EP1699383A1

EP1699383A1 - Control of the degradation of biodegradable implants using a coating

Info

Publication number: EP1699383A1
Application number: EP04765010A
Authority: EP
Inventors: Marc Kuttler; Claus Harder; Carsten Momma; Heinz Müller; Daniel Lootz
Original assignee: Biotronik VI Patent AG
Current assignee: Biotronik VI Patent AG
Priority date: 2003-12-24
Filing date: 2004-09-07
Publication date: 2006-09-13
Also published as: DE10361940A1; JP4861827B2; WO2005065576A1; JP2007518473A; US20090208555A1

Abstract

The invention relates to an endovascular implant, which is at least largely biodegradable and whose in vivo degradation can be controlled. To achieve this, the implant comprises a tubular base body, open on its end faces and consisting of at least one biodegradable material, said base body having an in vivo, location-dependent first degradation characteristic D1(x), in addition to a coating that covers the base body completely or in sections and consists of a biodegradable material, said coating having an in vivo, location-dependent second degradation characteristic D2(x). According to the invention, a location-dependent cumulative degradation characteristic D(x) in one location (x) is made up of the sum of the respective degradation characteristics D1(x) and D2(x) in said location (x) and the location-dependent cumulative degradation characteristic D(x) is predetermined by a variation of the second degradation characteristic D2(x) in such a way that the degradation in the given location (x) of the implant takes place over a predeterminable time period at a predeterminable degradation rate.

Description

Degradation control of biodegradable implants by coating

The invention relates to an at least largely biodegradable endovascular implant whose in vivo degradation can be controlled.

In medical technology, the implantation of endovascular support systems has become one of the most promising therapeutic measures for the treatment of vascular diseases. For example, in interventional therapy for stable angina pectoris in coronary artery disease, the introduction of stents has led to a significant reduction in the rate of restenosis and thus to better long-term results. The reason for the benefit of stent implantation is the higher primary lumen gain. The use of stents can achieve an optimal vascular cross-section that is essential for the success of the therapy, but the permanent presence of such a foreign body initiates a cascade of microbiological processes that can lead to a gradual growth of the stent. A starting point for The solution to the problem is therefore to manufacture the stent from a biodegradable material.

A wide variety of materials are available to the medical technician for realizing such biodegradable implants. In addition to numerous polymers, which are often based on natural origin for better biocompatibility or at least based on natural compounds, metallic materials, with their mechanical properties which are significantly more favorable for the implants, have recently been favored. In this context, particular attention is paid to materials containing magnesium, iron and tungsten. One of the problems to be solved in the practical implementation of biodegradable implants is the degradation characteristic of the implant in vivo. On the one hand, this is to ensure that the functionality of the implant is maintained at least for the period of time necessary for the therapeutic purposes. On the other hand, the degrada- tion should run as evenly as possible over the entire implant, so that fragments are not released in an uncontrolled manner, which can be the starting point for undesired complications. Known biodegradable stents do not show a locally coordinated degradation characteristic.

Based on the state of the art, the task is to provide a biodegradable implant whose degradation can be optimized depending on the location.

This object is achieved by the endovascular implant with the features mentioned in claim 1. Because of the implant

a tubular basic body made of at least one biodegradable material on its end faces, the basic body having a location-dependent first degradation characteristic D-ι (x) in vivo, and a coating of at least one biodegradable material that completely or possibly only partially covers the base body, the coating having a location-dependent second degradation characteristic D ₂ (x) in vivo, and

- Where at a location (x) a location-dependent cumulative degradation characteristic D (x) results from the sum of the degradation characteristics D-ι (x) and D ₂ (x) existing at the location (x) in each case and the location-dependent accumulation the degradation characteristic D (x) is predetermined by varying the second degradation characteristic D ₂ (x) in such a way that the degradation takes place at the specified location (x) of the implant in a predeterminable time interval with a predeterminable degradation curve,

the degradation characteristic of the entire stent can be locally optimized in the desired manner.

The invention accordingly includes the idea that the degradation of the base body of the implant is adapted by a suitable coating - in extreme cases, however, also by omitting the coating - so that the degradation characteristic existing at one location degrades the implant in a predefinable time interval and with a predeterminable course of degradation enables.

