Friction Measurements on Contact Lenses in Their

Tribol Lett (2011) 44:387–397
DOI 10.1007/s11249-011-9856-9
METHODS PAPER
Friction Measurements on Contact Lenses in Their Operating
Environment
M. Roba • E. G. Duncan • G. A. Hill
N. D. Spencer • S. G. P. Tosatti
•
Received: 14 April 2011 / Accepted: 5 September 2011 / Published online: 17 September 2011
Springer Science+Business Media, LLC 2011
Abstract An important issue concerning the use of soft
contact lenses is comfort, which, among other factors, has
been related to the level of friction between the anterior
side of the lens and the inner eyelid. Although several
studies have been carried out to investigate the frictional
properties of contact lenses, these have not taken the
physiological environment of the eye into account. In use,
lenses are in contact with proteins present in tears, with
corneal cells and with the palpebral conjunctiva (clear
membrane on inner eyelid). The focus of this study was to
establish a biologically relevant measurement protocol for
the investigation of friction of contact lenses that would
mimic the eye’s physiological environment. By optimizing
parameters such as the composition of the friction counter
surface, the lubricant solution, the normal load and the
velocity, an ideal protocol and setup for microtribological
testing could be established and used to perform a comparative study of various commercially available soft
contact lenses.
M. Roba E. G. Duncan G. A. Hill S. G. P. Tosatti (&)
SuSoS AG, Lagerstrasse 14, CH-8006 Duebendorf, Switzerland
e-mail: samuele.tosatti@susos.com
G. A. Hill N. D. Spencer
Department of Materials, Laboratory for Surface Science and
Technology, ETH Zurich, Wolfgang-Pauli-Strasse 10,
CH-8093 Zurich, Switzerland
G. A. Hill
VISTAKON, A Division of Johnson & Johnson Vision Care Inc,
Jacksonville, FL, USA
G. A. Hill
GHBiomaterials, LLC 1918 Hickory Lane, Atlantic Beach,
FL 32233, USA
Keywords Contact lens Microtribometer Mucin Friction Hydrogel
1 Introduction
The use of soft contact lenses is extremely widespread [1].
However, issues concerning end-of-day comfort still exist
[2]. Discomfort caused by contact lenses has been related
to several factors such as dryness, protein adsorption,
physiological factors and friction occurring during the
blinking process, especially between the anterior side of the
lens and the inner eyelid [3–8]. Clinical tests have been
performed to understand and improve comfort, and have
centered on the trial of different lens materials and ophthalmic solutions [9–13]. The dryness issue has been
investigated by studies of the incorporation of wetting
agents into contact lenses [8] and on the effect of water
content of the lens [14]. A few studies have been carried
out to investigate friction of the lens [4–7]. Blinking is the
primary physiological contributor to the forces exerted on
contact lenses and their consequent motion. Typical values
of contact pressure are in the range of 3–5 kPa, sliding
speed being around 12 cm/s [15, 16]. The friction studies
carried out to date have been based on these reported
values, despite the practical difficulties in achieving them
because of resolution limits of the microtribometers used.
However, the physiological environment has never been
taken into account. Rennie et al. [4] investigated the friction between glass and a contact lens by positioning the
lens taken from borate buffer on a special holder, without
additional lubricant being added. Nairn and Jiang [5]
looked at friction, for both the anterior and posterior side of
contact lenses, against polymethylmethacrylate (pMMA)
and polyhydroxyethylmethacrylate (pHEMA) counter
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388
surfaces, using ophthalmic solutions as lubricants. Dunn
et al. [7] carried out studies on friction between the anterior
side of a contact lens and corneal epithelial cells, while
Dong and Haugstad [17] looked at friction and adhesion of
polyvinylalcohol (PVA) contact lenses by scanning probe
microscopy (SPM). Lydon et al. [18] studied the coefficient
of friction (CoF) for contact lens versus glass and polyethylene (PE) and, lastly, Ngai et al. [6] investigated friction for a contact lens versus glass system in the presence
of saline solution. Interestingly, they also considered the
possible effect of protein adsorption from the tear film by
incubating some of their lenses in a lysozyme and albumin
solution. Apart from the work performed by Ngai et al.,
these studies generally did not take physiological aspects,
such as the eyelid counter surface and the presence of a tear
film, into account. Under the physiological conditions on
the eye surface, the contact lens is in contact with proteins
present in tears and the anterior side of the palpebral
conjunctiva (inner eyelid) [7].
