Smart ocular plug design and method of insertion for punctal and intracanalicular implants

An ocular plug design and method of insertion is described for the treatment of dry eye. This ocular plug is a narrow rod-like cylinder of appropriate diameter, which is tapered at one end, for insertion into an ocular channel. The plug is prepared from either (or both) of two specific classes of polymeric materials having both viscous and elastic properties. The first class of polymeric materials have a glass transition temperature (T.sub.g) at or below human body temperature (37.degree. C.). The second class of polymeric materials have a melting temperature (T.sub.m) at or below human body temperature (37.degree. C.). The plug is stored in a frozen, rigid, elongated state prior to insertion into an ocular channel. Once inserted into an ocular channel, the smart plug responds to an increase in temperature, due to the surrounding physiochemical environment, whereby it becomes soft and the plug subsequently expands to adapt to the size and shape of the patient's punctum or canaliculum. Once the plug expands to the size of the particular ocular channel, the plug is met with a resistance from the surrounding tissue. At this point, expansion of the plug ceases and the plug can effectively block tear drainage through either ocular channel.

TECHNICAL FIELD
 The present invention generally relates to a removable intraocular plug
 used to temporarily close the punctal or canalicular opening of the human
 eye to be utilized, for example, in the treatment of keratoconjunctivitis
 sicca (dry eye). Specifically, the present invention relates to a method
 of occluding ocular channels by using materials that can adapt to the size
 and shape of the individual's punctum or canaliculus by exploiting the
 rigid, viscous and elastic properties of the material composition.
 BACKGROUND OF THE INVENTION
 The human eye includes a complex composition in the form of a tear film.
 Tears include three basic components: (1) lipids; (2) an aqueous layer;
 and (3) mucin. The absence of any one of these components causes
 discomfort and can lead to a temporary or permanent condition known as
 keratitis sicca (or keratoconjuctivitis sicca, often referred to as dry
 eye). Dry eye can have a variety of causes but is generally attributed to
 one or two basic malfunctions. First, the tear ducts leading from the
 lacrimal glands can be clogged or malfunctioning so that an insufficient
 amount of tears reaches the eye. For many years, this was generally
 thought to be the main reason for dry eye. Artificial tears were developed
 in response to this need. However, the relief to patients using these
 artificial tears is short-lived and treatment must be readministered
 several times each hour.
 More recently, it has been discovered that, with increasing age, dry eye is
 caused by either insufficient or inadequate tears and tear components or
 the inability to maintain effective tear film. Accordingly, recent
 therapies have proceeded on the basis that tear production may be
 inadequate in some individuals and that a significant percentage of dry
 eye syndrome can be alleviated by slowing down the drainage of the tears
 through the lacrimal ducts.
 Tears are removed from the eye by draining through the upper and lower
 punctal openings which lead into the canalicular canals (See FIG. 1).
 Initial attempts at sealing the puncta and/or the canalicular canals
 involved stitching the puncta shut or using electrical or laser
 cauterization to seal the puncta and or canalicular canals. Although such
 methodology can provide desirable results, the procedure is not reversible
 without reconstructive surgery. Since it is sometimes difficult to
 determine whether in a particular patient, the drainage is too great or
 the tear production is too small, irreversible blockage is not without
 risk.
 One means of temporarily blocking the punctum and canaliculus for the
 treatment of dry eye is through the use of intracanalicular gelatin
 implants. Intracanalicular Gelatin Implants in the Treatment of
 Kerato-Conjunctivitis Sicca, Wallace S. Foulds, Brit J. Ophthal (1961)
 Vol. 45 pp 625-7. Foulds discloses that the occlusion of the lacrimal
 puncta can be performed by use of and insertion of a fine, water soluble
 gelatin rod into the punctal openings. The gelatin rod is formed from pure
 powdered gelatin to which a small quantity of distilled water has been
 added and is heated in a water bath until the gelatin dissolves and a
 thick gel results. By dipping a cold glass rod into the prepared gelatin,
 and withdrawing the same, fine solid rods of gelatin were formed. The
 gelatin rods were then inserted into the canaliculi to provide a temporary
 blockage. As such, the gelatin rod implants, although very fragile,
 provide an alternative means for temporarily blocking the canaliculus.
 Water-insoluble plugs which can be placed in the punctum openings and into
 vertical sections of the canalicular canals are disclosed in U.S. Pat. No.
