M. TOMI] et al.: LACUNARITY PROPERTIES OF NANOPHOTONIC MATERIALS BASED ON ...
145–151
LACUNARITY PROPERTIES OF NANOPHOTONIC MATERIALS
BASED ON POLY(METHYL METHACRYLATE) FOR CONTACT
LENSES
RAZPOREDITEV PRAZNIN NANOFOTONI^NEGA MATERIALA
NA OSNOVI POLI(METIL METAKRILATA) ZA KONTAKTNE LE^E
Marija Tomi}1, Bo`ica Bojovi}2, Dragomir Stamenkovi}3, Ivana Mileusni}1,
Djuro Koruga1,4
1University of Belgrade, Faculty of Mechanical Engineering, Department of Biomedical Engineering, NanoLab,
Kraljice Marije 16, 11120 Belgrade, Serbia
2University of Belgrade, Faculty of Mechanical Engineering, Department of Production Engineering,
Kraljice Marije 16, 11120 Belgrade, Serbia
3Optix LLC, Oracka 13, 11070 Belgrade, Serbia
4University for Peace established by the United Nations, Department of Biomedical Engineering and Nanomedicine,
ECPD, Terazije 41, 11000 Belgrade, Serbia
marija.m.tomic@gmail.com
Prejem rokopisa – received: 2016-01-17; sprejem za objavo – accepted for publication: 2016-02-05
doi:10.17222/mit.2016.014
The aim was to develop new materials that would, after appropriate machining processes, improve the surface roughness and
wettability of contact lenses. The samples used in this investigation were standard rigid gas-permeable (RGP) SOLEKO contact
lenses, made of poly-MMA-co-siloxy silane methacrylate material (known under the commercial name SP40TM), and its
modifications by adding three nanomaterials: fullerene C60 (designated as SP40-A), fullerol C60(OH)24 (designated as SP40-B)
and methformin hydroxylate fullerene C60(OH)12(OC4N5H10)12 (designated as SP40-C). Both atomic force microscopy (AFM)
and magnetic force microscopy (MFM) were used to measure the topography and gradient of the magnetic field of the
nanophotonic materials and the reference samples. According to the magnetic properties of all the materials yielded by MFM it
can be concluded that adding fullerene and its derivatives certainly reduces the spectrum of the phase shifts angle by almost
50 %, which increases the paramagnetic characteristics of the nanophotonic material. The positive result of nanophotonic
materials characterization is the fact that the roughness parameter values for all of these materials, are lower than those for the
basic material. A surface lacunarity analysis, based on in-house procedures for determining the lacunarity value of contact lens
surfaces, confirms the influence of surface topology on the tear film volume distribution and, consequently, the contact lens’
surface lubrication. The presence of carbon nanomaterials, according to the surface roughness parameters, are improved for
rigid gas-permeable (RGP) contact lenses made from nanophotonic polymer materials compared to those produced from the
basic material.
Keywords: fullerenes, polymer materials, surface roughness, atomic force microscopy, magnetic force microscopy
Namen je bil razviti nov material, ki bi po ustrezni strojni obdelavi zmanj{al hrapavost povr{ine in omo~ljivost kontaktnih le~.
Vzorci, uporabljeni v tej raziskavi so bile standardne toge, za plin prepustne (RGP) SOLEKO kontaktne le~e, narejene iz
poli-MMA-ko-siloksi silan metakrilatnega materiala (s komercialnim imenom poznanega SP40TM) in njihove modifikacije z
dodatkom treh nanomaterialov: fulerena C60 (ozna~enega kot SP40-A), fulerola C60(OH)24 (ozna~enega z SP40-B) in metformin
hidroksilat fulerena C60(OH)12(OC4N5H10)12 (ozna~enega z SP40-C). Uporabljeni so bili: mikroskopija na atomsko silo (AFM),
mikroskopija na magnetno silo (MFM) za merjenje topografije in gradient magnetnega polja nanofotonskih materialov in
referen~nih vzorcev. Skladno z magnetnimi lastnostmi vseh materialov, dobljenih z MFM, je mogo~e zaklju~iti, da dodajanje
fulerenov in njegovih izpeljank, mo~no zmanj{a spekter kotov faznega premika za skoraj 50 %, kar pove~a paramagnetne
zna~ilnosti nanofotonskih materialov. Pozitivni rezultati karakterizacije nanofotonskih materialov so, da so vrednosti parametra
hrapavosti pri vseh teh materialih ni`je kot pri osnovnem materialu. Analiza povr{inske razporeditve praznin, na osnovi doma
razvite tehnologije za dolo~anje vrednosti razporeditve praznin na povr{ini kontaktnih le~, je pokazala vpliv topologije povr{ine
na razporeditev plasti solz in posledi~no na mazanje povr{ine kontaktnih le~. Prisotnost ogljikovih nanomaterialov je glede na
parametre hrapavosti povr{ine, izbolj{ala plinsko propustnost togih, za plin prepustnih (RGP) kontaktnih le~, izdelanih iz
nanofotonskega polimernega materiala, v primerjavi s tistimi izdelanimi iz osnovnega materiala.
