Patent Application
20060287710
The subject matter of the invention is a temporary stent made from biodegradable shape memory polymers (SMP) for use in the non-vascular or vascular field. The stent may be implanted in compressed form by means of minimal invasive surgery and takes its desired size at the location of use caused by the shape memory effect. The stent gradually resolves caused by biological degradation which makes further surgery for removing the stent dispensable. A further subject matter of the invention is a method of implanting and removing the stent and for manufacturing and programming the stent.
To treat clogged vessels or constricted tubular organs or after surgical procedures, tubular tissue supports (stents) are inserted into the tubular organ. They serve for keeping open the constriction portion or for taking over the function of the injured tubular organ to re-enable normal passage or discharge of body liquids. Stents are also inserted into the blood vessel to treat clogged or constricted blood vessels, said stents keeping open the constricted portion and re-enabling normal blood flow.
Stents are usually cylindrical structures made of a kind of wire netting (wire coil design) or tubes, which may be perforated or which may not be perforated (slotted tube design). Conventional stents have a length of 1 and 12 cm and may have a diameter of 1 to 12 mm.
The mechanical demands on a stent are contradictory. On the one hand, a stent must exert high radial forces onto the tubular organ to be supported. On the other hand it is required that the stent can be radially compressed to be able to easily insert it into a tubular organ without injuring the vessel wall or the surrounding tissue.
This problem was solved in that the stents are inserted in compressed form and are mounted only after having reached the correct position. In the compressed state the diameter is smaller than in expanded state. This process can basically also be used for the minimal invasive removal of the stent. A possible problem is, however, that the metallic materials usually used do not always completely regularly expand and cannot be folded again, which is a potential risk of injury for the bordering tissue.
For the minimal invasive insertion of a stent, two different technologies have established (market report “US peripheral and vascular stent and AAA stent graft market” (Frost & Sullivan), 2001):
The removal of expanded stents is problematic, as was already indicated above. If the stent must be pulled out of a tubular cavity, there is a risk of injuring the surrounding tissue by abrasion, because the stent is too large and has sharp edges. The shape memory effect is therefore also applied to reduce the diameter of the stent if a stent must be removed again. Examples for removable implants (stents) made of shape memory metals are known from the prior art: U.S. Pat. No. 6,413,273 “Method and system for temporarily supporting a tubular organ”; U.S. Pat. No. 6,348,067 “Method and system with shape memory heating apparatus for temporarily supporting a tubular organ”, U.S. Pat. No. 5,037,427 “Method of implanting a stent within a tubular organ of a living body and of removing same”; U.S. Pat. No. 5,197,978 “Removable heat-recoverable tissue supporting device”.
Nitinol cannot be used in the case of a nickel allergy. The material is also very expensive and can only be programmed by laborious methods. This programming methods need comparatively high temperatures so that a programming within the body is not possible. The SM material is therefore programmed outside the body, i.e. it is brought to its temporary shape. After implantation, the shape memory effect is activated and the stent expands, i.e. it regains its permanent shape. A removal of a stent by again utilizing the shape memory effect is then not possible. A frequent problem in metallic stents not only in the vascular area is above that the occurrence of a restenosis.
Other metallic stents of SM materials, such as from U.S. Pat. No. 5,197,978 on the other hand enable a utilization of the shape memory effect to remove the stent. However, these metallic materials are very laborious to manufacture, and the tissue compatibility is not always ensured. Due to the inadequately adapted mechanical properties of the stents, inflammations and pain often occur.
The temporary stent described in U.S. Pat. No. 5,716,410 “Temporary stent and method of use” is a coil made of a shape memory plastic material. The SMP material has an embedded heating wire. The heating wire is connected via a catheter shaft to an electrical controller, wherein the shaft end being a hollow tube is put over the end of the coil. If the implanted stent is heated, which is in its expanded, temporary shape, above the switching temperature Ttrans, the diameter of the coil reduces. This shall enable a simple removal of the stent. A disadvantage of the coil structure is that the radial forces are too low to expand the tubular cavities. The radial forces of the coil spread only over a small contact surface to the tissue. There is even a risk of a local mechanical overload by pressure, possibly by incision into the tissue. Moreover, the attachment of the catheter shaft (heating element) to the heating wire of the implanted coil proves to be difficult, since the catheter shaft must only be put over the one end of the coil.
U.S. Pat. No. 4,950,258 describes a device for expanding a constricted blood vessel. The device is made of biodegradable polymers based on L-lactide and/or glycolide and exists in the form of a coil or tube. Caused by the shep memory effect, the diameter enlarges so that a vessel can be expanded. A disadvantage of the materials used is the embrittelement thereof during degradation and the generation of particles that may lead to vessel occlusions released from the device.
EP 1033145 also describes biodegradable stents made of shape memory polymers for use in blood vessels, lymphatic vessels, in the bile or in the ureter. The stent is composed of a thread of homopolymers or copolymers or of their blends based on L-lactide, glycolide, ε-caprolacton, p-dioxanon or trimethylenecarbonate. The thread is interwoven as mono-filament or multi-filament to form a mesh structure. The shape memory effect is utilized for enlarging the diameter of the stent and to fix it at the location of use. The switching temperature is a glass temperature not higher than 70° C. Active substances or diagnostics may be added to the SMP or may be superficially applied.
