The biomechanics of blade shaving

International Journal of Cosmetic Science, 2016, 38 (Suppl. 1), 17 23 doi: 10.1111/ics.12330 Review Article The biomechanics of blade shaving Gillette Reading Innovation Centre, Reading, Berkshire, UK Received 7 March 2016, Accepted 22 March 2016 Keywords: beard, blade, friction, irritation, male, razor, shaving Abstract The shaving challenge represents a technical contradiction, whereby many of the requirements to improve hair removal efficacy can also result in increased damage to the surrounding skin. Poor-quality shaving can cause a significant inflammatory response of the skin, which may consequently lead to skin irritation and soreness. This study aims to explore and quantify the forces that act upon the skin and hair during the shave and suggests that careful management of these forces is essential in optimizing the shaving process. Various razor features are discussed and their impact on the resulting biomechanical forces is considered. Recent data are included comparing a basic low-tier disposable razor with a more technologically advanced premium system razor and show significant differences in both subjective shave attribute scores and resulting blood flux in the skin. Resume Le defi du rasage represente une contradiction technique, alors que bon nombre des exigences visant a ameliorer l efficacite de l epilation peut egalement se traduire par une augmentation des dommages a la peau environnante. Une mauvais qualite du rasage peut provoquer une reponse inflammatoire importante de la peau, ce qui peut par consequent entra^ıner une irritation de la peau et de la douleur. Ce document vise a explorer et quantifier les forces qui agissent sur la peau et les poils pendant le rasage et suggere que la gestion attentive de ces forces est essentielle pour optimiser le processus de rasage. Diverses caracteristiques des rasoirs sont discutees et leur impact sur les forces biomecaniques resultantes est considere. Des donnees recentes sont inclus, comparant un rasoir bas de gamme jetable avec un rasoir premium de technologie plus avancee et montre des differences significatives dans les appreciations subjectives du rasage et aussi dans les flux sanguin dans la peau qui en resultent. Introduction Safe and effective removal of beard hair has been an ongoing requirement for men for many centuries. Beard hairs grow at an average rate of around 0.27 mm day [1], with the average beard estimated to contain between 6000 and 25 000 hairs [2]. Shaving Correspondence: Dr. Kevin Cowley, Gillette Reading Innovation Centre, 460 Basingstoke Road, Reading, RG2 0QE, Berkshire, UK. Tel. no.: +44 (0)118 9231622; fax: +44 (0)118 975822; e-mail: cowley.k@pg.com is a complex process to slice and remove this continuously growing stubble using highly sharpened metal edges. In the late 19th century, the safety razor was developed and this forms the basis for modern wet shaving systems today. The past 100+ years has seen continuous advances in razor technology, with corresponding improvements to the consumer experience of the shaving process. Despite this, relatively little work has been published on the biomechanics of the shave, detailing the forces that impact skin and hair as a consequence of razor interaction. It is evident that detailed understanding of these forces is critical in optimising hair removal without associated skin discomfort. The shaving challenge arises in effectively removing stiff, individual terminal hairs, whilst not impacting the soft, flexible supporting skin. This can be thought of as a technical contradiction, whereby many of the requirements to improve hair removal efficacy will likely also result in increased damage to the surrounding skin. Effective razor design requires a careful balance of these requirements and consideration of a number of conflicting parameters. Whilst both skin and hair elements are comprised largely of keratin proteins, the difference in their structures results in vast differences in their relative mechanical moduli. In addition to the fundamental disparity in hair and skin material properties, there is also a wealth of biological variability present. Each person exhibits a unique pattern of facial hair growth, which can vary in hair growth direction, coverage and density, alongside differences in facial curvature and shape. Variability is also apparent in comparing different facial sites within one individual and even adjacent hairs, where growth rate and cross-sectional shape have been shown to be highly divergent [3]. As a consequence, many men struggle to achieve hair removal through shaving without resulting irritation. Shaving-related irritation