Trends Paper Aesthetics of Topical Semi-Solid Products: A Rheology Approach

Trends Paper Aesthetics of Topical Semi-Solid Products: A Rheology Approach : 작성자, 카테고리, 작성일, 조회수, 원문,출처, 정보 제공
작성자	관리자	카테고리	전문가 인사이트
작성일	2016-11-10	조회수	4,313
원문
출처

글 Richard Buchta: GPKOL위원

컨설팅 분야

R&D기획
기술마케팅

주요 약력

Present: Formulytica Pty Ltd/Contract Research & Development Organisation/Director

2014-2016: RBR Pharma Consulting/ CEO
2001-2014: Stiefel/ R&D & Operations Sr. Director
1999-2001: Wyeth/ New Product Project Manager
1998-1999: Astra Zeneca/ New Product development/ R&D Project Manager
1990-1998: Fort Dodge Australia Pty Ltd/ Project Manager & Sr. Research Scientist

ABSTRACT

Semi-solid (topical) products are developed to apply to the skin. The skin being a sense organ provides the individuals response to the applied formulation. Different sites on the skin, face, forehead, neck, hands, feet, arms and legs are of different thickness and innervation. To characterise the aesthetics of the applied formulation, qualitative and quantitative measures and scales are used, typical; a cart wheel tool. However, these scales are subjective to each individual’s skin condition and their response. In an approach that has been developed and is increasing in use, typical physical chemistry measures are being applied to the formulation to characterise flow, viscosity, density, and correlated with the sense of skin feel evaluation.

INTRODUCTION

One of the aspects in semi solid or topical pharma products, often overlooked is the skin feel of the product. That insight tends to rest with cosmetic and personal care product development. Aesthetics play a role, in reacting to the experience of applying the product, ease of spreading, rubbing in and compliance. It also creates additional noise in the market place for potential to increase sales of the product where there is a direct competitor and clinical efficacy is similar.

So how do we evaluate aesthetics of topical pharmaceutical products? Typically from market research we apply qualitative systems to focus on consumer response. That is the domain of personal care and cosmetic products. While aesthetics of topical pharma products are often forgotten over the need for efficacy and safety, aesthetics is of increasing importance. There is increasing interest in applying more quantitative analysis to aesthetics that assess the physical properties of topical products, which can also be used to measure physico –chemical stability. This is the domain of topical pharma products which must meet physical and stability requirements for registration. Rheology is not a new science, however there is increasing interest in pharmaceutical aesthetics as rheology has been used in food and other materials properties to quantify the flow and textural properties. Thus, an approach to evaluate rheological aesthetics can then be related to the market research to develop a basis of applying a quantitative system to aesthetics.

Rheological knowledge is required in a wide variety of pharmaceutical and cosmetic areas including (Park & Song 2010) : (a) quality control of semi-solid products ;(b) storage stability of semi-solid products ; (c) correlation of physical parameters with sensory assessment and consumer evaluation ; (d) effects of consistency on the percutaneous absorption of drugs ; (e) effects of formulation on consistency and homogeneity (e.g., action of self–bodying agents) ; and (f) prediction of flow behaviour under shear deformation conditions encountered in manufacture (eg. pumping, milling, filling, packaging).

This paper describes recent trends in rheological approaches to aesthetics of topical pharmaceutical products.

RHEOLOGY PROFILING OF SEMI-SOLID PHARMACEUTICALS

The spreadability of topical formulations (http://www.rheologylab.com), with a variability in thickness, such as gels, creams, ointments, lotions and foams will vary depending on surface area, disease, hair bearing or non hair bearing), rheology of the formulation and manipulation on application (typically by finger to the indication skin site. The formulation rheology also has an impact on expelling a formulation from its packaging eg. small or large orifice, packaging materials (HDPE vs aluminum), pump pack vs tube.

