Biodegradable stent

A biodegradable, biocompatible, resorbable infusion stent comprising a terpolymer of: PA1 (a) L(-)lactide, PA1 (b) glycolide, and PA1 (c) epsilon-caprolactone. This invention includes a method for treating ureteral obstructions or impairments by utilizing a biodegradable, biocompatible, resorbable infusion stent, and a method for controlling the speed of resorption of the stent.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the preparation of ureteral stents from 
biodegradable polymers of lactic acid. 
2. Description of the Prior Art 
Ureteral stents are often used to maintain fluid drainage from the renal 
pelvis to the bladder when the ureter is obstructed or otherwise impaired, 
and also for providing support to a collapsed or restricted ureter. 
Very often, ureteral stents are positioned in a patient on a temporary 
basis to provide drainage from the kidney to the bladder following 
surgery. The stent is generally coiled or looped at opposite ends to 
prevent upward or downward migration from a predetermined position in the 
ureter caused by peristaltic action or other body motion that would impose 
forces on the stent to move it from its predetermined position. 
Certain ureteral stents have the capability of infusing fluids into the 
kidney and are commonly referred to as "infusion stents". 
In many situations where the ureteral stent is installed for short term 
usage, an additional surgical procedure must be employed to remove the 
stent after its purpose has been fulfilled. 
A ureteral stent that is made of a biodegradable and biocompatible material 
would assure its safe and innocuous disappearance without the need for a 
second surgical procedure for its removal after it has completed its 
function. 
Canadian Patent No. 808,731 to Fouty discloses the preparation of high 
molecular weight polylactides with an anionic coordination catalyst 
containing a divalent metal of Group II of the Periodic Table, to produce 
a polymer containing the divalent metal as part of the polylactide. Either 
optical isomer of lactide may be used, and the lactide can be 
copolymerized with other cyclic esters having from 6 to 8 carbon atoms in 
the ring, such as glycolide or tetramethyl glycolide. 
U.S. Pat. No. 4,045,418 to Sinclair discloses thermally stable copolymers 
of optically inactive lactide and epsilon caprolactone with a tin ester of 
carboxylic acid serving as a catalyst to produce throwaway thermoplastic 
objects that are environmentally attractive because they slowly degrade to 
harmless substances. Cyclic esters such as glycolide, lactide and the 
lactones are also disclosed as being used to produce thermoplastics. U.S. 
Pat. No. 4,057,537 also to Sinclair discloses the copolymerization of 
glycolide with lactide and various lactones to form copolymers which are 
reported as useful in making absorbable sutures. Sinclair's primary 
objective is to produce a non-gummy, high impact, non-brittle, thermally 
stable copolymer of an optically active lactide and epsilon-caprolactone 
which can be fabricated into various thermoplastic objects that are 
disposable and environmentally attractive since they degrade into harmless 
substances. 
U.S. Pat. No. 3,844,987 to Clendinning et al, discloses shaped containers 
fabricated from biodegradable thermoplastic oxyalkanoyl polymers, such as 
epsilon-caprolactone polymers, and naturally occurring biodegradable 
substances to serve as containers in which to germinate and grow seed or 
seedlings. 
U.S. Pat. No. 3,636,956 to Schneider discloses copolymers of L(-)lactide 
with up to 35% glycolide for use in surgical applications such as sutures 
and ligatures. U.S. Pat. No. 3,739,773 to Schmitt et al, discloses 
polyglycolic acid or polyhydroxyacetic ester can be surgically used for a 
solid prosthesis or a protective gauze and is absorbable by living 
mammalian tissue. 
U.S. Pat. No. 3,736,646 to Schmitt discloses a copolymer containing 15 to 
85 mole % of both glycolic and lactic acid can be formed into 
biodegradable surgical structures such as tubes or sheets or spun as 
filaments to prepare sutures. 
U.S. Pat. No. 4,300,565 to Rosensaft et al, discloses a method for 
producing sterile surgical articles from a synthetic absorbable copolymer 
formed by copolymerizing glycolide monomer with a cyclic ester monomer 
other than a glycolide, such as a lactone, oxalate or carbonate. 
