If consumers think that brushing their teeth with toothpaste is a simple hygiene procedure, they are mistaken. Mouth care is a complex strategy and modern cosmetics do much more than simply eliminate microbes. The oral cavity is a special environment, assembling entities including: living tissues, i.e., the gums, tongue and oral cavity mucosa; saliva, an ionic wetting fluid with plenty of enzymes; solid mineral/organic solid structures, i.e. teeth; nerve endings; bacteria and mold colonies that gather and form plaque; a two-way moving gas phase—the breath; regular supplies of nutrients via food and beverages; the arrival of other bacterial species from the environment and close human contacts; and subsequent temperature fluctuations. Bearing all these factors in mind, this article looks at one of the most common cosmetic products aimed to insure the adequate care of this intriguing body part.
Overall, oral health is improving worldwide. Nutritionists attribute this trend to the growing refinement of diets including less sugar, while dentists relate this improvement to advances in the quality and quantity of oral health services. Health authorities find that water fluorination has supported oral health improvement and toothpaste producers say the use of technically advanced toothpastes and their active ingredients are the reason. Regardless, the benefits of modern toothpastes are indeed multiple. Not only do they support oral hygiene, they also strengthen tooth structure, improve the functioning and physiology of gums, encourage consumers to maintain hygienic practices, decrease tooth sensitivity, improve the appearance of teeth, and provide a fresh breath. With so many variables in one product, how can the cosmetic chemist incorporate multiple ingredients into a stable, efficient formula? To answer this question it is important to know the goals of a particular formula and build it from the ground up, starting with the physical form.
First, consider the most common physical toothpaste form, which the name itself suggests—paste. There are alternatives to this standard besides those having colored stripes or suspended microspheres, but paste is the most common. Pastes are generally profiled as soft, smooth, thick mixtures, although one could debate about up until what viscosity level a paste is a paste. In the current market there are semi-fluid, easy flowing pastes and viscous gels that can even be dispersed in water to prepare ad hoc mouthwashes.
As a semisolid ribbon, the paste must be designed for easy measurement and distribution onto the massaging tool, i.e., the toothbrush. For distribution into the oral cavity, a viscous pseudo-plastic, shear-thinning fluid is ideal. The ribbon also should stay in place on the toothbrush bristle tips for the period of time before brushing begins, and be thin enough to allow for easy dispersion in the mouth without tearing or forming lumps under the mechanical action of the brush, or upon dilution by water. Finally, it should produce a controlled amount of foam, in comparison with the oral cavity volume, under both brushing and dilution.
More recently, there has been a revamp of solid toothpastes1, 2 in response to alarming statistic that 2 million toothpaste tubes are thrown away in the UK every year.3 Although used, each discarded tube still contains 4% of its original contents, including 1% to 2% each of flavors, surfactants and cellulose gum—i.e., from 3–6% organic material that cannot be squeezed out by consumers, amounting to 70,000 tons of toothpaste ending up in landfills each year. One possible solution to this problem is the longstanding powder form of toothpaste. Another is the more recently introduced compressed solid tabs based on sodium bicarbonate, cream of tartar and di-calcium phosphate, which are more convenient than loose powders. A self-foaming paste, similar to a self-foaming shave cream, also has been launched that contains a dissolved volatile hydrocarbon that expands to gas bubbles upon exposure to mouth temperatures, forming a foam with enhanced cleansing properties.4 This product form can be accomplished using bag-on valve packaging or pressurized cans. Yet another recent form of toothpaste is chewing gum, which claims tooth cleansing and mouth-refreshing properties.
After a given length of time brushing, the mouth-dispersed toothpaste should easily and quickly rinse off with water and the consumer should perceive a sense of well-being and freshness in the mouth. Gums should be non-irritated in spite of the massage, while the perception of teeth surfaces, as sensed by gliding the tongue over them, should be as even and polished. Further, since rinsing is never complete, residual traces of toothpaste should be harmless if ingested.