Klju~ne besede: fulereni, polimerni materiali, hrapavost povr{ine, mikroskopija na atomsko silo, mikroskopija na magnetno silo
1 INTRODUCTION
The tear film is a liquid layer that covers the cornea
and conjunctiva keeping the surface of the cornea
smooth and making it optically clear. While blinking, the
tear film lubricates the friction area between lids and
ocular surface1. The precorneal tear film thickness,
which is related to dry eye and corneal integrity, is
reported in range from 45 μm2 to 3 μm.3–5 When a rigid
gas-permeable (RGP) contact lens is placed on the eye,
the tear film is divided into two layers, the pre-lens and
post-lens tear film. The importance of tear exchange
behind an RGP contact lens remains an ongoing debate.6
Once a contact lens is inserted, the value of the
precorneal tear film thickness doubles, resulting in a 2–5
μm thickness for pre and post lens tear films. Some
authors7 determined the post lens tear film below 3 μm.
Materiali in tehnologije / Materials and technology 51 (2017) 1, 145–151 145
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
UDK 67.017:620.3:617 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(1)145(2017)
J. L. Creech et al.8 have concluded that tear film
thickness is an important parameter that varies among
individuals. The presence of any contact lens on the front
surface of the eye influences and disturbs tear film
stability. These changes in the tear film are caused by
contact lens design, surface, material and applied
solution for conditioning. The quality of a machined
surface is determined by its roughness, which is
described by the surface topography (i.e., the height
differences between specific points of the surface), as
well as by their exact position in regard to some
reference area. The surface roughness of a contact lens
has various implications, such as comfort, post lens tear
film disruption and tear component deposition.
The aim is to test the response of a material’s surface
roughness quality to retain the tear film on the nano-level
using a gliding-box method for lacunarity analysis.
Surface lacunarity analysis is an innovative approach
based on the work of B. Mandelbrot9, R. Voss10 and R. E.
Plotnick11. B. Mandelbrot introduced the term lacunarity
from the Latin word šlacuna’, which is related to the En-
glish šlake’.9 Lacunarity describes the degree of
šgappiness’ of the fractal and is a complement to fractal
dimension, because fractal dimension measures how
much space is filled, whereas lacunarity measures how
the fractal fills space. Intuitively, more and larger gaps
give a higher lacunarity.12 Although originally developed
for further classification of fractals as a counterpart to
the fractal dimension10, lacunarity application is more
general, as presented and illustrated in R. E. Plotnick.11
Lacunarity analysis is broadly applicable to many data
sets used in natural sciences13,14, as a very useful multi-
scaled method for describing patterns of spatial dispersion.
The fractal analysis can also be applied to biological
samples such as microglia (the brain’s immuno-inflam-
matory cell). This analysis may contribute significantly
to the next steps forward in engineered tissues and 3D
models in neuroscience.15
Taking these issues into account, the idea was to
develop new contact lens materials that would, after an
appropriate lathing and polishing process, improve the
roughness parameters. There is a need for more precise
measurements of the surface roughness as well as for
detecting the physical properties of surfaces. To measure
the topography and gradient of the magnetic field of
nanophotonic materials, as well as of reference samples,
both atomic force microscopy (AFM) and magnetic force
microscopy (MFM) were used.