This study has shown that measuring contact lens frictional properties without taking the physiological environment into account can lead to misleading results. It is,
therefore, important to perform such measurements under
biologically relevant conditions. Friction tests on a glass
versus contact lens system were performed by means of a
microtribometer. The measurement protocol was optimized
by varying the functionalization of the glass counter surface, with the aim of mimicking the on-eye environment.
A comparative study was carried out by measuring the
coefficient of friction of several commercially available
contact lenses using the newly developed measurement
protocol.
Tribol Lett (2011) 44:387–397
Fig. 1 Silicone-rubber cover (left hand side) and PMMA ring (right
hand side). Silicone cover was cast to perfectly match the geometry of
the Teflon chamber, rounded holder and contact lens system. A hole
was made on the top part of the silicone cover to allow the contact
lens anterior surface to be exhibited
2 Materials and Methods
2.1 Instrument and Setup
Friction tests were performed with a microtribometer
(Basalt Must, Tetra, Germany). Cantilevers (Tetra,
Germany) with different ranges of spring stiffness (N/m)
were used: kn = 23, kt = 23, ±10% and kn = 15, kt =
15, ±10% (kn is the normal force spring constant and kt is
the tangential force spring constant). The contact lens was
placed inside a Teflon chamber on top of a sand-blasted
rounded plastic holder (cyclo olefin polymer, Johnson &
Johnson Vision Care Inc., USA), matching the internal
radius of curvature of the lens, and was held in position by
a cast silicone-rubber cover (polyvinylsiloxane, Provil
Novo, Germany) and plastic ring (poly(methyl methacrylate), PMMA) (Figs. 1 and 2). Silicone cover and PMMA
ring were screwed to the Teflon chamber by two screws
placed at 180 to one another. The anterior surface of the
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Fig. 2 Experimental setup for tribological tests: The contact lens is
placed on a rounded holder inside a Teflon chamber. The green
silicone-rubber cover and PMMA ring that hold the lens in position
are screwed into the Teflon chamber
lens was facing upward. The counter surface consisted of a
functionalized 5-mm diameter glass disk (cover glass,
Thermo Scientific, Germany). Functionalization protocols
are described in the following. A 6-mm long glass rod was
glued onto the tip of the tribometer cantilever. In turn, the
functionalized glass disk was glued to the glass rod, being
the latter as centered as possible (Fig. 3). Gluing was
Tribol Lett (2011) 44:387–397
389
covered with lubricant solution during the friction test.
Figure 5 shows an example of contact lens and counter
surface mounted in the microtribometer. Contact area and
pressure between the flat glass counter surface and the soft
contact lens (Table 1) were estimated using the Hertzian
contact model as described by Chaudhri and Yoffe [19].
They related the radius of the contact area ah to the
mechanical properties of the materials by
Fig. 3 Example of cantilever modified for friction experiment: The
glass rod is glued to the cantilever tip; second, a glass disk is glued on
the other end of the glass rod
a3h ¼ RP½ð1 m21 Þ=ðE1 Þ þ ð1 m22 Þ=ðE2 Þ
ð1Þ
R being the radius of the contact lens, P the load, mi
Poisson’s ratio and Ei Youngs’s modulus.
French reported the Young’s modulus for a series of
contact lenses materials [20]. For the purposes of this
publication, values of contact area and pressure were
estimated for the contact lens materials with the highest
and lowest moduli. These materials are:
ACUVUE
NIGHT & DAY
Fig. 4 Measurement setup: The functionalized glass disk, attached to
the cantilever through a glass rod, is gently lowered until contact with
the lens occurs. Arrows in the drawing indicate normal force and
lateral force directions
performed using a cyanoacrylate-based glue (UHU GmbH
& Co. KG, Germany). The cantilever was then mounted in
the tribometer.
Just before the test, the cantilever was slowly lowered so
that the glass disk approaches the center of the contact lens
surface (determined by eye) (Fig. 4). The contact lens was
0.3 MPa
1.5 MPa
ACUVUE brand contact lens is a cast-molded polyHEMA contact lens manufactured by VISTAKON, a
division of Johnson & Johnson Vision Care Inc., Jacksonville, FL, USA. The water content is around 58%. Focus
Night & Day is a cast-molded silicone hydrogel contact
lens manufactured by CIBA VISION. It has a water
content of about 24% and has a plasma-treated surface.
Poisson’s ratio has not been reported for these materials.
A value of 0.3 was used in the calculations. The values for
the glass counter surface are not important for this analysis.
The Young’s modulus is much greater than that of the
contact lenses; therefore, 1/E becomes negligible in Eq. 1.
