Rheological fingerprinting as an effective tool to guide development of personal care formulations

Predicting tactile sensory attributes of personal care emulsions based on instrumental characterizations: A review

Emulsions in the form of creams, lotions, gels or foams are the most widely used personal care formulations to improve the condition and feel of the skin. Achieving an optimal balance between their performance, effectiveness and sensory profile is essential, with the sensory profile being a key factor in consumer satisfaction and the success of these products in the market. Well-established methods using highly trained and semi-trained panels (e.g. Spectrum descriptive analysis, Flash Profile method, Quantitative Descriptive Analysis method and 'Check-all-that-apply') are available and commonly used for the sensory assessment of personal care products. Nevertheless, a common drawback among all these methods is their inherent cost, both in terms of financial resources and time requirements. In recent years, research studies have emerged to address this limitation by investigating potential correlations between tactile sensory attributes and instrumental data associated with the physical characteristics of topical formulations. In other words, significant efforts have been invested in the development of robust instrumental methods specifically designed to accurately predict the sensory description that a panel of assessors could establish. These methods are not only faster, cheaper and more objective compared to traditional sensory testing, but they can also be applied to formulations that have not undergone extensive safety and toxicological testing. This review summarizes the most relevant findings, trends and current challenges in predicting tactile sensory attributes of personal care emulsions based on instrumental parameters.

Bioactive Materials Based on Hydroxypropyl Methylcellulose and Silver Nanoparticles: Structural-Morphological Characterization and Antimicrobial Testing

The progress achieved in recent years in the biomedical field justifies the objective evaluation of new techniques and materials obtained by using silver in different forms as metallic silver, silver salts, and nanoparticles. Thus, the antibacterial, antiviral, antifungal, antioxidant, and anti-inflammatory activity of silver nanoparticles (AgNPs) confers to newly obtained materials characteristics that make them ideal candidates in a wide spectrum of applications. In the present study, the use of hydroxypropyl methyl cellulose (HPMC) in the new formulation, by embedding AgNPs with antibacterial activity, using poly(N-vinylpyrrolidone) (PVP) as a stabilizing agent was investigated. AgNPs were incorporated in HPMC solutions, by thermal reduction of silver ions to silver nanoparticles, using PVP as a stabilizer; a technique that ensures the efficiency and selectivity of the obtained materials. The rheological properties, morphology, in vitro antimicrobial activity, and stability/catching of Ag nanoparticles in resulting HPMC/PVP-AgNPs materials were evaluated. The obtained rheological parameters highlight the multifunctional roles of PVP, focusing on the stabilizing effect of new formulations but also the optimization of some properties of the studied materials. The silver amount was quantified using the spectroscopy techniques (energy-dispersive X-ray fluorescence (XRF), energy-dispersive X-ray spectroscopy (EDX)), while formation of the AgNPs was confirmed using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), and dynamic light scattering (DLS). Also, the morphological examination (Atomic Force Microscopy (AFM) and Scanning electron microscopy (SEM)) by means of the texture roughness parameters has evidenced favorable characteristics for targeted applications. Antibacterial activity was tested against Escherichia coli and Staphylococcus aureus and was found to be substantially improved was silver was added in the studied systems.

A Torsion-Based Rheometer for Measuring Viscoelastic Material Properties

Rheology and the study of viscoelastic materials are an integral part of engineering and the study of biophysical systems. Tissue rheology is even used in the study of cancer and other diseases. However, the cost of a rheometer is feasible only for colleges, universities, and research laboratories. Even if a rheometer can be purchased, it is bulky and delicately calibrated, limiting its usefulness to the laboratory itself. The design presented here is less than a tenth of the cost of a professional rheometer. The design is also portable, making it the ideal solution to introduce viscoelasticity to high school students as well as for use in the field for obtaining rheological data.

Physicochemical Properties of Cellulose Ethers

Cellulose ethers are naturally derived ingredients that are commonly used in personal care products as rheology modifiers, film formers, stabilizers, and sensorial agents. In this work, we investigated the physicochemical properties of various grades of hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), hydroxypropylmethylcellulose (HPMC), methylcellulose (MC), and sodium carboxymethylcellulose (CMC). In addition, we also studied the influence of hydrophobic modification on the structure of HEC by carrying out experiments with cetyl hydroxyethylcellulose (HMHEC). Rheological, friction coefficient, dynamic vapor sorption (DVS), surface tension analysis, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) data were generated for the cellulose ethers in order to obtain information about their viscosity, lubricity, moisture absorption, solubility in the bulk solution phase, physical properties, and thermal degradation profile, respectively.

Stringiness of hyaluronic acid emulsions

Objective

Cosmetic emulsions containing hyaluronic acid are ubiquitous in the cosmetic industry. However, the addition of (different molecular weight) hyaluronic acid can affect the filament stretching properties of concentrated emulsions. This property is often related to the “stringiness” of an emulsion, which can affect the consumer's choice for a product. It is thus very important to investigate and predict the effect of hyaluronic acid on the filament stretching properties of cosmetic emulsions.

Methods

Model emulsions and emulsions with low and high molecular weights are prepared and their filament stretching properties are studied by the use of an extensional rheometer. Two different stretching speeds are employed during the stretching of the emulsions, a low speed at 10 µm/s and a high speed at 10 mm/s. The shear rheology of the samples is measured by rotational rheology.

Results

We find that filament formation only occurs at high stretching speeds when the emulsion contains high molecular weight hyaluronic acid. The formation of this filament, which happens at intermediate states of the break‐up, coincides with an exponential decay in the break‐up dynamics. The beginning and end of the break‐up of high molecular weight hyaluronic acid emulsions show a power law behaviour, where the exponent depends on the initial stretching rate. At a lower stretching speed, no filament is observed for both high molecular weight and low molecular weight hyaluronic acid emulsions and the model emulsion. The emulsions show a power law behaviour over the whole break‐up range, where the exponent also depends on the stretching rate. No significant difference is observed between the shear flow properties of the emulsions containing different molecular weights hyaluronic acid.

Conclusion

In this work, we underline the importance of the molecular weight of hyaluronic acid on the elongational properties of concentrated emulsions. The filament formation properties, for example the stringiness, of an emulsion is a key determinant of a product liking and repeat purchase. Here, we find that high molecular weight hyaluronic acid and a high stretching speed are the control parameters affecting the filament formation of an emulsion.