2 MATERIALS AND METHODS
2.1 Nanophotonic materials
New nanophotonic materials based on fullerene C60
and its derivatives16–18 have been developed for the
production of RGP contact lenses. Fullerenes and their
derivatives show strong electron affinity, acting like
šradical sponges’, and they easily enter addition reac-
tions with nucleophiles. One of the main disadvantages
of fullerenes is their low solubility in water. In order to
make them soluble, they have to be functionalized with
polar groups such as –OH and –COOH. Of all the
water-soluble fullerenes the most important are those
with –OH groups attached, called fullerols or fullerenols,
(nano-C60(OH)24). These fullerene derivatives have
shown promise as drug carriers to bypass the brain and
ocular barriers and have been approved for use in
nano-cosmetics, although fullerols have been found to be
cytotoxic to HaCaT keratinocytes, a transformed epider-
mal human cell line and human dermal fibroblasts.19
The properties of C60 can also be modified by its
incorporation into polymers, making it a material that
can be easily handled and produced. Combining both
systems has led to a wide variety of new materials with
appealing features based on the possibility of tuning their
properties by modifying the chemical nature of the
components or the chemical linkage between them.20 The
easiest method of incorporation of C60 into a polymer is
radical copolymerization, which can produce materials
of widely varied fullerene content, but the degree of
substitution of the fullerene is hard to control, and the
structures are heterogeneous and hard to determine.21
The aforementioned nanomaterials were incorporated
into the standard RGP SOLEKOTM material (Pontecorvo,
Italy) known by the commercial name SP40TM. This
material belongs to the group of polymers known as
poly-MMA-co-siloxy silane methacrylate, whose optical
characteristics are given in Table 1. The polymerization
was made separately on three samples by adding 1 g of
each nanomaterial to the basic Soleko SP40™. There-
fore, three new materials were produced, designated
SP40-A (fullerene C60), SP40-B (fullerol C60(OH)24) and
SP40-C (methformin hydroxylate fullerene
C60(OH)12(OC4N5H10)12). The percentage of nanoma-
terials in the solution was 0.33 % and although they were
not completely dissolved in methylmethacrylate, the
polymerization was homogeneous in all the samples. The
fourth polymerization was conducted without nano-
materials so the regular Soleko SP40™ RGP material
was used as a reference sample. The preparation of the
samples was conducted at the Soleko, Italy laboratories.
Each material was cut into a cylindrical sample of
12.8 mm diameter and 5.2 mm height. The samples were
cut in a row from a rod produced by polymerization,
providing optimal repeating. The contact lenses were
manufactured by using a computer numerical control
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Table 1: Optical characteristics of material SP40
Tabela 1: Opti~ne zna~ilnosti materiala SP40
Optical characteristics Value Test
Transmission in the visible
part of the spectrum >90 % ISO 8599
Transmission in the UV part
of the spectrum
<60 %
(290–330 nm) ISO 8599
Refractive index 1.472±0.003 ISO 9914
(CNC) lathe (Politech 1800 Aspheric-Toric) and po-
lishing as a finishing process.
2.2 Atomic force microscopy and magnetic force mi-
croscopy
Both surfaces of the contact lens can be characterized
by AFM, which allows the characterization of surface to-
pography and surface roughness. AFM images are ob-
tained by scanning a sharp probe across a surface while
monitoring and compiling the tip–sample interactions to
provide an image.22 The atomic force microscope used in
this study was a JSPM-5200 (JEOL, Japan). MFM mode
allows us to obtain the topography of the sample, but on
the other hand provides information about the magnetic
properties (magnetic gradient) of the material. In order to
obtain both topography and magnetic gradient images,
MFM operates in a so-called šlift mode’.23,24 The lift
mode or a two-pass technique is used to separate the ef-
fects of the magnetic and mechanical tip–sample forces.
The probe (cantilever) used in this study was pro-
duced by MikroMasch (Estonia) under the trade name
NCS18/Co-Cr. The length of the cantilever is 230 μm,
and a typical spring constant is 3.5 N/m. The tip
curvature radius is 10 nm, but that is the radius of the
uncoated tip. It is necessary to add a coating of ferro-
magnetic material to provide the magnetic field. This
coating consists of a Co layer about 60 nm thick on the
tip and backside of the probe. The Co coating is pro-
tected from oxidation by a 20-nm Cr layer. This leads to
a total tip radius of 90 nm. All measurements were made
at room temperature (around 23 °C) under the atomic
force microscope’s glass bell to minimize the effects of
temperature fluctuations. In phase imaging, the phase lag
of the cantilever’s oscillation relative to the drive signal
is simultaneously monitored with topography data.
