Preethi et al. [
12] defined a shampoo classification, listing different types of products: powder shampoo, liquid shampoo, lotion shampoo, solid gel shampoo, liquid herbal shampoo, solid cream shampoo and aerosol foam shampoo [
12]. Among them, in this review, the attention has been focused on solid shampoo and herbal shampoo, reporting the main important information known in literature [
12] and the novelty introduced by a clay shampoo.
3.2.1. Herbal Shampoo
Since ancient times, people have been using herbs and herbal extracts for cleaning, beautifying and managing hair. Nowadays, in accordance with an actual trend, cosmetic products formulated with natural ingredients are starting to be used again [
16,
17].
The interest towards the herbal products is also justified because, on one hand, they are considered less expensive; on the other hand, they present negligible side effects [
17], as confirmed in literature in the toxicological studies reported by Middha et al. [
18]. In fact, the authors performed the in vivo toxicological estimation in order to evaluate the effects of the extract, the object of study (
Emblica officinalis Fruit Extract), on cells. The LD
50 was measured to check lethal toxicity, and it was found to be 1125 mg/kg. In particular, the treatment, by using the extract at a dose of 200 mg/kg and 400 mg/kg to their respective group of mice, for one month, did not show toxic side effects. The selection of these two doses was based on an initial toxicity study where, until one month after administration of the extract, the animals were healthy without any visual signs or symptoms of illness [
18].
Interestingly, there is a large number of plants having beneficial effects on hair and being commonly used in shampoos for their content of vitamins, amino acids, sugars, glycosides, phyto-hormones, bioflavonoids, fruit acids and essential oils [
2]. Accordingly, various studies on shampoos based on natural ingredients have been developed. In particular, these products must be safe and efficient for long-time use [
17], assuring mild detergency and an aesthetic appealing. However, the formulation of cosmetics using natural raw material is a difficult task, since the main challenge regards the selection of these natural ingredients and new formulation techniques [
3].
In this regard, Bellare et al. [
3] reported as commercially available herbal shampoos, both liquid and powder, are still based on synthetic functional ingredients, boosted with natural raw materials or extracts [
2]. As reported in several Indian literature studies [
17], herbal shampoos are usually formulated incorporating some common traditional drugs normally employed for hair washing by Indian people [
17,
19]. Although these plant products can be used either in their powdered form, crude form, purified extracts, or derivative form [
2,
18], the preparation of an herbal shampoo using only natural materials (milder and safer than the synthetic ingredients), able to compete with traditional shampoos in terms of foaming, detergency and solid content, ends up being very difficult [
17].
Al Badi et al. [
17] formulated herbal shampoos mainly using the pericarp of
Sapindus mukorossi, commonly known as Soapnut or Reetha, the fruits of
Phyllanthus emblica (Amla), and dried pods of
Acacia concinna (Sheekakai), that have been traditionally used in India for centuries to wash hair [
17]. Authors described the formulated herbal shampoo being evaluated for conditioning performance by administering a blind test to 20 student volunteers. The shampoo formula showed good cleansing and detergency, low surface tension, small bubble size and good foam stability after five minutes. Furthermore, several tests were performed to evaluate and compare successfully the physicochemical properties of both prepared and marketed shampoos [
17].
Thanks to their high content in saponins, Reetha and Sheekakai ingredients produce a rich lather when shaken with water, also giving advantageous effects on skin that make them important ingredients for cosmetic applications [
20]. In addition, Reetha, Amla and Sheekakai are each characterized by peculiar properties. For example, the Amla extract, nourishing hair, can help hair growth and, thanks to the presence of fatty acids penetrating through the scalp, can remove dryness and dandruff, while its antioxidant properties strengthen hair roots [
21]. Reetha, instead, among all its properties, could be considered a mild cleansing agent, which, thanks to its antimicrobic properties, can remove any microorganism responsible for infections. Furthermore, it makes hair shine, restoring the natural hair texture. [
21] About Sheekakai, it is reported that it retains the natural oil of hair, keeps hair lustrous, reduces hair loss, adds volume, gives hair strength, and is a powerful antidandruff and conditioning agent [
21].
