U.S. patent application number 10/444827 was filed with the patent office on 2004-05-13 for method of coating a stent with a polysaccharide layer and associated stents.
This patent application is currently assigned to Biotronik Mess- und Therapiegeraete GmbH & Co Ingenieurbuero Berlin. Invention is credited to Bayer, Gerd, Borck, Alexander, Nagel, Markus.
Application Number | 20040091605 10/444827 |
Document ID | / |
Family ID | 29285692 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040091605 |
Kind Code |
A1 |
Bayer, Gerd ; et
al. |
May 13, 2004 |
Method of coating a stent with a polysaccharide layer and
associated stents
Abstract
The invention concerns methods of coating stents and stents
produced in accordance therewith. The object of the invention is to
provide methods of coating stents with a polysaccharide layer which
has improved adhesion capacity on the substrate surface of the
implant, and to afford correspondingly functionalized stents. That
is achieved inter alia by covalent bonding of a non-crosslinked
hyaluronic acid to a substrate surface of the stent with the
formation of hyaluronic acid layer and crosslinking of the
hyaluronic acid layer.
Inventors: |
Bayer, Gerd; (Erlangen,
DE) ; Nagel, Markus; (Forchheim, DE) ; Borck,
Alexander; (Aurachtal, DE) |
Correspondence
Address: |
HAHN LOESER & PARKS, LLP
TWIN OAKS ESTATE
1225 W. MARKET STREET
AKRON
OH
44313
US
|
Assignee: |
Biotronik Mess- und Therapiegeraete
GmbH & Co Ingenieurbuero Berlin
|
Family ID: |
29285692 |
Appl. No.: |
10/444827 |
Filed: |
May 23, 2003 |
Current U.S.
Class: |
427/2.27 ;
623/1.46 |
Current CPC
Class: |
A61L 31/10 20130101;
Y10S 623/90 20130101; A61L 31/10 20130101; C08L 5/08 20130101; Y10S
623/901 20130101; A61L 2420/02 20130101; Y10T 428/31536 20150401;
A61F 2/82 20130101 |
Class at
Publication: |
427/002.27 ;
623/001.46 |
International
Class: |
A61L 002/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 24, 2002 |
DE |
102 23 310.1 |
Claims
What is claimed is:
1. A method of coating a surface of a stent with a polysaccharide
layer, the method comprising the steps of: providing a stent with a
surface prepared for covalent bonding of a polysaccharide thereto;
and bonding a non-crosslinked polysaccharide covalently to the
prepared surface, thereby providing a polysaccharide surface
layer.
2. The method of claim 1, comprising the further step of: forming a
cross-linked polysaccharide outer layer.
3. The method of claim 2, wherein: the non-crosslinked
polysaccharide is hyaluronic acid; and the crosslinked
polysaccharide outer layer is formed by crosslinking the
polysaccharide surface layer.
4. The method of claim 3, wherein: the step of providing the stent
with a surface prepared for covalent bonding comprises bonding a
bonding agent layer to the substrate surface.
5. The method of claim 2, wherein: the non-crosslinked
polysaccharide is hyaluronic acid; and the crosslinked
polysaccharide outer layer is formed by covalently bonding a layer
of a crosslinked hyaluronic acid to the polysaccharide surface
layer.
6. The method of claim 2, wherein: the non-crosslinked
polysaccharide is hyaluronic acid; and the crosslinked
polysaccharide outer layer is formed by the steps of covalently
bonding a layer of a non-crosslinked hyaluronic acid to the
polysaccharide surface layer and crosslinking the non-crosslinked
hyaluronic acid layer.
7. The method of claim 1, wherein: the step of providing the stent
with a surface prepared for covalent bonding comprises bonding a
bonding agent layer to the substrate surface; and the step of
bonding a non-crosslinked polysaccharide covalently to the prepared
surface is accomplished by the steps of: bonding a non-crosslinked
chitosan covalently to the bonding agent; and bonding a layer of
crosslinked hyaluronic acid covalently to the chitosan.
