Abstract:
Prosthetic implants designed to be implanted in the cornea for modifying the cornea curvature and altering the corneal refractive power for correcting myopia, and myopia with astigmatism, such implants formed of a micro-porous hydrogel material.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation of application Ser. No. 10/047,726, filed Jan. 15, 2002, which is a continuation of U.S. patent application Ser. No. 09/385,103, filed Aug. 27, 1999, now U.S. Pat. No. 6,361,560, which is a continuation-in-part of U.S. patent application Ser. No. 09/219,594, filed Dec. 23, 1998, now U.S. Pat. No. 6,102,946.  
     
    
     FIELD OF THE INVENTION  
       [0002]     The field of this invention relates to prosthetic implants designed to be implanted in the cornea for modifying the cornea curvature and altering the corneal refractive power for correcting myopia, hyperopia, astigmatism, and presbyopia, and, in addition, to such implants formed of a micro-porous hydrogel material.  
       BACKGROUND OF THE INVENTION  
       [0003]     It is well known that anomalies in the shape of the eye can be the cause of visual disorders. Normal vision occurs when light that passes through and is refracted by the cornea, the lens, and other portions of the eye, and converges at or near the retina. Myopia or near-sightedness occurs when the light converges at a point before it reaches the retina and, conversely, hyperopia or far-sightedness occurs when the light converges a point beyond the retina. Other abnormal conditions include astigmatism where the outer surface of the cornea is irregular in shape and effects the ability of light to be refracted by the cornea. In addition, in patients who are older, a condition called presbyopia occurs in which there is a diminished power of accommodation of the natural lens resulting from the loss of elasticity of the lens, typically becoming significant after the age of  45 .  
         [0004]     Corrections for these conditions through the use of implants within the body of the cornea have been suggested. Various designs for such implants include solid and split-ring shaped, circular flexible body members and other types of ring-shaped devices that are adjustable. These implants are inserted within the body of the cornea for changing the shape of the cornea, thereby altering the its refractive power.  
         [0005]     These types of prostheses typically are implanted by first making a tunnel and/or pocket within the cornea which leaves the Bowman&#39;s membrane intact and hence does not relieve the inherent natural tension of the membrane.  
         [0006]     In the case of hyperopia, the corneal curvature must be steepened, and in the correction of myopia, it must be flattened. The correction of astigmatism can be done by flattening or steepening various portions of the cornea to correct the irregular shape of the outer surface. Bi-focal implants can be used to correct for presbyopia.  
         [0007]     It has been recognized that desirable materials for these types of prostheses include various types of hydrogels. Hydrogels are considered desirable because they are hydrophilic in nature and have the ability to transmitting fluid through the material. It has been accepted that this transmission of fluid also operates to transmit nutrients from the distal surface of the implant to the proximal surface for providing proper nourishment to the tissue in the outer portion of the cornea.  
         [0008]     However, while hydrogel lenses do operate to provide fluid transfer through the materials, it has been found that nutrient transfer is problematic because of the nature of fluid transfer from cell-to-cell within the material. Nutrients do not pass through the hydrogel material with the same level of efficacy as water. Without the proper transfer of nutrients, tissue in the outer portion of the cornea will die causing further deterioration in a patient&#39;s eyesight.  
         [0009]     Thus, there is believed to be a demonstrated need for a material for corneal implants that will allow for the efficacious transmission of nutrients from the inner surface of a corneal implant to the outer surface, so that tissue in the outer portion of the cornea is properly nourished. There is also a need for a more effective corneal implant for solving the problems discussed above.  
       DESCRIPTION OF THE PRIOR ART  
     SUMMARY OF THE INVENTION  
       [0010]     The present invention is directed to a corneal implant formed of a biocompatible, permeable, micro-porous hydrogel with a refractive index substantially similar to the refractive index of the cornea. The device, when placed under a lamellar dissection made in the cornea (such as a corneal flap), to relieve tension of Bowman&#39;s membrane, alters the outer surface of the cornea to correct the refractive error of the eye. By relieving the pressure and subsequent implantation of the device, the pressure points which typically are generated in present corneal surgeries are eliminated, and hence reduced risk to patients of extrusion of implants.  