"Biodegradation" means hydrolytic, enzymatic and other metabolic degradation processes in the living organism, which lead to a gradual dissolution of at least large parts of the implant. The term biocorrosion is often used synonymously. The term bioresorption also includes the subsequent absorption of the degradation products. Materials suitable for the base body can be, for example, polymeric or metallic in nature. The base body can also consist of several materials. A common feature of these materials is their biodegradability. Examples of suitable polymeric compounds are first of all polymers from the group cellulose, collagen, albumin, casein, polysaccharides (PSAC), polylactide (PLA), poly-L-lactide (PLLA), polyglycol (PGA), poly-D, L-lactide -co-glycolide (PDLLA / PGA), polyhydroxybutyric acid (PHB), polyhydroxyvaleric acid (PHV), polyalkylcarbonates, polyorthoesters, polyethylene terephthalate (PET), polymalonic acid (PML), polyanhydrides, polyphosphazenes, polyamino acids and their copolymers and hyaluronic acid. Depending on the desired properties of the coating system, the polymers can be in pure form, in derivatized form, in the form of blends or as copolymers.

Metallic biodegradable materials are based on alloys of magnesium, iron or tungsten. The biodegradable magnesium alloys in particular show extremely favorable degradation behavior, are easy to process and show little or no toxicity, but rather seem to stimulate the healing process positively.

The basic body of a stent is generally composed of a large number of support elements arranged in a specific pattern. Depending on the application - be it, for example, during dilatation or due to the obstruction of the surrounding tissue - the support elements are loaded with different mechanical forces. In the case of biodegradable materials, this can mean, among other things, that the areas of the support elements which are under stress or which are at least temporarily exposed to high mechanical loads are broken down more quickly than areas which are less loaded. Among other things, the present invention allows to counteract this phenomenon. The coating can also be formed from the aforementioned biodegradable materials. Of course, several different materials can also be used in one implant, for example at different locations or as multi-layer systems at a specific location of the implant.

"Location-dependent degradation characteristic" in the sense of the invention means the time course (degradation course) and the time interval in which this degradation takes place. For the sake of simplicity, the first point of reference for the time interval is the time of the implantation itself. Of course, other times can also be used. An end of the time interval is understood in the sense of the invention as the point in time at which at least 80% by weight of the biodegradable implant mass has been broken down or the mechanical integrity of the implant no longer exists, i.e. the implant can no longer perform its supporting function. The degradation curve indicates the speed at which the degradation takes place at specific times in the time interval. For example, by means of appropriate modifications according to the invention, the degradation of the implant is greatly delayed in the first two weeks after the implantation by means of a suitable coating and only progresses rapidly after the coating has been removed due to the faster degradation of the base body. In order to allow the degradation processes to take place in a suitable manner, it is therefore necessary on the one hand to know the degradation characteristics of the base body at the specific location of the implant and on the other hand to influence the overall degradation behavior of the implant at this location by applying a coating with a second degradation characteristic. The degradation characteristics of the base body and the coating can be estimated in advance using in vitro tests.

The degradation characteristic of the coating is preferably determined by - varying its morphological structure, - material modification of the material and / or

- Adjustment of a layer thickness of the coating

reached.

The location-dependent degradation characteristics of the implant can be influenced by adjusting the layer thickness of the coating. Here too, the focus is on controlling the degradation at a specific location in terms of time and scope. Depending on the medical requirements, it may be necessary to maintain the support function of the implant over a certain period of time, possibly also depending on the location. With an increased layer thickness, the removal of the implant at a certain location can be delayed.

“Morphological structures” in the sense of the invention mean the conformation and aggregation of the compounds forming the coating, in particular polymers. This includes the type of molecular order structure, the porosity, the surface quality and other intrinsic properties of the carrier, which influence a degradation behavior of the biodegradable material on which the coating is based. Molecular order structures include amorphous, (partially) crystalline or mesomorphic polymer phases, which can be influenced or generated depending on the manufacturing process, coating process and environmental conditions used. The porosity and surface quality of the coating can be influenced by targeted variation of the manufacturing and coating process. In general, the degradation takes place more quickly with increasing porosity of the coating. Amorphous structures show similar effects to (partially) crystalline structures.

