A stain/topcoat system for non-porous thermoset and/or thermoplastic articles comprises a pigmented stain, and a topcoat comprising one or more non-siloxane film-forming polymers, an emulsion of one or more curable organopolysiloxanes, a weatherability agent having a functional group that is reactive with carboxylate functional sites and water. The topcoat displays exceptional adhesion and weatherability to pigmented stained surfaces.

The present invention pertains to weatherable coating systems suitable for application to thermoset or thermoplastic composite surfaces having minimal surface porosity. More particularly, the present invention pertains to a topcoat, and to a stain and topcoat system which can be easily applied by the consumer to produce an aesthetic, weatherable surface, particularly on exterior doors and door entry components.
Conventional finishing methods for exterior doors and door system components, where a traditional finely finished wood grain appearance is desired, have involved staining a bare wood surface with dye and/or pigment-bearing stains followed by application of one or more coats of exterior varnish. A seal coat is sometimes applied prior to the stain and topcoat to seal the pores of the wood and to promote a more adherent and uniform stain and topcoat. The stain is virtually always solvent borne, as water based stains have the effect of raising the grain, thus requiring an intermediate sanding step which is labor and time intensive. In addition to the solvent contained in the stain, stains contain drying oils as carriers, for example heat-bodied linseed oil. Generally, several different oils are provided to promote penetration of the stain into the wood surface.
After the stain has dried, one or more transparent topcoats are applied. Traditional topcoats are varnishes containing dissolved resins such as copal, often in conjunction with a drying oil. More recently, chemically modified natural resins and synthetic polymers have replaced all or part of natural resins, especially for outdoor use. In addition, to provide superior weatherability, ultraviolet absorbers have been added. Despite the advances made in such finishes, varnishes still generally require light sanding or abrading with steel wool or the like between coats, or necessitate the addition of the second coat prior to the full drying of the previous coat. Otherwise, delamination between varnish coats may occur. Sanding and similar treatments are time consuming, and judging the proper time to apply a second coat onto a not fully-cured prior coat requires some skill and experience. Despite these drawbacks, stain/varnish systems continue to be used on exterior door systems.
Recently, high quality exterior door systems employing molded thermoset and/or thermoplastic composite skins have become commercially available. These door systems are compression molded to exhibit a wood grain appearance, and when properly finished, are difficult to distinguish from natural wood doors. These composite doors offer several advantages over natural wood products, such as freedom from warpage, lower cost, greater insulation efficiency, and the like. Unfortunately, the stain/varnish systems in use for finishing natural wood doors are only marginally acceptable for composite doors. A satisfactory finish may often be achieved under production conditions where skilled workers, sophisticated spray equipment, drying ovens and the like are available. However, such systems are ill-suited for application by the consumer.
The difference between the ease with which finishes may be applied to natural wood products on the one hand and composite exterior door systems on the other, is due to the nature of the materials involved. Natural wood is a porous and absorbent product. Hence, dyes may penetrate the wood fibers as well as into the wood pores. The latter may also serve to trap and retain pigment, as may partially abraded fibers generated in sanding the wood prior to finishing. However, composite door skins are constructed of sheet molding compound (SMC) or other thermoset or thermoplastic matrices, often containing fillers which may range from wood flour to finely ground minerals and/or glass fibers. Despite the presence of fillers, the composite door skins are essentially non-porous. Thus, stains containing dyes as the colorant are largely ineffectual, and pigmented stains require a higher amount of pigment than would be required for a conventional wood stain. The lack of porosity and the generally smooth surface of such products require the stain pigments to be physically adhered to the surface.
Furthermore, the drying oils used in conventional stains, particularly those of lower viscosity, act as subsurface emollients in wood, but remain as an exterior thin film on composite skins. Lower molecular weight oils, being unable to penetrate the essentially non-porous surface, may remain tacky or require lengthy drying times or oven cure. During weathering due to light exposure, these oils experience scission of naturally occurring unsaturated molecular sites. The scission produces a variety of lower molecular weight, liquid and often volatile components, which causes the subsequently applied topcoat to blister, coming off in sheets.
The detrimental effects of drying oil vehicles may be minimized by substituting quickly evaporating solvents such as mineral spirits, naphtha, or the like. However, such stains tend to dry rapidly, leaving application marks, i.e., xe2x80x9cbrush marksxe2x80x9d where overlapping coats of stain are applied. Elimination of drying oil components results in little adherence of pigment to the surface. This minimal adherence may be acceptable in porous products, but is highly problematic when non-porous surfaces are involved.
Adkins, et al., in U.S. Pat. No. 4,923,760, discloses water-borne emulsions employing two different maleinized linseed oils together with cosolvents such as propylene glycol tertiary butyl ether and a variety of hydrocarbons. However, such stains are still formulated for porous wood surfaces and are not well suited to non-porous composite door skins. Lowering the amount of the oil vehicles would be appropriate, but the stains then dry too fast and have only a limited capacity to stably disperse pigment.
Pigmented stains, whether solvent-borne or water borne, are traditionally protected by aromatic polyurethane topcoats. When the previously deposited stain contains low levels of scissionable drying oils, failure by blistering and peeling is substantially minimized. However, aromatic polyurethanes are subject to severe degradation by light. Exposure to sunlight causes degradation which ultimately results in ablative loss of the coating when applied to non-porous surfaces. Polyurethane varnishes often contain sacrificial ultra-violet (UV) protectants. However, the ultraviolet light protection package is typically consumed in as few as eight months exposure to sunlight. As the stain coat is not strongly adhered to non-porous substrates, failure of the topcoat results in ablative loss of the stain layer, typically in less than two months after the topcoat is breached.
Aqueous topcoats have been used with some success, as indicated by the disclosures of U.S. patents to Gobel et al. (U.S. Pat. No. 5,342,882) and Sanders et al. (U.S. Pat. No. 4,509,981). Water-based topcoats employing aliphatic urethane-acrylic latexes are commercially available. The use of a non-sacrificial UV protectant package in these coating compositions provides a substantial increase in durability of underlying resin-bound solid pigmented coating relative to the degree of protection provided by aromatic polyurethane finishes. However, these finishes are incompatible with pigmented stains having little or no resin binder and high pigment loading. This incompatibility is believed due to the relative differences between the somewhat hydrophilic pigment particles and the hydrophobic topcoat.
It would be desirable to provide a pigmented stain suitable for use on relatively non-porous thermoset and thermoplastic composite surfaces, particularly door skins. It would be further desirable to provide such pigmented stains in a formulation which can easily be applied by the consumer in a uniform manner, without resort to the use of expensive and not readily available production equipment. It would be yet further desirable to provide a topcoat which is compatible with the pigmented stain, and which exhibits a high, continuing degree of ultraviolet protection and weathering resistance. The stain/topcoat system should not employ two-component coatings with limited pot life.
The present invention pertains to a weatherable stain and topcoat system which is highly effective to stain and weatherize thermoset and/or thermoplastic, essentially non-porous surfaces. The system comprises a pigmented stain and an aqueous, essentially transparent topcoat which is compatible with the stain. The stain/topcoat system provides a finish system which is suitable for consumer application, and which, following application, presents a highly aesthetic, weatherable surface.
The pigmented stain of the present invention contains a high loading of pigments necessary to impart the degree of coloration required for non-porous thermoset and/or thermoplastic composite materials, particularly door skins, trim, etc., and a specific resin binder, in a solvent system which preferably employs a tail vehicle in addition to conventional fast drying solvents. The resin binder is a dicyclopentadiene-modified drying oil as disclosed by U.S. Pat. No. 5,288,805, herein incorporated by reference, and commercially available as DILULIN(trademark) copolymer resins from Cargill, Inc. It has been surprisingly found that one copolymer resin is adequate not only to provide the requisite adhesion of pigment to substrate, but also, in conjunction with the formulation of the topcoat, to allow for the use of large amounts of pigment in the stain without compatibility problems. Thus, it is preferred that the dicyclopentadiene-modified drying oils constitute the largest part, and preferably all or substantially all of the resin binder.
The dicyclopentadiene-modified drying oil is preferably prepared by reacting dicyclopentadiene in an amount of from about 10 weight percent to about 40 weight percent with a drying oil or mixture of drying oils having relatively high iodine numbers, preferably above 150, and more preferably in the range of 170 to 195. Linseed oil is the preferred drying oil, however other oils such as tung oil, dehydrated castor oil, and other oils may be used as well, or in admixture with linseed oil. The dicyclopentadiene is added to the drying oil in small increments at relatively high temperature, i.e., 250-270xc2x0 C. with stirring. It is believed that the dicyclopentadiene is converted to cyclopentadiene which then reacts with the unsaturated sites of the drying oils in a Diels-Alder reaction. The preparation of such modified oils and their use with oil-modified polyurethanes in polyurethane varnish is disclosed in U.S. Pat. No. 5,288,805.
The dicyclopentadiene-modified linseed oil is present in the stain in an amount of from about 20 weight percent to about 45 weight percent, more preferably 30 to 40 weight percent, and most preferably about 32 to 38 weight percent. The pigment loading is present in the stain from about 15 weight percent to about 35 weight percent of the stain, more preferably 20 weight percent to about 30 weight percent, and most preferably about 25 to 30 weight percent. Solvents are present in the stain in amounts of about 20 weight percent to 40 weight percent of the stain, more preferably 25 to 35 weight percent, and most preferably about 30-35 weight percent. Of the total solvents, approximately 10 weight percent to about 30 weight percent, preferably 15 weight percent to about 25 weight percent are slowly evaporating solvents commonly known as xe2x80x9ctail solvents.xe2x80x9d The remainder of the solvents exhibit fast to moderate evaporation, such as mineral spirits, naphtha, solvent 142 petroleum distillate, and the like. It is preferred to use combinations of these faster evaporating solvents such that the solvents flash off at different intervals. The majority of solvent, i.e.,  greater than 50%, should flash off within a period of from 2 to 10 minutes. Most preferably, solvent 142 petroleum distillate, mineral spirits, and tridecyl alcohol tail solvent are employed in a weight ratio of about 10:10:5, with the weight ratio of dicyclopentadiene-modified linseed oil to tail solvent being approximately 3:1.
