UNIVERSIDADE ESTADUAL DE CAMPINAS
FACULDADE DE ODONTOLOGIA DE PIRACICABA
RAFAEL SOARES GOMES
CERÂMICAS ODONTOLÓGICAS MODIFICADAS POR
VIDRO BIOATIVO - PROPRIEDADES FÍSICAS E
MECÂNICAS
DENTAL CERAMIC MODIFIED BY BIOACTIVE GLASS -
PHYSICAL AND MECHANICAL PROPERTIES
Piracicaba 2020
RAFAEL SOARES GOMES
CERÂMICAS ODONTOLÓGICAS MODIFICADAS POR
VIDRO BIOATIVO - PROPRIEDADES FÍSICAS E
MECÂNICAS
DENTAL CERAMIC MODIFIED BY BIOACTIVE GLASS -
PHYSICAL AND MECHANICAL PROPERTIES
Tese apresentada à Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas como parte dos requisitos exigidos para a obtenção do título de Doutor em Clínica Odontológica, na Área de Prótese Dental.
Thesis presented to the Piracicaba Dental School of the University of Campinas in partial fulfillment of the requirements for the degree of Doctor in Clinical Dentistry, in Prosthetic Dentistry area.
Orientadora: Prof. Dra. Altair Antoninha Del Bel Cury
ESTE EXEMPLAR CORRESPONDE À VERSÃO FINAL DA TESE DEFENDIDA PELO ALUNO RAFAEL
SOARES GOMES E PELA PROFA. DRA. ALTAIR ANTONINHA DEL BEL CURY.
Piracicaba 2020
Ficha catalográfica
Universidade Estadual de Campinas
Biblioteca da Faculdade de Odontologia de Piracicaba Marilene Girello - CRB 8/6159
Gomes, Rafael Soares,
G585c GomCerâmicas odontológicas modificadas por vidro bioativo - propriedades físicas e mecânicas / Rafael Soares Gomes. – Piracicaba, SP : [s.n.], 2020.
GomOrientador: Altair Antoninha Del Bel Cury.
GomTese (doutorado) – Universidade Estadual de Campinas, Faculdade de Odontologia de Piracicaba.
Gom1. Cerâmica odontológica. 2. Materiais dentários. 3. Vidros bioativos. I. Del Bel Cury, Altair Antoninha, 1948-. II. Universidade Estadual de Campinas. Faculdade de Odontologia de Piracicaba. III. Título.
Informações para Biblioteca Digital
Título em outro idioma: Dental ceramics modified by bioactive glass - physical and
mechanical properties
Palavras-chave em inglês:
Dental ceramics Dental materials Bioactive glasses
Área de concentração: Prótese Dental Titulação: Doutor em Clínica Odontológica Banca examinadora:
Altair Antoninha Del Bel Cury [Orientador] Raissa Micaella Marcello Machado Flavio Henrique Baggio Aguiar Edmara Tatiely Pedroso Bergamo João Henrique Lopes
Data de defesa: 29-01-2020
Programa de Pós-Graduação: Clínica Odontológica
Identificação e informações acadêmicas do(a) aluno(a)
- ORCID do autor: https://orcid.org/0000-0002-7989-0098 - Currículo Lattes do autor: http://lattes.cnpq.br/4318324775782720
A Comissão Julgadora dos trabalhos de Defesa de Tese de Doutorado, em sessão pública realizada em 29 de Janeiro de 2020, considerou o candidato RAFAEL SOARES GOMES aprovado.
PROFª. DRª. ALTAIR ANTONINHA DEL BEL CURY
PROF. DR. JOÃO HENRIQUE LOPES
PROFª. DRª. EDMARA TATIELY PEDROSO BERGAMO
PROF. DR. FLAVIO HENRIQUE BAGGIO AGUIAR
PROFª. DRª. RAISSA MICAELLA MARCELLO MACHADO
A Ata da defesa, assinada pelos membros da Comissão Examinadora, consta no SIGA/Sistema de Fluxo de Dissertação/Tese e na Secretaria do Programa da Unidade.
DEDICATÓRIA
Dedico esse trabalho aos meus pais, Gilberto Alves Gomes e Maria de Conceição Soares Gomes. Sempre com apoio irrestrito à minha trajetória. Um caminho de muita renúncia, de distância, de saudades, mas compreendendo e apoiando sempre. À minha amada Carol, que juntos crescemos, pessoalmente e profissionalmente, trilhando esse árduo caminho. Inteligente, dedicada, determinada e companheira, faz com que as mais difíceis decisões da vida se tornem mais amenas ao seu lado. Obrigado por tudo.
Aos meus irmãos Patrícia e Júnior, que mesmo distantes torcem e vibram a cada conquista. Entendem a distância, mas fazem dos curtos momentos juntos momentos únicos.
Aos meus sobrinhos Arthur, Davi e Cauã. Estar ausente e perder boa parte do crescimento é um pouco triste. Porém é revigorante mesmo receber uma foto do novo dente que está “nascendo” e chegar em casa após longos meses fora vendo a incrível evolução desses pequenos em tão pouco tempo.
À minha orientadora Profa. Dra. Altair Antoninha Del Bel Cury, por acreditar em mim desde o meu ingresso no mestrado, sempre incentivando o crescimento profissional e pessoal.
AGRADECIMENTOS
O presente trabalho foi realizado com apoio da Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Código de Financiamento 001 bem como pelo Programa de Doutorado Sanduíche no Exterior, processo n° 88881.187691/2018-01. Agradeço à CAPES pela concessão da bolsa do Programa de Doutorado Sanduíche no Exterior.
O presente trabalho foi realizado com apoio do Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), processo n° 141065/2016-8. Agradeço ao CNPq pela concessão da bolsa de doutorado, durante a vigência no país. À Universidade Estadual de Campinas (UNICAMP), na pessoa do Magnífico Reitor, Prof. Dr. Marcelo Knobel.
À direção da Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas, na pessoa do Diretor Prof. Dr. Francisco Haiter Neto e do Diretor Associado Prof. Dr. Flávio Henrique Baggio Aguiar.
À Profa. Dra. Karina Gonzales Silvério Ruiz, coordenadora dos cursos de Pós-Graduação e ao Prof. Dr. Valentim Adelino Ricardo Barão, coordenador do Programa de Pós-Graduação em Clínica Odontológica.
Agradeço à Profa. Dra. Altair Antoninha Del Bel Cury pelos anos de trabalho, orientação e dedicação à graduação e pós-graduação. Espelho de dedicação e luta por uma instituição, emana sabedoria e instiga seus alunos a superarem todas as barreiras da vida pessoal e acadêmica. Estar ao lado da professora Altair é sempre uma oportunidade ímpar de aprender, seja em momentos de reuniões, clínica de graduação e até em conversas informais.