U.S. Pat. No. 3,531,561 to Trehu discloses the use of high molecular weight 
polylactides extruded to form a surgical suture. 
U.S. Pat. No. 4,539,981 to Tunc discloses an absorbable bone fixation 
device made from a polymer of L(-)lactide with an inherent viscosity above 
4.5. 
U.S. Pat. No. 4,181,983 to Kulkarni discloses an assimilable, porous, 
hydrophilic prosthesis composed of a polymer of hydroxycarboxylic acid, 
with the preferred polymer being a lactic acid. 
U.S. Pat. No. 4,137,921 to Okuzumi discloses the formation of highly 
crystalline, fiber-forming addition copolymers of lactide and glycolide 
having from 50 to 75% glycolide. The lactide-glycolide addition copolymers 
are highly crystalline and useful in forming fibers for surgical sutures. 
U.S. Pat. No. 3,839,927 to Wasserman et al, discloses the formation of a 
high molecular weight 1-lactide/glycolide copolymer using a stannous 
octoate catalyst. The copolymer may be extruded to form filaments useable 
as absorbable sutures. 
European Patent Application No. 0204931 to Pertti et al, discloses a 
synthetic polymeric surgical osteosynthesis material absorbable by the 
body composed of such polymers as a polylactide. 
Other patents of interest relating to the preparation of polylactides 
include U.S. Pat. Nos. 2,703,316 to Schneider; 2,890,208 to Young et al; 
2,362,511 to Teeters; 3,169,945 to Hostettler et al; 3,284,417 to 
Hostettler et al; 2,758,987 to Salzburg et al and Canadian Patent 779,291 
to Kleine. 
SUMMARY OF THE INVENTION 
The present invention is based upon the discovery of a biodegradable, 
biocompatible, resorbable infusion stent comprising a terpolymer of: 
(a) L(-)lactide, 
(b) glycolide, and 
(c) epsilon-caprolactone. 
This invention is also based upon a method for treating ureteral 
obstruction or impairment by utilizing a biodegradable, biocompatible, 
resorbable infusion stent, and a method for controlling the rate of 
biodegradation of the stent. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the present invention, it has been found that a 
biocompatible, biodegradable, resorbable infusion stent can be made from a 
terpolymer of: 
(a) L(-)lactide, 
(b) glycolide, and 
(c) epsilon-caprolactone. 
The inventive infusion stent has the following properties: 
(i) a minimum tensile strength of at least about 500 psi, preferably at 
least about 650 psi, 
(ii) an elongation greater than about 10%, preferably greater than about 
100%, 
(iii) Shore A hardness of about 50 to 100, preferably about 75 to 95 
In addition, the biodegradable stent is pliable, and can be fabricated or 
extruded into tubing with an inside diameter that can vary from about 
0.050 to about 0.075 inches, and an outside diameter than can vary from 
about 0.075 to about 0.120 inches. 
The inventive stent can be made transparent and is biocompatible. Because 
it is also biodegradable, the stent disintegrates in mammalian body 
tissue, within a few weeks to a few months, without interfering with 
urinary function. 
The inventive stent can be fabricated with a pliable curl set at each end 
by heat setting techniques, can be sterilized, and is capable of being 
compounded with radiopaque materials such as barium sulfate. The stent 
should have a minimum curl strength of at least about 4 grams, and a 
minimum break strength of about 1.7 pounds. The stent can also be 
imprinted with biocompatible inks. 
It has been found that the controlling factor in the stiffness of the 
terpolymer composition used in making the stent is the amount of 
epsilon-caprolactone which can vary between about 15 and about 25% by 
weight of the terpolymer composition. At about 15 weight % or less, the 
terpolymer composition becomes too stiff, and at about 25 weight % or 
higher, the composition becomes too pliable and weak to construct the 
stent. A 20 weight % caprolactone terpolymer is most preferred for its 
pliability characteristics. 