As you push your finger on the formulation and across the skin to spread you can observe how easy or difficult it can be depending on the formulation thickness and viscosity. A level of residue remains or appears to increase as volatile solvents evaporate or the formulation is absorbed and the formulation becomes thicker and more difficult to spread.

Rheology profiling for a semi solid topical provides a useful methodology for formulation and process design Rheology testing is useful for :

Predicting spreadability, aesthetics of skin feel and sensory properties
Comparing a generic formulation with the Reference Listed Drug
Adding meaningful, quantifiable metrics to laboratory, pilot and production batches for scale up
Identifying the impact of temperature regimes, cooling rates and holding times on product texture
Tracking viscosity/rheology for stability batches following manufacture
Early insight into instability in stability studies
Identifying the impact of excipient changes

of which predicting the spreadability and aesthetics can provide important insight of user acceptability.

Topical creams and ointments are typically Non-Newtonian, shear-thinning, and their viscosity is high at low shear rates and low at high ones. Any viscosity measurements should be performed at shear rates that are representative of spreading formulation on the skin the case of spreadability predictions for topical products, application shear rates tend to be in the upper hundreds to lower thousands of reciprocal seconds. In addition the assessment should ideally be non-destructive prior to the measurement

Viscosity testing of topical pharmaceuticals is often performed with a viscometer fitted with either standard RV or LV spindles or a Small Sample Adapter for “flowable” samples such as lotions or a T-Bar/Helipath accessory for soft solid samples such as creams and ointments. While this approach provides a low cost solution for quick comparative testing of batches there are some major attributes of pharma topical products that the viscometer cannot measure. This requires a rheometer, some key definitions and terminologies of which are necessary to understand:

Shear stress or yield stress is a measure of the stress that must be applied to elicit minimum flow from a semi- solid such as a cream or ointment. Shear stress contributes significantly to the attributes such as the ability to squeeze a sample from a tube and the spreadability onto the skin. A sample with a high yield stress will resist squeezing and spreading.

Shear Strain is a measure of the deformation due to stress. A high shear process is carried out by application onto the skin, with shear rates increasing to higher values as the product thickness decreases with ongoing spreading. To gain a reliable understanding of the spreadability and ease of application viscosity profiles recorded at high shear are essential.

Zero-shear viscosity is the viscosity of a formulation when effectively at-rest. As products spend are typically in an at-rest condition they therefore spend this time at their zero-shear viscosity, not at the viscosity under the shear conditions imposed in a viscometer measurement.

The rate at which a material undergoes structural breakdown and shear thinning contributes to the ease with which it can be applied. Some semi-solid products require “working” to shear them down during application whereas others shear-thin to a free flowing liquid with the slightest touch.

Recovery after shear determines how rapidly a material will return to a structured, high zero-shear viscosity condition and, as such, will influence behaviour over the period following application of creams and lotions. This has implications for product in a package, related to temperature as well as manufacturing transfer, ie product performance.

Dynamic oscillation testing is a useful tool to reveal the microscopic structures of a viscoelastic material; (Herh et al 1998). A sinusoidal shear stress (or strain) smaller than the critical value is imposed on a semi-solid and the amplitude of the resulting strain (or stress) and the phase angle between the imposed stress and the output strain are measured. Experimentally, this can be accomplished by setting a small sinusoidal displacement or strain (within the linear viscoelastic range) on the emulsion under a controlled frequency or temperature sweep. In general, the material can respond to this type of deformation through two mechanisms: elastic energy storage and viscous energy dissipation. Quantitatively, these responses can be represented as storage modulus (G), energy stored per unit volume, and loss modulus (G), energy dissipated per unit deformation rate per unit volume. Storage modulus (G) is proportional to the extent of the elastic component (contributed by crosslinking, entanglement, and/or aggregation) of the system, and loss modulus (G) is proportional to the extent of the viscous component (contributed by the liquid like portion) of the system. Typically, the strength of interaction of internal microstructure of an emulsion is measured by the magnitude of the ratio G/ G= tan which is called the damping factor (is the phase angle). The smaller tan (or the greater G), the stronger the interaction.