 3,949,750, Freeman, issued Apr. 13, 1976. The punctum plug (10) of Freeman
 is a rod-like plug formed with an oversized tip (11) that dilates and
 blocks the vertical canaliculus (see FIG. 2). The punctum plug has a
 relatively large, smooth head portion (12) which functions to prevent the
 punctum plug from passing into the horizontal portion of the canaliculus.
 Although these plugs are reversible, they tend to become dislodged quite
 easily. Further, they are somewhat difficult to insert, and occasionally
 their size and shape can cause tissue damage during insertion or, if they
 protrude from the puncta, they can cause irritation to the sclera. The
 tissue of the punctum can also be damaged by being dilated by the plugs
 over extended periods of time.
 An improvement on the Freeman plugs is disclosed in U.S. Pat. No.
 4,959,048, Seder et al., issued Sep. 25, 1990. Seder et al. disclose a
 preformed plug or channel occluder which is somewhat conical in shape,
 making it possible to insert the occluder into the opening of the punctum
 more easily than the devices disclosed by Freeman. Further, Seder et al.
 disclose that variations in the anatomy of individuals make it desirable
 to provide a series of occluders in different lengths and/or widths in
 order to accommodate anatomical differences. Therefore, ophthalmologists
 need to measure the actual size of the punctal opening to determine the
 best size of punctum plug to be used for each patient and manufacturers
 must then provide five or more different sizes of punctum plugs to meet
 the ophthalmologist's needs.
 Accordingly, using the prior art plugs, doctors must follow a number of
 procedures that are not only time consuming but also require a high level
 of skill. First, doctors need to measure each patient's punctum diameter
 since this size will vary from patient to patient, and for some patients,
 there will even be variances in punctum size in the left eye versus right
 eye (see FIG. 3). This is done by inserting a sizing gauge (13) into the
 punctum (2). An oversized plug will cause the patient discomfort while an
 undersized plug will fall out of the patient's eye. Second, doctors need
 to dilate the punctum (2) and quickly insert the plug, usually within 30
 seconds or less (see FIG. 4). The dilation needs to be repeated if the
 plug fails to be inserted within the 30 seconds, and, since the plug is so
 soft and small, it is often very difficult to complete the insertion
 within this 30 second time window. FIGS. 4 and 5 show tools which may be
 used to enlarge the punctum for insertion of the plug.
 From the foregoing discussion, there exists a clear need for a new punctal
 plug design which would greatly simplify or eliminate the current
 time-consuming surgical dilation and insertion procedures. A
 "one-size-fits-all" plug design would not only eliminate the need for
 manufacturers to provide doctors with plugs of various dimensions, but
 also eliminate the need for doctors to measure the patient's punctal size
 prior to surgery.
 SUMMARY OF THE INVENTION
 The above-described objectives are achieved with the method and ocular plug
 design of the present invention. This invention involves a "smart" punctal
 and canalicular plug for blocking lacrimal flow through ocular channels.
 This smart plug is a narrow rod-like cylinder of appropriate diameter for
 insertion into an ocular channel. It is tapered at one end and is prepared
 from either (or both) of two specific classes of polymeric materials
 having both rigid, elastic and viscous properties. The first class of
 polymeric materials have a glass transition temperature (T.sub.g) at or
 below human body temperature (37.degree. C.). The second class of
 polymeric materials have a melting temperature (T.sub.m) at or below human
 body temperature (37.degree. C.). The polymeric materials of the present
 invention can also be the blended with wax-like materials to form a
 composition with a T.sub.g and/or T.sub.m at or below 37.degree. C. Since
 the plug is stored in a frozen, rigid, elongated state prior to insertion,
 doctors should find it easier to insert this plug into the punctum or
 canaliculus of the eye as compared with a soft plug and the need for a
 special inserter during surgery is eliminated (see FIG. 5).
 Once inserted into an ocular channel, the smart plug responds to an
 increase in temperature, due to the surrounding physiochemical
 environment, whereby it becomes soft and the plug subsequently expands to
 adapt to the size and shape of the patient's punctum or canaliculum. Once
 the plug expands to the size of the particular ocular channel, the plug is
 met with a resistance from the surrounding tissue, and at this point,
 expansion of the plug ceases. This externally applied resistance by the
 surrounding tissue in turn activates the elastic and viscous properties of
 the plug which function to fill any void space between the plug and
 punctum or canaliculus (see FIG. 8). Thus, the plug can effectively block
 tears from being drained through either ocular channel.