2.1.1 Sample Preparation
Contact lenses were removed from their packaging
immediately before the test and installed on the sample
holder, which had previously been cleaned with water and
Table 1 Contact area and contact pressure estimated values for ACUVUE and NIGHT & DAY pressing against a flat glass surface
Normal force (mN)
Contact area (mm2)
ACUVUE
Contact pressure (kPa)
NIGHT & DAY
ACUVUE
NIGHT & DAY
2.50E - 01
5.00E - 01
0.093
0.15
0.032
0.057
2.7
3.4
7.8
9.5
1.00E ? 00
0.24
0.081
4.2
12.4
2.00E ? 00
0.37
0.13
5.4
15.6
3.00E ? 00
0.49
0.17
6.1
17.9
4.00E ? 00
0.59
0.20
6.7
19.7
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390
Tribol Lett (2011) 44:387–397
Table 2 Advancing contact angle measurements for treated glass
surfaces and ellipsometry thickness measurements for SiO2 model
surfaces
Treatment
Contact angle ()
Adlayer thickness (nm)
Plasma
\10
n/a (substrate base)
Hexamethyldisilazane
[75
0.25 (±0.04)
Mucin
\45
1.2 (±0.1)
Adlayer thickness measurements were performed on two separate
occasions with a total of six measurements
Fig. 5 Contact lens and cantilever mounted in the microtribometer.
The cantilever is slowly lowered until the glass disk is in contact with
the contact lens
detergent (1:1) (hydrochloric acid 300 mmol/L, detergent
1%, Cobas Integra, Roche, Switzerland) and wetted with
lubricant solution. Excess storage solution (from lens
packet) trapped between the lens and the holder was
avoided by gently tapping the edge of the lens on a clean
surface. The contact lens was immediately covered with
lubricant solution to avoid drying.
2.1.2 Counter Surface Preparation
Before fixation onto the cantilevers, glass disks were oxygen
plasma cleaned for 2 min (Diener Electronic, Germany) and
hydrophobized with hexamethyldisilazane (Alfa Aesar,
Germany) from the gas phase for 30 min in a vacuum desiccator. To assess successful hydrophobization, the dynamic
water contact angle ([75) was measured on hydrophobized glass model surfaces. Contact angle (advancing
(wCAadvancing) measurements were performed with a Krüss
contact angle–measuring system (G2/G40 2.05-D, Krüss
GmbH, Germany) with a drop speed of 15 lL/min. A movie
with 100 images was recorded for the advancing contact angle
and the analysis was carried out by means of the tangent
method 2 routine of the Krüss Drop-Shape Analysis
program (DSA version 1.80.0.2 for Windows 9x/NT4/2000,
1997-2002 KRUESS). Hydrophobized glass disks were
mounted on the cantilever as previously described. Immediately before the friction tests, the glass disk mounted on the
cantilever was incubated in a 1 mg/mL mucin solution
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(from bovine submaxillary glands type I–S, Sigma-Aldrich,
Germany) in HEPES 1 (10-mM N-2-hydroxyethylpiperazineTM
N-2 ethanesulfonic acid, pH 7.4, BDH , UK) for 30 min and
rinsed with ultra-pure water to remove any non-adsorbed
species. Additionally, hexamethyldisilazane and mucin layer
thickness in the dry state were evaluated by ellipsometry (J.A.
Woollam Co., Inc., Ellipsometry Solutions, USA) on oxidized
silicon wafer model surfaces. The surface modification protocol applied to the silicon wafers was the same as for glass
model surfaces, with additional initial pre-cleaning with toluene and isopropanol. Adlayer thickness measurements were
conducted under ambient conditions at an angle of incidence
of 70 with respect to the surface normal, averaging 50 measurements at each point. The spectral range was 370–995 nm.
Data were fitted with the WVASE32 analysis software using a
three-layer model (Si/SiO2/Cauchy), where Si was assumed to
be constant for all wafers (1 mm). The SiO2 layer was fitted
with the SiO2 model before hydrophobization with silanes and
mucin adsorption; the silane and mucin layers were fitted
using the Cauchy (organic) layer model. The dry film thickness of the silane layer and of the mucin layer was assumed to
have a refractive index of 1.45. Characterization results are
shown in Table 2.
