The wetting theory mainly applies to liquids or substances with low viscosity (hereby referred to as “adhesive candidate”), endowed with a high affinity to the mucus layer and, thus, a good capability to spontaneously spread onto the mucosa surface. The adhesive candidate-mucosa affinity can be evaluated through the contact angle method: the lower the contact angle, the higher the affinity between the substance and the mucosa. The contact angle indicates the degree of wetting when a liquid (i.e., the adhesive candidate) and a solid (i.e., the mucosa) interact: a contact angle equal or close to zero indicates an adequate spreadability of the adhesive candidate onto the mucosal tissue, that is a prerequisite for the mucoadhesion (Peppas and Sahlin, 1996; Mathiowitz et al., 1999; McBain and Hopkins, 2002; Shaikh et al., 2011).

The electronic theory postulates that the adhesive candidate and the mucosal tissue are characterized by different electronic structures. Thus, after contact of adhering surfaces, an electron transfer occurs leading to the formation of a double electronic layer at the adhesive candidate-mucus interface; the result is an electrostatic attraction between the two surfaces that is responsible for mucoadhesion (Mathiowitz et al., 1999; Asati et al., 2019).

The adsorption theory predicts that mucoadhesion is achieved via specific interactions (primary and secondary bonds) between the adhesion candidate and the mucosal tissue. After contact of adhering surfaces, they mainly interact by hydrogen bonds and Van der Walls forces; hydrophobic interactions may play an important role especially when the adhesive substance (i.e., a polymer) has an amphiphilic nature. According to this theory, such interactions, although they are individually weak, are the main contributors to the mucoadhesion; a great number of interactions at the adhesive candidate-mucus interface can result in an intense adhesive phenomenon (Ahuja et al., 1997; Mathiowitz et al., 1999; Huang et al., 2000; Lee et al., 2000; Hägerström et al., 2003; Salamat-Miller et al., 2005).

The diffusion theory postulates that the polymeric chains of the adhesive candidate and the mucus glycoproteins (i.e., mucins) interpenetrate to a sufficient depth to create semi-permanent adhesive bonds. The interdiffusion phenomenon mainly depends on the diffusion coefficient and the time of contact between the adhesive candidate and the mucus layer; it is generally enhanced when polymeric chains and mucins have similar chemical structures and are mutually soluble. The existence of concentration gradients is the driven force that promotes the diffusion of the polymeric chains within mucus network and, in turn, the mucin chains into the adhesive polymeric matrix until an equilibrium interpenetration depth is achieved. According to the literature, the interpenetration degree of the polymeric chains is also affected by certain properties of the polymer, such as MW, flexibility, hydrophilicity and cross-linking density (Peppas and Buri, 1985; Duchene et al., 1988; Jimenez-Castellanos et al., 1993).

The mechanical theory describes the effect of the surface roughness of the mucosal tissue on the adhesion of liquids due to their interlocking with the mucosa itself; more in detail, such systems fill the irregularities of the rough surface, which are responsible for an increase in the interfacial area available for adhesive interactions (Lee et al., 2000).

The fracture theory relates the strength of the adhesive bonds to the forces required to detach the two adhering surfaces after contact. This theory is different to the others: the mucoadhesion phenomenon is described by the fracture (or detachment) strength at the interface and is not related to the factors (i.e., polymer chemical structure, molecular weight, hydrophilicity, chain flexibility) that could affect the formation of adhesive bonds between the adhesive candidate (i.e., solid system) and the mucus layer. Since such a theory is based on mechanical considerations, it is also necessary to consider the other theories to better understand the mucoadhesion phenomenon in its entirety (Gu et al., 1988; Asati et al., 2019).