8. The method of claim 1, wherein: the step of providing the stent
with a surface prepared for covalent bonding comprises bonding a
bonding agent layer to the substrate surface; and the step of
bonding a non-crosslinked polysaccharide covalently to the prepared
surface is accomplished by the steps of: bonding a non-crosslinked
chitosan covalently to the bonding agent; and bonding a layer of
non-crosslinked hyaluronic acid covalently to the chitosan.
9. A stent, comprising a polysaccharide surface layer applied by
the method of claim 1.
Description
[0001] The invention concerns methods of coating stents, in
particular cardiovascular implants, with a polysaccharide layer or
polysaccharide derivative layer, and stents produced in accordance
with such methods.
BACKGROUND OF THE ART
[0002] In regard to the background of the invention it is to be
stressed that polysaccharides are known to be biocompatible.
Typical representatives in this connection are heparin, chitosan,
alginate or hyaluronic acid. The latter have proven on the one hand
to be highly body-compatible while on the other hand coatings of
hyaluronic acid are hydrophilic and consequently the devices
provided therewith can be well implanted.
[0003] Implants coated with polysaccharides in general and
hyaluronic acid in particular and methods of coating them with
hyaluronic acid are known from the state of the art in many
different forms. Thus, U.S. Pat. No 6,042,876, to Deem (Mar. 28,
2000), discloses a guide wire for implantation purposes, which is
coated with such a polysaccharide such as hyaluronic acid or
chondroitin sulfate.
[0004] Della Valle, in U.S. Pat. No. 4,957,744 (Sep. 18, 1990),
teaches the crosslinking of esters of hyaluronic acid which are
used for the most widely varying medical and cosmetic articles, as
well as pharmaceutical compositions. The crosslinked esters result
from the esterification of polyvalent alcohols with two or more
carboxy groups of hyaluronic acid. Such crosslinked esters can be
used in particular in the field of bioresorbable plastic materials
for medical and surgical articles.
[0005] Finally, PCT publication WO 8802623 A1, by Guire at
Bio-Metric Systems, Inc., relates to biomaterials with a
biocompatible surface, wherein among a large number of starting
materials and binding mechanisms there is disclosed inter alia the
use of hyaluronic acid for the production of a biocompatible
contact lens. This publication is related to U.S. Pat. Nos.
4,979,959 and 5,263,992.
[0006] Insofar as the above-mentioned publications concern coating
methods for medical equipment and in particular stents, they suffer
from the disadvantage that the polysaccharide layers produced do
not achieve adequate levels of adhesive strength on the substrate
surface.
[0007] Accordingly the object of the present invention is to
provide a method of coating stents with a polysaccharide layer
which enjoys improved adhesion on the substrate surface of the
implant, and to afford correspondingly fuctionalized stents.
SUMMARY OF THE INVENTION
[0008] That object is attained by the alternative methods having
the features of the appended claims as well as the associated
stent. Specifically, the object is attained by the following
characterizing method steps:
[0009] covalent bonding of a non-crosslinked hyaluronic acid to the
substrate surface of the stent forming the polysaccharide layer,
and
[0010] crosslinking of the hyaluronic acid layer (variant I).
[0011] In an alternative configuration of the method of the
invention, instead of crosslinking of the applied non-crosslinked
hyaluronic acid layer, a further layer of a crosslinked hyaluronic
acid is applied to the first non-crosslinked hyaluronic acid layer
(variant II).
[0012] In accordance with a third variant (III) according to the
invention the method is carried out as follows:
[0013] covalent bonding of a non-crosslinked hyaluronic acid to a
substrate surface of the stent forming a first hyaluronic acid
layer,
[0014] covalent bonding of a second non-crosslinked layer of
hyaluronic acid, and
[0015] crosslinking of the second hyaluronic acid layer.
