Formulating Oral Hygiene Products for Healthy Teeth and Total Care

black man adult and child brushing teeth in mirror
When developing oral hygiene products, the formulator must balance ingredients that are both gentle to teeth and effective for polishing.
Image by Irshaad Majal/peopleimages.com at Adobe Stock

The mouth, teeth and oral cavity are key structures of the body, as well as instruments of verbal communication, passages for breathing and essential routes for nutrition. Their balance is closely linked to the overall health of our organism. Over time, all characteristics of the oral cavity change significantly, especially as progressive aging makes physiological damage repair mechanisms less efficient. Cosmetic oral care products and correct hygiene are therefore valuable allies to keep lips, teeth, gums and the oral cavity general in a healthy condition.

Many different tissues are present in the oral cavity, including one of the few mineral substances produced by the human body: hydroxyapatite, the material forming teeth and bones. This is a crystalline calcium phosphate with variable composition and hardness depending on carbonate, fluoride or hydroxyl ions in the crystals, making it more or less susceptible to acid attacks and mechanically resistant. Greater resistance and hardness are provided by a high fluoride ion content, whose salts have proven to be effective in the prevention of dental caries.1

The part of the teeth in direct contact with the oral cavity is the enamel, the most calcified and hardest biological tissue in humans, yet it exists in a precarious balance. It is constantly subject to mineral loss, mainly due to acids introduced through food or produced by bacteria in dental plaque through the metabolism of dietary sugars.

A bacterial film comprising a mixture of bacteria, starches, proteins and lipids forms on the tooth surface. In the presence of both bacteria and sugars, caries can develop. The consequences depend on individual susceptibility to corrosion, the type and quantity of bacteria, and the quality and amount of saliva secretion. When dental tissue is corroded by dietary acids in a process without bacterial involvement, the loss of materials, known as erosion, is progressive and hardly reversible.2

While mechanical factors like toothbrushing have almost no or little effect on hard enamel, incorrect oral hygiene habits (e.g., the over-brushing of teeth) can result in abrasion and severe wear of the enamel.3 For these reasons, when developing oral hygiene products, the formulator must balance ingredients that are both gentle to teeth and effective for polishing.

This article describes the formulation of oral care products including toothpaste and mouthwash. It outlines key functional ingredients along with those for added benefits ranging from anti-tartar and desensitizing effects to strengtheners, whiteners and more. Prototype formulations also are presented.

Toothpaste Abrasives

Choosing abrasives: When formulating toothpaste, abrasive powders should not be too powerful, to avoid the loss of dental material – especially if the enamel surface is compromised or the softer underlying dentin is exposed by gum recession or vigorous brushing. The key characteristics of abrasives are: hardness and size of the particles.

In terms of hardness, it must be lower than that of enamel, which is about 6-7 on the Mohs scale. This scale refers to the qualitative ordinal scale of mineral hardness and scratch resistance, from 1 to 10, where talc = 1 and diamond =10. Ideally, toothpaste abrasives should be around 3.

The particle size should not exceed 20 µm to prevent excessive roughness and damage to teeth surface and gums. The dosage of abrasives in toothpastes varies between 20% and 45%, depending on the amount of humectants, cost, particle size and aqueous phase absorption.

Abrasive ingredients: Calcium carbonate, being a less expensive material, has been widely used in toothpaste although its use has declined, supplanted by calcium phosphate, which is less abrasive. Calcium phosphate occurs in anhydrous and dehydrated form. The anhydrous form is harder and consequently less frequently used while the dehydrated variant has medium abrasiveness.

Silica also is widely used and allows for the formulation of low-abrasive cleansing toothpastes. It has the advantage of being compatible with fluoride salts.

Sodium bicarbonate is another option that slowly dissolves in water during brushing. It is considered a gentle temporary polish with low abrasive power. Its alkaline reaction also prevents the aggressive action of acids.

Finally, polishing powders like zeolite, kaolin and microcrystalline cellulose can be used to formulate very mild toothpastes or those for sore gums.

Other Basic Toothpaste Ingredients

Built around abrasive powders, toothpaste formulations (see Formula 1) are typically pseudo-plastic viscous hydrocolloids, additionally containing thickeners, surfactants and hydrotropes along with functional specific additives.

