| Background and objectivesSilicone elastomers are most widely used in restoration of maxillofacial defects functionally and aesthetically due to their good biocompatibility, suitable mechanical properties, chemical inertness and pigmentability. Considerable efforts have been focused on the structure of maxillofacial prosthetic silicone elastomers and related properties.However, traditional maxillofacial silicone elastomers need further improvement for clinical applications. For example, the hardness and elasticity of traditional maxillofacial silicone always make patients feel uncomfortable together with the high density which could even cause bad retention. The relative high thermal conductivity may lead to cicatricial contraction at the surgical site because of the fast heat transfer. Moreover, the poor shock absorption could resistant neither external forces nor the movements of facial muscles or chewing. Thus, maxillofacial silicone elastomers with low density, low heat conductivity and improved flexibility and shock absorption properties are urgently required.Hollow microspheres are spherical particles with diameter from tens of nanometers to several hundreds of microns. They are usually used as light weight fillers, high quality insulating materials, damping materials, drug delivery systems and so on because of low density, high specific surface area, huge inner space, good thermal and chemical stability, high resistance, low gas permeability and great elasticity.In this study, the first explore of introducing hollow microspheres into the silicone elastomer is presented to obtain optimal maxillofacial prosthetic materials with low density, low heat conductivity and better flexibility. The biocompatibility and physical and mechanical properties of silicone elastomer filled with hollow microspheres were investigated.Materials and methodsThe two-component silicone elastomer (SILASTIC MDX4-4210 BioMedical Grade Elastomer) and the dilution agent hexamethyldisiloxane (Q7-9180 Silicone Fluid,0.65 cSt) were purchased from Dow Corning Chemical Company, USA. Hollow microspheres (461 DET 40 d25, Expancel) were provided by Akzo Nobel, Sweden.The elastomer component (mentioned as MB) of MDX4-4210 was first mixed with Silicone Fluid Q7-9180 (Q) for dilution by magnetic stirring (300 rpm) at a mass ratio of 1:1 for 10 min at room temperature. A given amount of hollow microspheres was added to the mixture and then stirred for 30 min at 800 rpm. The curing agent component (mentioned as MA) was then added (The mass ratio of MB: MA was kept at 10:1) to the mixture and then stirred for 10min at 800rpm.The mixture was vacuum deaerated for 30 min before pouring into Teflon mould. At least a 4 h curing at 25℃was necessary for allowing the residue traces of gas to escape and preventing the bubbles to form. Further curing was carried out at 60℃for 2 h, and another 24 h curing in room temperature was performed before test. Biocompatibility of the silicone/hollow microspheres composites was analyzed by cytotoxicity test (filter dissusion test) according to ISO 7405:1997.Hollow microspheres were observed by optical microscope (OLYMPUS-BX51, Japan) by transmission mode. Morphologies of the hollow microspheres and the fracture surfaces of the composites in tensile test were observed by field emission scanning electron microscopy (JSM-6700F, JEOL) operating at 5 kV.Contact angle (CA) of the pure silicone elastomer and the silicone/hollow microspheres composites were measured by a homemade instrument under ambient circumstance with a water drop of 5μL. All the samples were tested on both sides and the value of the contact angle was the average of five measurements at different positions at each side.Density (p, g/cm3) of the composites was measured with Quantachrome pycnometer (ULTRAPYC 1200e, USA). At least three samples were tested for each composite.Shore A hardness was measured with a durometer (SLX-A, SUNDOO) according to ISO 7619-1:2004. More than three specimens (30 mm×30 mm×6 mm) of each composite were tested.Thermal conductivity (κ, W/K-m) of the composites was measured from 300 to 320 K by a physical properties measurement system (PPMS-9T, Quantum Design). All the samples were made into 5 mm×5 mm X3 mm, and the heating rate was 0.3 K/min.Shock absorption property was evaluated with the dynamic mechanical analyzer (DMA 800, TA Instruments). Rectangle thin film samples of each composite (width 8 mm, thickness 1 mm, length 30mm) were tested from-140℃to 20℃under stretch mode with a heating rate of 5℃/min, a constant frequency of 1 Hz and a uniform amplitude of 10μm. The tensile test and tear test were carried out on the universal testing machine (3365, INSTRON) according to ISO 37:2005 and ISO 34-1:2004, respectively. A 500 mm/min cross-head speed was performed when testing.All data were analyzed by One-Way ANOVA (SPSS 13.0) for significant differences between the composites(P< 0.05). Levene's test of homogeneity of variance was used (α= 0.1), following the assumption of equal variance. Bonferroni post-hoc test was used when equal variances were assumed (P> 0.1), and Dunnett's T3 post-hoc test was utilized when equal variances were not assumed(P< 0.1).ResultsWhen Expancel hollow microspheres added into the silicone elastomer, a smooth and bubble-free composite was gained. At the contents of 5% and 15%v/v, the hollow microspheres disperse rather uniformly and separately in the matrix. When the content of the hollow microspheres increases to 30%v/v, some of the microspheres aggregate with each other in the composite.Cytotoxicity test (filter dissusion test) showed that cytotoxicity is grade 0. The silicone elastomer filled with hollow microspheres had good biocompatibility. Before coming into service, further animal and clinical trials need to be done.The hydrophobicity ensures the composites a good isolation from the surrounding pollution. Both MDX4-4210 and hollow microspheres are hydrophobic. The apparent contact angles of the hollow microspheres and the pure silicone elastomer are more than 90°, respectively. The composites filled with different content of hollow microspheres have smooth surfaces, and the average contact