Silicones as innocious Materials - Dow

English EditionInternational Journal for Applied Science • Personal Care • Detergents • Specialties

12-2014

61st SEPAWA Congress and 10th European Detergents Conference

Influencing Dendritic Cells for an Increased Tolerance Threshold of Sensitive Skin

Silicones as Innocuous Materials to Keep Natural Moisture Balance and Protect Skin Against Particle Pollution

Probiotic Fractions, a New Solution to Improve Skin Health by Strengthening Barrier Function, Enhancing Skin Hydration and Preventing Inflammation

Antipollution Molecule to Prevent Premature Ageing

Cationic Surfactants as Antimicrobial Ingredients in Detergents and Cleaning Agents: Blessing or Curse?

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26 SOFW-Journal|140|12-2014

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Silicones have a long use history in skin care and topical healthcare applica-tions, thanks to some of their typical properties such as skin protectancy (1) or biocompatibility (2), (3). This article specifically discusses their non-occlusive behavior on skin as well as the potential of some silicone technologies to protect from pollution particulates. While those are two distinct topics, both are actu-

ally relevant to sensitive skin applica-tions, when considering sensitive skin as hyper-reactive in response to external factors and changes in the environment. In-vitro and in-vivo evaluations of typi-cal silicones are first reviewed. Work highlighting the potential of some hy-brid technologies to reduce the adhesion of pollution particulates equivalents is then described.

■■ Polydimethylsiloxanes Chemical Structure and Non-Occlusivity

When applied to the skin, silicones typi-cally form water vapor permeable films. Misconceptions however still exist and a common belief is that silicones are occlu-sive. This may be rooted in the fact they are hydrophobic, like hydrocarbons, and can improve the protectiveness and water resistance of creams and lotions, which might be misinterpreted as occlusiveness (4), (5). However, the specific structure of silicones makes these materials very distinct from their organic counterparts. Fig. 1 shows the structure of PolyDiMeth-ylSiloxanes (PDMS), also known under its INCI name dimethicone. These polymers are built from a polysiloxane backbone, consisting of silicon and oxygen atoms, surrounded by methyl groups. The silox-ane bond has a low barrier to rotation so is flexible. The side methyl groups are very organic in character and often associated with low surface energy (6). Thanks to its intrinsic flexibility, the polysiloxane chain typically adopts a conformation enabling a maximum number of methyl groups to be exposed to the outside, creating a chain-shielding effect (3). As a result, chain-to-chain interactions remain low, accounting for the polydimethylsiloxanes low surface tension. Consequences in-clude a low glass-transition temperature as well as high free volume compared to hydrocarbons, resulting in high perme-ability to water vapor. However, the world of silicones is not lim-ited to polydimethylsiloxanes. Expanding from the PDMS structure, a large variety of materials can be generated, sometimes having very different properties from

J.-L. Garaud*, A. Sieg*, M. Le Meur*, H. Baillet*, I. Van Reeth**, S. Massé*

Silicones as Innocuous Materials to Keep Natural Moisture Balance and Protect Skin Against Particle Pollution

Abstract

This paper discusses the non-occlusive behavior of typical silicones on skin as well as the potential of some hybrid technologies to protect from pollution particulates, in light of the interest of those benefits

for hyper-reactive, sensitive skin. Permeability was assessed via an in-vitro ASTM standard-based method as well as an in-vivo corneometry and tran-sepidermal water loss study. Pollution protection potential was evaluated using a new, internally-developed in-vitro method assessing pollution particulates equivalents adhesion on cosmetic coatings and scanning elec-tron microscopy imaging to estimate surface microroughness levels. Evalu-ations demonstrated the non-occlusive behavior of three commonly used silicones and identified some pollution protection potential for specific copolymers. Those results highlight the relevance of silicones for use in sensitive skin applications.

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SOFW-Journal|140|12-2014 27

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PDMS. For instance, while polydimethylsiloxanes have high permeability to wa-ter vapor, grafting on the chain long aliphatic groups that are known to be occlusive will reduce the permeability of the corresponding silicone-organic hy-brid material created. It should be kept in mind that those materials, alkyl silicone copolymers, remain a specific sub-class and should not be considered representa-tive of typical silicones. In order to clarify the myth about the oc-clusivity of silicones, an in-vitro and an in-vivo study were performed. Three silicones commonly used in personal care applica-tions were assessed: 350 cSt PDMS as well as a silicone gum and a silicone elastomer, both in a 5 cSt PDMS carrier. Petrolatum was used as an occlusive benchmark.

