Leading researchers from the Faculty of Dentistry, the University of Hong Kong (HKU), Wuhan University (WHU), and the Peking University Shenzhen Hospital have found that a compound found in mussels helps increase the durability of a dental filling.
In a journal article published in Materials Today Bio titled ‘Enhancing resin-dentin bond durability using a novel mussel-inspired monomer’, they explain why this is a promising clinical finding for the future of dental filling treatments.
A dental filling is commonly used to restore tooth decay and broken teeth. Its durability highly depends on the longevity and stability of the bond between the compound (resin) and the hard tissue of the tooth (dentin). Here is where mussels play a role.
Small shellfish widespread in the marine environment, mussels offer unique wet adhesion properties which have long been of interest to the scientific community. Thus, the interaction between mussel plaques and substrates under humid environments has been extensively studied for insights on potential clinical applications. The study revealed that a compound found in an adhesive protein in mussels could strengthen the resin-dentin bond.
“Mussels need to maintain their adhesiveness under harsh marine environments, including humidity, drastic change of water temperature and pH value, sudden shocks and so on. These are similar to the daily activities that happen in the oral cavity. Our research aimed to understand the adhesive properties of the compounds from mussels, which may improve the durability and longevity of dental fillings,” explained Professor Cynthia Kar Yung Yiu, Clinical Professor in Paediatric Dentistry, HKU who is leading the research team. Other members include Dr James Kit Hon Tsoi, Associate Professor in Dental Materials Science and Mr Kang Li from the Faculty of Dentistry, HKU; Professor Cui Huang from WHU, and researchers from the Peking University Shenzhen Hospital.
In a usual dental filling procedure, the dentist first removes the decayed tooth structure and fills the cavity with a tooth-colored restoration using a dental adhesive to glue the filling to the tooth structure. However, the durability of this bond can be affected by several factors, such as the humidity inside the oral cavity and repeated mechanical stress induced by chewing. Therefore, it remains a clinically significant challenge for the dentist as well as the patient as it leads to frequent replacement of the dental fillings at extra costs.
The study revealed that the wet adhesive property of mussels is attributed to the amino acid Dopa which they secrete. Based on the result, the team successfully applied N- (3,4-dihydroxyphenethy) methacrylamide (DMA), a mussel-derived compound, as a dental adhesive. The team further tested the durability of this resin-dentin interface versus the new DMA bond.
The control group and those with distinct concentrations of DMA underwent different tests including thermocycling aging, a process where dental materials are exposed to varied temperatures. The international standard for testing dental adhesives requires test specimens to be held repeatedly first in 5 °C cold water and then in 55 °C hot water for a large number of cycles. The results after subsequent testing invariably show a decrease in adhesive strength.
The researchers then employed the nanoleakage evaluation method in which an acid is added to measure the quality of the bond. The team used a silver nitrate solution to observe the patterns of nanoleakage.
In the resin-dentin interface, the thermocycling aging process caused the formation of cracks and fissures which further led silver particles to infiltrate and settle along the bonded interface. The silver deposition therefore clearly reflected the water-filled and destructed areas along the interface. In the control group, silver particles were observed spreading along the resin-dentin interface and infiltrating inside dentinal tubules after aging (nanoleakage deposition increased from 36.57% to 50.41%) . On the contrary, no obvious change could be detected for the DMA-treated groups (nanoleakage deposition around 20%). The team thus deduced that DMA could strengthen the resin-dentin bond and its durability and is believed to increase the longevity of a dental filling.
“This research discovered that DMA is effective in strengthening the resin-dentin bond and improves its durability. The cytotoxicity is also similar to the resin monomers in traditional dental adhesives. It is believed that this compound may be commercialized in the future,” said Dr Tsoi.
Mussels need to maintain their adhesiveness under harsh marine environments
Comparison of the resin-dentin interface using DMA added adhesive and conventional dental adhesive. (set of photos on the left) DMA added adhesive results in a more stable interface. (set of photos on the right) In nanoleakage evaluation, in the control group, silver particles were observed spreading along the resin-dentin interface and infiltrating inside dentinal tubules after aging.
Images of nanoleakage expression from different groups. (A–D) The immediate groups. (a–d) The 10,000 times thermocycling aged groups. The first column is the Control. Groups on the right are treated with increasing concentrations of DMA. In the control group, nanoleakage deposition increased from 36.57% to 50.41% after thermoaging, while no obvious change could be detected for the DMA-treated groups.