The application of a topical product produces a formulation layer on the skin. The thickness of the layer progressively decreases due to evaporation, absorption and rub in. The rheology that exists where two surfaces are separated by a layer of formulation transitions to tribology properties. The squeezing load force and the sliding speed contribute to the friction generated. The lubricating qualities of topical semi solid products can be investigated by measuring the frictional force of artificial elastomeric surfaces in sliding contact under defined loads and sliding speeds. The tribology properties of topical pharmaceuticals is not part of this trends paper.

A number of publications over the past 50 years have tried to relate sensorial and aesthetics to rheological characterization. To some extent this has shown some relationship but further work to define the rheological property is dependent on the property being measured and how it is interpreted, as well as composition.

Barry (1970) and Barry & Grace (1971a) reviewed the rheological test methods of various pharmaceutical vehicles and materials used at the time on penetrometry, continuous shear viscometry, oscillatory testing and creep analysis. The chemical nature, microscopic structure, miscellaneous methods of standardisation, formulation and work softening of petrolatum and their textural properties of soft paraffin, a master curve of the rheological conditions operative during spreading on the skin was developed.

Continuous shear and creep viscometry were sued to investigate the effect of softening and recovery on the rheological properties of four grades of white soft paraffin (Barry & Grace, 1971b). I a continuous shear mode, using a cone and plate rheometer, the apparent viscosities of two samples increased with recovery time after manipulation. This indicated partial structural recovery. Apparent viscosities of non-manipulated samples were generally lower than those of the manipulated samples due to elastic recovery of the samples during shear. Viscoelastic results were analyzed to obtain initial elastic and residual viscosities. These data indicated that the loss of material consistency during manipulation and recovery after manipulation were mainly viscous phenomena. Manipulation of the samples caused minimal irreversible structural breakdown. A sensory evaluation of white soft paraffin in different physical states of manipulation was correlated with discrete viscoelastic parameters and continuous shear yield stresses of the material. A correlation was not found with continuous shear apparent viscosity data. The rheological conditions which operate during spreading of topical preparations on the skin were further investigated for a series of lipophilic preparations ranging from thick semi-solids to liquids (Barry & Grace 1972). Rates of shear were found to vary from approximately 400 to 2500 s−1 depending on the homogeneity of the preparation being spread; a rheological master curve was determined. Three types of scaling procedures (ordinal, preference, and ratio scaling), employed to investigate the ability of a panel to differentiate levels of the textural parameter spreadability, were found to be of similar outcome. The data derived were used to indicate a method for determining instrumental rheological conditions for use in routine control procedures and a spreadability screening test for application prior to market research or clinical trials.

Barry & Meyer, (1973) carried out a comparison of rheological and sensory properties for a series of aqueous gels and oil in water emulsions. The rheological conditions during spreading of topical preparations on the skin were determined. Data was derived from the rheological data obtained using a sensory test with a panel of 24 members. Rates of shear varied from 350 to 10,000 s−1, depending on the consistency and the type of preparation being spread. Two preference scoring techniques were used to assess the series of preparations in terms of spreadability. From the preference data, the shearing conditions with optimum patient acceptance potential were derived.

Wang et al (1999) showed that the overall rheology of creams had little direct effect on the moisturizing efficacy and the perceived perceptual attributes. Neither did the overall viscoelasticity or the aesthetic attributes of the preparations seem to affect the amounts of creams/lotions that panelists judged to be necessary for achieving satisfactory perceived hand moisturization.

Wortel et al (2005) investigated the relationship between rheological properties and sensory attributes to understand the influence of emulsion structure on sensory characteristics. The data showed cohesiveness can be predicted by two rheological properties: dynamic viscosity and yield stress. Some sensory and rheological properties were found to be dependent on the emulsion structure.