Agradeço ao apoio dos Laboratórios Multiusuários do Centro de Combustível Nuclear (CCN) com a colaboração do Dr. Rafael Henrique Lazzari Garcia, do Centro de Lasers e Aplicações (CLA) com a colaboração do Dr. Marcus Paulo Raele e do Centro de Ciência e Tecnologia de Materiais (CCTM) com a colaboração do Dr. Eguiberto Galego, pertencentes ao Instituto de Pesquisas Energéticas e Nucleares (IPEN-CNEN/SP) pela utilização dos equipamentos Difratômetro de raios X,
Perfilômetro Óptico e Calorímetro de Varredura Diferencial.
Agradeço à Oregon Health & Science University (OHSU) pela oportunidade de realizar meu estágio de Doutorado no Exterior. Grande experiência de crescimento não só profissional como também pessoal.
Agradeço à Profa. Dra. Carmem Pfeifer pela orientação durante o período de estágio de doutorado no exterior. Pessoa de inteligência rara e acolhedora. Agradeço por toda a disponibilidade e oportunidade de colaborar com os trabalhos dessa instituição e ver a seriedade com que são desenvolvidos. Fez desse pequeno tempo um dos períodos de maior aprendizado pessoal e profissional.
Agradeço ao Prof. Dr. Jack Ferracane por todos os ensinamentos, e das grandes contribuições nas reuniões de laboratório. Um ser humano gentil e generoso, exemplo a ser seguido.
Agradeço ao Prof. Dr. Harry Davis pelos ensinamentos e disponibilidade em colaborar sempre. A confraternização de Natal organizada pelo professor Harry será sempre um evento guardado na memória, que nos faz sentir mais acolhidos longe do nosso país e família.
Agradeço aos professores Dr. Wander José da Silva, Dra. Vanessa Cavalli Gobbo e Dra. Raissa Micaella Marcello Machado pelo aceite e colaboração como banca de qualificação de tese desse trabalho.
Agradeço aos professores Dr. João Henrique Lopes, Dra. Edmara Tatiely Pedroso Bergamo, Dra. Raissa Micaella Marcello Machado e Dr. Flávio Henrique Baggio Aguiar pelo aceite e colaboração como banca de defesa de tese desse trabalho, além dos professores Dr. Ney Diegues Pacheco, Dr. Dimorvan Bordin e Dr. Wander José da Silva como membros suplentes.
Agradecimento especial à Caroline Mathias Carvalho de Souza, por toda a ajuda e parceria no desenvolvimento não só desse trabalho, mas como de diversos outros ao longo da pós-graduação tanto no Brasil como durante estágio no exterior.
Agradeço aos professores, amigos, colegas e companheiros de trabalho durante o estágio de doutorado sanduíche na Oregon Health & Science University que fizeram
parte desse período especial: Caroline Mathias, Ana Paula Fugolin, Oscar Navarro, Steven Lewis, Matthew Logan, Luciana Barcelos, Marcela Borges, Julia Rontani, Yui, Hiroshi Morisaki, Nara Rodrigues, Clarissa Mota, Christina Hipfinger-Panton, André Faria, Anthony Tahayeri e Luiz Bertassoni.
Agradeço às amizades incríveis de todos os colegas de laboratório que passaram durante esses anos e aos que ainda permanecem. Agradecimento especial aos amigos de orientação desse período: Raissa Machado, Edmara Bergamo, Dimorvan Bordin, Ney Pacheco, Marco Carvalho, Priscilla Lazari, Louise Dornelas, Victor Muñoz, Mariana Itaboraí, Mirelle Ruggiero, Loyse Martorano e Raíra Brito. Às não menos especiais e queridas Olívia Figueiredo e Mariana Barbosa. À Talita Carleti, Mayara Pinheiro, Camilla Fraga e Ingrid Meira. Ao membro da Dentística, mas PPR de coração Caroline Mathias. A todos os demais colegas da PPR e sem esquecer da Gislane Piton e Eliete Aparecida Ferreira Lima Marim, o braço esquerdo e direito desse laboratório! Além dos professores da área Altair Antoninha Del Bel Cury, Renata Cunha Matheus Rodrigues Garcia, Wander José da Silva e Raissa Micaella Marcello Machado.
Agradecimento especial à ex-colega de turma de pós-graduação, atual professora colaboradora da FOP e amiga Raissa Machado e ao seu marido Hamilton Pereira, por todos os momentos de lazer e descontração além da FOP. Quando estamos longe da família de sangue é importante nos sentirmos acolhidos em uma família de amigos, e vocês representaram muito bem isso.
Agradecimento aos demais amigos feitos nas demais áreas da FOP, especialmente Suelem Chasse e Sameh Brglah.
Agradeço a todos os Docentes e servidores técnico-administrativo da FOP que contribuem para a excelência dessa instituição em todos os níveis.
RESUMO
O comportamento bioativo em materiais odontológicos tem sido investigado em diversos campos da odontologia. Alguns estudos modificaram com sucesso cerâmicas odontológicas adicionando vidro bioativo (VB), tornando esses materiais capazes de exibir um comportamento bioativo. No entanto, essas composições têm apresentado propriedades mecânicas inferiores que podem estar relacionadas a um tratamento térmico inadequado. Além disso, outras propriedades como solubilidade química, dureza, rugosidade de superfície e estabilidade de cor, que podem ser afetadas, não foram ainda avaliadas. Assim, o objetivo deste estudo foi avaliar propriedades físicas e mecânicas de uma cerâmica feldspática reforçada por leucita modificada por vidro bioativo. Para isso, três composições de cerâmica odontológica (CO) modificada por vidro bioativo 58S foram sintetizados pelo método de sol-gel (VB20: 80% em peso de CO + 20% em peso de VB, VB30: 70% em peso de CO + 30% em peso de VB e VB40: 60% em peso de CO + 40% em peso de VB). Inicialmente, as composições foram caracterizadas por calorimetria exploratória diferencial e sua resistência à flexão biaxial foram testadas após sinterização a diferentes temperaturas (910, 950, 1000 e 1050 °C). Um grupo composto por 100% de CO, sinterizado de acordo com as recomendações do fabricante (910 °C), foi utilizado como grupo de controle. A temperatura de sinterização que promoveu os maiores valores de resistência à flexão para cada composição (VB20 a 950 °C, VB30 a 1000 °C e VB40 a 1050 °C) foram utilizadas para a avaliação do comportamento bioativo, solubilidade química, microdureza, rugosidade de superfície e estabilidade de cor. Para verificação do comportamento bioativo os grupos foram avaliados por difração de raios-X (DRX) e espectroscopia de infravermelho por transformada de Fourier (FTIR) com e sem imersão em fluido corporal simulado (Simulated Body Fluid, SBF) por 21 dias. Para a microdureza (n=5), foram avaliados em um microdurômetro com identador Knoop. A solubilidade (n=10) foi avaliada por imersão em solução de ácido acético a 4% por 16 horas, a rugosidade de superfície (n=10) por um perfilômetro óptico 3D e a estabilidade de cor (n=10) pelo sistema CIEL*a*b*. As composições experimentais mostraram indícios de comportamento bioativo após a imersão em SBF. Todos os grupos experimentais apresentaram resistência à flexão menor do que o grupo controle (p <0,05; ANOVA / teste de Tukey) e foram semelhantes entre si. A dureza foi maior apenas no grupo VB40 em comparação com o grupo controle. A solubilidade
química aumentou com o aumento do teor de VB nas composições. Por outro lado, a rugosidade da superfície e a estabilidade de cor tendem a ser melhores nos grupos com maior quantidade de VB. Nenhuma composição experimental conseguiu obter resultados desejáveis em todas as propriedades estudadas. Estudos adicionais são necessários para que a composição e o tratamento térmico ideal possam contribuir para desempenhos mecânicos adequados aliados a um equilíbrio entre o comportamento bioativo e a solubilidade química, bem como baixa rugosidade de superfície e baixa alteração de cor.