is one of the most frequently mentioned male cosmetic complaints in Europe and the United States [4]. Previous studies have shown that only 12% of men are reported to never experience irritation after shaving [5], with two-thirds of men expectant of some level of irritation. This difficulty is exacerbated on the neck, where a number of physiological factors combine to make this a particularly challenging facial area, including multiple hair growth directions, hairs with lower elevation angles, rougher skin topography and looser skin attachment [6]. These are coupled with more difficult access and reduced visibility, and hence, the neck is recognized as an area which is more prone to problems such as erythema, irritation and shaving nicks. 2016 Society of Cosmetic Scientists and the Societe Francßaise de Cosmetologie 17

a b c Figure 1 Forces applied during shaving: (a) applied load perpendicular to the skin, (b) drag force parallel to the skin and (c) force to cut through the hairs. Skin issues associated with poor-quality shaving Shaving with poor-quality products causes insult to the skin, which if poorly controlled, can result in a significant inflammatory response. The act of cutting through the hair can cause significant lateral displacement, angular rotation and extension of the hair out of its follicle. Additionally, there is corresponding disruption to the surrounding skin, due to the softness of the supporting epithelial tissue and the relative hardness of the hair itself. This can trigger neurotransmission in the skin, experienced by the user as discomfort or pain. Similarly, if the skin and hair are not managed correctly by the razor cartridge, the use of sharpened edges to cut the hair can result in harsh skin abrasion, particularly around the follicle opening [7]. Excessive removal of immature corneocytes from the stratum corneum will result in impaired skin barrier, inflammation and resultant hyperproliferation of keratinocytes [8]. Finally, skin penetration by the blade can occur if the loading of the blade tip is excessive, the angle of the blade to the skin is overly high or the blade edge is displaced laterally. Penetration to the level of the underlying dermis can result in injury to the vasculature, experienced by the user as nicks or cuts. For these reasons, there is a requirement to explore and quantify the forces that act upon skin and hair during the shaving process. Correct management of these forces is suggested to lead to a more optimized shaving event, thus minimizing associated skin discomfort and irritation. Cartridge forces The force applied to the face during a shave can be thought of as having three primary components: a force perpendicular to the skin surface to load the razor cartridge onto the face, a force parallel to the skin to move the cartridge across the surface and a force to cut through the hairs (Fig. 1). For the following discussion, the nomenclature of the key cartridge components is highlighted in Fig. 2. Mul ple closelyspaced blades Front pivot Figure 2 Razor nomenclature. Fin guard Blade springs Lubrastrip Narrow blade edge Forces perpendicular to the skin Applied load The first of these forces, the applied load arising from the user pressing the razor onto the face, has been found to vary tremendously between individuals. This is driven by differences in facial physiology and nerve sensitivity, but also by differing consumer attitudes, experience and preferences. The choice of razor itself has also been shown to make a significant difference to the usage behaviour [9]. The applied load for a premium system razor (whereby the handle is retained, but the cartridge is able to be replaced) can be measured using an instrumented shaving device [10]. Load cells are used to measure the forces in two orthogonal directions, with a potentiometer to record the simultaneous cartridge swivel (Fig. 3). By measuring these three components, the instantaneous perpendicular load and drag force on the skin can be calculated, compensating for variations by the user in the angle of the razor handle. The average applied load is found to be around 2 N, but can vary from 0.5 N up to as high as 4 N for some individuals [6]. If a typical cartridge footprint is estimated to be 40 mm 9 15 mm in size, the resulting pressure under the cartridge can be calculated to be of the order of 3.5 kpa. 18 2016 Society of Cosmetic Scientists and the Societe Francßaise de Cosmetologie

