UNIVERSIDADE ESTADUAL DE CAMPINAS FACULDADE DE ODONTOLOGIA DE PIRACICABA
VALÉRIA BISINOTO GOTTI
INFLUÊNCIA DA HIDROFILICIDADE DE NANOGÉIS INCORPORADOS A SISTEMAS ADESIVOS NO DESEMPENHO MECÂNICO E ADESÃO À DENTINA
INFLUENCE OF HYDROPHILICITY OF NANOGELS INCORPORATED INTO DENTAL ADHESIVE FORMULATIONS ON MECHANICAL PERFORMANCE AND ADHESION TO
DENTIN
PIRACICABA 2016
VALÉRIA BISINOTO GOTTI
INFLUÊNCIA DA HIDROFILICIDADE DE NANOGÉIS INCORPORADOS A SISTEMAS ADESIVOS NO DESEMPENHO MECÂNICO E ADESÃO À DENTINA
INFLUENCE OF HYDROPHILICITY OF NANOGELS INCORPORATED INTO DENTAL ADHESIVE FORMULATIONS ON MECHANICAL PERFORMANCE AND ADHESION TO
DENTIN
Tese apresentada à Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas como parte dos requisitos exigidos para obtenção do título de Doutora em Materiais Dentários.
Thesis presented to the Piracicaba Dental School of the University of Campinas in partial fulfillment of the requirements for the degree of Doctor in Dental Materials area.
Orientador: Prof. Dr. Américo Bortolazzo Correr
Este exemplar corresponde à versão final da tese defendida pela aluna Valéria Bisinoto Gotti e orientada pelo Prof. Dr. Américo Bortolazzo Correr
PIRACICABA 2016
Agência(s) de fomento e nº(s) de processo(s): CAPES
Ficha catalográfica
Universidade Estadual de Campinas
Biblioteca da Faculdade de Odontologia de Piracicaba Marilene Girello - CRB 8/6159
Gotti, Valéria Bisinoto,
G715i GotInfluência da hidrofilicidade de nanogéis incorporados a sistemas adesivos no desempenho mecânico e adesão à dentina / Valéria Bisinoto Gotti. – Piracicaba, SP : [s.n.], 2016.
GotOrientador: Americo Bortolazzo Correr.
GotTese (doutorado) – Universidade Estadual de Campinas, Faculdade de Odontologia de Piracicaba.
Got1. Polímeros. 2. Nanotecnologia. 3. Hidrólise. 4. Resistência de materiais. 5. Adesivos dentinários. I. Correr, Americo Bortolazzo,1981-. II. Universidade Estadual de Campinas. Faculdade de Odontologia de Piracicaba. III. Título.
Informações para Biblioteca Digital
Título em outro idioma: Influence of hydrophilicity of nanogels incorporated into dental
adhesive formulations on mechanical performance and adhesion to dentin
Palavras-chave em inglês: Polymers Nanotechnology Hydrolysis Material resistance Dentin-bonding agents
Área de concentração: Materiais Dentários Titulação: Doutora em Materiais Dentários Banca examinadora:
Americo Bortolazzo Correr [Orientador] Marcelo Giannini
Roberto Ruggiero Braga Regina Maria Puppin Rontani William Cunha Brandt
Data de defesa: 04-02-2016
Programa de Pós-Graduação: Materiais Dentários
Dedicatória
A Deus, minha fonte inesgotável de força e sabedoria, por me guiar, me proteger e me capacitar para mais essa caminhada.
Aos meus pais, Eduardo e Fátima, que me possibilitam a realização de tantos sonhos. Obrigada pelo abraço carinhoso, conselho sincero e apoio incondicional. Obrigada por acreditarem sempre no meu potencial e compreenderem minhas escolhas, mesmo quando o instinto de proteção e a saudade falavam mais alto. O exemplo de simplicidade e caráter que vocês são pra mim faz com que eu me orgulhe de tê-los comigo nesta vida. Toda minha gratidão, respeito e amor.
À minha irmã, Giselle, pela amizade, paciência e apoio, mesmo à distância. Obrigada por me proporcionar a oportunidade de aprender a ser uma pessoa melhor, frente a tantos erros e desafios da convivência. A você, meu carinho e os melhores desejos de felicidade.
Agradecimentos
Ao meu orientador, Prof. Dr. Américo Bortolazzo Correr, por mais uma oportunidade de aprendizado e crescimento pessoal. Obrigada pelo empenho, dedicação e paciência, em todos os momentos. Eu me sinto imensamente grata por poder conviver com você e receber seu apoio e respeito. Saiba que o admiro e me inspiro na sua competência e jeito calmo e esforçado de conduzir a vida. Muito obrigada.
À Faculdade de Odontologia de Piracicaba da Universidade Estadual de Campinas, na pessoa do Diretor Prof. Dr. Guilherme Elias Pessanha Henriques, pela acolhida e suporte para aproveitar as oportunidades a mim concedidas.
Ao especial Prof. Dr. Simonides Consani, por compartilhar conosco tanta experiência e sabedoria. Obrigada pela chance de poder aprender com um verdadeiro mestre.
Aos professores da Área de Materiais Dentários, Prof. Dr. Lourenço Correr Sobrinho,
Prof. Dr. Mario Alexandre Coelho Sinhoreti, Prof. Dr. Marcelo Giannini, Profa. Dra. Regina Maria Puppin Rontani, Profa. Dra. Fernanda Miori Pascon e ao Prof. Dr. Mario Fernando de Goes, pelos conhecimentos transmitidos, incentivo de buscar sempre o melhor e
constante disponibilidade em ajudar.
Ao Dr. Jeffrey W. Stansbury, supervisor do meu doutorado sanduíche no exterior, pela oportunidade de aprender tanto durante um ano de estágio na Universidade do Colorado. Foi um prazer imensurável poder conviver com um pesquisador sensacional e uma pessoa com o maior coração que eu já pude encontrar. Obrigada por compartilhar comigo tanto conhecimento e pelo apoio em todas as partes da realização deste projeto. Meu muito obrigada também ao técnico Steven Lewis, por todo auxílio que recebi no laboratório durante este período.
À querida amiga Ima Yaghoubi-Rad pelo amor e carinho com que fui recebida em Denver e na Universidade do Colorado. Obrigada pela ajuda, apoio e por me ensinar tanto, sempre com
simplicidade e paciência. Meus dias com certeza foram mais especiais, mais felizes e mais animados, pois eu pude dividi-los com você!
À família que construí em Denver, Marina e Vinícius, Lucy e Axl, por serem meu porto seguro quando eu estava tão longe de casa. Vocês me fizeram sentir parte da vida de vocês e, em tão pouco tampo, eu pude perceber que vocês são parte da minha vida também! Obrigada pelo amor, preocupação, carinho, ajuda! Obrigada por me apresentarem tantas coisas novas e bonitas, pelo bom humor, pela rotina e pela oportunidade de aprender tanto!