Bacterial flora of the mouth cavity should be reduced by the whole cleaning operation and the breath should be nicely flavored, but not so much as to interfere excessively with the next intake of food or beverages. Finally, a quick glance in the mirror should show good light reflection off the tooth enamel—in ideally a whiter shade, if possible. All these perceptions are preferably long-lasting; up to one hour is usually the maximum.
In order to accomplish good distribution in the mouth and deliver all the necessary ingredients to their targets after dilution with water and saliva, the toothpaste vehicle must be an aqueous solution. These generally include water-dissolving poly-hydric humectants such as glycerol, sorbitol, xylitol, maltitol, myo-inositol, polyethylene glycols and their blends. Humectants insure: good and fast miscibility of the paste with the saliva; adequate powder wetting—i.e., of abrasives and solid thickeners; proper swelling of the thickeners; water retention capability of the paste; and a basic sweet taste. The water portion of the vehicle is the object of fierce binding competition among all water-seeking ingredients in the formula, including the hydrotropes, thickeners and abrasives. Consequently, a sufficient amount of water to coordinate to the polyols, to wet the abrasives, and to swell the thickeners must also be present.
On the contrary, however, phase separation and shrinkage can take place over time. A high concentration, i.e., 20–40%, of humectants used in the vehicle certainly exerts strong osmotic pressure on the bacteria of the mouth but also on the cell walls of the gums. Successive dilutions by rinsing with water will decrease such osmotic stress. Recently, the use of osmoprotectants such as betaine and ectoines has been proposed5 to reduce the impact of osmotic stress on the equilibrium of the mucosa. Further, the pH of saliva is naturally buffered between 6.6 and 6.9, therefore toothpaste vehicles are usually formulated at slightly higher values in order to counteract the acidity of drinks and bacterial fermentations of food and beverages; phosphate buffers may also be used when required.
In order to polish enamel surfaces, abrasives generally are incorporated into toothpaste. Tooth surfaces become coated by a very adherent, thin layer of protein that must be eliminated or reduced to hinder the growth of bacteria on it. While the underlying tooth enamel surface is hard enough so as not to be noticeably scraped by abrasives, erosion problems can arise from the continuous contact of abrasives with the dentine, which is a much softer material than enamel. Dentine becomes exposed to toothpaste activities when gums are retracted either with aging or improper brushing.
In ancient yoga tradition, ground marine salt was used as an abrasive and freshly mixed with olive or sesame oil, then applied to the teeth with two fingers until a special, “clean sound” was produced.6 It is hypothesized that the salt entering the gum cavities may have imparted some antibacterial activity, and that the layer of lipophilic triglycerides may have inhibited bacterial growth. Successively, volcanic pumice was used in conjunction with vegetal sticks to mechanically detach plaque.7, 8 Crushed marble was then used with natron from salt lakes, and successively, precipitated calcium carbonate and calcium phosphates were adopted into formulae.9 With technological developments, the use of very hard abrasives like calcium carbonate and anhydrous or hydrated calcium phosphate has declined, even if these powders are still used in some countries.
Precipitated silica gels have progressively found a role in toothpastes as very mild abrasives, although alumina, i.e., aluminium oxide—a stable powder that provides gentle yet moderate abrasion to dentine and tooth enamel, is still used in some products. Vegetal abrasives include derivatives of the horsetail plant or ashes from burned eggplant peels.10 There are also toothpastes without abrasives; these are real gels whose polishing action is confined to the activity of humectants for wetting and surfactants for detaching food debris. The modern use of baking soda as an abrasive is an intelligent revolution. Besides being a fine solid with low abrasive power,11 sodium bicarbonate can be considered a “temporary abrasive” because during brushing, it slowly dissolves in water and its granules become more rounded. This spheronization effect creates less sharp-edged crystals. However, maintaining the original, fine crystal size of baking soda throughout the shelf life of the formula is a challenge.
The sensorial elements regulating the acceptability of toothpastes are important for long-lasting success in the market since they determine continuity of product use. Given the life-long use of this category of products, the degree of formulation sophistication expected by consumers is high. Several key characteristics lead to a “sensorial symphony” in toothpastes, including: the ribbon appearance on the brush—i.e., shine and evenness; the homogeneous dispersion of paste in the mouth; a light, abrasive feel in the mouth during brushing; a freshness stroke and perception of sweetness; equilibrated foam formation; easy rinse-off; long-lasting effects; and a pleasant, clean residual freshness feel.