         [0011]     The implant is preferably generally circular in shape and is of a size greater than the size of the pupil in normal or bright light, and can specifically be used to correct hyperopia, myopia, astigmatism, and/or presbyopia. Due to the complete non-elastic nature of the corneal tissue, it is necessary to place the implant in the cornea with Bowman&#39;s membrane compromised, such as through a corneal lamellar dissection, to prevent extrusion of the implant from the cornea over the lifetime of the implant. Extrusion is undesirable because it tends to cause clinical complications and product failure.  
         [0012]     Preferably, for the correction of hyperopia, the implant is formed into a meniscus-shaped disc with its anterior surface radius smaller (steeper) than the posterior surface radius, and with negligible edge thickness. This design results in a device-that has a thickness or dimension between the anterior and posterior surfaces along the central axis greater than at its periphery. When such an implant is placed under the corneal flap, the optical zone of the cornea is steepened and a positive optical power addition is achieved.  
         [0013]     For the correction of myopia, the implant is shaped into a meniscus lens with an anterior surface curvature that is flatter than the posterior surface. When the implant is placed concentrically on the stromal bed the curvature of the anterior surface of the cornea in the optic zone is flattened to the extent appropriate to achieve the desired refractive correction.  
         [0014]     For astigmatic eyes, implants are fabricated with a cylindrical addition along one of the axes. This device can be oval or elliptical in shape, with a longer axis either in the direction of cylindrical power addition or perpendicular to it. The implant preferably has a pair of markers such as, for example, protrusions, indentations or other types of visual indicators, in the direction of the cylindrical axis to easily mark and identify this direction. This indexing assists the surgeon in the proper placement of the implant under the flap with the correct orientation during surgery to correct astigmatism in any axis.  
         [0015]     For simple or compound presbyopia, the implant is made by modifying the radius of curvature in the central 11.5-3 mm, thereby forming a multi-focal outer corneal surface where the central portion of the cornea achieves an added plus power for close-up work. The base of an implant designed for compound presbyopia can have a design to alter the cornea to achieve any desired correction for the myopic, hyperopic, or astigmatic eye.  
         [0016]     The material from which any one or more of these implants are made is preferably a clear, permeably, microporous hydrogel with a water content greater than 40% up to approximately 90%. The refractive index should be substantially identical to the refractive index of corneal tissue. The permeability of the material is effected through a network of irregular passageways such as to permit adequate nutrient and fluid transfer to prevent tissue necrosis, but which are small enough to act as a barrier against the tissue ingrowth from one side of the implant to another. This helps the transmembrane tissue viability while continuing to make the implant removable and exchangeable.  
         [0017]     The refractive index of the implant material should be in the range of 1.36-1.39, which is substantially similar to that of the cornea (1.376). This substantially similar refractive index prevents optical aberrations due to edge effects at the cornea-implant interface.  
         [0018]     The microporous hydrogel material can be formed from at least one (and preferably more) hydrophilic monomer, which is polymerized and cross-linked with at least one multi- or di-olefinic cross-linking agent.  
         [0019]     The implants described above can be placed in the cornea by making a substantially circular lamellar flap using any commercially available microkeratome. When the flap is formed, a hinge is preferably left to facilitate proper alignment of the dissected corneal tissue after the implant is placed on the exposed cornea.  
         [0020]     The implants described above which can be used for correcting hyperopia or hyperopia with astigmatism are preferably made into a disc shape that is nominally about 4.5 mm in diameter and bi-meniscus in shape. The center of the lens is preferably no greater than 50 micrometers thick. The edge thickness should be less than two keratocytes (i.e., about 15 micrometers).  