Other common herbal ingredients used for shampoo formulation are:
Azadirachta indica (Neem) as antibacteric,
Ocimum sanctum (Tulsi) that contains vitamins, antioxidants, oligoelements, having also antibacterial properties,
Aloe vera (aloe) as conditioning agents,
Terminalia chebula (harda, haritaki) and
Terminalia bellirica (bahera), with proven efficacy in preparation for hair care [
17].
Utane et al. [
21] also reported that Nagarmotha (
Cyperus Rotundus) promotes hair growth, Bhringaraj (
Eclipta prostrata) rejuvenates the scalp, Brahmi (
Bacopa monnieri) nourishes hair and helps with better circulation on the scalp. The appropriate pH of shampoo also helps in minimizing eye irritation, enhancing hair wellness and maintaining the physiology of the scalp [
2,
17,
22].
In accordance with the quality and security of the products, when commercial shampoos are compared with those formulated in laboratory (also in solid form), specific parameters must be evaluated. For this purpose, when herbal-based shampoos are taken into account, special tests, reported in
Table 1, need to be performed [
2,
16,
17,
22].
3.2.2. Solid Shampoos
As mentioned before, the consumers’ demand, related to a rapid hair and scalp cleansing, with additional benefits for hair softness and shine, is increasing the interest for the development of new shampoo formulations as, for example, the solid one. However, the innovations regard not only the product form (solid), but also the packaging and the use of novel interesting ingredients. These ingredients range from clays (mainly the Rhassoul clay) to herbs (Sheekakai, Reetha), as alternative washing bases, being already used as natural remedies in Moroccan and Indian traditions, up to washing flours (the most commonly used are chickpeas, millet, oats, rice starch or corn) [
6].
Solid shampoos present some additional advantages compared to the traditional ones. In particular, they are easy to transport and can be used for a longer time, thanks to more microbiological stability than liquid formulations. Indeed, among ingredients, water is largely present in liquid shampoos and, for that reason, the use of preservatives is necessary. Solid shampoos, instead, could decrease or even eliminate water in the formula, thus reducing also the preservatives amount. Some cosmetic industries have already done this, especially in order to preserve water as a primary human resource, with respect to the eco-sustainability idea of a new era of cosmetic products. Among the first attempts to formulate a solid shampoo, some patents describe both the formulation of a solid bar shampoos and of dry shampoos. The first ones are based on SDS, as main anionic surfactant (from 70 to 90%), vegetable oils, preservatives, dyes, conditioning agents, perfumes and binding agents (as consistency factors), while the latter formulations presented are based on baking soda and corn starch [
7]. In detail, it provides a dry shampoo composition comprising from 4 to 35% of baking soda and from 65 to 96% of starch. The product is proposed for a quick cleansing of children’s hair, when wet washing is not desirable, and for sensitive and/or young skin with a sensitive scalp, avoiding frequent hair washes [
23].
Clay Minerals as Natural Ingredients
Interestingly, the study of Carretero et al. [
24] reports that clay minerals can be powerful in cosmetic products and especially for hair care, due to their peculiar properties. An overview about the use of clay in skin and hair care cosmetics is reported in the following, presenting also a general introduction regarding these minerals, and its wide-ranging potential application in the biomedical field, which could be useful for formulating novel solid shampoo formulations.
General Uses of Clays
For years, clay minerals were used for their beneficial effects on human health as active agents or excipients in many pharmaceutical preparations, utilized in different contexts [
24]. In particular, as active agents, clays can be orally administered (for example in gastrointestinal protectors, osmotic oral laxatives, and antidiarrheals) or topically applied (dermatological protectors). As excipients, clays can be considered as a multitasking system, being lubricants, carriers, inert bases, viscosizing agents, stabilizing for suspensions and emulsions, as protection against environmental agents, for adhesion on skin and as grease adsorption agents, also as heat release controllers, and so on [
25]. Interestingly, clays are also used in synergy with other rheological modifiers for influencing the stability and/or other properties of health care products [
25].