Ao querido colega Prof. Dr. Victor Feitosa, pela oportunidade de poder continuar a contar com você em todos os momentos. Trabalhar e aprender com você é e sempre será um prazer. Obrigada por sua atenção, confiança, opinião sincera e ajuda de sempre, mesmo a distância.
À Selma Aparecida Barbosa Segalla e Marcos Blanco Cangiani, do Laboratório de Materiais Dentários da Faculdade de Odontologia de Piracicaba, pelo carinho e respeito. Obrigada pela boa vontade, bom humor e sabedoria transmitidos a mim desde que ingressei na FOP para o mestrado.
Ao Adriano L. Martins, responsável pelo Laboratório de Microscopia Eletrônica de Varredura da Faculdade de Odontologia de Piracicaba, pela paciência, bom humor e colaboração especial na realização de microscopias.
À CAPES pela concessão das bolsas de estudo que possibilitaram a realização deste curso de doutorado no Brasil e nos Estados Unidos.
À amiga Bruna Fronza, por compartilhar tantos momentos especiais desta parte da minha vida em Piracicaba. Obrigada pela convivência prazerosa, pela paciência inesgotável, pelo apoio nas situações difíceis e por vibrar comigo em todas as minhas conquistas. Obrigada por fazer de São Paulo uma extensão da minha casa em Minas Gerais, com um toque do Rio Grande do Sul! Foi tudo sempre muito bom!
Aos queridos amigos, Ana Paula Ayres, Pedro Freitas e Carolina Bosso, meu time insubstituível! Vocês são pessoas maravilhosas que levarei sempre em minha memória e
coração. Muito obrigada pela amizade, momentos de diversão e ajuda em todas as situações. Obrigada pelo companheirismo, incentivo e carinho. Estarei sempre torcendo por vocês!
Aos amigos de sempre, Anna Isabel Tiveron, Marina Stark, Angela Mendes, Juliana
Guide, Rhaiza Naves, Isadora Guimarães, Patrícia Makishi, Mariana Moreira, Narayane Kohl e Dudu Rezende, obrigada por se fazerem presentes mesmo à distância e por
compartilharem comigo a felicidade e a satisfação de alcançar mais um objetivo.
Aos colegas de mestrado e doutorado, deste e de outros programas, obrigada pelo carinho, aprendizado e troca de experiências.
Aos meus familiares, que torceram por mim e à todos que, direta ou indiretamente, contribuíram para mais essa conquista.
RESUMO
O objetivo neste estudo foi comparar nanogéis (NG) com diferentes níveis de hidrofilia e avaliar o desempenho mecânico desses materiais e os efeitos de sua incorporação em um adesivo dental sobre a resistência da união (RU) à dentina. Três nanogéis foram sintetizados: NG1 hidrófobo - IBMA e UDMA; NG2 anfifílico- HEMA e BisGMA; NG3 hidrófilo- HEMA e TE-EGDMA. As partículas foram caracterizadas quanto ao tamanho, peso molecular e temperatura de transição vítrea. O módulo de flexão (MF) foi avaliado para todos os nanogéis, dispersos em etanol (NG2 e NG3) ou 4-N,N-dimetilaminobenzoato (NG1); HEMA e BisGMA/HEMA 40:60 wt%. As amostras foram armazenadas por 7 dias em condição seca (SE) ou em água (condição úmida - UM) e o MF obtido por ensaio de flexão de três pontos. O grau de conversão (GC) das amostras foi avaliado antes do armazenamento. Para a análise da RU à microtração foram utilizados 40 molares humanos. Um adesivo experimental convencional de 1 frasco à base de BisGMA/HEMA (40:60 wt%) e etanol foi utilizado como controle (sem adição de nanogel) e com incorporação dos nanogéis. Blocos foram confeccionados usando o compósito Filtek Z100. As amostras foram cortadas em palitos, que foram armazenados por 24 horas ou por 3 meses em água destilada e tracionadas com velocidade de 1mm/minuto. Após o teste de microtração, a análise do padrão de fratura foi realizada nos palitos. Os resultados foram estatisticamente (α=0,05) analisados por ANOVA e teste de Tukey e para MF meio de dispersão HEMA, foram utilizados os testes Kruskal-Wallis e Student Newman Keuls. Para SE, NG2 apresentou o maior MF quando disperso em solvente e HEMA. Para o meio de dispersão BisGMA/HEMA, o grupo controle apresentou o maior MF. Para UM, NG1 apresentou o maior MF em HEMA e BisGMA/HEMA. Dispersos em solvente, NG1 e NG2 apresentaram os maiores resultados. Após armazenamento em água, NG1 e NG2, dispersos em solvente, mostraram aumento no MF em relação à SE, enquanto que NG3 apresentou significativa diminuição dos valores. Para os meios de dispersão HEMA e BisGMA/HEMA, todos os grupos apresentaram menor MF quando em água. NG2 apresentou o maior GC em solvente e BisGMA/HEMA. Em HEMA, NG1 e NG3 obtiveram os maiores resultados. Para o teste de RU, em 24 horas, o grupo controle e NG2 apresentaram os maiores valores. Foi observado aumento nos valores de RU após o armazenamento em água para NG2, enquanto que os grupos controle, NG1 e NG3 tiveram seus valores mantidos após 3 meses. Concluiu-se que os NG com diferentes níveis de hidrofilia apresentaram diferentes desempenhos mecânicos e foram capazes de
interferir na RU à dentina. De modo geral, o NG hidrófobo IBMA-UDMA apresentou maiores resultados mecânicos, mas os melhores valores de RU à dentina foram obtidos com o nanogel anfifílico à base de BisGMA/HEMA.
Palavras-chave: Polímeros, Nanotecnologia, Hidrólise, Resistência de materiais, Adesivos dentinários
ABSTRACT
The aim of this study was to compare nanogels (NG) with different levels of hydrophilic and evaluate the mechanical performance of these materials and the effects of their addition to a dental adhesive on bond strength (BS) to dentin. Three nanogel copolymers were synthesized: NG1 hydrophobic - IBMA and UDMA; NG2 amphiphilic - HEMA and BisGMA; NG3 hydrophilic- HEMA and TE-EGDMA. The particles were characterized to obtain information such as size, molecular weight and glass transition temperature. The flexural modulus (MF) was evaluated for all nanogels, dispersed in ethanol (NG2 and NG3) or 4-N,N-dimethylaminobenzoate (NG1); HEMA or mixture of BisGMA/HEMA 40:60wt%. The samples were stored in dry condition (CD) or in water (wet condition-CW) during 7 days, and the MF was obtained by three-point bending test. The degree of conversion (DC) of the materials was evaluated before the storage. For the microtensile BS analysis, 40 extracted human molars were used. An model ethanol-solvated, BisGMA/HEMA (40:60wt%) etch-and-rinse 1 bottle adhesive was used as a control (without addition of nanogel) and with the incorporation of nanogels. Blocks were made using Filtek Z100 composite. The samples were cut into sticks, which were stored for 24 hours or 3 months in distilled water and tested under tensile force at a speed of 1mm/min. After microtensile testing, the analysis of the fracture was performed on the sticks. The results were statistically (α=0,05) analyzed using ANOVA and Tukey’s test, and for MF, dispersion medium HEMA, Kruskal-Wallis and Student Newman Keuls were used. For CD, NG2 had the highest MF when dispersed in solvent and HEMA. For the dispersion medium BisGMA/HEMA, the control group had the highest MF. For CW, NG1 had the highest MF in HEMA and BisGMA/HEMA. Dispersed in the solvent, NG1 and NG2 showed the highest results. After storage in water, NG1 and NG2, dispersed in solvent, showed higher modulus compared to CD, while NG3 showed a significant decrease in values. For HEMA and BisGMA/HEMA, all groups had lower modulus in CW. NG2 showed the highest DC in solvent and BisGMA/HEMA. In HEMA, NG1 and NG3 obtained the highest results. For microtensile BS test, the control group and NG2 showed the highest values at 24 hours. An increase in bond strength values after storage in water for NG2 was observed, while in the control group, NG1 and NG3 the results were maintained after 3 months. In conclusion, the nanogels with different characteristics of hydrophilicity showed different mechanical performances and were able to interfere with the bond strength to dentin.