Formula 1. Standard Toothpaste FormulaFormula 1. Standard Toothpaste Formula

Humectant polyols: Humectant polyols (e.g., glycerin, sorbitol, xylitol, maltitol, polyethylene glycols) ensure easy miscibility with saliva, wetting of abrasive solids, swelling of thickeners, maintenance of the formula water content and a basic sweet taste. A high concentration of humectants provides significant osmotic pressure on the bacterial cell walls and gingival mucosa. The use of osmo-protectors like betaine and ectoine has been proposed to reduce such effect. In a clinical trial, it was demonstrated that betaine reduces the irritating effects of sodium lauryl sulfate-containing products for oral hygiene in subjects with dry oral mucosa.4

Common viscosity values for toothpastes range between 100,000 and 200,000 mPa.s at low shear rates (2.5 rpm). Hydrophilic polymers like carboxymethylcellulose, methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, sodium alginate, xanthan gum and carrageenan are typically used at high percentages. Mineral thickeners are usually constituted by precipitated silica gels and bentonite.

Foaming agents: A crucial sensory element of toothpaste is the formation of voluminous foam that also must easily be rinsed off. This function is usually achieved using high-purity sodium lauryl sulfate at 1.0-1.5%, which is also needed to wet the solids. Alternatively, there are a few surfactants with acceptable taste; alkyl glucosides, cocamidopropyl betaine, disodium cocoamphodiacetate and olivoyl fructoside have proven efficient, with good taste and moderate foam production.

The surfactant emulsifier steareth-30 can also be used in products with low mucosal aggressiveness containing the cationic antibacterial chlorhexidine gluconate, which is incompatible with anionic surfactants. Recently, saponins extracted from quillaja bark or yucca also have been proposed for their vegetal origin and low irritant potential.

Flavor enhancers: The pleasantness of toothpaste flavor is fundamental for consumers’ acceptability and depends on local taste and product claims (e.g., protection, breath freshness, medicinal effects, infant care, etc.). Some sweeteners like sodium saccharin or Stevia rebaudiana extract can be added to give a sweet taste or mask bitter actives.

Ammonium glycyrrhizate from licorice offers multiple benefits: sweetening as well as mild foaming, soothing and anti-inflammatory effects. Lasting refreshing breath is achieved with menthol derivatives. Salty toothpaste flavors have also found a market niche.

pH buffering: Since the pH of saliva is physiologically buffered between 6.6 and 6.9, a good toothpaste should also be buffered (e.g., with phosphates) at slightly higher values to counteract the effects of acidity due to carbohydrate fermentation.

Added Benefit Toothpaste Ingredients

Today’s oral care products are highly differentiated according to individual needs, age and habits. Therefore, a wide variety of added benefit ingredients, actives and flavors can play a role.

Antibacterials and anti-tartar agents: Antibacterials are intended to reduce the oral bacterial load and their proliferation. Bacteria are the cause of plaque and tartar formation that, if neglected, can lead to serious health consequences. However, their use is controversial because they can lead to alterations in the beneficial resident bacterial flora. Antibacterial enzymes like lactoperoxidase, lysozyme and lactoferrin, or plant extracts from pine bark can be used as alternatives to the highly debated triclosan or traditional chlorhexidine gluconate, which can cause tooth staining at use levels above 0.15%.

The most-used and highly effective ingredients against tartar formation are tetrasodium and tetrapotassium pyrophosphates, which exert this function by binding to the surface of the tooth and preventing the interaction with calcium and phosphate ions and the formation of crystals.

Astringents: Actives like stannous fluoride and zinc citrate counteract the inflammation of gums caused by bacterial biofilms that can degenerate into more severe gingivitis. Numerous plant extracts are also known for their soothing properties and are traditionally used in products intended to calm inflammation and irritation, and for the hygiene of sensitive teeth and gums: e.g., mallow, calendula, witch hazel, chamomile, etc.

Desensitizers: The erosion of dentin leads to contact between nerve endings inside the tooth and fluids contained in dentin microtubules, a phenomenon causing pain called dental sensitivity. Certain salts such as potassium nitrate, strontium chloride and stannous fluoride can be included to inhibit these nerve signals.

Enamel strengtheners: To strengthen enamel, fluorides (sodium, stannous or amine) are commonly used, along with sodium monofluorophosphate, preferred for its compatibility with abrasives and greater bioavailability of fluoride ions. The acceptable concentration of fluoride released by these salts has a maximum limit of 1,500 ppm.

The main action of fluorine is related to the ability of F- ions to replace OH- ions in the crystal lattice of hydroxyapatite, thus transforming it into fluoroapatite, which has lower solubility in acidic environments. Moreover, it is deposited in the tooth enamel in the form of calcium fluoride, thus acting by releasing fluoride during periods of low pH and inhibiting the formation of caries.

