The aim of this study was to investigate the effect of silanization of biostable and bioactive glass fillers in a polymer matrix on some of the physical properties of the composite.
The water absorption, solubility, flexural strength, flexural modulus and toughness of different particulate filler composite resins were studied in vitro . Five different specimen groups were analyzed: A glass-free control, a non-silanized bioactive glass, a silanized bioactive glass, a non-silanized biostable glass and a silanized biostable glass groups. All of these five groups were further divided into sub-groups of dry and water-stored materials, both of them containing groups with 3 wt%, 6 wt%, 9 wt% or 12 wt% of glass particles ( n = 8 per group). The silanization of the glass particles was carried out with 2% of gamma-3-methacryloxyproyltrimethoxysilane (MPS). For the water absorption and solubility tests, the test specimens were stored in water for 60 days, and the percentages of weight change were statistically analyzed. Flexural strength, flexural modulus and toughness values were tested with a three-point bending test and statistically analyzed.
Higher solubility values were observed in non-silanized glass in proportion to the percentage of glass particles. Silanization, on the other hand, decreased the solubility values of both types of glass particles and polymer. While 12 wt% non-silanized bioactive glass specimens showed −0.98 wt% solubility, 12 wt% silanized biostable glass specimens were observed to have only −0.34 wt% solubility.
The three-point bending results of the dry specimens showed that flexural strength, toughness and flexural modulus decreased in proportion to the increase of glass fillers. The control group presented the highest results (106.6 MPa for flexural strength, 335.7 kPA for toughness, 3.23 GPa for flexural modulus), whereas for flexural strength and toughness, 12 wt% of non-silanized biostable glass filler groups presented the lowest (70.3 MPa for flexural strength, 111.5 kPa for toughness). For flexural modulus on the other hand, 12 wt% of silanized biostable glass filler group gave the lowest results (2.57 GPa).
The silanization of glass fillers improved the properties of the glass as well as the properties of the composite. Silanization of bioactive glass may protect the glass from leaching at early stage of water storage.
Synthetic hydroxyapatites, bioactive glass (BG) and glass-ceramics have been used in recent years to transform biostable implants made of metals and polymer composites into bioactive materials . After the introduction of the first BG system, “Bioglass ® 45S5” by Prof. Hench, the system has been modified by many researchers, and it has been introduced to the field of tissue engineering . Modifications to the composition of BG were undertaken by Andersson et al. , and presently BG S53P4 is used in applications where bioactivity and antimicrobial properties are required . The most prominent feature of BGs is their bioactivity. Bioactivity occurs through the union of calcium and phosphate groups and the subsequent formation of a calcium phosphate (CaO-P 2 O 5 , CaP, in short) layer. CaP formation is a tissue-dependent process, and in vitro bioactivity correlates with in vivo bioactivity .
Both biodegradable and biostable polymers are used widely as biomaterials in medicine and dentistry . Poly(methyl methacrylate) (PMMA) is a commonly used biostable polymer used for example in bone cements and dentures . PMMA has been combined with BG fillers to be used as long bone segmental defect repair materials and in calvarial implants . Adhesion between filler and polymer, however, is important in the transfer of load from the matrix to the filler particles. The BG filler particles, as they are leached out of the matrix over time, could cause considerable changes to the properties of the composite under moist conditions. Silanes as bi-functional compounds can bind the filler particles to the polymer matrix regardless whether the glass is biostable or leachable. In the latter case, silanization may also provide protection for leaching, and thus the mechanical properties of the composite may be retained for a longer period of time. Gamma-3-methacryloxypropyltrimethoxysilane (MPS), the silane used in this study, is a trialkoxysilane and one of the most studied silane compounds .
The aim of the study was to evaluate some of the physical characteristics composites containing both bioactive and biostable glass with regard to the silanization and filler loading of the glass.
Materials and methods
The resin system for the matrix of the composites was based on an autopolymerizing methyl methacrylate and ethylene glycol dimethacrylate (95:5, w/w) monomer system with a powder component of PMMA (Palapress ® , Heraeus-Kulzer, Wehrheim, Germany). Bioactive (particulate size from 315 to 1000 μm, Vivoxid LTD., Finland) and biostable glass particles (granular size from 915 to 1000 μm, Vivoxid LTD., Finland), both silanized and non-silanized, were used as fillers in the composites ( Table 1 ).
|BG particulates||S53P4 glass system, particulate size 315–1000 μm||Vivoxid Ltd., Turku, Finland||SiO 2 53 wt%, Na 2 O 23 wt%, CaO 20 wt% and P 2 O 5 4 wt%|
|Biostable glass particulates||Particulate size 915–1000 μm||Vivoxid Ltd., Turku, Finland||SiO 2 70 wt%, Na 2 O 17 wt% and CaO 13 wt%|
The BG and biostable glass particles were measured with the aid of precision scale of 1 mg (Mettler PM100, Toledo, USA) and added to the resin in PMMA powder to prepare 3 wt%, 6 wt%, 9 wt%, and 12 wt% composites. Silanization of glass particles was done before adding them in to the resin as follows: 2 wt% MPS-silane (98% MPS, lot.0182EH-497, Aldrich) and for the hydrolysis of the MPS-silane, double the amount of glass of toluene (≥99.5%, A.C.S reagent lot.03334ME-157, Sigma–Aldrich) were mixed with the glass particles in a decanter. The silanization decanter was left in a fume hood for 24 h to evaporate the toluene, and the glass powder was then dried in ∼90 °C for 3 h.