Because the phase imaging highlights the edges and is
not affected by large-scale height differences, it provides
a clearer observation of the fine features, which can be
obscured by rough topography.
The imaging area for RGP contact lenses was 2×2 μm,
256×256 pixels. Images obtained during the AFM/MFM
scanning were analyzed using JEOL SPM Processing
Software and WinSxM.25 The parameters used for
materials comparison are the height difference between
the lowest and the highest point on the topography image
(Sz) and the areal arithmetic mean deviation (Sa).
Parameter Sa is calculated using Equation (1):26
S
S
f x y Z x ya
yx
= −∫∫
1
0
0
00
( , )
maxmax
d d (1)
where f(x,y) is the function of surface asperities on the
image in pixels denoted by (x,y), S0 is the image area
and Z0 is an average topography height. On the mag-
netic field gradient (phase images), relevant parameters
are the maximum phase shift angle (), minimum  and
the entire spectrum of phase shift angles.
2.3 Lacunarity analysis
Based on the gliding box method proposed by R.
Voss10 and R. E. Plotnick11, the procedures for deter-
mining the lacunarity values of the contact lens surfaces
were developed using Matlab software. All the necessary
image processing was also performed in Matlab. First,
the surface topography image was considered as a matrix
filled by surface height in each pixel. Such a matrix
represents an intensity image type with a grayscale map.
Second, the maximum height is divided into 100 levels.
Surface topography images are sliced on these levels by
planes starting from the top and all the way down. For
each section the pixels that belong to a surface are
colored white and considered as binary 1. The rest of the
surface image belongs to valleys, so they represent an
empty space. Those pixels are colored black and are
considered as binary 0.
This method considers the gliding box as a window
systematically moving through the binary image. The
gliding box could be moved either in a fixed scan or in
an overlapping window sliding manner. Regardless of
the sliding manner, the box mass value m is determined
for each of the gliding boxes as a number of black pixels
occupied by the gliding box. The gliding box size r is a
variable and can take values of 2n pixel length. Accord-
ing to R. E. Plotnick11, lacunarity L(r) is defined by
Equation (2):
L r
M r
M r
( )
( )
( )
= 2
1
2 (2)
where M1(r) and M2(r) are the first and second moments
of the distribution of black pixels in the gliding box. The
moments are defined by Equations (3) and (4):
M r m P m r
m
r
1
1
2
( ) ( , )= ⋅
=
∑ (3)
M r m P m r
m
r
2
2
1
2
( ) ( , )= ⋅
=
∑ (4)
The probability P(m,r) that the gliding box of size r
contains m black pixels is defined by Equation (5):
P m r
n m r
N r
( , )
( , )
( )
= (5)
where n(m,r) is the number of box size r with mass m,
and N(r) is the total number of boxes of size r.
If the calculated lacunarity L(r) is plotted in a double
logarithmic diagram then fractals will have straight-line
plots, in other words, they have a similar appearance
across scales. Otherwise, multi-fractals will have a diffe-
rent plot with few distinct areas, which means that they
do not have a similar appearance across scales. The
lacunarity plot for fractals can be fitted by a single line,
but the multi-scaled object has to be fitted with several
straight lines. Research done by R. E. Plotnick11 offers
double logarithmic plots for visual lacunarity distinction
concerning either fractals or non-fractals. It provides
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
visual information about the pixel entropy of the image
representing the surface. The double logarithmic plot
shown in Figure 1 can serve as a basis for a comparison
of different object plots.
The line slope as a numerical value p is suggested22
as a parameter that can be used for the AFM image
lacunarity comparison. In the case of surface topography,
particularly for binary images that are made by
sectioning on levels of interest, the lacunarity double log
plot (Figure 1 (left)) can be used for a surface section
comparison. However, the number p as a slope value for
every section can serve as a parameter for the surface
section characterization. Additionally, if parameter p is
plotted for every section made by slicing the surface
AFM image, then the diagram shown in Figure 1 (right)
can be considered as the AFM image lacunarity signa-
ture. The plots shown in Figure 1 have been made
according to Equation (6):
lg lgL r c p r( ) = + ⋅ (6)
The diagram, which consists of three sloped curves,
can be connected to randomly distributed surface aspe-
rities generated by the machining process, as demon-
strated in the work by B. Bojovic et al.27
Lacunarity analysis offers two types of p-diagrams,
which were presented in our previously reported work.