Other hypothesized mechanisms include the inhibition of bacterial enzyme systems of glycolysis, reduced production of bacterial acids and polysaccharides, as well as decreased adsorption of salivary glycoproteins from the tooth and plaque formation.5, 6

Whiteners: Whitening benefits can be achieved both by the physical removal of stains and by chemical reactions; i.e., by the degradation of chromophores. Many substances that release oxygen, besides reducing the bacterial load, oxidize the protein film covering enamel, thus enhancing its brightness.

Formula 2. Whitening Oral Gel-emulsionFormula 2. Whitening Oral Gel-emulsion

Hydrogen peroxide and its precursor, urea peroxide, can be used within the concentration limits imposed by regulations and safety criteria (up to 0.1%).7 An alternative to hydrogen peroxide is phthalimido peroxycaproic acid, which has demonstrated a bleaching effect after a single application, eliminating the problem of tooth sensitivity and irritation of the mucous membranes associated with traditional oxidizing agents.8, 9 Mixtures based on calcium carbonate and perlite abrasives also boast strong whitening effects, while activated charcoal, thanks to its adsorptive properties, can confer superficial tooth whitening.

Formula 2 shows a tooth cleansing and whitening gel-cream innovation. This viscous o/w emulsion is based on the whitening active ingredient phthalimido peroxycaproic acid and food-grade emulsifiers and oily lipids. The formula performs a gentle cleansing action with beneficial soothing and protective effects for gums.

Toothpaste Production, Quality Controls and Efficacy Tests

It is worth noting some of the challenges faced when producing toothpastes, as well as critical quality controls and efficacy tests.

Dissolution of surfactant: One difficulty in producing toothpaste is the dissolution of solid surfactant, e.g., sodium lauryl sulfate, in hot water. This can be improved by pre-wetting the surfactant with glycerin.

Minimizing air entrapment: Adding abrasive powders generally requires aspiration from the bottom of the vessel under vacuum, even if this operation entraps significant amounts of air, in addition to that adsorbed on the surface. Air bubbles are stabilized by the surfactant and their elimination under vacuum can be time-consuming.

One possible solution is adding powders from above, layering them on the liquid surface and creating maximum vacuum without moving blades or the turbine, in order to eliminate all air. Subsequent mixing and homogenization will incorporate the powders and residual air removal will be much easier.

Avoiding clumps: Thickeners should be added by pre-wetting them with a suitable fluid (flavor or liquid humectants) to avoid clumps. The viscosity at the production discharge should be lower than that of the product ready to be sold, considering that it tends to increase and stabilize within one month from the manufacturing.

Visual appearance: The typical appearance of toothpastes is opaque white, due to abrasives. Titanium dioxide can be added as a whitener to mask the semi-opalescent effect of formulas based on silica gels. Colored stripes or suspended microspheres make the pastes even more appealing. The stripe effect is obtained by filling the tubes with colored pastes of different viscosities.

Organoleptic properties: pH, density, viscosity, foam quality and quantity, and ease of rinsing are typical analytical controls requiring quality checks.

Cleaning and plaque removal efficacy: The cleaning (abrasive) power of toothpaste is typically evaluated on extracted radioactive teeth to measure the amount of dentin abraded. A specialized machine uses a toothpaste dispersion for such measurements. Plaque removal is measured on extracted molars treated with saliva, soy, sucrose, serum albumin and a reagent sensitive to removed proteins.

Breath freshening, enamel strengthening efficacy: Breath freshening effects are usually assessed by a trained sensory panel, while whitening is measured by a standard reflection colorimeter or comparison with color scales in ceramic teeth developed in twenty natural-colored shades. Enamel strengthening is determined with a microhardness analysis. Fluoride incorporation is conducted with standard analytical chemistry methods on treated enamel samples.

Mouthwash Ingredients

Mouthwashes are designed to reduce the bacterial load in the oral cavity and provide an immediate refreshing effect.

Vehicle, flavor, foaming, antibacterials, dye, astringent, soothing agent, etc.: The vehicle is commonly an aqueous or hydroalcoholic solution with humectants like glycerin, xylitol or sorbitol, containing a suitable flavor (1-2%) for a strong, long-lasting breath-refreshing effect. Also included are foaming surfactants at a low percentage (0.2-0.5%), antibacterials (often cetylpyridinium chloride, chlorhexidine gluconate or other cationic molecules), a water-soluble light-stable dye for visual appeal, and, if necessary, astringent and soothing plant extracts.

Surfactant, solubilizer, sweetener, buffering agents, etc.: The surfactant can be Sodium lauryl sulfate, as well as non-ionic ethoxylates, vegetal fructosides or plant saponins from Quillaja saponaria, with soothing and antibacterial properties. Non-ionic solubilizers are used to enhance the solubility of the flavor, especially if the formula does not contain enough alcohol to obtain a clear microemulsion.

