The authors have declared that no competing interests exist.
Conceived and designed the experiments: PX JH. Performed the experiments: PY XS JL YH. Analyzed the data: PY PX. Contributed reagents/materials/analysis tools: XS MZ. Wrote the paper: PY XS PX.
In order to endow environmental protection features to dentifrice, hydroxyapatite (HA) was added to ordinary dentifrice. The effects on dentinal tubule occlusion and surface mineralization were compared after brushing dentine discs with dentifrice with or without HA. The two types of dentifrice were then added to 100 µg/ml of hexavalent chromium cation (Cr6+) solution in order to evaluate their capacities of adsorbing Cr6+ from water. Our results showed that the dentifrice containing HA was significantly better than the ordinary dentifrice in occluding the dentinal tubules with a plugging rate greater than 90%. Moreover, the effect of the HA dentifrice was persistent and energy-dispersive spectrometer (EDS) revealed that the atomic percentages of calcium and phosphorus on the surface of dentine discs increased significantly. Adding HA to ordinary dentifrice significantly enhanced the ability of dentifrice to adsorb Cr6+ from water with the removal rate up to 52.36%. In addition, the sorption was stable. Our study suggests that HA can be added to ordinary dentifrice to obtain dentifrice that has both relieving dentin hypersensitivity benefits and also helps to control environmental pollution.
Hydroxyapatite (HA) is the main component of human bones and teeth
Dentine hypersensitivity refers to the transient and severe pain arising from stimulation of exposed dentine with cold, heat, and mechanical pressure. Increase in human life expectancy at the same time increases the lifetime of teeth in the mouth. Many diseases, including physiological wear, enamel hypoplasia, wedge-shaped defects, and gingival recession, can lead to exposed dentine
Currently, the most widely accepted mechanism of dentine hypersensitivity is the hydrodynamic theory proposed by Brannstrom in 1963
As to environment protection, economic development is accompanied with increased emissions of heavy metals to the environment. Many investigations revealed that the concentration of heavy metal ions always remained a high level in municipal sewage
There are a variety of methods to remove heavy metal ions from liquid, such as chemical precipitation, ion exchange, electrochemical method, sorption, reverse osmosis, and so on
We thus raise the possibility of mixing HA with dentifrice so that in addition to its role in dental care, HA can also adsorb heavy metal ions from the environment after the liquid waste containing dentifrice enters the sewer after tooth brushing, reducing the environmental heavy metal pollution. If feasible, it undoubtedly makes our daily oral care behaviors important for environmental protection, which has important social and economic benefits and opens a new gate for the field of environmental protection.
The SEM observations of the premolar dentine discs after 7 days treatment of dentifrice were shown in
(SEM, 5000×) (a, d) Dentinal tubules were empty without any occluding materials. (b, e) The majority of dentinal tubules were empty and a small number of dentinal tubules showed a sparse existence of occluding materials. The chunky occluding materials locate in the middle of the dentinal tubules with clear boundaries between the precipitate and dentin. (c, f) The majority of the dentinal tubules were blocked by materials. The occluding materials adhered to the dentinal wall with blurred boundaries between part of the precipitate and dentin.
Similar SEM observations were obtained for the premolars and molars dentine discs after treating the dentine discs with dentifrice for 7 days and then mechanical brushing with distilled water for extra 7 days (
(SEM, 5000×) (a, d) Dentinal tubules were empty with no occlusion. (b, e) Almost all dentinal tubules were empty with very few dentinal tubules showing little occluding materials. (c, f) Most of the dentinal tubules showed various levels of sediment blockage.
The dentinal tubule plugging rates of the premolar dentine discs treated with dentifrice for 7 days were shown in
The dentinal tubule plugging rate (
The major elements in each group detected by EDS include calcium (Ca), phosphorus (P), nitrogen (N), carbon (C) and oxygen (O). The groups with dentifrice treatment also had trace amounts of silicon (Si) and sodium (Na) that were used in friction modifiers. Ca and P elements, the main components reflecting the degree of mineralization, were analyzed statistically.