Flavor ingredients account for freshness and agreeableness while dispersion in the mouth is due to the quality and performance of the thickening system, which is associated with the surfactant action. The purity of the surfactant used, its amount and taste can interfere strongly with the development of flavor in the mouth. Besides flavor, the sweetness and “roundness” of the taste, i.e., what brings the aroma notes together and connects them into a unified taste, is related to poly-alcohol type and content in the vehicle, supported by the addition of artificial sweeteners such as sodium saccharin and the like. Saline toothpastes require special flavors to develop an acceptable and pleasant taste in the mouth. Long-lasting flavors are intended to mask breath odor between brushings, and menthol derivatives or other “cooling” molecules are used for this purpose.
The appearance of toothpaste is just as important in the eyes of the consumer, with the color white being considered the most suitable since it relates to the whiteness of teeth. When pastes are translucent, such as those obtained from silica gels, adding approximately 1% micronized titanium dioxide provides this white appearance. Other common colors are pink, blue and green; pink relates to healthy gums while blue and green refer to minty, fresh effects. A good deal of energy also has been invested in obtaining appealing-colored stripes in dentifrices, which are usually accomplished using special machinery to fill tubes with two high-viscosity, different colored pastes. The color system must be based on insoluble micronized pigments in order to avoid the bleeding of color during the product’s shelf life.
The usual viscosity values of toothpastes are between 100,000– 200,000 mPa.s at low shear rates, i.e., 2.5 rpm. Thickeners used to achieve these viscosities belong to two main chemical categories, the hydrophilic polymers and the mineral colloids. The most well-known polymers used are modified cellulose types such as sodium carboxy-methyl cellulose or hydroxyl-ethyl cellulose, but also xanthan or guar gums. The thickener must easily swell in the aqueous solution of the vehicle without forming lumps and provide a thick, pseudo-plastic gel that holds the abrasive powder in suspension and is easily actuated from the tube by simple pressure. Of course, the thickener also must not form an unattractive film when the ribbon of toothpaste is detached from the paste in the tube during use. Today’s mineral thickeners are mainly represented by very light, precipitated, thickening-grade silica gels with or without the addition of fumed silica. Magnesium aluminium silicate also has been used. Zea mays starch derivatives are not typically used as they are easily fermented by the bacterial species in saliva.
Foam is an important sensory signal in dentrifices. It ensures that consumers are using products correctly and is generally provided by the well-known surfactant sodium lauryl sulfate (SLS), used at concentrations between 1% and 1.5%. At this level, most of the surfactant is strictly bound to the abrasive granules, which are wetted in part by the vehicle. Therefore, the degreasing action of the surfactant in the oral cavity is just sufficient to develop some foam and to help detach food residue and bacteria from the teeth.
The purity of surfactant used must be very high because even small traces of lauryl alcohol provide an unpleasant taste. Alternative surfactants to SLS are rarely used because they tend to taste bitter. Recently, however, highly purified alkyl glucosides have been identified as potentially suitable replacements.12 The surfactant emulsifier steareth-30 has been used for years in an SLS-free Danish toothpaste, which claims to be less aggressive to the mucous membranes and to exhibit improved compatibility with the cationic antibacterial agent chlorhexidine.13 More recently, vegetal surfactant saponins extracted from Quillaja (Panama tree) bark or from yucca have been proposed for their low irritation potential.14
The acceptance of flavor in toothpaste is strictly bound to regional market preferences and habits concerning foods and spices, as well as to the claims of the product. Thus, these preferences must be carefully investigated. Mint notes including peppermint and spearmint are most frequently encountered, although wintergreen and some spices are as well. In medicated or especially innovative toothpastes, flavors may be added to hide off-tastes due to bitter active ingredients. When employing flavor, the chemist must be certain to consider a product’s flavor evolution with increasing shelf life.