The amounts of L(-)lactide can vary from about 45 to 85 weight %, 
preferably about 55 to 75 weight % and most preferably about 60 to 70 
weight % of the terpolymer composition. 
The amounts of glycolide can vary from about 5 to 50 weight %, and 
preferably about 10 to 30 weight % of the terpolymer composition. The 
blending of those components produces a pliable, transparent, 
thermoplastic elastomer that is biodegradable and biocompatible. 
The mechanism of biodegradation of the inventive stent is essentially one 
of hydrolysis; that is, the destruction, decomposition, or alteration of 
the chemical composition of the stent by water to the point where the 
stent disintegrates and is harmlessly excreted from the body in the urine. 
At the same time, certain portions of the stent which are in contact with 
the body tissues are resorbed into the tissues. For purposes of this 
invention, the terms "biodegradation, biodegradable" and the like are 
intended to also include resorption of the stent in the body tissues. 
It has been found that when the glass transition temperature (T.sub.g) of 
the biodegradable composition which comprises the stent is less than about 
37.degree. C., biodegradation proceeds at a more rapid rate than when the 
T.sub.g is about 37.degree. C. or greater. 
T.sub.g is defined as a second order transition temperature which results 
in a discontinuity of properties of a polymer composition. At the T.sub.g, 
the polymer will change from a stiff to a more flexible state, and its 
density and molecular free volume will increase. 
Thus, when the T.sub.g is less than about 37.degree. C., the polymeric 
composition comprising the stent becomes more susceptible to penetration 
by body fluids and the time of biodegradation proceeds more rapidly. 
The process of biodegradation of the stent begins from the time the stent 
is initially implanted between the kidney and bladder. However, the rate 
at which biodegradation occurs can be controlled to assure that the stent 
will function for the desired period of time which can range from a matter 
of weeks to two or three months or even longer as the requirement demands, 
based upon the individual condition and needs of the patient. Most 
preferably, the useful life of the stent, which is the time during which 
the stent continues to function and operate, will vary from about 3 to 7 
weeks. 
Factors which are influential in controlling the rate of biodegradation, 
which directly relate to the useful life of the stent, include the 
molecular weight of the stent composition and the amorphous nature of the 
stent composition. A reduction in molecular weight is indicative of 
biodegradation. The more amorphous the stent terpolymer composition is, 
the faster it will biodegrade. 
The inventive terpolymer should have a weight average molecular weight of 
about 20,000 to 1,000,000. preferably about 50,000 to 400,000, and is 
generally monomodal with respect to molecular weight distribution. 
The inventive terpolymer can be melt processed without decomposing at 
temperatures of 230.degree. C. and below. The terpolymer is thixotropic 
and most readily processable at temperatures from about 135.degree. to 
150.degree. C. 
Important factors involved in tube fabrication include the shear rate in 
the extruder and temperature. The preferred processing temperature for 
tube extrusion varies from about 135.degree. to 150.degree. C. Processing 
at all conditions reduces the average molecular weight and affects all 
molecules similarly. The shear rate in the extruder should be maintained 
as low as possible to reduce the amount of melt fracture and molecular 
weight degradation. Thus, it is important in the processing to preferably 
maintain the shear rate in the extruder to about 500 to 1000 sec.sup.-1 to 
maintain physical properties as close to the original material as 
possible. 
It is also desirable to incorporate or blend radiopaque materials such as 
barium sulfate with the terpolymer in amounts varying from about 5 to 30 
weight %, preferably about 10 to 20 weight % of the terpolymer 
composition. The barium sulfate is finely divided to a particle size which 
makes it homogeneous and compatible with the terpolymer, without affecting 
its light transmission properties. A suitable particle size is where 99% 
of the particles pass through 325 mesh or a 45 micron opening. 