ABSTRACT
The bioactive behavior in dental materials has been investigated in several dentistry field. Some studies have successfully modified dental ceramics by adding bioactive glass (BG), making those materials able to exhibit a bioactive behavior. However, those compositions have presented poor mechanical properties which could be related to an inappropriate thermal treatment. Additionally, other properties as chemical solubility, hardness, surface roughness and color stability, which may be affected, have not been tested. Thus, the objective of this study was to evaluate the physical and mechanical properties of a leucite-reinforced glass ceramic modified by bioactive glass. For this, different compositions of dental ceramic (DC) modified by 58S bioactive glass were synthesized by sol-gel method (BG20: 80wt% DC + 20wt% BG, BG30: 70wt% DC + 30wt% BG and BG40: 60wt% DC + 40wt% BG). First, the compositions were characterized by differential scanning calorimetry and had its biaxial flexural strength assessed after different sintering temperature (910, 950, 1000 and 1050 °C). A 100wt% DC group sintered according to the manufacturer’s recommendation (910 °C) was used as a control group. The sintering temperature that promoted the highest flexural strength values for each composition (BG20 at 950 °C, BG30 at 1000 °C and BG40 at 1050 °C) were used to evaluate the bioactive behavior, chemical solubility, microhardness, surface roughness and color stability. To investigate the bioactive behavior, the groups were evaluated by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) with and without Simulated Body Fluid (SBF) immersion for 21 days. For microhardness (n=5), the groups were evaluated by a microhardness tester with a Knoop indenter Knoop. Chemical solubility (n=10) was evaluated by immersion in 4% acetic acid solution for 16 hours, surface roughness (n=10) by a 3D optical profiler and color stability (n=10) by CIEL*a*b* system. Experimental groups showed indication of bioactive behavior with SBF immersion. All the experimental groups presented flexural strength lower than the control group (p < 0.05; ANOVA/Tukey’s test) and were similar among them. The hardness was only higher in the BG40 group compared to the control group. The chemical solubility increased with the increase of the BG content in the compositions. On the other hand, the surface roughness and color stability tend to be better in the groups with major BG amount. No experimental composition was able to obtain desirable results in all properties studied. More studies need to be performed so that the composition and heat treatment could
contribute to suitable mechanical performances allied a balance between bioactive behavior and chemical solubility as well as low surface roughness and low color change.
SUMÁRIO
1 INTRODUÇÃO 14
2 ARTIGO: Effect of thermal treatment of dental ceramic modified by bioactive
glass in the physical-mechanical properties and bioactive behavior 17
3 DISCUSSÃO 43
4 CONCLUSÃO 46
REFERÊNCIAS 47
ANEXOS 54
ANEXO 1: Submissão do artigo ao periódico 54
1 INTRODUÇÃO
Os vidros bioativos são materiais comumente utilizados na área odontológica e ortopédica (Abbasi et al., 2015; Boccaccini et al., 2010; Vitale-brovarone et al., 2012). São criados a partir de modificações na composição de vidros bioinertes, capazes de estimular uma resposta biológica ao ser inserido no corpo humano e de se ligar aos seus tecidos sem produzir toxicidade, inflamação ou respostas imunológicas (Abbasi et al., 2015). Na área odontológica, tem sido utilizado principalmente para a manutenção da crista óssea alveolar após extração dentária (Stanley et al., 1997; Zhong and Greenspan, 2000).
O primeiro vidro bioativo desenvolvido, nomeado como 45S5 (46,1 mol% SiO2,
24,4 mol% Na2O, 26,9 mol% CaO e 2,6 mol% P2O5) foi capaz de ligar-se ao osso de
forma que para ser removido, o próprio osso também deveria sê-lo (Hench, 2006; Hench et al., 1971). Essa forte ligação ocorre por meio da formação de fosfatos de cálcio sobre a superfície do vidro e também do estímulo de crescimento ósseo longe da interface osso-implante (Abbasi et al., 2015; Hench, 2006; Hench et al., 1971). Durante anos, o método de processamento dos biovidros foi realizado por meio da fundição dos componentes do vidro, sobre cadinhos de platina, com temperaturas de até 1400 °C (Li et al., 1991; Sepulveda et al., 2001). Esse método, muitas vezes, gerava impurezas e bioatividade do material diminuída (Abbasi et al., 2015).
O processo de síntese pelo método sol-gel favoreceu o melhor controle de fabricação do vidro bioativo, com pureza maior, redução da temperatura e de custos, sendo o vidro bioativo 58S um dos mais conhecidos obtido por esse método (Goudouri et al., 2011b). A síntese de vidros bioativos pelo método sol-gel é capaz de produzir vidros com bioatividade aumentada em comparação com vidros de mesma composição fabricados pela técnica de fundição (Sepulveda et al., 2001). Para a garantia da bioatividade, as composições dos vidros fabricados pelo método convencional devem possuir uma quantidade de SiO2 de até 60%. Valores superiores
não favorecem a ligação do vidro aos tecidos naturais (Li et al., 1991). Enquanto que para as composições produzidas pelo método sol-gel, a bioatividade pode ser observada mesmo em concentrações de SiO2 de 90% (Li et al., 1991; Pereira et al.,
15
Nos últimos anos, a síntese de cerâmicas bioativas tem sido estudadas no intuito de empregá-las não apenas como material de enxerto, mas também como material restaurador, participando da composição de próteses odontológicas (Abbasi et al., 2016; Chatzistavrou et al., 2010, 2012; Goudouri et al., 2011b, 2014; Manda et al., 2012). A existência de gaps entre o dente e a cerâmica é um achado relativamente comum que pode favorecer maior retenção de biofilme nesse local (Jacobs e Windeler, 1991). O acúmulo de bactérias nessa região, especialmente em pacientes com má higienização oral, pode proporcionar a ocorrência de inflamação com perda de inserção e degradação de tecido, cárie, irritação e necrose pulpar, sendo razões que levam à falha de uma restauração de prótese fixa (Bergenholtz and Syed, 1982; Felton et al., 1991; Jokstad, 2015; Lang et al., 1983). A cerâmica dental convencionalmente utilizada, apesar de biocompatível, não é bioativa. Dessa forma, ela é capaz de restaurar a morfologia e função de uma estrutura coronária perdida, mas não é capaz de desenvolver qualquer ligação verdadeira aos tecidos (Kontonasaki et al., 2003). Estudos apontam que os vidros bioativos podem, além de ligar-se ao tecido ósseo, ligar-se com certos tipos de tecido conjuntivo por meio da fixação do colágeno na superfície do vidro (Abbasi et al., 2015; Craig and LeGeros, 1999; Goudouri et al., 2014; Zhong and Greenspan, 2000). Estudos relatam que a modificação de cerâmicas convencionais de forma a estimular um comportamento bioativo poderia levar a fixação dos tecidos periodontais criando vedação em um gap existente na região marginal (Abbasi et al., 2016; Chatzistavrou et al., 2010, 2012; Goudouri et al., 2011b, 2014). Esta vedação poderia diminuir a ocorrência de falhas das restaurações fixas ao diminuir a micropenetração e adesão das bactérias orais (Craig and LeGeros, 1999). Além disso, é também relatado que os vidros bioativos seriam capazes de aumentar o pH da solução intersticial através da dissolução de íons Na+ ou Ca2+ e incorporação de H+ pelo material, formando silanóis, o que
provocaria uma diminuição de H+ livre e isto, por sua vez, afetaria a viabilidade de
diversas espécies bacterianas (Vallittu et al., 2015).