To mimic the surface of the eyelid, glass disks with different functionalizations were evaluated: oxygen plasma
cleaning (hydrophilic), silanization (hydrophobic) and protein or polymer attachment. Proteins used were fibrinogen
(from bovine plasma, fraction I type I–S, Sigma-Aldrich,
Germany) and lysozyme (AppliChem, Germany), whereas
poly(L-lysine)-graft-poly(ethylene glycol) (PLL(20)-g[3.5]PEG(2), SuSoS AG, Switzerland) was used for polymer
coating. Plasma cleaning and silanization were carried out
as described above. Fibrinogen and lysozyme coatings were
achieved by immersing plasma-cleaned glass disks, already
glued onto the cantilevers, in 1.5 mg/mL fibrinogen in
phosphate-buffered saline (PBS) or 1 mg/mL lysozyme in
HEPES 1 for 30 min. Finally, PLL-g-PEG functionalization
was performed by immersing plasma-cleaned cantilevermounted glass disks into 0.1 mg/mL PLL-g-PEG solution in
HEPES 1 for 30 min. After incubation, the disks were rinsed
with ultra-pure water and dried with nitrogen. Results are
discussed in Sect. 3.
Tribol Lett (2011) 44:387–397
391
2.1.3 Lubricant Solution
3 Results
A 0.9% sodium chloride solution with a borate buffer
(pH * 7.4) was used as a control (packing solution [PS]
supplied by Johnson & Johnson Vision Care Inc. [JJVCI]).
As a lubricant solution, a tear-mimicking solution containing PS plus serum (Precinorm U, Roche, Germany)
(20:1) plus lysozyme (5 mg/mL) was used to perform
comparative friction tests described in Sect. 3.
3.1 Optimization Measurements
2.1.4 Experimental Conditions
All experiments for the optimization step were carried out
TM
using 1•DAY ACUVUE and 1•DAY ACUVUE MOIST
brand contact lenses. In particular, the influence of sliding
cycles and velocity was determined using tear-mimicking
solution as a lubricant and mucin-coated silanized glass disks
as counter surfaces. The effect of glass disk functionalization
to mimic the eyelid was evaluated using packing solution as
lubricant. The comparative study was performed when a
model to mimic the natural eye environment was established:
All contact lenses were tested in tear-mimicking solution
against mucin-coated silanized glass disks.
2.1.5 Measurement Program
The measurement program used for the optimization step
and the comparative friction tests involved applying three
sets of seven normal forces varying from 0.25 to 5 mN.
Two cycles, backward and forward, were recorded for each
target normal force value. Only data points obtained from
the second cycle were recorded and subsequently analyzed.
Ageing was simulated by performing 50 additional cycles
at 2-mN normal force between each set of seven normal
forces. In detail, the measurement protocol consisted of the
following steps: measurement of coefficient of friction
(CoF) with seven normal forces, 50 ageing cycles, measurement of CoF with seven normal forces, 50 ageing
cycles to give a total of 100 ageing cycles and again a final
CoF determination. The measured stroke length was 1 mm
and the sliding speed was 0.1 mm/s. During optimization
of the measurement protocol, 1 and 10 mm/s sliding speeds
were also investigated. For data processing, lateral and
experimentally determined normal force values were calculated by averaging 20 data points at around 0.5-mm
distance, and then plotted in a graph. Trace and retrace
curves obtained in this way were averaged and a coefficient
of friction was determined from the slope.
2.1.6 Contact Lenses
The lenses used in this study are listed in Table 3. The
lenses were supplied by JJVCI.
3.1.1 Effect of Number of Sliding Cycles
Preliminary tests run on 1•DAY ACUVUE and 1•DAY
TM
ACUVUE MOIST brand contact lenses to investigate
the influence of ageing on friction revealed that CoF at 100
cycles at a normal load of 2 mN (maximum ageing) is
equal to or not significantly different than CoF at 0 cycles
(no ageing) (Fig. 6), indicating that there is no change in
the coefficient of friction for either lens type within the
limits of this experimental protocol. 1•DAY ACUVUE
TM
MOIST contact lenses exhibited significantly lower CoF
than 1•DAY ACUVUE contact lenses. Significant differences in the results were evaluated by means of statistical analysis (p value \0.05).
3.1.2 Effect of Sliding Velocity
CoF was shown to significantly increase (p value \0.05)
with sliding velocity for both 1•DAY ACUVUE and
TM
1•DAY ACUVUE MOIST
brand contact lenses
(Fig. 7). Above 1 mm/s, distinguishing between the two
lenses investigated was difficult because of high and
overlapping standard deviations.