■■ Confirming Non-Occlusivity In-vitro

The in-vitro screening study was per-formed using an internally-developed protocol based on ASTM standards (7). Candidates were coated onto a collagen sheet and fixed on a Payne cup device containing water. The ability of water to evaporate outside of the cup through the coating in controlled temperature and humidity conditions was assessed and converted into water vapor trans-mission rate (WVTR) values. Fig. 2 shows the corresponding results: Despite a small reduction compared to the non-occlusive control (untreated collagen), all three silicones led to high permeability values. As expected, very low values were observed for petrolatum, as an illustration of its occlusive behavior.

■■ Confirming Non-Occlusivity In-vivo: TEWL & Corneometry

The in-vivo study assessed the effects of the same three silicones on transepider-mal water loss (TEWL) and corneometry. In addition to the neat silicones, corre-sponding formulated systems consisting of each silicone in a water-in-oil chassis were included. The measurements were performed on 26 young female panelists forearms. After a baseline measurement and a single application of the product,

measurements were performed at three time points: 15 minutes after applica-tion, 4 hours after application and after dry wiping.

TEWL results (Fig. 3) suggested an overall similar trend across timepoints, with increased effects after 15 min-utes, as one would logically expect.

Fig. 1 Typical structure and key physico-chemical properties of PDMS.

Fig. 2 In-vitro occlusivity screening results.

Untreated collagen

PDMS 350 cSt

Gum in 5cSt

Elastomer in 5 cSt

Petrolatum

Water vapor permeability of common silicones

Rel

ativ

e w

ater

vap

or

tran

smis

sion

rat

e (%

)

100

80

60

40

20

0

Fig. 3 Transepidermal water-loss results.

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The observed TEWL decreases for the silicone materials were not significant-ly different from the untreated skin control zone and were significantly lower than petrolatum. The petrola-tum results were expected and have been described in the scientific litera-ture (8), (9).

Fig. 4 shows the results for skin capacitance measurements. The 15 minutes time point da-taset should be considered with caution since it has been described that electrical methods do not work adequately when measurements are performed shortly after application of li-pophilic compounds with occlusive proper-ties (10), (11), (12). Statistical analysis on the

4 hours timepoint showed that corneometer readings for all three silicones were not dif-ferent from the untreated skin control. There was no statistical difference in the hydration levels induced by the different W/(O + Si) creams, which all contained the same ingre-dients except for the type of silicone incor-porated, the higher readings being attributed to glycerin. This contrasts with the expected (8), (9), (10) higher skin capacitance values observed for the petrolatum-treated site.As a conclusion, both in-vitro and in-vivo results confirmed the non-occlusive be-havior of the commonly used silicones considered.

■■ Pollution

When it comes to air pollution, people are regularly exposed to »scary signals« that raise their awareness of the associated is-sues. In various European countries, such as Germany, air quality index information is provided along with weather forecasts and smartphone applications are available informing real time about local pollution levels. In parallel, consumers are getting more and more educated in the field of skin care. In the light of recently published work (13), (14), there is an increased focus on the actual impact of pollution on skin, most specifically skin aging. Whether un-der the form of gases or particulate matter, pollution generates free radicals when in contact with skin (Fig. 5). Supplying skin with anti-oxidants is a curative approach fighting against free radicals once they are already in skin. The work described hereaf-ter focused on a preventive approach, aim-ing at avoiding or limiting the formation of these noxious species (Fig. 6). Could sili-cones form on skin a film helping prevent the adhesion of pollution particulates?

■■ Superhydrophobicity: Inspiring Examples Found in Nature

Many examples of low particle-surface interactions exist in nature. The lotus leaf (Nelumbo Nucifera) is famous for stay-ing clean, preventing water or dirt from developing strong interaction with it. The associated extreme water repellency be-havior is called superhydrophobicity and stems from specific surface character-

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Fig. 6 Work hypothesis.