In addition to pigments, dicyclopentadiene-modified linseed oil, and solvents, the stain may contain other known additives, for example, leveling agents; drying agents, i.e., metal naphthenates such as cobalt naphthenate, calcium naphthenate, and magnesium naphthenate; ultraviolet absorbers, preferably of the non-sacrificial type, i.e., hindered amine stabilizers such as TINUVIN(copyright) 292 and TINUVIN(copyright) 328; wax; thickeners such as various clay minerals; and the like. The stain is preferably free of ordinary drying oils such as unmodified or bodied linseed oil, tung oil, and the like; and is preferably also free of other secondary resins such as urethane resins and oil-modified alkyd polymers.
The preparation of the stain is not overly critical, and may be performed by conventional paint preparation techniques. For example, the pigments, in standard commercial grade, are added to a first portion of the dicyclopentadiene-modified linseed oil and well blended in a mixing tank, three roll mill, Cowles mixer, etc. Sand milling or ball milling may also be used. In some cases, pigment size reduction is desired in addition to full wetting of the pigment-surface. A surfactant such as BYK(trademark) 156 may aid in dispersing and wetting the pigment with the resin binder. Following wetting of the pigment and any desired size reduction, the resulting mill base is diluted with solvent, additional resin added, and various driers, leveling agents, thickeners, and the like are added. Blending of the mixture is continued until a uniform dispersion is obtained.
The stain may be applied to the non-porous thermoset and/or thermoplastic composite by conventional means, i.e., by brushing, spraying, sponging, rolling, wiping, and the like. In general, excess stain is removed by wiping with a clean lint free rag or the like, but preferably by means of a china bristle brush. The stain is generally applied at temperatures between 5xc2x0 C. and 45xc2x0 C., and should be allowed to dry for approximately 48 hours at room temperature under dry conditions. Drying time may be adjusted depending upon the temperature and humidity. After the stain is dry, the topcoat may be applied.
The topcoat exhibits unexpectedly excellent compatibility with the inventive stain layer, particularly so in view of the fact that the topcoat is an aqueous composition. In addition, the topcoat exhibits unexpectedly excellent durability with regard to abrasion resistance and damage caused by ultraviolet light.
The topcoat comprises a non-siloxane, film-forming polymer comprising an acrylic polyurethane polymer latex. The term xe2x80x9clatexxe2x80x9d as used herein pertains to an emulsion, microemulsion, or dispersion of the respective polymer as a discontinuous phase in water as a continuous phase. The particle size is not overly critical, and may vary, for example, from smaller than 0.001 xcexcm to about 2 xcexcm or larger, preferably from about 0.01 xcexcm to about 0.5 xcexcm. While the dispersed phase should remain stably dispersed in the continuous phase, if the dispersed phase settles or separates, it should be readily redispersible by means of simple agitation, as by stirring or the use of a paint mixer or the like. Latexes with small particle sizes are preferred. By the term xe2x80x9cdispersiblexe2x80x9d as it applies to the non-siloxane, film-forming polymer, is meant that the polymer may be dispersed or emulsified in water to form a latex as that term is used herein.
Most preferably, the acrylic polyurethane polymer latex is a FLEXTHANE(copyright) urethane-modified acrylate resin latex, available from Air Products, of Allentown, Pa. The pH of the latex must be suitable for the dispersed phase, and is preferably between 8 and 11, adjusted by addition of suitable acid or alkaline additives, particularly with ammonium hydroxide. FLEXTHANE(copyright) 620 is particularly preferred as the acrylic polyurethane polymer latex.
The topcoat composition may contain conventional additives such as cosolvents, leveling agents, flatting agents, emulsifiers and emulsion stabilizers, biocides, thickeners, suspending agents, and in particular, ultraviolet protection additives. Examples of suitable cosolvents include the various glycol ethers and acetates available from the Dow Chemical Company under the tradename DOWANOL(copyright) glycol ethers and acetates. Aliphatic alcohol cosolvents such as methanol, ethanol, n-propanol, 2-propanol, n-butanol, 2-ethylhexanol, tridecyl alcohol, and the like may be used, as can also ketones such as methylethylketone, 2-pentanone, cyclohexanone; and aliphatic and aromatic hydrocarbons. The latter may be used in quantities such that stable emulsions may be maintained.
The key to the compatibility of the topcoat with the stain, and one of the keys to the improved durability of the topcoat, however, is due to the presence of an organopolysiloxane emulsion, preferably a microemulsion containing one or more curable or crosslinkable organopolysiloxanes. The presence of this microemulsion is believed to alter the hydrophobicity of the topcoat, rendering the latter compatible with the stain formulation of the present invention. Although organopolysiloxane microemulsions are preferred for use herein, stable emulsions containing larger particle sizes are useful as well.
The organopolysiloxanes in the microemulsion are curable organopolysiloxanes which are readily available. Such curable organopolysiloxanes have reactive functional groups such as alkoxy, silanol, chloro, hydrido, acetoxy, and the like. The functional groups may be terminal groups or may be distributed along the polysiloxane chain. Preferably, the average functionality of the organopolysiloxanes is greater than two such that a crosslinked thermoset polymer is obtained upon cure. However, lower functionality organopolysiloxanes are also suitable, so long as the improved topcoat weatherability is maintained. Suitable organopolysiloxanes are identified in U.S. Pat. No. 4,913,972, which is herein incorporated by reference.
A preferred organopolysiloxane microemulsion is Wacker BS(copyright) 43A methylsilicone resin microemulsion, available from Wacker Silicones, of Adrian, Mich.
Organopolysiloxanes which contain unsaturated organic groups such as allyl-, vinyl-, vinylether, and other unsaturation-functional groups are also useful. These organopolysiloxanes may further crosslink under the influence of light exposure.
The term xe2x80x9ccurablexe2x80x9d includes resins which are crosslinkable as well. The term xe2x80x9ccurablexe2x80x9d is intended to reflect a curing reaction which creates a higher molecular weight cured polymer as compared to the molecular weight of the uncured organopolysiloxane. By the term xe2x80x9cmolecular weightxe2x80x9d is meant number average molecular weight.
In addition to the curable organopolysiloxane, the topcoat may contain other organopolysiloxanes which promote water beading. These organopolysiloxanes may be curable, may be essentially inert, or may interact with other system components such as the film-forming polymer either by chemical reaction or polar or ionic attraction. Examples of water beading agents are trimethylsiloxy end-capped polydimethysiloxanes, xcex1, xcfx89-dihydroxypolydimethylsiloxanes, and in particular, amino-functional organopolysiloxanes, for example xcex1, xcfx89-[3-aminopropyl]polydimethylsiloxanes. A preferred beading agent is Wacker Silicones(copyright) BS 1306 organopolysiloxane, available from Wacker Silicones.
The key to the unexpectedly excellent durability of the topcoat with regard to abrasion resistance and damage caused by ultraviolet light, however, is due to the presence of an aqueous weatherability agent. Without wishing to be bound to any particular theory, it is believed that the weatherability agent reacts with the acrylic polyurethane polymer latex, the organopolysiloxane microemulsion, or both to increase the durability of the topcoat by at least 20% over a similar topcoat not containing the weatherability agent. More particularly, and while still wishing not to be bound to any particular theory, it is believed that the aqueous weatherability agent reacts with carboxylate functional sites in the acrylic component of the urethane-acrylic hybrid polymer resulting in a crosslinking reaction between the aqueous weatherability agent and the acrylic component of the urethane-acrylic hybrid polymer. For this reason the weatherability agent could also be considered a xe2x80x9ccrosslinking agentxe2x80x9d and is preferably chosen so as to have a functional group that is relatively reactive with carboxylate functional sites. The term xe2x80x9ccarboxylate functional sitesxe2x80x9d include carboxylic acids and derivatives of carboxylic acids, such as carboxylic acid salts.
Aqueous weatherability agents may be used in the form of aqueous or partially aqueous dispersions, suspensions, emulsions, or solutions containing weatherability agents chosen from a list of agents that could perform nucleophilic substitutions, and which would preferably have two functional groups or bonding sites to participate in a crosslinking reaction. Preferably, the weatherability agent has only one oxirane functional group, however, the weatherability agent could also have more than one oxirane functional group as long as the viscosity of the agent remains relatively low.
Preferred weatherability agents that are reactive with carboxylate functional sites are the following: aqueous aliphatic epoxy dispersions, most preferably aqueous aliphatic epoxy emulsions such as Chempol(copyright) 020-1642, which is available from Cook Composites, Inc. of Kansas City, Mo.; aziridines, and preferably trifunctional aziridines; epoxides and compounds containing blocked isocyanate groups, preferably with aliphatic or cycloaliphatic backbones, which have been tailored to be compatible with water through means known in the art; alcohols, including glycols and polyols; epoxy resins such as tetraglycidyl-methylenedianiline and bisphenol A diglycidyl ether; and divalent metals with weakly bonded ligand complexes which are soluble in polar solvents miscible with water, preferably zirconium(II) acetate or zirconium(II) propionate.