         [0021]     An improvement over the lenses described above for correcting myopia with astigmatism includes forming a lens in the shape of a ring with one or more portions in the center being solid and defining voids in the center section for shaping the astigmatic component by providing solid portions under the flatter meridian of the astigmatic myopic eye. An example of such a shape includes a ring with a rib extending across the center that is either squared off or rounded where it contacts the ring. Another example is a ring with one or more quadrants filled in, with the other ones forming voids. Other shapes can used to provide a solid portion under the flatter meridan. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     A better understanding of the invention can be obtained from the detailed description of exemplary embodiments set forth below, when considered in conjunction with the appended drawings, in which:  
         [0023]      FIG. 1  is a schematic illustration of a horizontal section of a human eye;  
         [0024]      FIG. 2  is a schematic illustration of an eye system showing adjustment of the cornea to steepen the corneal slope to correct for hyperopia;  
         [0025]      FIG. 3  is a schematic illustration of an eye system showing adjustment of the cornea to flatten the corneal slope to correct for myopia;  
         [0026]      FIGS. 4   a  and  4   b  are sectional and plan views of a solid corneal implant for correcting hyperopia;  
         [0027]      FIGS. 5   a  and  5   b  are sectional and plan views of a solid corneal implant for correcting myopia;  
         [0028]      FIGS. 6   a  and  6   b  are sectional and plan views of ring-shaped corneal implant for correcting myopia;  
         [0029]      FIGS. 7   a  and  7   b  are schematic representations of a lamellar dissectomy, with  FIG. 7   b  showing in particular the portion of the dissected cornea being connected through a hinge to the intact cornea;  
         [0030]      FIG. 8  is a schematic representations of a cornea in which an implant has been implanted for a hyperopic correction;  
         [0031]      FIGS. 9 and 10  are schematic representations of a cornea in which solid and ring-shaped implants, respectively, have been implanted lamellar for a myopic correction;  
         [0032]      FIGS. 11   a,    1   b,  and  11   c  are plan and sectional views of an implant useful for correcting astigmatism where two axes have different diopter powers;  
         [0033]      FIGS. 12   a ,  12   b , and  12   c  are plan and sectional views of an second implant for correcting astigmatism where the implant is elliptical in shape;  
         [0034]      FIG. 13  is a plan view of an implant with a pair of tabs used to identify an axis for astigmatic correction;  
         [0035]      FIG. 14  is a plan view of a second implant for astigmatic correction where indentations are used instead of tabs;  
         [0036]      FIGS. 15 and 16  are schematic representations showing implants with tabs orientated along the astigmatic-axis for correcting astigmatism;  
         [0037]      FIG. 17  is a sectional view of a corneal implant shaped to correct for compound presbyopia with an additional power in the center of an implant for correcting hyperopia;  
         [0038]      FIG. 18  is a sectional view of another corneal implant shaped to correct for compound presbyopia with additional power in the center of an implant for correcting myopia;  
         [0039]      FIG. 19  is a sectional view of a corneal implant with additional power in the center for correcting simple presbyopia;  
         [0040]      FIG. 20   a  is a schematic representation of a corneal implant for an astigmatic correction with a central power add for correcting presbyopia, showing in particular a pair of tabs for proper alignment of the lens;  
         [0041]      FIG. 20   b  is a schematic representation of a another corneal implant with a center power add for non-astigmatic correction, which shows in particular a steep transition between the central add and the remainder of the implant;  
         [0042]      FIGS. 21   a  and  21   b  are schematic representations showing the use of a lamellar dissection for implanting a lens of the type shown in  FIG. 20   b ; and  
         [0043]      FIGS. 22 and 23  are schematic representations of several lenses useful for correcting myopia with astigmatism formed in the shape of a ring with a rib extending across the center of the lens; and  
         [0044]      FIG. 24  is another schematic representation of another lens for correcting myopia with astigmatism where the ring-shaped lens has one quadrant that is solid, while the rest of the center portion forms a void. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0045]     Referring first to  FIG. 1  of the drawings, a schematic representation of the globe of the eye  10  is shown, which resembles a sphere with an anterior bulged spherical portion  12  that represents the cornea. The eye  10  is made up of three concentric coverings that enclose the various transparent media through which light must pass before reaching the light sensitive retina  14 .  
         [0046]     The outer-most covering is a fibrous protective portion that includes a posterior layer which is white and opaque, called the sclera  16 , which is sometimes referred to as the white of the eye where it is visible from the front. The anterior ⅙th of this outer layer is the transparent cornea  12 .  
         [0047]     A middle covering is mainly vascular and nutritive in function and is made up of the choroid  18 , the ciliary  20  and the iris  22 . The choroid generally functions to maintain the retina. The ciliary muscle  21  is involved in suspending the lens  24  and accommodating the lens. The iris  22  is the most anterior portion of the middle covering of the eye and is arranged in a frontal plane. The iris is a thin circular disc corresponding to the diaphragm of a camera, and is perforated near its center by a circular aperture called the pupil  26 . The size of the pupil varies to regulate the amount of light that reaches the retina  14 . It contracts also to accommodate, which serves to sharpen the focus by diminishing spherical aberrations. The iris  22  divides the space between the cornea  12  and the lens.  24  into an anterior chamber  28  and posterior chamber  30 .  