A pictural representation of the different theories is provided in Figure 3. None of these theories fully explain mucoadhesion, but several of these theories can be combined to obtain a comprehensive picture of the phenomenon. Depending on the nature of the system (i.e., liquid or solid), some theories are more applicable than others, but the relevance of the various theories is also strictly related to mucus layer. The high variability in mucus properties, such as viscoelasticity, thickness (from 50 to 450 μm in the stomach to less than 1 μm in the oral cavity), pH and sensitivity to various external stimuli, depends on both the location in the organism (ocular, nasal, buccal, respiratory, gastric, intestinal, cervico/vaginal mucus layers) and the physio-pathological conditions (i.e., over-production of mucus in SARS-CoV-2 infections) and it must be taken into account throughout the various development phases of mucoadhesive materials (Bayer, 2022).

SB-MD, like mucoadhesive products, belong to the broad family of medical devices that have been on the market for a long time under the scope of the previous legislation (European Union, 1993). They fall under the definition of medical devices as already present in Directive 93/42/EEC and adopted by the new Regulation, in art. 2, c. 1: “Medical device means any instrument, apparatus, appliance, software, implant, reagent, material or other article intended by the manufacturer to be used, alone or in combination, for human beings for prevention, treatment or alleviation of disease, or of an injury, and which does not achieve its principal intended action by pharmacological, immunological or metabolic means (abbreviated Ph.I.M), in or on the human body.”

According to the expert opinion expressed by Racchi et al. (2016), the term pharmacological means underlies all the Ph.I.M. mechanisms of action since immunological and metabolic modes of action are specific pharmacological actions. Presently the regulatory document uses the term pharmacological means (Ph. means) to indicate Ph.I.M means. Since the above definition of MD was released, it was clear that the definition of SB-MD overlaps in part with the definition of medicinal products. For this reason, since then they had been watched by MEDDEV (MEDical DEVices) Document (MEDDEV 2.1/3 rev. 3), being considered as borderline with medicinal products. According to MEDDEV guideline, “The borderline nature stems from the fact that both have the same presentation (dosage form) and composition (one or more substances). Medical devices may also be intended to treat and prevent disease, along with other specific medical purposes. Therefore, the decisive criterion for the demarcation between the two categories is the principal mode of action of the product.,” (MEDDEV 2.1/3 rev.3). In other words, for medical devices, it is necessary to demonstrate that they do not achieve their purpose by any pharmacological means.

Therefore, it is interesting to cite again the article by Racchi et al. since it contains interesting hints that might be useful when discussing the mechanisms of action of the mucoadhesive products thus defined. In the present article, the authors want to emphasize that the ‘mechanism of action’ and the ‘therapeutic effect’ of a product are different concepts. The same therapeutic effect might be achieved, in some cases through a pharmacological mode of action in other cases through a not pharmacological one. To quote an example relevant to the mucoadhesive materials (that is applicable to buccal, oropharyngeal, nasal, vaginal, etc.), the same effect (i.e., anti-inflammatory) may be reached by both a pharmacological means (inhibition of cyclooxygenase (NSAIDS)) and a physical mode of action (formation of a protective barrier to limit contact between tissue and external or internal irritating agents) (Racchi et al., 2016).

SB-MD are generally considered invasive devices according to Art. 2, c.1 of the New Regulation: “invasive device means any device which, in whole or in part, penetrates inside the body, either through a body orifice or through the surface of the body”, which is the case of mucoadhesive products. The New Regulation (Art. 51) as well as the old regulation requires to appropriately classify the medical device based on the intended purpose of the devices and their inherent risks in accordance with Annex VIII. Such a classification, in the case of an invasive medical device should be based on appropriate consideration on the level site and duration of invasiveness, but especially on the type of functioning, hereafter implicitly referring the mechanisms involved in the interactions with the specific part of the body.