The topcoat preferably contains, in weight percent, from about 30 to about 55 percent water, preferably from about 35 to about 50 percent water, and more preferably from about 40 to about 50 percent water; from about 20 to about 50 percent of an acrylic polyurethane polymer latex, preferably from about 30 to about 45 percent, and more preferably from about 30 to about 40 percent; from about 2 to about 25 percent of an organopolysiloxane emulsion, preferably from about 3 to about 10 percent, and more preferably from about 3 to about 8 percent; from about 1 to about 25 percent of a weatherability agent, preferably from about 5 to about 20 percent, and more preferably from about 10 to about 15 percent; and sufficient surfactant to provide a stable dispersion, preferably from about 1 to about 8 percent, more preferably from about 1 to about 5 percent, and most preferably from about 2 to about 4 percent. The term xe2x80x9cweight percentxe2x80x9d, as used in the previous sentence with respect to the latex, the emulsion and the weatherability agent, refers to the total weight of the latex, the emulsion and the weatherability agent, respectively, used in formulating the topcoat and not to the weight percents of the solids or polymers used in the latex, emulsions, or weatherability agent. The surfactants may be anionic, cationic, amphoteric, or non-ionic. Mixtures of various surfactants may be used. The type of surfactant should be tailored to the type of polymer latex. The selection of suitable surfactants is within the skill of the art once the general formulation is known.
The acrylic polyurethane polymer latex preferably contains in weight percent from about 20% to about 60% solids, and preferably from about 37% to about 40%, based on the weight of the latex. The organopolysiloxane emulsion preferably contains in weight percent from about 20% to about 60% solids, and preferably from about 42% to about 46%, based on the weight of the emulsion. The aqueous weatherability agent preferably contains in weight percent from about 40% to about 60% solids, more preferably from about 49% to about 51%, and most preferably about 50%, based on the weight of the weatherability agent.
In addition to the above ingredients, the topcoat preferably contains an ultraviolet protection package. This package may contain one or more ultraviolet absorbers, preferably of the non-sacrificial type. Examples are titanium dioxide, silica, and various ceramic materials. Such UV absorbing particles may be used in amounts up to about 5 percent by weight, based on the weight of the topcoat, preferably from about 0.1 weight percent to about 2.5 weight percent, and more preferably about 0.2 weight percent to about 1.8 weight percent.
The UV absorbing particles are preferably passivated, very finely ground anatase or rutile titanium dioxide having a mean particle size of about 20 nm (nanometers). Even though the passivated anatase or rutile titanium dioxide is very finely ground, a number of particles could exist having a particle size about 80 nm, and thus, it may still be opaque and could diminish the translucency of the topcoat. Thus, it is preferred that the very finely ground anatase or rutile titanium dioxide be present in an amount of from about 0.12 weight percent, based on the weight of the topcoat, to about 1.2 weight percent, and more preferably in an amount of about 0.12 weight percent. It is contemplated that a greater amount of passivated anatase or rutile titanium dioxide could be used, such as in an amount of from about 0.12 weight percent, based on the weight of the topcoat, to about 2.5 weight percent, and more preferably in an amount of about 1.8 weight percent, if the opacity of the titanium dioxide were reduced by employing passivated anatase or rutile titanium dioxide having a relatively narrow particle size distribution which would exclude all but a minimal amount of opaque particles. The preferred particle size of the passivated anatase or rutile titanium dioxides is 20 nm, plus or minus 10 nm.
The titanium dioxide is passivated to prevent the titanium dioxide from attacking the carbon atoms present in the other components of the topcoat. The choice of passivating coating for the translucent rutile titanium dioxide and some methods of application are known in the art. Selection of the passivating coating salts is determined by color and pigment wetting requirements in the formulation. Calcium mixed fatty acid carboxylate derivatives are most preferred, such as OM GROUP(copyright) 5 percent Calcium Hydro-Cem, which is available from Mooney Chemicals Inc., of Cleveland, Ohio.
In addition to finely divided inorganic particle UV absorbers, soluble or dispersible organic absorbers such as hindered amines, cinnamic acid esters, and the like may be used, generally in amounts up to about 5 weight percent, based on the weight of the topcoat, more preferably 0.1 to about 3 weight percent, and most preferably from about 0.5 to about 2 weight percent. Preferred UV absorbers are the various TINUVIN(copyright) absorbers available from Ciba Geigy.
The stain/topcoat system of the present invention displays excellent weatherability, as measured by ASTM G53-88 accelerated weathering test. Moreover, the topcoat itself, even when used over conventional oil-based stains on a porous surface, also displays excellent weatherability. The weatherability of both the stain/topcoat system, and the topcoat by itself, should exceed 3500 hours minimally by the above test on a non-porous surface, should preferably exceed 4000 hours, more preferably 4500 hours, and most preferably exceeds 5000 hours.
The preparation of the aqueous topcoat may be performed in numerous ways. However, the addition of the organopolysiloxane microemulsion must be accomplished in such a manner so as to prepare a stable microemulsion. Simply adding the entire amount of organopolysiloxane microemulsion at one time is generally ineffectual. It has been found advantageous to prepare a mill base containing the acrylic polyurethane polymer latex and other non-siloxane additives and utilize water or a portion of the mill base to form a organopolysiloxane microemulsion xe2x80x9cpreblend.xe2x80x9d The preblend is then added to the remaining portion of the mill base to form a xe2x80x9cmain topcoat mixturexe2x80x9d.
The preblend is added to the remaining portion of the mill base in slow continuous or incremental additions with modest agitation such that the total addition takes place over a period of 25 minutes to 10 hours, preferably 40 minutes to 6 hours. When incremental addition is utilized, it has been found advantageous to add up to about 8 percent by weight of the preblend, more preferably 0.5 to 2.5 weight percent of the preblend at each increment, the increments spaced approximately 1-5 minutes apart, more preferably a minimum of 2 minutes, and most preferably 3 minutes apart. Thus, with 2.5 weight percent increments spaced 3 minutes apart, the entire addition of preblend to mill base will take approximately 2 hours. The preblend preferably consists of 20-80 weight percent organopolysiloxane microemulsion and 80-20 weight percent water and/or mill base, more preferably 30-65% organopolysiloxane microemulsion, and most preferably about 40-60% organopolysiloxane microemulsion, the remainder preferably being mill base and/or water.
Within a week prior to application of the topcoat, the aqueous weatherability agent is added to and mixed with the main topcoat mixture to form the topcoat of the present invention.
The topcoat is preferably applied as two or more layers with a total dry thickness of 10-150 xcexcm, preferably 13-75 xcexcm, and more preferably 30-60 xcexcm. The topcoat is preferably applied at ambient temperatures in the 5xc2x0 C. to 45xc2x0 C. range, with three hours passing between coat applications. While the topcoat is preferably applied over a pigmented stain on a non-porous thermoset and/or thermoplastic surface, the inventive topcoat may be applied to other surfaces devoid of stain, and to porous surfaces, stained or unstained, as well.
The preferred non-porous surface to which the stain and topcoat of the present invention is applied is a compression molded thermoset and/or thermoplastic composite skin which comprises an outerface of an exterior door. The compression molded skin may preferably comprise a modified unsaturated polyester sheet molding compound, such as, acid modified propylene glycol maleate, a low profile additive, such as polyvinyl acetate, condensation reaction catalyst, filler and chopped fiberglass reinforcement. Examples of suitable exterior doors having suitable compression molded thermoset and/or thermoplastic composite skin are identified in U.S. Pat. No. 5,537,789, which is incorporated herein by reference.
Performance in the early stages of product life of the topcoat may be enhanced by further drying or heating the applied coating, once it is dry to the touch, at a temperature of about 30xc2x0 C.-121xc2x0 C., preferably 65xc2x0 C.-121xc2x0 C., for approximately 15 minutes. Performance can be measured by coating softening resistance during solvent rubs with methyl ethyl ketone rubs using test method ASTM D5402-93. Acceptable performance herein means greater than 100 rubs, preferably greater than 200 rubs. When no heat is applied, the curing of the topcoat to maximum hardness occurs over several months. Without heat, the topcoat is suitable for normal service handling within six hours after applying two coats of topcoat.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.

Weatherable coating and stain system for thermoset or thermoplastic composite surfaces


In accordance with the present invention, there are provided three bars of light to help a user maneuver a vehicle into a space accessed by a garage door that is controlled by an electric door opener. The three bars of light are powered by an electric garage door opener using one of the two electric lamp outputs of the garage door opener. A simple screw-in plug base and triple tap adapter allows one outlet to power the three bars of light. When a vehicle approaches the garage and signals for the garage door to open the three bars of light are activated. The driver of the vehicle need only keep the center bar of light centered between the outside bars of light to properly align the vehicle.

1. Field of Invention
The present invention relates to precision parking and, more particularly, to parking a vehicle into an optimal parking position within a parking space by visually centering one middle bar of light between two others.
2. Prior Art
The process of parking a vehicle in a garage is becoming more difficult. Originally designed for storage of vehicles, many people now find the room a convenient one for other stuff. Often items competing with the vehicle include bicycles, lawn mowers, ping-pong tables, fishing gear, holiday decorations and outdoor grills. In addition, a problem often surfaces with the width of vehicles increased due to large outside rearview mirrors. On occasion these mirrors have been damaged as drivers tried to park their vehicles through narrow garage doors. Other damage has occurred as a result of vehicles hitting bicycles or ping-pong tables. This has resulted in some drivers merely leaving their vehicles parked outside to avoid the problems of precise positioning when entering the garage.
Many attempts have been made to provide devices which enable a driver of a vehicle to precisely align and position a vehicle within a garage.
Examples include:
U.S. Pat. No. 3,874,322, issued to Brauer on Apr. 1, 1975, is a movable vehicle parking position indicator which moves in conjunction with a garage door used to close off the parking area. As the garage door is opened, a ball-like indicator is lowered into a position clearly visible from the driver position of a vehicle entering or properly parked in the garage. As the door is closed, the indicator is retracted. No assistance is given to the driver of the vehicle in proper aligning for entry into the garage.