 In particular, the present invention relates to a method for inserting a
 plug into an ocular channel, in which a biocompatible composition is
 supplied that is rigid at room temperature, becomes elastic when warmed to
 a temperature above either its melting temperature, T.sub.m, or its glass
 transition temperature, T.sub.g, and becomes rigid once again when cooled
 to temperature below either its T.sub.m or T.sub.g. The material for this
 composition consists of polymeric materials such as homopolymers,
 cross-linked polymers and copolymers of silicones, acrylic esters,
 polyurethanes, hydrocarbon polymers, silicone elastomers, and mixtures of
 these polymeric materials with waxes. This biocompatible composition is
 then warmed to a temperature at which it becomes elastic and subsequently
 is formed, through stretching, into a rod having dimensions suitable for
 insertion into an ocular channel. The resulting composition is allowed to
 cool and re-solidify in its stretched, rigid form at which point it is
 inserted into an ocular channel. The composition is then warmed by the
 body, becoming viscous and elastic, and subsequently conforms to the shape
 of the ocular channel.
 The present invention also relates to a removable rod-like plug for
 blocking lacrimal flow through the punctum or canaliculus of the human
 eye. It is constructed from a biocompatible composition that is rigid at
 room temperature, becomes elastic when warmed to a temperature above
 either its melting temperature, T.sub.m, or its glass transition
 temperature, T.sub.g, and becomes rigid once again when cooled to
 temperature below either its T.sub.m or T.sub.g. Materials suitable for
 this composition generally consists of polymers, homopolymers,
 cross-linked polymers and copolymers of silicones, acrylic esters,
 polyurethanes, hydrocarbon polymers, silicone elastomers, and mixtures of
 these polymers with waxes. The composition is formed into a cylindrical
 shape of diameter and length which is sufficient to fully occlude the
 ocular channel and has a tapered end to facilitate insertion into the
 punctum or canaliculus. The plug is stretched along its length and
 maintained in its frozen, elongated form prior to insertion.

DETAILED DESCRIPTION OF THE INVENTION
 To facilitate the understanding of the present invention, a brief
 description of the human eye 1 and the associated lacrimal system showing
 the paths of the tears from the sources is presented. FIG. 1 illustrates
 the lacrimal system for a human eye 1. Tears flow into small openings
 called puncta located in the lids of the eye. Both upper punctum 2 and
 lower punctum 3 lead to corresponding upper canaliculus 4 and lower
 canaliculus 5. The upper canaliculus 4 and the lower canaliculus 5 merge
 into the lacrimal sac 6 from which tears travel into the nasal lacrimal
 duct 7 and drain into the nose. The majority of the tears drain through
 the lower punctum 3 via canaliculus into the nasal passage. The implant is
 to be inserted into either the punctal opening or the horizontal portion
 of the canaliculus.
 Definitions
 Throughout the disclosure, unless the context clearly dictates otherwise,
 the terms "a" "an" and "the" include plural referents. Thus, for example,
 a reference to "a polymer" includes a mixture of polymers and statistical
 mixtures of polymers which include different weight average molecular
 polymers over a range. Reference to an "occluder" includes one or more
 occluders or plugs, and reference to "ocular channel" includes the punctum
 and canaliculum.
 Unless defined otherwise, all technical terms and scientific terms used
 herein have the same meaning as commonly understood by one ordinarily
 skilled in the art to which this invention belongs. Although any methods
 and materials similar or equivalent to those described herein may be used
 in the practice or testing of the present invention, preferred methods and
 materials are described below. All publications mentioned herein are
 incorporated by reference. Further, specific terminology of particular
 importance to the description of the invention is defined below.
 The terms "occluding" or "blocking" refer to the process of partially
 and/or completely filling at least a portion or section of an ocular
 channel, passage, opening, cavity or space with a substance that hinders
 and/or completely prevents the transport or movement of another substance
 through the channel. This "other substance" is generally tears. In
 preferred embodiments, the channel is completely blocked to prevents the
 flow of tears.
 The term "biocompatible" is intended to mean that no acute physiological
 activity is observed in response to the presence of the material or
 substance described as possessing such a property. Examples of
 unacceptable physiological activity would include surface irritation,
 cellular edema, etc.
 The terms "polymer" and "polymeric material" are used interchangeably
 herein to refer to materials formed by linking atoms or molecules together
 in a chain to form a longer molecule, i.e., the polymer. The polymers used
 in the present invention are preferably biologically inert, biocompatible
 and non-immunogenic. The particularly preferred polymeric materials are
 biocompatible, non-immunogenic and not subject to substantial degradation
 under physiological conditions.