The rule 21 of the new/old Regulations defines the level of risk associated to SB-MD as follows: “Devices that are composed of substances or of combinations of substances that are intended to be introduced into the human body via a body orifice or applied to the skin and that are absorbed by or locally dispersed in the human body are classified as: class III if they, or their products of metabolism, are systemically absorbed by the human body in order to achieve the intended purpose;—class III if they achieve their intended purpose in the stomach or lower gastrointestinal tract and they, or their products of metabolism, are systemically absorbed by the human body;—class IIa if they are applied to the skin or if they are applied in the nasal or oral cavity as far as the pharynx, and achieve their intended purpose on those cavities; and—class IIb in all other cases”. If we should consider this rule, we could superficially conclude that, since the mucoadhesive materials are typically high molecular weight macromolecules and as such they are not systemically absorbed, the mucoadhesion products, independently on the site of application, should fall into the risk categories IIa or IIb, without prejudice to being considered as medical devices to all effects. It is reminded that what does make the difference between the class IIa and IIb is the level of clinical evaluation needed in support according to Chapter VI of the New Regulation.

In view of the above, it is necessary to consult the New Regulation (European Regulation for medical devices 2017/745 (MDR)), in particular the premise 59 that reads: “Rules under the old regime applied to invasive devices do not sufficiently take account of the level of invasiveness and potential toxicity of certain devices which are introduced into the human body. To obtain a suitable risk-based classification of devices that are composed of substances or of combinations of substances that are absorbed by or locally dispersed in the human body, it is necessary to introduce specific classification rules for such devices. The classification rules should consider the place where the device performs its action in or on the human body, where it is introduced or applied, and whether a systemic absorption of the substances of which the device is composed, or of the products of metabolism in the human body of those substances occurs”.

This statement was alerting but not helpful pending an updated definition of Pharmacological means. Indeed, independently of whether a substance or any metabolite are systemically absorbed or not, it is required to elaborate further on their mechanism of action, not to risk falling into the category of medicinal products. The New Regulation in Art. 4 (European Union, 2017) delegates the definition of Pharmacological means to the new panel of experts of the Medical Device Coordination Group (MDGD) established under Art. 103.

Eventually, in April 2022, the new MDCG 2022—5 “Guidance on borderline between medical devices and medicinal products under Regulation (EU) 2017/745 on medical devices” was published. This guideline has further elaborated the concept of pharmacological, immunological, or metabolic means as follows “Pharmacological means is understood as an interaction typically at a molecular level between a substance or its metabolites and a constituent of the human body which results in initiation, enhancement, reduction or blockade of physiological functions or pathological processes.” Examples of constituents of the human body (cell membranes, intracellular structures, RNA, DNA, proteins, i.e., membrane proteins, enzymes) as well as components of extracellular matrix, components of blood and components of body fluids. Examples of immunological means and metabolic means are also given in non-exhaustive lists. Based on the above, it is not yet understood whether the new rules will help experts in correctly elaborating the principal mode of action of their intended devices, and how the implications of such new definitions will be received by the Notified Bodies. By the way, it seems that the intended simplification tends to make the definition of the mode of action more complicated.

HA if often combined with other polymers either natural, such as guar gum, or semi-synthetic ones like hydroxyethyl cellulose or synthetic ones, like carbomers or poloxamers. This combination allows for the formation of additional intermolecular interactions which strengthen the three-dimensional network. Interesting examples of combinations are given in the literature (Jiménez et al., 2007; Mayol et al., 2008). Especially polysaccharides that exhibit a water-binding capacity due to their high content in hydroxyl groups, should provide a synergistic effect leading to a moisturized and appropriately wetted mucosa and acting as a protective barrier for the entry of exogenous noxious agents (Hamza et al., 2008).

This section will summarize the main conclusions about the principal mode of action of HA as used in mucoadhesive products. The cited cascade of effects (hydration, swelling, lubrication, that in turn trigger phenomena such as wetting of the mucosal surface, spreading of the formulation, interpenetration of the polymeric and mucin chains, interlocking due to weak and reversible bonds, strengthening of the mucosal interface, rheological synergism between polymers) all together exhaustively describe and explain the mucoadhesion phenomenon. Nobody would question that these are mere physical mechanisms. The result of the formation of this mucoadhesive interface is the prolongation of the residence time of the formulation on the target mucosa promoting the desired effect, i.e., optimal hydration of the tissue, protection against the insult of the acidic environment or any other noxious event. According to MDCG document (April 2022), the physical barrier such as a film is one of the mechanisms recognized by the legislator as a physical mean.