Furthermore, among minerals used in pharmaceutical formulations that can be classified in different groups, i.e., oxides (for example rutile), carbonates (such as calcite, magnesite, etc.), sulphates, chlorides, hydroxides, elements (sulphur), sulphides (greenockite), phosphates (hydroxyapatite), nitrates, borates (as borax), there are phyllosilicates (smectite, palygorskite, sepiolite, kaolinite, talc, mica) [
26]. Among the various important properties of these clay minerals, there are the high specific surface areas, great sorption capacity, favorable rheological characteristics, chemical inertness, and low or null toxicity, with the further advantage of lower price [
24]. However, although the use of these natural minerals could be considered safe and rich with positive properties, the presence of some elements, even if in trace quantities, may pose a potential threat for the patient due to their natural origin [
27].
Clay Minerals and Clays
There is some misunderstanding in literature regarding “clay minerals” and “clays”. If the first ones are a mineralogical term referring to part of a family (the phyllosilicates) consisting of hydrated aluminosilicates containing considerable amounts of Mg, K, Ca, Na and Fe and less common ions such as Ti, Mn, or Li, the latter word “clays” is frequently used in the same sense and, sometimes, it also refers to natural materials composed of very fine-grained minerals, with some plasticity when mixed with water and hardening on drying [
28]. Clay minerals and clays are used in Pharmaceutical Technology and Dermopharmacy as excipients and as substances with interesting biological activity in solid (tablets, capsules and powders), liquid (suspensions, emulsions) or semisolid (ointments, creams) formulations [
28]. Overall, in nature, it is possible to find clays of different colors: red, green, yellow, purple, blue, white, depending on the presence of iron and its chemical state. For example, if it is bivalent iron, the clays have a green color, while, if it is trivalent iron, the color turns to red; when the clay is white instead, it does not contain iron [
15].
Chemical Composition and Impurities
The chemical composition of the natural clays used both in pharmacy and cosmetics is variable and it is mainly associated to the geological origin and to the associated minority minerals’ content, not always easily detectable. Trace elements detected in these clays are also variable. For example, if elements as As, Sb, Cd, Co, Cu, Pb, Ni, Zn, Hg, Se, Te, Tl, Ba, etc, are considered toxic, Li, Rb, Sr, Cr, Mo, V, Zr, Rare-earth element (REE), are, instead, less dangerous. These trace elements could be located inside clay minerals structure and/or adsorbed on the surface [
28]. The work of Rosselli et al. is interesting [
27], in which essential and toxic elements (Al, Si, P, S, K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, As, Br, Rb, Sr, Ba, Cd, Ce, Nd, Pb, U, Th, and La) were determined by energy-dispersive polarized X-ray fluorescence spectrometry in 15 samples of clay materials for pharmaceutical and cosmetic use. The investigated samples were grouped according to their mineralogical composition determined by X-ray powder diffraction. Samples consisting of smectites showed the lowest content of K, Zn, La, Ce, Nd, Pb, Ti, and Th and highest quantity of Sr, Br, and U. The sample containing smectite and kaolinite displayed the lowest content of Ca, Fe, Mn, Cu, Ni, and Sr and the highest amount of Al, Si, Ba, Zn, As, La, Ce, Pb, and Th. Samples composed of illite demonstrated minimal amounts of Br and maximal content of K, Rb, Ti, and Fe. In all samples analyzed, Cd and Hg levels were below 2 mg/kg [
27].