[0016] A fourth variant (IV) provides that the method steps are to
be carried out as follows:
[0017] bonding of a bonding agent layer to a substrate surface of
the stent,
[0018] covalent bonding of a non-crosslinked hyaluronic acid to the
bonding agent layer forming a hyaluronic acid layer, and
[0019] crosslinking of the hyaluronic acid layer.
[0020] Finally in accordance with a fifth variant (V) the coating
operation is effected in the following manner:
[0021] bonding a bonding agent layer to a substrate surface of the
stent,
[0022] covalent bonding of a non-crosslinked first layer of
chitosan to the bonding agent layer forming a chitosan layer,
and
[0023] applying a second layer of crosslinked or non-crosslinked
hyaluronic acid.
[0024] Basic variants I and V of the methods according to the
invention, by virtue of covalent bonding of the non-crosslinked
polysaccharide, provide for a significant increase in the adhesive
capability of the polysaccharide layer, which can be demonstrated
by experiment. In that respect the further layer can be applied in
the form of a non-crosslinked polysaccharide and can then be
crosslinked or it can be applied directly as a crosslinked
polysaccharide.
[0025] Further advantages in particular of variant II lie in the
primary application of a uniform polysaccharide layer and coupling
thereto of a secondary, preferably thicker layer which, in contrast
to other hydrogel films of comparable thickness, has a low swelling
capacity. By virtue of their physical and chemical properties
polysaccharide layers such as hydrogel films or polymer matrices
are suitable for embedding active substances in order to enhance
the biocompatible action by means of local active substance
liberation or locally to achieve a pharmacological action. In
comparison with conventional hydrogel films or polymer matrices,
polysaccharide layers of glycosaminoglycans, in particular,
hyaluronic acid, additionally have their own pharmacological
action.
[0026] An especially suitable polysaccharide for use in the method
according to the invention is hyaluronic acid, which has already
been referred to above, and which can be applied to the most widely
varying substrate surfaces of implants. Alloplastic vessel wall
supports--referred to as "stents"--are usually coated with
amorphous silicon carbide (a-SiC:H) which involves a particularly
intimate and adhesively strong bond to hyaluronic acid.
[0027] Finally, functional coating can be achieved by the alternate
application of a respective plurality of layers of non-crosslinked
and crosslinked polysaccharides.
[0028] Further features, details and advantages of the invention
will be apparent from the embodiments hereinafter.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0029] The method according to the invention is described by
reference to the coating of a substrate surface of amorphous
silicon carbide which is applied for example to a stent with a
basic structure consisting of a tantalum alloy. The essential
features of activation of the silicon carbide substrate surface can
be found in that respect from the present applicants' German patent
application DE 195 33 682 A1, which discloses the application and
immobilization of heparin on a silicon carbide coating.
[0030] I. Bonding of Polysaccharides
EXAMPLE 1
Bonding by Way of a Benzophenone Derivative
[0031] In accordance therewith the substrate surface was flushed
with water and incubated in a 20.times.10.sup.-6-molar
Fmoc-p-Bz-Phe-OH-soluti- on in N,N'-dimethylformamide (DMF). The
Fmoc-p-Bz-Phe-OH-solution, which is effective as a photoactive
spacer substance, can be obtained as a commercial product
"Fmoc-p-Bz-Phe-OH", product number B 2220 from Bachem Biochemica
GmbH, of Heidelberg, Germany. Reduction of the benzophenone was
initiated by irradiation with UV light. After the UV irradiation
operation, the reaction solution was poured off and the substrate
surface rinsed a plurality of times with distilled water.
[0032] The next step involves cleavage of the Fmoc protective group
with 25% piperidine solution in DMF. Bonding of the hyaluronic acid
takes place at the amino group which is now exposed and reactive.
For that purpose, non-crosslinked hyaluronic acid was firstly
covalently bound to the substrate surface treated in that way. The
polysaccharide layer formed in that fashion can then be
crosslinked.