The present invention also provides a method for treating and remedying a 
ureteral obstruction or impairment with a ureteral stent without the 
necessity for an additional surgical procedure to remove the stent after 
it has performed its function and is no longer needed. The use of the 
inventive biodegradable, biocompatible and resorbable ureteral stent 
assures its safe and innocuous disappearance by biodegradation at a 
controlled and predictable rate after the stent has fulfilled its 
function. The controlled predictable rate of biodegradation is based upon 
such factors as molecular weight and extent of the amorphous nature of the 
terpolymer composition. Thus, the only surgical procedure necessary is the 
initial insertion and positioning of the ureteral stent between the kidney 
and the bladder. Removal is accomplished by biodegradation of the stent.

The following examples illustrate specific embodiments of the present 
invention. In the examples and throughout the invention all parts and 
percentages are by weight, unless otherwise indicated. 
EXAMPLE 1 
Starting Materials 
High-purity L(-)lactide is available from commercial sources, under the 
trademark Crystallization 3.TM. from Purac Inc., affiliated with CCA 
biochem bv. of The Netherlands, and under the trademark L-Lactide S.TM. 
from Henley and Co., a subsidiary of Boehringer Ingelheim of Germany. 
Glycolide is available under the trademark Glycolide S.TM. from Henley and 
Co. 
Epsilon-caprolactone having a purity above 99% is purchased from commerical 
sources, such as Aldrich Company Catalog No. 16736-3, and is further 
purified by vacuum distillation through a Claisen head at 10 to 20 torr, 
to a water white cut at 90.degree. to 115.degree. C. with a boiling point 
range of about .+-.2.degree. C. The distillation i discontinued when the 
pot supply is low and with the temperature rising at constant pressure. 
The distillate is stored under a moisture free nitrogen or argon 
atmosphere. 
The catalyst, stannous octoate, is available from M&T Chemicals, Rahway, NJ 
as an anhydrous solution. About 10 milliliters of dried CP (certified 
pure) or AP (analyzed pure) grade toluene and 20 milliliters of stannous 
octoate was pipetted into a 200 to 300 ml flask, equipped with either an 
argon or nitrogen purge and a Dean-Stark type trap that was capped with a 
Drierite drying tube. The empty apparatus was previously flame-dried and 
cooled under nitrogen. The toluene solution was brought to reflux under a 
nitrogen trickle and 10 milliliters was distilled, to insure that the last 
few milliliters were clear. 
EXAMPLE 2 
Preparation of Terpolymer 
65 parts of L(-)lactide (L-lactide S.TM., Henley and Co.), 15 parts of 
glycolide (Glycolide S.TM., Henley and Co.), and 20 parts of purified 
epsilon-caprolactone, (Aldrich Catalog No. 16736-3; Chemical Abstracts No. 
502-44-3) were placed in an ampoule followed by the addition of 0.10 
milliliters of a 20% stannous octoate solution in toluene. The amount of 
stannous octoate catalyst solution was 0.10 milliliters per 100 grams of 
total lactide, glycolide and epsilon-caprolactone. The ampoule was 
evacuated with a vacuum pump for at least 10 minutes and sealed at its 
constriction. The contents were melted by placing the ampoule in a 
140.degree. to 160.degree. C. oil bath, while mixing the melt by swirling 
until the melt became viscous. Heating continued for about 16 to 72 hours 
at 140.degree. to 160.degree. C. The ampoule was removed from the oil 
bath, and cooled to room temperature. The terpolymer product was removed 
from the ampoule and stored in a desiccator. The terpolymer was 
transparent and nearly colorless. Its weight average molecular weight, as 
measured by gel permeation chromatography (GPC) was greater than 100,000. 
The preparatory procedure was again repeated using different amounts of the 
components as tabulated in Table 1. 
TABLE 1 
______________________________________ 
TERPOLYMER COMPOSITION, Weight % 
Sample No. 
L(-)lactide 
Glycolide Epsilon-caprolactone 
______________________________________ 
1 60 15 25 
2 37.5 37.5 25 
3 15 60 25 
4 65 15 20 
5 85 0 15 
______________________________________ 
Each of the samples was then tested for various physical properties 
tabulated in Table 2. 
