POLY(DIMETHYL SILOXANE)/TETRAETHYL ORTHOSILICATE MODIFIED
HYDROXYAPATITE COMPOSITES: SURFACE ENERGY, ROUGHNESS AND
MECHANICAL PERFORMANCE
Bareiro, O.(1); Santos, L. A.(1)
(1)
Laboratorio de Biomateriais, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brasil
oscarbafer@hotmail.com, luis.santos@ufrgs.br
Abstract: Poly(dimethyl siloxane) (PDMS) has been extensively applied in artificial organs and
implants for more than 50 years, due to its excellent softness, biostability and bioinertness, in
some cases it’s even used in medical devices to be contacted directly with blood. However,
PDMS is known to induce thrombogenic reactions when in contact with blood. Although there is
a controlling effect of the surface properties on the biomaterial thrombogenicity, this
dependence remains unclear. The purpose of this study was to investigate the effect of
hydroxiapatite (HAp) particles surface modification, using tetraethyl orthosilicate (TEOS), on
the surface energy, roughness and mechanical properties of PDMS/HAp composites. The
processing of the composite involved the surface modification of HAp with 5 or 10 %(wt/wt)
TEOS solutions, followed by mixing in a two roll open mixer with the silicone. Results showed
that the addition of unmodified particles led to higher surface energy values, on the other hand,
modified particles led to lower surface energy values. The roughness (Ra) values of the
composites were higher than those of the PDMS, regardless the surface modification applied to
the HAp particles. In tensile property measurement, the PDMS/modified-HAp composite showed
higher values of tensile strength (2.41 MPa) and lower elongation at break (139.62 %) than the
PDMS/unmodified-HAp composite, 2.20 MPa and 365.58 % respectively. In both cases, the
composites showed higher values of tensile strength than the original silicone (1.97 MPa).
Keywords: composite, poly(dimethyl siloxane), hydroxiapatite, surface modification, mechanical
performance.
1. INTRODUCTION
Silicone rubber (SR) has been widely used in clinic as implant for a long time. It is well
known that SR has good biocompatibility and physiological inertness. Its well workable property
promises a great convenience in clinical applications. However, SR is not able to bond
organically with tissues owing to its inertness. In some cases inflammation and foreign-body
reaction occur after the implantation [1]. Many serious attempts have been made to improve the
biocompatibility of SR [2]. It has been demonstrated that these modifications can improve
significantly the biological properties of SR. But so far these techniques have not been so
successful in enhancing the bioactivity of SR [3].
Bioceramics have excellent biological performance that can encourage new bone formation
by self-degradation under the microenvironment of the organism to produce a pore-like structure,
facilitating adherence to tissues and growth of peripheral tissues [4]. In this regard,
hydroxyapatite (HAp) is the preferred calcium phosphate because the primary constituent of
bone comprises inorganic crystallites similar to HAp. It can form a bonding with bone tissues
and posses excellent biocompatibility. However, in contrast to SR, HAp demonstrates poor
processing properties due to its brittleness [5]. At present, either HAp or SR can be utilized as
plastic material in clinic, but neither is ideal. Naturally, it would be desirable to combine the
advantages of HAp and SR and to form a new material system [4].
A number of studies have focused on improving the mechanical, physicochemical and
biological properties of polymers such as SR and chitosan scaffolds through incorporation of
bioactive inorganic substances such as calcium phosphate, especially HAp [Ca10(PO4)6(OH)2], to
achieve porous tissue ingrowth and high mechanical strength [6]. The resulting composite can be
viewed as representing a new class of nanostructured biomaterial. It has the potential to exhibit
excellent physical and biological properties, with the SR providing the desired fine mechanical
properties and HAp acting synergistically to promote bioactivity. The compounding of SR with
HAp will provide bioactive sites that are bioresorbable and favorable to tissue ingrowth [7].
Consequently, one can expect that SR/HAp composites can be made with improved
mechanical and biological properties. In this study, HAp powder was prepared by the
precipitation method, its surface was modified with tetraethyl orthosilicate (TEOS) solutions and
the surface energy, surface roughness and mechanical properties of the SR/HAp-TEOS
composites were assessed.