a b Figure 5 Representation of applied forces for (a) a centre-pivoting twoblade razor and (b) a front-pivoting five-blade razor. Figure 3 Instrumented shaving device used to measure forces during shaving. a b c Figure 4 Finite element computer modelling of cartridges indented into skin substrate with a load of 2 N: (a) a two-bladed cartridge, (b) a five-bladed male premium razor and (c) a five-bladed female premium razor. It can be seen from Fig. 4, using finite element computer modelling to visualise the skin response [11], that this is exacerbated at the edges of the cartridge due to the heavy indentation into the skin. The peak stress in the skin is consequently a function of cartridge shape and size, with a larger cartridge (Fig. 4b and c) providing a more even stress distribution than a smaller cartridge (Fig. 4a). There is a balance to be achieved in razor design, however, between enlarging the cartridge to effectively spread the applied load onto the skin and reducing the size of the cartridge to optimize cartridge fit into areas of difficult facial topography and curvature, such as under the nose, the chin and under the jawline. The majority of modern razors are designed to allow the cartridge to pivot front-to-back relative to the razor handle, and this enables the applied load to be spread evenly onto the skin. The use of a front-pivoting system on modern razors (also called a frontloaded razor), whereby the pivot point is located within the leading elastomer fin element at the front of the cartridge, is particularly beneficial. This enables excessive force to be transmitted through the soft elastomer fins (Fig. 5b), rather than through the blades themselves, as can occur with a central pivot position (Fig. 5a). additional blades into a given space by narrowing the interval between each blade will result in less load on each blade and less bulging of the skin between the blades (akin to the effect provided by a bed of nails) [2]. The challenge arises in creating a system with closely spaced blades, but also maintaining a sufficient gap between the blades to enable effective rinsing of shave debris from the cartridge. Some razor systems also incorporate miniature plastic springs (Fig. 6) upon which the blades rest, which are able to move if loaded excessively. These can assist in avoiding overly high blade loads, which could result in damage to the skin. The first springloaded blades were introduced in 1990 and showed a significant benefit in large-scale consumer testing for comfort and safety [13]. Skin protection can be further enhanced through using a shave prep which provides a protective layer to cushion and dampen peak blade loads. A minimum level of blade loading is required to provide effective hair engagement and cutting, dependent on the direction of hair growth, the hair physiology and the blade edge itself. As previously, there is a balance to be achieved between increasing blade loading to ensure effective hair cutting and reducing blade loading to minimize impact to the skin. Forces parallel to the skin Shaving drag In addition to the applied load, the skin also experiences a stretch parallel to the stratum corneum as the razor is dragged across the skin surface. Skin is an anisotropic, highly non-linear material, hence when it is stretched, its stiffness rapidly increases to resist further elongation [14]. This can be attributed to its non-homogeneous structure, particularly in the dermis where the connective tissue comprises a network of interwoven collagen and elastin Forces perpendicular to the skin Blade loading It can be shown via computational modelling and instrumented razor tests that a significant proportion of the applied load is transmitted through the blades, with the remaining part carried by the rest of the cartridge [12]. The number of blades and their associated spacing are key in determining the respective loading on each blade. If blades are simply added without reducing their relative spacing, the cartridge becomes larger and total blade drag increases as a result of increased skin interaction. In contrast, adding Figure 6 Cartridge with blades removed to highlight miniature plastic springs upon which the blades sit. 2016 Society of Cosmetic Scientists and the Societe Francßaise de Cosmetologie 19