A confecção de cerâmicas odontológicas bioativas tem sido proposta a partir de modificação de cerâmicas convencionais com adição de porcentagens em peso de biovidro (Abbasi et al., 2016; Chatzistavrou et al., 2010, 2012; Goudouri et al., 2011b, 2014). A adição de biovidros na cerâmica convencional, apesar de garantir sua bioatividade, tende a diminuir suas propriedades mecânicas quando mantida a temperatura de sinterização da cerâmica pura (Abbasi et al., 2016; Goudouri et al.,
2011b). A cristalização do vidro bioativo, ao passo que melhora suas propriedades mecânicas, tende a diminuir sua bioatividade. Além disso, a sua cristalização pode ocorrer a temperaturas superiores às das porcelanas odontológicas (Cacciotti et al., 2012; Dehaghani et al., 2015).
Ainda não há um consenso sobre qual seria a composição ideal que satisfaça minimamente as propriedades mecânicas e bioativas e principalmente não há estudos que avaliam a influência da temperatura de sinterização sobre essas composições. Sabe-se também, que a não boatividade das cerâmicas convencionais garante algumas propriedades desejáveis às próteses fixas. Elas tendem a possuir uma alta estabilidade de cor, com pouco manchamento, além de baixa solubilidade (Karaokutan et al., 2015; Kvam and Karlsson, 2010; Spyropoulou et al., 2016; Swain, 2014). Tais características podem ser afetadas negativamente ao torná-las bioativas. A reatividade maior com o meio oral poderia facilitar o manchamento da cerâmica, comprometendo sua estética. Além disso, a liberação de íons ao meio poderia aumentar a sua solubilidade (Abbasi et al., 2016; Cerruti et al., 2005), degradando a sua superfície mais rapidamente e tornando-a mais rugosa (Cerruti et al., 2005; Swain, 2014).
Considerando que há escassez na literatura sobre as propriedades físicas e mecânicas de cerâmicas odontológicas modificadas por vidro bioativo, nesse estudo foram avaliadas a resistência à flexão, o comportamento bioativo, solubilidade química, microdureza, rugosidade de superfície e estabilidade de cor destes materiais.
17
2 ARTIGO: Effect of thermal treatment of dental ceramic modified by bioactive glass in the physical-mechanical properties and bioactive behavior.
Rafael Soares Gomes, Caroline Mathias Carvalho de Souza, João Henrique Lopes, Celso Aparecido Bertran, Carmem Silvia Pfeifer, Altair Antoninha Del Bel Cury.
AUTHOR INFORMATION R. S. Gomes
Department of Prosthodontics and Periodontology, Piracicaba School of Dentistry (FOP), University of Campinas – UNICAMP, 13414-903 Piracicaba, SP, Brazil
E-mail: [email protected] C. M. C. de Souza
Department of Restorative Dentistry, Piracicaba School of Dentistry (FOP), University of Campinas – UNICAMP, 13414-903 Piracicaba, SP, Brazil
E-mail: [email protected] J. H. Lopes
Department of Chemistry, Division of Fundamental Sciences (IEF), Aeronautics Institute of Technology (ITA), 12228-900, Sao Jose dos Campos-SP – Brazil. E-mail: [email protected]
C. A. Bertran
Department of Physical Chemistry, Institute of Chemistry, University of Campinas – UNICAMP, P.O. Box 6154, 13083-970, Campinas, SP, Brazil.
E-mail: [email protected] C. S. Pfeifer
Department of Biomaterials and Biomechanics, School of Dentistry, Oregon Health & Science University (OHSU), 97201 Portland, OR, USA.
E-mail: [email protected]
A. A. Del Bel Cury (Corresponding Author)
Department of Prosthodontics and Periodontology, Piracicaba School of Dentistry (FOP), University of Campinas – UNICAMP, 13414-903 Piracicaba, SP, Brazil
E-mail: [email protected].
*Este trabalho foi realizado no formato alternativo conforme Deliberação da Congregação da Faculdade de Odontologia de Piracicaba n° 306/2010 da Universidade Estadual de Campinas e submetido ao periódico Journal of The
Highlights
Dental ceramic/bioactive glass mixtures were produced and showed bioactive behavior
An individualized heat treatment improves flexural strength of the compositions All desirable properties have not yet been achieved by a single composition
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In this study, different compositions of dental ceramic (DC) modified by 58S bioactive glass (BG) were synthesized by sol-gel method (BG20: 80wt% DC + 20wt% BG, BG30: 70wt% DC + 30wt% BG and BG40: 60wt% DC + 40wt% BG). A 100wt% DC group sintered according to the manufacturer’s recommendation (910 °C) was used as a control group. First, the compositions were characterized by differential scanning calorimetry, and had its biaxial flexural strength assessed after different sintering temperature (910, 950, 1000 and 1050 °C). The sintering temperature that promoted the highest flexural strength values for each composition (BG20 at 950 °C, BG30 at 1000 °C and BG40 at 1050 °C) were used to evaluate the bioactive behavior, microhardness, chemical solubility, surface roughness and color stability. For bioactive behavior, the groups were evaluated by X-ray diffraction and Fourier-transform infrared spectroscopy with and without Simulated Body Fluid (SBF) immersion for 21 days. For microhardness (n=5), the groups were evaluated by a microhardness tester with a Knoop indenter. Chemical solubility (n=10) was evaluated by immersion in 4% acetic acid solution for 16h, surface roughness (n=10) by a 3D optical profiler and color stability (n=10) by CIEL*a*b* system. Experimental groups showed indication of bioactive behavior with SBF immersion. The BG-containing groups presented lower flexural strength than the control group and were similar among themselves. The microhardness was only higher in the BG40 group compared to the control group. The chemical solubility increased with the increase of the BG content. Conversely, the surface roughness and color stability tended to be better in the groups with major BG amount. No experimental composition was able to obtain desirable results in all properties studied.
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Keywords: Dental ceramic; bioactive glass; 58S; bioactivity; solubility; mechanical performance.