Formula 3. Biphasic Alcohol-free Mouthwash; *final product = 70% aqueous phase, 30% oil phaseFormula 3. Biphasic Alcohol-free Mouthwash; *final product = 70% aqueous phase, 30% oil phase

Sweeteners are added to improve the taste of the product while buffering agents keep the pH around 7, similar to the physiological environment of the oral cavity. Note that sequestering agents are usually not added, as they could remove calcium ions from the enamel structure.

Novel formats: Besides standard liquid rinses, some innovative mouthwashes have entered the market for novel user experiences and benefits. Toothpaste tablets, for example, impart cleansing activity by chewing and crushing them with consequent brushing. Additional formulas are based on unique textures like mousses and biphasic solutions, examples of which are provided here.

Formula 3 is a biphasic alcohol-free mouthwash wherein the cleaning effect of the oils is an alternative to the abrasives of traditional toothpastes, minimizing the aggressiveness to mucous membranes. In addition, salt exerts osmotic action on the gums with an astringent effect and stimulates saliva production.

Packed in a suitable foamer, Formula 4 is a liquid mouthwash-toothpaste that mixes with air to create a soft mousse. It can be used as mouthwash or dosed onto a toothbrush to clean the teeth.

Formula 4. Gentle Mousse for Oral HygieneFormula 4. Gentle Mousse for Oral Hygiene

Conclusions

As this article outlines, typical oral care formulas are toothpastes in the form of abrasive cleansing hydrogels and hydroalcoholic solutions with antibacterial, refreshing and soothing ingredients. These products can be highly functionalized to meet the unique needs of consumers, e.g., whitening, enamel strengthening, and caries and malodor prevention. However, particular attention must also be paid to users who are sensitive to potential irritation who require new solutions with low abrasive power that are gentler to the gums, e.g., oral gel-creams.

The protection of the oral microbiome is also an indispensable requirement, which can be achieved using ingredients with prebiotic or probiotic functions as alternatives to classic powerful antibacterials such as triclosan or chlorhexidine. Salivary enzymes, such as lactoferrin and lysozyme, components of saliva immunity, are also useful in balancing the oral microbiota, as well as supporting the natural defenses of the oral cavity and improving the inflammatory state of the mucosa.

Finally, innovative products such as biphasic alcohol-free mouthwashes and hybrid foaming mouthwash-toothpastes (in mousse) provide users with unique experiences and benefits, examples of which are provided here. Taken together, such oral care products help to keep lips, teeth, gums and the oral cavity in general in a healthy condition.


References

1. Rugg-Gunn, A. and Bánóczy, J. (2013 Nov). Fluoride toothpastes and fluoride mouthrinses for home use. Acta Med Acad, 42(2) 168-78.

2. Gecy de Sousa Né, Y., Souza-Monteiro, … Paz Alvarenga, M.O., et al. (2022, Nov 8). Treatment for dental erosion: A systematic review of in vitro studies. Peer J, 10 e13864.

3. Wu, Y., Arsecularatne, J.A. and Hoffman, M. (2017 Dec). Attrition-corrosion of human dental enamel: A review. Biosurface and Biotribology, 3(4), 196-210.

4. Söderling, E., Le Bell, A., Kirstilä, V. and Tenovuo, J. (1998 Apr). Betaine-containing toothpaste relieves subjective symptoms of dry mouth. Acta Odontol Scand, 56(2) 65-9.

5. Rugg-Gunn, A. and Banoczy, J. (2013). Fluoride toothpastes and fluoride mouthrinses for home use. Acta Medica Academia, 42 pp 168-178.

6. Lussi, A., Hellwig, E. and Klimek, J. (2012). Fluorides - Mode of action and recommendations for use. Schweiz Monatsschr Zahnmed, 11(122) pp. 1030-1036.

7. SCCP (2007, Dec 18). Scientific Committee on Consumer Safety (SCCP) opinion on hydrogen peroxide, in its free form or when released, in oral hygiene products and tooth whitening products. European Commission. Available at https://ec.europa.eu/health/ph_risk/committees/04_sccp/docs/sccp_o_122.pdf

8. Pascolutti, M., and De Oliveira, D. (2021, Dec 9). A radical-free approach to teeth whitening. Dent J (Basel), 9(12) 148.

9. Bizhang, M., Domin, J., Danesh, G. and Zimmer, S. (2017). Effectiveness of a new non-hydrogen peroxide bleaching agent after single use — A double-blind placebo-controlled short-term study. Appl Oral Sci, 25 pp 575 – 584.


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