[0033] As an alternative to the above-described photochemical
reaction, it is possible for polysaccharides, and, in particular,
hyaluronic acid, to be covalently bound in a wet-chemical process
to silanized benzophenones, epoxysilanes and aminosilanes as spacer
substances to the substrate surface, in particular to the silicon
carbide substrate surface.
EXAMPLE 2
Bonding by Way of Silanized Benzophenone Derivative
[0034] Wet-chemical covalent bonding of a silanized benzophenone,
in particular 4-(3'-chlorodimethylsilyl)propyloxybenzophenone, was
effected by a wet-chemical procedure in an organic solvent such as
toluene at ambient temperature overnight in the presence of
Et.sub.3N as a catalyst. After the incubation time, the substrates
were rinsed in chloroform and then in methanol. Thereafter the
layer system of substrate spacer was wetted with a 0.1% -2% aqueous
hyaluronic acid solution and then dried. Covalent bonding of the
hyaluronic acid to the benzophenone present was effected under the
action of UV radiation at a wavelength of 340 nm which initiates
the reduction of the benzophenone. Alternatively the photochemical
reaction can also be implemented in aqueous hyaluronic acid
solution. That photochemical reaction resulted in covalent bonding
between the benzophenone and a C--H-group of the polymer chain, in
particular of the hyaluronic acid. That polysaccharide layer, which
was covalently bound to the substrate surface, was then
crosslinked.
EXAMPLE 3
Bonding by Way of Epoxysilanes
[0035] For wet-chemical coating of silicon carbide substrates with
epoxysilanes, the substrates were firstly cleaned and then dried
for an hour at a temperature of 75.degree. C. Silanization of the
warm substrate was effected with
(3-(2,3-epoxypropoxy)-propyl)-trimethoxysilane with immersion in
organic solvent. The silanized substrates were then dried and
washed in the organic solvent. Subsequent covalent bonding of the
hyaluronic acid was effected in an aqueous solvent overnight with
agitation at ambient temperature. Alternatively bonding of
hyaluronic acid can be effected by incubation in a 0.25% hyaluronic
acid solution in a 0.1 m HCl at 65.degree. C. for 1 h. Chitosan can
be bound by incubation in a 0.2% chitosan solution in a 1-2% acetic
acid solution at 65.degree. C. for 1 h. The stents were then rinsed
with deionized water and thereafter dried. The polysaccharide
layer, which is covalently bound to the substrate surface, was then
crosslinked.
EXAMPLE 4
Bonding of Chitosan
[0036] A good adhesive effect can be achieved by a covalently bound
chitosan layer (monolayer). The glycosaminoglycan chitosan (Mw:
100,000 to 1,000,000 Daltons) was covalently bonded by means of a
spacer in a chemical multi-stage reaction to an amorphous silicon
carbide layer (a-SiC:H) which covers the basic body of the
stent.
[0037] In the first step of the coating process the spacer--the
photoactive benzophenone component Fmoc-p-Bz-Phe-OH
(N-(9-Fmoc)-l-(4-benzoyl)-phenylalanine; 2 ml; 10 mmol/l; available
from Bachem)- was covalently bonded to the silicon carbide by
photochemical reaction in the solvent N,N'-dimethylformamide (DMF).
After rinsing with DMF, cleavage was effected in respect of the
Fmoc protective group of the spacer with a 20% piperidine solution
in DMF. The amino group of the spacer was then free.
[0038] The stent was then incubated in a 0.2% solution of chitosan
in 1% acetic acid and
N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (50
mg/ml) for at least 12 hours in ice-cold water. Covalent coupling
of the chitosan was effected by linkage of a peptide bond between
the activated carboxylic acid of the spacer and the amino group of
the chitosan or the formation of an ester bond between the
activated carboxylic acid of the spacer and the hydroxyl group of
the chitosan. After the end of the reaction the sample was
repeatedly rinsed with deionized water and then dried.