Figure 7 Measurement of skin surface strains during a shaving stroke using optical displacement analysis. fibres. Under in-plane stretching, the initial skin response is governed by the elastin fibres and is broadly independent of collagen. As the force increases, straightening of the collagen fibres occurs which causes stiffening of the overall response. It has been estimated from in vivo surface optical displacement analysis [15] that skin strains of up to 20% occur during shaving (Fig. 7), hence the initial elastin response is likely to be most relevant. The drag force acting to stretch the skin is a function of both the applied load by the user and the coefficient of friction at the razor/skin interface. To minimize shaving drag, good shaving practices should rely on the use of lubricating ingredients. These are typically formulated in shaving gels and in the lubricating strips that can be found on many modern razor technologies. The presence of shaving prep and lubrication onboard the cartridge (e.g. the lubricating strip) can act to reduce the friction and hence minimize the amount of skin drag experienced [16]. A key lubricant ingredient that has been abundantly used as a shave lubricant is polyethylene oxide, a highly water-binding long-chain polymer molecule. When pulled as a result of the drag force, this molecule is able to elongate longitudinally and maintains a cushioning protective film under the cartridge, and in particular under the blade edges, through the formation of a partial elastohydrodynamic film [17]. The presence of this protective layer is key in minimizing attrition of the skin from the sharpened blades. Lubricating strips onboard the razor typically consist of a nonwater-soluble polymer mesh (normally polystyrene) and water-soluble lubricants (typically a blend of polyethylene oxide of different chain lengths) which leach out of the strip during shaving to reduce drag and enhance shave comfort [18]. Lubricating strips were first introduced onto razors in 1985. As with cartridge size and blade loading, the drag force cannot be excessively reduced without negatively affecting the shave experience. The requirement for smooth glide needs to be balanced with a minimal level of drag, so that the user maintains a feeling of control of the razor. For this reason, lubricating strips have been traditionally located at the rear of the cartridge, after the blades (Fig. 8a). Placing the strip before the blades typically left men feeling they had less control over their razor. However, recent 5-blade razor technologies now also include additional onboard lubrication at the front of the cartridge, or before the blades (Fig. 8b). Advances in modern manufacturing and the ability to mould these strips with high precision in a specific shape have allowed further enhancement of friction reduction benefits, without negatively affecting the feeling of being in control of the razor. An important aspect in defining the strip shape is to ensure the contact of the fin guard with the skin is not impeded [19]. The drag force experienced on the razor blades can be attributed to two sources: skin interaction and hair interaction/cutting events. The guardform at the front of the cartridge (Fig. 8) often takes the form of miniature elastomer fins which provide stretch of the skin to flatten the local skin topography. This reduces the previously mentioned skin bulge between the blades and correspondingly reduces blade drag [6]. The distribution of drag forces across the cartridge varies considerably with different razors, dependent on their number of blades, guardform, lubrication, surface finish and so on. Shaving prep and hairs are removed by repeated shaving strokes, whilst the cartridge itself deposits lubrication from the onboard chemistry; hence, the drag profile changes dynamically during the shave. This, in turn, will also vary over the lifetime of the cartridge as the onboard chemistry becomes depleted and the blades become worn. Hair cutting forces Due to the width of the cartridge and the density of typical facial hair, each blade typically cuts several hairs simultaneously. Beard hair was shown to have a peak cutting force of around 0.05 N [20] for a double-edge blade, but this value will vary considerably as a function of both blade edge and hair properties. It is, however, a small fraction of the force needed to extract a hair fully from its follicle [21]. a Lubrica ng strip Elastomeric fin guard Figure 8 Commercially available cartridges highlighting the elastomer fin guard at the front to provide skin stretch and the lubricating strips to reduce friction: (a) traditional technologies (pictured: Gillette MACH3) only have the strip at the rear, (b) modern 5-blade technologies (pictured: Gillette Fusion ProShield) also include a strip at the front of the cartridge. b 20 2016 Society of Cosmetic Scientists and the Societe Francßaise de Cosmetologie

