Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-05-11
2003-08-19
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S061000, C521S064000, C264S041000, C264S042000
Reexamination Certificate
active
06608117
ABSTRACT:
The invention disclosed herein deals in one embodiment with cellular hydrogels and methods for their preparation. The hydrogels of this invention can be colored, rendered radio opaque, or can be complexed, for example, with iodine and/or other germicides to yield useful materials.
The hydrogels can be formed into essentially any shape, size, or surface texture, and can have a wide range of desired degrees of porosity, that is, have any pore size, or pore geometry, or any pore size/geometry distribution.
The methods for preparing the hydrogels require dissolution of the precursor polymers in either single or mixed solvents capable of dissolving the polymer. The polymer solution is then loaded with a material (described infra) that creates the continuous polymer network structure with the embedded material. The mixture is then subjected to conditions that cause crystallization, gellation, or coagulation, or a mixture of crystallization, gellation, or coagulation, of the polymer through formation of physical sites. Thereafter, the material is removed to provide a cellular hydrogel.
The cellular hydrogels can then be subjected to a solvent treatment and/or a heat treatment to modify and further tailor the physical properties.
In an alternative method, the polymer is first dissolved in a solvent for the polyvinyl alcohol (PVA), or a mixture of solvents for the PVA, and a stable froth is prepared through the use of surface active agents or a mixture of surface active agents. The froth is then subjected to conditions that cause crystallization, gellation, coagulation or a mixture of crystallization, gellation, or coagulation, of the polymer through the formation of physical sites. The hydrogels formed in this manner can then be subjected to a solvent treatment or heat treatment to modify and further tailor the physical properties of the hydrogels.
BACKGROUND OF THE INVENTION
The published literature is abundant with references to various types of cellular materials made from polymers such as polyurethanes, polystyrenes, polyolefins, polyvinylchloride, epoxies, urea-form aldehyde, latices, silicones, fluoropolymers, and a number of other polymers. Numerous methods for the preparation and controlling the physical properties of cellular materials have been disclosed in the literature.
Manufacturing processes for making cellular polymers are well known to those skilled in the art with regard to bulk (solid) polymers. Typically, cellular polymers are made either by mechanically entrapping gas bubble in a polymer matrix or by incorporating removable, materials. Commonly, gas bubbles of nitrogen and carbon dioxide are mechanically entrapped either under normal atmospheric pressure or generated by sudden expansion of gas dissolved in the polymer matrix upon decrease of the pressure. Cellular structures can also be created by entrapping gas generated through a chemical reaction of an expansion agent or blowing agent. For instance, one can entrap carbon dioxide released during the chemical reaction of sodium bicarbonate and an acid. Usually, the chemical foaming methods are preferred over mechanical, that is, physical foaming methods. This is because, when physical foaming methods are used, it is, for instance, difficult to ensure homogeneous distribution of entrapped gas in the polymer matrix, control reduction of gas pressure, and control the diffusion rate of a gas out of the polymer matrix.
Those methods that have been reported in the literature for the preparation of PVA hydrogels can be divided into methods that rely on covalent cross-linking in one approach, and those methods that involve physical cross-linking.
The first method, covalent cross-linking, also known as chemical cross-linking, includes the use of multi-functional reactive molecules, that is, cross-linkers, such as aldehydes, maleic acid, dimethylurea, diisocyanates, boric acid and also includes ionizing radiation, ultra-violet, or any other agent capable of creating covalent cross-links between molecules. This method has been used to prepare bulk (non-porous) and cellular (porous) hydrogels.
The second, or alternative method, also known as physical or reversible cross-linking, includes cross-linking through crystallites, hydrogen bonding and complexing. Physical cross-linking through formation of crystallites in situ is the most desirable and can be accomplished by single freezing and then de-freezing; repeated freezing and de-freezing; partial or complete freeze-drying; controlled low temperature crystallization and the like.
A review of the prior art shows that physical cross-linking methods have been used only to prepare bulk hydrogels. No references related to the preparation of cellular hydrogels by physical cross-linking were found. The first of these references is U.S. Pat. No. 2,609,347, which issued to Wilson in 1952. This reference teaches the preparation of covalently cross-linked porous hydrogels by cross-linking the polymers with formaldehyde at temperatures between 20° C. and 60° C. in the presence of acid catalysts, such as sulfuric acid. The method is a frothing method, in that, porous structures are created by entrapping gas bubbles in the polymer solution in the presence of a wetting agent which stabilizes bubbles and helps to disperse the bubbles uniformly throughout. This patent also discloses the possibility of using cross-linked polymer hydrogels in a number of applications including the use as implants in the human body.
Since the publication of that patent, a number of methods, based on a covalent cross-linking of PVA as disclosed in the '347 patents have been reported for making cellular materials. In all cases of the prior art, the first step in the preparation of cellular hydrogels is dissolution of the polymer or its copolymers in an appropriate solvent, typically water. The next step is entrapment of air bubbles in the polymer solution in the presence of a surfactant and finally, cross-linking the polymer by treating it with di- or multi-functional cross-linkers.
All cross-linking agents used in the prior art render the sponges intractable and thus making them insoluble in any solvent due to formation of covalent bonds between molecules. Typically, cross-linking agents for the PVA were selected from the aldehyde family, such as formaldehyde, glyoxal, glutaraldehyde, terephthaldehyde and hexamethylenealdehyde that leads to formation of highly acetalized cellular PVA networks. PVA can also be cross-linked with unsaturated nitrites, di-diisocyanates, trimethylolmelamine, epichlorohydrin, polyacrylic acid, dimethylolurea, maleic anhydride, boric acid, sodium tetraboratedecahydrate (Borax) or by exposure to high-energy radiation.
Covalently cross-linked PVA sponges and bulk PVA hydrogels have a relatively long history of use in a wide variety of applications. Covalently cross-linked PVA sponges have already established themselves as very useful materials in numerous applications such as in packaging, thermal and acoustic insulation, construction, furniture, transportation, aerospace, food industry, household, textile, medical cosmetics and a number of other areas. For example, covalently cross-linked cellular PVA are used commercially as filters for water, air filters in intakes of compressors, engines, and air conditioners, oil filters, and the like. Large numbers of uses of PVA sponges are also based on their ability to readily absorb and hold water such as household washing sponges, absorbent cloths, industrial dehydrating rollers, paint rollers, acoustic filters, and the like.
The use of PVA sponges and PVA hydrogels in the medical field is especially important because of unique physico-chemical properties of PVA hydrogels. In spite of some incompatibility concerns and physical property limitations, acetalized PVA sponges have readily found significant use in medical fields such as, in cardio-vascular applications. Some of the important unique properties of acetalized PVA sponges are, for instance, being impervious to attack by body fluids such as, for example, blood, urine and other secre
Foelak Morton
McKellar Robert L.
McKellar Stevens
Nanosystems Research Inc.
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