2. MATERIALS AND METHODS
2.1 Hydroxyapatite precipitation and surface modification
In order to obtain hydroxyapatite (HAp) via the precipitation reaction, an aqueous solution of
Ca(OH)2 (95% Synth) was placed in a balloon and heated up to 90 °C, while continuously
stirred, an aqueous solution of H3PO4 (85% Nuclear) was dropped. The system was continuously
stirred for 24 h at 90 °C, the obtained precipitate was then washed with ethylic alcohol (98%
Vetec), filtered and dried. The powder was calcined at 800 °C for 2 h. For the HAp surface
modification, an aqueous solution of ethylic alcohol at 25 %(wt/wt) was prepared, to whom
tetraethyl orthosilicate (TEOS) (98% Sigma-Aldrich) was added in order to obtain 5 %(wt/wt)
and 10 %(wt/wt) TEOS solutions, the pH was adjusted to 4.0 with acetic acid (99% Synth). The
solution was stirred for 30 min at 50 °C to allow hydrolysis, then, 5 %(wt/wt) of HAp was added,
the systems were agitated once again for 30 min at 50 °C. Finally, the modified calcium
phosphate was washed with ethanol (95% Vetec) and dried.
2.2 Composite elaboration and characterization
In order to prepare the composites, 10 %(v/v), 20 %(v/v) or 30 %(v/v) of modified or
unmodified HAp was incorporated into the commercial poly(dimethylsiloxane) (PDMS) NE-140
(Wenda Co.), which has equivalent mechanical performance compared to medical silicone, in a
open two-roll mixer. Afterwards, 0.8 %(wt/wt) of the curing agent, dicumil peroxide (98%
Aldrich), was added to the system, then, the composites were placed between metal plates and
pressed to obtain ~ 3 mm thick samples, which were cured at 185 °C for 35 min.
To assess the dispersion degree of the hydroxyapatite particles in the elastomeric matrix,
SEM fracture surface micrographs of the composite were obtained. In order to evaluate the
influence of the addition of modified and unmodified particles in the PDMS surface energy,
static contact angles were measured using water as prove liquid. The reported angles consist of
an average of 7 independent measurements, which were performed at room temperature using
the sessile drop method. The contact angle measurements were made using the ImageJ software.
The values of surface free energy were obtained by the Neumann’s method [8]. The surface
roughness (Ra) of the composite was assessed using a surface roughness measurer (Mitutoyo SJ400), the reported values consist of 5 independent measures maid at room temperature. In order
to evaluate the tensile properties of the composite, tensile tests were made in a universal testing
machine (Instron 3369), following the standard method ASTM D412. Measurements were at
room temperature with samples 10 mm wide, 33 mm long, and 3 mm thick, the rate of elongation
was 30 mm/min.
3. RESULTS AND DISCUSSION
SEM surface micrographs of the PDMS/20%(v/v)HAp composite, made with HAp treated
with 0, 5 and 10 %(wt/wt) TEOS solutions, are shown in Fig. 1. These results show that the
distribution state of the particles in the silicone matrix was homogeneous; on the other hand, the
dispersion state was poor, agglomerates of HAp particles were formed on the composite surface,
those features were seen regardless the surface treatment applied to the fillers.
Figure 1. SEM surface micrographs of composites made with 20 %(v/v) of HAp modified with 0, 5 and 10
%(wt/wt) TEOS solutions. (a), (b) HAp 0 %, (c), (d) HAp 5 %, (e), (f) HAp 10 %.
The results of static contact angle measurements of the composites filled with 20 %(v/v) and
30 %(v/v) of modified and unmodified HAp are shown in Fig. 2. It is possible to observe that the
incorporation of unmodified calcium phosphate into the silicone matrix led to lower contact
angle values, compared to the unfilled silicone; the grater the volume fraction tested, the lower
the contact angles measured. This could be due to the modification of the silicone surface
structure, which altered the surface charge and produced a hydrophilic SR–nHAp composite [3].
On the other hand, the incorporation of modified calcium phosphate with a 5 %(wt/wt) TEOS
solution did not change significantly the static contact angle values, whereas the incorporation of
modified calcium phosphate with a 10 %(wt/wt) TEOS solution led to higher contact angle
values. This is possibly due to the formation of a silicone layer on the hydroxyapatite surface
during the modification process, which led to an even more hydrophobic surface.
Specific surface energy values of the composites are shown in Tab. 1. The results show that
the incorporation of unmodified HAp led to higher specific surface energy values, compared to
the unfilled silicone, the grater the volume fraction tested, the higher the surface energy values.
