an in-vitro procedure for evaluation of lens-on-tissue friction...(results at different stages of...
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An in-vitro ProcedureFor Evaluation of
Lens-on-Tissue Friction
Anne E. MeyerUniversity at Buffalo (Buffalo, NY, USA)
Supported byAlcon Research Ltd. (Fort Worth, TX, USA)
Industry/University Center for Biosurfaces (Univ. at Buffalo)School of Dental Medicine (Univ. at Buffalo)
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coef
ficie
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f fric
tion
Ge-on-GeA B C
Tissue-on-GeA B C
Tissue-on-TissueA B C
Lubrication Testing: friction couple materials make a difference
metal-on-metal
tissue-on-metal
tissue-on-tissue
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μ and γc
Coefficient of Frictionand Critical Surface Tensiondirectly proportional for dry, synthetic materials
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surface tension [mN/m]
cosi
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vera
ge c
onta
ct a
ngle
μ γc(mN/m)
PTFE 0.04 18.5PE 0.2 32.0Steel 0.6 50+Steel +stearic acid
0.1 24.0
γc
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μ and γc
Coefficient of Frictionand Critical Surface Tensiondirectly proportional for dry, synthetic mat’ls
What about …
-- hydrated materials?
-- biological tissues?
-- biological lubricants?
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BIOLOGICAL TISSUES
hardsoft
fluid (e.g. blood, saliva, tear film)
osseointegration…thromboresistance……adhesive…cohesive…abhesive…
…load-bearing…nonload-bearing…shear…
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γc(mN/m)
environment
blood vessel 22-25 fluid shear
cornea 22-24 tissue/fluid shear
tooth enamel 30-35 load & shear
articular cart. 30-35 load & shear
skin – human 35-40 load & shear (air)
skin - orca 24-26 fluid shear
eel grass 24-28 fluid shear
Exam
ples
…
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Measured coefficients of friction for tissue/Unisol
HUVG (n=51): 0.450 ± 0.155 (2007)
HUVG (n=21): 0.312 ± 0.118 (2003)
HUVG/cornea (n=7): 0.089 ± 0.054 (2004)
Fresh Oral Mucosa (n=6): 0.697 ± 0.022 (2007)
Bovine Cartilage (n=11): 0.082 ± 0.008 (2007)
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coefficient of friction
hydrodynamic lubrication
boundary lubrication
contacting asperities & adhesion
…making & breaking “bonds”
elastohydrodynamic lubrication
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On the molecular scale…
biological fluids…
shear v. adsorbed film properties
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Exposure Time (minutes)
Crit
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Sur
face
Ten
sion
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/m)
460 sec-15 sec-1
Whole Saliva Flow over Germanium:Influence of fluid shear on adsorbed film γc
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Critical Surface Tensions of Contact Lenses
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FSA SA Hgel2 Hgel4Lens Type
criti
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urfa
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N/m
]
pre-exposurecontrolsafter 8-hr wear
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Relative Coefficient of Friction (metal-on-metal implant components)
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HEMA HUVG RGP Hi-Shine control
interpositional material or implant treatment
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Corneal epithelium (human)no fixative, still moist
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Human Umbilical Vein Graft [HUVG] used in vascular surgery [“Biograft”]
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Human Umbilical Vein Graft with mechanical damage
elastic lamina
underlying collagenousfibers
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Linkage to strain gauge & chart recorder
Reciprocating sample stage
Lower half of friction couple
Upper half of friction couple
Rigid connection to rotating diskSide View
Friction Apparatus
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Ge/Ge Friction Expt: Unisol control
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Ge/Ge Friction Experiments: saline control
(results at different stages of experiment)
saline only
more saline added
load increased
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again
after saline rinse
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Ge/Tissue Friction Expt: Unisol only
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Ge/HUVG Friction Experiments: saline control
(results at different stages of experiment)
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Tissue/Tissue Friction Experiments: Unisol Control
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HUVG/HUVG Friction Experiments: saline control
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HUVG Control (no friction)
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HUVG-on-HUVG: Saline Control
Note: this EDXray spectrum is representative of all EDXray spectra for all samples
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Tissue/Tissue Friction Experiments: FID104706
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HUVG/HUVG Friction Experiments: experimental
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in‐vitro study of “artificial tears” lubricity [3 formulations]
Meyer, et al. (2006) J Adhesion 82:607Meyer, et al. (2007) J Biomed Mater Res 82B:74Baier, et al. (2007) Trans Soc Biomaterials XXX, abst#124
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Ge-on-GeA B C
Tissue-on-GeA B C
Tissue-on-Tissue A B C
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Tissue/Cornea Friction Experiment: FID104706
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HUVG/Cornea Friction Experiments: experimental
(results at different stages of experiment)
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Observation:
HUVG/cornea condition: good mimic of healthy eye,
but, range of coefficients of frictionis very narrow
can it discriminate amongdifferent lubricating fluids?
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Obstacles to continuing with HUVG:
• Diminishing & aging in-house HUVG supply
• HUVG thickness can vary
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Obstacles to continuing with HUVG:
• Diminishing & aging in-house HUVG supply
• HUVG thickness can vary
• HUVG no longer commercially available
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new model tissue:- bovine pericardium [PeriGuard, Synovis Surgical]
- very flat, quite smooth
- easy to handle
- tough, elastic
- much thinner than the HUVG
- cGMP process quality controlled for human implantation
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Linkage to strain gauge & chart recorder
Reciprocating sample stage
Lower half of friction couple
Rigid connection
Top View
Rotating disk
Linkage to strain gauge & chart recorder
Reciprocating sample stage
Lower half of friction couple
Upper half of friction couple
Rigid connection to rotating diskSide View
contact lens
tissue
apparatus conditions:-- “blink” rate – approx. 30 full cycles/min-- velocity – approx. 2.5 cm/sec-- pressure – approx. 8 kPa
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baseline post-soak
Unisol additions
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Lessons Learned:
- Use reliable source of tissue
- Modeling most interfacial properties of thecornea & conjunctiva can be too limiting
- Control tissue hydration
- Obtain tissue “baseline” data for each experiment
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O2Optix-on-pericardium (experimental, 30sec soak, 1st replicate)
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Unisol baseline O2Op after 30sec soak 50ul Unisol added 50ul formulation added
Example of lens‐on‐tissue experimental sequence
addition of saline to partially desiccated tissue temporarily restores low coefficient of friction
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PureVision-on-Pericardium (benchmark product, 30sec soak, 2nd replicate)
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Unisol baseline PV after 30sec soak 50ul Unisol added 50ul CompleteMoisturePlus addedbenchmark prod. added
friction increase due to addition of saline
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Acuvue Advance-on-pericardium (experimental, 30sec soak, 1st replicate)
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Unisol baseline AcAd after 30sec soak 50ul Unisol added 50ul formulation added
Example of experimental sequence
slight friction increase due to addition of saline
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Biosurface Friction & Lubrication
Restoration of Function in-vivo
- Filling space with the “right” material
- Getting the “right” surface properties
- Consideration of interface remodeling