TABLE 2 
______________________________________ 
SUMMARY OF PHYSICAL PROPERTIES 
Elong- 
Sample 
Tensile.sup.(a) 
ation.sup.(a) 
No. Strength, psi 
percent Modulus.sup.(b) 
Shore, A 
______________________________________ 
1 1627 596 506 51 
2 439 600 377 52 
3 1383 35 8791 96.sup.(c) 
3 1693.sup.(d) 
40 9908 96 
4 1511 954 4035 94 
4.sup.(e) 
1654 564 382 60 
5 4558 275.sup.(f) 
110,368 (Shore D,75) 
______________________________________ 
Footnotes for Table 2 
.sup.(a) Average of 5 specimens, ASTM D638, 70 mil thickness, crosshead 
speed 20 in./min. 
.sup.(b) Plastic or initial tangent modulus. 
.sup.(c) Shore D: 54 
.sup.(d) Crosshead speed 2 in./min. 
.sup.(e) Tested at 37.degree. C. 
.sup.(f) To failure, but 5% to yield. 
EXAMPLE 3 
Compression Molding of Terpolymers 
Sheets of approximately 75 mil were compression molded in accordance with 
the following procedure: 
60 grams of each terpolymer sample prepared in accordance with Example 2 
were placed between silicone release paper in a polished, stainless steel 
hinged mold preheated in a press to the temperatures shown in Table 3. 
TABLE 3 
______________________________________ 
Sample No. 
Molding Temp. .degree.F. 
Platen Pressure (psi) 
______________________________________ 
1 250 5,000 for 2 min. 
10,000 for 3 min. 
2 250 same as sample No. 1 
3 205 " 
4 266 " 
5 330 20,000 for 1 min. 
______________________________________ 
Contact pressure was maintained on the mold for approximately 2 to 5 
minutes until the polymer flowed into the mold cavity. Platen pressure of 
5,000 to 2,000 pounds was applied for 1 to 3 minutes as shown in Table 2. 
The mold was then put into a cooling press under the same platen pressure 
and held until cooled to room temperature. The sheet of polymer for each 
sample was removed from the mold and release paper and specimens were cut 
from the sheet for tensile tests, with the results tabulated in Table 4. 
TABLE 4 
______________________________________ 
ELASTOMER MODULI AT 100 AND 200% ELONGATION.sup.(a) 
Sample No. 100% Modulus (psi) 
200% Modulus (psi) 
______________________________________ 
1 155 227 
2 162 188 
3 446 489 
4 214 294 
5 2130 3160 
______________________________________ 
.sup.(a) Elastomer modulus is psi load at 100% and 200% elongation. 
EXAMPLE 4 
Characterization of Molecular Weight and Thermal Properties 
A molecular weight analysis and thermal characterization of several samples 
of L(-)lactide/glycolide/epsilon-caprolactone terpolymer with component 
ratios of 65/15/20 in parts by weight was conducted. 
Molecular weight distribution and averages were determined using a Waters 
Model 150 C ALC/GPC SEC with a Model 820 data station and Maxima software. 
Operating parameters used to determine the molecular weight are listed in 
Table 5. Table 6 shows molecular weights calculated for the samples. 
Table 5 
Operating Parameters for Molecular Weight Analysis 
Columns: 10.sup.6 -10.sup.5 -10.sup.4 -10.sup.3 .ANG. .mu. Styragel 
Solvent: Burdick & Jackson DIG Tetrahydrofuran 
Flow Rate: 1 ml/min 
Injection Volume: 100 .mu.l 
Temperature: 23.degree. C. (RT) 
Nominal Concentration: 2 mg/ml 
Detector: Refractive Index 
Standards: Narrow distribution polystyrene 
TABLE 6 
______________________________________ 
Molecular Weights of Terpolymer Samples 
Before and After Extrusion Into Tubing 
Description 
M.sub.n, 1000's 
M.sub.w, 1000's 
M.sub.z, 1000's 
M.sub.w /M.sub.n 
______________________________________ 
Before 
processing - 
Sample 1 106 197 342 1.88 
Sample 2 145 310 591 2.14 
Beginning of 
extrusion - 
Sample 1 86 173 300 2.01 
Sample 2 104 228 526 2.18 
Middle of 
extrusion - 
Sample 1 90 177 303 1.97 
Sample 2 103 204 364 1.98 
End of 
extrusion - 
Sample 1 85 171 302 2.00 
Sample 2 97 200 375 2.07 
______________________________________ 
The terpolymers were monomodal with respect to molecular weight 
distribution. Although the polymers showed a decrease in molecular weight 
upon melt fabrication, the decrease was not significant in terms of loss 
of physical properties. 