U.S. Pat. No. 3,998,285, issued to Cooper on Dec. 21, 1976, is a vehicle indicating mechanism, in the form of a parking guide, which gauges the back clearance of an automobile or similar vehicle. The parking guide includes a hollow, translucent guide rod, which is movable responsive to a motor within a control box, from a substantially horizontal position, adjacent to the back bumper of the vehicle, to a substantially vertical position, extending upwardly from the corner of the bumper. The guide rod is illuminated by a light within the control box when the rod is in the vertical position thereby also enabling the rod to be viewed by the driver and used as a guide for parking the automobile at night. No assistance is offered in terms of aligning the vehicle.
U.S. Pat. No. 4,145,681, issued to Bubnich on Mar. 20, 1979, is a parking guide and signaling device for cars and trucks to assist the driver parking a vehicle in a designated parking area which includes a housing section having a window in the front wall thereof which is closed by a pane of translucent sheet material through which indicia can be seen clearly when the indicia are illuminated from the rear surface of the pane by an electric light with electrical apparatus within the housing for illuminating the pane and the housing section having pivotally mounted thereon an actuating lever biased so that the electrical apparatus is normally "off" but when the vehicle moves into a designated parking area the electrical apparatus is turned "on" and an intense beam of light illuminates the inner surface of the pane. No alignment assistance is provided to the driver of the vehicle
U.S. Pat. No. 4,218,157, issued to Moxness on Aug. 19, 1980, consists of a visual vehicular parking aid which includes a plurality of vertically suspended position indicators to direct a person driving an automobile into a parking space when the normal pavement markings are not visible or present. This parking space marker system does help insure the vehicle is in a designated space, but provides no advance assistance in properly aligning a vehicle as it approaches the parking space.
U.S. Pat. No. 4,257,706, issued to Smith, Mar. 24, 1981, describes a motor vehicle back-up limit gauging method and apparatus. This apparatus enables a driver of a vehicle to gauge distances when backing-up. The apparatus includes a pair of sights which are attached to the vehicle. These sights are then used by aligning them with the ground line of a wall or with another marker. Alignment is accomplished by viewing the sights through a side-view mirror. This apparatus has several drawbacks. It is only useful when a vehicle is being driven in reverse. It requires additional structure to be added to the side of a vehicle. It also requires training and skill to use.
U.S. Pat. No. 4,808,997, issued to Barkley on Feb. 28, 1989, consists of a photoelectric vehicle position indicating device which is for use in parking and otherwise positioning vehicles and is employed to assist in parking and otherwise positioning a vehicle on a supporting surface. A photoelectric control unit is mounted on an overhead structure spaced above the supporting surface and has a beam emitting device for directing a light beam downwardly for interception by the vehicle. The light beam is initially reflected back to a photoelectric transducer on the control unit by a reflective device or mirror. The light beam is pulsed so that the transducer supplies electrical pulses to an amplifier that is correspondingly gated. When the vehicle interrupts the light beam, the amplifier produces a beam interruption output signal which causes a one-shot timer to energize an alarm device, through an output relay. The operator then stops the vehicle in the desired position. The one-shot timer de-energizes the alarm device after a brief interval. Alternatively, the mirror is not employed, and the light beam is not significantly reflected back to the transducer until the light beam is intercepted by the vehicle, whereupon reflection from the vehicle produces a reflected light beam to the transducer. The corresponding pulsed signals from the transducer operate the amplifier, which is modified so that it actuates the timer, whereby it energizes the alarm device for a timed interval. The device allows for positioning in terms of how far the vehicle is allowed to enter the desired parking space but does not precisely provide aligning assistance.
U.S. Pat. No. 4,813,758, issued to Sanders Mar. 21, 1989, is a guide that enables a driver to position a vehicle within a garage. The guide is positioned at an end wall of a garage so that it faces the garage door. It comprises lateral positioning members and a longitudinal positioning member. The lateral positioning members comprise lateral positioners which are offset from each other. Alignment of the lateral positioning members indicates that the operator of a vehicle is in the correct lateral position within a garage. Longitudinal alignment is indicated by the reflection of a headlight in a driver's eyes. This guide provides very limited alignment assistance to the driver of a vehicle prior to entering the garage. The alignment depends upon reflections of the headlights requiring them to be activated even during the day.
U.S. Pat. No. 4,965,571 issued to Jones on Oct. 22, 1990, is a motor vehicle driver-indicator guide-device for signifying when it is safe to advance including a support structure mounting a light signaling structure and mechanism therefore, that displays a signal light viewable by a driver of a motor vehicle with an actuating lever which the bumper actuates. A green light of the light signaling structure has an electrical circuitry and an electrical switch that is normally closed providing a go signal evidencing that further advancement of the motor vehicle may be continued, actuation of the actuating lever serving to open the normally closed electrical switch when the actuation switch is pressed beyond a predetermined position indicative of maximum allowable forward advancement permissible for the advancing motor vehicle, the support structure having a tape mounted thereon carrying an adhesive backing for mounting the mounting structure on an upright surface of a wall or the like, the green light carrying a symbol viewable when the switch is closed indicative of permissive further advancement. A major problem with this device is that no assistance is offered in aligning the vehicle properly in the garage.
U.S. Pat. No. 5,177,479, issued to Cotton on Jan. 5, 1993, is a garage parking position indicator which is operable on principles of wave energy. A wave energy detector such as an infrared receiver is mounted to one side of the garage access door at a point displaced from the rear wall of the garage by a distance greater than the length of the vehicle. A wave energy source, such as an infrared transmitter, is mounted on the opposite side of the garage access door from the receiver. The transmitter and the receiver are mounted at a height suitable for cross-sectioning the traveling vehicle from this leading edge to its trailing edge, preferably from bumper to bumper. The transmitter constantly transmits and directs wave energy toward the receiver so that the presence of any portion of the vehicle between the transmitter and the receiver will interrupt reception of the wave energy by the receiver. No alignment assistance is provided to the driver of the vehicle.
U.S. Pat. No. 5,189,802, issued to Bergfield on Mar. 2, 1993, is a vehicle parking guide that comprises a rotary mechanical arm which is attached to a string. As a bumper of a vehicle encroaches upon the string, the arm rotates to indicate relative positioning of the vehicle. This guide does not assist in the alignment of the vehicle when the vehicle is approaching the garage.
U.S. Pat. No. 5,127,357, issued to Viscovich on Jul. 7, 1992, utilizes a mirror mounted on a surface in a garage to reflect light to the side rear-view mirror on a vehicle. The invention operates by reflecting the illuminated brake lights into the side rear-view mirror when the vehicle is properly positioned. The shortcomings of this invention involve requiring the driver's attention to be focused on the rear-view mirror, thus distracting the driver from other pertinent surroundings. In addition, the tolerance of proper positioning is inherently large as the mirrors will reflect all incident light and the driver must judge the magnitude or intensity of the reflected light in order to properly position a vehicle. This could lead to imprecise positioning of the vehicle.
U.S. Pat. No. 5,227,785, issued to Gann on Jul. 13, 1993, is a vehicle parking assist apparatus which includes a hollow mounting base, an elongated signaling device, such as an upright pole having a signal light mounted thereto, supported on the base for pivotal movement from a normal upright non-signaling position to a tilted signaling position upon being contacted by an advancing vehicle, and an electrical circuit connected to the signal light of the elongated signaling device and capable of being switched from an open circuit condition to a closed circuit condition for electrically actuating the signal light to provide a visible alerting signal for a driver of the advancing vehicle. A coupling device supports the pole upright on the base and also couples the signal light to the electrical circuit. The coupling device is capable of retaining the electrical circuit in an open circuit condition so long as the signaling device is disposed in the upright non-signaling position. The coupling device is further capable of switching the electrical circuit to the closed circuit condition in response to the elongated signaling device being contacted and tilted by the advancing vehicle to the tilted signaling position. No assistance is provided in vehicle alignment.
U.S. Pat. No. 5,285,205, issued to White on Feb. 8, 1994 is a vehicle-guidance and positioning system. The system utilizes a laser. The emitted beam from the laser is directed over the path along which a vehicle is to be guided. The laser beam impinges on a target area located on the vehicle in such a manner that the impingement of the laser beam on the target is continuously observable by the vehicle's operator. The operator steers the vehicle so that the laser beam continuously impinges on the target area until the vehicle reaches a pre-selected position. One problem with this system is the narrow and hard to see path that is provided by the laser light. In inclement weather, rain or snow, the laser light could be impacted on the target and thus be hard to see and would direct attention away from obstacles directly ahead of the vehicle.
U.S. Pat. No. 5,406,395, issued to Wilson on Apr. 11, 1995, is an optical parking alignment system which includes at least one projector located on a vehicle for projecting a respective image forwardly of the vehicle. The alignment of the vehicle is indicated when each respective image is in focus on a screen disposed in front of the vehicle. The projector includes either a transparency and an imaging lens or a hologram and a narrow-band light source. This is a fairly complex system and assistance provided for alignment is minimal at best.
U.S. Pat. No. 5,617,087, issued to Scott on Apr. 1, 1997, is a parking device for vehicles entering a ceiling-covered parking garage which includes a housing. The housing substantially encloses an electrical circuit. The electrical circuit includes a light-emitting bulb and a switch. The switch opens and closes the electrical circuit to light and shut off the bulb. A power supply and a cord suspend the housing from the ceiling of the parking garage so that the housing may intercept the vehicle to be parked, close the switch of the circuit, thereby emitting a light signal to the driver of the vehicle to let the driver know that the vehicle has reached the predetermined parking position. This parking aid helps place the vehicle in a predetermined position; however no assistance in alignment is provided.
