How to Choose an Emollient? Pharmaceutical and Sensory Attributes for Product Selection

Emollients have been in use for centuries. Records from ancient Greeks describe the use of wool fat to make the skin soft and smooth [1]. Meanwhile, emollients have become more sophisticated and user friendly. Today, emollients are widely used, are part of daily body care, and have become indispensable therapy adjuvants for the treatment of dry skin conditions such as atopic dermatitis, psoriasis, or dry skin as an adverse effect of cancer treatment. The term emollient, which is a Latin derivation and implies a material that softens and smoothens skin, has become a standalone term for a wide range of skin care products with varied characteristics [2‒5]. Simultaneously, the term moisturizer is used to refer to the same product category. Both terms describe the effects of softening and moisturizing. The mechanism of action of these products is based on an increase in the water content of the stratum corneum. As a result, typical symptoms of dry skin conditions, such as scaling, crackling, inflammation, and pruritus, are alleviated. Most marketed products include ingredients that are occlusive (e.g., paraffin, petrolatum reducing transepidermal water loss) and, at the same time, others that are humectant (e.g., urea, glycerin attracting/retaining water). The water contained in most emollients may have a noticeable instant moisturizing effect. For better readability, we used the term emollient as a collective superordinate denominator.

Adherence to topical medicated and non-medicated treatments, notably for dry skin conditions, is reportedly low [6‒10]. The underlying reasons for this are manifold and may include insufficient medical and nursing support for product selection, sensory product attributes (e.g., smell), aspects of product application (e.g., spreadability), product appearance (e.g., glossiness), and product feel (e.g., stickiness) on the skin. Attempts have also been made to portray lipid content, rheological attributes, or galenic product format as adherence-promoting or adherence-preventing properties [11, 12].

As national guidelines and recommendations remain generally broad and do not mention use- or application-relevant product attributes, personal experience with a particular product, advertisements, and recommendations of healthcare professionals, relatives, or friends influence such product choices. A crucial factor in any such choice is the sensory attributes of the product. In medicated treatments of dry dermatoses, numerous studies have investigated the importance of simple sensory product attributes and other adherence-relevant properties, such as packaging, product format, or application instructions. However, the descriptions of sensory product attributes are often limited to variables, such as the product absorbs well/badly, the product is sticky/smooth, and queries are often combined with other attributes such as effects or adverse effects [12, 13]. In the treatment of dry dermatoses with emollients, there is little information describing and relating to these various features.

We explored whether common pharmaceutical product attributes, such as lipid content, viscosity, or galenic product format, are associated with sensory attributes of selected emollients and discuss the extent to which this information is useful for product selection.

Based on administrative claim data from a large Swiss health insurance company, ten emollients frequently used in Switzerland for medical skin care were selected as exemplary samples. These products are marketed by various Swiss and international pharmaceutical companies (Table 1). All emollients are authorized as medicinal products in Switzerland.

The lipid content (%) and the galenic product format are summarized in Table 1. The information is based on authorized labelling text.

We used a rotational rheometer (Modular Compact Rheometer MCR 302, Anton Paar, plate 25 mm) to determine the viscosity η [mPas] at 22°C of the selected emollients at shear rates between 1 and 100 s⁻¹.

For each product, we measured three independent samples and calculated mean values with standard deviations. We conducted a one-way analysis of variance (ANOVA) and Fisher’s Least Significant Difference (LSD) test to examine whether there were statistically significant differences between the mean viscosities η of the analysed emollients at a shear rate of 1 s^-1 using the statistical software XLSTAT for Microsoft Excel 2018. Statistical significance was defined at an alpha level of 0.05.

Nine trained panellists (eight women and one man) independently evaluated the selected emollients in a blinded fashion based on 18 predefined sensory attributes (Table 2). They rated the attributes against a reference sample on a linear scale, where 100 were the highest and most intense and 0 was the lowest and least intense values. Each panellist evaluated the products twice. All tests were performed in the sensory laboratory facilities of the Zurich University of Applied Sciences (ZHAW) using the ZHAW standard guide for sensory descriptive analysis, which is based on the ISO Norm on general guidance for establishing sensory profiles and the ASTM Standard E1490-03 guide for descriptive analysis methods for skin creams and lotions [14‒17]. For sample coding and electronic data management, FIZZ sensory analysis software version 2.51 (Biosystemes, Couternon, France) was used.

For each product and sensory attribute, we calculated mean values from the individual ratings of the nine panellists. We conducted an ANOVA and Fisher’s Least Significant Difference (LSD) test to examine whether there were statistically significant differences between the mean values of the different sensorial attributes of the analysed emollients, using the statistical software XLSTAT for Microsoft Excel 2018 and SenPAQ from Qi Statistics (2005–2014, West Malling, Kent, UK). Statistical significance was defined at an alpha level of 0.05.