U.S. Pat. No. 5,964,744 describes implants such as tubes and catheters, for the urogenital tract or the gastrointestinal tract, made of polymer shape memory materials, which include a hydrophilic polymer. In an aqueous medium the material absorbs moisture, softens thereby and changes its shape. As an alternative or additionally the material softens when being heated. In the uretheral stent the effect is utilized to bend the straight ends of the stent at the place of use (e.g. kidney or bladder). Thus, the uretheral stent is fixed at the place of use so that the stent is not displaced in the case of peristaltic movements of the tissue.
WO 02/41929 describes tubular vessel implants with shape memory, which are e.g. also suitable as bile stents. The material is an alipathic, polycarbonate-based thermoplastic polyurethane with bio-stable property.
A disadvantage of the materials used in the prior art is that they are not biodegradable. The implant must be removed from the body in a second operation.
U.S. Pat. No. 6,245,103 describes bio-absorbable, self-expanding stents of braided filaments. The stent is compressed by application of an outer radial force. The stent is mounted on a catheter and is held by an outer sleeve under tension in compressed condition. If the stent is pressed out of this arrangement, its diameter automatically enlarges due to the resetting force of the elastic material. This is not the shape memory effect that is activated by an external stimulus, e.g. an increase in temperature.
U.S. Pat. No. 6,569,191 describes self-expanding stents of biodegradable interwoven threads. Several strips of an elastic, biodegradable polymer are adhered onto the outside of the stent. The stents have shape memory properties. When heated to body temperature or when absorbing moisture they contract. Thus, the stent is also contracted; at the same time the diameter of the stent enlarges. The elastic strips enforce the radial forces of the stent towards the outside. The strips are e.g. made of a shape memory polymer based on lactic acid and/or glycol acid.
The biodegradable materials, i.e. materials that can usually be hydrolyzed, used in the prior art partially reveal a problematic degradation behavior. A degradation takes place that leads to the generation of small particles that are a potential risk. The particles may clog the channels or tubes (e.g. the urethra). Moreover, a degradation may also change the structure/nature of an implant in a manner that an incompatibility with blood and/or tissue occurs.
Further problems that often occur are pain caused by the insufficient mechanical adaptation of the stent to the surrounding tissue and the displacement of the stent.
Since stents have increasingly captured an extending field of use in medicine, endeavors must be made to overcome the above-mentioned disadvantages. Thus, stents for the non-vascular or vascular use are needed which enable a minimal invasive implantation and at the same time enable the gentle removal thereof. The materials for the stent shall above that be adaptable to the respective place of use, e.g. in view of varying mechanical loads. The materials shall preferably enable a further functionalization of the stent, e.g. by embedding further medically useful substances.
To overcome disadvantages of the prior art, the following is required:
This object is solved by the subject matter of the present invention, as it is defined in the claims. These stents comprise a shape memory material (SMP material), preferably a biodegradable SMP material, preferably an SMP material, which reveals a thermally induced or light-induced shape memory effect. The SMP materials to be used according to the invention may remember one or two shapes in the memory. Preferred embodiments are defined in the dependent claims.
Stents of this type solve the above-mentioned problems, either on the whole or at least partially. Thus, the present invention provides stents, comprising an SMP material, which can be inserted minimally invasively and atraumatically by the use of the shape memory effect, which are tissue-compatible and haemo-compatible in their gegradation behavior and which have a sufficient stability/strength so that they reveal a sufficient stability despite the fact that a degradation takes place. Stents of this type manufactured by the materials to be used according to the invention particularly reveal a particle-free degradation behavior. This is important, since particles, which are produced during degradation, may lead to problems, such as clogging or injury of ureters etc. However, the stents of the present invention do not reveal such problems, since they exist in the form of hydrogel particles, which are soft and elastic so that the above-mentioned problems do not occur.
Since stents must exist in their temporary shape before placing in the body, they must be stored at sufficiently low temperatures and in a manner sufficiently protected against irradiation, also during transport to prevent an unintended activation of the shape memory effect.
In preferred embodiments, the object is solved by a stent of SMP, characterized in that
A possible procedure for the minimal invasive insertion of a stent, comprises the following steps (
Method of programming the stent according to the invention (
The present invention will now further be described.
The stent of the present invention comprises an SMP material. Thermoplastics, blends and networks are suitable. Composites of biodegradable SMP with inorganic, degradable nano-particles are also suitable. A heating element is preferably not embedded into the SMP material. The shape memory effect may be activated thermally by means of a heatable medium, by the application of IR or NIR irradiation, by applying an oscillating electrical field or by UV irradiation.
The definition that the stent according to the invention comprises an SMP material shall define that the stent on the one hand substantially consists of an SMP material, but that on the other hand the stent may also have a basic frame made of a biodegradable plastic material, embedded or coated with an SMP material. These two essential constructions offer the following advantages.
Stents, which essentially consist of SMP materials, use the SMP material to determine the mechanical properties of the stents. By the fact that the materials, which will now be described, are used for this purpose, a favorable tissue compatibility is ensured. Furthermore, such stents, as described above, may be implanted and removed by minimal invasive surgery. The SMP materials may also be relatively easily processed, which facilitates manufacture. Finally, the SMP materials can be compounded or layered with further substances so that a further functionalization is possible. In this connection, reference is made to the following statements.