Toothpastes are available for the whole family that conform to the formula described thus far, although other market offerings are differentiated according to individual needs, age and habits. The general toothpaste formula (see Formula 1) remains quite constant; the actives employed are what vary according to specialized functions.
Enamel strengtheners: The role of enamel strengthening is mainly played by fluorides including sodium fluoride, stannous fluoride and amines fluoride, but more frequently by sodium mono-fluoro-phosphate, which is not rendered inactive by abrasives. It also supplies a more bio-available form of fluoride ions. Tooth enamel is made of calcium hydroxyapatite containing variable amounts of substituting anions, i.e., hydroxyl and carbonate fluoride, in equilibrium with the corresponding solvated ions in the saliva. It has been demonstrated that brushing with a fluoride ion-containing solution can insert increasing amounts of fluoride ions into the apatite lattice.15, 16
Apatites of higher fluorine content are more acid-resistant and have a stronger crystalline lattice than low fluorine, high hydroxyl or high carbonate apatites. Stannous fluoride also has been found to reduce caries, together with anti-plaque and anti-gingivitis benefits. The maximum allowed fluoride ion content in toothpastes in Europe is 0.15%, which is considered an effective concentration against caries. In the United States, the standard content in over-the-counter products is between 0.1% and 0.15%, with a few exceptions for prescription products (up to 0.5%). In Japan, the limit is 0.1%.17
Indeed, the presence of fluoride ions inhibits the corrosion of enamel even in acidic solutions. However, more important than the absolute concentration of fluoride is the diffusion capability of the fluoride ions into the crystalline lattice of the enamel and dentine. Recent discoveries suggest the use of a special grade nano-sized calcium apatite, used also in bone surgery, as a biologically compatible filler for tooth crevices and an enamel strengthener to protect against acid corrosion.18–20 Antibacterials: Antibacterials are used to reduce the always-abundant bacterial charge in the mouth and to reduce the risk of bacterial colonies that bring rise to tartar and plaque formation. Their use is frequently discussed in the context of not only their influence, but also their potential harm to the resident bacterial flora of the mouth. Nevertheless, the bacterial charge in the mouth, if left uncontrolled, may represent a risk to the health of the whole body.
Bacterial-killing enzymes like lactoperoxidase, lysozime, lactoferrine and IgA antibodies have been proposed as alternatives to more standard and frequently challenged antibacterial agents like triclosan. Vegetal antibacterial agents, e.g., those extracted from pine bark, have been proposed for the same aim. Chlorhexidine gluconate is also very common but scarcely compatible with anionic foaming agents, as are most cationic bactericides. Therefore, it requires alternative surfactants. Care should be taken to maintain chlorhexidine gluconate at levels < 0.15% in formulas to avoid brown staining on the teeth.
Whiteners: Anything that releases oxygen in the mouth, besides killing a part of the bacterial colonies, oxidizes the protein film on tooth enamel, making them appear more luminous. Hydrogen peroxide, urea peroxide and the like have been proposed for such a purpose. Maximum amounts in toothpastes are regulated by each country, for safety reasons. Complex polyphosphates also have demonstrated whitening activity by interacting with the crystalline lattice of tooth enamel.21
Astringent substances: Again, stannous fluoride typically is used to reduce gingivitis, but zinc citrate also has been reported to have this action, alone or in association with complex phosphates. Those who believe that tooth cleaning can be performed with a toothbrush alone, without any dentifrice,22 probably have never had gingivitis (nor a fiancée). The inflammation of gums induced by bacterial proliferation, i.e., biofilm, can lead to more severe gum and teeth diseases. To provide soothing as well as astringent activity, antibacterial agents such as triclosan are suggested.
Tartar control: Calculus, or tartar, is a hardened plaque formed on the teeth by the solutes in saliva, debris and insoluble minerals. Bacteria can easily develop on tartar, producing acids that corrode the enamel. To help reduce tartar, pyrophosphates are considered the most suitable toothpaste ingredients.