The properties of the products are listed in Table 1. Lipid content usually includes all ingredients that can be incorporated into the lipid phase of an emollient. This implies that this phase comprises not only oils and waxes but also lipid-soluble substances, such as emulsifiers or structure givers. The proportion of the latter ingredients is usually relatively small. The assignment of substances to a specific phase is not strictly defined in a scientific context and may, therefore, be subject to different interpretations. The galenic product formats, as indicated by the manufacturers (authorized labelling text), were ointments, creams, and lotions.

All analysed emollients were non-Newtonian pseudo-plastic fluids. The application of a shear rate of 1 s^-1 best described product consistency and initial spreading (resulting from the internal structure) because the measured dynamic viscosity reaches its highest values and leads to better product discrimination [18]. The products were arbitrarily assigned to three viscosity groups according to the mean viscosity η at a shear rate of 1 s^-1: high > 100′000 mPas, medium 50′000-100′000 mPas, and low < 50′000 mPas (Table 3; Fig. 1).

The sensory profile values of all the products are summarized in Table 4, and the selected sensory attributes are depicted in spider diagrams (Fig. 1).

The lipid content did not seem to affect the peaking and firmness values (Table 4, e.g., Pruri-med Lipolotion [40%], Linola Emulsion [16%]). Products with medium-to-high-viscosity (Excipial Fettcreme [54%], Excipial Mandelölsalbe [96%], Linola Fett [60%], and Nutraplus Crème [25%]) are characterized by increased peaking and firmness values.

Lipid content does not necessarily result in different spreadabilities (Table 4, e.g., Dexeryl Crème [21%], Excipial U Hydrolotion [11%]). Within and between viscosity groups, significant differences in spreadability exist (e.g., within viscosity group: Excipial Mandelölsalbe (96%) vs. Linola Fett (60%) and between viscosity groups: Linola Fett (60%) [high-viscosity group] or Nutraplus Crème (25%) (medium-viscosity group) and Dexeryl Crème (21%) (low-viscosity group). In most cases, oil-in-water (o/w) emulsions elicit more pronounced cooling sensations than water-in-oil (w/o) emulsions because the outer phase is aqueous (Excipial U Hydrolotion (11%) o/w emulsion vs. Excipial U Lipolotion (36%) w/o emulsion). Within the same emulsion type group (see Table 1, galenic format), marked differences may occur (Excipial U Hydrolotion (11%) o/w emulsion, Linola Emulsion (16%) o/w emulsion).

A lipid content of >50% is an indication of poor absorbency (Linola Fett [60%], Excipial Mandelölsalbe [96%], Excipial Fettcreme [54%]). Products with low lipid content had high absorbency values (Excipial U Hydrolotion [11%]; Linola Emulsion [16%]). The lipid content is not a clear indicator of tackiness, oily residue, greasy residue, and smoothness silky.

Although consumers and patients are influenced by many factors when making their product choice, individually perceived sensory product attributes have a decisive impact on product choice and, more importantly, on adherence to product use. The latter is crucial for therapeutic success. We explored whether the sensory properties of selected emollients are associated with pharmaceutical product characteristics, such as lipid content, viscosity, or galenic product format, all of which also serve as selection criteria and discuss the extent to which this information is useful for product selection.

The pick-up phase provides a sensory impression during the initial product handling. Lipid content and viscosity are often assumed to be indicative of sensory product attributes. In our investigation, lipid content did not seem to affect peaking and firmness values. This is related to the varying physicochemical properties of lipid-phase ingredients. The peaking and firmness values increased with medium- to high-viscosity products. A key feature of the rub-out phase is spreadability. Low product viscosity is a reasonable indication of good spreadability (e.g., Excipial U Hydrolotion). Lipid content was not indicative of spreadability. Contrary to widespread expectations, high lipid content does not necessarily imply poor spreadability (e.g., Excipial Mandelölsalbe [96%]). Cooling sensations, unless ingredients such as menthol, menthol-like derivatives, or ethanol are present, depend on the type of emulsion and the proportion of the water phase that extracts heat from the skin by evaporation. In most cases, o/w emulsions elicit more pronounced cooling sensations than w/o emulsions because the outer phase is aqueous. Marked differences may also occur within the same emulsion type group. For patients and consumers, the evaporation-based cooling sensation is usually short-lasting and not as pronounced as sometimes implied. Key attributes during the after-feel phase are absorbency, tackiness, oily and greasy residue as well as silky smoothness. Lipid content is a poor indicator of any of these sensory attributes. Tackiness and greasy residue of the two products with very different lipid contents (Excipial Mandelölsalbe [96%] and Excipial U Hydrolotion [11%]) were comparable.

As exemplified above, our data showed that both lipid content and viscosity were not exclusively indicative of sensory perceptions in the pick-up, rub-out and after-feel phases. Although the quantitative and qualitative composition (oils, waxes, emulsifiers, and structure givers used) of the lipid (and water) phase(s) are not accurately known, one may conclude that specific physicochemical properties of the individual phase ingredients, with their different polarities, melting points, and hence structure-giving properties, are responsible for the overall sensory perceptions [19, 20].