The test results for the atomic percentage of Ca in the premolar dentine discs were shown in
The lines on top of the bars stand for standard deviation. *:
The Cr6+ concentration in water was gradually decreased with increasing amount of added dentifrice. The removal rate of the HA dentifrice group is always higher than that of the control. Cr6+ sorption of the HA dentifrice group did not show any obvious change with time. There were little changes when comparing the sorption curves after standing for 14 days or 28 days to that of standing for 1day. The ordinary dentifrice group showed unstable Cr6+ sorption. At low concentrations, the sorption capacities showed large fluctuations with the standing time, where no significant changes were observed for high concentrations (
The aqueous hexavalent chromium cations removal rate of the HA dentifrice group is always higher than that of the ordinary dentifrice group.
EDS detected the existence of chromium in the two groups of precipitates from dentifrice sorption (
There is chromium element in the precipitates from both ordinary and HA dentifrice after sorption.
The in vitro dentine disc dentinal tubule blockage experiment has become the gold standard of dentine hypersensitivity
As seen from the SEM images, ordinary dentifrice with added HA showed a significantly increased effect of dentinal tubule occlusion after only 7 days of brushing. The effect is similar to commercially available anti-dentin sensitive dentifrices (data not shown) and that reported by the commercial researches of such products
An outstanding anti-sensitivity dentifrice should exert continuous good occlusion effects of the dentinal tubules against all adverse external environments, thus achieving durable anti-sensitivity effects. It has been demonstrated that the mechanical brushing by toothbrush has strong abrasion effect on teeth
In recent years, heavy metal pollution in waste water has become one of the major international environmental problems to be solved. Heavy metal ions in polluted water mainly consist of copper (Cu2 +), zinc (Zn2 +), lead (Pb2+), cadmium (Cd2+), nickel (Ni2+) and Cr6+, etc
Chromium is a cumulative poison with strong irritation and corrosiveness. It can invade the body through the respiratory tract, the gastrointestinal tract and the skin, which causes contact dermatitis and ulcers in the contact areas, chemical pneumonia, difficulty in breathing, liver damage, acute renal failure and other adverse reactions. Chromium poison presents longer duration and delayed healing
This study found that adding HA to ordinary dentifrice significantly enhanced the sorption capacity of dentifrice for Cr6+ in water, with a removal rate up to 52.36%. In addition, all concentration groups showed stable sorption effects of Cr6+ without any significant fluctuations and desorption as the standing time increased, indicating that the HA dentifrice is a good Cr6+ adsorbent. Further analysis of the centrifuged precipitate after sorption via EDS test revealed that the powder contains chromium, confirming the role of dentifrice for the sorption of Cr6+. Since the sorption is stable, it can be inferred that the heavy metal ions adsorbed by HA and precipitated with the dentifrice particles is environmentally friendly.
As shown in the Cr6+ sorption curves of the HA dentifrice, the amount of adsorbed Cr6+ by the HA dentifrice initially increased with increasing amount of dentifrice. As the amount of dentifrice continued to increase, the sorption capacity plateaued and the sorption reached a balance. This hyperbolic saturation curve is consistent with the sorption isotherm curves obtained from previous studies using HA for the removal of heavy metal ions
This is the first report on environmentally friendly dentifrice. We believe that environmentally friendly dentifrice refers to dentifrice with the dual functions of benefiting oral health and reducing environmental pollution. Our findings are not limited to the development of dentifrices. Based on the logic of this article, the philosophy of environmental protection can also be applied to the development of detergents used in our daily lives to form an industry of environmentally friendly commodities, thus adding more elements in harmony with nature to human life.
Our current study is only a preliminary step. In the future, we will focus on the HA concentration in the mixture, the particle size and surface modification of HA, and the sorption capacity of HA for multiple heavy metal ions mixture. We hope to continuously improve the effects of the HA dentifrice in occluding the dentinal tubules, while also achieving the goal of efficient sorption of a variety of heavy metal ions from water.
Dentifrice containing HA could be effective in occluding the dentinal tubules and a good absorbent for Cr6+. It is a promising way to add HA in ordinary dentifrice to obtain a valuable product that has both relieving dentin hypersensitivity benefits and also helps to prevent heavy metal ions pollution.