Desensitizers: The progressive consumption of dentine structure leads to modified contacts of the nerve endings inside the teeth with fluids in the dentine microtubules, which are 0.5–2 microns wide. This phenomenon is transformed into a signal to the nerves that is generally referred to as sensitivity. Some ions have been discovered that can inhibit these signals, the most common being potassium nitrate, strontium chloride and, again, stannous fluoride, which act by modifying the ion flux inside the microtubules.
Other ingredients: Many vegetal ingredients are also used in dentifrices for marketing benefits, flavoring properties, and to maintain the equilibrium and proper functioning of the gums and mouth tissues. The most frequent targets of action are for gum protection, anti-inflammatory properties, a refreshing feel, antibacterial and astringent benefits, and tartar control. Sodium chloride, preferably in the non-purified form of sea-salt, is increasingly being used in toothpastes for its mild antibacterial action and capability to decrease gum inflammation.
How, then, does one blend all these toothpaste ingredients to create an effective formulation? The production procedure is not trivial. If it were just a matter of water and humectants, it would be no problem but difficulties arise in particular with the dissolution of the surfactant and addition of the powders and thickeners. Use of the SLS surfactant requires warming it in a side vessel so it can be dissolved in water, and since it is normally a powder that is easily airborne and can aggravate the lungs, extreme care is necessary when using it. Pre-wetting SLS with glycerin can simplify its addition. Moreover, if the water-hydrotropes solution to which the SLS solution is added has previously been accurately deaerated, the number of air bubbles entrapped will be very low and simplify successive operations.
Usually, the surfactant solution is added to the aqueous phase before adding abrasive powders in order to help their wetting but this initiates another big issue: If powders are sucked into the mixer under vacuum, a large amount of air is incorporated while the air absorbed by the powder granules adds to it. The large amount of air bubbles formed, stabilized by the presence of the surfactant, is a real nightmare for the production operators since air elimination under vacuum is time-consuming. One interesting solution is to add the powders carefully from the top of the open mixer, then create the maximum vacuum possible without blending the system until all the air is eliminated from the powders floating on the surface of the liquid. Blending and homogenizing can then begin, and the small amount of residual entrapped air will be easily eliminated.
Polymeric thickeners cannot simply be added into the mixer at this point, as they will form lumps that are difficult to break. To decrease their speed of hydration, pre-blending them with flavor has been proposed.
Indeed, this avoids the formation of lumps as it reduces the hydrophilicity of the particles. Some flavor will get lost in the vacuum pump but the efficiency of swelling is almost perfect, with the help of some homogenization time. Important to note is the fact that the rheology of a toothpaste at the end of the production cycle is much lower than is seen in the tubes on the market. Usually, it takes almost one month for the viscosity of the product the reach its maximum level.
The cleaning and polishing performance of toothpaste can be easily assessed on extracted teeth. Abrasion is studied by irradiating dentine, then brushing with water and measuring the radioactivity extracted. In vivo studies are typically carried out by qualified dentists who care for all aspects of mouth and gum health and disease; magnified photographic records of the teeth and the gums should accompany the expert’s evaluation. Breath refreshing power is usually tested by an expert panel of olfactive evaluators who detect the residual intensity of the flavor at increasing intervals.
Innovation and the Future
The growing green tide is leading to the use of more ingredients from vegetal sources; preferably exotic, both as purified actives or as whole extracts. Marigold, fennel, horse chestnut, glycyrrhetinic and rosmarinic acid are recent examples of this trend. The active search for new surfactants is also exploring vegetal fields, with little success thus far.
What seems to be a promising and expanding sector, due aging of the world’s population, is toothpaste for the “aging mouth.” Indeed, such factors as reduced saliva production and the subsequent decrease of immune defenses, frequent general inflammation, reduced sense of taste, presence of more or less important mouth prostheses, need for strong but fine crystals for re-mineralization, and need for socially acceptable mouth appearance draw a special picture that until now has scarcely been considered by the toothpaste industry. Hopefully, new developments also will reveal alternative ways to brush teeth since the instruments of today do not adequately reflect the levels of technology available in this second millennium.
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This content is adapted from an article in GCI Magazine. The original version can be found here.