The possible rationale of DMA as a functional monomer in dentin bonding. DMA could be deemed as a “bridge” connecting the upper adhesive network via carbon-carbon double bond polymerization and lower dentin collagen fibrils via hydrogen bond, unifying them as one whole structure
The chemical structure of N-(3,4-dihydroxyphenethyl)methacrylamide (DMA)
For media enquiries:
Ms Melody Tang, Senior Communications Officer of the Faculty of Dentistry, HKU
Tel: 2859 0494
Email:
melodytang@hku.hk
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香港大學（港大）牙醫學院、武漢大學和北京大學深圳醫院的共同研究發現，青口分泌的具黏性蛋白質中的一種複合物，能加强補牙黏合劑的黏合度，讓補牙物料能持久穩定地黏合於牙齒象牙質表面，有效提升物料耐用度及補牙效果，為補牙技術帶來突破。
研究結果已於學術期刊《Materials Today Bio》刊登，論文題為「青口仿生單體提升象牙質黏接長期耐久性」。
補牙療程常用於修補蛀牙、處理牙齒斷裂等口腔問題。牙醫一般先去除蛀壞的牙體組織，然後利用黏接劑，把樹脂複合材料等與牙齒顏色相近的填補物料，接合至牙齒的象牙質表面作修補。補牙物料與象牙質表面會形成微機械鎖結構，達至穩固的黏合效果。然而，由於口腔內長期濕潤的環境，加上咀嚼時撞擊等因素，傳統的牙科黏接劑日久效能可能降低，致補牙物料脫落或出現微滲漏、再蛀牙的情況，因而需要更換補牙物料等跟進治療。
青口是常見的海洋生物，在濕潤的環境中能牢牢黏著石塊等依附物，抵擋海浪衝擊不被冲走，其分泌物質黏附力之强，尤其在潮濕的環境下，一直引起科學家的興趣，探討其臨床應用的可能。
研究團隊發現，青口在濕潤環境中的特强黏力，源自其黏性蛋白質中的多巴（Dopa）成分，啟發團隊從中成功提取出仿生單體「多巴胺甲基丙烯酰胺」（DMA）。團隊將不同濃度的DMA作為新成份，加進黏接劑內以接合樹脂填補物料與象牙質，再以國際標準的攝氏5度和55度冷熱循環老化法，模擬牙齒在日常生活中反復遇上冷熱溫差環境，補牙物料有機會出現納/微米級裂縫和裂紋的情況，測試黏接劑的穩定性和耐用度。團隊利用硝酸銀測試結合界面的納米滲漏狀態，觀察銀粒子在裂縫和裂紋空隙沿着黏合面滲透和沉積的情況，以評估黏合面的注水和破壞幅度。
結果顯示，沒有加入DMA的傳統黏接劑對照組經老化測試，銀粒沿樹脂與象牙質接合面擴散，並滲入到象牙質小管内（納米滲漏沉積由36.57%增加至50.41%）。所有DMA組別都沒有明顯的變化（納米滲漏沉積保持在20%左右）。團隊從而推斷DMA能加強補牙物料與象牙質間的黏合強度及持久度，相信能延長補牙物料的壽命。
「青口在濕潤的生長環境、海水多變的溫度及酸鹼度，以及種種撞擊下維持黏合度，這些都跟我們的口腔環境類似。以青口作爲研究對象，有助我們理解如何運用這個特質，解決補牙物料不夠耐用的問題。」領導研究的港大牙醫學院兒童齒科臨床教授姚嘉榕教授解釋説。團隊成員還包括港大牙科物質學副教授徐傑漢博士及博士生李康、武漢大學口腔修復科黃翠教授和北京大學深圳醫院的研究人員。
徐傑漢博士指出：「研究結果顯示，DMA有效維持樹脂黏接強度和界面穩定性，抑制蛋白酶活性，細胞毒性亦跟通用樹脂單體近似，相信未來有商品化的可能。」
青口在濕潤的環境中能牢牢黏著石塊等依附物，抵擋海浪衝擊不被冲走
加入DMA的黏接劑與傳統黏接劑對照組比較。（左邊組圖）加入DMA的黏接劑的界面穩定性較傳統黏接劑對照組高。（右邊組圖）硝酸銀滲漏測試，銀粒在傳統黏接劑對照組沿樹脂與象牙質接合面擴散，並滲入到象牙質小管内
加入不同濃度DMA的黏接劑與傳統黏接劑對照組納米滲漏測試比較。上排未經冷熱循環老化過程，下排經歷10,000次冷熱循環。最左排是對照組，右排實驗組，加入DMA濃度低至高。沒有加入DMA的傳統黏接劑對照組經老化測試，納米滲漏沉積由36.57%增加至50.41%，所有DMA組別都沒有明顯的變化
多巴胺甲基丙烯酰胺 (DMA) 作為象牙質黏接的功能單體的可能原理。DMA視為「橋樑」，連接黏合劑和象牙質膠原纖維，並將它們結合為一個整體結構。
青口仿生單體: 多巴胺甲基丙烯酰胺 (DMA)
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