The hair follicle is innervated by a network of nerve fibres that run around and parallel to the outer root sheath and hence user sensitivity is very high to forces on the hair shaft itself [22]. Similarly, the adjacent skin is highly innervated with free nerve endings which terminate in the stratum granulosum and is therefore sensitive to stresses associated with hair deflection and lateral displacement. These nerve sensations from cutting hair will be experienced as tug and pull on the hairs and/or discomfort and scraping of the skin (Fig. 9). Minimising the force to cut the hair is essential in reducing negative sensations from hair interaction. As the hair is hydrated in wet shaving, the cutting force correspondingly reduces through the disruption of hydrogen bonds within the keratin structure. The force to cut beard hair has been shown to decrease by around 40% as the hair becomes fully hydrated [2]. Additional hydration time prior to the shave can therefore make a significant difference to the cutting force and consequently the sensation of pull and tug experienced. For this reason, it is recommended that men wash their face, or shower, prior to shaving. Similarly, hair cutting force can be reduced by modifying the blade itself, either by thinning of the edge profile or through the use of surface coatings to improve the ability of the blade to pass easily through the hair (Fig. 10 compares two different blades and illustrates a cutting force reduction through use of a narrower blade edge profile). Measuring skin changes due to shaving As mentioned previously, a poor-quality shave has a consequential impact on the skin, experienced by the consumer as soreness and irritation. Uncontrolled high biomechanical forces can lead to a complex inflammatory response within the skin, as neurotransmittors and pro-inflammatory cytokines signal insult to the skin. These, in turn, lead to vasodilation of the small blood vessels within the dermis, leading to hyperaemia and potentially erythema [23]. The inherent complexity of the shaving process brings new specific challenges to measuring skin condition by conventional instrumental methods. Devices such as Corneometer, Skicon and TEWL can be reapplied in a shaving context only with careful consideration of the resulting impact on the measurement obtained [24]. During the shaving process, skin and hair are transiently hydrated. This can influence traditional skin measures; therefore, it Figure 10 Comparison of cutting force profiles of a blade cutting through beard hair, as measured on an in-house-built in vitro rig. The graphs show a lower cutting force for narrower edges (bottom blade pictured is one of the first four blades of Gillette Fusion ProGlide) compared to edges which are significantly less sharp (top blade pictured is a Gillette Fusion blade). is important to understand the relative contribution from shaving products compared to transient hydration effects. The presence of surfactants within the prep can also lead to drying of the skin due to removal of sebum and lipids [25]. Skin condition is highly variable between panellists, but also shows high variability within a single face as a function of surface position [24], hence accurate facial repositioning between measurements is essential. The most obvious and impactful change to the skin during the shaving process is the removal of hair, along with surface corneocytes from the stratum corneum. This can lead to changes in values obtained from skin measures by impacting the physical contact of the instrument, along with modification to the surface of the skin. In this context, traditional skin measures of hydration and barrier integrity require carefully controlled testing. A measure of skin irritation response is available in the emerging technology of laser speckle contrast imaging (LSCI), which provides non-contact, real time imaging of blood flux: the product of average speed and concentration of moving red blood cells in the tissue sample. Laser light is used to generate a random speckle pattern from the upper layers of the skin, and blurring of this pattern provides a measure of the movement of erythrocytes in the superficial vessels [26]. The contrast of the speckle pattern is used to generate a colour-coded image that correlates with blood flux in the tissue. One of the primary applications of the device has been in examining skin microvascular function; hence, it has clear potential in reapplication for the measurement of changes in blood flux induced by shaving. LSCI applied to shaving Figure 9 Schematic representation of skin stresses arising from hair cutting events. A comparison was made of panellists shaving a premium 5-blade system razor with a front-pivoting cartridge and low cutting force edges (commercially available as Gillette Fusion ProGlide) against a low-tier 2-blade disposable razor with a non-pivoting head (Gillette Blue II). Panellists free-shaved with each product for 1 week with a commercially available shave gel (Gillette Series), using a balanced, randomized study design. Before each treatment regime, panellists 2016 Society of Cosmetic Scientists and the Societe Francßaise de Cosmetologie 21