U.S. Pat. No. 5,841,368, issued to Bryant on Nov. 24, 1998, consists of a parking assist device which is removable affixed in relation to a surface and is adapted to engage the on coming wheels of a vehicle and provide upon being engaged by the wheels of a vehicle a signal to the operator to stop the vehicle. The signal may be produced by either a mechanical mechanism or an electrical apparatus. Again, no assistance is offered in aligning the vehicle properly in the garage.
U.S. Pat. No. 5,945,907, issued to Yaron on Aug. 31, 1999, is a sensing and indicating device mounted at a fixed location to determine the distance between the sensor and an approaching vehicle. The sensor provides visual and/or audible indication of the distance between the vehicle and the sensor. The driver of the vehicle uses that displayed or audible information to locate the vehicle within a defined space or envelope, e.g., within a garage. The sensor and indicator may also be located laterally with respect to the vehicle so that the distance between the vehicle side and the sensor can be determined whereby the vehicle can be located laterally within the defined space or envelope. Once more, no assistance is offered in aligning the vehicle properly in the garage.
U.S. Pat. No. 6,199,287, issued to Rankila on Mar. 13, 2001, is a device for positioning a vehicle in a predetermined location. This device is comprised of two spatially separated but conjoined planar surfaces, the first of which is mounted on a wall and the second is extended toward the driver. While this device will assist the driver in positioning the vehicle when the second planar surface is aligned with and obscures a portion of the first planar surface, it has several drawbacks. In its rigid form, the device actually provides another obstruction that could cause damage to a vehicle because the distance required to separate the first and second surfaces must be of more than negligible in order for this device to operate properly (too short of a distance would provide too great of a tolerance in the desired parking position). In its retractable embodiment, the device would require a driver to exit the vehicle in order to reset the device into its operable position before attempting to position the vehicle. This is extremely inconvenient to the driver. Finally this device requires the driver's attention to be focused on the device and not on the pertinent surroundings in the parking space. This lack of attention on the pertinent surroundings could cause more extensive damage than what the device is attempting to correct.
U.S. Pat. No. 6,776,117, issued to D'Onofrio on Aug. 17, 2004, utilizes the peripheral vision of a driver of a vehicle in assessing the proper position of a vehicle relative to a confined parking space. The device is comprised of two surfaces, the first obscuring the second until the vehicle has attained the desired position within the parking space. Upon attaining this position, the second surface comes within the operator's field of vision thus notifying the operator to stop the vehicle. Alternately, the operator will stop the vehicle when the second surface obscures the first surface. This device requires the use of peripheral vision, careful attention to details and requires the driver to divert attention from the front of the vehicle which could cause an impact with obstacles in front.
The complexity of other solutions involving parallel vertical and horizontal bars, mirrors, pulleys, strings, breaking lights, rotating arms, alarm devices, and actuating cords, as well as the apparent difficulty of installing them properly, has not proven them in many instances acceptable to marketing. All of the previous attempts do not adequately solve the vehicle positioning problem. Thus, what is needed is a device and method for using that device that 1) is easily understood, 2.) requires little orientation or training in operation, 3) positions the vehicle precisely within a confined parking space, 4) operates automatically with the opening of the garage door, and 5) turns off automatically at the end of its assistance.


A beverage dispensing system further cools the beverage where it is dispensed at a considerable distance from the beverage storage container. The beverage conduit between the storage and dispensing sites is carried in a bundle located within an insulated jacket conduit. The bundle also contains parallel chilled liquid lines through which chilled liquid is circulated. A concentric coil is located at the dispensing site. A manifold connects the parallel beverage and chilled liquid lines to the concentric coil and to the dispensing valve.

1. Field of the Invention:
This invention relates in general to dispensing cold beverages from a tap, and in particular to an additional cooling unit used to reduce the temperature of the beverage at the tap.
2. Description of the Prior Art:
In restaurants, bars, stadiums and other public facilities, soft drinks and beer are commonly dispensed on tap. In many cases, the supply containers, such as the beer kegs, will be located in a large walk-in cooler or refrigerator. The drinks may be dispensed at several locations considerable distances away from the supply containers, as much as up to five hundred feet.
In the case of beer, this is handled by connecting the beer keg to a carbon dioxide container, which applies pressure to a beer line leading to the valve or tap. To keep the temperature of the beer in the beer line from warming too much, a chilled liquid line containing glycol runs in parallel alongside the beer line. The glycol line and the various beer lines are all located in a parallel bundle surrounded by an insulated jacket conduit. The glycol is kept at a cool temperature by a glycol refrigeration unit which cools the glycol in a glycol tank located near the beer kegs.
In the case of soft drinks, where no beer is being served, carbonated water will be circulated through a chilled liquid line in the parallel bundle instead of glycol. The carbonated water is mixed at the dispensing site with the soft drink syrup.
While these systems are successful, warming of the beverage is still a problem. The parallel chilled liquid lines do not adequately maintain the temperature of the beverage in some cases where the lines are very long. This can be partiuclarly a problem with beer. If the temperature goes above approximately 40-45 degrees Fahrenheit, the beer will foam excessively at the tap.


A device for attaching connective tissue to bone has a longitudinal axis and comprises an annular toggle member and a body member disposed distally of the toggle member, such that there is an axial space between the toggle member and the body member. The toggle member is movable between an undeployed position wherein the toggle member has a smaller profile in a direction transverse to the axis and a deployed position wherein the toggle member has a larger profile in the direction transverse to the axis. When installed in a desired procedural site, in suitable bone, suturing material extends axially through a center aperture in the annular toggle member, without being secured to or contacting the toggle member. This approach permits a suture attachment which lies entirely beneath the cortical bone surface, and which further permit the attachment of suture to the bone anchor without the necessity for tying knots, which is particularly arduous and technically demanding in the case of arthroscopic procedures.

This invention relates generally to methods and apparatus for attaching soft tissue to bone, and more particularly to anchors and methods for securing connective tissue, such as ligaments or tendons, to bone. The invention has particular application to arthroscopic surgical techniques for reattaching the rotator cuff to the humeral head, in order to repair the rotator cuff.
It is an increasingly common problem for tendons and other soft, connective tissues to tear or to detach from associated bone. One such type of tear or detachment is a xe2x80x9crotator cuffxe2x80x9d tear, wherein the supraspinatus tendon separates from the humerus, causing pain and loss of ability to elevate and externally rotate the arm. Complete separation can occur if the shoulder is subjected to gross trauma, but typically, the tear begins as a small lesion, especially in older patients.
To repair a torn rotator cuff, the typical course today is to do so surgically, through a large incision. This approach is presently taken in almost 99% of rotator cuff repair cases. There are two types of open surgical approaches for repair of the rotator cuff, one known as the xe2x80x9cclassic openxe2x80x9d and the other as the xe2x80x9cmini-openxe2x80x9d. The classic open approach requires a large incision and complete detachment ofthe deltoid muscle from the acromion to facilitate exposure. The cuff is debrided to ensure suture attachment to viable tissue and to create a reasonable edge approximation. In addition, the humeral head is abraded or notched at the proposed soft tissue to bone reattachment point, as healing is enhanced on a raw bone surface. A series of small diameter holes, referred to as xe2x80x9ctransosseous tunnelsxe2x80x9d, are xe2x80x9cpunchedxe2x80x9d through the bone laterally from the abraded or notched surface to a point on the outside surface of the greater tuberosity, commonly a distance of 2 to 3 cm. Finally, the cuff is sutured and secured to the bone by pulling the suture ends through the transosseous tunnels and tying them together using the bone between two successive tunnels as a bridge, after which the deltoid muscle must be surgically reattached to the acromion. Because of this maneuver, the deltoid requires postoperative protection, thus retarding rehabilitation and possibly resulting in residual weakness. Complete rehabilitation takes approximately 9 to 12 months.
The mini-open technique, which represents the current growing trend and the majority of all surgical repair procedures, differs from the classic approach by gaining access through a smaller incision and splitting rather than detaching the deltoid. Additionally, this procedure is typically performed in conjunction with arthroscopic acromial decompression. Once the deltoid is split, it is retracted to expose the rotator cuff tear. As before, the cuff is debrided, the humeral head is abraded, and the so-called xe2x80x9ctransosseous tunnelsxe2x80x9d, are xe2x80x9cpunchedxe2x80x9d through the bone or suture anchors are inserted. Following the suturing of the rotator cuff to the humeral head, the split deltoid is surgically repaired.
Although the above described surgical techniques are the current standard of care for rotator cuff repair, they are associated with a great deal of patient discomfort and a lengthy recovery time, ranging from at least four months to one year or more. It is the above described manipulation of the deltoid muscle together with the large skin incision that causes the majority of patient discomfort and an increased recovery time.
Less invasive arthroscopic techniques are beginning to be developed in an effort to address the shortcomings of open surgical repair. Working through small trocar portals that minimize disruption of the deltoid muscle, a few surgeons have been able to reattach the rotator cuff using various bone anchor and suture configurations. The rotator cuff is sutured intracorporeally and an anchor is driven into bone at a location appropriate for repair. Rather than thread the suture through transosseous tunnels which are difficult or impossible to create arthroscopically using current techniques, the repair is completed by tying the cuff down against bone using the anchor and suture. Early results of less invasive techniques are encouraging, with a substantial reduction in both patient recovery time and discomfort.
Unfortunately, the skill level required to facilitate an entirely arthroscopic repair of the rotator cuff is inordinately high. Intracorporeal suturing is clumsy and time consuming, and only the simplest stitch patterns can be utilized. Extracorporeal knot tying is somewhat less difficult, but the tightness of the knots is difficult to judge, and the tension cannot later be adjusted. Also, because of the use of bone anchors to provide a suture fixation point in the bone, the knots that secure the soft tissues to the anchor by necessity leave the knot bundle on top of the soft tissues. In the case of rotator cuff repair, this means that the knot bundle is left in the shoulder capsule where it can be felt by the patient postoperatively when the patient exercises the shoulder joint. So, knots tied arthroscopically are difficult to achieve, impossible to adjust, and are located in less than optimal areas of the shoulder. Suture tension is also impossible to measure and adjust once the knot has been fixed. Consequently, because of the technical difficulty of the procedure, presently less than 1% of all rotator cuff procedures is of the arthroscopic type, and is considered investigational in nature.