         [0048]     The inner-most covering is the retina  14 , consisting of nerve elements which form the true receptive portion for visual impressions that are transmitted to the brain. The vitreous  32  is a transparent gelatinous mass which fills the posterior ⅘ths the globe  10 . The vitreous supports the ciliary body  20  and the retina  14 .  
         [0049]     Referring to  FIG. 2  of the drawings, the globe of an eye  10  is shown as having a cornea  12  with a normal curvature represented by a solid line  34 . For people with normal vision, when parallel rays of light  36  pass through the corneal surface  34 , they are refracted by the corneal surfaces to converge eventually near the retina  14  ( FIG. 1 ). The diagram of  FIG. 2  discounts, for the purposes of this discussion, the refractive effect of the lens or other portions of the eye. However, as depicted in  FIG. 2 , when the eye is hyperopic the rays of light  36  are refracted to converge at a point  38  behind the retina.  
         [0050]     If the outer surface of the cornea  12  is caused to steepen, as shown by dotted lines  40 , such as through the implantation of a corneal implant of an appropriate shape as discussed below, the rays of light  36  are refracted from the steeper surface at a greater angle as shown by dotted lines  42 , causing the light to focus at a shorter distance, such as directly on the retina  14 .  
         [0051]      FIG. 3  shows a similar eye system to that of  FIG. 2 . except that the normal corneal curvature causes the light rays  36  to focus at a point  44  in the vitreous which is short of the retinal surface. This is typical of a myopic eye. If the cornea is flattened as shown by dotted lines  46  through the use of a properly-shaped corneal implant, light rays  36  will be refracted at a smaller angle and converge at a more distant point such as directly on the retina  14  as shown by dotted lines  48 .  
         [0052]     A hyperopic eye of the type shown in  FIG. 2  can be corrected by implanting an implant  50  having a shape as shown in  FIGS. 4   a ,  4   b . The implant  50  is in the shape of a meniscus lens with an outer surface  52  that has a radius of curvature that is smaller than the radius of curvature of the inner surface  54 . When a lens of this type is implanted using the method discussed below, it will cause the outer surface of the cornea to become steeper in shape as shown by reference numeral  40  in  FIG. 2 , correcting the patient&#39;s vision so that light entering the eye will converge on the retina as shown by the dotted lines  42  in  FIG. 2 .  
         [0053]     The lens  50  shown in  FIGS. 4   a  and  4   b  is formed with a bi-meniscus shape, with the anterior and posterior surfaces having different radii of curvature.  
         [0054]     The anterior surface has a greater radius than the posterior surface. The lens  50  preferably has a nominal diameter of about 4.5 mm. The center of the lens is preferably no greater than 50 micrometers thick to enhance the diffusion characteristics of the material from which the lens is formed, which allows for more effective transmission of nutrients through the lens material and promotes better health of the anterior corneal tissue. The outer edge of the lens  50  has a thickness that is less than the dimensions of two keratocytes (i.e., about 15 micrometers) juxtaposed side-by-side, which are the fixed flattened connective tissue cells between the lamellae of the cornea. An edge thickness as specified prevents stacking and recruitment of keratocytes in the lens material so that keratocyte stacking and recruitment does not take place. This in turn eliminates unorganized collagen that forms undesirable scar tissue and infiltrates the lens, which tends to compromise the efficacy of the lens.  
         [0055]     On the other hand, in order to cure myopia, an implant  56  having the shape shown in  FIGS. 5   a,    5   b , can be used where an outer surface  58  is flatter or formed with a larger radius than that of the inner surface  60  which is formed with a radius of curvature substantially identical to that of the corneal stroma bed generated by the lamellar dissection described below. The implant  56  has a transition zone  62  formed between the outer and inner surfaces  58 ,  60 , which is outside of the optical zone. In this way, the curvature of the outer surface of the cornea, as shown in  FIG. 3 , is flattened to an extent appropriate to achieve the proper refractive correction desired so that light entering the eye will converge on the retina as shown in  FIG. 3 .  
         [0056]     Alternatively, instead of using a solid implant as shown in  FIGS. 5   a ,  5   b , for correcting myopia, a ring  64  of the type shown in.  FIGS. 6   a ,  6   b  could be used. This ring has substantially the same effect as the implant shown in  FIGS. 5   a ,  5   b , by flattening the outer surface of the cornea shown in  FIG. 3 . The ring  64  has a center opening  66  that is preferably larger than the optical zone so as not to cause spherical aberrations in light entering the eye.  