Fig. 5 Schematic representation of pollution impact on skin.

Fig. 4 Corneometry results.

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30 SOFW-Journal|140|12-2014Finding derived from a representative TNS market survey.

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istics at a microscopic level, that have been widely described in the literature (15), (16), (17). This research showed that beyond a smooth and low energy sur-face, microroughness was required to reach the superhydrophobicity domain. The interdependence between particle adhesion and hydrophobic surfaces mi-croroughness was also investigated.The understanding of these physical prin-ciples was leveraged in several industrial applications, especially in the coatings or textiles area (18), (19), (20). Microrough-ened surfaces were successfully gener-ated using a variety of routes, achieving the desired low water-wetting behavior.

■■ The Challenge of Leveraging the Lotus Effect in Cosmetic Applications

However, applying those to Beauty Care is associated with a number of challenges. One potentially viable route to recreate such type of surface in cosmetic applications is to combine small-sized solid particles with a film-forming technology (21). After appli-cation on skin and evaporation of the vola-tile ingredients, solids may remain embed-ded into the dried film-former matrix and create an irregular surface. However, this approach sets the following constraints: First, the high solid levels required may detrimentally affect sensory performance. Second, high amounts of large particle size solids may lead to a visible deposit on skin and impact product consumer acceptance. Finally, the technology providing this ben-efit should be incorporated at a high use level in order to counteract the dilution effect associated with formulating it into a consumer product. To help go around those challenges, a screening test protocol was set up to identify promising candidates. It included water contact angle measurement as well as an in-vitro test assessing the adhe-sion of pollution particulates equivalents on cosmetic coatings. Carbon black was selected to mimic pollution, based on a chemical nature similarity criterion. Cos-metic coatings were prepared on collagen and exposed to carbon black. The adher-ent and non-adherent fractions were then separated from each other by grav-ity and the quantity of adhering particles

estimated. In parrallel, scanning electron microscopy (SEM) measurements were carried out to investigate coating sur-face topography and confirm either the presence or the absence of microrough-ness. Screening a variety of silicone tech-nologies led to identifying a new route to generate a microroughened coating not requiring the use of solid particles. By carefully selecting a silicone film-forming technology and adjusting its in-teraction with the rest of the formulation ingredients, it was possible to generate surface microroughness without the use of solids and their associated constraints. The mechanism relies on a differential interaction with the other components of the formulation, resulting in micror-oughness formation upon evaporation of the volatile ingredients. The effect being formulation-based, comparative-only as-sessments were carried out, evaluating a

silicone-containing formulation against a control, consisting of the same formula-tion with no silicone technology. Effects were observed in a variety of formulation chassis (oil-in-water, water-in-oil, hydro-gel) at a ~2 % active level. As an example, the system below (Fig. 7) describes the water-in-oil chassis formulation.Fig. 8 shows the corresponding results obtained for the control as well as two silicone technologies, Acrylates/Polytri-methylsiloxymethacrylate Copolymer and C30-45 Alkyldimethylsilyl Polypro-pylsilsesquioxane. SEM images show that the formulation containing Acrylates/Polytrimethylsi-loxymethacrylate Copolymer was char-acterized with some microroughness in comparison with the control that led to a smooth surface. This microroughness was associated with low particle adhe-sion in comparison with the control.

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Fig. 8 Particle adhesion and surface aspect results.

Fig. 7 Formulation details for the water-in-oil chassis.

Phase Ingredients Control (%)

With technology (%)

AWater 72.2 72.2Sodium Chloride 1 1Glycerine 5 5

B

Technology (active) 1.95Volatile carrier (isododecane, other) 11.8 9.85Lauryl PEG-10 Tris(trimethylsiloxy)-silylethyl Dimethicone 2 2