Another significant difficulty with current arthroscopic rotator cuff repair techniques is shortcomings related to currently available suture anchors. Suture eyelets in bone anchors available today, which like the eye of a needle are threaded with the thread or suture, are small in radius, and can cause the suture to fail at the eyelet when the anchor is placed under high tensile loads.
There are various bone anchor designs available for use by an orthopedic surgeon for attachment of soft tissues to bone. The basic commonality between the designs is that they create an attachment point in the bone for a suture that may then be passed through the soft tissues and tied, thereby immobilizing the soft tissue. This attachment point may be accomplished by different means. Screws are known for creating such attachments, but existing designs suffer from a number of disadvantages, including their tendency to loosen over time, requiring a second procedure to later remove them, and their requirement for a relatively flat attachment geometry.
Another approach is to utilize the difference in density in the cortical bone (the tough, dense outer layer of bone) and the cancellous bone (the less dense, airy and somewhat vascular interior of the bone). There is a clear demarcation between the cortical bone and cancellous bone, where the cortical bone presents a kind of hard shell over the less dense cancellous bone. The aspect ratio of the anchor is such that it typically has a longer axis and a shorter axis and usually is pre-threaded with a suture. These designs use a hole in the cortical bone through which an anchor is inserted. The hole is drilled such that the shorter axis of the anchor will fit through the diameter of the hole, with the longer axis ofthe anchor being parallel to the axis ofthe drilled hole. After deployment in to the cancellous bone, the anchor is rotated 90B that the long axis is aligned perpendicularly to the axis of the hole. The suture is pulled, and the anchor is seated up against the inside surface of the cortical layer of bone. Due to the mismatch in the dimensions of the long axis of the anchor and the hole diameter, the anchor cannot be retracted proximally from the hole, thus providing resistance to pull-out. These anchors still suffer from the aforementioned problem of eyelet design that stresses the sutures.
Still other prior art approaches have attempted to use a xe2x80x9cpop rivetxe2x80x9d approach. This type of design requires a hole in the cortical bone into which a split shaft is inserted. The split shaft is hollow, and has a tapered plug leading into its inner lumen. The tapered plug is extended out through the top of the shaft, and when the plug is retracted into the inner lumen, the tapered portion causes the split shaft to be flared outwardly, ostensibly locking the device into the bone.
Other methods of securing soft tissue to bone are known in the prior art, but are not presently considered to be feasible for shoulder repair procedures, because of physicians"" reluctance to leave anything but a suture in the capsule area of the shoulder. The reason for this is that staples, tacks, and the like could possibly fall out and cause injury during movement. As a result of this constraint, the attachment point often must be located at a less than ideal position. Also, the tacks or staples require a substantial hole in the soft tissue, and make it difficult for the surgeon to precisely locate the soft tissue relative to the bone.
As previously discussed, any ofthe anchor points for sutures mentioned above require that a length of suture be passed through an eyelet fashioned in the anchor and then looped through the soft tissues and tied down to complete the securement. Much skill is required, however, to both place the sutures in the soft tissues, and to tie knots while working through a trocar under endoscopic visualization.
There have been attempts to solve some of the problems that exist in current anchor designs. One such approach is disclosed in U.S. Pat. No. 5,324,308 to Pierce. In this patent, there is disclosed a suture anchor that incorporates a proximal and distal wedge component having inclined mating faces. The distal wedge component has two suture thread holes at its base through which a length of suture may be threaded. The assembly may be placed in a drilled hole in the bone, and when tension is placed on the suture, the distal wedge block is caused to ride up against the proximal wedge block, expanding the projected area within the drilled hole, and locking the anchor into the bone. This approach is a useful method for creating an anchor point for the suture, but does not in any way address the problem of tying knots in the suture to fix the soft tissue to the bone.
The problem of placing sutures in soft tissues and tying knots in an endoscopic environment is well known, and there have been attempts to address the problem and to simplify the process of suture fixation. One such approach is disclosed in U.S. Pat. 5,383,905 to Golds et al. The patent describes a device for securing a suture loop about bodily tissue that includes abead memberhaving a longitudinal bore and an anchor member adapted to be slidably inserted within the bore of the bead member. The anchor member includes at least two axial compressible sections which define a passageway to receive two end portions of a suture loop. The axial sections collapse radially inwardly upon insertion of the anchor member within the bore of the bead member to securely wedge the suture end portions received within the passageway.
Although the Golds et al. patent approach utilizes a wedge-shaped member to lock the sutures in place, the suture legs are passing through the bore of the bead only one time, in a proximal to distal direction, and are locked by the collapsing of the wedge, which creates an interference on the longitudinal bore of the anchor member. Also, no provision is made in this design for attachment of sutures to bone. The design is primarily suited for locking a suture loop, such as is used for ligation or approximation of soft tissues.
A prior art approach that includes tissue attachment is described in U.S. Pat. No. 5,405,359 to Pierce. In this system, a toggle wedge is comprised of a two piece structure comprising a top portion characterized by the presence of a barbed tip and a bottom portion. The suturing material extends through apertures in each of the two toggle portions, and is maintained in position by means of a knot disposed in the suture at a lower edge of the bottom toggle portion. To anchor the suture into adjacent soft tissue, the two toggle portions are rotated relative to one another, as shown for example in FIG. 33. The disclosure states that the device could be used to anchor suture in bone, as well as soft tissue, if two embodiments are utilized in tandem. However, the system is disadvantageous in that it is complex, difficult to manipulate, and still requires the tying of a knot in the suture.
Another approach that includes bone attachment is described in U. S. Pat. No. 5,584,835 to Greenfield. In this patent, a two part device for attaching soft tissue to bone is shown. A bone anchor portion is screwed into a hole in the bone, and is disposed to accept a plug that has been adapted to receive sutures. In one embodiment, the suture plug is configured so that when it is forced into its receptacle in the bone anchor portion, sutures that have been passed through an eyelet in the plug are trapped by friction between the wall of the anchor portion and the body of the plug portion.
Although there is some merit to this approach for eliminating the need for knots in the attachment of sutures to bone, a problem with being able to properly set the tension in the sutures exists. The user is required to pull on the sutures until appropriate tension is achieved, and then to set the plug portion into the bone anchor portion. This action increases the tension in the sutures, and may garrot the soft tissues or increase the tension in the sutures beyond the tensile strength of the material, breaking the sutures. In addition, the minimal surface area provided by this anchor design for pinching or locking the sutures in place will abrade or damage the suture such that the suture""s ability to resist load will be greatly compromised.
A disclosure that incorporates bone attachment and eliminates knot tying is set forth in U.S. Pat. No. 5,702,397 to Goble et al. One embodiment, in particular, is shown in FIG. 23 of that patent and includes a bone anchor that has a threaded body with an inner cavity. The cavity is open to one end of the threaded body, and joins two lumens that run out to the other end of the threaded body. Within the cavity is disposed a gear, joumaled on an axle. A length of suture is threaded through one lumen, around the gear, and out through the other lumen. A ball is disposed within the cavity to ride against a tapered race and ostensibly lock the suture in place. What is not clear from the patent disclosure is how the force D shown as the tension in the suture would lock the ball into the race. Although this embodiment purports to be a self-locking anchor adapted for use in blind holes for fixing sutures into bone, the construct shown is complicated, and does not appear to be adequate to reliably fixate the suture.
U.S. Pat. No. 5,782,863 to Bartlett discloses a suture anchor including bone attachment, which simply comprises a conical suture anchor having an anchor bore through which a length of suture is threaded. The anchor is inserted into a bore within a portion of bone using an insertion tool having a shape memory insertion end. As the anchor is inserted, because of its conical shape, it will re-orient itself by rotating in order to fit into the bore, bending the end of the insertion tool. However, once the proximal edge of the bone anchor enters cancellous bone, the shape memory insertion end of the insertion tool will begin resuming its natural straight orientation, thus rotating the anchor back into its original orientation. The corners of the conical body thus protrude into the soft cancellous bone, and the anchor body is prevented from exiting proximally from the bone bore through the hard cortical bone. The insertion tool is then removed.
The Bartlett patent approach, while innovative, is disadvantageous to the extent that it involves the use of a unique and complex insertion tool, and can be difficult to deploy. It also does not permit suturing of the soft tissue prior to anchoring the suture to bone, and thus does not permit tensioning of the suture to approximate the soft tissue to bone, as desired, at the conclusion of the suturing procedure. Additionally, in preferred embodiments, the suture is knotted to the anchor, a known disadvantage.
Yet another prior art approach is disclosed in U.S. Pat. No. 5,961,538 to Pedlick et al. In this patent, a wedge shaped suture anchor system is described for anchoring a length of suture within a bore in a bone portion, which comprises an anchor body having an offset suture opening for receiving the length of suture therethrough, and for creating an imbalance in the rotation of the device as it is inserted. A shaft portion is utilized to insert the wedge-shaped anchor body into the bone bore. Once the anchor body is in cancellous bone, below the cortical bone layer, the shaft is pulled proximally to cause the anchor body to rotate, thereby engaging the corners of the anchor body with the cancellous bone. The shaft then becomes separated from the anchor body, leaving the anchor body in place within the bone.