         [0057]     Implants of the type shown in  FIGS. 4, 5  and  6  can be implanted in the cornea using a lamellar dissectomy shown schematically in  FIGS. 7   a ,  7   b . In this procedure, a keratome (not shown) is used in a known way to cut a portion of the outer surface of the cornea  12  along dotted lines  68  as shown in  FIG. 7   a . This type of cut is used to form a corneal flap  70  shown in  FIG. 7   b , which remains attached to the cornea  12  through what is called a hinge  72 . The hinge  72  is useful for allowing the flap  70  to be replaced with the same orientation as before the cut.  
         [0058]     As is also known in the art, the flap is cut deeply enough to dissect the Bowman&#39;s membrane portion of the cornea, such as in keratome surgery or for subsequent removal of the tissue by laser or surgical removal. A corneal flap of 100 to 200 microns, typically 160 to 180 microns, will be made to eliminate the Bowman&#39;s membrane tension. This reduces the possibility of extrusion of the implants due to pressure generated within the cornea caused by the addition of the implant. Implants of the type shown in  FIGS. 4, 5  and  6  are shown implanted in corneas in  FIGS. 8, 9  and  10 , respectively, after the flap has been replaced in its normal position. These figures show the corrected shape for the outer surface of the cornea as a result of implants of the shapes described.  
         [0059]     Implants can also be formed with a cylindrical addition in one axis of the lens in order to correct for astigmatism, as shown in the implants in  FIGS. 11-16 . Such implants can be oval or elliptical in shape, which the longer axis either in the direction of cylindrical power addition or perpendicular to it. For example, the implant can be circular as shown in  FIG. 1  la where the, implant  72  has axes identified as x, y. In the case of a circular implant  72 , the axes of the implant have different diopter powers as shown in  FIGS. 11   b  and  11   bc,  which are cross-sectional views of the implant  72  along the x and y axes, respectively. The different thicknesses of the lenses in  FIGS. 11   b  and  11   c  illustrate the different diopter powers along these axes.  
         [0060]     Alternatively, as shown in  FIG. 5   a , an astigmatic implant  74  can be oval or elliptical in shape. The implant  74  also has axes x, y. As shown in the cross-sectional views of the implant  74  in  FIGS. 12   b ,  12   c , along those two axes, respectively, the implant has different diopter powers as shown by the different thicknesses in the figures.  
         [0061]     Because implants of the type identified by reference numeral  72 ,  74  are relatively small and transparent, it is difficult for the surgeon to maintain proper orientation along the x and y axes. In order to assist the surgeon, tabs  76   a ,  76   b  or indentations  78   a,    78   b  are used to identify one or the other of the axis of the implant to maintain proper alignment during implantation. This is shown in  FIGS. 15, 16  where, for example, indentations  76   a ,  76   b , are aligned with axis x which has been determined as the proper axis for alignment in order to effect the astigmatic correction. Alternatively, other types of markers could be used such as visual indicators such as markings on or in the implants outside of the optical zone.  
         [0062]     Referring to  FIGS. 17-21 , implants with presbyopic corrections are shown. In  FIG. 17 , an compound implant  80  is shown, which is appropriate for hyperopic correction, which has an additional power section  82  in the center. As shown, the implant  82  has anterior and posterior curvatures similar to those in  FIGS. 4   a ,  4   b , in order to correct for hyperopia. In  FIG. 18 , a central power add  84  is formed on another compound implant  86 , which has a base shape similar to the one shown in  FIGS. 5   a ,  5   b , and is appropriate for a myopic correction. In  FIG. 19 , a central power portion  88  is added to an simple planar implant  90  which has outer and inner surfaces of equal radii, which does not add any correction other than the central power.  
         [0063]     The central power add portions  82 ,  84 , and  88  are preferably within the range of 1.5-3 mm in diameter, most preferably  2 mm, and which provide a multi-focal outer corneal surface where the central portion of the cornea achieves an added plus power for close-up work. In addition to the based device having no correction, or corrections for hyperopia or myopia, the base device can have a simple spherical correction for astigmatism as shown in  FIG. 20   a  , where a central power add  92  is added to an implant  94  similar to the one shown in  FIG. 11   a,  which also includes tabs  76   a ,  76   b.    
         [0064]     As shown in  FIG. 20   b  in order to enhance the acuity of a presbyopic implant, a transition zone  96  can be formed around the central power add  98  for implant  100 . This transition zone  96  is a sharp zone change in power from central added power to peripheral base power and is anchored over a radial distance 0.5 to 0.2 mm start to from the end of the central zone.  