3.1.3 Choice of Counter Surface
The CoF was measured for different counter surfaces in
packing solution. Results show that the lowest CoF was
obtained for PLL-g-PEG-coated glass disks, whereas
higher CoF values were obtained for hydrophilic and
lysozyme-coated glass disks (Fig. 8). In the case of
hydrophilic glass disks, CoF for 1•DAY ACUVUE
TM
MOIST
was significantly higher than for 1•DAY
ACUVUE. In all other cases where there is a significant
TM
difference, 1•DAY ACUVUE MOIST contact lenses
exhibited lower CoF than 1•DAY ACUVUE. No post
measurement testing was performed on the mucin-coated
counter surface. As the coefficient of friction did not
change between the first ramp (0 cycles) and the final ramp
(after 100 cycles), the lens surface and counter surface
were deemed unchanged during the experiment.
3.2 Comparative Study
Various commercially available daily disposable and
reusable contact lenses were tested using the optimized
protocol, tear-mimicking solution as lubricant and mucincoated silanized glass as counter surface. Friction
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Tribol Lett (2011) 44:387–397
Table 3 Lenses used
Daily disposable lenses
Name of contact lens
TM
Manufacture
Silicone hydrogel?
Surface treatment?
Sauflon
No*
Yes
No
Johnson and Johnson
Vision Care, Inc.
Yes
No
No
(narafilcon B)
Johnson and Johnson
Vision Care, Inc.
Yes
Yes
No
(narafilcon A)
Johnson and Johnson
Vision Care, Inc.
Yes
Yes
No
1•DAY ACUVUE
Johnson and Johnson
Vision Care, Inc.
No
No
No
Focus DAILIES
TM
All Day Comfort
CibaVision
Proclear 1 day
CooperCooperVision
CLARITI
1 day
Contains PVP?
1•DAY ACUVUE MOIST
1•DAY ACUVUE TruEye
TM
1•DAY ACUVUE TruEye
TM
ClearSight
TM
1 Day
DAILIES AquaComfort Plus
No
No
No
No
No
No
CooperVision
No
No
No
CibaVision
No
No
No
Reuseable lenses
AVAIRA
CooperVision
No*
Yes
No
ACUVUE OASYS
Johnson and Johnson
Vision Care, Inc.
Yes
Yes
No
TM
CLARITI
Sauflon
No*
Yes
No
ACUVUE ADVANCE PLUS
Johnson and Johnson
Vision Care, Inc.
Yes
Yes
No
ACUVUE ADVANCE
Johnson and Johnson
Vision Care, Inc.
Yes
Yes
No
Biofinity
CooperVision
No*
Yes
No
ACUVUE
Johnson and Johnson
Vision Care, Inc.
No
No
No
[AIR OPTIX] NIGHT & DAY AQUA
CibaVision
No
Yes
Yes (methane air plasma)
PremiO
AIR OPTIX AQUA
Menicon
CibaVision
No
No
No
Yes
Yes
Yes (methane air plasma)
CibaVision
No
Yes
Yes (methane air plasma)
CibaVision
No
Yes
Yes (methane air plasma)
BAUSCH?LOMB
No
Yes
Yes (oxygen plasma)
TM
AIR OPTIX (formerly O2 OPTIX )
NIGHT & DAY
PureVision
TM
Optima
38
BAUSCH?LOMB
No
No
No
SOFLENS
BAUSCH?LOMB
No
No
No
* Indicates no PVP is reported in formulation but maybe formed in situ
coefficient was evaluated before (0 cycles) and after ageing
(50 and 100 cycles) (Tables 4 and 5; Figs. 9 and 10).
4 Discussion
The goal in the development of a model is to be able to use
the in vitro data to predict in vivo performance. To correlate clinical contact lens performance with the model
results and to evaluate potential sources of comfort-driven
performance, possible lens discontinuation and adverse
effects to the coefficient of friction, it is desirable to
123
measure the CoF of contact lenses in the most biologically
relevant conditions.