1. INTRODUCTION
Bioactive glasses are a class of biomaterials which are used in dentistry to repair bone defects, such as in periodontal diseases or in the maintenance of alveolar bone after teeth extraction (Boccaccini et al., 2010; Stanley et al., 1997; Vitale-brovarone et al., 2012). Currently, some studies have been done in order to incorporate bioactive glass (BG) in several dental materials as resin composite, adhesives, luting cements and dental ceramics (Al-eesa et al., 2019; Beketova et al., 2018; Davis et al., 2014; Sfalcin et al., 2017; Tezvergil-Mutluay et al., 2017). The addition of BG in dental ceramic (DC) powders by sol-gel method showed a bioactive behavior of this new powder by producing an apatite layer precipitation when immersed in simulated body fluid (SBF) (Abbasi et al., 2016; Chatzistavrou et al., 2012, 2010; Goudouri et al., 2014; Goudouri et al., 2011b; Manda et al., 2012). Those studies report that bioactive behavior around restoration margins could be beneficial, once it increases the pH of the interfacial solution due to ions dissolution, promoting an antibacterial effect (Abbasi et al., 2016; Chatzistavrou et al., 2012, 2010; Goudouri et al., 2014; Goudouri et al., 2011b; Manda et al., 2012; Mortazavi et al., 2010). Additionally, it has been reported that BG facilitates periodontal tissue attachment onto the bioactive material, which could be biologically favorable for subgingival restorations, once this condition could avoid bacterial penetration into marginal gaps, which in turn may promote radicular caries and affect the long-term success of the restoration (Abbasi et al., 2015).
In practice, those new mixtures have been showing poor mechanical properties when tested, achieving flexural strength below the acceptable by ISO 6872 for dental ceramics (Abbasi et al., 2016; Goudouri et al., 2011a). As the amount of BG increases in the mixture, the flexural strength tends to decrease (Abbasi et al., 2016; Goudouri et al., 2011a). This performance can be related to the sintering process, since the
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sintering temperature used for thermally treating those experimental materials are frequently the same to the commercial DC in several studies (Abbasi et al., 2016; Goudouri et al., 2014; Goudouri et al., 2011a; Goudouri et al., 2011b). The addition of BG, which has a different crystallization temperature to the DC, can alter the sintering process of the mixture and produce a new composition that is only partially sintered, which would explain the low mechanical properties (Cacciotti et al., 2012; Taghian Dehaghani et al., 2014). Moreover, the bioactivity test in SBF of this mixture has been evaluated in the powder form, before sintering procedure (Abbasi et al., 2016). This presentation enables an easier ion release compared with a crystallized bioactive glass (Cacciotti et al., 2012; Groh et al., 2014; Massera et al., 2012). This fact can induce a misunderstanding that the same could occur as easily in an oral environment with a sintered material.
The ion exchange between restoration and the buccal environment may promote undesirable results in other important properties of conventional dental ceramics such as its color stability, surface roughness and chemical solubility (Karaokutan et al., 2015; Kvam and Karlsson, 2010; Montazerian et al., 2015; Spyropoulou et al., 2016). It is well known that dental ceramics have low chemical solubility, low surface roughness and high color stability (Karaokutan et al., 2015; Kvam and Karlsson, 2010; Spyropoulou et al., 2016; Swain, 2014). Promoting bioactivity in a dental ceramic by adding bioactive glass is expected to increase their solubility. The amount of calcium in the bioactive glass is able to produce a porous network that allows for water penetration, increase of solubility and consequently increasing the surface roughness (Beketova et al., 2018; Cacciotti et al., 2012; Chatzistavrou et al., 2012; Goudouri et al., 2014; Huang et al., 2014). In addition, the interaction between a non-inert material and extrinsic pigments from diet can increase
the restorations staining, compromising aesthetic (Montazerian et al., 2015; Palla et al., 2018; Silva et al., 2018).
Therefore, this study had as objective to evaluate the effect of thermal treatment of dental ceramic modified by bioactive glass in physical-mechanical properties and bioactive behavior. For this, properties which directly affect the clinical performance of dental ceramics were investigated such as flexural strength, microhardness, chemical solubility, surface roughness and color stability.
2. MATERIAL AND METHODS
2.1 Preparation of dental ceramic (DC) modified by bioactive glass (BG) A precursor solution of a SiO2-CaO-P2O4 bioactive glass (60:36:4 mol% ratio)
was produced by sol-gel method following previous studies (Abbasi et al., 2016; Beketova et al., 2018; Montazerian et al., 2015; Zhong and Greenspan, 2000). Tetraethyoxysilane (TEOS, Si(OC2H5)4) was poured into a solution of DI water and
nitric acid. This mixture was stirred for 30 min to allow for TEOS hydrolysis. After that, triethyl phosphate (TEP, (C2H5)3PO4) was poured into the mixture and stirred for 20
additional min. Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) was added and stirred
for 1h to ensure its dissolution. The reagents were obtained from Sigma-Aldrich (St. Louis, MO, USA) and the amount used of TEOS, calcium nitrate and TEP followed the molar ratio of the 58S bioactive glass (60:36:4). After, different amounts (80, 70 and 60 wt%) of a leucite-based dental ceramic powder (DC) (IPS InLine Dentin, Ivoclar, Schaan, Liechtenstein) were gradually added to the bioactive glass solution. The mixing process was continued until gelation was observed, which took another 2h.
The formed gels were transferred to a porcelain crucible for the drying and stabilization process. First, the content was placed in a furnace at 180 °C for 6h to the
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drying process, where the excess of water and ethanol were removed. For the stabilization cycle the samples were heated at 700 °C for 18h (Abbasi et al., 2016). The solid product formed was pulverized in an agate mortar and sieved to powders of particle size < 45 mm.
2.2 Differential Scanning Calorimetry
Unsintered powder of the 3 experimental groups and a commercial control group (IPS Inline Dentin, Ivoclar, Schaan, Liechtenstein) were characterized by differential scanning calorimetry (DSC) in a DSC 404 F3 Pegasus (Netzsch, Selb, Germany). The measurements were performed on 80 mg samples in an alumina pan at a heating rate of 10 °C/min up to 1200 °C in air atmosphere.