In the case of PAA, hydration and swelling properties are strictly related. As in the case of HA, PAA and its derivatives are anionic polymers with hydrogel-forming moieties (carboxylic and hydroxyl groups), but contrary to HA, their polymeric backbones are more hydrophobic, meaning that PAA hydration and swelling properties are pH-dependent.

PAA is poorly soluble in water at low pH, causing structure shrinkage. It shows swelling properties under certain ionic strengths and salt concentrations in alkaline solutions. This is due to certain functional groups (-COOH) which are undissociated at acidic pH (less than 5) and dissociated at pH higher than 5.

In the early studies by Park and Robinson, (1987), it was envisaged that the mucoadhesive properties of PAA derivatives are due to the formation of hydrogen bonding between the carboxylic groups of PAA in the protonated (undissociated) form and the many hydrogen bonding sites of mucin macromolecules, which occurs mainly at acidic pH.

Thereafter, it has been confirmed (Patel et al., 2003) that, at pH < 4.0, the majority of the carboxylic groups are available in protonated form for the formation of conventional head-to-head H-bond dimers, whereas, at pH > 4, the majority of the carboxylic groups of both poly(acrylicacid) and the mucus glycoprotein are ionized.

On the contrary, the entanglement and the interpenetration of the bioadhesive polymer chains with the glycoprotein network are necessary for the formation of a mucoadhesive joint. These mechanisms require a tailored degree of swelling of the formed gel, which occurs at higher pH levels, thus explaining why polymer chain mobility and entanglements, and the consequent molecular interlocking, are favored in neutral/alkaline conditions.

A compromise between the number of available protonated carboxylic groups and the swelling properties was obtained by creating a series of cross-linked PAA derivatives. As it is often the case with polymers, the cross-linking, either chemically or physically induced, by reducing chain mobility, is functional to the modulation of the swelling properties.

PAA cross-linked derivatives, such as CP and PCP, are most commonly used as mucoadhesives. They show a high molecular mass and a high density of carboxylic acid groups. Therefore, at acidic pH, these polymers show minor swelling propensity due to a low percentage of dissociated acidic moieties. On increase in pH, additional charges result in electrostatic repulsion as well as osmotic forces within the polymeric backbone and uncoiling/expansion of the molecules lead to the desired degree of swelling of the polymer network (Zahir-Jouzdani et al., 2021). At the same time the cross-links prevent from an excessive swelling which would be detrimental to mucoadhesion.

Following the initial studies on PAA, the mechanisms of action of its derivatives have been thoroughly investigated and all the authors have come to the conclusion that hydrogen bonding significantly contributes to the mucoadhesion phenomenon, whereas swelling of the gel formed serves to consolidate the interlocking between polymeric chains (Shaikh et al., 2011). In an interesting paper (Patel et al., 2003), the authors have used various spectroscopic techniques (IR), nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS), and differential scanning calorimetry (DSC) to investigate at the molecular level the interactions between mucin and Carbopol 934P. The formation of hydrogen bonds between mucus and the mucoadhesive agent (CP) has been shown by the displacement of IR absorption bands and by NMR resonances. In addition, differences in surface atom concentrations, between mixtures and separate components, were consistent with H-bonding between the components.

CPs and PCPs are used in combination with other polymers such as hyaluronic acid in several medical devices (gels/suppositories) used for the treatment of vaginal dryness as shown in Table 2.

Besides that, the interaction between PAA derivatives and Poloxamers has been used to develop in situ gelling formulations, taking advantages of their pH dependent swelling properties (Zahir-Jouzdani et al., 2018). Other PAA derivatives, such as pegylated and thiolate polyacrylates have been proposed for optimizing the mucoadhesive properties (Leitner et al., 2003; Bayer et al., 2022).