Steady shear flow properties alone are not able to provide a sufficient information when considering an actual product application condition such as rubbing a pharmaceutical ointment or a cosmetic cream onto the human skin. This is because it is easily realized that most rubbing processes can reasonably be assumed to be in nature a periodically oscillatory motion with large strain (or stress) amplitudes and consequently the materials show a nonlinear behaviour in their responses. A study (Park & Song, 2010) on petrolatum, showed that as the strain amplitude increased, the difference between the storage modulus G’ and loss modulus G”, gradually decreased and a viscous property became superior to an elastic property at sufficiently large strain amplitude range. This suggested that petrolatum imparts a desirable rheological effect during an actual rubbing process because the product should smoothly flow ahead at large strain amplitudes.

Lukic et al (2012) and Korac et al 2016 formulated a number of model water-in-oil emulsions (creams) intended for hand care, varying in one emollient component. The formulations were characterised for rheological, sensory and textural properties. Their efficacy was evaluated by an in vivo study in human volunteers. A specific change restricted to the oil phase induced a change in all investigated characteristics, showing that each instrumental measurement can be used as a sensitive tool in the characterization of cream samples. Regarding the correlation between physical measurements and certain sensory attributes, it is possible to formulate a product with specific sensory characteristics by using pre-defined rheological or textural parameters. This study showed that a modified sensory study could be useful for fast in-line screening along with instrumental characterization of emulsion products and could be particularly helpful in the process of distinguishing a single formulation from several differing in one component.

Further work by Lukic et al 2013, showed that by modifying the oil phase composition in one component (mineral oil, isocetyl palmitate, special olive butter and dimethicone 200/350) they showed the influence of that alteration on emulsion rheological and textural properties. Additionally, continuous and oscillatory rheological measurements along with texture analysis were assessed in order to predict emulsion application behavior. The change of oil phase induced a change in the observed rheological and textural parameters. Samples were divided into two distinct groups: samples with special olive butter and dimethicone 200/350 and samples with isocetyl palmitate and mineral oil. Thus this approach was able to differentiate emulsions regarding textural properties and effect on usage and sensory properties. The influence of a small change in the composition of a cosmetic emulsion to its rheological and textural properties was evaluated using three cosmetic emulsions, water-in-oil body lotions, differing in a ratio of emulsifier. Samples were analyzed by rheological procedures and sensory analysis, and the particle size was evaluated (Moravkova & Filip 2013). The results were compared with the ingredients of the sample to assess the role of the emulsifier. Rheological analysis proved that a small change in emulsifier composition reflects on the behaviour of the emulsion; shown by the differences in the measured flow curves.

Prediction of the sensory texture of cosmetic emulsions using 8 oil-in-water emulsion (creams), only varying by their polymeric constituent and one “control” emulsion without any polymer were formulated and 6 texture attributes were selected with accurate sensory evaluation procedures and notation scales, gloss, integrity of shape, penetration force, compression force, stringiness and difficulty of spreading (Gilbert et al 2013). Rheological and textural characterization of the emulsions was carried out, including flow, creep, oscillation tests and tests of penetration, compression, extrusion, stretching, spreadability, respectively. The methodological approach was borrowed from the food domain. The gloss was also measured on emulsion films using a gloss meter. Predictive models were developed using linear simple, linear multiple and Partial Least Square (PLS) regressions. Univariate analysis showed that gloss, compression force and difficulty of spreading were very well predicted by instrumental measurements. Several good calibration models were built for each of the three other attributes.