The second embodiment that is possible in principle is a stent, which comprises a basic frame, such as a “wire netting structure” or a deformable tube. These basic frames are coated by an SMP material or they are embedded therein. Particularly wire netting constructions proved that the SMP materials may exert a sufficiently great power to deform the basic frame if the shape memory effect is activated. This embodiment therefore allows to combine the positive properties of the conventional stents with the above-mentioned positive effects of the SMP materials. Particularly, stents with a very high mechanical resistance can thereby be obtained, since the basic frame contributes to this. Thus, this embodiment is particularly suitable for stents that are exerted to high mechanical loads. Furthermore, the use of the basic frame enables the reduction of the quantity of SMP materials, which may help serve costs.
If the basic frame consists of a metallic material, it should preferably be biodegradable metals such as magnesium or magnesium alloys.
Stents of this type in accordance with the present invention enable a safe placing of the stent and a compatible degradation behavior. In an alternative the stent according to the inventions usually reveals a behavior, after placing, in accordance with a 3-phase model.
The intended use of the stent determines its design, e.g. the surface composition (micro-structuring) or the existence of coatings etc.
The following embodiments are possible in principle.
The surface of the stent is compatible in view of the physiological environment at the place of use, by suitable coating (e.g. hydrogel coating) or surface micro-structuring. In the stent design the basic conditions such as the pH value or the number of germs must be taken into consideration depending on the location of use.
Then a settlement of the surface by endothel cells takes place, which may possibly be supportee by a respective modification of the surface (e.g. coating). Thereby the stent is slowly grown over by endothel cells.
In the case of vascular stents the surface of the stent is formed in a haemo-compatible manner, by suitable coating (e.g. hydrogel coating) or by surface micro-structuring so that the stent enables the comparatively short period of time after placing in full blood contact without affecting the organism. Subsequently, the settlement of the surface takes place, as mentioned above, so that the sent is slowly absorbed by the vessel wall.
Finally, the hydrolytic degradation usually takes place, the stent degrades in contact with the soft tissue but it still exerts the desired support effect due to the above-mentioned degradation behavior (particle-free degradation, mechanical stability is not affected by degradation over a long period of time).
Another alternative is that the stent after placing shall remain outside of the endothel layer, which may be achieved by suitable measures, such as the selection of the surface, the selection of the segment for the SMP materials etc.
Suitable materials for the stents of the present invention will now be described.
SMP materials in the sense of the present invention are materials, which are capable, due to their chemical-physical structure, to carry out aimed changes in shape. Besides their actual permanent shape the materials have a further shape that may be impressed on the material temporarily. Such materials are characterized by two structural features: network points (physical or covalent) and switching segments.
SMP with a thermally induced shape memory effect have at least one switching segment with a transitional temperature as switching temperature. The switching segments form temporary cross linking portions, which resolve when heated above the transitional temperature and which form again when being cooled. The transitional temperature may be a glass temperature Tg of amorphous ranges or a melting temperature Tm of crystalline ranges. It will now in general be designated as Ttrans.
Above Ttrans the material is in the amorphous state and is elastic. If a sample is heated above the transitional temperature Ttrans, deformed in the flexible state and then cooled down below the transitional temperature, the chain segments are fixed by freezing degrees of freedom in the deformed state (programming). Temporary cross linking portions (non-covalent) are formed so that the sample cannot return to its original shape also without external load. When re-heating to a temperature above the transitional temperature, these temporary cross linking portions are resolved and the sample returns to its original shape, By re-programming, the temporary shape can be produced again. The accuracy at which the original shape is obtained again is designated as resetting ratio.
In photo-switchable SMP, photo-reactive groups, which can reversibly be linked with one another by irradiation with light, take over the function of the switching segment. The programming of a temporary shape and re-generation of the permanent shape takes place in this case by irradiation without a change in temperature being necessary.
Basically, all SMP materials for producing stents can be used. As an example, reference can be made to the materials and the manufacturing methods, which are described in the following applications, which by reference directly belong to the content of the application on file:
German patent applications: 10208211.1, 10215858.4, 10217351.4, 102173050.8, 10228120.3, 10253391.1, 10300271.5, 10316573.8.
European patent applications: 99934294.2, 99908402.3
SMP materials with two shapes in the memory are described in the U.S. Pat. No. 6,388,043 which is comprised herewith by reference.
To manufacture the stents according to the invention, thermoplastic elastomers can be used. Suitable thermoplastic elastomers are characterized by at least two transitional temperatures. The higher transitional temperature can be assigned to the physical network points which determine the permanent shape of the stent. The lower transitional temperature at which the shape memory effect can be activated can be associated to the switching segments (switching temperature, Ttrans). In the case of suitable thermoplastic elastomers the switching temperatures are typically approximately 3 to 20° C. above the body temperature.