The glass particle containing resin mixture was poured into 65 mm × 10 mm × 3.5 mm stainless steel molds and the polymerization was carried out according to manufacturers’ instructions (10 mL powder/7 mL liquid; under 55 °C, 200 kPa pressure, 15 min curing time) (Ivomat, Typ IP 2, Ivoclar AG., Schaan, Liechtenstein). Polymerized specimens were ground down by 180, 500, 1200-grit (FEPA) silicon carbide papers (Struers A/S, Rodovre, Denmark) under water cooling with 300 rpm speed (LaboPol-21, Struers A/S, Rodovre, Denmark) to a thickness of 3 ± 0.1 mm. The specimens’ dimensions were controlled by the means of an electronic caliper, and they were then kept in excicator for 1 week before testing. Test specimens were classified as shown in Table 2 . There were eight test specimens ( n = 8) in each of the groups.
|C-d||Dry||Control, no fillers|
|C-w||In water, 60 days||Control, no fillers|
|Non-silanized biostable glass filler groups|
|BS3-ns-d||Dry||Biostable glass, 3%-wt, not silanized|
|BS3-ns-w||In water, 60 days||Biostable glass, 3%-wt, not silanized|
|BS6-ns-d||Dry||Biostable glass, 6%-wt, not silanized|
|BS6-ns-w||In water, 60 days||Biostable glass, 6%-wt, not silanized|
|BS9-ns-d||Dry||Biostable glass, 9%-wt, not silanized|
|BS9-ns-w||In water, 60 days||Biostable glass, 9%-wt, not silanized|
|BS12-ns-d||Dry||Biostable glass, 12%-wt, not silanized|
|BS12-ns-w||In water, 60 days||Biostable glass, 12%-wt, not silanized|
|Silanized biostable glass filler groups|
|BS3-s-d||Dry||Biostable glass, 3%-wt, silanized|
|BS3-s-w||In water, 60 days||Biostable glass, 3%-wt, silanized|
|BS6-s-d||Dry||Biostable glass, 6%-wt, silanized|
|BS6-s-w||In water, 60 days||Biostable glass, 6%-wt, silanized|
|BS9-s-d||Dry||Biostable glass, 9%-wt, silanized|
|BS9-s-w||In water, 60 days||Biostable glass, 9%-wt, silanized|
|BS12-s-d||Dry||Biostable glass, 12%-wt, silanized|
|BS12-s-w||In water, 60 days||Biostable glass, 12%-wt, silanized|
|Non-silanized bioactive glass filler groups|
|BG3-ns-d||Dry||Bioactive glass, 3%-wt, not silanized|
|BG3-ns-w||In water, 60 days||Bioactive glass, 3%-wt, not silanized|
|BG6-ns-d||Dry||Bioactive glass, 6%-wt, not silanized|
|BG6-ns-w||In water, 60 days||Bioactive glass, 6%-wt, not silanized|
|BG9-ns-d||Dry||Bioactive glass, 9%-wt, not silanized|
|BG9-ns-w||In water, 60 days||Bioactive glass, 9%-wt, not silanized|
|BG12-ns-d||Dry||Bioactive glass, 12%-wt, not silanized|
|BG12-ns-w||In water, 60 days||Bioactive glass, 12%-wt, not silanized|
|Silanized bioactive glass filler groups|
|BG3-s-d||Dry||Bioactive glass, 3%-wt, silanized|
|BG3-s-w||In water, 60 days||Bioactive glass, 3%-wt, silanized|
|BG6-s-d||Dry||Bioactive glass, 6%-wt, silanized|
|BG6-s-w||In water, 60 days||Bioactive glass, 6%-wt, silanized|
|BG9-s-d||Dry||Bioactive glass, 9%-wt, silanized|
|BG9-s-w||In water, 60 days||Bioactive glass, 9%-wt, silanized|
|BG12-s-d||Dry||Bioactive glass, 12%-wt, silanized|
|BG12-s-w||In water, 60 days||Bioactive glass, 12%-wt, silanized|
The water absorption of the composite specimens was measured by determining the initial weights of the specimens before immersion ( m 1 ) to distilled water and comparing it with the weight of the specimen after immersion ( m 2 ). Specimens were removed from water on days of 1, 2, 3, 7, 14, 21, 30, 45, 60; dried and weighed after 1 min by the aid of precision scale of 0.1 mg (0.0001 g) (Mettler Toledo AT261 DeltaRang®, USA). Ten values were obtained in this way from each specimen. Water absorption percentages were determined using the formula below:
W a t e r a b s o r p t i o n % = m 2 − m 1 m 1 × 100
m 1 : initial weight before absorption test (g); m 2 : last measured weight (g).
The mechanical properties of the specimens were measured by a three point bending test performed by universal testing machine (Lloyd LRX Plus, United Kingdom). Samples were placed on supports 50 mm apart, and the force applying tip on the sample in the middle of the two supports. The speed of the device was set at 5 ± 1 mm/min till fracture occurred. Flexural strength, flexural modulus and toughness data were determined as previously . Formulas for calculation of strength, modulus of elasticity and toughness were:
T . S . = 3 × F × L 2 × b × h 2