Five commercial cosmetic emulsions were instrumentally and sensory assessed to test the robustness of each calibration model and cross-validation was used to evaluate their predictability. Stringiness was well predicted by the breaking length of filament. Rheology and texture analysis were shown to be excellent tools to evaluate sensory texture attributes of cosmetic emulsions. properties of 6 over-the counter vaginal products used as ex vivo microbicide surrogates. Shear-thinning behavior (n) and tan δ (10 rad s-1) showed no relationship with any perceptual attributes. Shear storage modulus, G’ (10 rad s-1) was correlated with some attributes, but did not appear to be a strong predictor of sensory properties. Storage loss modulus, G” (10 rad s-1) and the consistency coefficient, K, were correlated with several sensory attributes: stickiness, rubberiness, and uniform thickness for G’’ and stickiness, rubberiness, and peaking for K. This pilot study suggested rheological principles can be used to understand the sensory properties evoked by microbicide surrogates assessed ex vivo.

A study on the impact of thickening agents on the rheological properties of emulsions (creams) was correlated with the sensory attributes using a combined instrumental-sensorial approach (Estanqueiro et al 2016). Formulations contained different thickening agents at different concentrations. These formulations were observed under the microscope and then evaluated through textural, rheological and spreadability measurements during 6 months to assess the physical stability of the studied formulations. Sensory attributes, firmness, adhesiveness, cohesiveness, spreadability, consistency and adhesiveness post-application, were then tested by a trained panel. It was observed that an increase in the concentration of thickening agents improved the physical stability of formulations over time as expected. Besides, textural parameters (firmness and adhesiveness), viscosity and difficulty to spread also increased. A good correlation between mechanical characterization and sensorial analysis was verified, mainly for spreadability properties. This again confirmed that rheological methods for formulation characterization can be correlated with sensorial perception obtained from volunteers, thus representing a faster and less expensive alternative than sensory analysis.

Kitagawa et al 2016, investigated the effects of surfactants on the rheological properties of three brand name ointments and six equivalent generic ointments. They detected marked differences in hardness, adhesiveness, and spreadability among the ointments. Further examinations of model ointments consisting of white petrolatum, propylene glycol, and surfactants revealed that the abovementioned properties, especially hardness and adhesiveness, were markedly affected by the surfactants. Since steroidal ointments are often mixed with moisturizing creams prior to use, the mixing compatibility of the ointments with heparinoid cream and how this was affected by their surfactants was investigated. The ointments containing glyceryl monostearate demonstrated good mixing compatibility, whereas those containing non-ionic surfactants with polyoxyethylene chains exhibited phase separation.

Borrowing methodologies from the food industry to evaluate rheological and sensory properties of topical pharmaceutical properties has been variously investigated. The food industry has good examples of where emulsions and gels have similar performance requirements to that of topical products. The rheological behaviour of emulsion-filled gels is extremely important because it can measure interferences promoted by droplets or particle inclusion on the textural properties of the gelled systems (Geremias-Andrade et al 2016). Dynamic oscillatory tests, of small amplitude oscillatory shear, creep-recovery tests, and large deformation experiments, as techniques present in the literature to characterize rheological behaviour of emulsion-filled gels. The correlation of mechanical properties with sensory aspects of emulsion-filled gels demonstrating the applicability of these parameters in understanding mastication processes.

The relation between rheological properties and predictive sensory or aesthetic attributes, requires a suitable rheological protocol and required statistical and model comparisons. For example, a questionnaire arranged for sensory or aesthetic analysis comprises a number of characteristics. Four characteristics, were assessed on a qualitative scale (ease of pouring from the bottle, of spreading on a palm, thickness, and of spreading on back of hand). The results of the sensory analysis confirmed different attributes of the samples, which were verified by observing the microstructure, specified by the mean droplet size. To detect the relationship between rheological parameters obtained by the power law model and sensory variables from the analysis, a linear approximation was carried out. The correlation coefficients describe the relevance of the relationship. Tight relationship were found for the following combinations: droplet size, pouring from the bottle; pouring from the bottle; thickness; and, droplet size. These results allowed the prediction of some sensory parameters (pouring from the bottle and thickness) from the results of the rheological measurements. In contrast to pouring, there is no relationship between the consistency parameter and spreading tests. This is probably caused by the fact that for pouring from a bottle (shear rates lower than 100 s−1) the corresponding flow curves are more concave in comparison with the process of spreading (shear rates exceeding 100 s−1) where a more “linear” character dominates. Further work in analysing these lotions will be to do oscillatory measurements. In our own studies, we are using internal sensory panels to correlate formulation aesthetics with rheological properties defined by Brookfield CS rheometer with cone plate geometry. The intention is based on a range of products developed to build a database of sensory vs rheology of these vehicles. The measurement of viscosity and yield rate correlate well for a range of commercial creams, lotions and foams, which have different properties of shear thinning, high drag and low viscosity/ water like. There appears to be a range in which the ideal vehicle extrudes from packaging and its spreadability across the skin, that correlates with an ideal viscosity and shear rate range.

