et al., 2004).
However, in cements intended to medical applications, such as CPC, specific
requirements arise in the selection of the fibers: on one hand, they must be biocompatible; on
the other hand, they can be used not only as a reinforcement for the cement matrix but also as
pore-generating agents. In this second approach, fibers, in addition to the biocompatibility,
must also be biodegradable.
Theoretical models of the fibers reinforcement in cement systems generally assume that
the fibers are aligned and uniformly distributed in the matrix. It is considered that both, the
fiber and matrix, behaving elastically until failure. Thus, the fiber-matrix interface is modeled
as a uniform and continuous. Studies "in vitro" with fibers of PLGA (poly (lactic-co-glycolic
acid) showed a transient reinforcement of the implant, followed by polymer degradation,
facilitating bone in growth through the macropores. Ideally, the loss of mechanical strength
due degradation of the fiber would be compensated by the formation of bone.
Natural fibers (such as cellulose, sisal, jute, bamboo, asbestos, rock-wool, etc.) and
man-made fibers (such as steel, titanium, glass, carbon, polymers, etc.) have been used for the
purpose of enhancing the mechanical properties of cement, as cracking and micro cracking,
resistance in tensile, shear and bending, ductility, and energy absorption capacity (Naaman,
2007). A high fiber tensile strength is essential for a substantial reinforcing action. A high
ratio fiber elastic modulus to matrix elastic modulus improves stress transfer from the matrix
to the fiber.
In vitro studies (Xu and Quinn, 2002) showed that introduction of randomly oriented 8
mm length yarns at 25% fibers volume provided reinforcement for several weeks, and then
dissolved to create long cylindrical macropores allowing cell infiltration. A threefold increase
in flexural strength (up to 25 MPa) and two orders of magnitude in work-of fracture (up to 3.4
kJ/m2) were reported, but no significant differences were observed in the elastic modulus of
the materials.
MATERIALS AND METHODS
The dibasic calcium phosphate (DYNE 10596, lot 230696) was calcined at 550°C for 5
hours, producing the  - Ca2P2O7. This product was blended with CaCO3 (Quimex QX
258.0500 lot 29721) and calcined at 1500°C for 2 hours, using Maitec 4500W furnace.
The apparent porosity and apparent density was based on ASTM C20 - 00 (2010), by
Archimedes’ principle. The amount of the liquid with 2.5% Na2HPO4 used to obtain a paste of
proper consistency was 0.4mL/g. The mold used was stainless steel containing cavities 10mm
+/- 1mm in diameter and 20mm +/- 1mm. ATS universal testing machine, model 1105C at
0.1mm/min, was used for diametral compressive measurements. The samples were immersed
in SBF (Simulated Body Fluid) and measured at 0, 7 and 14 days of immersion.
For the preparation of fibers, the polymer used was poly (lactic-co-glycolide) (PLGA)
and the method for obtaining this was the electrospinning.
The PLGA was dissolved (5% m/v) in a mixture of acetone and chloroform (50/50) as
solvents, and with 0.75% oleic acid as a surfactant and 1.5% -TCP and stirred with magnetic
bar for total dispersion. Using a device for electrospinning was obtained a structure of
randomly oriented fibers with an average diameter of 1.41 microns with a height of the
syringe tip to the collector of 17 cm, flow rate 5 ml/h voltage of 12 kV.
Specimens for diametral strength (brazilian test) were obtained with fibers in different
directions and also without fibers. Specimens with 10 cm high and 20 cm in diameter were
manufactured and the added fibers in two layers perpendicular to the axis and two layers
parallel to the axis of the machine in another. These samples were immersed in SBF for 0, 7
and 14 days.
For microstructural analyzes of the fracture surface of the cements used the scanning
electron microscope, JEOL brand, model JSM 6060.
RESULTS AND DISCUSSIONS
The α-TCP obtained using high-temperature sintering, occurred due to alpha-beta
transition. Recently ceramic biphasic calcium phosphate consisting of HA/TCP-α or
HA/TCP-β has been evaluated in bone tissue. The results showed these biphasic ceramics
were biologically more active than the ceramics with only pure HA, the biological behaviors
of biphasic ceramics containing TCP were higher in the formation of new bone. This
phenomenon can also be related to the hydrolysis of α-TCP phase, in other words, the
variation of the phase structure and morphology during hydrolysis (Bohner, M. et al. 2005).
