Note: Descriptions are shown in the official language in which they were submitted.
CA 02398616 2002-07-29
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POLISHING METHOD FOR SOFT ACRYLIC ARTICLES
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods of manufacturing products
comprising acrylic materials. In particular, this invention relates to methods
for
,o polishing soft acrylic articles to remove rough surfaces and tool or
machining
marks.
2. Description of Related Art
,5 Soft acrylic materials are used in the manufacture of a wide variety of
products. Because soft acrylic materials are generally compatible with
biological
tissues and fluids, they can be particularly useful in making products for
biomedical applications. Examples of such soft acrylic products include soft
contact lenses and soft prosthetic implants, such as intracorneal and
intraocular
Zo lenses, corneal inlays used during refractive surgery, and intracapsular
rings
used to support the natural lens capsule during ophthalmic surgery.
A highly polished finish, free of sharp edges or surface irregularities, is
required in many biomedical applications. Implantable products, such as
25 intraocular lenses, are in direct contact with body tissues and the tearing
or
abrading of tissue by rough surfaces could result in rupture of blood vessels,
irritation or other trauma to the tissue. Even minute irregularities can cause
irritation of body tissues. This is a particularly serious problem with
contact
lenses and portions of intraocular lenses that contact the eye, where the
tissue is
3o extremely sensitive.
The use of soft acrylic materials for intraocular lenses is a relatively new
development. Intraocular lenses formed of soft acrylic material are
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advantageous in that they can be folded and inserted through smaller incisions
in
the cornea than previously possible, resulting in fewer post-operative
complications. Rough edges resulting from the cutting of lens blanks or
flashing
generated during molding can cause intraocular irritation.
In addition, soft contact lenses require a highly polished finish to prevent
irritation of the interior of the eyelid and corneal epithelium. The eye is
extremely
sensitive to imperfections in contact lenses, and even slight ridges resulting
from
the molding process can produce irritation and discomfort. Expensive molding
,0 procedures or individual hand-grinding techniques may be used to provide
the
desired finish for these lenses.
Mechanical devices utilizing smooth, frictionless movement also require
highly polished, smooth surfaces of their soft acrylic products. Obtaining
such a
,5 highly polished, smoothly-finished soft acrylic article is often difficult
as these
products are manufactured by curing acrylic material in molds, wherein even
the
most precise dies result in some flashing and/or irregular edges. The products
may be trimmed and polished, but these finishing procedures are generally done
by hand, and are both time consuming and expensive, as well as imprecise, so
z0 that they do not result in the totally smooth or regular surface required.
Further,
many of these articles, particularly those for biomedical applications, are
relatively small, and/or irregularly shaped, causing difficulties in obtaining
the
desired finish, and/or clarity.
z5 For silicone materials, such as silicone rubbers and silicone elastomers,
tumble polishing processes are known. See, for example, U.S. Patent No.
5,133,159. However, the tumble polishing methods known for articles made
from silicone materials are not adequately applicable to articles made from
soft
acrylic materials. The removal of imperfections from small and irregularly
3o shaped soft acrylic products is an unsolved problem in the art. It would be
of
great utility to provide a simple, economic, and effective method for
polishing
and/or clarifying soft acrylic articles for industrial, medical, and
mechanical
purposes.
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Commonly assigned, copending U.S. Patent Application Serial No.
08/962,604 discloses methods for tumble polishing soft acrylic articles. The
methods comprise a cryogenic polishing step and a cleaning step.
SUMMARY OF THE INVENTION
The present invention provides methods for polishing articles comprising
soft acrylic materials. The methods comprise two steps: a polishing step and a
,o cleaning step. The polishing step comprises charging a receptacle with a
polishing slurry and the articles to be polished, and agitating the receptacle
for a
period of time and at a speed sufficient to remove surface irregularities from
the
articles. The polishing slurry comprises polishing beads, alumina, and a
swelling
agent. The polishing step may be carried out under ambient conditions.
