Lecture program

Nanoparticles have a bad reputation among the public. There is great uncertainty as to whether the small particles permeate through skin and/or mucosal membranes, are absorbed by the blood, spread throughout the body and cause illness. In Regulation (EC) No 1223/2009 on cosmetic products, Article 2(1)(k), the term “nanomaterial” means an insoluble or bio-persistent and intentionally manufactured material with one or more external dimensions or an internal structure in the order of 1-100 nm. Cosmetic ingredients in this size range must fulfil special safety requirements and be labelled with (nano) on the packaging. One test point is the possible absorption of the particles through mucous and/or skin membranes.
In a BMBF-funded project we used 3D tissue models to investigate the penetration/permeation of nanocellulose into/through different types of mucosal tissue (mouth, stomach, intestine, lung) using electron microscopy. Au and Ag nanoparticles were used as reference. The research was extended to include Au nanoparticle penetration/permeation into/through the horny layer of the skin using Acrylat-stripping method and electron microscopy (TEM and SEM). Nanocellulose- as well as Au- and Ag-nanoparticles could be ultra-structurally visualized very well. In the 3D tissue models, both nanocellulose- and Au- and Ag-nanoparticles remained in the application medium, while in the skin model the Au- and Ag-nanoparticles were detectable on the skin surface. No particles could be detected in any tissues (penetration) nor in the receiver phase of the 3D models (permeation). In this model setting nanocellulose- (80-120nm) as well as Au- and Ag- (15-140nm) nanoparticles showed no penetration/permeation into/through the various tissues. Due to dimensions and physicochemical properties of nanocellulose, penetration/permeation into/through the various tissues is highly unlikely.

The skin barrier is the interface between the environment and the organism and protects the organism against environmental factors, the penetration of pathogens or irritants, and from water loss. It is therefore essential to protect and maintain the integrity of the skin barrier.

Various factors to which we are exposed daily can damage the skin barrier. For example, environmental pollution, UV light and blue light contribute to damage to the skin barrier through free radicals. In addition to strong acids and bases, surfactants and alcohol remove lipids from the stratum corneum (SC) and the hydrolipid film, denature proteins and trigger inflammatory reactions that can even lead to cell death. In contrast, moisturizing, pH-neutral, gentle, and emollient cosmetic products can help repair and protect the skin barrier and maintain its integrity.

Therefore, the product’s active ingredients must penetrate the skin. This penetration can be measured by using Raman Spectroscopy. Product effectiveness regarding the regeneration or protection of the skin barrier can be evaluated by transepidermal water loss (TEWL) measurements or on a more detailed level by line-field confocal optical coherence tomography (LC-OCT) or Lipbarvis® (Lipid Barrier Visualization). While LC-OCT visualizes upper cell layers and structures in cellular resolution, the lipids and lipid lamellae of the SC as well as the skin microbiome can be analyzed via the Lipbarvis® technique.

In the cosmetics industry, however, it is not just about effectiveness, but also about the proven tolerability of the products. Therefore, in addition to efficacy evaluation and in vitro compatibility tests (red blood cell test, HET-CAM test etc.), in vivo compatibility tests such as epicutaneous / patch tests or IN-USE tests with determination of skin barrier properties are also very important to protect the consumer from acute and chronic irritant reactions.

Sustainability is of key importance nowadays, leading to significant changes in the selection of ingredients for cosmetic O/W emulsions. Bio-based polymeric thickeners are one such example of a replacement that significantly affects the rheological properties and skin feel of cosmetic products.

However, consumer acceptance of product texture is an important issue and sensory analysis with a trained panel or lay panels is expensive and time consuming.

In the development of O/W emulsions at an early stage, rheological measurements are a useful tool to differentiate emulsions with different base ingredients such as emulsifiers, emollients and thickeners. In the specialist literature, these measurements were also associated with perception in sensory analysis.