In vitro studies revealed that the α-TCP was higher than the dissolution rate of β-TCP.
The order of relative solubility TCP-α, TCP-β and HAp has been presented as follows:
α-TCP > TCP-β >> HAp. Due to the phase of tricalcium phosphate present a high degree of
solubility, many published work showing the ability of these biomaterials have to deteriorate
when implanted in a biological medium, and forming a gradually absorbable new bone
structure (Lin 2006).
The major constraints of the mechanical performance of the CFC derived arise from the
intrinsic fragility of its composition and microstructure. The CFC ceramic are inherently
porous with porosities ranging between 20% and 50%, depending on the ratio of liquid to
powder used in preparing (Espanol et al., 2009). It is observed from Figure 1 that the sample
had without fibers a significant increase in porosity throughout their immersion time in SBF.
This occurs because the intrinsic porosity of the ceramic material and the behavior of CFC
during the immersion time, having a greater number of pores in the samples without fiber.
45
Apparent porosity
40
without fibers
parallel fibers
perpendicular fibers
35
30
25
20
15
0
7
14
Days
Fig. 1: Graphic apparent porosity vs. time of immersion in SBF.
2,0
1,9
without fibers
parallel fibers
perpendicular fibers
Apparent density
1,8
1,7
1,6
1,5
1,4
1,3
1,2
0
7
14
Days
Fig. 2: Graph apparent density vs. time of immersion in SBF.
It is observed that in Figure 2, the sample without fiber showed a lower density, because
of their high porosity. And the literature is a material with lower density will have a greater
porosity.
For forming the samples was used compared liquid/powder 4.0 ml/g Na2HPO4. The
mold used was stainless steel containing cavities 10mm +/- 1mm in diameter and 20mm +/1mm, after its manufacture to be testing the diametral compression test. This so-called
Brazilian disc test can be used to determine the failure behavior of natural flaws contained in
ceramic materials under multiaxial loading, as a stress state with both negative and positive
principal stresses is induced in a disc during the test. The samples were immersed in SBF
(Simulated Body Fluid) and measured at 0, 7 and 14 days of immersion.
The growing interest in the production of nanostructured fibers is due mainly to the
development of “electrospinning”. This process comprises forcing a fluid or molten polymeric
solution through a capillary, where a voltage order of 15-40 kV is applied between the exit the
capillary and collecting system, which are separated by a distance 5-15 cm. The tension
imposed on the surface of the electrostatic drop polymer in the capillary exit exceeds the
surface tension thereof, resulting in stretch polymer and, consequently, the formation of fiber
diameters in size nanometrics. However, the formation of the fibers occurs only under specific
conditions depending on the processing conditions and the physical properties of the fluid
polymer (Theron, SA, et. al., 2004; Ramakrishna, S., et. al., 2005; Andrady, A.L., 2008).
The PLGA was dissolved (5% m/v) in a mixture of acetone and chloroform (50/50) as
solvents, and with 0.75% oleic acid as a surfactant and 1.5% -TCP and stirred with magnetic
bar for total dispersion. Using a device for electrospinning was obtained a structure of
randomly oriented fibers with an average diameter of 1.41 microns with a height of the
syringe tip to the collector of 17cm, flow rate 5 ml/h voltage of 12 kV.
Fiber reinforcement has been extensively explored in the field of hydraulic cements and
concretes for civil engineering and building applications. The incorporation of fibers into a
brittle cement matrix has been proven to increase the fracture toughness of the composite by
the resultant crack arresting processes as well as the tensile and flexural strengths (Beaudoin,
1990). However, in cements intended for medical applications such as CPC, specific
requirements arise in the selection of the fibers; on one hand, they must be biocompatible. On
the other hand, they can be used not only as reinforcement for the cement matrix but also as
pore-generating agents. In this second approach, fibers, in addition to being biocompatible,
must also be biodegradable.