         [0065]     Implantation of the device shown in  FIG. 20   b , is illustrated in  FIGS. 21   a ,  21   b , where a flap  102  formed through a lamellar dissectomy is shown pulled back in  FIG. 21   a  so that the implant  100  can be positioned, and then replaced as shown in  FIG. 21   b  for the presbyopic correction. As shown, the formation of a sharp transition  96  on the implant  100  provides a well defined central power after implantation is complete.  
         [0066]      FIGS. 22 and 23  illustrate lenses  166 ,  168 , respectively, which are useful for correcting myopia with astigmatism. As shown, these lenses are ring-shaped, similar to the one in  FIGS. 6   a ,  6   b . However, the lenses  166 , 168  include rib sections  166   a ,  168   a,  respectively, which extend across the center of each lens and define voids between the ribs and the outer periphery of the lenses. These solid rib sections shape the astigmatic component by providing solid portions under the flatter meridian of the astigmatic myopic eye, when these flatter portions are located above the ribs. The ribs  166   a ,  168   a  can be formed in any suitable shape such as, by way of example, the rib  166   a  being squared off as shown in  FIG. 22  or the rib  168   a  being rounded s shown in  FIG. 23 , where they contact their respective rings.  
         [0067]     Another example of a design for correcting myopia with astigmatism is a lens  170  as shown in  FIG. 24 , which is also ring-shaped but has one its quadrants  170   a  filled in. This lens can be used where the flatter portion of an astigmatic eye is located in a position where the quadrant can be located beneath the flatter portion. The solid portion of the lens will tend to raise the flattened portion so that a smooth rounded outer surface is formed. As can readily be appreciated, lenses can be formed with solid portions located in any number of places where they can positioned under the flattened portion of an astigmatic eye to achieve the same end.  
         [0068]     The implants described above are preferably formed of a microporous hydrogel material in order to provide for the efficacious transmission of nutrients from the inner to the outer surface of the implants. The hydrogels also preferably have micropores in the form of irregular passageways, which are small enough to screen against tissue ingrowth, but large enough to allow for nutrients to be transmitted. These microporous hydrogels are different from non-microporous hydrogels because they allow fluid containing nutrients to be transmitted between the cells that make up the material, not from cell-to-cell such as in normal hydrogel materials. Hydrogels of this type can be formed from at least one, and preferably more, hydrophillic monomer which is polymerized and cross-linked with at least one multi-or di-olefinic cross-linking agent.  
         [0069]     An important aspect of the materials of the present invention is that the microporous hydrogel have micropores in the hydrogel. Such micropores should in general have a diameter ranging from 50 Angstroms to 10 microns, more particularly ranging from 50 Angstroms to 1 micron. A microporous hydrogel in accordance with the present invention can be made from any of the following methods.  
         [0070]     Hydrogels can be synthesized as a zero gel by ultraviolet or thermal curing of hydrophillic monomers and low levels of cross-linking agents such as diacrylates and other UV or thermal initiators. These lightly cross-linked hydrogels are then machined into appropriate physical dimensions and hydrated in water at elevated temperatures. Upon complete hydration, hydrogel prosthesis are flash-frozen to temperatures below negative 40° c., and then gradually warmed to a temperature of negative 20° c. to negative 10° c. and maintained at the same temperature for some time, typically 12 to 48 hours, in order to grow ice crystals to larger dimensions to generate the porous structure via expanding ice crystals. The frozen and annealed hydrogel is then quickly thawed to yield the microporous hydrogel device. Alternatively, the hydrated hydrogel device can be lyophilized and rehydrated to yield a microporous hydrogel.  
         [0071]     Still further, the microporous hydrogel can also be made by starting with-a known formulation of monomers which can yield a desired cross-linked hydrogel, dissolving in said monomer mixture a low molecular weight polymer as a filler which is soluble in said mixture and then polymerizing the mixture. Resulted polymer is converted into the required device shape and then extracted with an appropriate solvent to extract out the filled polymer and the result in a matrix hydrated to yield a microporous device.  
         [0072]     Still further and alternatively, microporous hydrogels can also be made by any of the above methods with the modification of adding an adequate amount of solvent or water to give a pre-swollen finished hydrogel, which can then be purified by extraction. Such formulation can be directly cast molded in a desired configuration and do not require subsequent machining processes for converting.