'Material modification' in the sense of the invention includes both a derivatization of the biodegradable material, in particular the polymers, and the addition of fillers and additives (additives) for the purpose of Understanding of the degradation characteristics understood. The derivatization includes, for example, measures such as networking or replacing reactive functionalities in these materials. It is well known, for example, that polymeric materials such as hyaluronic acid are broken down more slowly when increasing the degree of crosslinking. These measures must first be recorded quantitatively by means of established in vitro investigations in order to be able to provide an estimate of the degradation characteristics for the in vivo behavior.

The location-dependent degradation characteristic of the implant is preferably specified as a function of the pathophysiological and / or rheological conditions to be expected in the application. The pathophysiological aspects take into account the fact that the stent is usually placed in the vessel in such a way that it lies in the center of the lesion, ie. H. the adjacent tissue at the ends and in the middle area of the stent is of different nature and therefore the supporting function of the implant needs to be maintained for different times to optimize the healing process. Furthermore, the tissue resistances acting on the implant are unequal due to the pathophysiological change, which can lead to an accelerated degradation due to the resulting mechanical stress in places of greater resistance.

Rheological aspects in turn take into account the fact that the flow conditions, in particular in the area of the ends and in middle sections of the stent, are different. This can lead to accelerated dismantling of the implant at the ends of the stent due to the stronger flow. Rheological parameters can vary widely, particularly by specifying the stent design, and must be determined in individual cases. By taking the two parameters mentioned into account, optimal degradation over the entire dimension of the stent can be ensured for the desired therapy. The invention is explained in more detail below on the basis of exemplary embodiments and in the associated drawing. Show it:

1 shows a stent with a tubular base body which is open on its end faces and the peripheral wall of which is covered with a coating system;

2a, 2b a schematic cross section along a longitudinal axis of a stent to illustrate the coating according to a first variant and

3a, 3b show a schematic cross section along a longitudinal axis of a stent to illustrate the coating according to a second variant.

1 shows a highly schematic perspective side view of a stent 10 with a tubular base body 14 that is open at its ends 12.1, 12.2. A circumferential wall 16 of the base body 14 that extends radially about a longitudinal axis L consists of axially arranged side by side Segments, which in turn are composed of a plurality of support elements arranged in a specific pattern. The individual segments are connected to one another via connecting webs and, in summary, result in the base body 14. In FIG. 1, the depiction of a specific stent design was deliberately omitted, since this is not necessary for the purposes of illustrating the invention and, moreover, an individual adaptation for each stent design a coating to the given geometric factors and other parameters is necessary. A wide variety of stent designs are known from the prior art in a very large variety and are not explained in more detail here. All that remains to be said is that all common stents 10 have a tubular basic structure 14 of whatever kind, which comprises a circumferential peripheral wall 16. In the following, therefore, an outer circumferential surface 18 of the peripheral wall 16 with the possibly made up of a plurality of existing support elements, the outer circumferential surface of these support elements equated.

For example, the stent 10 can be formed from a biodegradable magnesium alloy, in particular WE43. As a result of the transition and its unexpanded state into its expanded state during the dilation of the stent 10 in the body, the individual support elements are subjected to different mechanical loads, in particular at their articulation points. This can lead to the fact that the metallic structure z. B. changed due to micro-cracking. As a rule, a particularly rapid degradation will take place at points where a particularly high mechanical stress occurs. Furthermore, the dimensions of the individual support elements are dimensioned differently depending on the stent design present. It goes without saying that supporting elements with a larger circumference are dismantled more slowly than correspondingly filigree structures in the basic structure. The objective for a satisfactory degradation behavior of the implant should therefore be to counteract a kind of fact formation due to these different degradation characteristics. The location-dependent degradation characteristic of the base body is expressed in the following with the abbreviation D ^ x).