On the other hand, the incorporation of surface modified HAp treated with a 5 %(wt/wt) did not
change significantly this values, meanwhile, the incorporation of particles treated with a 10
%(wt/wt) TEOS solution led to lower surface energy values.
Figure 2. Water contact angle measurements of pure silicone and composites filled with 20 %(v/v) and 30 %(v/v) of
hydroxyapatite, treated with 0 %(wt/wt), 5 %(wt/wt) and 10 %(wt/wt) TEOS solutions.
Table 1. Specific surface energy of pure silicone and composites filled with 20 %(v/v) and 30 %(v/v) of
hydroxyapatite, treated with 0 %(wt/wt), 5 %(wt/wt) and 10 %(wt/wt) TEOS solutions.
Specific surface energy (mJ/m2)
Sample
PDMS
PDMS/HAp
14.15 ± 0.01
0 % TEOS
20 % (v/v)
5 % TEOS
10 % TEOS
17.79 ± 0.37
14.67 ± 0.67
6.74 ± 0.38
0 % TEOS
30 % (v/v)
5 % TEOS
10 % TEOS
18.80 ± 0.80
16.90 ± 0.40
4.75 ± 0.16
The proposed sequence of chemical reactions involved in the surface modification of HAp by
means of a TEOS aqueous solution could be represented as follows:
3.1 TEOS hydrolyses
Si(OC2C3)4 + 4H2O  Si(OH)4 + 4(C3C2OH)
3.2 TEOS condensation
3.3 TEOS film formation on the HAp surface
HAp surface
HAp surface
Figure 3 shows Ra roughness values of pure silicone and PDMS/20%(v/v)HAp composites. It is
possible to observe that the addition of HAp into the silicone matrix led to higher values of Ra
roughness, this due to the presence of particles on the composite surface, as seen in Fig. 1. The
surface treatment applied to the particles did not influence the measured Ra roughness values.
The tensile strength of PDMS and composites are presented in Fig. 4.
Figure 3. Ra roughness values of pure silicone and PDMS/20%(v/v)HAp composites.
Figure 4. Tensile strength of pure silicone and PDMS/10,20,30 %(v/v)HAp composites, modified with 0, 5 and 10
%(wt/wt) TEOS solutions.
The incorporation of unmodified HAp into the PDMS matrix led to slightly higher values of
average tensile strength, as the volume fraction of HAp added was increased. The composite
PDMS/10%(v/v)HAp-0%(wt/wt)TEOS showed a 6 % increase in the average tensile strength,
once compare to the pure PDMS. The composites made with modified HAp showed higher
values of tensile strength than those made with unmodified HAp, regardless the volume fraction
of calcium phosphate added.
Higher values of tensile strength were achieved with HAp treated by a 5 %(wt/wt) TEOS
solution, once compared to HAp treated with a 10 %(wt/wt) TEOS solution. The composite
PDMS/10%(v/v)HAp-5(wt/wt)TEOS showed a 37 % increase in the tensile strength, once
compared to the pure PDMS. Therefore, the surface modification strengthened the interfacial
bond between the matrix and the fillers. This allowed loading to be more efficiently transferred,
leading to the composite reinforcement [9].
Mechanical properties, also got by means of the tensile test, are presented in Tab. 2. From
these results it is possible to elucidate that the values of elastic modulus increased monotonically
as the volume fraction of unmodified HAp added was increased. On the other hand, the
composites made with modified HAp showed higher values of elastic modulus, once compared
to the composites made with unmodified HAp, regardless the volume fraction of calcium
phosphate added. Higher values of elastic modulus were achieved with HAp treated by the 5
%(wt/wt) TEOS solution, once compared to the fillers treated by the 10 %(wt/wt) TEOS
solution.
Table 2. Mechanical properties of PDMS and 10, 20 and 30 %(v/v) HAp composites, modified with 0, 5 and 10
%(wt/wt) TEOS solutions.