Examples for thermoplastic elastomers are multiblockcopolymers. Preferred multiblockcopolymers are composed of the blocks (macrodioles) consisting of α,ω diol polymers of poly(e-caprolacton) (PCL), poly(ethylene glycol) (PEG), poly(pentadecalacton), poly(ethyleneoxide), poly(propyleneoxide), poly(propylene glycol), poly(tetrahydrofuran), poly(dioxanon), poly(lactide), poly(glycolid), poly(lactide-ranglycolid), polycarbonates and polyether or of α,ω, diol copolymers of the monomers on which the above-mentioned compounds are based, in a molecular weight range Mn of 250 to 500,000 g/mol. Two different macrodiols are linked by the aid of a suitable bi-functional coupling reagent (especially an alipathic or aromatic diisocyanate or di-acid chloride or phosgene) to form a thermoplastic elastomer with molecular weights Mn in the range of 500 to 50,000,000 g/mol. In a phase-segregated polymer, a phase with at least one thermal transition (glass or melt transition) may be associated in each of the blocks of the above-mentioned polymer irrespective of the other block.
Multiblockcopolymers of macrodiols on the basis of pentadeclaracton (PDL) and—caprolacton (PCL) and a diisocyanate are especially preferred. The switching temperature—in this case a melting temperature—may be set over the block length of the PCL in the range between approx. 30 and 55° C. The physical network points to fix the permanent shape of the stent are formed by a second crystalline phase with a melting point in the range of 87 to 95° C. Blends of multiblockcopolymers are also suitable. The transitional temperature can be set in an aimed manner by the mixing ratio.
To manufacture the stents according to the invention, polymer networks can also be used. Suitable polymer networks are characterized by covalent network points and at least one switching element with at least one transitional temperature. The covalent network points determine the permanent shape of the stents. In the case of suitable polymer networks, the switching temperature, at which the shape memory effect can be activated, are typically approximately 3 to 20° C. above the body temperature.
To produce a covalent polymer network, one of the macrodiols described in the above section is cross linked by means of a multifunctional coupling reagent. This coupling reagent may be an at least tri-functional, low-molecular compound or a multi-functional polymer. In the case of a polymer, it might be a star polymer with at least three arms, a graft polymer with at least two side chains, a hyper-branched polymer or a dendritic structure. In the case of the low-molecular and the polymer compounds, the final groups must be able to react with the diols. Isocyanate groups may especially be used for this purpose (polyurethane networks).
Amorphous polyurethane networks of trioles and/or tetroles and diisocyanate are especially preferred. The representation of the star-shaped pre-polymers such as oligo[(raclactate)-co-glycolat]triol or -tetrol is carried out by the ring-opening copolymerization of rac-dilactide and diglycolide in the melt of the monomers with hydroxy-functional initiators by the addition of the catalyst dibutyl tin(IV)oxide (DBTO). As initiators of the ring-opening polymerization, ethylene glycol, 1,1,1-tris(hydroxy-methyl)ethane or pentaerythrit are used. Analogously, oligo(lactat-co-hydroxycaproat)tetroles and oligo(lactate-hydroxyethoxyacetate) as well as [oligo(propylene glycol)-block-oligo(raclactate)-co-glycolat)]triole are manufactured. The networks according to the invention may simply be obtained by conversion of the pre-polymers with diisocyanate, e.g. an isomeric mixture of 2,2,4- and 2,4,4-trimethylhexane-1,6-diisocyanate (TMDI), in solution, e.g. in dichloromethane, and subsequent drying.
Furthermore, the macrodiols described in the above section may be functionalized to corresponding α,ω-divinyl compounds, which can thermally or photo-chemically be cross linked. The functionalization preferably allows a covalent linking of the macro-monomers by reactions that do not result in side products. This functionalization is preferably provided by ethylenic unsaturated units, particularly preferred acrylate groups and methacrylate groups, wherein the latter are particularly preferred. In this case the conversion to α,ω-macrodimethacrylates or macrodiacrylates by reaction with the respective acid chlorides in the presence of a suitable base may particularly be carried out. The networks are obtained by cross linking the end group-functionalized macro-monomers. This cross linking may be achieved by irradiation of the melt, comprising the end group-functionalized macromonomer component and possibly a low-molecular co-monomer, as will be explained further below. Suitable method conditions for this are the irradiation of the mixture in melt, preferably at temperatures in the range of 40 to 100° C., with light of a wavelength of preferably 308 nm. As an alternative, a heat cross linking is possible if a respective initiator system is used.
If the above-described macromonomers are cross linked, networks are produced having a uniform structure, if only one type of macromonomers is used. If two types of monomers are used, networks of the AB-type are obtained. Such networks of the AB-type may also be obtained if the functionalized macromonomers are copolymerized with suitable low-molecular or oligomer compounds. If the macro-monomers are functionalized with acrylate groups or methacrylate groups, suitable compounds, which can be copolymerized, are low-molecular acrylates, methacrylates, diacrylates or dimethacrylates. Preferred compounds of this type are acrylates, such as butylacrylate or hexylacrylate, and methacrylates such as methylmethacrylate and hydroxyethylmethacrylate.
These compounds, which can be copolymerized with the macromonomers, may exist in a quantity of 5 to 70 percent by weight related to the network of macromonomer and the low-molecular compound, preferably in a quantity of 15 to 60 weight percent. The installation of varying quantities of the low-molecular compound takes place by the addition of respective quantities of compound to the mixture to be cross linked. The installation of the low-molecular compound into the network takes place at a quantity that corresponds to that of the cross linking mixture.