The stent 10 of FIG. 1 shows in a highly schematic manner a coating 26 in which a plurality of sections 20.1, 20.2, 22.1, 22.2, 24 of the outer circumferential surface 18 of the peripheral wall 16 are formed from biodegradable materials which differ in their degradation characteristics D ₂ (x) , A polymer based on hyaluronic acid is given here as an example of a suitable material for the coating 26. Hyaluronic acid not only shows favorable degradation behavior, but is also particularly easy to process and also has positive physiological effects. The degradation characteristic D ₂ (x) can be influenced, for example, in such a way that a certain degree of crosslinking is predetermined by reaction with glutaraldehyde. The higher the degree of crosslinking, the slower the hyaluronic acid will decompose. Numerous processes have been developed for applying a coating to the stent, such as, for example, rotary atomization processes, immersion processes and spray processes. The coating at least in regions covers the wall or the individual struts of the stent that form the support structure.

The degradation characteristic D ₂ (x) differs in the individual sections 20.1, 20.1, 20.2, 22.1, 22.2, 24. Thus, as will be explained in more detail below, it can be provided that the sections 20.1 and 20.2 at the ends 12.1, 12.2 of the stent 10 show an accelerated degradation characteristic D ₂ (x), whereas the sections 22.1 arranged more in the middle , 22.2 and 24 degrade more slowly. This in turn has the consequence that, given the same degradation characteristics D ^ x) of the base body, degradation at the end of the stent 10 proceeds faster. This makes sense insofar as the lesion to be treated should be centered opposite sections 22.1, 22.2 and 24 if the stent 10 is applied correctly. Accordingly, the degeneration characteristics D -] (x) and D ₂ (x) add up to a cumulative location-dependent degeneration characteristic for the implant.

2a, 2b, 3a, 3b, 4 and 5 show - in each case in a highly schematic manner - a section along the longitudinal axis L of the stent 10 and in each case only one of the two sections resulting therefrom through the peripheral wall 16 however, the principles underlying the design of the coating are briefly discussed.

A degradation characteristic D ₂ (x) of a coating at a specific location (x) essentially depends on factors such as

a layer thickness of the coating,

- a morphological structure of the coating and a material modification of the coating.

Increasing the layer thickness of the coating extends the duration of the degradation. There are well-founded theoretical as well as practical model systems that allow an assessment of the later in vivo behavior.

Finally, the local degradation characteristic D ₂ (x) depends on the morphological structure and material modifications of the coating. In particular, the porosity of the coating can be varied, an increased porosity leading to accelerated degradation. For the material modification, it can be provided, for example, to add additives to the carriers which delay the enzymatic degradation. The degradation of coatings based on polysaccharide can also be delayed by increasing the degree of crosslinking.

To summarize, it should therefore be stated that the cumulative degradation characteristic D (x) of the coating 26 can be predetermined by suitable specification of the degradation characteristic D ₂ (x) of the coating 26, provided the degradation characteristic D-ι (x) of the base body is known.

The individual sections of the coating of the stent are also adapted depending on the pathophysiological and theological conditions to be expected in the application.

The pathophysiological conditions here mean the tissue structure changed by disease in the stented vascular area. As a rule, the stent is placed in such a way that the lesion, ie the fibroatheromatous plaque in coronary applications, is approximately in the central area of the stent. In other words, the adjoining tissue structures diverge in the axial direction over the length of the stent, and another therapy may also be indicated locally under certain circumstances. The theological conditions are understood to mean the flow conditions as they occur in the individual longitudinal sections of the stent after implantation of the stent. Experience has shown that there is a greater flow around the ends of the stent than the central regions of the stent. This can result in increased degradation of the carrier in the end regions.

Degradation that is too fast cannot support the healing process. Such an undesirable development can be prevented by specifically specifying the time interval in which mining is to take place at a specific location (x).