,5
After polishing, the articles may contain alumina particles or a surface film
or other residue causing a hazy appearance. Any alumina particles or other
residue is removed in the cleaning step. The cleaning step comprises
contacting
the polished article with a cleaning slurry in a receptacle and agitating the
20 receptacle for a period of time and at a speed sufficient to clean the
surface of
the polished articles. The cleaning slurry comprises cleaning beads, alumina,
a
solvent and a surfactant.
Examples of articles which may be polished according to the methods of
25 the present invention include one-piece intraocular lenses, intraocular
lens
haptics, intraocular lens optics, intracapsular rings, corneal inlays,
intracorneal
lenses, and contact lenses.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides methods for polishing articles comprising
soft acrylic materials. As used herein, "soft acrylic material" means
materials
comprising polymers or copolymers of acrylic acid, methacrylic acid, esters of
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these acids, or acrylonitrile, wherein the polymers or copolymers have a glass
transition temperature (T9) of about 35 °C or less, and a Shore A
Hardness value
of about 90 or less. Preferably the soft acrylic materials have a T9 of about
25 °C
or less, and most preferably 20 °C or less. The soft acrylic materials
preferably
have a Shore A Hardness value of about 60 or less, and most preferably about
45 or less.
Examples of soft acrylic materials suitable for making foldable intraocular
lenses include but are not limited to (i) the acrylic material made from
,o copolymerizing about 65 wt.% 2-phenylethyl acrylate, about 30 wt.% 2-
phenylethyl methacrylate, an ultraviolet absorber and a cross-linking agent,
and
(ii) the acrylic material made from copolymerizing about 80 wt.% 2-phenylethyl
acrylate, about 15 wt.% 2-hydroxyethyl methacrylate, an ultraviolet absorber
and
a cross-linking agent. These and other suitable soft acrylic materials are
,5 described in U.S. Patent No. 5,290,892 and in copending, commonly-assigned
U.S. Patent Application Serial No. 08/739,245. Other examples of soft acrylic
materials include, but are not limited to, those disclosed in U.S. Patent No.
5,331,073.
2o As used herein, the "Preferred Acrylic Material" means the soft acrylic
material obtained by copolymerizing about 65 wt.% 2-phenylethyl acrylate,
about
30 wt.% 2-phenylethyl methacrylate, about 1.8 wt.% o-methallyl Tinuvin P and
3.2 wt.% 1,4-butanediol diacrylate, using 1.8 wt.% Perkadox 16 as an
initiator.
25 The present methods comprise a polishing step and a cleaning step. The
polishing step may be, and is preferably, conducted under ambient conditions.
In the polishing step, a receptacle is charged with a polishing slurry and the
articles to be polished. The receptacle may be various sizes and shapes, and
may be formed of glass, polycarbonate, or other suitable material. The
3o receptacle is preferably a round glass container, such as a 1 liter
Wheaton~
glass jar with glass lid.
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The polishing slurry comprises a mixture of polishing beads, alumina and
one or more swelling agents. As one skilled in the art will appreciate,
however,
the exact composition of the polishing slurry will vary depending on a variety
of
factors, such as the identity of the acrylic material and the size and shape
of the
article to be polished. Preferably, the polishing beads are glass beads, which
are
relatively inexpensive and readily commercially available, but may be solid or
filled beads formed of any suitable material. Commercially available glass
beads
are available in a variety of sizes, for example 0.3, 0.5, 1 and 2 mm sizes.
The
glass beads contained in the polishing slurry have preferably been
,0 preconditioned so that they are not in a raw state; that is, they have
preferably
been slightly worn or conditioned so that they are less likely to harm the
surface
of the lens.
One way to condition raw glass beads received from commercial suppliers
,5 is to sequentially tumble the beads in acidic (e.g., 2N HCL for 10 minutes)
and
basic (1 % NaOH for 1 hour) washing solutions, respectively. After their rough
surfaces have been slightly worn, the beads may be utilized in the polishing
step.
