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?