 The terms "polymer", "polymer composition", "polymeric material",
 "composition", and "composite" are interrelated. The terms "polymer
 composition" and "polymeric material" are used interchangeably and refer
 to either the polymer or polymeric material itself as defined above or a
 composite as defined below. The term "composite" refers to a combination
 of a polymer with a biologically inert substance that need not qualify as
 a "polymer" but may have the special characteristics of having a melting
 point above body temperature and may have the ability to provide desirable
 properties to the polymer (such as to toughen or act as a heat sink for
 the polymer). Examples of these biologically inert substances or waxes
 are, for example, octadecane or oligomeric polyethylenes.
 The term "melting point" (T.sub.m) of the polymer refers to the temperature
 at which the peak of the endotherm rise is observed when the temperature
 is raised through the first order transition at standard atmospheric
 conditions. The first order transition is the melting point of the
 crystalline domains of the polymer. The peak developed in the trace of a
 differential scanning calorimeter (DSC) analysis experiment has been used
 to define this transition (see Encyclopedia of Polymer Science and
 Engineering, 2.sup.nd edition, vol. 4, pp. 482-519).
 The term "glass transition temperature" (T.sub.g) refers to the temperature
 at which the amorphous domains of a polymer take on the characteristic
 properties of the glassy state-brittleness, stiffness, and rigidity. At
 the glass transition temperature, the solid, glassy polymer begins to
 soften and flow (see Encyclopedia of Polymer Science and Engineering,
 2.sup.nd edition, vol. 7, pp. 531-544).
 The term "smart plug" and "plug" are used interchangeably and refer to the
 polymer, polymeric material, polymer composition or composite in its solid
 elongated form below the crystalline T.sub.m or T.sub.g (i.e. prior to
 insertion into the ocular channel) and in the shape and dimensions of the
 channel which it fills.
 Main chain crystallizable polymers (MCC polymers) are useful for this
 invention and are well-known, some of which, are commercially available.
 These are described by Robert W. Lenz, "Organic Chemistry of Synthetic
 High Polymers", John Wiley & Sons, New York, 1967, pp. 44-49, incorporated
 herein by reference. Generally, these polymers are characterized as having
 crystallizable structures, such as stiff repeating units or stereoregular
 repeating units, as part of the main polymer chains. The more persistent
 the crystalline structural units, the higher degree of crystallinity of
 the polymer.
 Side chain crystallizable polymers (SCC polymers) are also particularly
 useful for this invention and also are well-known, some of which are
 commercially available. These polymers are described in J. Polymer Sci.:
 Macromol. Rev. 8:117-253 (1974), the disclosure of which is hereby
 incorporated by reference. In general, these polymers are characterized as
 having a crystallizable cluster off to the side of the main backbone and
 can be made in several configurations, i.e. homopolymers, random
 copolymers, block copolymers and graft copolymers.
 Polymeric Materials
 In general, material compositions of the present invention can be divided
 into two classes. The first class contains at least one component which
 has a glass transition temperature (Tg) at or below human body temperature
 (37.degree. C.). The second class contains at least one component which
 has a melting temperature (T.sub.m) at or below human body temperature
 (37.degree. C.). Compositions containing both the first class and second
 class can also be used for the present invention as long as either (or
 both) the T.sub.g or T.sub.m of the mixture is below about 37.degree. C.
 The glass transition temperature of a polymer is the temperature above
 which the polymer is soft and elastic and below which the polymer is hard
 or glass-like. Examples of suitable T.sub.g polymeric materials include,
 but are not limited to, silicones, acrylic polymers, polyurethanes,
 hydrocarbon polymers, copolymers of the foregoing, and any combinations
 thereof. These polymers may be blended with wax-like materials, such as
 octadecane, or oligomeric polyethylenes to create a composite that
 contains both rigid, elastic and viscous properties and has a T.sub.g at
 or below 37.degree. C. Preferably, the T.sub.g -based polymeric material
 is an acrylic ester and more preferably it is a copolymer of
 laurylmethacrylate and methylmethacrylate.
 Generally speaking, the T.sub.g of a copolymer containing two or more
 monomers will be dependent on the percentage composition of the monomers.
 For example poly(methyl methacrylate) (PMMA) has a T.sub.g of 105.degree.