Among healthcare professionals, as well as among patients and consumers, it is common to assign certain sensory product attributes to galenic product formats [21, 22]. For example, ointments are often considered sticky and difficult to spread, whereas creams are considered smoother and easier to spread. This view may tend to be true for certain galenic product formats; many ointments are indeed sticky. However, our investigations showed that an ointment (Excipial Mandelölsalbe) can be spread better than a cream (Linola Fett). It is often overlooked that it is not the galenic product format in the primary container (sales container) that is responsible for sensory perception and, hence, a certain preference, but the sum of vehicle ingredients during and after application onto the skin. Except for Excipial Mandelölsalbe, all emollients lost their volatile components after application, depending on the emulsion type (w/o, o/w). The primary vehicle, as it is present in the primary container (sales container: tube, bottle), is crucial for sensory attributes during the pick-up phase (e.g., firmness). The secondary vehicle comprises vehicles during the rub-out phase (e.g., spreadability). During this phase, the volatile vehicle ingredients evaporate. After the evaporation of all volatile vehicle ingredients, the tertiary or final vehicle evolves [23]. The latter is responsible for the sensory attributes of the after-feel phase (e.g., absorbency). Because many topical products contain volatile ingredients that evaporate after application to the skin, thereby changing the galenic product format, it is obvious that the galenic product format referring to the product in the primary container should not be used to predict any sensory attributes. This phenomenon, coined as metamorphosis of the vehicle, is not only important in the present context but also in the topical bioavailability of active pharmaceutical ingredients [23].

Crucial for the sensory attributes are the physicochemical properties of the individual product ingredients in the product before, during, and after product application [22, 24]. Qualitative and quantitative information on product ingredients is publicly unavailable. As refined information on sensory product attributes, such as that compiled for this study, is also not readily available, it is important to consider patient preferences when determining the treatment regimen. This can be handled in individual cases when communicating with patients but is a challenge within larger institutions, as only a limited number of sensory-differentiated products can be kept in stock. This underlines the importance of drug commissions in which different representatives of health professions must agree on products. Regardless of the context, responsible healthcare professionals should consider their own sensory experience with the products in consideration [10, 21, 25].

When choosing emollients, the lipid content may also play a role in other contexts. The lipid content of products indicated for the direct and adjuvant treatment of desiccation dermatoses is often considered clinically relevant. It is assumed that the product occlusivity generated by lipids is responsible for skin hydration, and hence, for symptom relief. Some older studies have suggested that there is an association between skin hydration and occlusivity due to the lipid content and lipid type of the topical preparation [24, 26‒29]. However, a recent study showed that the occlusivity of two very similar emollients (same galenic product format, qualitatively and quantitatively the same lipid-phase ingredients, but different emulsifiers) is very different [30, 31]. Because these measurements have been applied in only a few studies and data on the validity of such measurements are not yet available, their clinical relevance remains unclear.

In conclusion, we show that widely held views on the importance of pharmaceutical attributes such as lipid content, viscosity, and galenic form of emollients, as currently understood and taught in product selection, are not universally valid and should therefore be reconsidered. For the time being, the gold standard “human” as a tester remains unrivalled. It underlines the importance of the dialogue between prescribers and patients.

When assessing rheological properties, we based our work solely on viscosity data obtained from a rotational rheometer. Other methods have not been evaluated. We dealt exclusively with sensory mapping. Preference mapping was deliberately excluded in this context, as it may be influenced not only by the skin disease itself but also by other factors such as disease status, ethnicity, location, or season. As the quantitative composition of the marketed products is unknown, we do not discuss the suitability of the ingredients used.

The panellists are dedicated experts from the Zurich University of Applied Sciences (ZHAW) in the assessment of sensory attributes of semi-solid forms. They are not participants in a clinical study. According to the Cantonal Ethics Committee, Zurich, this type of investigation does not fall within the scope of the Swiss Human Research Act (HFG; SR 810.30) and therefore does not require the approval of the Ethics Committee for its implementation. Hence, no written informed consent has been collected.

The authors declare no personal or financial conflicts of interest.

This study was not supported by any sponsor or funder.

Conceptualization: P.H., D.R., C.M., and C.S. Data curation, software, visualization, and project administration: P.H., D.R., and Z.B. Formal analysis, investigation, methodology, and validation: P.H., D.R., Z.B., and C.S. Supervision: P.H., C.M., and C.S. Writing – original draft: CS and D.R. Writing – review and editing: P.H., D.R., Z.B., C.M., and C.S.

Christian Surber is an associate editor of Skin Pharmacology and Physiology.

The data that support the findings of this study are not publicly available due to their extent and electronic format but are available from the corresponding author (C.S.) upon reasonable request.