Method for reducing tackiness of soft acrylic polymers

The tackiness associated with certain soft acrylic polymers can be reduced by plasma treatment of the polymer surface. This is particularly useful with regard to intraocular lenses (IOLs), especially foldable IOLs, which are made from such soft acrylic polymers.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for reducing the tackiness of 
certain soft acrylic polymers. The present invention is particularly 
useful in relation to ophthalmic lenses made of such soft acrylic 
polymers, especially foldable intraocular lenses. 
Intraocular lenses have been known since about 1950. They are used to 
replace the natural lenses of eyes which have been damaged by trauma or 
disease, such as cataracts. A typical intraocular lens (IOL) comprises an 
artificial lens ("optic") and at least one support member ("haptic") for 
positioning the IOL in the eye. The optic may be formed from any of a 
number of different materials, including polymethylmethacrylate (PMMA), 
and it may be hard, relatively flexible or even fully deformable in order 
to permit the IOL to be rolled or folded and inserted through a relatively 
small incision in the eye. The haptic is generally made of some resilient 
material, such as polypropylene or PMMA. The IOLs can have either a 
"one-piece" or a "multi-piece" design. A multipiece lens is so called 
because the optic and the haptic(s) are made separately and then the 
haptics are attached to the optic. In a one-piece lens, the optic and 
haptic(s) are formed out of one piece of material, and depending on the 
material, the haptics are then cut, or lathed, out of the material to 
produce the IOL. 
European Patent Publication No. 485 197 A1 (published 13 May 1992) 
discloses a class of soft acrylic polymers which are suitable for forming 
IOLs which have high refractive index, are flexible and transparent, can 
be inserted into the eye through a relatively small incision and recover 
their original shape after insertion. Although this type of lens material 
is advantageous in that thinner lenses may be crafted, which allow the 
IOLs to be folded and permit the use of smaller incisions, it has been 
found that this type of lens material has a tendency to adhere to itself, 
as well as to the handling tools. In fact, the inserted IOL can take 
several minutes to release from itself following insertion. A tacky lens 
requires more manipulations by the surgeon and increases surgery time 
(because of the manipulations and lengthy unfolding time). The reduced 
surface tack lens can be more easily manipulated during surgical 
implantation. The final outcome with a reduced surface tack lens is easier 
IOL placement, reduced surgery time, and greater safety for the patient. 
Both chemically non-reactive and chemically reactive plasmas have been used 
to treat various polymer surfaces for several years. See, for example: 
Inagaki et al., "Adhesion of Glow Discharge Polymers to Metals and 
Polymers," Journal of Applied Polymer Science, 26:3333-3341 (1981), and 
Inagaki et al., "Distribution of Polymer Deposition in Glow Discharge 
Polymerization in a Capacitively Coupled System," Journal of Applied 
Polymer Science, 26:3425-3433 (1981). In addition, plasma treatments have 
recently been used to treat silicone contact lenses and silicone 
intraocular lenses. See, for example: Ho et al., "Ultrathin Coating of 
Plasma Polymer of Methane Applied on the Surface of Silicone Contact 
Lenses," Journal of Biomedical Materials Research, 22:919-937 (1988); 
Hettlich et al., "Plasma-induced Surface Modifications on Silicone 
Intraocular Lenses: Chemical Analysis and In Vitro Characterization," 
Biomaterials, 12:521-524 (1991); and Tidwell, C. D., "The Development of a 
Surface-modified, Self-passivating Intraocular Lens," Master of Science 
Thesis, University of Washington (1990). 
While plasma treatments have been used to modify other types polymer 
surfaces, the Applicants are not aware of any prior use of plasma 
treatments to reduce polymer surface tackiness of soft acrylics. 
SUMMARY OF THE INVENTION 
It has now surprisingly been found that plasma treatment of the polymer 
surface of articles made from soft acrylic polymers greatly reduces the 
tackiness often associated with such articles. 
The method of the present invention involves placing the article to be 
treated into a radio frequency (RF) plasma reactor and treated with a 
plasma. The plasma may be formed from an inert gas or organic molecules. 
The conditions for formation of a stable plasma vary from reactor to 
reactor. The process parameters which are adjusted to create the plasma 
are: gas type, power, pressure, flow rate, time, and temperature. Factors 
which vary from reactor to reactor and can influence plasma process 
conditions are: reactor chamber geometry/volume (barrel, rectangular or 
cubic volume), electrode type/geometry/spacing, RF power/frequency, gas 
flow/pumping system. Other factors which influence the process are the 
material being treated, loading density of samples, and fixturing. 
DETAILED DESCRIPTION OF THE INVENTION 
The method of the present invention can be applied to any object made of 
soft acrylic polymers which have an inherent tackiness in air or in fluid 
medium, where such tackiness is undesirable. It is preferred to utilize 
the method of the present invention on the modified soft acrylic polymers 
which can be formed from monomers having the formula: 
##STR1## 
wherein: X is H or CH.sub.3 ; 
m is 0-10; 
Y is nothing, O, S, or NR wherein R is H, CH.sub.3, C.sub.n H.sub.2n+1 
(n=1-10), iso-OC.sub.3 H.sub.7, C.sub.6 H.sub.5, or CH.sub.2 C.sub.6 
H.sub.5 ; 
Ar is any aromatic ring, such as benzene, which can be unsubstituted or 
substituted with H, CH.sub.3, C.sub.2 H.sub.5, n-C.sub.3 H.sub.7, 
iso-C.sub.3 H.sub.7, OCH.sub.3, C.sub.6 H.sub.11, Cl, Br, C.sub.6 H.sub.5 
or CH.sub.2 C.sub.6 H.sub.5 ; and 
a cross-linking monomer having a plurality of polymerizable ethylenically 
unsaturated groups. 