The macromonomers to be used according to the invention will now be described in detail.
By variation of the molar weight of the macrodiols, networks with different cross linking densities (or segment lengths) and mechanical properties can be achieved. The macromonomers to be cross linked covalently preferably have a numeric average of the molar weight determined by GPC analysis of 2000 to 30000 g/mol, preferably 500 to 20000 g/mol and particularly preferred of 7500 to 15000 g/mol. The macromonomers to be covalently cross linked preferably have on both ends of the marcomonomer chain a methacrylate group. Such a functionalization allows the cross linking of the macromonomers by simple photo-initiation (irradiation).
The marcomonomers are preferably polyester macromonomers, particularly preferably polyester macromonomers on the basis of ε-carprolacton. Other possible polyester macromonomers are based on lactide units, glycolide units, p-dioxane units and the mixtures thereof and mixtures with ε-caprolacton units, wherein polyester macromonomers with caprolacton units are particularly preferred. Preferred polyester macromonomers are furthermore poly(caprocacton-co-glycolide) and poly(caprolacton-co-lactide). The transitional temperature as well as the degradation speed can be set through the quantity ratio of the co-monomers.
Particularly preferred are the macromonomers polyester to be used according to the invention, comprising the linkable end groups. An especially preferred polyester to be used according to the invention is a polyester on the basis of ε-caprolacton or pentadecalacton, for which the above-mentioned statements about the molar weight apply. The manufacture of such a polyester macromonomer, functionalized at the ends, preferably with methacrylate group, may be manufactured by simple syntheses, that are known to the person skilled in the art. These networks, without consideration of the further essential polymer components of the present invention, show semi-crystalline properties and have a melting point of the polyester component (determinable by DSC measurements) that depends on the type of polyester component used and which is also controllable thereby. As is known, this temperature (Tm1) for segments based on caprolacton units is between 30 and 60° C. depending on the molar weight of the macromonomer.
A preferred network having a melt temperature as switching temperature is based on the macromonomer poly(caprolacton-co-glycolide)-dimethacrylate. The macromonomer may be converted as such or may be co-polymerized with n-butylacrylate to form an AB-network. The permanent shape of the stent is determined by covalent network points. The network is characterized by a crystalline phase, whose melting temperature can be set e.g. by the comonomer ratio of caprolacton to glycolide in an aimed manner in the range of 20 to 57° C. n-butylacrylate as comonomer may e.g. be used for optimizing the mechanical properties of the stent.
A further preferred network having a glass temperature as switching temperature is obtained from an ABA tri-blockdimethylacrylate as macromonomer, characterized by a central block B of polypopyleneoxide and end blocks A of poly(rac-lactide). The amorphous networks have a very broad switching temperature range.
To manufacture stents with two shapes in the memory, networks having two transitional temperatures are suitable, such as inter-penetrating networks (IPNs). The covalent network is based on poly(caprolacton)-dimethacrylate as macromonomer; the inter-penetrating component is a multiblockcopolymer of macrodiols based on pentadecalacton (PDL) and ε-caprolacton (PCL) and a diisocyanate. The permanent shape of the material is determined by the covalent network points. The two transitional temperatures—melt temperatures of the crystalline phases—may be utilized as switching temperatures for a temporary shape. The lower switching temperature Ttrans may be set via the block length of the PCL in the range between approx. 30 and 5° C. The upper switching temperature Ttrans 2 lies in the range of 87 to 95° C.
To manufacture the stents according to the invention, photosensitive networks can also be used. Suitable photosensitive networks are amorphous and are characterized by covalent network points, which determine the permanent shape of the stent. A further feature is a photo-reactive component or a unit reversibly switchable by light, which determines the temporary shape of the stent.
In the case of the photosensitive polymers a suitable network is used, which includes photosensitive substituents along the amorphous chain segments. When being irradiated with UV light, these groups are capable of forming covalent bonds with one another. If the material is deformed and irradiated by light of a suitable wavelength λ1, the original network is additionally cross-linked. Due to the cross-linking a temporary fixing of the material in deformed state is achieved (programming). Since the photo-linking is reversible, the cross linking can be released again by further irradiation with light of a different wavelength λ2 and thus the original shape of the material can be reproduced again (reproduction). Such a photo-mechanical cycle can be repeated arbitrarily often. The basis of the photo-sensitive materials is a wide meshed polymer network, which, as mentioned above, is transparent in view of the irradiation intended to activate the change in shape, i.e. preferably forms an UV-transparent matrix. Networks of the present invention on the basis of low-molecular acrylates and methacrylates, which can radically be polymerized are preferred according to the invention, particularly C1-C6-meth(acrylates) and hydroxyderivatives, wherein hydroxyethylacrylate, hydroxyporpylmethacrylate, poly(ethyleneglycole)methacrylate and n-butylacrylate are preferred; preferably n-butylacrylates and hydroxyethylmethacrylate are used.
As a co-monomer for producing the polymer network of the present invention a component is used, which is responsible for the cross linking of the segments. The chemical nature of this component of course depends on the nature of the monomers.