Sample
Elastic Modulus (MPa)
Deformation (%)
PDMS
0.63 ± 0.07
525.28 ± 36.97
PDMS/10 %(v/v) HAp-0 % TEOS
1.11 ± 0.32
533.80 ± 90.73
PDMS/10 %(v/v) HAp-5 % TEOS
3.66 ± 0.60
163.98 ± 19.39
PDMS/10 %(v/v) HAp-10 % TEOS
2.77 ± 0.25
170.28 ± 14.29
PDMS/20 %(v/v) HAp-0 % TEOS
1.58 ± 0.32
365.58 ± 65.62
PDMS/20 %(v/v) HAp-5 % TEOS
3.37 ± 0.62
139.62 ± 10.95
PDMS/20 %(v/v) HAp-10 % TEOS
2.29 ± 0.22
73.44 ± 3.45
PDMS/30 %(v/v) HAp-0 % TEOS
2.66 ± 0.18
200.05 ± 47.41
PDMS/30 %(v/v) HAp-5 % TEOS
5.39 ± 0.30
113.43 ± 17.16
PDMS/30 %(v/v) HAp-10 % TEOS
5.30 ± 0.35
33.18 ± 6.62
Knowing that the composite tensile strength and elastic modulus depend on the matrix/filler
interface, these features are a measure of the interfacial strength. When the interfacial adhesion is
strengthened, by means of a coupling agent, in this particular case, reactive silane, polymer
chains surrounding the particles may bond to the particle surface and form a layer of
immobilized polymer chains. Thus, the tensile strength and elastic modulus of the composite
made with modified filler increased, once compared to the composite made with unmodified
filler, as a result of the interface strengthening [10].
Higher volume fractions of HAp added to PDMS led to lower values of deformation, once
compared to the values of pure PDMS. This behavior was intensified with the filler surface
modification. Higher TEOS concentrations led to lower values of deformation, for a constant
volume fraction of HAp. This feature could be a result of polymer chain bonding to the filler
surface, as previously described.
Fig. 5 shows SEM surface fracture micrographs of the composites made with modified and
unmodified HAp. Fig. 5(a), related to the composite made with unmodified HAp, shows a
particles detached from the matrix. The process of filler particle detaching from the matrix after
the composite deformation is known as debonding. The debonding normally occurs due to the
lack of adhesion between the matrix and the filler. Thus, the particle is unable to support any
loading and the composite tensile strength is reduced with the addition of any further amount of
filler [9].
The Fig. 5(b) and (c) are related, respectively, to the composites made with HAp modified
with 5 and 10 %(wt/wt) TEOS solutions. It’s possible to observe that the particles remained well
embedded in the matrix after the deformation, this as result of the matrix/filler interface
strengthening, achieved with the surface modification using TEOS solutions.
Figure 5. SEM surface fracture micrographs of 20 %(v/v) HAp composites, modified with (a) 0 %(wt/wt), (b) 5
%(wt/wt) and (c) 10 %(wt/wt) TEOS solutions.
4. CONCLUSION
Hydroxyapatite was synthesized by the precipitation method and its surface was modified by
means of TEOS aqueous solutions. The modified HAp was added to an elastomeric matrix of
PDMS in other do produce PDMS/HAp-TEOS composites. SEM surface micrographs showed
that the modified HAp particles were homogeneously distributed in the elastomeric matrix; still,
some agglomerates were formed. The incorporation of unmodified HAp into the PDMS led to a
less hydrophobic composite; on the other hand, the incorporation of modified HAp led to an even
more hydrophobic composite. The incorporation of unmodified HAp led to higher specific
surface energy values, compared to the unfilled silicone, the grater the volume fraction tested,
the higher the surface energy values. Meanwhile, the incorporation of modified HAp led to lower
surface energy values. A mechanism was proposed leading to the surface modification of HAp
particles using TEOS aqueous solutions.
The roughness (Ra) values of the composites were higher than those of PDMS, regardless the
surface modification applied to the HAp particles. Tensile measurements showed that the
composite PDMS/10%(v/v)HAp-5%(wt/wt)TEOS achieved a 37 % increase in the tensile
strength, once compared to the pure PDMS. Composites made with modified HAp showed
higher values of elastic modulus and lower values of elastic deformation, once compared to the
composites made with unmodified HAp. The surface modification strengthened the interfacial
bond between the matrix and the fillers. Therefore, the addition of TEOS modified HAp into the
PDMS led to a reinforced composite, but this was only achieved by means of withdrawing the
elastic properties of PDMS. SEM fracture micrographs showed that the HAp modified particles
remained well attached to the matrix after deformation.
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