Holly Sibley - Importance of understanding skin tribology

Report
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 1013
cm4 s.dyn 1013
cm4 s.dyn 1013
cm4 s.dyn 1013
cm4 s.dyn 1013
cm4 s.dyn 1013
cm4 s.dyn 1013
cm4 s.dyn 1013
cm4 s.dyn 1013
cm4 s.dyn 1013
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?

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