In general, the more hydrophobic NG IBMA-UDMA showed the highest mechanic results, but the highest BS values were obtained with the amphiphilic NG based on BisGMA/HEMA.
Key words: Polymers, Nanotechnology, Hydrolysis, Material resistance, Dentin-Bonding agents
SUMÁRIO
1 INTRODUÇÃO 14
2 ARTIGO: Influence of nanogel additive hydrophilicity on dental adhesive mechanical
performance and dentin bonding 18
3 CONCLUSÃO 35
REFERÊNCIAS 35
ANEXO 1 - Certificado do Comitê de Ética em Pesquisa 39
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1 INTRODUÇÃO
Os materiais sintéticos disponíveis no mercado para procedimentos restauradores adesivos apresentam excelente desempenho na adesão a curto prazo, entretanto a interface adesiva ainda permanece a região mais frágil das restaurações (Breschi et al., 2008). Isso tem sido reportado pois, verifica-se que a maioria dos sistemas adesivos apresenta reduções na resistência de união após o envelhecimento artificial prolongado, em função da degradação (Van Meerbeek et al., 2010).
Se a adesão em esmalte é bastante efetiva (Swift et al., 1995; Lopes et al., 2002), em função da morfologia homogênea e maior conteúdo mineral, o processo adesivo realizado em dentina ainda representa um desafio quando se trata da longevidade da união. Este fato se deve às características intrínsecas e estruturais do substrato dentinário, como maior conteúdo orgânico e presença da umidade (Perdigão, 2007). Além disso, a composição dos sistemas adesivos contribui para o agravamento do processo natural de degradação e consequente diminuição da resistência da união dentina-resina (Hashimoto et al., 2007; Inoue et al., 2012), como é o caso dos adesivos simplificados, com grandes quantidades de monômeros hidrófilos e solvente.
Em sistemas adesivos com condicionamento ácido prévio, uma dificuldade da técnica é estabelecer a quantidade da umidade residual ideal para a aplicação do adesivo. Na dentina extremamente ressecada, as fibrilas colágenas colapsam, os espaços interfibrilares estão diminuídos e há formação de pontes de hidrogênio entre as fibras, o que dificulta a reexpansão da rede de colágeno (Tay e Pashley, 2003; Pashley et al., 2011). A velocidade e a taxa de difusão dos monômeros em um substrato nessas condições são reduzidas, resultando na formação de inadequada zona de interdifusão pela resina (Pashley e Pashley, 1991) e deixando as fibrilas colágenas desprotegidas e vulneráveis à degradação por enzimas colagenolíticas secretadas por bactérias do biofilme ou pela ativação de metaloproteinases endógenas da matriz dentinária (MMPs) (Carrilho et al., 2007; Breschi et al., 2009; Pashley et al., 2011). Por outro lado, excessiva quantidade de água na camada superficial da dentina pode provocar a separação de fases do sistema adesivo, interferindo na adesão e polimerização do material (Tay et al., 1996) e, subsequentemente, na estabilidade e durabilidade do polímero formado.
Mesmo nos sistemas autocondicionantes, onde acredita-se que a desmineralização ocorra simultaneamente à infiltração dos monômeros, a lixiviação de componentes resinosos
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pela hidrólise e a posterior degradação das fibrilas colágenas da dentina desmineralizada pode equiparar à degradação dos adesivos autocondicionantes de 1 passo aos sistemas adesivos de condicionamento ácido prévio (Erhardt et al., 2011).
Foi demonstrado que os materiais simplificados funcionam como membranas semipermeáveis, permitindo a passagem de fluidos pela camada híbrida e adesiva (Tay et al., 2005). Esses sistemas adesivos apresentam grande concentração de solvente e monômeros hidrófilos (Tay e Pashley, 2003; Hiraishi et al., 2005), o que dificulta a polimerização e resulta em grau de conversão menor que o desejável (Cadenaro et al., 2005; Navarra et al., 2009; Ito et al., 2010). Estes fatores contribuem diretamente para a degradação hidrolítica do polímero resinoso.
A degradação hidrolítica dos polímeros ocorre pela clivagem das ligações presentes na cadeia polimérica por moléculas de água, resultando em cadeias menores e moléculas com elétrons de valência desemparelhados (Göpferich, 1996). Quando cadeias poliméricas são expostas aos solventes, dentre eles a água, um aumento do volume ocorre devido ao enfraquecimento das forças de coesão entre as cadeias, os monômeros residuais são liberados e as cadeias lineares são dissolvidas (Feitosa et al., 2012), gerando radicais livres que aceleram a degradação dos polímeros. Os radicais livres resultantes da degradação hidrolítica também são, em parte, responsáveis pela degradação da matriz de colágeno exposta pela desmineralização (Erhardt et al., 2011).
Apesar de muitos polímeros serem considerados insolúveis e quimicamente estáveis, todos os tipos de estrutura polimérica, a certo tempo, apresentarão algum nível de degradação. Os solventes estão diretamente relacionados com o processo de sorção e solubilidade dos polímeros, podendo causar alterações volumétricas, como embebição; alterações físicas, como plasticização e amolecimento; e alterações químicas, como oxidação e hidrólise, de acordo com as características de hidrofilicidade dos monômeros, qualidade da rede polimérica formada após a polimerização, presença de partículas de carga na rede polimérica e tipo de solvente. As propriedades do polímero podem ser permanentemente comprometidas por esses efeitos (Ferracane, 2006), enfraquecendo a união resina-dentina (Brechi et al., 2008).