underwent a 7-day pre-conditioning phase, where they were asked to shave with a Gillette Fusion ProGlide razor and Gillette Series gel, whilst refraining from using moisturizing or shave balm products. During testing, panellists were acclimatized for 15 min after arrival, and all measurements were taken in a controlled temperature environment of 20 C 2 C. Sixteen panellists completed the study, with LSCI measurements recorded each day pre-shave and 10 min post-shave using a 785-nm wavelength FLP-2 device (Moor Instruments, Axminster UK). Measurements were taken on the right side of the neck, over a region of interest of 24 9 18 mm, with a working distance of 25 cm. In each case, a stable measurement of 20 s duration was averaged to determine the mean blood flux. In addition, a detailed questionnaire was completed by each panellist immediately after each shave to examine the subjective response. A 5-point scale ranging from Poor to Excellent was used for key shaving attributes including Being comfortable while shaving, Gliding smoothly over your skin and Not leaving you with nicks and cuts. For attributes relating directly to skin soreness (including Visibly red, Sore and Burning ), a 10-point gauge scale was utilized, ranging from Not at all at one end of gauge scale to Extremely at the other. Consumer scores were analysed by averaging the results for each panellist over the four days of testing and then using a paired Student s t-test comparison. The results were found to be significantly in favour (at >95% confidence level) of the 5-blade system razor for the majority of attributes asked, including Overall score, Comfort During, Comfort After, Close Shave, Nicks & Cuts, Gliding Smoothly and Soreness. For all of these attributes, apart from Comfort During, the consumer scores for the disposable razor significantly decreased over the 4 days of testing, indicating a deteriorating shave with increasing levels of soreness. The LSCI data, expressed as a percentage increase in blood flux from pre- to post-shave, were analysed similarly and were shown to be significantly different between the razors at >95% confidence (Fig. 11). The disposable razor resulted in a higher increase in blood flux postshave, consistent with a higher level of skin impact from the shaving event [27]. The results of the study were combined with additional available LSCI data to explore further the correlation between consumer scores and blood perfusion. The data for each shaving attribute were grouped into appropriate bins for % blood flux change to facilitate weighted regression analysis. It was shown that a negative correlation existed between the change in blood perfusion due to shaving and consumer ratings for many of the key attributes, including Comfort During, Comfort After, Glide and Nicks & Cuts (Fig. 12). This study confirms a strong link between the razor, the resulting blood flux and consequently the consumer rating for skin comfort, driven largely by the ability of the razor to manage the biomechanics of the shave effectively. The 5-blade premium product includes many of the key features designed to promote a comfortable and safe shave: front-pivoting cartridge, elastomer fin guard, thinner blade edges and onboard lubrication. It is concluded that the inclusion of these features is sufficient to cause a meaningful and measurable difference in the inflammatory response of the skin. Conclusions Shaving comfort has been shown to be highly dependent on the biomechanics of skin and hair. Skin is a viscous, non-linear material with a stiffness several orders of magnitude below that of keratinised facial hair. The ability of the razor to manage % increase in blood flux (post-pre shave) 40% 35% 30% 25% 20% 15% 10% 5% 0% p=0.02 Premium 5-blade razor Low- er disposable razor Figure 11 Mean % increase in blood flux for a premium 5-blade razor (commercially available as Fusion ProGlide) compared to a low-tier disposable (Blue II), measured 10 min post-shave, analysed using a paired Student s t-test (error bars indicate 95% confidence intervals). Consumer score for 'Comfort A er' (1-5 scale) 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0-40% -20% 0% 20% 40% 60% 80% 100% % change in blood flux due to shaving p=0.068 R 2 =0.722 Figure 12 Correlation between Comfort After subjective scores and the % change in blood flux due to shaving, indicating a negative correlation at >90% confidence. The data have been grouped to provide a weighted regression, with