RHEOLOGY INSTRUMENTATION

A rotational rheometer, also known as a shear rheometer, is used to determine how a semi-solid flows and is capable of measuring viscosity, thixotropy, shear stress, and shear strain. It is typically used in the pharmaceutical and cosmetic industries for standalone measurements. Rotational rheometers help to maintain parameters such as consistency and fluidity, which in turn allow predictions to be made about stability, texture, and shelf life. Rotational rheometers apply a known force to the sample and measure how it reacts to the force and the force the sample then exerts on the rheometer. It may be possible to do this at controlled temperature.

From Brookfield

From Guide to Rheological Nomenclature: Measurements in Ceramic .ciks.cbt.nist.gov

Cone and plate geometries come in 3 main types as noted in the figure. The more viscous a material suggest a cone and plate geometry compared to parallel plate should be used. Of the geometries to be used will depend on the viscosity of the material, but with higher viscosity one would use the parallel plates. Vane and helipath spindles have seen increase use for viscometry applications. Heterogeneous samples, particularly those comprising solid fillers, highly structured materials that yield, and materials that slip, are some general classes in which these spindles are useful when other geometries do not provide acceptable measurements. Although vanes and T-spindles may be inserted into sample containers and tests run directly, the shear rates may be undefined as material behavior varies across large spindle-edge-to-container-wall gaps. A recent study (Kealy et al 2008) on using the vane spindle for foam rheology showed the rheological differences between foam types based on their formulation composition, although studies have yet to relate this to aesthetic attributes. These studies were not directly correlated with aesthetics of the foam products evaluated.

SUMMARY

Aesthetics of topical pharma products are an important payoff of the branding and marketing of a topical product, like the personal care and cosmetic counter parts. Depending on intended use for treating skin conditions where efficacy and dosing is important as in pharma where more beautifying aspects are important as in cosmetics, one should understand how the product can be applied and during application, spreading and after feel. Rheology is an important tool among others that can characterise and quantify the application spreading, texture, and potential performance of the topical product. Rheological instrumentation and geometries can vary and understanding of their application is ongoing. Many modern semi-solid pharmaceutical and cosmetic vehicles demonstrate complicated rheological behaviour that is difficult to characterize by means of the traditionally-used methods. Relating that to market research sensory panels is a challenge. However, with modern instrumentation and software and improved correlation between rheological characteristics and sensory panels this is progressing to predictive behaviour and understanding.

REFERENCES

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Barry, B.W. and Meyer, M.C. (1973) Sensory assessment of spreadability of hydrophilic topical preparations Journal Pharmaceutical Sciences 62, 1349–1354.
Estanqueiro, M., Amaral, M.H., and Sousa Lobo, J.M. (2016) Comparison between sensory and instrumental characterization of topical formulations: impact of thickening agents. International Journal Cosmetic Science 38, 389-398.
Geremias-Andrade, I.M., Souki, N.P.B.G., Moraes, I.C.F. and Pinho, S.C. (2016) Rheology of Emulsion-Filled Gels Applied to the Development of Food Materials, Gels, 2, 22-40.
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Trends Paper Aesthetics of Topical Semi-Solid Products: A Rheology Approach

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