In the physiological environment, contact lenses are in
contact with proteins present in tears, with corneal cells (on
the posterior side) and with the palpebral conjunctiva (on
the anterior side). Most studies carried out so far on the
tribology of contact lenses have not taken such physiological conditions into account [4–6, 18]. The coefficient of
friction is a property of the sliding pair; therefore, the
determination of the coefficient of friction using different
counter surfaces would be expected to produce different
results. When comparing the results found in this study
Tribol Lett (2011) 44:387–397
Fig. 6 Effect of number of sliding cycles (ageing) on friction using
TM
1•DAY ACUVUE and 1•DAY ACUVUE MOIST brand contact
lenses versus hydrophobized mucin-coated glass disks. Tear-mimicking solution was used as lubricant
Fig. 7 Graph showing the effect of sliding velocity on friction for
TM
1•DAY ACUVUE and 1•DAY ACUVUE MOIST brand contact
lenses versus hydrophobized mucin-coated glass disks in tearmimicking solution. Error bars are standard deviation of three repeat
experiments
with those from previous studies, the following observations can be made. Rennie et al. [4] evaluated Etafilcon A
using an untreated spherical glass counter surface at normal
forces of 3–20 mN. They report that the coefficient of
friction obeys a sliding speed–dependent power law. Using
their data, a coefficient of friction of about 0.15 would be
predicted for Etafilcon A. Our results, on the other hand,
show a coefficient of friction of 0.02–0.09 that was
invariant with normal force. Nairn and Jiang [5] evaluated
the coefficient of friction for SeeQuence (Polymacon)
brand contact lenses using a pin-on-disk tribometer. They
report coefficients of friction of 0.05–0.3 in saline. Our
TM
results for Optima 38, a Polymacon lens produced by the
same manufacture, were 0.55. Finally, Ngai et al. [6]
reported the coefficients of friction for Lotrafilcon A
and Polymacon A to be 0.27 (calculated from normal and
tangential forces in their study). Our results were 0.38 and
393
Fig. 8 Mimicking the eyelid: CoF at 100 cycles for various
functionalized glass counter surfaces measured in packing solution.
Sliding speed was 0.1 mm/s and normal forces ranged from 0.25 to
5 mN. Error bars are standard deviation of three repeat experiments
0.55, respectively. These variations show the need for a
standardized method for the measurement of the coefficient
of friction for contact lenses. Protein adsorption on contact
lenses was investigated by Garrett and Milthorpe [21] and
by Ngai et al. [6], who also performed friction tests. The
major finding was that CoF decreased at an early stage of
protein deposition, but it was hypothesized to increase in
time because of protein denaturation. Protein deposition on
contact lenses was also found to be related to comfort,
vision and increased inflammatory responses, as reviewed
by Jones [22]. In this study, the physiology of the eye was
taken into account by establishing a measuring protocol
that mimics the natural environment. For this optimization
step, not only parameters such as ageing and sliding speed
were considered but also the role of eyelid and tears. The
eyelid surface was mimicked by the counter surface, which
was required to meet several conditions. It had to be hard
and flat so that any lens deformation would be accommodated on contact. This would yield a constant contact area
and eliminate the modeling required when using a spherical
counter surface [4]. For this study, a hard–soft contact pair
was chosen rather than the soft–soft contact pair found in
the ocular environment. The hard–soft pair was chosen
because the goal of this study was to provide a method to
conveniently compare materials rather than to mimic the
eye’s mechanics exactly. It was felt that the hard glass
surface would produce more reproducible results and has
less experimental difficulty than a soft counter surface.
Additionally, it was required not to adhere components
from the contact lens and to provide a surface representative on the ocular environment. The various glass functionalization procedures that were investigated were
deliberately chosen with the aim of mimicking the eyelid.
PLL-g-PEG, well known for its lubricating properties in an
aqueous environment [23], gave rise to the lowest CoF,
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Tribol Lett (2011) 44:387–397
Table 4 Coefficients of friction measured on daily disposable contact lenses at 0.1 mm/s, arranged in ascending order of CoF at 100 cycles
Name of contact lens
TM
CLARITI
1 day
1•DAY ACUVUE MOIST
TM
0 Cycles
Standard
deviation (±)
50 Cycles
Standard