EXAMPLE 5
Bonding to Plasma-Deposited Polymer
[0039] An example of use for the application of a bonding agent
layer is described in greater detail hereinafter. A polymer layer
which is a few nanometers thick was applied to the cleaned
substrate surface. The polymer layer served as a bonding agent and
had functional groups at the surface, which are suitable for
subsequent covalent bonding of a polysaccharide layer. Such a
bonding agent layer can be produced by plasma polymerization of
N-heptylamine and acetaldehyde.
[0040] The plasma polymerization operation was effected with a 40
kHz plasma installation of Piko type from Diener electronics. As an
alternative to the above-mentioned precursor it is possible to use
acetaldehyde, amyl alcohol, allylamine, acetoacetic acid ester or
acrylic acid. The plasma-polymerized layers exhibited good wetting
with hyaluronic acid solutions by virtue of their hydrophilic
nature. In addition a thin layer of hyaluronic acid or other
polysaccharides can be coupled to the functional surfaces by means
of glutaraldehyde, epichlorhydrin or carbodiimides.
[0041] The container was flushed with oxygen for the definitive
removal of residual gas, in which case it is continuously
evacuated. A flushing gas flow of about 40 cm.sup.3/min was set.
The sample space was flushed for 10 minutes and the plasma was
ignited in the presence of a Teflon block for the surface
activation procedure. The power of the reactor was about 200 W and
an oxygen flow of about 40 cm.sup.3/min was maintained during the
surface treatment. Activation and simultaneous plasma purification
lasted for 5-10 min.
[0042] After activation and cleaning were effected, the power was
reduced to 80 W and the precursor is introduced into the container.
The polymerization period was 5 min with the aerometer open. After
it was been switched off, further surface activation was effected
with oxygen, but then the power was only 80 W with a duration of
about 30 sec. This short surface activation operation resulted in a
still further improvement in wettability of the surfaces.
[0043] Taking the deposited bonding agent layer comprising the
N-heptylamine plasma polymer, the hyaluronic acid was then
covalently bonded by means of a water-soluble carbodiimide to the
substrate-bonding agent layer complex. Covalent bonding of the
hyaluronic acid to the acetaldehyde plasma polymer was effected
directly by means of a diimidazole or with bonding of a
polyethylene imine intermediate layer which is applied by means of
reductive amination. Covalent bonding of the hyaluronic acid to
that substrate-bonding agent complex was effected by means of a
water-soluble carbodiimide. The polysaccharide layer which is
covalently bonded to the bonding agent layer was then
crosslinked.
EXAMPLE 6
Bonding by Way of Derivatized Polyhydroxybutyric Acid
[0044] As an alternative to the method of plasma polymerization,
the substrate surface was functionalized by derivatized
polyhydroxybutyric acid, which exhibits an experimentally
demonstrated good layer adhesion to silicon carbide and metals.
Functionalization of the polyhydroxybutyric acid was effected by
amination. Covalent bonding of the hyaluronic acid to the amino
group of the functionalized polyhydroxybutyric acid (bonding agent
layer) was effected by means of a water-soluble carbodiimide, with
the formation of a peptide bond.
EXAMPLE 7
Bonding of Chitosan by Way of an Aminosilane
[0045] A monolayer of chitosan was produced in the following
manner. The pre-cleaned stent was dried at 75.degree. C. for 30
minutes in a drying oven. Then, the stent, while still warm, was
incubated for 10 minutes in a silane solution of 3 ml of water-free
toluene, 20 .mu.l of
3-[2-(2-aminoethylamino)-ethylamino]-propyl-trimethoxysilane and 70
.mu.l of Et.sub.3N at ambient temperature with repeated slight
agitation. The stents were then dried at 75.degree. C. for 1 hour.