Mineral oil 3 3C12-15 Alkyl Benzoate 3 3

CDiazolidinyl Urea (and) Iodopropynyl Butylcarbamate 0.5 0.5

Propylene Glycol 1.5 1.5100 100

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Dermatological-clinical application tests. Co-operation with other medical

specialities (ophthalmologist, gynecologist, paediatrician, dentist etc.) |

Simple epicutaneous trials conducted in accordance with international

guidelines | TrichoScan hair analysis | Determining skin properties by

means of confocal laser scanning (VivaScope 1500 system) | Measuring

the elasticity of the skin (cutometry) | Determining the hydration and fat

content of the skin (corneometry, sebumetry) | Sun protection determi-

nation according to DIN and COLIPA | Photo patch tests | Long-term

(repetitive) epicutaneous tests | Safety assessment | Measuring TEWL |

Ultrasound examination of the skin (DermaScan C) | Determining skin

roughness using PRIMOS (optical 3-D assessment device) | UVA protec-

tion in accordance with COLIPA standard | Human full-thickness skin

model for cosmetic testing of efficacy and tolerance

121108_DER_Anz1/1.indd 2 09.11.12 11:55

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These results also suggested that creat-ing some level of microroughness was not a sufficient condition to lower par-ticle adhesion. The C30-45 Alkyldimeth-ylsilyl Polypropylsilsesquioxane-based formulation led to a microroughened coating as well, but larger size scaled. In this case, no reduction in particle adhesion compared to the control was observed. The proposed explanation is that the large-size microspaces al-low particulates to reach the surface and develop with the substrate inter-actions of similar intensity as the »no microspaces« situation. This work identified a technology to re-duce carbon particle adhesion from a cosmetic formulation in-vitro and led to proposing a new mechanism to create microroughness in-vitro without requir-ing the use of solid particles.

■■ Conclusion

The two examples described in this article highlight the relevance of silicone ma-terials for use in sensitive skin applica-tions. Based on their demonstrated non-occlusive behavior, typical silicones such as polydimethylsiloxanes, silicone gums or silicone elastomers are not expected to impact significantly the natural wa-ter vapor flux happening though skin when formulated in cosmetic or topi-cal applications. Potential to minimize the adhesion of pollution particulates was also identified, which could lead to minimizing the adverse pollution effects on skin and be of particular interest for sensitive skin care.

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(14) Pollution and skin: From epidemiological and mechanistic studies to clinical implications. Krutmann J, et al. 2014, Journal of Dermato-logical Science.

(15) Wettability of porous surfaces. Cassie, A. and Baxter, S. Trans Farad Soc, Vol. 40, pp. 546-551.

(16) Barthlott, C. Purity of the sacred lotus, or escape from contamination in biological sur-faces. 1997, Vol. 202.

(17) Quéré, D. and Ressat, M. Non-adhesive lotus and other hydrophobic materials. Philosophi-cal Transactions of the Royal Society. 2008, Vol. 366, pp. 1539-1556.

(18) Sto Belgique – Fonction de StoLotusan. (On-line) Sto, 2014. (Cited: 02 14, 2014.) http://www.sto.be/94742_FR-StoLotusan-Fonc-tion_de_StoLotusan.htm.

(19) Tegotop 210. (Online) 08 2011. (Cited: 02 20, 2014.) http://corporate.evonik.com/en/products/search-products/pages/product-details.aspx?pid=27384&pfcat=5066.

(20) Ultrahydrophobic Textiles Using Nanopar-ticles: Lotus Approach. Ramaratnam, K. et al. 4, 2008, Journal of Engineered Fibers and Fabrics, Vol. 3.

(21) Ranade Rahul, A. et al. Composition, useful e.g. to impart hydrophobic film on surface of animal, comprises water-in-oil emulsion comprising continuous oil- and discontinu-ous aqueous phase, emulsifier, hydrophobic film former, and hydrophobic particulate material. US20100266648 (A1) United States, October 21, 2010. Application.

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©Dow Corning Corporation, 2014, All Rights Reserved.

Authors´ addresses:*Jean-Luc Garaud

([email protected])Anke Sieg

([email protected])Morgane Le Meur

([email protected])Sylvain Massé

([email protected])Dow Corning Europe S.A.

Parc Industriel - Zone CRue Jules Bordet

7180 Seneffe, Belgium

**Isabelle Van Reeth([email protected])

Dow Corning China Holding Co., Ltd1077 Zhangheng Road

Zhangjiang Hi-Tech ParkShanghai, P.R.C., 201203

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Silicones as innocious Materials - Dow