The Pedlick et al. approach is conventional, in that the suture is attached to desired soft tissue after it is anchored within the bone. Consequently, there is no opportunity to tension the suture, as desired, to optimally approximate the soft tissue to the bone upon completion of the surgical procedure. Additionally, the approach is complex and limited in flexibility, since the suture is directly engaged with the bone anchoring body. There is also the possibility that the bone anchoring body will not sufficiently rotate to firmly become engaged with the cancellous bone before the insertion tool breaks away from the anchor body, in which case it will be impossible to properly anchor the suture.
U.S. Pat. No. 6,056,773 to Bonutti discloses a suture anchoring system which is somewhat similar to that disclosed by Pedlick et al. A cylindrical suture anchor body is provided which is insertable into a bone bore, using a pusher member which pushes distally on the anchor body from a proximal direction. As the anchor body proceeds into the bone bore, below the cortical bone surface, the suture extending through the lumen of the anchor body applies a proximal tensile force on the anchor body, to cause the anchor body to rotate relative to the pusher member, thereby anchoring the anchor body in cancellous bone. Of course, this system has similar disadvantages to those of the Pedlick et al. system, and requires the suture to be directly engaged with the bone anchoring body.
What is needed, therefore, is a new approach for repairing the rotator cuff or fixing other soft tissues to bone, wherein both the bone and suture anchors reside completely below the cortical bone surface, there is no requirement for the surgeon to tie a knot to attach the suture to the bone anchor, and wherein suture tension can be adjusted and possibly measured. The procedure associated with the new approach should better for the patient than existing procedures, should save time, be uncomplicated to use, and be easily taught to practitioners having skill in the art.
The present invention solves the problems outlined above by providing innovative bone anchor and connective techniques which permit a suture attachment which lies entirely beneath the cortical bone surface, and which further permit the attachment of suture to the bone anchor without the necessity for tying knots, which is particularly arduous and technically demanding in the case of arthroscopic procedures.
More particularly, in one aspect of the invention, a bone anchor device is provided for attaching connective tissue to bone, which has a longitudinal axis and comprises a toggle member and a preferably tubular body member disposed distally of the toggle member. An axial space is present between the toggle member and the body member. A connecting portion is disposed in the axial space, which joins the toggle member to the body member.
In operation, when it is desired to deploy the inventive bone anchor device, the toggle member is movable, in a pivoting or rotational fashion, between an undeployed position wherein the toggle member has a smaller profile in a direction transverse to the longitudinal axis, which is no wider than the transverse dimension or width of the body member and the hole into which the bone anchor device is disposed, and a deployed position wherein the toggle member has a larger profile in the direction transverse to the axis, which is substantially larger than the width of the hole, so that the outer edges of the toggle member become embedded in the cancellous bone which lies beneath the cortical bone surface, and so that there is no reasonable way, short of widening the hole through the cortical bone, ofwithdrawing the anchor proximally out of the hole.
When the toggle member is deployed, the connecting portion deforms such that the axial space is reduced in length.
The connecting portion preferably comprises a one or more struts having proximal ends joined to the toggle member and distal ends joined to the body member. In manufacture, the body member, struts, and toggle member, which is preferably annular and elliptical in configuration, may all be fabricated from a single piece, such as a hypotube.
Preferably, the inventive toggle member is disposed at an acute angle relative to the axis in the undeployed position, and is disposed in a substantially transverse orientation relative to the axis in the deployed position.
The inventors have discovered that, due to potential cyclic loading effects during usage of the affected body part after completion of the medical procedure, it is advantageous to form at least the connecting portion, and preferably the toggle member as well of a biocompatible relatively ductile material. In a presently preferred embodiment, the material comprises an annealed metal, such as stainless steel.
In a preferred embodiment, there is disposed a mandrel proximally of the toggle member, and a casing extending through the toggle member. The mandrel, together with the body, is useful in actuating the toggle member from its undeployed position to its deployed position.
In another aspect of the invention, there is provided a bone anchor device for attaching soft tissue to bone, which device has a longitudinal axis and comprises a toggle member being rotatable from an undeployed position wherein the toggle member has a smaller profile in a direction transverse to the axis and a deployed position wherein the toggle member has a larger profile in the direction transverse to the axis. The toggle member has no structure for attaching suture material thereto, since the suture material is to be attached to a body member disposed distally of the toggle member.
In yet another aspect of the invention, there is provided an apparatus for attaching connective tissue to bone, which apparatus has a longitudinal axis and comprises an annular toggle member and a body member disposed distally of the toggle member, such that there is an axial space between the toggle member and the body member. Advantageously, the toggle member is movable between an undeployed position wherein the toggle member has a smaller profile in a direction transverse to the axis and a deployed position wherein the toggle member has a larger profile in the direction transverse to the axis. When installed in a desired procedural site, in suitable bone, suturing material extends axially through a center aperture in the annular toggle member, without being secured to or contacting the toggle member.
In still another aspect of the invention, there is provided an apparatus for attaching connective tissue to bone, which apparatus has a longitudinal axis and comprises a toggle member and a body member disposed distally of the toggle member, such that there is an axial space between the toggle member and the body member. The toggle member is movable between an undeployed position wherein the toggle member has a smaller profile in a direction transverse to the axis and a deployed position wherein the toggle member has a larger profile in the direction transverse to the axis. A connecting portion is disposed in the axial space and joins the toggle member to the body member.
In another aspect of the invention, there is provided an apparatus for attaching connective tissue to bone, which comprises an anchor body having a longitudinal axis and having an anchoring structure for fixing the anchor body within a body cavity. The anchor body has a proximal end, a distal end, and a lumen opening at the proximal end, and further includes a suture return member disposed therein such that a length of suture may be introduced into the lumen from the proximal end, looped around the suture return member, and passed out ofthe lumen through the proximal end. A suture locking plug is movable within the lumen from a first position to a second position, and a bone anchoring member is attached to the anchor body, preferably at the proximal end thereof, and is movable between an undeployed position and a deployed position. In preferred embodiments of the invention, the suture return member comprises a shaft or pin which may be either fixed or rotatable. The bone anchoring member preferably comprises a toggle member, which, in the undeployed position has a smaller profile in a direction transverse to the longitudinal axis and in the deployed position has a larger profile in the direction transverse to the axis.
As noted supra, the toggle member is preferably disposed proximally of the anchor body such that there is an axial space between the toggle member and the anchor body. When the toggle member is moved from the aforementioned undeployed position to the aforementioned deployed position, the axial space is reduced in length. A connecting portion is disposed in the axial space and joins the toggle member to the anchor body. The connecting portion preferably comprises a pair of struts having proximal ends joined to the toggle member and distal ends joined to the anchor body.
In still another aspect of the invention, there is disclosed a method of using suture to secure soft tissue, preferably a tendon, with respect to a body cavity, preferably disposed in a portion of bone. The method comprises the steps of passing a length of suture material through soft tissue so that a loop of suture material is disposed in the soft tissue, resulting in two free ends, and providing an anchor body having an open proximal end and a lumen. A suture return member is disposed in the anchor body. Additional steps include passing the two free ends ofthe length of suture into the lumen of the anchor body through the open proximal end, and looping them about the suture return member such that the two free ends of the suture extend proximally from the lumen through the open proximal end. The anchor body is fixed with respect to the body cavity, and the loop of suture material is tensioned by pulling on one or both of the two free ends of the length of suture, to approximate the soft tissue with respect to the body cavity as desired. A further step is to fasten the two free ends of the length of suture with respect to the anchor body without knots.
In preferred approaches, the step of fixing the anchor body with respect to the body cavity comprises forming the body cavity, passing the anchor body into the body cavity, and radially expanding anchoring structure, preferably a deployable toggle member, on the anchor body. The anchoring structure is provided on a proximal end of the anchor body so as to engage the cortical layer of the bone and to prevent proximal removal of the anchor body from the body cavity.
In yet another aspect of the invention, there is disclosed a method of securing soft tissue to bone, comprising disposing an anchor body having a longitudinal axis and having a length of suture secured therein within a bore disposed in a portion of bone, and deploying a toggle member attached to a proximal end of the anchor body from an undeployed position wherein the toggle member has a smaller profile in a direction transverse to the axis to a deployed position wherein the toggle member has a larger profile transverse to the axis. The toggle member fixes the anchor body axially relative to the portion of bone. A connecting portion joins the toggle member to the anchor body, and is disposed in an axial space between the toggle member and the anchor body. The aforementioned deploying step includes deforming the connecting portion as the toggle member is moved from the undeployed position to the deployed position.
The invention, together with additional features and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying illustrative drawing.

Method and apparatus for attaching connective tissues to bone using a knotless suture anchoring device


Disclosed are a recombinant DNA molecule encoding a fusion protein comprising a SlpA chaperone and a target polypeptide wherein human FK506 binding proteins (FKBPs) are excluded as target polypeptides, a corresponding expression vector encoding said fusion protein as well as host cells transformed with said expression vector. Also disclosed are a method for producing the fusion protein, a recombinantly produced fusion protein comprising a SlpA chaperone and a target polypeptide. A further aspect of the invention is the use of the recombinantly produced fusion protein, and a reagent kit containing a recombinantly produced fusion protein comprising a SlpA chaperone and a target polypeptide.

Protein folding is a spontaneous process that is driven by the small difference in Gibbs free energy between the native and unfolded state. Within the folding process, a largely unstructured polypeptide chain adopts what is termed the native conformation or three-dimensional structure of a protein. Aggregation of incompletely folded molecules competes with productive folding, and this constitutes a major problem and affects the folding yields both in vivo and in vitro. In living cells, folding is assisted by helper proteins. Folding helpers are polypeptides that assist the folding and maintain the structural integrity of other proteins. They possess the ability to promote the proper folding of a polypeptide chain by reversibly interacting with their target, thereby preventing detrimental side reactions such as aggregation processes. They do so both in vivo and in vitro, and there is an ever increasing number of applications of these folding helpers in biotechnological problems. Generally, folding helpers are subdivided into folding catalysts and chaperones.
