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 SJ-400), 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.

Sample

Specific surface energy (mJ/m2)

 

 

 

PDMS

14.15 ± 0.01

20 % (v/v)

30 % (v/v)

0 % TEOS

5 % TEOS

10 % TEOS

0 % TEOS

5 % TEOS

10 % TEOS

PDMS/HAp

17.79 ± 0.37

14.67 ± 0.67

6.74 ± 0.38

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.

REFERENCES

[1] F. Abbasi, H. Mirzadeh and A.A. Katbab, Polym. Int. Vol. 50 (2001), p. 1279–1287.

[2] J. Wen, Y. Li, Y. Zuo, G. Zhou, J. Li, L. Jiang and W. Xub: Preparation and characterization of nano-hydroxyapatite/silicone rubber composite, Materials Letters Vol. 62 (2008), p. 3307–3309.

[3] W.W. Thein-Han, J. Shah, and R.D.K. Misra: Superior in vitro biological response and mechanical properties of an implantable nanostructured biomaterial: Nanohydroxyapatite–silicone rubber composite, Acta Biomaterialia Vol 5 (2009), p. 2668–2679.

[4] C. Zhou and Z. Yi: Blood-compatibility of polyurethane/liquid crystal composite membranes, Biomaterials Vol. 20 (1999), p. 2093-2099.

[5] T.M. Volkmer, L.L. Bastos, V.C. Sousa, and L.A. Santos: Obtainment of α-tricalcium phosphate by solution combustion synthesis method using urea as combustible, Key Engineering Materials (2009), p. 591-594.

[6] M. Zenkiewicz: Comparative study on the surface free energy of a solid calculated by different methods, Polymer Testing Vol. 26 (2007), p.14–19.

[7] J. Jovanovic, B. Adnadjevic, M. Kicanovic and D. Uskokovic: The influence of hydroxyapatite modification on the cross-linking of polydimethylsiloxane/HAp composites, Colloids and Surfaces B: Biointerfaces Vol. 39 (2004), p. 181–186.

[8] Neumann, A. W.; Good, J. R.; Hope, C. J.; Sejpal, M. An equation-of-state approach to determine surface tensions of low-energy solids from contact angles, J. Colloid Interface Sci. v. 49(2), p. 291, 1974.

[9] Fu, S. Y.; Feng, X.; Lauke, B.; Mai, Y. W. Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites, Composites: Part B, v. 39, p. 933–961, 2008.

[10] Wang, K.; Wu, J.; Ye, L.; Zeng, H. Mechanical properties and toughening mechanisms of polypropylene/barium sulfate composites, Composites: Part A, v. 34, p. 1199–1205, 2003.