Some Aspects of Skin Tribology: Friction Blister Holly Sibley MEng Supervisor: Dr. Georges Limbert University of Southampton Main Functions of Skin Barrier functions as the skin straddles both the external and internal environments Protective Function Examples Mechanical Abrasions Blunt Impacts Cutting Biological Invasion from foreign organisms Radioactive UV Chemical Penetration Thermal Insulation Main Functions Cont. What happens when the skin is broken? • Self-regeneration! – Allows the skin to remain an effective barrier • Deep skin wounds close spontaneously by – Epitheliasation – Wound contraction – Scar synthesis • Skin is one of the body’s main sensory interfaces – It contains many of the peripheral endings of the sensory nerve system Secondary Functions of Skin • Protection against UV damage • Production of Vitamin D3 • Production of pheromones Variability of Skin • • • • Location Age Ethnicity Hydration levels http://3.bp.blogspot.com/_kqkBbz54MBo/Skvc2XeY-4I/AAAAAAAAABk/nxpZYBcwYgk/s400/right+forearm.JPG (Accesseed 09/04/10) http://camelsnose.files.wordpress.com/2009/01/hand-left-500px.png (Accesseed 09/04/10) https://www.storesonlinepro.com/files/1862656/uploaded/baby%20face.jpg (Accesseed 09/04/10) http://www.tribuneindia.com/2005/20051017/ldh6.jpg (Accesseed 09/04/10) Multilayered Material Structure of Skin Cells at the Stratum Basale layer replicate by mitosis and end at the keratinised Stratum Corneum. http://wpcontent.answers.com/wikipedia/commons/thumb/2/20/Skinlayers.png/250px-Skinlayers.png (Accessed 24/03/10) Structures within skin Sweat Gland Hair Follicle Sweat Glands and Hair Follices Blood Vessels and Lymphatics http://www.relata.info/ (Accessed 25/03/10) Nerves Dermatological Problems BCC Melanoma Acne Allergies Cancer Cuts SCC Dermatological Problems Eczema Emergency Dermatology Infections Fungal Viral Psoriasis Infestations Blisters Blisters • There are different types of blisters and each ‘splits’ the skin at a different histological level Pemphigus Vulgaris Pemphigoid Bullosa http://www.bing.com/health/static/articles/mayo/1BCC967C5563FFF0270561FAAAFA8E5F.jpg (Accessed 10/04/10) http://meded.ucsd.edu/clinicalimg/skin_pemphigus2.jpg (accessed 10/04/10) Importance of Understanding Blisters Create a model that accurately predicts blister formation Ascertain the effect each variable has on the system Use the results to further knowledge in aiding blister prevention Friction Blisters • ‘Split’ occurs in the Stratum Spinosum • These blisters occur after frictional forces are applied to the skin • Self healing BUT... • Friction blisters can have a disastrous effect with the sports and military world http://feet.thefuntimesguide.com/images/blogs/bad-foot-blister-on-heel-by-Lady-Weaxzezz.jpg (Accessed 21/04/10) Questions to Answer • What are the main variable in promoting the formation of friction blisters? • If a force and repetition creates a blister, would doubling the force and halving the repetitions make the same blister? • What is the respective influence of hydrostatic and shear forces within blister formation? • How can this work improve blister prevention? Assumptions • • • • Linear Poroelastic Isotropic Simplified fluid flow in the skin – Blood flow and lymphatics • Boundary conditions As the model progresses these assumptions can be slowly modified to become more accurate. Poroelastic Model Porous Structure Fluid Fluid Infiltrated Porous Structure •Look at the cube of skin as a whole • Response of the fluid can be described by Darcy’s Law • Response of the solid can be demonstrated by the poroelasticty equations Poroelastic Model Fluid conservation Eqn: . v f 0 t Symbol Volume Fluid Fraction Velocity of Fluid 1 vf Velocity of Solid vs Volume Solid Fraction Coefficient of Growth Solid conservation Eqn: 1 .