deviation (±)
100 Cycles
Standard
deviation (±)
0.017
0.001
0.014
0.002
0.017
0.004
0.022
0.010
0.019
0.006
0.024
0.003
(narafilcon B)
0.032
0.013
0.030
0.006
0.034
0.009
(narafilcon A)
0.031
0.028
0.035
0.021
0.037
0.019
0.046
0.023
0.024
0.004
0.047
0.029
0.122
0.021
0.113
0.016
0.091
0.009
Proclear 1 day
TM
ClearSight 1 Day
0.090
0.078
0.033
0.018
0.081
0.141
0.039
0.036
0.125
0.181
0.073
0.037
DAILIES AquaComfort Plus
0.344
0.000
0.474
0.012
0.424
0.051
1•DAY ACUVUE TruEye
TM
1•DAY ACUVUE TruEye
TM
1•DAY ACUVUE
TM
Focus DAILIES All day comfort
Table 5 Coefficients of friction measured on reusable contact lenses at 0.1 mm/s, arranged in ascending order of CoF at 100 cycles
Name of contact lens
0 Cycles
Standard
deviation (±)
50 Cycles
Standard
deviation (±)
100 Cycles
Standard
deviation (±)
AVAIRA
0.011
0.002
0.018
0.003
0.018
0.001
0.016
0.014
0.024
0.018
0.018
0.015
0.034
0.014
0.023
0.005
0.022
0.009
0.022
0.005
0.021
0.001
0.024
0.004
0.029
0.002
0.028
0.002
0.042
0.010
ACUVUE OASYS
TM
TM
CLARITI
ACUVUE ADVANCE
TM
ACUVUE ADVANCE
TM
PLUS
Biofinity
0.033
0.009
0.057
0.004
0.050
0.002
0.093
0.108
0.023
0.051
0.069
0.110
0.010
0.093
0.090
0.166
0.010
0.058
PremiO
0.177
0.010
0.207
0.023
0.176
0.012
AIR OPTIX AQUA
0.178
0.061
0.187
0.045
0.222
0.073
AIR OPTIX (formerly O2 OPTIX )
0.343
0.038
0.361
0.048
0.292
0.031
NIGHT & DAY
0.391
0.081
0.394
0.040
0.382
0.027
0.415
0.037
0.443
0.041
0.423
0.032
38
0.203
0.045
0.587
0.010
0.551
0.002
SOFLENS
0.562
0.063
0.542
0.033
0.513
0.017
ACUVUE
[AIR OPTIX] NIGHT & DAY AQUA
TM
PureVision
TM
Optima
whereas hydrophilic glass led to an unusual behavior
(Fig. 8). Against a hydrophilic counter surface, 1•DAY
TM
ACUVUE MOIST brand contact lenses exhibited a
higher friction coefficient than 1•DAY ACUVUE despite
the presence of polyvinylpyrrolidone (PVP). This is
inconsistent with the tactile sensation felt when rubbing
TM
1•DAY ACUVUE MOIST lens surface between the
fingers. 1•DAY ACUVUE brand does not feel as slippery. Belyakova et al. [24] reported a strong attraction
between PVP and silicon dioxide. The higher coefficient of
TM
friction observed for 1•DAY ACUVUE MOIST brand
lens is attributed to an attractive interaction between PVP
TM
in the 1•DAY ACUVUE MOIST brand lens matrix and
hydrophilic glass surface. Therefore, the high tangential
force results from adsorption of the PVP on the counter
surface rather than from an intrinsically high coefficient of
TM
friction for 1•DAY ACUVUE MOIST brand. In the
case of PLL-g-PEG, no difference in terms of friction could
123
be observed between 1•DAY ACUVUE and 1•DAY
TM
ACUVUE MOIST brand, most likely masked by the
excellent lubricating properties of PEG. For these reasons,
the first two functionalization protocols were discarded. All
other protocols gave rise to the opposite trend, as expected.
Fibrinogen was chosen to have a more biologically oriented counter surface and because it readily adsorbs on
glass. The use of lysozyme went one step further, as it is
the major component of tears. Both proteins lead to a
clearly distinguishable behavior of 1•DAY ACUVUE
TM
and 1•DAY ACUVUE MOIST brand, although friction
coefficients were relatively high. Mucin, a natural lubricant
additive, became the surface coating of choice for performing friction tests on glass. It was chosen as it is a
glycoprotein found in the mucous membrane that constitutes the palpebral conjunctiva, and is therefore in contact
with lenses in their operating environment. In particular,
mucin from bovine submaxillary glands was used as it is a
Tribol Lett (2011) 44:387–397
395
Fig. 9 Comparative study: CoF
for various commercially
available daily disposable
contact lenses tested using the
optimized protocol, which is
tear-mimicking solution as
lubricant and mucin-coated
silanized glass as counter
surface. Contact lenses are
represented in ascending (mean
of 0, 50 and 100 cycles) friction
coefficient order, according to
Table 4. Error bars are standard
deviation of three repeat
experiments
Fig. 10 Comparative study:
CoF for various commercially
available reusable contact lenses
tested using the optimized
protocol, which is tearmimicking solution as lubricant
and mucin-coated silanized
glass as counter surface. Contact
lenses are represented in
ascending friction coefficient
order, according to Table 5.
Error bars are standard
deviation of three repeat
experiments
readily available mucin that functions at pH 7, as do
mucins in the eye. Hydrophobization of glass disks before
incubation in mucin solution was performed to enhance
mucin adsorption. The major finding with these experiments is that the CoF can significantly vary according to
the measuring protocol.