The first type represents p-diagrams with four line slopes
and we labeled them as contorted p-diagrams. In contrast
to them is a second type consisting of slanted p-diagrams
with three line slopes. Images associated with a specific
type of diagrams demonstrate similar surface topology.
The contorted p-diagram is related to topology with dis-
tinctive high hills and the slanted p-diagram is desig-
nated by regularly distributed asperities.
3 RESULTS AND DISCUSSION
The average results for the topography and gradient
of the magnetic field investigations into the 12 investi-
gated samples are presented in Figures 2 and 3. Topo-
graphy images are necessary to provide information
about the surface roughness and to see whether there are
any potential bumps on the surface that could cause
damage to the cornea or make wearing the contact lens
uncomfortable.
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Figure 1: a) Plot of logarithms of L(r) against r for arbitrary cutting
level of the engineering surface and b) slope value p vs. cutting level n
Slika 1: a) Diagram logaritmov L(r) v odvisnosti od r za arbitrarno
stopnjo rezanja in`enirske povr{ine in b) vrednosti naklona p v
odvisnosti od nivoja n
Figure 2: Average 3D topography image (left) and 3D magnetic force
gradient (right) of basic material SP40
Slika 2: Posnetek povpre~ne 3D-topografije (levo) in 3D-gradient
magnetne sile (desno) osnovnega materiala SP40
Figure 3: Average 3D topography image (left) and 3D magnetic force
gradient (right) of nanophotonic materials a) SP40-A, b) SP40-B and
c) SP40-C
Slika 3: Posnetek povpre~ne 3D-topografije (levo) in 3D-gradient
magnetne sile (desno) nano fotonskih materialov, a) SP40-A, b)
SP40-B in c) SP40-C
As presented in D. Stamenkovic28 all the materials
possess topography that is smooth enough to fit the
human eye. The maximum height difference of all mate-
rials can be seen on the topography of the basic material,
and nanophotonic materials have smoother topographies.
SP40 has the largest Rz value of 331.8 nm, which is
much lower than the human tear film, as expected from
commercially available materials. However, nanopho-
tonic materials are proven to have much smaller height
differences, where the second largest Rz of material
SP40-C is 157.2 nm, which is more than half the Rz of
SP40. The basic material has the largest surface rough-
ness of 23.4 nm. The second largest is material SP40-A,
and then material SP40-C with a surface roughness of
9.19 nm. It is interesting that material SP40-C, although
it has the second largest Rz, has a smoother surface than
material SP40-A.28
The images of the gradient value of the magnetic
field show that all materials have paramagnetic proper-
ties. The largest phase-shift angle difference is for the
basic material, suggesting there are particles inside as
impurities or that there are conglomerates of basic ma-
terial with a higher density that have the largest diffe-
rences in magnetic properties. Also, it can be seen that
the basic material possesses the highest paramagnetic
characteristics, with a minimum phase shift angle of
–121.6°. However, a maximum phase shift of –10.8°
shows a large variability in the paramagnetic properties.
All nanophotonic materials show much smaller diffe-
rences in the phase shift angles, which means that they
have more consistent magnetic properties and density
distributions (more homogenous). Also, it can be seen
that nanophotonic materials tend to shift magnetic pro-
perties even more to paramagnetism, which is more
strongly expressed in material SP40-A and material
SP40-C.
3.1 Lacunarity analysis
For the contact lens lacunarity analysis, the topogra-
phic image of each material was recorded and imported
to Matlab (Version R2013a, license number 853706) for
further calculation. Four surfaces of four RGP contact
lens materials were gathered. In order to make lacunarity
perceptible for the integral contact lens surface, the para-
meter p is determined for every section made by slicing
the AFM image.
The slanted form of p-diagrams can be seen in the
case of the RGP contact lenses made of SP40 base
materials (Figure 4). Topography images related to RGP
materials group of diagrams demonstrate a similar sur-
face topology (Figure 4). Consequently, the p-diagrams
look alike and have similar line slopes through the levels.