Clay Minerals in Cosmetic and Health Care
Minerals are mainly used in cosmetic products as masks, sunscreens, toothpastes, creams, powder and emulsions, bathroom salts and deodorants [
26]. However, due to the growing success of natural remedies, their use for wider cosmetic purposes is recently increased [
24]. In the following, a list of potential applications in this field is reported:
Clay minerals can be topically applied as dermatological protectors or for cosmetic reasons [
29]. Creams, powders, emulsions used as cosmetic products, applied on the external parts of the body, embellish, modify physical appearance, and/or preserve the physico-chemical skin conditions. For example, as reported by Carretero et al. [
26], clays with a high sorption capacity are present in creams, powders, emulsions for giving opacity, removing shine, and covering physical skin imperfections. Moreover, the ability of adhering to skin forming a protective film makes these materials able to absorb the excess sebum. For this purpose, the phyllosilicates group (such as palygorskite and sepiolite, in liquid preparations, kaolinite, smectites, and talc) is considered the most suitable [
26]. In the case of pharmaceutical applications as dermatological protectors, clays are generally used in the form of powders, creams and ointments to protect skin against external agents, and also in the case of exudations and liquid excretions. The most commonly used clays are kaolinite, talc and smectites, due to their high absorbent power [
26,
29]. Indeed, these minerals are capable of forming a film by adhering to skin and protecting it against external physical or chemical agents [
29]. By absorbing the skin’s secretions, they also have a refreshing action, producing a large surface for the evaporation, together with a gentle antiseptic action thanks to a water-poor environment, unfavorable for the development of bacteria. In this context, fibrous minerals (palygorskite and sepiolite) were considered the most suitable materials, but since there are doubts existing concerning the possible carcinogenic effect of palygorskite, if inhaled, their use as a dermatological protector is not desirable [
26]. However, some authors report that these minerals are neither toxic nor dangerous [
29].
As functional ingredients in hair care cosmetics and in skin care mask formulations, clay minerals are used due to their high absorbency level of substances such as greases, toxins, etc.) [
29]. Therefore, they are recommended for treating cutaneous inflammatory processes, such as seborrhoeic dermatitis, psoriasis, chronic eczemas or acne [
26,
29]. In particular, in the case of hair care applications, the addition of sulfur-containing minerals in shampoo formulations is considered an effective remedy against dandruff and seborrhoea [
26].
Clays and clay minerals are potential candidates as natural UV-protection agents in sunscreen formulations through mechanisms of absorption or reflection of UV radiation. Hoang-Minh et al. [
30] studied the protective role of kaolin, smectite, mixed-layer series-dominated clay and mica-dominated clay against ultraviolet (UV) radiation, in the range 250–400 nm, due to the bulk Fe
2O
3 content that lowers the UV-transmission level. Additionally, the UV-protection depends on the expandability of the clay or the combination of clay mineral with mixed ointment [
30]. Besides the direct use of clays as sunscreen, clays and clay minerals being used as delivery systems in cosmetic products, in order to improve the stability of an organic sunscreen like PABA (p-amino benzoic acid, an UV-B absorber in the range 200–313 nm), these new systems also open a novel horizon in the use of clays in this field as a promising, good, efficient way to protect other chemical filters. Moreover, as reported by Perioli et al. [
31], these formulations allow a very low sunscreen release avoiding the close contact between skin and filter preventing at least cutaneous reactions and allergy problems.
A clear distinction must be made between “healing clays” and those we have identified as “antibacterial clays”. The antiseptic and disinfectant activity of these minerals is mainly due to a high astringent capacity, largely dependent on their concentration. It is worth underlining that high concentrations of these materials can be considered highly toxic to organisms. For this reason, their continuous application over large skin areas or over non-intact skin should be avoided [
26]. While clays may heal various illnesses by means of their unique physical properties (e.g., high absorbance, high surface area, high heat capacity, high exchange capacity, etc.), only a few natural clays that kill pathogenic bacteria are reported. For example, among the healing clays, the researcher’s attention was drawn to a clinical use of French green clay (rich in Fe-smectite) for healing Buruli ulcers, a necrotizing fasciitis caused by
Mycobacterium ulcers [
32]. On the other hand, white, gray and yellow clays have an antibacterial effect against
Staphylococcus aureus (bacterial viability, colony-forming unit, CFU, of around 0 after 24 h) but have no effect against
Pseudomonas aeruginosa exhibiting bacterial viability CFU from 80 to 100. Instead, under the same condition, pink clay explained an antibacterial effect against
Pseudomonas aeruginosa (bacterial viability CFU of around 0) and has a low effect against
Staphylococcus aureus exhibiting bacterial viability CFU of around 40 [
33]. In spite of the use of geological mineral clay to heal skin bacterial infections having been evident since ancient times [
33], the antibacterial process displayed by the identified clays is not yet well known [
32]. Therefore, studies should be the focus in this direction since we are in the era of bacteria developing antibiotic resistance to existing pharmacological agents, and the discovery of new antibacterial agents, such as natural clay minerals, against pathogenic bacteria, could bring peculiar and great advantages [
32]. For example, Williams et al. [
32] reported that, under their conditions, the complete killing of
E. coli,
S. typhimurium,
P. aeruginosa, and
M. marinum by the investigated clays was observed. Thus, their use could provide an inexpensive treatment for skin infections, especially in areas with limited access to hospitals and medical resources [
32].