Ainda que a utilização de algumas substâncias como a clorexidina, (Carrilho et al., 2007) e antioxidantes (Bedran-Russo et al., 2008; Tezvergil-Mutluay et al., 2012) esteja consolidada na literatura como alternativas para a diminuição ou prevenção do processo de degradação da interface adesiva, especialmente no que diz respeito à degradação do colágeno, poucas opções enfocando a degradação hidrolítica da camada híbrida e da película de adesivo
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estão disponíveis (Sadek et al., 2008; 2010a; 2010b; Erhardt et al., 2011). Bons resultados têm sido obtidos na redução da degradação hidrolítica com o uso de nanomateriais incorporados aos sistemas adesivos (Morães et al., 2012).
A nanotecnologia tem atraído substancial interesse em pesquisa devido às suas estruturas versáteis, grande número de extremidades de cadeia e a capacidade de incorporar funcionalidades sensíveis a estímulos, com várias aplicações no carreamento de drogas, engenharia tecidual e compósitos poliméricos (Moraes et al., 2011; Liu et al., 2012). Em Odontologia, um dos possíveis uso dos nanomateriais é através dos nanogéis (Morães et al., 2012; Moraes et al., 2011). Nanogéis são partículas poliméricas única ou multi-cadeias internamente cíclicas e reticuladas tipicamente com tamanho bem abaixo da escala micrométrica (Moraes et al., 2011).
Nanogéis podem ser dispersos em monômeros, tais como HEMA e BisGMA, (Moraes et al., 2011) e solventes (de Groot et al., 2001), que no processo de união à dentina, são os responsáveis por transportar as nanopartículas pela dentina desmineralizada (Morães et al., 2012). Em consequência da remoção do solvente e da contração de polimerização da matriz resinosa, partículas de nanogel se aglutinam parcial ou totalmente para, potencialmente, reforçar a rede polimérica do adesivo, com o objetivo de aumentar a resistência de união e limitar a infiltração de água, bem como aumentar o volume da rede polimérica (Sahiner et al., 2006; Szaloki et al., 2008; Morães et al., 2012). O tamanho e a estrutura globular das partículas individuais de nanogel (Szaloki et al., 2008; Moraes et al., 2011) permitem a dispersão pelos túbulos dentinários e entre as fibras de colágeno.
Adesivos dentinários com a incorporação de nanogéis demonstraram propriedades mecânicas, como módulo flexural e resistência à flexão melhoradas tanto em condições secas quanto úmidas, em função do reforço da rede polimérica pela adição das nanopartículas (Morães et al., 2012). Além disso, a solubilidade em água foi reduzida e, a curto prazo, a resistência da união à dentina foi significativamente melhorada com a inclusão dos nanogéis, sem a necessidade de modificar as técnicas de aplicação já existentes (Morães et al., 2012). Os adesivos sem nanogéis e modificados com nanogéis apresentaram similar grau de conversão, uma vez que participam da polimerização e são parte da rede polimérica formada (Morães et al., 2012).
Ainda que a utilização de nanogéis seja uma opção comprovadamente interessante para a modificação de materiais odontológicos, os efeitos da combinação entre nanogéis com diferentes níveis de hidrofilia e um sistema adesivo ainda não foram estudados. A hidrofilicidade dos polímeros está diretamente ligada às propriedades e velocidade de
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degradação da rede polimérica. Além disso, a interação entre materiais e substratos é muito importante para o estabelecimento de um vínculo de adesão eficiente, e específicos tipos de nanopartículas podem ser capazes de influenciar esta interação de acordo com sua hidrofilicidade. Dessa forma, o objetivo neste estudo foi comparar três diferentes nanogéis e avaliar o desempenho mecânico desses materiais e os efeitos da incorporação dessas nanoestruturas à um adesivo dental na resistência da união à dentina, considerando-se a variação na hidrofilicidade das moléculas.
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Influence of nanogel additive hydrophilicity on dental adhesive mechanical
2 ARTIGO:
performance and dentin bonding
Artigo submetido ao periódico Dental Materials
ABSTRACT
Objectives: To assess the influence of hydrophilicity of reactive nanogels on the mechanical performance of dental adhesives and early-stage microtensile bond strength (µTBS) to dentin. Methods: A series of three nanogels were synthesized: NG1- IBMA/UDMA; NG2- HEMA/BisGMA; NG3- HEMA/TE-EGDMA. The nanogels were dispersed in solvent, HEMA or BisGMA/HEMA. The degree of conversion (DC) of the materials was measured and the flexural modulus of these polymers was evaluated in dry or wet conditions. For µTBS analysis, a model adhesive was used without nanogel (control) or with the incorporation of nanogels. µTBS was evaluated after storage in distilled water for 24 h or 3 months. The analysis of the fracture was performed after µTBS testing. Data were analyzed (α=0.05) using ANOVA and Tukey’s test, and for MF, dispersion medium HEMA, Kruskal-Wallis and Student Newman Keuls were used. Results: Water significantly increased the modulus of NG1 and NG2 dispersed in solvent, while significantly decreased the stiffness of NG3. All polymers dispersed in HEMA and BisGMA/HEMA had significantly lower modulus when stored in water. NG2 showed the highest DC in solvent and BisGMA/HEMA. In HEMA, NG1 and NG3 produced the highest DC. After three months, NG2 showed the best µTBS. The µTBS of NG2-containing adhesive resin significantly increased after after 3 months, while storage had no effect in the control group, NG1 and NG3. Conclusion: The more hydrophobic IBMA/UDMA nanogel showed higher bulk material mechanical property results, but the best dentin bond strength values, and notably strength values that improved upon storage, were obtained with the amphiphilic nanogel based on BisGMA/HEMA.
Keywords: Nanogel; dental material; hydrophilicity; adhesion; hydrolysis. 1. INTRODUCTION
Although high quality adhesive systems are available for applications in Restorative Dentistry, the great immediate results of bond strength typically decline with storage time. The adhesive interface remains the weakest component of dental restorations
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based on varied degrees of degradation over relatively short times [1]. This seems to be due to several factors such as substrate characteristics, technical limitations, aging conditions in the oral environment (i.e. temperature, moisture and chewing load) and the composition of adhesives [2-6].
To ensure appropriate hybridization of wet collagen matrix, increasing concentrations of hydrophilic and ionic monomers have been added to adhesives [7]. Furthermore, simplified adhesive systems also have a large concentration of solvents and hydrophilic monomers [8,9], which interferes in the polymerization and decreases the degree of conversion [10-12]. The addition of hydrophilic monomers forms a network with low cross-linking density, and increases water sorption/solubility and resin plasticization [13,14]. These factors are thought to contribute directly to the hydrolytic and potential enzymatic degradation of the polymer resin.
Different strategies have been developed to control and prevent the degradation of the hybrid layer, such as ethanol wet bonding [13-17], chlorhexidine [18], and antioxidants/crosslinking agents [19-23]. Moreover, good results have also been obtained using nanomaterials in adhesive formulations [24].
Nanotechnology in the form of reactive nano-scale prepolymeric particles that can be swollen by monomer has attracted substantial interest due to the versatile structures with multiple applications in the drug delivery, tissue engineering and polymer composites [25,26]. While a large number of biomedical applications envolve nanogels as freely dispersed particles, recent studies have applied nanogels as functional fillers or additives in the preparation of nano-composite polymer [24-27].