A high fiber tensile strength is essential for a substantial reinforcing action. A high ratio
of fiber elastic modulus to matrix elastic modulus facilitates stress transfer from the matrix to
the fiber. Fibers having large values of failure strain give high extensibility in composites
(Beaudoin, 1990). However, not only fiber type is important. Other factors, such as fiber
length, volume fraction, orientation or fiber/matrix adhesion among others, determine the
final properties of the composite. Both the fiber and the matrix are assumed to work together,
and provide the synergism needed to make an effective composite (Naaman, 2007). The load
is transferred through the matrix to the fiber by shear deformation at the fiber–matrix
interface.
There is that the samples with added fiber as the force applied in parallel diametral
compression test as figure 3, there was obtained a value greater traction. According to
literature, this behavior is due to the crash that brought the fiber not to crack propagation
during the test to the complete break of the specimen (Beaudoin, 1990).
We observed an increase in the toughness of the material with a large deformation zone,
followed by an increased load. This was reproduced in all specimens tested under these
parameters. It is not an isolated behavior. For specimens with immersion in SBF for 7 and 14
days, the traditional behavior was that of a ceramic material.
5,0
4,5
without fibers
parallel fibers
perpendicular fibers
Traction [MPa]
4,0
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0
7
14
Days
Fig. 3: Graph result traction vs. time of immersion in SBF.
The characterization by microscopy was used primarily to analyze the bioactivity of the
cement (precipitation of hydroxyapatite), the incorporation of fiber in the matrix and whether
hydroxyapatite nucleation on the fiber surface. Although there is an increased mechanical
strength specimens that were not immersed in SBF, no coupling to the polymer cement
matrix, as seen in the image. To increase the interaction between surfaces and having good
adhesion, it suggested the use of surfactants, such as chitosan, mannitol and glycerol (Zhang
and Xu, 2005; Zhao L 6L., 2010).
Using a device for electrospinning was obtained a structure of randomly oriented fibers
with an average diameter of 1.41 microns with a height of the syringe tip to the collector of
17cm, flow rate 5 ml/h voltage of 12 kV, as figure 4.
The addition of fibers with higher reabsorption rate than the CPC matrix would allow
creating macropores to favour cell colonization, angiogenesis, and eventually fostering bone
regeneration. Ideally, the loss of strength produced by fiber degradation should be
compensated by the formation of new bone (Espanol et. Al. 2009). This behavior can occur
with PLGA by the fiber to be biodegradable and absorbable “in vitro”.
Fig. 4: Microfibers PLGA dissolved in a solution of chloroform and acetone.
In Figure 5 is observed for a fiber surface of a sample of PLGA immersed for 7 days in
SBF, which notice cement particles adhered to the fiber surface. Not finding crystals of HA,
we have a weak coupling of the fiber.
Fig. 5: Electron microscopy of the fracture region (7 days SBF, parallel fiber).
Keeping the specimens for a longer period in SBF (14 days) there was no positive
change for the precipitation of hydroxyapatite on the fibers. Note the Figure 6.
Fig. 6: Cement particles adhered to the surface.
Taking into account that was used the same α-TCP and the same batch of fibers of
the same polymer, the position of the fibers within the cement does not affect the
precipitation of hydroxyapatite, and then the results obtained in a reproducible sample
to the other.
CONCLUSION
The immersion in SBF was efficient only in times shorter than 14 days in the case
of the increase in tensile strength for test specimens without fibers. For bodies with
fibers perpendicular, we had a similar behavior, in which the tension remained virtually
unchanged from 7 to 14 days. Regarding the use of perpendicular fibers, we obtained a
promising result, where there was a large increase in material toughness. However, this
behavior was not reproduced when there was precipitation of hydroxyapatite (7 and 14
days), brittle fracture occurs at low voltages.
As for vertical fibers, there were no satisfactory results, since the breakdown
voltage was similar to the fibers without (0 days SBF) and below the voltage of 7 days
in SBF, also without fibers.
ACKNOWLEDGEMENT
To coworkers and supervisors of Labiomat, CNPq, CAPES, PPGE3M, INCT and Biofabris
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