Biodegradable materials for the coating can include all polymeric matrices of synthetic nature or of natural origin are used in the sense of the invention, which are degraded in the living organism due to enzymatic or hydrolytic processes. In particular, polymers from the group cellulose, collagen, albumin, casein, polysaccharides (PSAC), polylactide (PLA), poly-L-lactide (PLLA), polyglycol (PGA), poly-D, L-lactide-co -glycolide (PDLLA / PGA), polyhydroxybutyric acid (PHB), polyhydroxyvaleric acid (PHV), polyalkylcarbonates, polyorthoesters, polyethylene terephthalate (PET), polymalonic acid (PML), polyanhydrides, polyphosphazenes, polyamino acids and their copolymers and hyaluronic acid are used , Depending on the desired properties of the coating system, the polymers can be applied in pure form, in derivatized form, in the form of blends or as copolymers.

If desired, pharmacologically active substances, which are used in particular to treat the consequences of percutaneous coronary interventions, can be added to the coating.

2a shows a highly schematic and simplified sectional view of the peripheral wall 16, with its coating 26 applied to the outer lateral surface 18. The coating 26 consists of two end sections 28.1 and 28.2 and a middle section 30. In the present case, the entire coating 26 is formed from a biodegradable material applied in a uniform layer thickness.

Sections 28.1, 28.2, 30 differ in that the final soapy sections 28.1, 28.2 degrade more slowly than the middle section 30. In the present exemplary case, this is used to compensate for logically induced accelerations of the digestion process at the stent ends used, d. H. The schematic stent shown in FIG. 2a will show a largely homogeneous degradation behavior over the entire length of the stent.

FIG. 2b discloses a second variant of the coating 26. The sections 28.1, 28.2 correspond to those in FIG. 2a. The section 30, however, is significantly reduced in its layer thickness. The result of this is that section 30 is broken down much more quickly than sections 28.1 and 28.2. Such degradation behavior of the implant can be useful if the artificial structure in the area of the lesion is to be removed as quickly as possible in order to eliminate any starting point for possible complications in this area as early as possible.

3a shows a coating system 26, in which two different materials with a different degradation behavior are applied to the sections 28.1, 28.2, 30 of the stent 10. The same applies to the variation of the system according to FIG. 3b.

According to the embodiment according to FIG. 3a, sections 28.1, 28.2 are covered by a material with a delayed degradation behavior compared to the material used in the middle section 30. Accordingly, the location-dependent degradation characteristic D (x) is influenced, ie generally delayed at the end. Such a design is always useful if the support structure has a longer end Period should be maintained and the rheological conditions otherwise lead to accelerated degradation.

3b shows in sections 28.1 and 28.2 a multilayer structure of the coating 26 in the radial direction. The material with the delayed degradation behavior is again applied in a first section 32, while a section 34 with the more rapidly degradable material is located radially outward.

The above-mentioned examples of FIGS. 2a, 2b and 3a, 3b, 4 and 5 represent only highly schematic exemplary embodiments of the invention. They can be combined with one another in a variety of ways. For example, it is conceivable to design a complex coating consisting of several materials in individual sections. The primary goal is always to optimize the local degradation of the implant.

Claims

claims

1. Endovascular implant with

a tubular base body, open on its end faces, made of at least one biodegradable material, the base body having a location-dependent first degradation characteristic D-t (x) in vivo, and

a coating of at least one biodegradable material that completely or possibly only partially covers the base body, the coating having a location-dependent second degradation characteristic D ₂ (x) in vivo, and

- Where at a location (x) a location-dependent cumulative degradation characteristic D (x) results from the sum of the degradation characteristics D ^ x) and D ₂ (x) existing at the location (x) and the location-dependent cumulative degradation characteristic D (x) is predetermined by varying the second degradation characteristic D ₂ (x) in such a way that the degradation takes place at said location (x) of the implant in a predefinable time interval with a predeterminable degradation curve.

2. Implant according to claim 1, characterized in that the degradation characteristic D ₂ (x) of the coating

- Variation of its morphological structure, material modification of the material and / or - adjustment of a layer thickness of the coating given is.

3. Implant according to claims 1 or 2, characterized in that the degradation characteristic D ₂ (x) of the coating is predetermined as a function of the pathophysiological conditions to be expected in the application.

4. Implant according to claims 1 or 2, characterized in that the degradation characteristic D ₂ (x) of the coating is predetermined as a function of the rheological conditions to be expected in the application.