2.3 Biaxial flexural strength
For the biaxial flexural strength test, 10 discs (12 mm diameter x 1.2 mm thickness) were made from each composition, by mixing the powders with distilled water to obtain slurries. The excess of water was removed with a paper towel. The three experimental groups: 80wt% DC + 20%wt BG (BG20), 70wt% DC + 30%wt BG (BG30) and 60wt% DC + 40%wt BG (BG40) were heat-treated (Dekema Austromat M, Dekema Service GmbH, Freilassing, Germany) in different temperatures (910, 950, 1000 and 1050 °C) and the control group (100wt% DC) at 910 °C following the manufacturer’s recommendation. The samples were polished by grit silicon carbide paper #400, #600, #1200 and #2000 under a steady stream of water in a rotatory polishing device (Arotec APL-4, Cotia, SP, Brazil) until a flat and parallel surface was obtained. After that, the specimens were polished with diamond paste with a grain diameter of 1 µm (Buehler, Lake Bluff, IL, USA). Finally, the samples were washed in an ultrasonic bath for 5 min and kept in a desiccator before use. Then, the discs were
subjected to biaxial flexural strength test in a universal testing machine (Instron 4411, Instron, Norwood, MA, USA) at a crosshead speed of 0.5 mm/min. The samples were concentrically positioned on a three hardened steel balls (4.5 mm diameter) positioned 120° apart on a support circle (11 mm diameter). The load was applied with a dowel flat pin with a diameter of 1.5 mm at the center of the specimen. The biaxial flexural strength (σ) was calculated in megapascal (MPa) according to the formula (ISO 6872):
Where P is the total load causing fracture, in newtons; X=(1+v)ln(r2/r3)2+[(1−v)/2](r2/r3)2; Y=(1+v)[1+ln(r1/r3)2]+(1−v)(r1/r3)2; ν is Poisson’s ratio
(considered as 0.25); r1 is the radius of support circle, in mm; r2 is the radius of loaded
area, in mm; r3 is the radius of specimen, in mm; b is the specimen thickness in mm.
The sintering temperature that promoted the highest flexural strength values for each composition (BG20 at 950 °C, BG30 at 1000 °C and BG40 at 1050 °C) were used to evaluate the bioactive behavior, microhardness, chemical solubility, surface roughness and color stability.
2.4 Bioactivity behavior by calcium-phosphate forming ability
For the qualitative analysis of bioactive behavior, two discs (12 mm diameter x 1.2 mm thickness) of each one of the four groups (DC at 910 °C, BG20 at 950 °C, BG30 at 1000 °C and BG40 at 1050 °C) were evaluated by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) with and without simulated body fluid (SBF) soaking (in 10-3 mol: 142 Na+, 5 K+, 1.5 Mg2+, 2.5 Ca2+, 147.8 Cl-, 4.2 HCO3-, 1.0
HPO42-, 0.5 SO42, pH=7.4) (Kokubo and Takadama, 2006). For XRD analysis was used
an X-ray Diffractometer (XRD, Bruker D8 Advance 3kW, Karlsruhe, Germany) equipped with a CuKα tube, 35 kV voltage and 35 mA. Data were obtained at step size
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of 0.04° in the 2θ range of 12–80°. After, the same samples used in the XRD were grinded for FTIR analysis by KBr pellet technique (0.1mg of the sample was mixed with 0.002mg of KBr) and evaluated in a Nicolet 6700 FTIR (Thermo Scientific, Pittsburgh, PA, USA). The spectra were recorded in transmittance mode in the mid-infrared range of 4000–400 cm-1 with 32 scans at 4 cm-1 resolution. For the discs which were
immersed in SBF, they were kept in the solution for 21 days, and the solution replaced every 3 days. At the end of the 21 days, the discs were removed from SBF, gently washed with distilled water, dried in a desiccator and also evaluated by FTIR and XRD as previously described.
2.5 Chemical solubility
For the chemical solubility analysis, 10 discs (12 x 1.2 mm) from each group were used. They were washed with distilled water and dried in an oven at 150 °C for 4h. After, the samples were weighed on an analytical balance with accuracy of 0.1 mg for determination of initial mass and then immersed for 16h in a solution of 100 ml of 4% acetic acid heated to 80 °C. Later, the samples were washed and dried again in an oven at 150 °C for 4h and reweighed until constant mass was achieved. The chemical solubility was calculated by mass loss in micrograms per square centimeter of the samples (ISO 6872:2015).
2.6 Knoop microhardness
The microhardness was measured using a microhardness tester with a Knoop indenter (FM-100; Future-Tech Corp., Kawasaki, Japan) with a load of 5 N and a dwell time of 10 s. Five measurements were performed on 5 samples (7 mm diameter x 1.2 mm thickness) from each group (Zhang et al., 2017).
2.7 Surface roughness
The surface roughness was evaluated in a non-contact 3D optical profiler (ZeGage; Zygo Corporation, Middlefield, CT, USA). Ten samples of each group (7 mm diameter x 1.2 mm thickness) were analyzed using the arithmetical mean height (Sa) and root mean square height (Sq) parameters.
2.8 Color stability
For the color stability test, 10 samples of each group were prepared (7 mm diameter x 2mm thickness). Initial color values of all specimens were measured in CIEL*a*b* color space with a spectrophotometer (Konica Minolta CM-700D, Tokyo, Japan) using a white background and recorded by the software SpectraMagic NX (Konica Minolta, Tokyo, Japan). According to the Commission Internationale D’
Eclairage (CIE) the coordinate L* represents the lightness value (a gray scale from
darkest black at L*= 0 to the brightest white at L*= 100), the parameter a* shows the chromaticity ranging from greenness (negative) to redness (positive), and the b* parameter shows the chromaticity ranging from blueness (negative) to yellowness (positive). After the initial measurements (L0, a0, b0), the specimens were immersed in a coffee solution. 15 g of coffee powder (Nescafé Clasico dark roast, Nestlé, Glendale, CA, USA) was poured into 500 mL of boiled water and stirred. The specimens were immersed separately in the solution at 37°C in 24-well cell culture plates and stored in an oven at the same temperature for 7 days (Silva et al., 2018). The solution was refreshed each 3 days by the same operator. After 7 days of coffee storage, the specimens were rinsed under running water, hand-dried, and the final color (L1, a1, b1) measurements were achieved as before. The distance metric which demonstrate difference between the color before and after coffee storage was calculated using the following formula:
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2.9 Statistical analysis
For statistical analysis the IBM SPSS Statistics 20 software (IBM Corp., Armonk, New York, USA) was used. Normal data distribution and homoscedasticity were confirmed for the flexural strength, hardness and color stability by Shapiro-Wilk test and Levene's test, respectively. A two-way ANOVA was performed to compare the flexural strength of the experimental groups taking in to account the composition and sintering temperature as factors. Dunnett's test was performed to compare the experimental groups to the control group. To analyze hardness and color stability data, a one-way ANOVA and Tukey's HSD test were performed. For chemical solubility and surface roughness data, since ANOVA assumptions were not met, a Kruskal-Wallis test was performed. The overall significance level was set at 5% (α=0.05).
3. RESULTS
Curves of heat flow as a function of temperature for the studied groups analyzed by differential scanning calorimetry are shown in figure 1a and 1b. From these curves it is possible to observe a slight change in the baseline curve approximately at 580 °C (control) and about 630 °C for the experimental groups, which could be attributed to the glass transition temperature. In addition, exothermic peaks indicating crystallization events are observed after the temperature reaches 900-950 °C. In BG30 and BG40, a second exothermic peak is evident in the DSC curve.
Figure 1. Differential scanning calorimetry curves of the DC, BG20, BG30 and BG40 groups. (a) Full DSC curves and (b) its magnified region around glass transition of 58S bioactive glass.