In general, incorporation of nanogels in dental adhesive systems and composites reduced shrinkage and improved mechanical properties such as flexural modulus and flexural strength, both in dry and wet conditions, due to the strengthening of the polymeric network by the presence of the crosslinked particles [24]. Furthermore, water solubility was reduced, and short-term bond strength to dentin was improved significantly with the inclusion of the nanogels, without need to modify the existing application techniques [24].
Despite the use of nanogels as an interesting option for the modification of dental materials, the effects of comonomer combinations within the nanogels that produce different levels of hydrophilicity have not been studied with regard to adhesive formulation and bonding to a dentin substrate. The interaction between materials and substrates is very important for the establishment of efficient and effective adhesive bonds. It is reasonable to expect that specific types of nanoparticles may be able to influence the properties and
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durability of polymers. Thus, the aim of this study was to compare three nanogels that systematically differ in terms of hydrophilicity and assess how the incorporation of these nanostructures into dental adhesives will influence the mechanical performance of materials and bond strength to dentin over early storage times. The null hypotheses were: (1) nanogels with different hydrophilicity would not affect the mechanical performance when exposed to water; (2) nanogels with different hydrophilicity would not interfere with dentin bond strength.
2. MATERIALS AND METHODS
2.1. Nanogel synthesis
Three nanogel (NG) copolymers were synthesized at a 70:30 molar ratio of: isobornyl methacrylate (IBMA; TCI America, Portland, OR, USA) and urethane dimethacrylate (UDMA; Sigma-Aldrich Co., St. Louis, MO, USA) (NG1); 2-hydroxyethyl methacrylate (HEMA; TCI America) and bisphenol A glycerolate dimethacrylate (BisGMA; Esstech, Essington, PA, USA) (NG2); 2-hydroxyethyl methacrylate (HEMA) and tetraethylene glycol dimethacrylate (TE-EGDMA; TCI America) (NG3). 2-Mercaptoethanol (ME; Sigma-Aldrich) was added (10 mol% for NG1, 40 mol% for NG2, and 15 mol% for NG3 relative to monomers) as a chain-transfer agent to avoid macrogelation, control molecular weight/nanogel particle size, and provide sites for post-polymerization refunctionalization with reactive groups. Free radical polymerization was conducted in solution (six-fold excess for NG1, eight-fold excess for NG2, seven-fold excess for NG3 of methyl ethyl ketone (MEK; Fisher Scientific, Waltham, MA, USA) relative to monomer) with 1 wt% 2,2'-azobisisobutyronitrile (AIBN; Sigma-Aldrich) as thermal initiator. A 100 mL round-bottom flask was used as the reactor with monomer batch sizes of approximately 10 g with reaction conditions of 80°C and a stirring rate of 200 rpm.
Methacrylate conversion during nanogel synthesis was calculated from mid-IR spectra (Nicolet 6700, Thermo Scientific, Waltham, MA, USA) before and after polymerization. Conversion around 90 % was achieved for NG1 after about 4 h of reaction, 78 % for NG2 after about 2 h and 30 min, and 90 % for NG3 after about 3 h.
Nanogels were purified by precipitation from hexanes (10-fold excess; Fisher Scientific) and filtration. Resulting precipitates were re-suspended in dichloromethane (BDH Chemicals, VWR Analytical, Radnor, PA, USA) for NG1 and NG3, and acetone (Fisher Scientific) for NG2, and reacted at room temperature with a 10 mol% for NG1, 10 mol% for NG2, and 15 mol% for NG3 of 2-isocyanoethyl methacrylate (IEM; TCI America) with a trace amount of dibutyltin dilaurate (Sigma-Aldrich) as catalyst. The polymer precipitation
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method was repeated to isolate the methacrylate-functionalized reactive nanogel. Residual solvent was removed completely under vacuum until the nanogels were obtained as dry powders.
2.2 Nanogel particle characterization
Polymeric nanogels were characterized by triple-detector (refractive index, viscosity, light scattering) gel permeation chromatography - GPC (GPCmax; Viscotek, Malvern Instruments, Malvern, UK) in tetrahydrofuran (EMD Millipore, Billerica, MA, USA). The glass transition temperature (Tg) of nanogel powders was determined by dynamic mechanical analysis (DMA 8000, Perkin Elmer, Waltham, MA, USA) by sandwiching 10 mg of nanogel in a thin metallic pocket that was then subjected to single cantilever cyclic displacement of 50 µm at 1 Hz. The nanogel was heated to 150°C with tan δ data collected as the sample was cooled to 0°C at 2°C/min in air.
2.3 Mechanical strength characterization
The flexural modulus was evaluated for all nanogels dispersed only with inert solvent or in HEMA or BisGMA/HEMA. 2,2-Dimethoxy-2-phenylacetophenone (Sigma-Aldrich) was used at 0.2 wt% as photoinitiator. For the solvated nanogel groups, NG1 was dispersed in N-N dimethylformamide (DMF) (Sigma-Aldrich), while NG2 and NG3 were dispersed in ethanol (Fisher Scientific), according to their solubility characteristics. Slight differences in the weight percentages of nanogels were established according to the specific viscosity characteristics of the blends. This was done as needed to normalize the viscosities between formulations and avoid difficulties related to flow in handling and specimen preparation. When in solvent, nanogels were dispersed at 50:50 wt% NG:solvent; when nanogels were dispersed in HEMA, a 50:50 wt% NG:monomer proportion was used for NG2 and NG3, while a 48:52 wt% NG:monomer formulation was used for NG1; and when the nanogels were dispersed in BisGMA/HEMA (40:60 wt%) in a 40:60 wt% NG:resin ratio. Viscosity measurements of the nanogel-modified materials were performed using a cone-plate digital viscometer (CAP2000+; Brookfield, Middleboro, MA, USA) at ambient temperature.
Flexural modulus tests were performed using bar specimens (n = 8) with dimensions of 25 mm×2 mm×2 mm, light-cured between glass slides in an elastomer mold by exposure to UV light at 60 mW/cm2 for 2 min on each side (mercury arc lamp 365 nm - Acticure 4000, EXFO, Richardson, TX, USA). The degree of conversion (Nicolet 6700) of all the bar specimens was evaluated following the preparation of the samples, before storage.
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Methacrylate conversion was calculated by following the area of the methacrylate absorption band in near-IR (6165 cm−1) before and after photoactivation. The bars were stored in an oven (Single-Wall Transite Oven; Blue M Electric Company, Blue Island, IL, USA) overnight at 37°C and then randomly divided into two groups (n = 8): dry storage condition and wet storage condition. After one-week additional storage in dark containers (dry or in distilled water) at room temperature, the flexural modulus was obtained in three-point bending on the MTS testing machine (MTS Mini Bionix II, MTS, Eden Prairie, MN, USA) using a span of 20 mm and a cross-head speed of 1 mm/min.