The necessary number and size distribution of polishing beads will vary
2o with the number and size of the articles to be polished, but a suitable
selection
can be easily determined without undue experimentation. Generally, a mixture
of polishing beads of different sizes is preferred. In the case of the
Preferred
Acrylic Material, for example, the polishing slurry would comprise 0.5 mm and
1.0
mm glass beads in a 1:3 ratio. For example, the polishing beads added to a 1
z5 liter receptacle would comprise approximately 1000 g of polishing beads as
follows: about 250 g of 0.5 mm glass beads and about 750 g of 1.0 mm glass
beads.
The polishing slurry also contains alumina as a polishing agent. Alumina
30 polishing powder is commercially available. Available mesh sizes of alumina
range from less than 0.05 micron to 3.0 micron and larger. The optimum amount
and mesh size of alumina will depend on other process parameters, including
the
identity of the soft acrylic material. In general, however, the polishing
slurry
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contained in a 1 liter receptacle will require a minimum of approximately 0.2
alumina to swelling agent (w/w) to achieve satisfactory results, regardless of
the
size and shape of the articles to be polished. Increasing the amount of
alumina
seems to increase the efficiency of the polishing slurry, especially as the
number
of articles to be polished increases, up to a maximum of about 2 % (alumina to
solvent, w/w). Higher concentrations of alumina may result in damage to the
articles) being processed and do not appear to provide any significant
improvement in polishing efficiency or results. In the case of the Preferred
Acrylic Material, the polishing slurry contains about 1 % of 0.5 micron
alumina to
,° swelling agent (w/w).
In addition to polishing beads and alumina, the polishing slurry also
comprises one or more swelling agents. The swelling agent ingredient slightly
swells the article to be polished, making its surfaces) more brittle, thereby
,5 facilitating and improving the polishing results. In addition to slightly
swelling the
article to be polished, the swelling agent also serves as the medium for the
alumina and polishing beads. Any agent capable of swelling the article to be
polished without irreversibly damaging it will be suitable.
z° Suitable swelling agents include solvents, such as, but not limited
to,
alcohols, aliphatic hydrocarbons, chlorinated solvents, and aromatic
hydrocarbons. Examples of suitable alcohol solvents include short chain
alcohols (approximately 10 total carbon atoms or less), such as methanol,
ethanol, and isopropanoi. Examples of suitable aliphatic hydrocarbon solvents
25 include pentane, hexane, heptane and mineral spirits. Examples of suitable
chlorinated solvents include methylene chloride and trichloromethane. Examples
of suitable aromatic hydrocarbon solvents include benzene and toluene.
Preferred solvents include mineral spirits with flash points of about 90 - 145
°F,
such as the mineral spirits fraction known as stoddard solvent.
Just as in the case of determining the optimum amount of alumina to
include in the polishing slurry, the identity and amount of swelling agent
which
should be included in the polishing slurry will depend upon the identity of
the
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chosen soft acrylic material. For certain acrylic materials and polishing
conditions, swelling agents having specific boiling point ranges or glass
transition
temperatures will function better than others in the polishing slurry. In the
case
of the Preferred Acrylic Material, the preferred swelling agent is a
commercially
available mineral spirits fraction known as "stoddard solvent." Other mineral
spirit fractions or other swelling agents may be more suitable for soft
acrylic
materials other than the Preferred Acrylic Material. In general, a suitable
amount
of swelling agent is that amount which is sufficient to cover the standing
volume
of polishing beads contained in the polishing slurry. Thus, in a one liter
,o receptacle containing approximately 1000g of glass polishing beads, the
preferred polishing slurry for the Preferred Acrylic Material comprises about
250
mL of stoddard solvent as the mineral spirits ingredient.
The polishing slurry is preferably formulated in the receptacle in the
,5 absence of the acrylic articles that are to be polished. For example,
alumina and
the chosen swelling agent ingredient are first added to the receptacle
containing
the polishing beads. The polishing slurry is then mixed before the articles to
be
polished are added. In the case of a 1 liter receptacle and the polishing
slurry for
the Preferred Acrylic Material, for example, the complete polishing slurry is
Zo tumbled for a short time (approximately 30 minutes) at room temperature
before
the articles to be polished are added.
