An in-vitro Procedure For Evaluation of Lens-on-Tissue Friction...(results at different stages of...
Transcript of An in-vitro Procedure For Evaluation of Lens-on-Tissue Friction...(results at different stages of...
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
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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
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
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ngle
μ γc(mN/m)
PTFE 0.04 18.5PE 0.2 32.0Steel 0.6 50+Steel +stearic acid
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γc
μ and γc
Coefficient of Frictionand Critical Surface Tensiondirectly proportional for dry, synthetic mat’ls
What about …
-- hydrated materials?
-- biological tissues?
-- biological lubricants?
BIOLOGICAL TISSUES
hardsoft
fluid (e.g. blood, saliva, tear film)
osseointegration…thromboresistance……adhesive…cohesive…abhesive…
…load-bearing…nonload-bearing…shear…
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
…
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)
coefficient of friction
hydrodynamic lubrication
boundary lubrication
contacting asperities & adhesion
…making & breaking “bonds”
elastohydrodynamic lubrication
On the molecular scale…
biological fluids…
shear v. adsorbed film properties
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Exposure Time (minutes)
Crit
ical
Sur
face
Ten
sion
(mN
/m)
460 sec-15 sec-1
Whole Saliva Flow over Germanium:Influence of fluid shear on adsorbed film γc
Critical Surface Tensions of Contact Lenses
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FSA SA Hgel2 Hgel4Lens Type
criti
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urfa
ce te
nsio
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N/m
]
pre-exposurecontrolsafter 8-hr wear
Relative Coefficient of Friction (metal-on-metal implant components)
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HEMA HUVG RGP Hi-Shine control
interpositional material or implant treatment
rela
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coef
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Corneal epithelium (human)no fixative, still moist
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surface tension [mN/m]
cosi
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vera
ge c
onta
ct a
ngle
Human Umbilical Vein Graft [HUVG] used in vascular surgery [“Biograft”]
Human Umbilical Vein Graft with mechanical damage
elastic lamina
underlying collagenousfibers
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
Ge/Ge Friction Expt: Unisol control
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coef
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Unisol only more Unisol after Unisol rinse
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rinse
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ith U
niso
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Ge/Ge Friction Experiments: saline control
(results at different stages of experiment)
saline only
more saline added
load increased
load increased
again
after saline rinse
Ge/Tissue Friction Expt: Unisol only
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coef
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Unisol only Unisol after Unisol rinse
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Ge/HUVG Friction Experiments: saline control
(results at different stages of experiment)
Tissue/Tissue Friction Experiments: Unisol Control
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HUVG/HUVG Friction Experiments: saline control
HUVG Control (no friction)
HUVG-on-HUVG: Saline Control
Note: this EDXray spectrum is representative of all EDXray spectra for all samples
Tissue/Tissue Friction Experiments: FID104706
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ID10
4706
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HUVG/HUVG Friction Experiments: experimental
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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coef
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Ge-on-GeA B C
Tissue-on-GeA B C
Tissue-on-Tissue A B C
Tissue/Cornea Friction Experiment: FID104706
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coef
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Uni
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FID
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HUVG/Cornea Friction Experiments: experimental
(results at different stages of experiment)
Observation:
HUVG/cornea condition: good mimic of healthy eye,
but, range of coefficients of frictionis very narrow
can it discriminate amongdifferent lubricating fluids?
Obstacles to continuing with HUVG:
• Diminishing & aging in-house HUVG supply
• HUVG thickness can vary
Obstacles to continuing with HUVG:
• Diminishing & aging in-house HUVG supply
• HUVG thickness can vary
• HUVG no longer commercially available
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
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
baseline post-soak
Unisol additions
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
O2Optix-on-pericardium (experimental, 30sec soak, 1st replicate)
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elapsed friction time (minutes)
coef
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f fric
tion
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
PureVision-on-Pericardium (benchmark product, 30sec soak, 2nd replicate)
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elapsed friction time (minutes)
coef
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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
Acuvue Advance-on-pericardium (experimental, 30sec soak, 1st replicate)
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coef
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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
Biosurface Friction & Lubrication
Restoration of Function in-vivo
- Filling space with the “right” material
- Getting the “right” surface properties
- Consideration of interface remodeling