2.4 Dentin-bonding study
A dentin-bonding study was conducted with a model ethanol-solvated (30 wt%) BisGMA/HEMA (40:60 wt%) etch-and-rinse 1 bottle adhesive as a control or containing nanogels 1, 2 or 3 at 40 wt% relative to the BisGMA/HEMA monomer content. Camphorquinone (Sigma-Aldrich) and ethyl 4-N,N-dimethylaminobenzoate (Sigma-Aldrich) were added at 0.2 and 0.6 wt%, respectively, as co-initiators.
Forty extracted human molars were obtained under approval of the institutional ethics committee (protocol 022/2014) and stored in distilled water at 4 °C no longer than 6 months. The roots were removed 2 mm beneath the cement-enamel junction (CEJ), while the occlusal enamel was cut 2 mm above the CEJ using a slow-speed water-cooled diamond saw (Isomet, Buehler; Lake Bluff, IL, USA), exposing a flat surface of dentin. The exposed dentin surface was abraded before bonding with a 600-grit SiC paper for 60 s under running water, with the intent to create a standardized smear layer.
Prepared teeth were randomly divided into four groups (n = 10) according to adhesive systems (with or without NG). The samples were subjected to acid etching with 35 % phosphoric acid (Scotchbond Universal Etchant, 3M ESPE) for 15 s to dentin and 30 s to enamel. All adhesive systems were applied in 2 coats. Light activation was performed for 10 s using a LED Elipar Deepcure-S lamp (1000-1200 mW/cm², 3M ESPE, St Paul, MN, USA). Composite buildups were constructed only in dentin using Filtek Z-100 (3M ESPE) in 2 layers (each layer 2 mm thick). Approximately 20-25 sticks (bonding area 1 x 1 mm2) were obtained from each molar by sectioning the bonded teeth. The sticks from the periphery showing remaining enamel were excluded. Half of the sticks were tested after 24 h of restoration and the other half after 3 months of storage in distilled water at 37°C.
The sticks were fixed to a jig using a cyanoacrylate gel and tested to failure using MTS testing machine (MTS Mini Bionix II, MTS) at a crosshead speed of 1.0 mm/min. The
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cross-sectional area of the sticks was measured before failure with a digital caliper. The µTBS of the sticks from the same bonded tooth were averaged and used for the statistical analysis as the statistical unit. Means and standard deviations were calculated and expressed in MPa. A small number of the bonded specimens failed prematurely and were not included in the statistical analysis. This defect exclusion is supported by the very limited numbers involved and that the range of bond strengths achieved were all relatively narrow and well above that associated with spontaneous failure.
The fractured specimens were analyzed using SEM JSM-5600LV (JEOL; Tokyo, Japan). In brief, the specimens were mounted on aluminum stubs and subsequently dehydrated in silica gel. Finally, the specimens were gold-sputter coated and examined using SEM, operated at 15 kV. The fractures were classified as adhesive, cohesive in composite, cohesive in adhesive, cohesive in dentin or mixed.
2.5 Statistical analysis
The data were statistically analyzed by equal variance and Kolmogorov-Smirnov normality tests and, after proving the normality of data and homogeneity of variances, the results were analyzed using one-way ANOVA (nanogel, for degree of conversion) and two-way ANOVA (nanogel and condition - dry or wet, for flexural modulus; and nanogel and storage time – 24 h and 3 months, for microtensile test) and Tukey’s test (α=0.05). For flexural results, dispersion medium HEMA, Kruskal-Wallis and Student Newman Keuls were used because of the absence of normality of data and homogeneity of variances. The analyses were performed for each dispersion medium separately (solvent, HEMA and BisGMA/HEMA), for flexural results and degree of conversion.
3. RESULTS
Number and weight-averaged molecular weight (Mn and Mw, respectively), polydispersity index (PDI), hydrodynamic radius (Rh), and Mark- Houwink α exponent (MH-α) were obtained by GPC analysis (Table 1). Dynamic mechanical analysis of NG1, NG2 and NG3 provided bulk nanogel Tg values of 96°C, 55°C and 40°C, respectively (Table 1).
Results for the polymeric flexural modulus (MPa, Table 2) demonstrate that the interaction between the factors was significant for all dispersion media (solvent, HEMA, BisGMA/HEMA; p<0.001). For dry condition, NG2 presented the highest modulus in solvent and dispersed in HEMA. BisGMA/HEMA control showed the highest modulus compared to nanogels groups dispersed in BisGMA/HEMA 40:60 wt%.
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For wet condition, NG1 presented the highest modulus in HEMA and BisGMA/HEMA. When in solvent, NG1 and NG2 were not statistically different, and showed the highest modulus values. After storage in water, NG1 and NG2 solvent groups showed significantly higher modulus in wet conditions compared to dry condition, while NG3 had a significant drop when in saturated with water. When mixed with HEMA and BisGMA/HEMA, all nanogels groups including the nanogel-free controls showed significantly lower modulus in wet conditions compared to dry condition.
The degree of conversion (DC) ranged from 82.9 % to 91.4 % (Table 3). For solvent and BisGMA/HEMA groups, NG2 had the highest DC. When mixed in HEMA, NG1 and NG3 were not statistically different and showed the highest DC.
In the 24 h µTBS testing (MPa, Table 4) with ethanol-solvated BisGMA/HEMA adhesives, the control group showed the highest bond strength value, but not statistically different from group with NG2. After 3 months of storage in distilled water, NG2 presented the highest results. NG2 had statistically higher bond strength after 3 months, compared to 24 h. Control, NG1 and NG3 maintained their values after storage.
The failure (Figure 1) was predominantly adhesive for control groups and NG3 at 24 h (52.9% adhesive, 23.6% mixed, and 23.5% cohesive in dentin, for control, 62.5% adhesive, and 37.5% mixed, for NG3) and after 3 months (90.47% adhesive, and 9.53% mixed, for control; 53.84% adhesive, 38.47% mixed, and 7.69% cohesive in resin, for NG3). For NG1 and NG2, the failure pattern changed from mixed at 24 h (60% mixed, 26.7% cohesive in adhesive, and 13.3% cohesive in resin, for NG1; 81.82% mixed, 18.18% adhesive, for NG2) to adhesive after storage (53.4% adhesive, and 46.6% mixed, for NG1 58.4% adhesive, and 41.6% mixed, for NG2).