The protocol was approved by the Academic and Medical Ethic Committee of Guangdong Provincial Stomatological Hospital, Southern Medical University (approval number: 2011014).
Twenty-four premolars and twenty-four molars with complete crowns and without decay, cracks, or previous reparative therapies that were extracted for orthodontic treatment or treatment of periodontal disease were collected after written informed consent. The teeth were cleaned of all surrounding soft tissues, scrubbed clean, and stored in neutral 10% formaldehyde solution for future use
The dentine disc preparation method improved by Mordan et al
In order to remove the smear layer, dentine discs were etched with 10% citric acid for 120 seconds, washed with a large volume of deionized water, and ultrasonicated in deionized water for 3 times with 10 minutes each time. The dentinal tubules were exposed, thus generating dentine discs suitable for the research of dentine hypersensitivity.
The common calcium carbonate dentifrice and the HA/calcium carbonate dentifrice containing 3%(w/w) of HA with a particle size of 600 nm were made by the Foshan Engineering Center for Oral Care Products (Guangdong, China) according to national standards.
Twenty-four premolar dentine discs were randomly divided into 3 groups: the blank group treated with distilled water, the control group treated with calcium carbonate dentifrice, and the experimental group treated with HA/calcium carbonate dentifrice containing 3%(w/w) of HA. Each group contained 8 dentine discs.
Twenty-four molar dentine discs were similarly grouped and were used for parallel experiments.
Dentin discs were fixed on glass slides, and kept wet with distilled water. 0.2 g dentifrice was spread evenly on the entire dentin surface of the control group and the experimental group, and brushed with electric toothbrushes (Oral B, USA) at 7600 rpm and 100 g force for 2 minutes at room temperature. Dentin discs of the blank group did not receive dentifrice treatment, and distilled water was used instead. After standing for 2 minutes, dentine discs were rinsed with running water for 30 seconds to remove dentifrice on the surface. Slides were placed in 10 mL of artificial saliva containing Ca and P ions
The other half of the dentine disc was brushed by electric toothbrushes with distilled water for 2 minutes each time and twice a day for extra 7 days, followed with SEM observation. The persistence of the dentinal tubule occlusion effects was then analyzed.
Based on the method of Ahmed et al
Potassium dichromate powder (K2Cr2O7) was dried in a 110°C oven for 2 hours. 2.83 g potassium dichromate was weighted and dissolved in 100 mL water. Five milliliters of the solution was diluted with distilled water to 500 mL and mixed well to prepare the 100 µg/mL Cr6+ solution.
The ordinary calcium carbonate dentifrice was taken as the control group and the HA/calcium carbonate dentifrice containing 3%(w/w) of HA was taken as the experimental group. Dentifrice suspensions of 0.25, 0.50, 1.00, 2.00, 3.00, 4.00, 5.00, 6.00, 7.00, 8.00, 9.00, or 10.00 mg/mL were prepared. One milliliter of dentifrice suspension of each concentration was added to 50 mL of the Cr6+ solution described above, stirred at 25°C and 300 rpm for 3 hours, and stand for 1 day, 14 days, and 28 days. After centrifuged at 3000 rpm for 10 minutes, 25 mL of the supernatants were taken, added with 3 mL of diphenylcarbohydrazide, and mixed well. After standing for 10 minutes, absorbance at 540 nm was measured by a UV spectrophotometer using distilled water as the reference solution. The precipitates after centrifugation were dried to powder in a 120°C oven, sputtered with gold, and the elements in the powder were detected by EDS
The Cr6+ mass concentration in water was calculated according to formula (2):
The Cr6+ removal rates by the dentifrice suspensions were calculated according to the formula (3):
The SPSS16.0 (PASW, USA) package was used for statistical analyses. The difference in dentinal tubule plugging rates between dentine discs treated with ordinary dentifrice and HA dentifrice was examined using Wilcoxon-Mann-Whitney test. Wilcoxon-Kruskal-Wallis test was used to analyze the data for surface mineralization. The test level is α = 0.01.
The authors were especially grateful to Mr. Jianbo He and Mr. Changqing Zou (Foshan Engineering Center for Oral Care Products, Guangdong, China) for their precious contribution to making the dentifrice used in this study.