 C. Therefore, it is soft and rubbery, and it can be molded into various
 shapes above 105.degree. C. At room temperature, however, PMMA is hard and
 this is due to the short C-1 side chain. This hardness enhances the
 elasticity of the copolymer and is the driving force for the stretched
 polymer to return to its initial shape after the temperature increases
 above its T.sub.g. On the other hand, poly(lauryl methacrylate) (PLMA) has
 a Tg of -65.degree. C., and is soft at room temperature, due in part to
 the C-12 side chain. Thus, a copolymer containing various ratios of PMMA
 and PLMA can be designed to achieve any T.sub.g in the range of
 -65.degree. C. to 105.degree. C.
 For instance, a copolymer in a molar ratio of 40% lauryl methacrylate and
 60% methyl methacrylate, as described in Example 1, has a T.sub.g of
 19.degree. C. This particular side-chain copolymer has a number of
 desirable properties for the smart punctal plug design. Because the
 T.sub.g of this copolymer is 19.degree. C., at room temperature it is
 elastic and can be stretched. When the stretched sample is placed into ice
 water for about one minute, it remains in the stretched, rigid form as
 long as the surrounding temperature is maintained below 19.degree. C.
 However, those skilled in the art realize that the glass transition
 temperature for a polymer occurs over a temperature range, possibly
 10.degree. C. or even larger, rather than a single sharply defined
 temperature. Also, since this copolymer has C-12 alkyl side chains, there
 is a high degree of freedom associated with the various rotational
 perturbations the molecule may undergo. Such a copolymer is superior to
 the main chain crystallizable polymers as well as crosslinked polymers
 since these have much more restricted modes of rotational movement. Thus,
 the flexibility of the C-12 side chain of the LMA component enables this
 copolymer to readily conform to the shape of the ocular channel. The MMA
 component of this copolymer is relatively hard and elastic. This
 elasticity is the driving force for the stretched plug to return to its
 initial shape. Additionally, the LMA/MMA copolymer can be crosslinked
 using appropriate crosslinkers (see Example 2). Crosslinking further
 enhances the elastic properties of this copolymer. Finally, this copolymer
 is an acrylic ester and polymers of this chemical composition have been
 most widely used in ophthalmic implants because of their long-term
 stability and biocompatability.
 A second class of polymers which can also serve as an ideal material for
 this smart plug design are those polymers which have a T.sub.m lower than
 about 37.degree. C. The T.sub.m of these polymers is a function of the
 crystalline structure resulting from the nature of the main chain or side
 chain. The group of T.sub.m materials includes, but is not limited to,
 those compositions which have a crystalline structure based upon one or
 more side chains which contain at least 10 carbon atoms, or alternatively,
 any compositions whose crystalline structure is a function of the
 polymeric main chain structure.
 Examples of side chain crystalline materials are, but not limited to,
 homopolymers or copolymers that contain one or more monomeric units
 (wherein n=at least 1 monomer unit) having the general formula:
 ##STR1##
 wherein
 X is H, or a C.sub.1 -C.sub.6 alkyl radical;
 R is a linear C.sub.10 -C.sub.26 alkyl radical.
 For example, poly(stearylmethacrylate) (PSMA) is a white solid which has an
 observed melting temperature of 34.degree. C. (see Table 1). This melting
 temperature is mainly attributed to the crystalline structure of the
 polymer due to the presence of the pendant 18-carbon side chain. Upon
 warming the PSMA up to the human body temperature (ca. 37.degree. C.),
 this white solid is transformed into a clear elastic polymer. Furthermore,
 the elastic properties of PSMA can be altered by copolymerization with one
 or more other monomers. Also, whether the PSMA copolymer becomes more
 elastic or more rigid than PSMA alone is determined by the nature of the
 added monomers. Table 1 illustrates the properties of various copolymer
 compositions of stearylmethacrylate (SMA) with methylmethacrylate (MMA).
 As illustrated in Table 1, when the percentage of MMA increases in the
 copolymer composition, the copolymer become more rigid and its elasticity
 increases. For the present invention, preferably this composition is a
 copolymer constituting at least 95% SMA/5% MMA, and more preferably 97.5%
 SMA/2.5% MMA.
 TABLE 1
 SMA/MMA Polymer Compositions and Their Melting Temperature
 Melting
 Weight of SMA Weight of MMA Temperature
 ID (grams) (grams) (.degree. C.)