Biaxial flexural strength means and standard deviation are summarized in table 1. The control group, which was tested only in the temperature recommended by the manufacturer, had statistically higher flexural strength than the BG-containing groups (Dunnett’s test, p < 0.01). Among the experimental groups, the increase of temperature increased the flexural strength. However, a threshold in temperature was identified for each material (1000 °C for BG20 and 1050 °C for BG30), above which degradation of the material was observed, which led to complete deformation of the disc during the sintering procedure and preventing the flexural strength measurement at these temperatures. Comparing the groups at the same temperature, it is possible to observe a decrease in the flexural strength as the BG content increases.
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Table 1. Biaxial flexural strength means (standard deviation) in MPa according to the group distribution and sintering temperature.
Composition 910 °C 950 °C 1000 °C 1050 °C
DC 73.4 (5.8) - - -
BG20 34.2 (2.5)Ba* 41.1 (3.0)Aa* n. a n. a
BG30 30.0 (2.9)Cb* 33.1 (3.0)Bb* 40.2 (3.7)Aa* n. a BG40 26.7 (2.2)Cc* 28.6 (3.5)Cc* 35.5 (4.0)Bb* 41.3 (3.7)A*
Different uppercase letters in the same row and different lowercase letters in the same column indicate statistical difference between the experimental groups (two-way ANOVA/Tukey’s HSD test, p < 0.05)
*Indicates difference with commercial dental ceramic (Dunnett’s test, p < 0.05)
To evaluate the bioactive behavior, microhardness, chemical solubility, surface roughness, and color stability, the temperature which led to the highest values for each group were used (BG20 – 950 °C, BG30 – 1000 °C and BG40 – 1050 °C). At this temperature, the flexural strength among them is similar but lower than the control group (figure 2).
Figure 2. Mean and standard deviation of the biaxial flexural strength of the highest values for each group showing statistically difference of the control group (DC) to the experimental groups (ANOVA/Tukey’s HSD test).
The crystallographic long-range structure and the ability to form a calcium phosphate layer in SBF assays was evaluated by X-ray diffraction (Figure 3a). Both control and experimental groups presented an amorphous silicate network with calcium silicate phase. The calcium silicate phase of the control group was identified as leucite, a common phase in commercial feldspathic dental ceramics, with major peaks at 16.5°, 25.9°, 27.16°, 30.5° and 31.5° 2θ (Sinmazişik and Öveçoǧlu, 2006). In the BG-containing groups, calcium phosphate and calcium silicate phases were present, of which wollastonite was the dominant calcium silicate type at around 30° 2θ (Chatzistavrou et al., 2012; Goudouri et al., 2014). In the group with the highest BG content, a hydroxyapatite phase was assigned at 32.92° 2θ, which can be present even before soaking in SBF (Goudouri et al., 2014). After SBF immersion, a hydroxyapatite peak was found in BG20 and BG30 and an overall increase of calcium phosphate peaks are seen for all experimental groups.
The FTIR spectra reveals all groups present a broad band at 1025-1050 cm-1
and 800 cm-1, which are attributed to the Si-O-Si stretching vibration modes Lopes et
al., 2019) (Figure 3b). Additionally, the spectra show a band at 470 cm-1 assigned to
the Si-O bending mode and at 640 and 720 cm-1 assigned to Si-O-Al of the leucite
(Abbasi et al., 2016; Chatzistavrou et al., 2010; Goudouri et al., 2014; Hatzistavrou et al., 2010). The BG containing groups shows a shoulder at 950 cm-1 related to the
Si-O-Ca vibration mode of the wollastonite. They also present a double peak at 566 and 602 cm-1 assigned to P-O vibration mode (Abbasi et al., 2016; Chatzistavrou et al.,
2010; Goudouri et al., 2014; Hatzistavrou et al., 2010). After SBF immersion, the bioactive behavior indication might be related to the shifting and sharpening of the broad peak at 1000-1100 cm−1 and the slight enhancement of bands at around 566 cm−1 and 602 cm-1 (Beketova et al., 2018; Lopes et al., 2019).
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Figure 3. (a) XRD pattern of the groups with and without soaking in SBF. L: Leucite, W: Wollastonite, CP: Calcium phosphate, HAp: Hydroxyapatite. (b) FTIR spectra of the DC, BG20, BG30 and BG40 before and after SBF immersion (-SBF suffix). ★: Si-O-Si bending, ⬢: P-O, ■: Si-O-Al (leucite), ●: Si-O-Si stretching, ○: Si-O-Ca (wollastonite).
For the chemical solubility test, the groups BG30 and BG40 showed higher chemical solubility than the control group and BG20 had similar chemical solubility to the control group (Figure 4a). The Knoop microhardness showed statistically similar results among DC (4.76 ± 0.26 GPa), BG20 (4.79 ± 0.41 GPa) and BG30 (5.01 ± 0.41 GPa) (p > 0.05). Only BG40 (5.21 ± 0.23 GPa) presented a statistically higher hardness compared to the DC (p = 0.044).
The mean surface roughness for Sa parameter were lower for the DC and BG40 groups, which were statistically similar between them. For the BG20 and BG30, Sa parameter was higher than the DC (p < 0.05) but similar to the BG40. For the Sq parameter only BG20 group presented statistically higher values than the other groups, which were similar among them (Figure 4b).
Figure 4. (a) The boxplot shows a trending upward of the chemical solubility as the BG content increases. Lower amounts of BG in the composition (20%) are not statistically different to the control group, whereas greater amounts (40%) are. Different letters indicate statistical difference (Kruskal-Wallis test, p < 0.05). (b) Boxplot diagram of the Sa and Sq parameter surface roughness. Different uppercase and lowercase letters indicate statistical difference between groups for Sa and Sq parameters, respectively (Kruskal-Wallis test, p < 0.05).
The control group presented the lowest color change after immersion in coffee solution (DE = 0.64±0.13), and therefore the best color stability among all groups, apart from being the only group where DE < 3.3, value considered as an unacceptable color change which can be eye-visible detected. A decrease of the DE values is observed as the BG amount increase (Figure 5).
Figure 5. Mean and standard deviation of DE values. Different letters indicate statistical difference (ANOVA/Tukey’s HSD test, p < 0.05).
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4. DISCUSSION
The temperature at which a BG is sintered affects the crystalline phase and its microstructure (Cacciotti et al., 2012; Groh et al., 2014). Studies have shown that when a composite is made by mixing dental ceramic powders with bioactive glass in sol-gel solution, there is a linear relationship between the decrease of flexural strength and the increase of the amount of BG (Abbasi et al., 2016; Beketova et al., 2018; Goudouri et al., 2011a). In accordance with that, the first part of this study showed a clear negative influence of the BG amount in the flexural strength when they were sintered at the same temperature. One of the strategies to control the decrease in flexural strength is to heat treat the specimens at different temperatures since, as already mentioned, the increase of the temperature can modify the crystalline components of the composition and lead to an improvement in its mechanical properties (Cacciotti et al., 2012). In fact, that approach had a statistically significant improvement in the flexural strength results of this study.