Thereafter the stent was rinsed with toluene or chloroform and
dried again. The next step was covalent coupling of adipinic acid
by way of a solution of 10 mg/ml of adipinic acid in water for the
production of functional carbonyl functions, to the surface of the
implant. The adipinic acid had been previously activated in THF or
DMF with a carbodiimide or diimidazole. After rinsing in deionized
water and drying, the operation of bonding chitosan took place. For
that purpose the implant was incubated in a 0.2% solution of
chitosan in a 1-2% acetic acid solution at ambient temperature for
1-4 hours. That was followed by rinsing with deionized water and
drying.
[0046] II. Crosslinking and Coating Methods
[0047] Crosslinking and coating methods of hyaluronic acid on
implant surfaces will now be described in greater detail. The
described methods are suitable in this respect for:
[0048] crosslinking a non-crosslinked polysaccharide layer which is
covalently bonded to the substrate,
[0049] covalently bonding a non-crosslinked polysaccharide to a
crosslinked or non-crosslinked polysaccharide layer, or
[0050] covalently bonding a crosslinked polysaccharide to a
crosslinked or non-crosslinked polysaccharide layer.
EXAMPLE 8
Crosslinking with Glutaraldehyde
[0051] The crosslinking of hyaluronic acid with glutaraldehyde can
be implemented. The implant was coated with a 0.1-2% hyaluronic
acid solution and then subjected to the action of a crosslinker
solution for several hours. The crosslinker solution comprised 240
ml of acetone, 80 ml of glutaraldehyde in 25% solution in water and
1.6 ml of HCl 3 M. Thereafter the crosslinker solution was replaced
by a fresh solution and incubation was again effected at ambient
temperature for several hours. The hyaluronic acid crosslinked by
means of glutaraldehyde was washed several times in distilled
water. The sample was incubated in a 0.5-3% solution of sodium
cyanoborohydride for one hour at ambient temperature. The fixer
solution was removed and the procedure then involveed a plurality
of washing steps in doubly distilled water and isotonic saline
solution.
[0052] Crosslinking of the hyaluronic acid with bifunctional
aldehydes and formaldehyde was effected in a method similarly to
crosslinking of the hyaluronic acid with glutaraldehyde.
EXAMPLE 9
Crosslinking with Epichlorhydrin
[0053] 0.38 g of hyaluronic acid was dissolved in 90 ml of water
for the crosslinking of hyaluronic acid by epichlorhydrin. 10 g of
NaOH and 6.8 ml of aqueous ammonia solution (25%) were added to the
solution. The temperature of the reaction solution was set at
20.degree. C. After that temperature was reached, 19.6 ml of
epichlorhydrin was added thereto. The solution was stirred at
20.degree. C. for 24 hours. The crosslinked hyaluronic acid was
then dialyzed in relation to doubly distilled water. The dialysis
hoses used have an exclusion limit of 120,000 DA.
EXAMPLE 10
Crosslinking with Divinyl Sulfone
[0054] For the crosslinking of hyaluronic acid by divinyl sulfone,
2 g of hyaluronic acid was dissolved in 50 ml of 0.1 m aqueous NaOH
solution, giving a 2% solution. The solution was put on ice. When
temperature equalization was effected, 2 ml of divinyl sulfone was
added. The resulting two-phase mixture was agitated for 15 minutes
on ice. After 5 minutes, only one phase was still to be observed.
The implants were immersed in that solution and then dried.
EXAMPLE 11
Crosslinking with Ethylene Glycol Diglycidylether
[0055] For the crosslinking of hyaluronic acid with ethylene glycol
diglycidylether, a 0.1-2% hyaluronic acid solution in a 0.9%
isotonic saline solution was produced. The reaction was conducted
at 25.degree. C. As the crosslinking agent, up to 10 molar percent
of ethylene glycol diglycidylether was added, with respect to the
repetition unit of the hyaluronic acid.