 PSMA 1.0 0 34
 97.5% PSMA 9.75 0.25 28
 95% PSMA 9.50 0.50 26
 90% PSMA 9.0 1.0 22
 80% PSMA 8.0 2.0 18
 (SMA = stearylmethacrylate monomer; MMA = methylmethacrylate monomer)
 Examples of the main chain crystallizable materials of the T.sub.m family
 include, but are not limited to, silicone elastomers derived from the
 general structure of poly[methyl(3,3,3-trifluoro-propyl)siloxane].
 Examples of such silicone elastomers are disclosed in U.S. Pat. No.
 5,492,993, Saam et al., issued Feb. 20, 1996, and also described in
 Strain-Induced Crystallization in
 Poly[methyl(3,3,3-trifluoropropyl)siloxane] Networks, Battjes et al.,
 Macromolecules, 1995, 28, 790-792, both of which are incorporated by
 reference herein.
 Description of the Methodology
 As discussed above, it is possible to engineer materials with balanced
 rigid, elastic and viscous properties. A smart plug made from these
 materials is elongated at temperatures above its T.sub.g or T.sub.m and
 the plug subsequently frozen in its elongated form at temperatures below
 its T.sub.g or T.sub.m. Upon insertion into the ocular channel, the plug
 "senses" an increase in its external environmental temperature. In
 response to this increase in temperature, the elongated rigid plug becomes
 soft and rubbery, which in turn triggers the shape recovery motion caused
 by the elastic properties of the plug material. Once the plug
 approximately corresponds to the size of the ocular channel, resistance
 from surrounding tissue stops further expansion of the plug and the plug
 will "rearrange" itself to the size and the shape of the patient's ocular
 channel now based upon the inherent viscosity of the composition. It is
 noted that such movement by the composition due to its viscosity at a
 molecular level and results from the presence of pendant hydrocarbon side
 chains on the polymer. Thus, this is a "smart plug" since it is able to
 adapt to the size and the shape of each patient's ocular channel.
 When a plug of this new design is fabricated, its initial dimension (15) is
 designed to fit the large size of the punctum, which is approximately 1 mm
 to about 5 mm in length and approximately 0.5 mm to a maximum of about 2.5
 mm in diameter. This large size punctal plug is then elongated into a
 thin, needle-like rod (16), at temperatures above the T.sub.g or T.sub.m,
 to a length of about twice that of the initial plug length. The diameter
 is thus reduced to about 70% or less of its initial diameter. Cooling the
 elongated plug to temperatures below its T.sub.g or T.sub.m will freeze
 the needle-like shape as long as the temperature remains below the T.sub.g
 or T.sub.m of the plug material. Consequently, doctors can simply insert
 the elongated plug into an ocular channel. Since this elongated plug is
 rigid and has a reduced diameter, there is no need to dilate the punctum
 as disclosed in prior art (for example, see U.S. Pat. No. 3,949,750,
 Freeman, issued Apr. 13, 1976). Upon warming, this plug becomes soft and
 rubbery and the elastic component of the plug material will cause the
 elongated needle-like solid rod to return to its original larger size and
 shape (15) if no restriction force is applied to the plug. This shape
 transformation is illustrated in FIG. 6. FIG. 8 shows the initial shape of
 the plug (15), the elongated shape (16) after it is stretched and frozen,
 and the adaptation of the plug to the shape of the punctum (17) after the
 plug is inserted and warmed to body temperature.
 However, for in vivo applications, the surrounding ocular tissue will
 supply a resisting force to the expanding plug once the plug reaches the
 size of the particular ocular channel in which it resides, achieving a
 "one-size-fits-all" plug design. Where and when the plug stops expanding
 is controlled by the balance between the elastic properties of the plug
 material and the resistance supplied by the surrounding tissue. In terms
 of polymer rheology, how well the needle-like rod will adapt to the size
 and shape of the patient's punctum or canaliculus is determined by the
 ratio of the elastic and viscous components of the polymeric material. The
 higher the percentage of the viscous component, such as a C-18 side-chain
 crystallizable polymer, the more likely the plug will conform to the size
 and the shape of the ocular channel. In order to provide a sufficient
 quantity of the viscous component to the polymer composition, the side
 chain crystallizable polymer should be a C.sub.10 or higher carbon
 radical; alternatively, if the composition is a copolymer, 50% of the
 copolymer composition should contain a C.sub.10 or higher side chain
 crystallizable polymer.