Clay Minerals Safety Specifications
About the use of these natural products, Pharmacopoeias and regulations report important information [
28], devoted to different safety aspects associated with their processing, handling and administration. Since clay properties are also related to their colloidal size and crystalline structure, to a high specific surface area, to optimum rheological characteristics and/or to excellent sorption ability [
28], their use depends on both structure and chemical composition. Overall, both as active ingredients or excipients, these minerals must conform with textural and compositional requirements, i.e., grain size, degree of mineral purity, water content, trace elements and microbial contamination, and appropriate technical properties [
28]. It is worth mentioning that the material safety data sheets include relevant information established by the European Commission [
28], including the identification of the substance, its principal intended or recommended uses, the composition/information on ingredients, hazards identification, handling and storage, physical and chemical properties, stability and reactivity, toxicological information, and a concentration range allowed in cosmetic formulations [
28]. Although clays are considered by consumers as a non-toxic and a non-irritant when applied topically in low concentrations, it is important to recognize that cosmetic firms must verify the safety of their products [
28,
32]. Safety must also be guaranteed in terms of preservation of clays not only before the use, but also during the preparation of a cosmetic product; indeed, the use of metal containers should be avoided during the preparation and conservation of product containing clays.
Toxicological Evaluation of Clays
López-Galindo et al. [
25] described some safety considerations about the use of clays starting from the consideration that clays used in pharmacology (treatment) or cosmetics (care and beauty) are usually taken as one, although their application field should be specified because it determines both technical aspects of their preparation as well as questions regarding code of practice and legal matters [
28]. In particular, the Cosmetic Ingredient Review Expert Panel evaluation reported the safety assessment of the most commonly used clays, focusing also on their biological effects, being specifically interested in studies about clays adsorption, absorption, distribution, metabolism and excretion, and in-vitro assays [
28]. The detailed review also commented on the results obtained from the experiments for testing acute toxicity, short-term and sub-chronic oral and parenteral toxicity in animals, inhalation toxicity, different types of irritation on skin, eyes and mucous membrane and the genotoxicity and carcinogenicity of such products [
28]. Despite the inhalation toxicity being reported for animals (with particular attention to granulometry, particle shape, concentration and mineral composition, showing the greatest effect), since most of the formulations are not respirable and the clay concentration is very low, the analysis concluded that the available data were enough to evaluate clay safety used in cosmetic products [
28]. As these substances vary widely in composition, texture and crystallinity, with significant effects on their properties, some of the tests, included in the main Pharmacopoeias, may be obsolete or imprecise since they are usually qualitative or semi-quantitative. Moreover, other properties, such as specific surface area or ion exchange capacity, determine the suitability of phyllosilicates, but are rarely taken into account [
28]. About the toxicological data, additional information are also reported in literature [
34]. For example, Tokarský et al. [
34] describe the adsorbing properties of clays regarding heavy metals and organic compounds (pollutants and dyes) from water or soil. Thus, this could justify the reason why the clays should not be stored or handled in metal containers. About this aspect, some authors, in their study, determined the presence of heavy metals such as Lead, Cadmium and Copper inside samples of clays from different origin. Results are important to guarantee the safety of cosmetic products as well as the quality and effectiveness [
35].