For the preferred networks on the basis of the acrylatemonomers described above as being preferred, suitable cross linking agents are bi-functional acrylate compounds, which are suitably reactive with the starting materials for the chain segments so that they can be converted together. Cross linking agents of this type comprise short, bi-functional cross linking agents, such as ethylenediacrylate, low-molecular bi- or polyfunctional cross linking agents, oligomer, linear diacrylate cross linking agents, such as poly(oxyethylene)diacrylates or poly(oxypropylene)diacrylates and branched oligomers or polymers with acrylate end groups.
As a further component the network according to the invention comprises a photo-reactive component (group), which is also responsible for the activation of the change in shape that can be controlled in an aimed manner. This photo-reactive group is a unit which is capable of performing a reversible reaction caused by the stimulation of a suitable light irradiation, preferably UV radiation (with a second photo-reactive group), which leads to the generation or resolving of covalent bonds. Preferred photo-reactive groups are such groups that are capable of performing a reversible photodimerization. As a photo-reactive component in the photosensitive networks according to the invention, different cinnamic acid esters (cinnamates, CA) and cinnamylacylic acid ester (cinnamylacylates, CAA) can preferably be used.
It is known that cinnamic acid and its derivatives dimerize under UV-light of approx. 300 nm by forming cyclobutane. The dimeres can be split again if irradiation is carried out with a smaller wavelength of approx. 240 nm. The absorption maximum can be shifted by substituents on the phenyl ring, however they always remain in the UV range. Further derivatives that can be photodimerized, are 1,3-diphenyl-2-propene-1-on (chalcon), cinnamylacylic acid, 4-methylcoumarine, various orthos-substituted cinnamic acids, cinammolyxysilane (silylether of the cinnamon alcohol).
The photo-dimerization of cinnamic acid and similar derivatives is a [2+2] cyclo-addition of the double bonds to a cyclobutane derivative. The E-isomers as well as the Z-isomers are capable of performing this reaction. Under irradiation the E/Z-isomerization proceeds in competition with the cyclo-addition. In the crystalline state the E/Z-isomerization is, however inhibited. Due to the different possibilities of arrangement of the isomers with respect to each other, 11 different stereo-isomeric products (truxill acids, truxin acids) are theoretically possible. The distance of the double bonds of two cinnamic acid groups to one another required for the reaction is approximately 4 Å.
The networks are characterized by the following properties:
On the whole, the networks are favorable SMP materials, with high reset values, i.e. the original shape is also obtained in the case of running through a cycle of changes in shape several times at a high percentage, usually above 90%. A disadvantageous loss of mechanical property values does not occur.
Since the above-mentioned materials are based on alipathic polyesters, the SMP materials used can be hydrolyzed and are biodegradable. Surprisingly it was proven that these material on the one hand degrade in a biocompatible manner (i.e. the degradation products are not toxic) and at the same time the mechanical integrity of the stent is upheld during the degradation process which ensures a sufficiently long functionality of the stent.
To increase the haemocompatibility, the chemical structure of the SMP-materials used according to the invention can be modified, e.g. by the installation of the above-mentioned poly or oligoether units.
Processing of the Polymers to Become Stents
To process the thermoplastic elastomers to form stents, e.g. in the form of a hollow tube or the like (
In the manufacture of stents of polymer networks it must be taken care that the form in which the cross linking reaction of the macromonomers takes place corresponds to the permanent shape of the stent (casting method with subsequent curing). Especially the network materials according to the invention require, for further processing, special milling and cutting methods. The perforation or the cutting of a tube by the aid of LASER light of a suitable wavelength is suggested. By the aid of this technology—especially in the case of a combination of CAD and pulsed CO2 or YAG lasers—shapes up to a size of 20 μm can be worked down without the material being exposed to a high thermal load (and thus undesired side reactions on the surface). As an alternative, a chip removing processing to obtain a ready stent is suggested.
The second embodiment is obtained by coating or embedding a conventional material (see above) into an SMP material by a suitable method.
The required mechanical properties of the stent depend on the place of use and require an adapted design. If the implanted stent is exposed to strong mechanical deformations, a very high flexibility is required without the stent collapsing during the movements. Basically, the “wire coil design” is more suitable. In other areas of organs that are located deeper the stent is less loaded mechanically by deformations but rather by a relative high external pressure. A stent suitable for this purpose must be characterized by high radial forces onto the ambient tissue. In this case the “slotted tube design” seems to be more suitable. Tubes with perforations enable the inflow of liquid from the ambient tissue into the stent (drainage).
Since drainage effects are in the fore in the case of stents that shall be used on the non-vascular area, particularly a design with embedded conventional basic frame is favorable for such stents, or a design basically consisting of SMP material (perforated tube or network body), since in these designs the permeability for liquids necessary for the drainage is very simple while at the same time revealing a sufficient mechanical strength.
The prior art particularly revealed problems with blood vessels with small diameters, since the known stents are not flexible and adaptable enough for such vessels, The stents of the present invention, however, also enable a safe use in such vessels, since the superior elastic properties of the SMP materials, i.e. high elasticity at small deflections and high strength at large expansion, protects the vessel for instance in the case of pulsatile movements of the arteries.
Functionalization of the Stents
For a more convenient insertion of the stent, this stent may possibly be provided with a coating which increases slippage (e.g. silicones or hydrogels).