EXAMPLE 12
Crosslinking with Diimidazole
[0056] It is also possible to implement crosslinking and covalent
bonding of the hyaluronic acid to layer systems comprising
amorphous silicon carbide spacer and an amorphous silicon
carbide-spacer-polysaccharide monolayer with diimidazole. The
implant with bound spacer or with a polysaccharide layer was
immersed in a diimidazole-bearing acetone solution. The
substrate-spacer complex or the polysaccharide layer was activated
for at least 30 minutes in the diimidazole-bearing acetone solution
and then immersed in an aqueous hyaluronic acid solution or sprayed
with a hyaluronic acid solution. For the spray coating operation,
the stent was sprayed for 0.5-1 sec at a pressure in respect of the
carrier air of 2-4 bars. Between the spraying steps, the stent was
dried for 15-30 sec with a supply of warm air. Repeating the steps
makes it possible to produce a desired layer structure on the
stent. In order to achieve layer growth, that process was repeated
a plurality of times.
EXAMPLE 13
Crosslinking with Acid Dichlorides or Phosphorus Oxychloride
[0057] The crosslinking of the OH- and NHR-groups of
polysaccharides was effected by means of acid dichlorides or
phosphorus oxychloride with the formation of ester or amide groups
and with the liberation of HCl in an organic solvent.
[0058] III. Derivatization of the Polysaccharide Layer
[0059] Derivatization of the coated hyaluronic acid hydrogel on the
implant can also be implemented.
EXAMPLE 14
Sulfatization
[0060] By virtue of polymer-analogous transformation of hyaluronic
acid, for example, by means of an SO.sub.3*pyridine complex,
enzymatic decomposition of hyaluronic acid in vivo was delayed or
hyaluronic acid was stabilized in the body, as the following
example of use shows.
[0061] Hyaluronic acid was suspended in dry pyridine in a nitrogen
atmosphere in a thermostatizable double-wall reactor with a reflux
condenser and an agitator. A sulfur trioxide-pyridine-complex was
added to that suspension and heated to the desired reaction
temperature. After 3 hours, the reaction was terminated and the
suspension, when cooled to ambient temperature, was poured into
five times the amount of methanol. The precipitated polymer was
filtered off, dissolved in water and dialyzed in relation to
de-ionized water. As the product was partly in the form of
pyridinium salt and the polymer chains were intermolecularly
esterified with each other, the pH-value of the polymer solution
after dialysis was adjusted to 11 by the addition of 0.1 N soda
lye. Dialysis and titration were repeated three times. At a
pH-value of 7.3 the polymer was freeze-dried.
[0062] A variation in the degree of sulfatization in this
polymer-analogous transformation procedure is possible by virtue of
the amount of added sulfatization reagent SO.sub.3* pyridine, the
reaction time and the reaction temperature.
[0063] IV. Embedding Drugs
[0064] Active substance loading with suitable drugs was generally
effected after crosslinking and fixing of the polysaccharide layer
in the swollen condition. Alternatively the active substance can be
furnished by means of a spraying or immersion process prior to the
coating step or during coating with the polysaccharide. Active
substance embedding was generally effected by way of diffusion
processes.
EXAMPLE 15
Embedding Cyclosporin
[0065] The active substance was embedded by way of an immersion
process in a hyaluronic acid layer as can be obtained in accordance
with one of the preceding examples. For that purpose, the implant
was immersed in a solution of 15 mg of cyclosporin per ml of a
paritetic ethanol-water mixture. The ratio of 1:1 of ethanol to
water has proven to be surprisingly effective for implementation of
the diffusion process. Other ratios, especially those with an
elevated ethanol content, slow down the embedding effect. Depending
on the respective layer thickness and degree of crosslinking of the
polysaccharide layer the implant remained in the solution for at
least one hour. The implant was then removed and dried. With a
coating amount of 0.5 mg of hyaluronic acid, the amount of
cyclosporin which can be incorporated in that way was at least 0.2
mg.
* * * * *