Only the p-diagram for the SP40-C material exhibits a
line slope change in the range 20–32 % of maximum
height. All p-diagrams reach asymptotic zero value after
a certain level where materials become predominant,
which is related to empty space in deep valleys. The
surfaces of all four RGP contact lens materials include
large regions of material in the bottom half (20 %) of the
maximum height, which makes the object very dense and
the p-value obtains a constant value.
The frontal surface of a contact lens is in direct con-
tact with the inner part of the upper eyelid. With each
blink of the eye the upper eyelid covers the frontal sur-
face with a tear film, thus filling in the gaps and smooth-
ing the surface roughness. The surface of a contact lens
with too many irregularities (high roughness) is undesir-
able because the tear film would not be sufficient to
cover them, particularly at the end of the day when post
lens tear film gets significantly thinner. According to the
lacunarity analysis and observed line slope change in the
range 20–32 % of maximum height, the nanophotonic
lens surface made of SP40-C material exhibits a less de-
sirable topography. However, by combining the surface
maximal height with an unwanted elevations distribution,
it could be concluded that they are significantly lower
compared to the post lens tear film thickness.
From an optics (refraction) point of view, a surface
with minor irregularities is not a disadvantage. Further-
more, šover polished’ surfaces have decreased wetta-
bility, which results in rapid šbreakage’ of the tear film
on the frontal surface of the lens. Besides aforemen-
tioned, šover polished’ surfaces have lower adhesion
force which keeps the RGP lens in eye. Considering
these specific requirements for functional behavior it
could be concluded that nanophotonic lenses have an
even more suitable topography compared to the conven-
tional one.
Surface topography with deep valleys causes the con-
tact lens to attract more proteins, lipids and other compo-
nents of a tear film. These deposits have a negative effect
on the optical characteristics of a contact lens, as well as
on comfort. The results presented in the p-diagrams that
is related to empty space in deep valleys indicate that all
four RGP contact lens materials are appropriate for di-
minishing deposits.
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Materiali in tehnologije / Materials and technology 51 (2017) 1, 145–151 149
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Figure 4: Plot of parameter p vs. cutting level n for RGP contact lens
surface
Slika 4: Diagram parametra p v odvisnosti od nivoja rezanja n na
povr{ini RGP kontaktnih le~
4 CONCLUSION
The results presented here suggest the future
commercialization of nanophotonic materials is affirma-
tive. As suggested in D. Stamenkovic30 the AFM method
has shown that the quality of polished surfaces for all
three nanophotonic materials completely satisfies the
standards for the production of contact lenses since they
have a smoother surface than SP40. The positive result is
the fact that the roughness parameter values for all nano-
photonic materials are lower than those for the basic
material.30
According to the magnetic properties of all the
materials yielded by MFM it can be concluded that the
basic material had very large phase shift angle differen-
ces, which are reduced by adding fullerene and its deri-
vatives. The incorporation of these carbon nanomaterials
caused the spectrum of phase shift angles to be reduced
by almost 50 %. Also, although even the basic material
has demonstrated paramagnetic properties it can be seen
that adding carbon nanomaterials increases the paramag-
netic characteristics of the material.
Topography images show that bumps significantly
influence the surface lacunarity, because they embrace a
great deal of lubricant volume. Therefore, the origins of
the contorted p-diagram are bump-like entities. Con-
sidering the functional behavior of the contact lens
surface, irregularities like bumps are unwanted, and thus
this kind of contact surface cannot be considered accept-
able. The surface lacunarity influences the tear film
volume distribution and, consequently, the contact lens
surface lubrication.
The surface lacunarity behavior can be related to the
surface topology. The results of the surface lacunarity
analysis confirm the sample surface state as belonging to
an appropriate (slanted p-diagram) roughness concerning
the tear film stability. Regarding the tribological beha-
vior that depends on lacuna distribution on the surface,
the RGP contact lenses made of three nanophotonic
materials and machined in the usual manner are en-
hanced compared to commercially available ones. Those
lacunas preserve a critical amount of tear film in the case
of dry eye and therefore improve the lubrication process
during RGP contact lens wearing.
Acknowledgements
This research was funded by the Ministry of Edu-
cation, Science and Technological Development of the
Republic of Serbia, through Project III45009. We would
like to thank Soleko, Italy, for turning our initial ideas
about new materials into a reality by producing them
with us in their laboratories.
5 REFERENCES
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