Further possibilities of improving haemocompatibility comprise the possibility that a coating is provided (the materials necessary for this purpose are known to the person skilled in the art), or a micro-structuring of the surface can be made. Suitable methods of surface modification are for instance the plasma-polymerization and graft polymerization.
To localize the stent more easily by visual diagnostic procedures, the shape memory plastic material can be screened by a suitable x-ray contrast agent (e.g. BaSO4). A further possibility can be seen in the installation of metal threads (e.g. stainless steel) into the stent. These metal threads do not serve stabilization purposes (but localization purposes); it is their only object to increase the x-ray contrast. A third possibility is seen in the screening with metals, which besides their high x-ray contrast also have virostatic, fungicidal or bactericidal properties (e.g. nano silver). A further alternative in this respect is the installation of x-ray opaque chromophores such as triiodine benzene derivatives into the SMP-materials themselves.
In a further embodiment, the SMP may be compounded with inorganic, biodegradable nano-particles. Examples are particles made of magnesium or magnesium alloys or magnetite. Particles made of carbon are also suitable. SMP functionalized in this way may be heated in an oscillating electrical field to active the shape memory effect.
The stent according to the invention may also be charged with a number of therapeutically effective substances, which support the healing process, which suppress the restenosis of the stent or which also prevent subsequent diseases. The following may especially be used:
The stent according to the invention can be charged with active substances in different ways.
The active substances can either be directly screened with the plastics or they may be attached onto the stent as a coating.
Stents of this kind may also be used in the field of genetic therapy.
If the active substances are introduced into the hydrophilic coating, these active substances are released as long as the stent enables a diffusion-controlled release. It must be taken care that the diffusion speed of the active substances from the hydrophilic coating must be higher than the degradation speed of the material of the stent.
If the active substances are introduced into the material of the stent according to the invention, the release of the active substances takes place during degradation, possibly after the stent is grown over by endothel cells and is in contact with the soft tissue. The release of the active substance involves the degradation of the stent; thus, it must be taken care that the diffusion speed of the active substance from the stent must be lower than tzhe degradation speed of the material of the stent.
For vascular stents, the following applies:
If the active substances are introduced into the hydrophilic coating, these active substances are released as long as the stent is in contact with flowing bood. It must be taken care that the diffusion speed of the active substances from the hydrophilic coating must be higher than the degradation speed of the material of the stent.
The following applications are especially possible:
Iliac Stents
These stents have a length of 10 to 120 mm, usually 40 to 60 mm. They are used in the abdominal area. Usually, two stents are used, since the use of long stents is difficult. The stents of the present invention are, however, characterized by a favorable flexibility and enable a very gentle minimal invasive application and removal, so that the stents of the present invention can also be used on lengths that are considered not to be feasible in the prior art.
Renal Stents
In this case a high radial strength is required, due to high elastic load in the kidney artery, which possibly requires an increased mechanical reinforcement of the stent. In this case the “slotted tube design” is suitable. This embodiment allows the use of radio-opaqwue markers. In this case it is important to ensure a safe installation of the stent on the balloon of the catheter and a precision during insertion. Due to the different anatomy of all creatures, adapted, variable lengths and diameters are required. Furthermore, the combination with a distal protective device and a plaque filter is advisable.
Carotid Artery Stents
High radial strength due to high elastic load in the blood vessel, which possibly requires an increased mechanical reinforcement. In this case the “slotted tube design” is rather suitable, particularly the use of two long stents is conceivable.
Coronal Stents
The essential fields of application are the entire gastrointestinal tract, trachea and esophagus, bile duct, ureter, urethra and oviduct. Accordingly, stents in various sizes are used. The different pH values of the body liquids and the occurrence of germs must individually be taken into consideration in the stent design.
Independent of the location of use, non-vascular stents are substantially used for the drainage of body liquids such as bile juice, pancreas juice or urine. Thus, the design of a perforated hose is advisable, which on the one hand may safely discharge the liquid to be discharged from the cavity, but which on the other hand absorbs the liquid across the entire way. Furthermore, the polymer material used must have a high flexibility to ensure wearing comfort. For a better identification in x-ray examinations, the starting material may be screened by x-ray contrast substances such as barium sulfate, or x-ray opaque chromphores are integrated into the SMP materials, e.g. by polymerization of suitable monomers. If stents are to be used in fields in which germs occur, the integration of antibiotic active substances into the material might be sensible.
The encrustation of the stents frequently occurring particularly in the uretheral area can be reduced by suitable coating or surface modification.
Fixing of the stent substantially depends on the location of use. In the case of a uretheral stent, the proximal end is located in the renal pelvis, the distal end is located in the urinary bladder or also outside of the body. The proximal end forms a loop after termination of the expansion in the renal pelvis and therefore ensures a safe hold.
Another possibility of fixing the stent is that the stent is tightly pressed to the surrounding tissue via radial forces towards the outside, or that it contains anchoring elements serving for fixing.
In the case of bile or kidney stents, an atraumatic placing and removal is an indispensable condition. It must particularly be ensured during placing that the tissue is not injured by abrasive effects thus causing inflammations. A stent used in this area does not have any retaining elements that could injure the tissue.