Another important aspect of the physiology of the eye is
the presence of tears, which play a fundamental role in
lubrication. For the evaluation of different contact lens
materials, the use of a lubrication solution that is closely
related to human tears is preferred. Human tears contain a
protein array that is similar to serum but is lacking lactoferrin, lysozyme and tear-specific prealbumin [25]. Yoon
et al. [26] reported the use of diluted antilogous serum as
eye drops for severely diseased patients. In their studies,
they diluted the serum 1:20 with saline, and did not add any
of the tear-specific proteins. Hill et al. [27] described a
synthetic tear fluid of 5% blood plasma supplemented with
lysozyme at 4.5 g/L and lactoferrin at 1.7 g/L. The tearmimicking fluid used in this study was modified increasing
the lysozyme to 5 g/L and eliminating the lactoferrin.
Lipids are an important part of the dacruon [28] and
have been shown to adsorb onto contact lenses [29]. Cher
[28] has proposed dacruon as a new description of the tear
film. The historical model describes the tear film as a lipid
layer, an aqueous layer and a mucus layer. The dacruon
model describes it as a continuous concentration gradient
of mucin from the ocular surface to the lipid layer of the
123
396
tear film. Lipids are reported to be floating on the surface of
the dacruon [28], and therefore would not be involved in
the lubrication of the lid on the contact lens unless a break
in the tear film would occur, allowing lipids to adsorb on
the surface of the lens. This eventuality can take place as a
consequence of eye dryness, irritation and on insertion of
the contact lens. Our model does not contemplate such
situations, but rather focuses on mimicking the eyelid–
contact lens system in standard conditions, where the tear
film is intact and lipids are not adsorbing on the contact
lens surface. Moreover, the inclusion of lipids into the tearmimicking fluid results in a heterogeneous lubricating
fluid, which could affect the reproducibility of measurements. For these reasons, lipids were not included in the
described model.
For the determination of our eye-mimicking system,
contact pressure and sliding speed were also taken into
account. Typical in vivo pressure and speed values are
reported to be around 3–5 kPa and 1.2 mm/s, respectively.
We chose normal loads that allow our setup to be in the
contact pressure range described in literature. Contact
pressures for 1•DAY ACUVUE were estimated by using
the Hertzian contact model and were found to be in the
range of 2.6–6.5 kPa for the normal loads used in this study.
The purpose of this model is to evaluate the coefficient
of friction of contact lenses against a realistic counter
surface. The data in Fig. 7 suggest that there is a convergence of the coefficients of friction as the sliding speed is
increased. This is thought to be the result of an increased
importance of hydrodynamic lubrication. However, the
goal is to determine the materials properties, and this can
be achieved by measuring the coefficient of friction in the
boundary–lubrication regime. The sliding speed of
0.1 mm/s, which is one order of magnitude lower than
literature values, was chosen to reduce the possibility of
hydrodynamic effects and to ensure that measurements
were made in the boundary–lubrication region.
Once optimization of the measurement protocol was
achieved, with respect to the physiological environment,
commercially available contact lenses were tested for frictional properties and compared with each other (Figs. 9 and
10). PVP-containing lenses exhibited the lowest friction
coefficients, typically less than 0.050, whereas friction coefficient values for non-PVP-containing lenses ranged from
TM
0.100 to 0.600. The exceptions are Sauflon CLARITI ,
which contains the monomer N-vinylpyrrolidone (NVP),
and AVAIRA, which contains a monomer analogous
TM
to NVP. However, Sauflon (CLARITI ) claims that the
N-vinylpyrrolidone in the formulation homopolymerizes to
form PVP in situ [30]. In the United States Adopted Names
Council (USAN) document for AVAIRA (enfilcon A),
it is reported that the lens contains a homopolymer of
N-etheny-N-methylacetamide, which can homopolymerize
123
Tribol Lett (2011) 44:387–397
to produce a PVP analog in situ. This frictional behavior
was expected because of the lubricating properties of PVP
or its analogs and confirmed the validity of our measurement protocol, which proved to be successful in distinguishing between contact lens frictional properties.
5 Conclusion
Friction tests were run on commercially available contact
lenses with a microtribometer. The measurement protocol
was optimized according to sliding speed, counter surface
and lubricant–solution composition, to achieve a set-up that
mimics the physiological environment. The best combination was found to consist of 0.1 mm/s sliding speed,
mucin-coated glass as a counter surface and a lubricant
based on packing solution containing lysozyme and serum.
Measuring contact lens frictional properties without taking
physiological conditions into account was shown to lead to
different and potentially misleading results, demonstrating
the importance of establishing a broadly valid biologically
relevant measurement protocol.
Acknowledgments This study was supported by VISTAKON, a
division of Johnson & Johnson Vision Care Inc., Jacksonville, FL, USA.
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