4. DISCUSSION
According to the GPC analysis (Table 1), the three nanogels have particles with size of approximately 10 nm and globular structure such that they potentially can be dispersed into dentinal tubules and interfibrillar collagen spaces [24,25,28]. The roughly spherical or globular structure of the nanogel is confirmed by the Mark–Houwink α exponent (0.28 to 0.36) as compared with a typical value of approximately 0.7 associated with a random coil linear polymer structure [29]. The higher concentration of chain transfer agent and solvent used in the synthesis of NG2 was necessary to avoid macrogelation. The thermal scan of the bulk nanogel powder samples by dynamic mechanical analysis provided Tg values of 96 °C, 55 °C and 40 °C, respectively for NG1, NG2 and NG3. Isobornyl methacrylate contributes
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toward the relatively high Tg of NG1 while the BisGMA crosslinking component of NG2 increases its Tg compared with TE-EGDMA in NG3. The Tg of the crosslinked nanogel particles potentially can affect solvent/monomer uptake and dispersion in direct relation to its proportional reduction of the overall matrix volume fraction [14]. As macromers, the nanogels provide network structure as well as mechanical reinforcement of secondary polymeric networks [24,28,30].
When the solvent-dispersed nanogels were mechanically tested, NG1 and NG2 notably showed significantly higher modulus after storage in water compared to their corresponding dry (solvent-free) modulus values (Table 2). For hydrogen bonding nanogel structures that do not swell excessively in water, as may be expected with internal crosslinks associated with UDMA or BisGMA, the introduction of water may provide bridging hydrogen bonding reinforcement between the urethane or hydroxyl groups, respectively, that can account for the improved wet strength observed. However, despite the higher bulk nanogel Tg found for NG1, the polymers from both NG2 and NG3 displayed significantly higher dry modulus results, probably due to the dry-state hydrogen bonding reinforcement associated with the HEMA component in these nanogels. Although all dry samples were subjected to overnight solvent evaporation in a vacuum oven, residual DMF may have also contributed to the low modulus values for NG1. DMF is a solvent with lower evaporation rate compared to ethanol. The residual DMF present at the point of polymerization may have hindered the formation of polymeric network structure.
As nano-scale gels with varying degrees of hydrophilic functional groups attached to the polymeric backbone, these materials may act like a hydrogel at a certain level. Hydrogels are polymeric materials that exhibit the ability to swell and retain a significant fraction of water within their network structure, but will not dissolve in water [31]. Nanogels cannot be dissolved, but depending on their particular solubility parameter, they can be dispersed in water or other solvents as well as monomers, with swelling that is balanced by affinity and crosslink-imposed elastic limitations. While all the polymerizations of the controls and nanogel-containing formulations reached high conversion (Table 3), it can be assumed that there would be trapped radicals that during the seven days of water storage could produce some additional polymerization within these polymers. The flow of water into the polymer structure may be capable of plasticizing the polymer, weakening the strength of the bonds between the chains. Residual monomers can be leached or can react into the polymer network if free radicals persist [21]. The nanogel particles are both physically and covalently retained in the final polymers with little chance for leaching.
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On the other hand, the polymer produced from the solvent-based polymerization of NG3 presented a dramatic decrease in modulus when stored in water, most likely due to its high hydrophilicity. HEMA is a hydrophilic, water miscible monomer widely used in dental materials. It leads to rapid water uptake before and after polymerization in a wet or humid environment with deteriorating properties [32]. Furthermore, TE-EGDMA is a relatively hydrophilic cross-linker that also has ester-based crosslinks that may be hydrolytically susceptible groups when present in a highly swollen state [14]. The hydrophilic character of this NG3 polymer material could certainly contribute to an excessive absorption of water resulting in short-term plasticization and potentially leading to longer-term hydrolytic damage.
For the polymers created in the HEMA dispersion medium, the HEMA control and all nanogels had lower flexural modulus after water storage. The poly(HEMA) control presented the lowest modulus and degree of conversion in this case. The polymerization of HEMA produces a very loosely crosslinked network that has a high affinity for water while offering little elastic resistance to aqueous swelling [32]. The substantial water uptake into the polymer overcomes the hydrogen bonding reinforcement of the many pendant hydroxyl groups and accounts for the greater loss in wet modulus compared with the other materials in this study.
Although the polymer from HEMA/NG1 also experienced a significant decrease in the value of flexural modulus when in water, the deterioration occurred in lesser magnitude. NG1 is the most hydrophobic nanogel tested, and its UDMA cross-linker absorbs less water compared to BisGMA or TE-EGDMA [14]. Furthermore, the NG1 nanogel had the highest Tg, which can be related to reduced plasticization by water and/or solute [33]. UDMA is a very reactive dimethacrylate monomer that generates strong polymeric networks [26], and IBMA provides a very high glass transition temperature and hydrophobic character to the copolymer [26]. Because of its higher Tg, NG1 might have been expected to allow for lower conversion because of the lower chain mobility; however, no reduction in conversion was seen. In fact, all of the nanogel-modified resins showed enhanced conversion during polymerization. In particular, the slightly lower nanogel content used for the HEMA/NG1 formulation to normalize the viscosity may have facilitated mobility and reaction in this material. In the wet storage condition, this HEMA/NG1 sample showed significantly better mechanical properties than the other HEMA-containing compositions. It should be noted that while the dry modulus results for all the HEMA-dispersed nanogel materials were significantly greater than the dry modulus of the solvent-dispersed nanogel polymers, the wet
27
modulus of the polymers from the solvent-dipersed NG1 and NG2 materials, which were produced with 50 wt% inert solvent, are higher than that of the wet HEMA-dispersed analogs. For the material groups with nanogel dispersed in BisGMA/HEMA 40:60wt%, as with HEMA, the same superior wet strength performance was observed with NG1 as the reactive additive. Despite the deterioration in stiffness after water storage, the NG1-modified resin continued to show the highest flexural modulus values. All the BisGMA/HEMA resin formulations had higher dry and wet modulus results compared with all other groups, which is probably due to the additional BisGMA cross-links in the dispersion material [14], making it both more hydrophobic and more resistant to swelling. The addition of BisGMA, a monomer of high viscosity and molecular weight, may also have hindered the polymerization due to the decreased mobility between the chains, which slightly reduced the degree of conversion, but did not compromise the material properties.
Possibly, the higher degree of conversion for the resins with NG2 and NG3 may be associated with the lower Tg’s and greater mobility of chains. Nevertheless, the greater hydrophilicity of these nanogel additives may have contributed to prejudice the wet flexural modulus whereas the dry modulus results are all fairly similar but lower than that of the unmodified resin control.
Thus, the null hypothesis that nanogels with different hydrophilicity would not affect the mechanical performance when exposed to water must be rejected.
At 24 h microtensile bond strength analysis, ethanol-solvated BisGMA/HEMA control group and this adhesive resin with NG2 presented the highest results. It should be noted that NG2 has the same monomer composition of the adhesive control. BisGMA and HEMA are two of the most common monomers used in dental adhesives [14,32]. Furthermore, HEMA is long known for good wetting, diffusion, and penetration properties, which are the primary mechanisms to obtaining micro-mechanical retention on demineralized wet dentin [32]. In this way the amphiphilic character obtained with control and NG2 formulations appears to have positively influenced the interaction with the substrate. However, the predominance of mixed failure in the NG2 group compared to the predominance of adhesive failure in the control group can be an indicative of a better adhesion of the material with inclusion of nanogel. One distinction between the BisGMA/HEMA adhesive resin and the BisGMA/HEMA nanogel is that in wet bonding conditions, there is not potential for water-induced phase separation within the covalently interconnected nanogel structure.
