Rick Carvalho

Professor

Research Classification

Biomaterials
Dental Materials

Relevant Degree Programs

 
 

Recruitment

Postdoctoral Fellows
Any time / year round

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2019)
Evaluation of distortion of monolithic zirconia crowns under the influence of different preparation designs and sintering techniques (2020)

Zirconia has seen a marked increase in its use in dentistry. The sintering of soft milled zirconia is accompanied by high shrinkage, approximately 20–30%. Sintering shrinkage is usually estimated as a single value for each blank, and manufacturers do not provide information on how shrinkage percentage is calculated and whether the estimated shrinkage percentage is based on linear or volumetric changes. In order to compensate for sintering shrinkage, the dimensions of the milled frameworks are enlarged by an appropriate factor, which supposedly corresponds to the estimated shrinkage upon sintering. Since dimensional changes are unavoidable during the processing of zirconia, the purpose of this series of studies was to understand how different preparation designs may affect dimensional changes during sintering, especially when using different sintering protocols, and how that would affect the fitting of the crown (Chapter 1). A systematic overview of how altering sintering protocol could affect the microstructure, mechanical and optical properties of zirconia material was conducted (Chapter 2). Then a systematic review of literature on the factors affecting the marginal fit of zirconia crowns was assessed qualitatively (Chapter 3). Subsequently, the effects of different preparation designs and sintering protocols on the marginal fit of zirconia crowns were investigated (Chapter 4). Afterward, the linear and volumetric dimensional differences between the virtual, milled and sintered copings as a result of the two different sintering protocols were measured (Chapter 5). Our search demonstrated that fast sintering improved the optical properties of zirconia but decreased its flexural strength. There was a lack of studies investigating the effects of different sintering protocols on marginal fit and dimensional changes of zirconia prostheses. There was a significant interaction between the crown thickness, finish line width and sintering protocol on the marginal fit of zirconia crowns. There was also a significant interaction between the coping design, processing stage and sintering protocol on linear and volumetric dimensions of zirconia copings. The combined outcome of this series of experiments allowed the proposition of the ideal combination of design and sintering protocol that results in minimal distortion and improves fitting of zirconia crowns.

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Master's Student Supervision (2010 - 2018)
Experimental 4-step braided glass fiber reinforced composite for dental CAD/CAM applications (2018)

Introduction: Computer-aided design/computer-aided manufacturing in dentistry has lead to rapid expansion of new dental materials. A new product from Shofu (Shofu, Japan), TRINIA CAD/CAM blocks and pucks, offers an alternative to the existing resin composite CAD/CAM materials. The claimed mechanical properties of TRINA provide merit to further research and development of 3D fiber reinforced composites for CAD/CAM dentistry. Objective: To design and produce 3D braided fiber-reinforced composites for dental CAD/CAM applications. Materials and Methods: Experimental groups were designed and produced in conjunction with the Department of Materials Engineering, UBC. The proposed design parameters included: 50:50 volume fraction fibers, 45-degree braiding angle, and 14 mm*14 mm cross sectional area, the approximate size of currently available CAD/CAM blocks. Continuous S-2 glass fiber roving was used as the reinforcing agent. Three experimental 3D glass fiber preforms were produced with internal structural variations using a 4-step braiding technique. A 50:50 UDMA:TEGDMA resin matrix blend with a thermal curing agent was used for infiltration via submersion under vacuum, followed by thermal curing. An unreinforced resin blend and a unidirectional fiber-reinforced composite, with the same volume fraction and dimensions, served as control groups. Samples were prepared to 4 mm*4 mm*45 mm beams for three-point-bend testing. An Instron 5969 Dual Column Material Testing System was used to test the flexural properties of the samples. Scanning electron microscopy (SEM) was used to evaluate the fractured samples. Results: Results showed that incorporation of anisotropic unidirectional fibers had the most significant effect in reinforcing the resin blend. The unidirectional control group produced the greatest flexural strength and elastic modulus at 336.6 MPa and 37.3 GPa, respectively. The 2-ply+axial group followed at 236.52 MPa and 20.75 GPa. The 2-ply had values of 196.2 MPa and 11.83 GPa. The 4-ply had values of 96.48 MPa and 4.906 GPa. The 4-ply group failed to reinforce a resin control group, which had values of 102.65 MPa and 2.467 GPa.Conclusion: Incorporation of anisotropic reinforcing fibers has most significant influence on the experimental materials flexural properties.

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Experimental dental composites with electrospun nanofibers and nanofibrous composites. (2016)

Electrospun nanofibers with and without nanoparticles are poorly explored in dental research. Nanocrystalline cellulose is a nanoparticle with distinguished properties that has already been associated with nanofibers, but yet not applied to any dental aspect.The objective of this work was to investigate the use of polyacrylonitrile (PAN) nanofibers containing nanocrystalline cellulose (NCC) in the light of the mechanical behavior of fibrous mat and experimental dental composites. Three experiments were performed to answer the following research questions: 1) Can nanocrystalline cellulose improve mechanical properties of polyacrylonitrile nanofiber meshes? 2) Does the method of dispersion (simple mixture vs. with solvent exchange) of NCC in PAN solution affect fiber formation and the respective properties of the meshes? and 3) Can NCC-containing PAN electrospun nanofibers affect flexural properties of experimental dental composites?Results showed that nanocrystalline cellulose, at low concentrations, significantly increases PAN nanofibers tensile properties (chapter 2). Dispersion methods affected both the morphology and mechanical properties of the fibers (chapter 3). Finally, when NCC-containing PAN nanofibers were used to produce experimental dental composites, there was a significant improvement in flexural strength and work of fracture (chapter 4). In conclusion, the findings indicated that the use of electrospun nanofibers and nanofibres containing nanoparticles is a promising approach to reinforce dental composites.

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