European Patent Publication No. 485 197 A1 published 13 May 1992 ("the 197 
Application"), discloses methods for preparing these soft acrylic polymers 
and discusses these polymers in greater detail; therefore, they are not 
further discussed herein. In addition, the '197 Application discloses 
methods for forming IOLs from these polymers. The entire contents of the 
'197 Application are hereby incorporated by reference herein. 
Although Applicants do not intend to be bound by any particular theory, 
Applicants believe that there are three primary mechanisms involved in 
reducing tackiness of these soft acrylic articles by inert gas plasma 
treatment: 
1) The ionic or charged particle bombardment on the extended polymer chains 
of the IOL surface create clipped chains (the pendant chemical groups are 
physically removed by the ion bombardment on the polymer surface), 
resulting in a smoother surface and therefore less intertwining of these 
chains when the IOL is contacted to either itself or other surfaces (e.g., 
instruments). 
2) During plasma generation, short wavelength UV radiation is also produced 
which, under high vacuum, will promote enhanced surface cross-linking of 
the polymer structure. This higher density cross-linking of the polymer 
surface will contribute to reduced surface tack. 
3) The increase of the surface polymer network structure results in a 
surface having both enhanced polarity and wetting properties (i.e., a more 
hydrophilic surface); such properties have been proven to have major 
impact on the tack properties of soft acrylics when immersed in water. 
When using reactive molecules to treat soft acrylics, it is believed that 
the primary mechanism for tackiness reduction is a function of the 
properties of the particular polymer and particular gas plasma. For 
example, with methane gas treatment, the treated polymer is not only 
highly hydrophobic, but is very highly cross-linked, resulting in a 
smooth, compact surface which minimizes the potential for surface 
interactions (i.e., molecular chain entanglement). 
It should be noted that, depending on the type of plasma being utilized, 
the tackiness reduction may only occur when the treated article is in 
aqueous medium (e.g., in water, saline, or aqueous humor) or is wet (if 
there is residual water on the polymer surface). For example, the 
tackiness of IOLs is reduced only in aqueous medium when argon gas plasma 
is used, but when methane gas plasma is used, tackiness is reduced in air, 
as well as in aqueous medium. 
Plasma chambers which are suitable for use in the present invention are 
commercially available, for example, from Advanced Plasma Systems, Inc. 
(St. Petersburg, Fla.), GaSonics/IPC, Inc. (Hayward, Calif.), Plasma 
Science, Inc. (Foster City, Calif.) and Advanced Surface Technology, Inc. 
(Billerica, Mass.). A particularly useful plasma chamber is the B- Series 
Plasma System, available from Advanced Plasma Systems, Inc. ("Advanced 
Plasma"). Fixtures and/or holders for the articles may be desired in order 
to ensure equal treatment of the surfaces; such fixtures and holders are 
also available from Advanced Plasma. 
The articles to be treated are placed in appropriate fixtures, if necessary 
or desirable, and loaded into the plasma chamber. The chamber is 
evacuated, and gas is then introduced into the chamber and adjusted to the 
desired pressure. The RF power generator is then turned on. A plasma forms 
within the chamber and is allowed to treat the articles for the desired 
length of time. After treatment is complete, the RF power generator is 
turned off and the articles removed from the chamber. 
The processing conditions for plasma treatment of soft acrylic articles 
will vary depending on the plasma type and reactor specifications. In 
general, the types of plasmas useful in the present invention include 
inert gases such as argon and helium (and mixtures thereof), as well as 
reactive gases, such as various hydrocarbons (e.g., methane). For treating 
soft acrylic IOLs, it is preferred to use argon plasma. For the argon 
plasma process, the preferred range of process conditions (using a cubic 
reactor having primary electrodes) are as follows: power=300-400 watts; 
pressure=150-225 milliTorr (mTorr); and time=3-5 minutes. These settings 
are approximate only and may vary somewhat, depending on the individual 
reactor chambers used.

The following examples are presented to illustrate further various aspects 
of the present invention, but are not intended to limit the scope of the 
invention in any respect. 
EXAMPLE 1 
The following is an example of a typical procedure for Argon plasma 
treatment of soft acrylic optics. 
The lens holder and plasma chamber were first cleaned with an oxygen plasma 
according to procedures well known to those skilled in the art. The optics 
to be treated were loaded onto the lens holder and the lens holder placed 
into the plasma chamber. The chamber was then sealed and evacuated. Argon 
gas was pumped into the chamber until the pressure reached 160 milliTorr 
("mTorr"). The RF power generator was then turned on to 400 watts for 5 
minutes, at which time the power was turned off. The gas was allowed to 
flow for 5 minutes after the power was turned off, then the chamber was 
purged with Argon gas and vacuum broken to atmospheric pressure. Samples 
were then removed from the chamber. 
EXAMPLE 2 
In another typical example of Argon plasma treatment of soft acrylic 
optics, the procedure of Example 1 was followed, except that the chamber 
was filled with Argon gas to a pressure of 225 mTorr, and the power was 
applied at 300 watts for a period of 3 minutes. 
EXAMPLE 3 
In a typical procedure for methane plasma treatment of soft acrylic optics, 
the procedure of Example 1 was followed, except that the chamber was 
filled with methane gas to a pressure of 50 mTorr, and the power of 50 
watts was applied for a period of 10 minutes. 
The invention has been described by reference to certain preferred 
embodiments; however, it should be understood that it may be embodied in 
other specific forms or variations thereof without departing from its 
spirit or essential characteristics. The embodiments described above are 
therefore considered to be illustrative in all respects and not 
restrictive, the scope of the invention being indicated by the appended 
claims rather than by the foregoing description.