( vs (1 )) s t Variable s t Permeability Pressure Stress Strain Darcy’s Law: vf vs kh. p Bulk Modulus kh p Viscosity K Biot’s Coefficient Poroelastic Model Strain: 1 ui u j ij 2 x j xi Poroelastic Term Growth Term . ij 0 Volume Fluid Fraction Velocity of Fluid 1 vf Velocity of Solid vs Coefficient of Growth 2 ij 2 ij K kk ij ij p K ij 3 Force Balance Equation: Symbol Volume Solid Fraction Poroelastic Response: Elastic Response Variable Permeability Pressure Stress Strain Bulk Modulus kh p Viscosity K Biot’s Coefficient Which Value to Use? Hydraulic Conductivity Values Units Source Subject Material Publication Notes 5.33 cm ×10-8 mmHgs (Swartz and Fleury 2007) Rat Dermis Review Article Measurement made Ex Vivo. 15 to 78 cm2 ×10-8 mmHgs (Swartz and Fleury 2007) Rat Abdominal Muscle Review Article Measurement made In Vivo. 70 to 150 cm2 ×10-8 mmHgs (Swartz and Fleury 2007) Mouse Tail Skin Review Article Measurement made In Vivo. (Swartz, Kaipainen et al. 1999) Mouse Tail Skin Journal Article Measurement estimated from the fluorescent characteristic length approximation. (Swartz and Fleury 2007) Mesentery Review Article Measurement made Ex Vivo. (Intaglietta and de Plomb 1973) Mesentery Journal Article Flow across tissue slice In Vitro (Winters and Kruger 1968) Mesentery Journal Article Flow across tissue slice In Vitro (Swabb, Wei et al. 1974) Hepatoma Journal Article Flow across tissue slice In Vitro (Granger, Dhar et al. 1975) Wharton’s Jelly Journal Article - (Swabb, Wei et al. 1974) Rat Hepatoma Journal Article Darcy’s law analysis of In Vitro filtration data. (Swabb, Wei et al. 1974) Rat Subcutaneous Tissue Journal Article Darcy’s law analysis of In Vitro filtration data. (Swabb, Wei et al. 1974) Rabbit and Human Sclera Journal Article Darcy’s law analysis of In Vitro filtration data. (Swabb, Wei et al. 1974) Human and Rabbit corneal Stroma Journal Article Darcy’s law analysis of In Vitro filtration data. (Swabb, Wei et al. 1974) Human Articular Cartilage Journal Article Darcy’s law analysis of In Vitro filtration data. (Swabb, Wei et al. 1974) Pig Aorta Journal Article Darcy’s law analysis of In Vitro filtration data. 70 41 to 253 190 31 21 20 8 5 9 2.5 7.75 7 2 cm2 ×10-8 mmHgs cm2 ×10-8 mmHgs cm4 s.dyn 1013 cm4 s.dyn 1013 cm4 s.dyn 1013 cm4 s.dyn 1013 cm4 s.dyn 1013 cm4 s.dyn 1013 cm4 s.dyn 1013 cm4 s.dyn 1013 cm4 s.dyn 1013 cm4 s.dyn 1013 COMSOL Model 50 N/m2 Free Pressure Atmospheric Inward1000 Flux:MPa 0.01 mm/s 0.085mm 1.150mm 1.750mm Zero Flux/ Free Symmetry 5 MPa Inward Flux: 0.01 mm/s 0.025 MPa Fixed Zero Flux/ Symmetry 10mm Zero Flux/ Free Symmetry COMSOL Results Surface: First Principal Stress [Pa] Arrow: Velocity Field Deformation: Displacement COMSOL Results Surface: First Principal Strain Arrow: Velocity Field Deformation: Displacement COMSOL Results Surface: Pressure [Pa] Arrow: Velocity Field Deformation: Displacement COMSOL Results Surface: Velocity Field [m/s] Deformation: Displacement Limitations with the Model • Deciding on realistic boundary conditions for the COMSOL model • Value of the flux • Meshing...needing to use 1D element – not necessarily a necessity to model the stratum corneum. General Difficulties in Modelling Skin • Age and location of the skin • Issues in obtaining the values for the model: • Young’s modulus: How do you find this in living tissue? • Other mechanical values Future Work • Refine the model • Obtain more realistic boundary conditions for fluid flow • Add other properties beyond poroelasticity • Take the model to the next stage: ‘The split’ • Verification of the model through experimental work Other Applications of Skin Tribology Dr. Georges Limbert, ‘Multi-layer finite element model of skin’ presentation http://missinglink.ucsf.edu/lm/DermatologyGlossary/img/Dermatology%20Glossary/Glossary%20Clinical%20Images/Eczema_Nummular-L.jpg (Accessed 19/04/10) http://www.enaturalhealth.com/blog/wp-content/uploads/2009/12/tips-to-improve-psoriasis1.jpg (Accessed 19/04/10) http://www.holytrinitytn.org/images/shaving_face.jpg (Accessed 19/04/10) http://www.bioinf.uni-sb.de/DN/Projects/Skin%20Penetration/Skin%20Penetration/skin_logo.png (Accessed 19/04/10) http://content.answers.com/main/content/img/elsevier/dental/f0329-01.jpg (Accessed 19/04/10) http://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/3187.jpg (Accessed 19/04/10) Conclusions • Skin remains a difficult material to model but computational modelling can help to unravel the mechanics behind it • Modelling can help us understand what we can’t investigate experimentally • Blister modelling is a way of testing different hypotheses • Important to correlate computational work to experimental data Thank you Any Questions?