angles of the surfaces exposed to air and contacted with the PTFE mould are also more than 90°. That means the composites are also hydrophobic. However, the hydrophobic surfaces could also cause a low adhesion to non-silicone-based adhesives. Surface modification must be done at the thin margin of the final prostheses.Density of the maxillofacial prostheses has directly impact on the comfort and satisfaction of patients. A prosthesis that is over weight will also lead to a bad retention between the prosthesis and the human body. The hollow microspheres are closed cells encapsulating light gas. A density decline of the composites was shown as expected when 5%,15%and 30%v/v hollow microspheres were filled into the silicone elastomer matrix. There were significant differences between these data tested by One-Way analysis of variance (P<0.05). Therefore, the final prostheses made of the composites will be lighter and more comfortable than the traditional maxillofacial prosthetic silicone elastomers.Shore A hardness indicated the softness of the elastomers, and also directly impacted the comfort of the patients. Results show that the hardness of the composites first decreased then increased with the filler contents. The results may due to the variation of cross-link density and filler content. The modulus of the fillers was higher than that of the silicone elastomer. The fillers dispersed uniformly in the matrix when the filler contents were 5% and 15%. The cross-links of organic silicon molecular chain was decreased as the hollow microspheres dispersed well in the continuous phase. Thus, the hardness of the composite declined. As the filling volume increases, the amount of microspheres was high enough to touch each other. There are two continuous phases in the composites and the hardness of the fillers played a dominant role. Therefore, the hardness of the composite increased.Thermal conductivity (κ, W/K-m) of composite materials depends on the type, shape, size and loading level of the filler together with the intrinsic thermal conductivity of filler and matrix. The intrinsic thermal conductivity of silicone elastomer is about 0.20 W/K-m. Low thermal conductivity of the composites is expected for preventing cicatricial contraction of the residual tissues. As shown in this study, the pure silicone elastomer (0%v/v) presented a thermal conductivity of about 0.21 W/K·m, while the composites of 5%,15%,30%v/v showed the thermal conductivity of 0.20,0.17,and 0.13 W/K·m, respectively. The thermal conductivity decreased due to the longer heat-pathway caused by the interface of the microspheres and matrix. Poor thermal conductivity of gas in the hollow microspheres also plays an important role in descending the thermal property.Maxillofacial prostheses are under sustaining forces subjected to the movements of the facial musclegroups, masticatory or other external forces. Therefore, excellent shock absorption is needed for the maxillofacial materials. High loss factor (tanδ) and wide damping temperature range result in high shock absorption of materials. The 5%v/v composite presents higher loss modulus and low storage modulus leading to a slightly higher tanδ. The appropriate amount of hollow microspheres filling into silicone elastomer could maintain good shock absorption for the prosthetic application.Good physical and mechanical properties are necessary for maxillofacial silicone elastomers. Nevertheless, the silicone elastomers filled with hollow microspheres were fabricated by using dilution to make uniform and smooth composites which could lead to a lower cross-link density of the matrix than the pure PDMS without addition of diluent during the curing process. Therefore, obvious decline of mechanical and dynamic mechanical properties was observed as anticipated. All the samples were made in the same condition with identical matrix/diluent ratio.Tensile strength of silicone/hollow microspheres composites first increased and then decreased. Fillers as disperse phase were distributed into the continuous polymeric phase. Effective area of cross section of the composite was less than that of the pure polymeric matrix even if there were no cracks or bubbles between the matrix and fillers. When the filler volume was low, the tensile strength of the composite was higher than that of the pure matrix as the fillers may move together with the matrix and the effective cross-section area was enlarged. When the content of fillers increased to a certain amount, the microspheres were close to each other or conglomerated and then the micro-cracks turned into macro-defects, which resulted in tensile strength decreasing.Elongation at break of silicone elasomer filled with hollow microspheres also assumes first increasing but later decreasing because of the similar reason of the changes of tensile strength. However, the elongations at break of the composites were much higher than that of the pure elastomer.5%v/v composite presents the highest elongation, which was more than twice of the pure silicone elastomer. The reason may be due to the cross-link density decreased when the microspheres were added into the system, leading to the increase of the rebound resilience and the decrease of the permanent deformation of the elastomer. That meant the elasticity of the elastomer has totally been improved.After adding the fillers, the tear strength of the composites exhibited a significant decrease. It was maybe because the interfaces of the fillers and matrix were not strong enough to prevent the composites from being ruptured. When the tear test begins, crack propagated not only along the place pre-cut, but also along the micro-cracks between the interfaces of the two phases rapidly.ConclusionsHollow microspheres filled maxillofacial silicone elastomer with good biocompatibility was fabricated with MDX4-4210 silicone elastomer and Expancel hollow microspheres. The particles can be homogeneously dispersed in the matrix. Different composition ratios were investigated to evaluate the properties of the composites. The density and thermal conductivity decreased as expected. Higher... |
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