Clays Tests
For pharmaceutical or cosmetic applications, clays must fulfil various chemical (stability, purity, chemical inertia), physical (texture, water content, particle size) and toxicological (toxicity, safety and microbiological purity) requirements. Therefore, the Pharmacopeias recommend different tests because their usage for a specific application is dependent on its chemical and mineralogical composition (for example the type of clay mineral) and on its mineral structure (1:1 or 2:1 layer type) [
36]. Indeed, the technical behavior of a clay could change due to the presence different mineral phases determined by different cations in the octahedral sheet, or by isomorphic substitutions in the octahedral and tetrahedral sheets [
36]. Regarding this aspect, Desideri et al. [
36] used the examples of kaolinite and talc, which present low cation-exchange capacities, due to the reduced layer charges. Instead, the smectites are characterized by high ion-exchange capacities thanks to octahedral and tetrahedral substitutions. The most significant tests are related to identification of clay minerals, pH, microbial limit, water content, quantity of acid soluble substances, and the presence of impurities as trace elements, since the presence of some elements may pose a potential risk for the user [
27]. However, differently from the herbal shampoos, there are no descriptions in the literature about the evaluation parameters of clay and clay minerals in solid shampoo formulations. To guarantee greater consumer safety, as well as the quality and effectiveness of these products, it would be important to implement the academic studies in the evaluation of these parameters, also by comparing it with herbal shampoos.
Clays’ Cleaning Properties
As already said, clays are versatile ingredients in cosmetic field due to their many properties, including the detergent one. In fact, some clays, if wet with water, behave like detergents and so, they were usually used for hand and body hygiene well before the introduction of soaps, on the industrial-scale [
15]. Indeed, these substances, through an adsorption process due to the ability of the particles that compose them to attract and fix some greases on their surface, are able to remove impurities. In particular, this process occurs also at a cutaneous level when clays, after being applied in gel form, subtract fats and impurities [
15]. Accordingly, in recent times, hair care cosmetic formulation technologies have focused the attention on washing clays, among which the use of Rhassoul clay in shampoos is highlighted, together with washing herbs. With regard to this, there are various patents (see, for example, [
37]), in which authors report solid shampoo formulation using Rhassoul clay, glycerin and natural extracts as conditioning agents [
37]. In particular, the Williams’ invention relates to compositions for a multi-benefit hair care product, acting as a shampoo or a cleanser, as smoothing conditioner, as deep conditioner and as a leave-in conditioner.
Rhassoul Clay
The Rhassoul clay is an outstanding naturally found reddish-brown clay that originates from Morocco, traditionally used in skin and hair care formulations as soap, shampoo and conditioner [
8,
38]. It is also known by the names Red Clay, Red Moroccan Clay, Ghassoul Clay and Oxide Clay [
38]. Its etymology derives from the Arabic verb Rhassala which means ‘to wash’ [
38]. Not surprisingly, due to its diffused use, the official Moroccan Pharmacopoeia reports specifications and uses of Rhassoul [
38]. Due to its good detergent property, Rhassoul clay can be used as a washing base in several shampoo formulations (especially against oily hair), but, thanks to its simplicity of use, it is often also used as a mask in hair treatments [
8]. Moreover, Rhassoul, containing natural minerals that act as natural detoxifying and nourishing, is also used in several skin care products, from masks to anti-acne creams, as a principal ingredient [
8]. However, it is worth mentioning that the natural Rhassoul clay is characterized by a basic pH, so, when employed in a shampoo formulation, the use of a pH neutralizer for reaching a pH value that is suitable for hair and scalp physiology is necessary [
37]. The Rhassoul clay composition, related to the nature of the pure mineral clay fraction which is claimed stevensite (belonging to the smectite group), is controversial. Overall, the XRD data revealed that the raw Rhassoul clay consists mainly of a Mg-rich trioctahedral smectite, with the presence of impurities such as quartz and dolomite [
35,
38]. Rhouta et al. [
39] reported that the mineral fraction of the clay contains also a larger amount of Al (>1 wt.%) if compared with other stevensite. The Moroccan official bulletin (No. 3202; March 1974) defined Rhassoul clay as a product made up of 90% of stevensite and Al
2O
3 that could reach 5 wt.% [
39]. According to the first article of the Moroccan Decree nº 2-73-370, dated 5 March 1974, the products can only be marketed under the name ‘Rhassoul’, if they contain at least 90% w/w of the clay mineral known as stevensite and Li (Hectorite) [
38].