Once the articles to be polished are added to the mixed polishing slurry,
the receptacle is agitated for a time and at a speed sufficient to remove any
z5 rough spots, sharp edges, and any tool or machining marks from the
articles'
surfaces. Agitation is preferably accomplished by placing the receptacle on a
rotational apparatus such as a commercially available tumbling machine (e.g.,
Model 3BAR from Topline Mfg. Co., Fullerton, CA). The optimal time and
rotation speed will vary with the batch size, identity of the soft acrylic
material, the
3o size and shape of the articles to be polished, etc. When the methods of the
present invention are used to polish intraocular lenses, a typical batch size
will be
on the order of 50 - 100 lenses for a 1 liter receptacle. In the case of the
Preferred Acrylic Material, excellent polishing results are obtained when the
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article is tumble-polished for approximately 3 - 10 days, with the rotation
speed of
the receptacle being approximately 80 rpm.
After the polishing step, the articles may be separated from the polishing
beads by emptying the contents of the polishing jar into a sieve stack so that
the
swelling agent is drained away. The articles then may be rinsed by flushing
with
either fresh solvent or deionized water to separate the lenses from the beads.
The polished articles will appear very smooth, but may contain alumina
particles
on the surface and/or may appear hazy or frosty, as if there is a residue on
the
,0 surface of the polished articles. Any alumina particles and any surface
residue is
removed in a cleaning step.
After the articles are polished but before they are subjected to the
cleaning step, it may be advantageous to reduce or eliminate any residual
,5 swelling agent from the articles' surfaces in the event that the chosen
cleaning
slurry contains a solvent different than the swelling agent chosen for the
polishing slurry. This can be accomplished by briefly cleaning the articles in
a
commercially available ultrasonic cleaner. Cleaning solutions suitable for use
in
ultrasonic cleaners generally include solvents, detergents, water and mixtures
Zo thereof. The exact composition of the cleaning solution is not critical,
though it
may be desirable to adjust the composition's ingredients based upon the
identity
of the chosen solvent. In the case where the swelling agent is chosen to be
stoddard solvent, a suitable stock ultrasonic cleaning solution comprises a
mixture of water, 2-butoxyethanol, Micro~ detergent, and ammonium hydroxide.
Following ultrasonic cleaning, if any, the polished articles are cleaned in
an agitating receptacle containing a cleaning slurry comprising cleaning
beads,
alumina, a solvent and a surfactant. The solvent is preferably the same as any
solvent utilized in the polishing slurry. As in the polishing step above, the
agitation is preferably achieved by means of a rotational machine. The
receptacle is preferably a 1 liter round glass jar with a glass lid, though
any
receptacle having a shape in which the articles to be cleaned do not stick
(e.g., in
corners of a square jar) or get caught (e.g., in a neck area connecting the
body
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cavity to the lid) would be suitable. The cleaning beads may be of the same
type
as those suitable for use as polishing beads. As in the case of the polishing
beads, the cleaning beads are preferably conditioned glass beads. fn general,
a
mixture of cleaning beads of different sizes is preferred.
The necessary number and size distribution of cleaning beads will vary
with the number and size of the articles to be polished, but a suitable
selection
can be easily determined without undue experimentation. In the case of the
Preferred Acrylic Material, for example, the cleaning slurry added to a 1
liter
,o receptacle would comprise approximately 1000 g of cleaning beads as
follows:
about 250 g of 0.5 mm glass beads and about 750 g of 1.0 mm glass beads
(larger sizes of beads are generally avoided in an attempt to reduce the
possibility that the cleaning beads will damage the surface of the articles).
,5 The cleaning slurry contains alumina, a surfactant, and a solvent in which
the alumina is not soluble. The most preferred solvents are mineral spirits
solvents having a flash point of about 90 - 145 °F, such as stoddard
solvent. In a
preferred embodiment, the polishing slurry contains a solvent as the swelling
agent and the cleaning slurry contains the same solvent. This simplifies the
20 polishing and cleaning process by eliminating the need to recover and rinse
the
lenses between the polishing and cleaning steps. In this preferred embodiment,
the cleaning step may begin after simply adding a suitable surfactant directly
to
the polishing slurry in the receptacle used for polishing once the polishing
step is
complete.