To conclude about the principal mode of action of PAA derivatives, as mentioned above, all authors concur to the conclusion that hydrogen bonding formation significantly contributes to mucoadhesion, whereas swelling of the gel formed serves to consolidate the interlocking between polymeric chains. At the same time, the peculiar pH dependent solubility of these polymers makes them suitable to formulate both solid and semisolid dosage forms. For similar solubility reasons the lubrication mechanism cannot be claimed for these materials. Nobody would question that the quoted mechanisms are mere physical mechanisms according to MDCG document (April 2022).

As a final consideration, both pharmaceutical product/medical devices are already present on the market whereby PAA derivatives are used as functional excipients with no prejudice with respect to their merely physical mechanism of action.

The gel used as a mouthwash helps the management of painful symptoms deriving from oropharingeal mucositis. Due to its mucoadhesive properties, it forms a protective film thus alleviating relief against pain caused by many insults, including chemio- and radiotherapy.

The composition of Gelclair (Caramella et al., 2015a) comprises, besides the vehicle, flavouring agents and preservatives, and the following mucoadhesive substances: PVP, sodium hyaluronate, maltodextrin. PVP and sodium hyaluronate are certainly the prevailing ones as responsible for the mechanisms of bioadhesion and barrier film formation, as pointed out in the following papers (Buchsel, 2008; Vokurka et al., 2011). The increased residence time of the formulation was also proved by means of a washability test using fluorescence markers (Rossi et al., 1999; Sandri et al., 2007; Caramella et al., 2015b).

The mode of action of mucoadhesive polymers, specifically hyaluronate sodium and PVP, is in line with the concept of physical means as required for a medical device. Besides that, it is expected that the level of risk assigned to Gelclair, based on rule 21, and related the level of injury of the underlying mucosa, could be higher in comparison to other mouthwashes.

The product is a combination of hyaluronic acid, chondroitin sulfate, PVP, poloxamer 407, and contains xylitol C, sodium benzoate, potassium sorbate as preservative, flavoring agents and purified water. The gel acts as a physical barrier, protecting the gastro-esophageal mucosa against the attach of gastric juice (Di Simone et al., 2012). As for the composition, it is noted that PVP is considered an adhesive substance in general as suggested by its chemical nature, whereas chondroitin sulfate mainly favors the reparative processes of the underlying mucosa. Instead, the combination of hyaluronic acid with poloxamer represents a rather innovative solution since a synergic effect has been demonstrated between the two polymer types in promoting jellification and gel strengthening. Mayol and colleagues (2008) have demonstrated that the mechanical and mucoadhesive properties of poloxamer blends (F127 and F68), known to possess thermo-gelling properties, could be influenced by the addition of low molecular weight (150 kDa) HA. The authors evidenced that the presence of HA did not hinder the self-assembling process of poloxamers into micelles and, thus, did not significantly affect the thermo-gelling properties of poloxamer blends. On the counterpart, the addition of HA was responsible for an increase in poloxamer gel strength; the authors supposed that HA formed secondary bonds, in particular hydrogen ones, with micelles during poloxamer gelation, reinforcing the gel structure. Such a hypothesis was confirmed by PCS (Photon Correlation Spectroscopy) analysis: aggregates with hydrodynamic diameters higher than those of poloxamer micelles were measured when HA was added to poloxamer blends. Such results demonstrated that HA, in the hydrated state, allow micelles formation, movement and packing and interact with micelles through hydrogen bonds, improving the mechanical properties of the resulting poloxamer gels (gel network strengthening). In addition, a rheological synergism between poloxamer/HA gels and mucin dispersion was observed proving the mucoadhesive potential of the gels.

To note: the regulatory status of the medical devices registered in Italy can be found on the website of the Ministry of Health using link cited above. It is worth mentioning that presently the majority of products are still registered according to the old Directive, pending their re-evaluation according to the new Regulation.