However, studies on combinations of bio-based polymeric thickeners in O/W emulsions are still lacking, in which rheological and sensory properties are correlated.

The chemical structure of the rheology modifiers used today and their mode of action differ from conventional viscosity-increasing compounds such as partial glycerides and fatty alcohols and even polyacrylates, but certain consumer expectations regarding sensory performance and flow properties must be met. Therefore, hydrocolloids such as Xanthan Gum and other polysaccharides are combined to achieve optimal performance and synergy in viscosity enhancement and skin feel.

In this study, green O/W emulsions with different combinations of bio-based polymeric thickeners were produced and characterised in flow and oscillation measurements as well as in a sensory assessment with a trained group of students. The stability and the microscopic image of the emulsions were also analysed.

The results obtained provide indications of the specific influence of combinations of the bio-based thickeners used in O/W emulsions and the correlation of instrumental rheological measurements with sensory perception for high product acceptance.

Massage rituals both contribute to the effectiveness of skin care and the occurrence of positive emotions in the user. Developing a minimalist emulsion tailored to the massage routine is one way to protect our ecosystems by using a minimum of ingredients, but it requires controlling the long-term perception of the formulation. A reliable method for selecting functional ingredients is essential for ensuring the formulation stability and mandatory playtime. Consumer experience is usually anticipated in the lab using sensory descriptive evaluation with trained panelists but this does not permit the dynamic monitoring of skin feel over a suitable massage duration, nor understanding the interactions between ingredients.
The study aimed to investigate a sensory approach in two steps to customize a lightweight robust O/W emulsion base. A sensorial Pivot© test was firstly carried out to select the most impactful “sensory ingredients” when added one by one in the emulsion base (15 expert panelists). Then, the skin feel of the optimized emulsion was dynamically assessed over a suitable period for massage using Temporal Dominance of Sensations (TDS) methodology, initially developed to evaluate food products (12 trained expert panelists; 30 to 40 seconds massage; scoring from application to 2 minutes).
Pivot® methodology allowed the identification of three ingredients with specific sensory effects. TDS method highlighted four dominant effects, known to facilitate massage, with the sequencing of descriptors: 1) thickness between fingertips and face skin; 2) gliding; 3) richness 4) presence on skin. The combination of both methods permitted to decode the temporal effects of each “sensory ingredient” and how sensory attributes interfere with each other during the dynamic perception process. This approach opens a new dimension to reliably assess skin feel of cosmetic formulations requiring long-term perception. In addition It helped to select strictly necessary ingredients.

Cosmetics are claimed to be able to improve various skin physiological parameters such as the skin barrier function, stratum corneum hydration (SC hydration) and skin relief. But is that true?
To get more insight into these recurring question meta-analyses from efficacy studiese were performed.

First, the overall effect size, which is a quantitative measure of the size of an effect and thus shows the strength of the effect, was calculated.
Secondly, the influence of the baseline value of each parameter at baseline on the overall effect was estimated.
The meta-analysis showed that skin care cosmetics have a significant effect on improving the skin barrier. This effect proved to be particularly strong in subjects with a high baseline TEWL value. Furthermore, the overall effect on skin hydration also resulted in high strength. But this was improved irrespective of the skin hydration at baseline.

Interestingly, the application of skin care has no effect on the improvement of skin roughness. An influence of the baseline value on the overall effect could also not be confirmed. Nevertheless, the skin scaliness improved after the application of skin care cosmetics, and the overall effect was pronounced in subjects with both high and low baseline values. As expected for skin wrinkliness there was no overall effect. The overall effect, however, in terms of skin volume, which describes the depth of wrinkles, calculated to be of high strength. Both low and high baseline values in subjects showed an influence on the overall effect. It was thus confirmed that wrinkles can be minimized in their expression by skin care cosmetics, but not disappear completely.
Thus, skin care cosmetics can do it! But attention – the results show also the importance for the correct recruitment of test subjects within cosmetic efficacy studies.