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The adhesive resin modified with NG1 and NG3 showed the lowest values of bond strength after 24 h as compared to control. The more hydrophobic characteristic of NG1 may have affected the interaction with wet dentin by increasing the contact angle of the material to the surface and decreased wettability. An intimate contact (i.e. wettability) between the adhesive resin and the dental hard tissues plays an important role in establishing effective bonding [34]. Furthermore, considering the unfavorable mechanical performance obtained with the NG3 and the presence of solvent and water in the restorative process, these factors may have influenced the degree of conversion, strength and adhesive properties of the adhesive with the more hydrophilic nanogel. Large concentrations of hydrophilic monomers may impair the polymerization of the adhesive system and reduce the degree of conversion [10-12].
After 3 months of storage in distilled water, the group NG2 presented the best dentin bond strength results. The good performance of BisGMA/HEMA combined with NG2 appears to have contributed to a better interaction with the substrate. The enhancement in µTBS after water storage is not completely understood but this surprising result was independently reproducible. There is a possibility that the denser polymer network structure associated with the BisGMA/HEMA resin plus nanogel may lead to more trapped radicals than in the case of the resin alone. This could result in an extended post-cure as water infiltrates the polymer structure. This outcome would not necessarily be limited to the NG2 structure but taken together with appropriate surface wetting behavior may be necessary to achieve an improving bond strength. There may also be differences in the long-term swelling character of this nanogel-modified resin matrix that leads to more favorable interactions with the substrate. These details will certainly be explored in greater detail in subsequent studies.
The adhesive resin control and the formulations modified with NG1 and NG3 had bond strength values that were maintained after storage. It is likely that this limited 3 month time was not sufficient to obtain significant degradation of the bond strength [35]. However, the predominance of adhesive failures in all groups may indicate the first signs of degradation of this model adhesive.
Thus, the second null hypothesis that nanogels with different hydrophilicity would not interfere with dentin bond strength must be also rejected.
Dental formulations with reactive nanogels able to interact better with the substrate and that require no modification of conventional bonding techniques, can combine ease of use and the positive effects of stronger materials. This study has demonstrated that nanogel additives likely need to be tailored to specific adhesive resins and bonding conditions
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to achive the most favorable results in terms of bond strength and longevity of the bonded interface. The synthesis of new nanogels will be aimed at the development of adhesive materials that allow for simple dentin-bonding procedures while providing long-term stability of well-bonded interfaces. Additional studies should emphasize other aspects, such as other compositions of nanogels, the analysis of the degree of conversion at the bond interface with dentin to monitor the polymerization process over time and a longer storage time of experimental adhesives with nanogel additives.
5. CONCLUSION
Nanogels with different hydrophilicity were able to affect the flexural modulus of materials when exposed to water and the dentin bond strength. In general, the more hydrophobic IBMA/UDMA nanogel showed higher bulk material mechanical property results, but the highest dentin bond strength values, and notably strength values that improved upon storage, were obtained with the amphiphilic nanogel based on BisGMA/HEMA.
ACKNOWLEDGMENTS
We are grateful to Matthew Barros for providing the GPC results and Adriano Martins for technical assistance for SEM analysis. This study was funded by Capes Brazil (BEX: 4566/14-9) and NIH/NIDCR R01DE023197. The authors declare no potential conflicts of interest with respect to the authorship and/or publication of this article.
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Table 1. Gel permeation chromatography parameters* and glass transition temperature (Tg).
Nanogel Sample Mn (Da) Mw (Da) PDI Rh (nm) MH-α Tg
NG1 15,460 42,760 2.76 4.76 0.360 96°C
NG2 12,550 51,490 4.10 4.44 0.281 55°C
NG3 3,580 18,870 5.28 3.12 0.299 40°C
*Number and weight-averaged molecular weight (Mn and Mw, respectively), polydispersity index (PDI), hydrodynamic radius (Rh), and Mark-Houwink α exponent (MH-α). The data represent a single analysis.
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Table 2. Mean (standard deviation) of flexural modulus (MPa). DISPERSION MEDIUM GROUP CONDITION DRY WET Solvent NG1 127.6 (23.4) c B 936.7 (49.4) a A NG2 662.5 (59.9) a B 937.1 (128.2) a A NG3 362.7 (62.5) b A 36.7 (19.3) b B HEMA HEMA control 2338.0 (346.6) b A 0.3 (0.1) d B NG1 2485.1 (142.7) b A 512.9 (20.6) a B NG2 2809.6 (230.8) a A 93.6 (7.8) b B NG3 2228.1 (126.4) b A 17.9 (10.4) c B BisGMA/HEMA BisGMA/HEMA control 3088.8 (123.4) a A 1062.6 (46.6) b B NG1 2632.7 (94.2) b A 1465.3 (116.1) a B NG2 2546.1 (90.7) bc A 656.0 (47.7) c B NG3 2399.9 (131.6) c A 481.3 (40.9) d B
Mean values with the same uppercase letters (row) and lowercase letters (column) for each dispersion medium are not significantly different (p > 0.05).
Table 3. Mean (standard deviation) of degree of conversion (%) before storage. DISPERSION MEDIUM GROUP DEGREE OF CONVERSION Solvent NG1 83.1 (3.1) b NG2 88.9 (3.9) a NG3 84.8 (5.6) b HEMA HEMA control 86.3 (0.4) c NG1 90.9 (1.5) a NG2 89.3 (0.9) b NG3 90.9 (1.1) a BisGMA/HEMA BisGMA/HEMA control 82.9 (1.1) d NG1 84.1 (0.8) c NG2 91.4 (0.9) a NG3 85.9 (1.0) b
Mean values with the same lowercase letters for each dispersion medium are not significantly different (p > 0.05).
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Table 4. Mean (standard deviation) of microtensile bond strength (MPa).
GROUP STORAGE PERIOD
24 HOURS 3 MONTHS
Control 22.5 (3.1) a A 20.4 (1.6) b A
NG1 15.4 (0.6) b A 16.3 (4.7) b A
NG2 18.0 (2.1) ab B 27.5 (4.9) a A
NG3 15.4 (3) b A 16.4 (3.6) b A
Mean values with the same uppercase letters (row) and lowercase letters (column) are not significantly different (p > 0.05).
Figure 1. Failure mode analysis of the debonded specimens (%) tested by microtensile in 24 hours (h) and 3 months (m) of storage in water.
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3 CONCLUSÃO
Nanogéis com diferentes hidrofilicidades foram capazes de afetar o módulo flexural dos materiais quando expostos à água e a resistência de união a dentina. Em geral, o nanogel mais hidrófobo IBMA/UDMA apresentou os maiores resultados de propriedades mecânicas, mas os maiores valores de resistência de união, e valores que notavelmente aumentaram após armazenamento, foram obtidos com o nanogel anfifílico BisGMA/HEMA.
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