Suitable surfactants for use in the cleaning slurry are those that dissolve
in the chosen solvent and aid in suspending the alumina particles in the
solvent.
The preferred amount of surfactant in the cleaning slurry is that amount which
provides about a 1:1 ratio (w/w) with the amount of alumina in the cleaning
slurry.
3o Reducing the ratio of surfactant to alumina may allow the alumina particles
to
become imbedded in, or cause damage to, the surface of the articles being
cleaned. Increasing the ratio of surfactant to alumina may reduce the
effectiveness of the cleaning process. The preferred surfactant for use with
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stoddard solvent is dioctyl sulfosuccinate ("DSS"), a commercially available
surfactant.
The preferred amount and "mesh size" of the alumina for use in the
cleaning slurry will depend on other process parameters, including the
identity of
the soft acrylic material. In general, however, the cleaning slurry contained
in a 1
liter receptacle will require a minimum of approximately 0.2 % of alumina to
solvent (w/w) to achieve acceptable cleaning results, regardless of the size
and
shape of the articles being cleaned. Increasing the amount of alumina and
,o surfactant seems to increase the efficiency of the cleaning slurry, up to a
maximum of about 2 % (w/w of each alumina and surfactant). Higher
concentrations may damage the articles being cleaned and do not appear to
significantly improve cleaning results. In the case of the Preferred Acrylic
Material, the cleaning slurry contains 1 % of 0.05 micron alumina, and 1 % DSS
,5 to solvent (w/w).
The volume of liquid which should be included in the cleaning slurry
depends upon the volume of cleaning beads, the number and size of the articles
to be tumble-cleaned, etc. In general for tumble cleaning, however, the
cleaning
z0 slurry should contain a liquid level sufficient to prevent the cleaning
beads from
becoming too dry and riding along the inside surface of the cleaning
receptacle
to the extent that they fall sporadically or too violently. Instead, the
cleaning
beads should tumble relatively smoothly. Likewise, the volume of liquid in the
cleaning slurry should not be too great that the articles to be cleaned float
or
z5 remain outside the stream of tumbling cleaning beads. For example, in the
case
of the Preferred Acrylic Material, a 1 liter receptacle containing 1000 g of
glass
cleaning beads as described above, will contain approximately 250 mL of
liquid.
The cleaning step is preferably conducted at a temperature sufficiently
30 above the soft acrylic material's glass-transition temperature to insure
that the
article in cleaning slurry is soft and at least slightly flexible to aid in
the removal of
any frosty or hazy residue. In the case of the Preferred Acrylic Material, the
cleaning step is preferably conducted at about 18 °C or higher.
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As in the case of the polishing step, the optimal time and rotation speed
for the cleaning step will vary with the batch size, identity of the soft
acrylic
material, the size and shape of the articles to be cleaned, the condition of
the
cleaning beads, etc. When the methods of the present invention are used to
polish intraocular lenses, a typical batch size will be on the order of 50 -
100
lenses for a 1 liter receptacle. In the case of the Preferred Acrylic
Material,
excellent polishing results are obtained when the article is tumble-polished
for
approximately 3 - 10 days, with the rotation speed of the receptacle being
,o approximately 80 rpm. After approximately 1 - 3 days of tumble cleaning
with a
rotation speed of the receptacle being approximately 80 rpm, intraocular
lenses
made of the Preferred Acrylic Material are very clean (no frosty appearance)
and
optically clear with little or no cosmetic blemishes.
,5 Certain embodiments of the present invention are illustrated in the
following examples.