the size of each bubble relating to the number of datapoints represented. appropriately the forces applied onto skin and hair is key to an effective and comfortable shave. In many cases, technical contradictions are at play and shaving devices need to manage a balance of opposing requirements. At the highest level, this can be seen in the need to manage effective hair removal against protection of the surrounding skin. It is also present in the need for effective rinsing of debris from the razor, control when moving the razor over the face, access to tricky areas and so on. A firm understanding of shaving biomechanics is essential in continuing to develop future razor technologies which can manage and break these trade-offs, to deliver superior quality shaves with great skin comfort. 22 2016 Society of Cosmetic Scientists and the Societe Francßaise de Cosmetologie

Acknowledgements This work was fully funded by Procter & Gamble. The authors are both employees of Procter & Gamble. The authors would like to thank Leigh Knight, Nick Sardar, Daniel Coe, David O Callaghan, Alison Stephens (all of Gillette Reading Innovation Centre, Reading, UK), along with Pam Zupkosky and William Jolley (both of Gillette South Boston Innovation Centre, Boston, USA) for providing material for inclusion in this manuscript. References 1. Saitoh, M., Uzuka, M. and Sakamoto, M. Rate of hair growth. In: Advances in Biology of the Skin, Vol IX (Montagna, W. and Dobson, R.L., eds.), pp. 183 201. Pergamon Press, Oxford (1969). 2. Ertel, K. and McFeat, G. Blade shaving. In: Cosmetic Dermatology: Products and Procedures (Draelos, Z.D., ed.), pp. 156 164. Blackwell, Oxford, UK (2010). 3. Thozhur, S., Crocombe, A.D., Smith, P.A., Cowley, K.D. and Mullier, M. Structural characteristics and mechanical behaviour of beard hair. J. Mater. Sci. 41, 1109 1121 (2006). 4. Elsner, P. Overview and trends in male grooming. Br. J. Dermatol. 166, 2 5 (2012). 5. Gillette data held on file. 6. Cowley, K. and Vanoosthuyze, K. Insights into shaving and its impact on skin. Br. J. Dermatol. 166, 6 12 (2012). 7. Bhaktaviziam, C., Mescon, H. and Matoltsy, A.G. Shaving. I. Study of Skin and Shavings. Arch. Dermatol. 88, 874 879 (1963). 8. Evans, R.L., Marriott, R.E. and Harker, M. Axillary skin: biology and care. Int. J. Cosmet. Sci. 34, 389 395 (2012). 9. Gillette data held on file. 10. Cloke, CS, Howell, DM and Wain, KJ. Razor cartridge measurement apparatus. US Patent 2008/0168657 A1. (2008). 11. Cowley, KD and O Callaghan, DJ. Skin modelling methods and systems. US Patent 8,306,753 B2 (2012). 12. Gillette data held on file. 13. Gillette data held on file. 14. Silver, F.H., Freman, J.W. and DeVore, D. Viscoelastic properties of human and processed dermis. Skin Res. Technol. 7, 18 23 (2001). 15. Doughty, DG and Zupkosky, PJ. Optical measurement method of skin strain during shaving. US Patent 7,344,498 B1 (2008). 16. Draelos, Z.D. Male skin and ingredients relevant to male skin care. Br. J. Dermatol. 166, 13 16 (2012). 17. Vicente, J., Stokes, J.R. and Spikes, H.A. Lubrication properties of non-adsorbing polymer solutions in soft elasohydrodynamic (EHD) contacts. Tribol. Int. 38, 515 526 (2005). 18. Coope-Epstein, J and Jannusch, L. Shaving aid material. US Patent 2009/0223057 (2009). 19. Gillette data held on file. 20. Deem, D.E. and Rieger, M.M. Observations on the cutting of beard hair. J. Soc. Cosmet. Chem. 27, 579 592 (1976). 21. Yamada, H. and Evans, F.G. Strength of Biological Materials. Williams and Wilkins, Philadelphia, PA, U.S.A. (1970). 22. Winkelmann, R.K. The innervation of a hair follicle. Ann. N. Y. Acad. Sci. 83, 400 407 (1959). 23. Campbell, N.A. and Reece, J.B. Biology, 7th edn. Benjamin Cummings / Pearson, Boston, USA (2005). 24. Knight, L, Cowley, KD, Vanoosthuyze, K and Zupkosky, PJ. Hydration and skin condition in the context of shaving. Poster presented at: Stratum Corneum VII, 10 12 September 2012, Cardiff, UK (2012). 25. F orster, T., Issberner, U. and Hensen, H. Lipid/surfactant compounds as a new tool to optimize skin-care properties of personal-cleansing products. J. Surfactants Deterg. 3, 345 352 (2000). 26. O Doherty, J., McNamara, P., Clancy, N.T., Enfield, J.G. and Leahy, M.J. Comparison of instruments for investigation of microcirculatory blood flow and red blood cell concentration. J. Biomed. Opt. 14, 034025 (2009). 27. Zupkosky, P, Knight, L. Use of blood flow imaging as an instrumental measure of skin comfort and irritation attributes in male shaving. Poster presented at: World Congress of Dermatology, 8 13 June 2015, Vancouver, Canada (2015). 2016 Society of Cosmetic Scientists and the Societe Francßaise de Cosmetologie 23