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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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]

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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

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Sur

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(mN

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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

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urfa

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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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Corneal epithelium (human)no fixative, still moist

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surface tension [mN/m]

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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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Unisol only more Unisol after Unisol rinse

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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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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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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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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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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)

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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

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

Acuvue Advance-on-pericardium (experimental, 30sec soak, 1st replicate)

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elapsed friction time (minutes)

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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