EXAMPLE 1
Zo TUMBLE POLISHING OF INTRAOCULAR LENSES
(Preferred Acrylic Material)
A 1000 mL round glass tumbling jar was filled with approximately 1000 g
of a mixture of glass polishing beads. The mixture contained approximately 25%
25 0.5 mm, and 75% 1.0 mm glass beads. To this was added 2.0 g of 0.05 micron
Alumina polishing powder (Baikowski International Corp., Charlotte, NC) and
250
mls of stoddard solvent (EM Scientific, Gibbstown, NJ). The jar and its
contents
were placed on a modified 3BAR tumbler (Topline Mfg. Co.). The tumbling unit
was switched on low speed (80 RPM), and the jar was tumbled for approximately
30 15 minutes at Ambient temperature (21.0 ° C) to allow the contents
to mix prior to
adding one piece intraocular lenses made from Preferred Acrylic Material.
After
adding 10 lenses, the jar which was placed back into the tumbler and tumbled
for
days. After tumbling, the jar was removed and its contents were poured into a
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No. 6 sieve to separate the lenses from the glass beads. They were then rinsed
with deionized water for 2 minutes, and dried with compressed air. A sample
lens was inspected at 16X for roundness and surface finish quality. It
appeared
highly polished. All machining lines had been removed and the optic and
haptics
edges were smooth and rounded. A very slight haze covering the entire surface
of the lens was also observed post tumbling.
EXAMPLE 2
TUMBLE CLEANING OF INTRAOCULAR LENSES POLISHED IN EXAMPLE 1
,0 (Solvent-based Cleaning Slurry)
A 1000 ml round glass jar was filled with approximately 1000 g of a
mixture glass cleaning beads. The mixture contained approximately 25% 0.5
mm and 75% 1.0 mm glass beads. To this was added approximately 1.0 g of
,5 0.05 mesh size alumina polish, and 1.0 g of dioctyl sulfosuccinate (DSS).
Finally, about 250 ml of stoddard solvent was added to the jar to complete the
cleaning slurry mixture. The polished lenses from example 1 were then added to
the cleaning jar, which was sealed and placed onto the 3BAR tumbler (Topline
Mfg. Co.). The jar and its content were tumbled at 80 r.p.m. for approximately
3
20 days at room temperature (21.0° C). At the conclusion of the
cleaning cycle, the
jar was removed from the tumbler and its contents poured into a No. 6 sieve to
separate the lenses from the cleaning beads. The lenses were rinsed briefly
with
deionized water and allowed to air dry. A sample lens was inspected at 16X to
determine whether the surface haze was successfully removed. The lens
ZS surface appeared clean and clear.
EXAMPLE 3
TUMBLE POLISHING/CLEANING OF INTRAOCULAR LENSES
(Preferred Acrylic Material)
A 1000 mL round glass tumbling jar was filled with approximately 1000 g
of a mixture of glass polishing beads. The mixture contained approximately 25%
0.5 mm, and 75% 1.0 mm glass beads. To this was added 1.5 g of 0.05 micron
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Alumina polishing powder (Baikowski International Corp., Charlotte, NC) and
250
mls of stoddard solvent (EM Scientific, Gibbstown, NJ). The jar and its
contents
were placed on a modified 3BAR tumbler (Topline Mfg. Co.). The tumbling unit
was switched on low speed (80 RPM), and the jar was tumbled for approximately
15 minutes at Ambient temperature (21.0 ° C) to allow the contents to
mix prior to
adding one piece intraocular lenses made from Preferred Acrylic Material.
After
adding 10 lenses, the jar which was placed back into the tumbler and tumbled
for
9 days. After an acceptable polishing level of the lens samples had been
achieved, the jar was removed from the tumbler and opened. To it was added
,0 1.0 g of dioctyl sulfosuccinate (DSS), which essentially transformed the
polishing
slurry into a cleaning slurry. The jar was then re-sealed, and returned to the
tumbler and tumbled at low speed (80 rpm), for an additional 3 days. At the
completion of the cleaning cycle, the jar was again removed from the tumbler
and its contents were poured into a No. 6 sieve to separate the lenses from
the
,5 glass beads. They were then rinsed with deionized water for 2 minutes, and
dried
with compressed air. A sample lens was inspected at 16X for roundness and
surface finish quality. All machining lines had been removed and the optic and
haptics edges were smooth and rounded. The lens surface appeared clean and
highly polished, with no trace of surface haze.
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
25 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.
13