Suitable materials that are for instance suitable of being used in the present invention will now be stated as an example:
Examples for Multiblockcopolymers
The multiblockcopolymer was manufactured from macrodiols on the basis of pentadecalacton (PDL) and ε-caprolacton (PCL) and a diisocyanate. PDL defines the portion of pentadecalacton in the multiblockcopolymer (without consideration of the diisocyanate bridges) as well as the molecular weight of the polypentadecalacton segments. PCL defines the respective data for caprolacton units.
The mechanical properties depending on the temperature for example 8 are as follows:
Examples for Polymer Networks
Suitable polymer networks are obtained by copolymerisation of a macrodimethacrylate, on the basis of glycolide units and ε-caprolacton units with n-butylacrylate. The weight proportion of glycolide in the macrodimethylacrylate is 9 percent by weight (or 11 percent by weight in example 13). The molecular weights of the macrodimethacrylates are approximately 10000 to 11000 g/mol.
Examples for Amorphous Polymer Networks
The amorphous networks were manufactured from ABA triblockdimethacrylates, wherein A stands for segments of poly(rac-lactide) and B stands for segments of atactic poly(propyleneoxide) (Mn=4000 g/mol).
PD = Polydispersity
* Sample polymerized in the DSC-measurement
**values above 100 are to be ascribed to impurities
The polymer amorphous networks were examined in view of their further thermal and mechanical properties. The results of these examinations are combined in the following tables.
*determined by DMTA;
n.d.—not detectable
*thermal transition at Tg2
Examples for Photosensitive Networks
10 mmol n-butylacrylate (BA), a cinnamic acid ester (0.1-3 mmol) and possibly 2 mmol hydroxyethylmethacrylate (HEMA) are mixed in a flask. 1 mol % AiBN and 0.3 mol % poly(propyleneglycol)dimethacrylate (Mn=560) are added to the mixture. The mixture is filled by means of a syringe into a mould of two silylated object carriers, between which a Teflon seal ring of a thickness of 0.5 mm is located. The polymerisation of the mixture takes place for 18 hours at 80° C.
The mould in which the cross linking takes place corresponds to the permanent mould. The mixture can also be cross linked in any other shapes.
After polymerization the network is removed from the mould and is covered by 150 mL hexane fraction. Subsequently, chloroform is gradually added. This solvent mixture is exchanged several times within 24 hours to solve out low-molecular and non cross linked components. Subsequently, the network is cleaned by means of hexane fraction and is dried over night in a vacuum at 30° C. The weight of the extracted sample relative to the preceding weight corresponds to the gel content. The two following tables show the quantities of the monomers used as well as the moisture expansion in chloroform and the gel content G thereof.
BA = butylacrylate;
cinnamic acid ester: CA = cinnamic acid;
HEMA = hydroxyethylmethacrylate;
HEA = hydroxyethylacrylate;
HPMA = hydroxypropylmethacrylate;
HPA = hydroxypropylacrylate;
PEGMA = poly(ethyleneglycol)methacrylate
In a further series, a portion of 2 mmol hydroxyethylmethacrylate (HEMA) is additionally added to the binary polymer systems, since by this comonomer a further possibility of controlling the mechanical properties of the polymer networks can be expected.
Manufacture of the Inter-Penetrated Networks IPN
n-butylacrylate is cross linked with 3 percent by weight (0.6 mol %) poly(propyleneglycol)dimethacrylate (molecular weight 560 g/mol) in the presence of 0.1 percent by weight of AiBN, as described above. Subsequently, the film is welled in THF to solve out unused monomer, and is then dried again. Then the firm is welled in a solution of the star-shaped photo-reative macromonomer in THF (10 percent by weight) and is subsequently dried again. The charging of the network with the photo-reactive component is then approx. 30 percent by weight.
Manufacture of the Star-Shaped Photosensitive Macromonomers
Star-shaped poly(ethyleneglycol) with 4 arms (molecular weight 2000 g/mol) is solved in dry THF and triethylamine. For this purpose cinnamyliden acetylchloride slowly solved in dry THF is dripped. The reaction mixture is stirred for 12 hours at room temperature, then it is stirred for three days at 50° C. Fallen out salts are filtered off, the filtrate is concentrated and the product obtained is washed with diethylether. H-NMR measurements resulted in a conversion of 85%. From the UV-spectroscopic point of view, the macromonomer has an absorption maximum at 310 nm before photoreaction, after photoreaction it has an absorption maximum at 254 nm.
The polymer amorphous networks were examined in view of their further thermal and mechanical properties. The results of these examinations are combined in the following table.
*network of n-butylacrylate; 0.3 mol % cross linking agent; without photo-reactive component
**IPN; 0.6 mol % cross linking agent, physically charged with photo-reactive component
The shape memory properties were determined in cyclical photo-mechanical experiments. For this purpose, punched-out, barbell-shaped sheet pieces having a thickness of 0.5 mm and a length of 10 mm and a width of 3 mm were used.
Examples for shape memory polymers with two shapes in the memory are described in U.S. Pat. No. 6,388,043, which is comprised by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 103 26 782.4 | Jun 2003 | DE | national |
| 103 26 778.6 | Jun 2003 | DE | national |
| 103 57 747.5 | Dec 2003 | DE | national |
| 103 57 744.0 | Dec 2003 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP04/06261 | 6/9/2004 | WO | 6/19/2006 |