Chaperones are known to reversibly bind to denatured, partially denatured or, put simply, hydrophobic surfaces of polypeptides and thus help to renature proteins or to keep them in solution. Chaperones lower the concentration of aggregation-prone folding intermediates and aggregation-prone folded proteins by reversibly binding and masking hydrophobic surfaces. They thus exert a mere binding function. In contrast, folding catalysts such as disulfide oxidoreductases and peptidyl-prolyl cis/trans isomerases accelerate rate limiting steps in protein folding and thus shorten the lifetime of folding intermediates. Folding catalysts thus lower the concentration of aggregation-prone folding intermediates due to their catalytic function. An important class of folding catalysts is referred to as peptidyl prolyl cis/trans isomerases (PPIases).
Based on sequence similarity, protein topology and binding of immunosuppressant molecules, prolyl isomerases are distinguished into three distinct families, the cyclophilins, the parvulins and the FK506 binding proteins (hence the acronym FKBPs). FKBPs bind to and are inhibited by FK506, rapamycin and related macrolide derivatives, which have been used as immunosuppressant drugs.
A putative folding helper that belongs to the FKBP family of peptidyl prolyl cis/trans isomerases in E. coli is SlpA, SlpA being the acronym for "SlyD-like protein A" (Hottenrott et al. 1997, JBC 272/25, 15697-15701). Up to now, information on SlpA and its physiological role in E. coli has been scarce. Although a poor prolyl isomerase activity of SlpA has been reported, this protein has hitherto remained rather enigmatic. So far, information on the physico-chemical or possible chaperone properties of SlpA has been lacking, and the function of SlpA in the E. coli cytosol has not even been addressed.
In many diagnostic applications recombinantly produced proteins are used as binding partners, e.g., as antigens in an immunoassay designed for the detection of a specific immunoglobulin analyte. These antigens may be produced as fusion proteins containing one part that makes up the target portion or antigenic polypeptide which is intended to recognize and bind a specific moiety present in the sample or in the assay mixture under study. The other part of the recombinantly produced fusion protein is a polypeptide portion that is fused to the specificity-conferring antigenic part in order to facilitate its cloning, expression, overproduction, folding/refolding and purification, and to increase its solubility, its stability or its reversibility of folding. The synthesis of recombinantly produced fusion proteins is well described in prior art. It is also well-established that it is advantageous to use chaperones as that part of the fusion protein that serves a role as a helping molecule for the expression, folding, purification, solubilization, and the increase in the overall stability of the target polypeptide.
U.S. Pat. No. 6,207,420 discloses a fusion protein system for the expression of proteins, in which the amino acid sequences of the target polypeptide part and the fused peptide part originate from different organisms. Recently it could be shown that FkpA and SlyD are suitable as fusion modules for the production of recombinant proteins. Both chaperones increase the expression rate of their client proteins in a prokaryotic host, support correct refolding and increase the overall solubility of even extremely aggregation-prone proteins such as retroviral transmembrane proteins (Scholz et al. 2005, JMB 345, 1229-1241 and WO 03/000877).
While FkpA and SlyD are particularly useful in helping difficult or aggregation-prone proteins to adopt and maintain their native structure in diagnostic reagents and, more generally speaking, biotechnological applications, there remains the challenge of thermal stability. The native conformation of proteins is stabilized by a carefully balanced network of van-der-Waals contacts, hydrogen bonds, salt bridges and hydrophobic interactions. These contacts are optimized for the microenvironment of the respective protein, and changes in pH, ionic strength or temperature do perturb and shift the equilibrium between folded and unfolded molecules. An increase in temperature is particularly well suited to denature proteins, which often results in aggregation of the fully or partially unfolded molecules. Thermally induced aggregation of proteins with the concomitant loss of function constitutes a major problem of any protein formulation. It is well conceivable that elevated temperatures, or, more generally speaking, thermal stress may occur during inappropriate shipment or storage of protein reagents or formulations.
A chaperone fusion module such as SlyD, for instance, shows an onset of thermally induced unfolding at a temperature around 42° C., a temperature which is easily exceeded, e.g., when the cooling system is defective in a container used for transportation, shipment or storage of a protein formulation. In case the target protein X is highly hydrophobic and fully depends on the chaperoning activity of its fusion partner, the complete fusion polypeptide will aggregate as soon as the SlyD module unfolds and concomitantly loses its solubilizing function. In other words, the stability of SlyD limits the overall stability of a SlyD-X fusion polypeptide when X is a very hydrophobic and aggregation-prone client protein.
Fusion proteins comprising FkpA show a slightly increased stability, probably due to the higher intrinsic thermostability of the dimeric FkpA carrier module. The melting temperature of E. coli SlyD has been determined at around 42° C., whereas FkpA is rather stable up to around 50° C. Yet, for reasons that are outlined in the following section, there remains the urgent need to provide alternative functional chaperone variants with high intrinsic stability.
In a heterogeneous immunoassay of the double antigen sandwich (DAGS) format, for instance, two variants of an antigen are employed on either side of the assay. One of these variants bears a label with a high affinity for the solid phase, the other bears a signaling moiety in order to generate a signal output. Each of these antigen variants may be fused to a helper sequence, i.e., a carrier or fusion module. At least one chaperone (or a functional polypeptide binding domain, i.e., a chaperone domain) is attached or fused to the target polypeptide and facilitates folding, increases stability and solubility and maintains the target polypeptide in a suitable conformation so that the antibody analyte to be determined can specifically recognize and bind the target polypeptide. Preferably, different chaperones are used as fusion partners on either side of an immunological bridge assay, in order to break the inherent symmetry of the assay. An assay format containing different carrier or fusion modules but identical or similar target polypeptides on either side (i.e., on the capture and the detection side) may also be termed an asymmetric DAGS format. By using different fusion modules on each side of a DAGS assay, the risk of immunological cross-reactions due to the carrier modules and, concomitantly, erroneously high signals may be substantially reduced.
Clearly, the overall stability of the assay is limited by the immunological component with the lowest inherent stability. When using FkpA and SlyD as fusion partners in an asymmetric DAGS, SlyD is the fusion partner that limits the overall stability. Thus, there is an obvious need to find other chaperones, which can fully replace SlyD functionally and which are inherently more stable towards thermal stress. Even though a wealth of SlyD homologues from thermophilic or hyperthermophilic organisms have been described, there is a caveat in simply using these proteins as fusion partners: Since they have been evolved and optimized for temperatures far beyond 60° C., they possess an extremely high thermodynamic stability. As a consequence, stable and hyperstable proteins often tend to become rather rigid at ambient temperature, i.e., they lose the flexibility which is a prerequisite for dynamic binding to target molecules. It is widely accepted that the stability of a protein can only be increased at the expense of both its flexibility and function, which often precludes highly stable proteins from applications at ambient temperature. An object of the present invention is therefore to identify thermostable folding helpers from mesophilic organisms. A further object of the present invention is to provide polypeptides suitable for diagnostic and biotechnological applications that possess an increased thermal stability and prolong the shelf life of diagnostic reagents and protein formulations.
A few proteins of E. coli are stable and soluble at temperatures far beyond 49° C. as reported recently by Kwon et al. (BMB reports 2008, 41(2), 108-111). The proteins that were soluble upon exposure to elevated temperature were identified by SDS polyacrylamide gel electrophoresis. The study was carried out with sonicated extracts of E. coli after incubation at various temperatures. Amongst the 17 heat-stable proteins that were identified, 6 proteins turned out to be putative folding helpers (GroEL, GroES, DnaK, FkpA, trigger factor, EF-Ts). It is noteworthy that the experiment was performed with a cell-free lysate of E. coli and that the solubility of the respective protein was taken as the sole criterion for stability.
There is, however, a substantial difference between the solubility and the stability of a protein. It is well known in the art that the solubility of a protein often reaches a minimum at conditions of maximal stability. For instance, the thermodynamic stability of a protein reaches a maximum when the pH of the buffer solution coincides with the pI of the respective protein. Yet, under these very conditions, the protein solubility reaches a minimum. Another popular example is the salting-out of proteins by means of ammonium sulfate or other cosmotropic agents: here also, the solubility of a protein decreases as its stability is increased (ammonium sulfate is a strongly cosmotropic agent, i.e., it stabilizes protein structures).
WO 2007/077008 discloses recombinantly produced chimeric fusion proteins that contain the polypeptide binding segment of a non-human chaperone like, e.g., E. coli SlyD, and N- and C-terminally fused thereto a human FKBP type peptidyl-prolyl-cis/trans isomerase. A similar fusion polypeptide has been disclosed using a chaperone segment of SlpA.
Surprisingly, SlpA, in particular E. coli SlpA, is able to confer thermal stability on other target polypeptides when used as a fusion partner. As reported by Hottenrott et al. (supra) SlpA is an enzyme with a rather poor peptidyl-prolyl cis/trans isomerase activity. Unexpectedly, SlpA exhibits also pronounced chaperone features and, even more surprisingly, SlpA possesses an high intrinsic stability and confers thermal stability on a fused target polypeptide thereby making the target polypeptide less susceptible to heat-induced aggregation. Whereas the closely related SlyD exhibits only a marginal stability with a midpoint of thermal unfolding at around 42° C., SlpA retains its native fold at least up to 50° C. and shows a midpoint of thermal unfolding (defined as the melting temperature) at around 56° C. This is indeed puzzling given the close relationship between SlyD and SlpA (which stands for SlyD-like protein) and given the fact that both are monomeric proteins from a mesophilic organism such as E. coli with a maximum growth temperature of 49° C. Most surprisingly, the mesophilic organism E. coli harbors a putative folding helper such as SlpA that combines outstanding thermostability and chaperone features.

SlpA as a tool for recombinant protein and enzyme technology