Some Aspects of Skin Tribology: Friction Blister
Holly Sibley MEng
Supervisor: Dr. Georges Limbert
University of Southampton
Main Functions of Skin
Protective Function Examples
MechanicalAbrasions
Blunt ImpactsCutting
Biological Invasion from foreign organisms
Radioactive UV
Chemical Penetration
Thermal Insulation
Barrier functions as the skin straddles both the external and internal environments
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
Blood Vessels and Lymphatics
Sweat Gland
Hair FollicleSweat Glands and Hair Follices
Nerves
http://www.relata.info/ (Accessed 25/03/10)
Dermatological Problems
Dermatological Problems
Allergies
Cuts
Eczema
Psoriasis
Infestations
Infections
Fungal Viral
Acne
Emergency Dermatology
Cancer
BCC
SCC
Melanoma
BlistersBlisters
Blisters
• There are different types of blisters and each ‘splits’ the skin at a different histological level
Pemphigoid BullosaPemphigus Vulgaris
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
. 0t
fv
1.( (1 )) s
t
sv st
Variable SymbolVolume Fluid FractionVolume Solid Fraction
Velocity of Fluid
Velocity of Solid
Coefficient of Growth
Permeability
Pressure
Stress
Strain
Bulk Modulus
Viscosity
Biot’s Coefficient .hk p f sv v
1
fv
sv
hkpK
Fluid conservation Eqn:
Solid conservation Eqn:
Darcy’s Law:
Poroelastic Model
. 0ij Force Balance Equation:
Variable Symbol
Volume Fluid Fraction
Volume Solid Fraction
Velocity of Fluid
Velocity of Solid
Coefficient of Growth
Permeability
Pressure
Stress
Strain
Bulk Modulus
Viscosity
Biot’s Coefficient
Poroelastic Term Growth TermElastic Response
22
3ij ij kk ij ij ijK p K
Poroelastic Response:
1
fv
sv
hkpK
1
2ji
ijj i
uu
x x
Strain:
Which Value to Use?Hydraulic Conductivity
Values Units Source Subject Material Publication Notes
5.33 (Swartz and Fleury 2007) Rat Dermis Review Article Measurement made Ex Vivo.
15 to 78 (Swartz and Fleury 2007) Rat Abdominal Muscle Review Article Measurement made In Vivo.
70 to 150 (Swartz and Fleury 2007) Mouse Tail Skin Review Article Measurement made In Vivo.
70 (Swartz, Kaipainen et al. 1999) Mouse Tail Skin Journal Article Measurement estimated from the fluorescent characteristic length approximation.
41 to 253 (Swartz and Fleury 2007) Mesentery Review Article Measurement made Ex Vivo.
190 (Intaglietta and de Plomb 1973) Mesentery Journal Article Flow across tissue slice In Vitro
31 (Winters and Kruger 1968) Mesentery Journal Article Flow across tissue slice In Vitro
21 (Swabb, Wei et al. 1974) Hepatoma Journal Article Flow across tissue slice In Vitro
20 (Granger, Dhar et al. 1975) Wharton’s Jelly Journal Article -
8 (Swabb, Wei et al. 1974) Rat Hepatoma Journal Article Darcy’s law analysis of In Vitro filtration data.
5 (Swabb, Wei et al. 1974) Rat Subcutaneous Tissue
Journal Article Darcy’s law analysis of In Vitro filtration data.
9 (Swabb, Wei et al. 1974) Rabbit and Human Sclera
Journal Article Darcy’s law analysis of In Vitro filtration data.
2.5 (Swabb, Wei et al. 1974) Human and Rabbit corneal Stroma
Journal Article Darcy’s law analysis of In Vitro filtration data.
7.75 (Swabb, Wei et al. 1974) Human Articular Cartilage
Journal Article Darcy’s law analysis of In Vitro filtration data.
7 (Swabb, Wei et al. 1974) Pig Aorta Journal Article Darcy’s law analysis of In Vitro filtration data.
2-8cm
×10mmHgs
4 13cm s.dyn 10
2-8cm
×10mmHgs
2-8cm
×10mmHgs
2-8cm
×10mmHgs
2-8cm
×10mmHgs
4 13cm s.dyn 10
4 13cm s.dyn 10
4 13cm s.dyn 10
4 13cm s.dyn 10
4 13cm s.dyn 10
4 13cm s.dyn 10
4 13cm s.dyn 10
4 13cm s.dyn 10
4 13cm s.dyn 10
COMSOL Model
10mm
1.750mm
1.150mm
0.085mm
Free
Fixed
Free Free
1000 MPa
5 MPa
0.025 MPa
50 N/m2
Inward Flux: 0.01 mm/s
Inward Flux: 0.01 mm/s
Atmospheric Pressure
Zero Flux/ Symmetry
Zero Flux/ Symmetry
Zero Flux/ Symmetry
COMSOL ResultsSurface: First Principal Stress [Pa]
Arrow: Velocity FieldDeformation: Displacement
COMSOL ResultsSurface: First Principal Strain
Arrow: Velocity FieldDeformation: Displacement
COMSOL ResultsSurface: Pressure [Pa]Arrow: Velocity Field
Deformation: Displacement
COMSOL ResultsSurface: 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’ presentationhttp://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?
Top Related