 In typical applications, the use of homopolymers or copolymers (see Table
 1) is routinely used for the smart punctal plug design. For example, a
 solid rod of the copolymer polystearylmethacrylate/methylmethacrylate (1
 mm in diameter, 100 mm in length) is warmed to a temperature above its
 T.sub.m so that it becomes soft. The rod is subsequently stretched to
 approximately 0.5 mm in diameter and 300 mm in length, and immersed in ice
 water. After approximately 1 minute of cooling, the rod will remain in its
 elongated, rigid form. The rod is maintained at a temperature below its
 T.sub.m until it becomes opaque, indicating that crystallization of the
 polymer side chain has occurred. At this point, the stretched rod is
 almost as hard as a solid crystal. This hardness is required for ease of
 insertion of the rod into the ocular channels. This elongated rod is then
 cut into pieces which are 6 mm in length. One end of shortened rod is then
 sharpened to form a tapered point for ease of insertion. If necessary, a
 flared neck may be also formed at the opposite end of the rod. A total of
 approximately 45-50 pieces may be formed in this manner. Upon insertion,
 the rod is warmed by the surrounding body tissue, and the elongated form
 of the rod will begin to deform, and take on the shape of the ocular
 channel for occlusion. The final dimensions of the plug will conform to
 the size and shape of the individual's ocular channel.
 Finally, if doctors wish to remove the punctal plug, an ice patch is
 applied to the outside area surrounding the punctum. In a few minutes, the
 smart punctal plug becomes hard again. Doctors can use regular tools, such
 as forceps, to grasp the plug and extract it. This procedure eliminates
 any risks involved in removing a soft plug hat may break into small pieces
 when pulled out using forceps.
 EXAMPLES
 In order that the present invention may be more fully understood, the
 following examples and other comparative results are given by way of
 illustration only and are not intended to be limiting.
 Example 1
 To a round-bottomed flask, under N.sub.2 atmosphere, equipped with a
 magnetic stirring bar, is added a mixture of 8.89 g of methylmethacrylate,
 15.39 g of laurylmethacrylate, and 0.02 g of benzoyl peroxide. The
 reaction mixture is heated to approximately 100-110.degree. C. After
 approximately 20 minutes, evolution of O.sub.2 (g) is observed, indicating
 decomposition of the benzoyl peroxide for initiation of the polymerization
 reaction. After about 5 minutes from the initial evolution of gas, the
 reaction mixture becomes viscous, indicating the polymerization has
 started. Before the reaction mixture becomes too viscous, it is
 transferred to a Teflon plate equipped with a Teflon gasket. A second
 Teflon plate is then placed on top of the reaction mixture in order to
 sandwich the polymer between the two Teflon plates. This set of Teflon
 plates containing the polymer is then placed in between two glass plates,
 clamped together, and heated in an oven at 90.degree. C. for 15 h. The
 temperature is then raised to 130.degree. C. for an additional 3 h. At
 this point, the glass plates containing the polymer are removed from the
 oven and produce a transparent elastic sheet of the polymer measuring 3.5
 inches.times.4.5 inches. The polymer has a T.sub.g of 19.degree. C.
 Mechanical properties of the polymer as measured by ASTM D412 are as
 follows: Tensile strength: 292 psi; Elongation at break: 531%. Since there
 are no crosslinkers in this composition, the copolymer is soluble in
 organic solvents, such as chloroform. By .sup.1 H NMR, there are no
 visible signals caused by the vinyl protons, indicating the polymerization
 is complete (see FIG. 7). Quantitative analysis of .sup.1 H NMR indicates
 that there is approximately 56% mole percentage of lauryl methyacrylate in
 this copolymer. This copolymer is a thermoelastomer. A unique
 characteristic for a thermoelastomer is that it can be injection molded at
 temperatures above its class transition temperature.
 Example 2
 The same procedure as in Example 1 is followed with the exception of the
 reactants: 7.8 g of methylmethacrylate is combined with 13.2 g of
 laurylmethacrylate, and 0.07 g of a crosslinker, ethylene dimethacrylate,
 is also added to the reaction mixture. The copolymerization is initiated
 by benzoyl peroxide. The resulting copolymer has a T.sub.g of 9.degree. C.
 Due to its crosslinking, this copolymer is not soluble in any organic
 solvents. In fact, due to the high degree of crosslinking, this copolymer
 behaves like a typical elastomer with a very small viscosity component.
 Mechanical properties of the polymer as measured by ASTM D412 are as
 follows: Tensile strength: 550 psi; Elongation at break: 488%.