On the other hand, the literature reports that as the crystalline phase increases in size, the hydroxyapatite layer formation over the material decreases; additionally, at high enough temperatures, the bioactive glass can even become an inert material (Cacciotti et al., 2012). Because of this, the high sintering temperatures before material degradation, which promoted the higher flexural strength values, were used for the other tests in order to observe if they would exhibit bioactive behavior and good physical-chemical properties relevant to clinical use. The FTIR and XRD results reveal that all groups exhibited some indication of bioactivity. Notwithstanding, while the literature describes that the increase of BG content increases the bioactivity (Abbasi et al., 2016; Goudouri et al., 2011a; Goudouri et al., 2011b), this was not observed in this
study. This is probably related to the sintering temperature, where the major BG content was followed by an increase of the sintering temperature. Thus, some sort of compensation mechanism may have been developed, creating a limit to the bioactive behavior.
In spite of the higher sintering temperature that could make the ion release more difficult (Cacciotti et al., 2012), the chemical solubility seems to have been affected by the presence of bioactive glass. The worse results were observed for the groups with greater BG content, with BG40 mean values being considered above than the acceptable according to the ISO 6872. The solubility can be related not only to the BG content and sintering temperature, but also to morphological characteristics such as particle size, pore size and surface area (Sepulveda et al., 2002). Although BG40 showed higher microhardness values compared with the control group, all groups presented mean values in the range of the reported values of hardness of dental enamel, of 3 to 6 GPa (Beketova et al., 2018). Moreover, they also presented mean hardness values compatible with several dental ceramic systems (Sinmazişik and Öveçoǧlu, 2006). This indicates that the materials proposed here would likely be clinically acceptable from the hardness standpoint. Regarding surface area, major surface roughness would be an aspect which could increase de surface area to solution and consequently the ion release (Sepulveda et al., 2002). However, in this study the BG-containing groups had statistically similar mean surface roughness. Therefore, this is not seen as a factor which influenced the chemical solubility in this study.
Conversely, the Sq parameter, which shows the dispersion of the set of data values, shows that there is a tendency to the data points of the surface roughness to be spread out over a wider range of values for the groups that contains BG in the lesser
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amount. Thus, differently from the BG amount, which clearly influenced the chemical solubility (increasing it), the real influence of the surface roughness parameter on the chemical solubility is not clear. It is known that differences among the surface roughness, regardless of the BG content, can be related to the sintering temperature which influence the pore size and material densification (Cacciotti et al., 2012). This could be the reason for a different surface roughness.
The intrinsic bioactive glass degradation is pointed out as a disadvantage for certain clinical applications (Cacciotti et al., 2012). The color stability and staining resistance are considered properties as important as fracture resistance and other mechanical properties of restorations (Sagsoz et al., 2016). In this study, the surface degradation and absorption of staining agents promoted by a bioactive surface led to visual changes, which would likely lead to early replacement of the restoration in clinical use (Cacciotti et al., 2012; Sagsoz et al., 2016; Silva et al., 2018). In this study the DE was lower in the control group, for which low surface roughness and chemical solubility were also observed. Among the experimental groups, the color stability was better as the BG amount increased. This could be explained by the surface roughness, property which can increase the extrinsic staining (Palla et al., 2018).
Considering no other further optimizations, none of the experimental groups tested here would be acceptable for clinical use, since they presented, in some of the properties studied, values considered undesirable. None of them presented flexural strength above 50 MPa, recommended by ISO 6872 and the BG40 group presented mean chemical solubility higher than 100 µm/cm2. The surface roughness of the
experimental groups showed mean values above 0.2 µm, considered as the threshold for bacterial adherence and plaque accumulation (Lima et al., 2006). Furthermore, all of them presented DE values higher than 3.3, considered unacceptable in several
studies, what could compromise the esthetic and long-term success (Palla et al., 2018; Silva et al., 2018).
As a limitation of this study, not only does the sintering temperature but also the dwell time at that temperature could be investigated followed by a dilatometry analysis. Assessments of the longevity of the bioactive behavior are needed as an ion release profile besides a cytotoxicity evaluation and antibacterial effect.
5. CONCLUSION
No experimental composition was able to obtain desirable results in all properties studied at the same time. Although an improvement in the mechanical properties has been achieved in this study with an individualized heat treatment for each composition, additional studies are necessary to improve these materials.
ACKNOWLEDGMENTS
We thank the support given by the Center for Lasers and Applications' Multiuser Facility (CLA), the Nuclear Fuel Center (CCN) and the Materials Science and Technology Center (CCTM) at Nuclear and Energy Research Institute (IPEN-CNEN/SP). The authors also thank the National Council for Scientific and Technological Development (CNPq grant #141065/2016-8) and Coordination for the Improvement of Higher Education Personnel (CAPES grant #88881.187691/2018-01) agencies for their funding.
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3 DISCUSSÃO
O comportamento bioativo tem sido desejado para diversos materiais odontológicos como cimentos, adesivos, resinas e cerâmicas odontológicas (Al-eesa et al., 2019; Sfalcin et al., 2017; Tezvergil-Mutluay et al., 2017). O termo bioatividade para odontologia tem sido definido como um processo biológico benéfico ativo nos quais os materiais restauradores, além de sua função principal de restaurar ou substituir a estrutura dentária perdida, teriam que ser capazes de estimular ativamente ou direcionar respostas celulares ou teciduais específicas, ou ambas, ou controlar interações com espécies microbiológicas (Hench, 2006; Vallittu et al., 2018). De forma ampla, o termo bioativo deve descrever materiais restauradores que possuem algumas características tais quais: promover a formação de tecido reparador, dissolver componentes que estejam associados a atividade antimicrobiana, incluindo os compostos que elevem o pH, possuir superfície condutiva para ligação celular e superfície que promovam a formação de fosfatos de cálcio (Hench, 2006; Vallittu et al., 2018).
Estudos vem mostrando que a adição de cerâmica em pó em solução precursora de vidro bioativo produz um tipo de material cerâmico bioativo, porém estes apresentam um padrão linear de diminuição da resistência à flexão quanto mais vidro bioativo compõe a formulação (Abbasi et al., 2016; Beketova et al., 2018; Goudouri et al., 2011b). No presente estudo foi observada tal relação, onde, em uma mesma temperatura de sinterização, a resistência à flexão diminuiu conforme mais vidro bioativo foi inserido.
Um mecanismo que eleva a resistência do material pode ser a alteração da cristalinidade e a microestrutura do material, onde a temperatura na qual ele é sinterizado pode acarretar em um material mais ou menos resistente (Cacciotti et al., 2012; Groh et al., 2014). No presente estudo, o aumento da temperatura de sinterização de uma composição também aumentou linearmente a sua resistência à flexão, sendo que os grupos com maiores proporções de vidro bioativo conseguiram ser sinterizados à maiores temperaturas sem serem degradados. Ainda que uma melhor resistência à flexão tenha sido alcançada quando comparado à estudos anteriores (Abbasi et al., 2016; Beketova et al., 2018; Goudouri et al., 2011a), estas são consideradas abaixo do ideal de 50 MPa pela ISSO 6872 para cerâmicas odontológicas. Uma possível solução seria a utilização desse material estratificado