 Example 3
 The same procedure as in Example 1 is followed with the following
 exceptions: a mixture of 9 g of stearylmethacrylate is combined with 1 g
 of methylmethacrylate, using benzoyl peroxide as the initiator, and the
 reaction mixture is heated to 110.degree. C. for 15 h. The melting
 temperature of the resulting copolymer is 22.degree. C. (see Table 1). The
 other compositions listed in Table 1 are also prepared in a similar
 fashion.
 Example 4
 To a round bottom flask, under a N.sub.2 atmosphere, is added a mixture of
 9 g stearylmethacrylate, 1 g of methylmethacrylate and 0.02 g of benzoyl
 peroxide. The reaction mixture is gently stirred until all the benzoyl
 peroxide is dissolved. The resulting solution is injected into a glass
 capillary tube (ca. 1 mm in diameter and 100 mm in length) using a 100
 .mu.L syringe. The glass capillary is sealed and heated to 100.degree. C.
 overnight. The tube is then cooled to 10.degree. C. for 15 h during which
 time the copolymer turns white due to crystallization of the side chain
 structure. The glass tube is carefully broken to yield a solid copolymer
 measuring ca. 1 mm in diameter and 100 mm in length.
 The resulting copolymer rod is warmed to 40.degree. C. in a water bath,
 stretched to approximately 300 mm in length and then cooled for 1 minute
 in an ice bath to allow the stretched rod to re-solidify. The stretched
 rod is subsequently cut into pieces 6 mm in length with one end sharpened
 and the opposing end optionally having a flared neck.
 Example 5
 The same procedure as in Example 4 with the following exceptions: 0.02 g of
 ethylene dimethacrylate is added as a crosslinker. The addition of the
 crosslinker makes this copolymer more rigid and therefore more elastic so
 that it has a better ability to recover from the stretched solid form back
 to its initial shape. Also, the crosslinking of the copolymer reduces the
 viscous properties of the copolymer so that a larger externally applied
 force is needed in order to stop the shape recovery process. Therefore, by
 controlling the amount of crosslinking agent used in a composition, it is
 possible to achieve a balance of properties for desirable viscous and
 elastic characteristics.
 Example 6
 Preparation of copolymer of 62% cis- and 38% trans- isomers of
 1,3,5-trimethyl-1,3,5-tris(3',3',3'-trifluoropropyl)cyclotrisiloxane and
 curing (crosslinking):
 (1) Preparation of the initiator: In a 10 mL round-bottom flask, under Ar,
 equipped with magnetic stir bar is added 0.5 g diphenyldihydroxysilane, 5
 .mu.L of styrene and 4 mL of dry THF. At this point, 2.5M n-butyllithium
 (in hexanes) is added dropwise until the reaction mixture becomes yellow
 in color (ca. 1.8 mL of n-butyllithium was added).
 (2) Polymerization: In a 50 mL round-bottom flask, under Ar, is added 12.4
 g of 1,3,5-trimethyl-1,3-5-tris(3',3',3'-trifluoropropyl)cyclotrisiloxane
 (with 62% cis isomer and 38% trans isomer) in 12 mL of dry THF. To the
 siloxane is added dropwise 0.7 mL of the freshly prepared initiator
 solution. The reaction mixture is stirred at room temperature for 3 h. At
 this point a mixture of 0.5 mL of dimethylvinylchlorosilane and 0.5 ml of
 triethylamine is added and the reaction mixture stirred for an additional
 5.5 h. At this point, 25 mL of H.sub.2 O is added to the solution and
 after work-up of this reaction mixture, a copolymer with an average
 molecular weight of 74,200 and number average molecular weight of 49,600
 is obtained. DSC experiments indicate that the cis- enriched copolymer has
 a T.sub.m of 12.degree. C. and a T.sub.g of -69.degree. C.
 (3) Curing: To 0.2 g of the 62% cis- and 38% trans- isomers of
 poly-1,3,5-trimethyl-1,3,5-tris(3',3',3'-trifluoropropyl)trisiloxane is
 added 8 .mu.L of tetrakis(dimethylsiloxyl)siloxane and 2 drops of a Pt
 catalyst. The reaction mixture is stirred for 5 minutes, transferred to a
 glass capillary tube and sealed. The tube is placed in an oven at
 100.degree. C. for 15 h and subsequently cooled in an ice water bath until
 the silicone hardens, as indicated by it turning slightly cloudy. The
 glass tube is carefully broken to yield a solid copolymer measuring ca. 1
 mm in diameter and 100 mm in length.
 Example 7
 The following copolymers are prepared as described in Example 4.