Thermal gravimetric analysis (TGA) and differential scanning calorimetry 
(DSC) were performed on the terpolymers. Inhomogeneity, as evidenced by 
melting points of monomers, or weight loss on programmed heating, as well 
as melting points of homopolymers was not detectable. The terpolymers were 
pure and homogeneous, and contained at most ppm quantities of unreacted 
monomer. The terpolymer could be melt processed without decomposition at 
230.degree. C. and below. The material was thixotropic (shear thinning) 
and processible at approximately 138.degree.-148.degree. C. 
EXAMPLE 5 
Formation of Small Diameter Tubes 
A Brabender single screw 3/4 inch diameter extruder with 30 L/D was used 
with a die to manufacture small tube diameters. The take up device, a 
Univex Take-off from C. W. Brabender, was placed after a 6 foot water 
bath. The terpolymer composition of Example 4 was used. The initial 
processing temperatures for tube extrusion were 138.degree. to 148.degree. 
C. The shear rate in the extruder was maintained in the range of 500 to 
1000 sec..sup.-1 to minimize the amount of melt fracture and molecular 
weight degradation. The molecular weights were determined using Maxima 820 
GPC analysis, with results shown in Table 7 as follows: 
TABLE 7 
______________________________________ 
Molecular Weights 
Temperature M.sub.n M.sub.w 
.degree.C. Number Average 
Weight Average 
______________________________________ 
Unprocessed 126412 295015 
control 
138 103062 203780 
148 90005 176878 
______________________________________ 
The extrusion temperature profile from the feed zone to the die was as 
follows: 
______________________________________ 
Zone Temperature .degree. C. 
______________________________________ 
1 (feed) 138 
2 143 
3 146 
4 (die) 148 
______________________________________ 
The terpolymer was extruded to produce a tube with an inside diameter of 
0.072 inch and outside diameter of 0.111 inch. This test demonstrated that 
the biodegradable terpolymer can be extruded into small tubes with the 
desired diameter using the processing conditions described. Important 
factors involved in tube fabrication were shear history and temperature. 
Processing at all conditions reduced the molecular weight averages and 
affected all molecules similarly. Thus, it was important in the processing 
to maintain the shear rate in the extruder at a minimum to maintain 
original material properties. The processing temperature also affected the 
final molecular weight. The higher the processing temperature, the lower 
the calculated molecular weight averages. Therefore, it is preferable to 
operate the extruder at the low end of the processing temperature range of 
the terpolymer (138.degree. to 148.degree. C.). 
EXAMPLE 6 
Blending of Terpolymer with Barium Sulfate Followed by Extrusion 
A terpolymer having the same composition as that in Example 5 was melt 
blended with 12 percent by weight of small, micron size particulate 
BaSO.sub.4 on a two roll mill at 280.degree. F. The BaSO.sub.4 was 
homogeneous and compatible with the terpolymer. The terpolymer resin 
filled with the BaSO.sub.4 was then ground with dry ice and placed in an 
oven at 100.degree. F. for 1 hour to remove excess moisture. To complete 
the drying, the material was placed in a vacuum oven at room temperature 
overnight. 
The filled copolymer was extruded into a tube using a 3/4 inch Brabender 
extruder with 30:1 length to diameter ratio. Additional die parts were 
used for the small diameter requirements. A 6-foot water bath and a take 
up device followed the extruder to cool and control the size of the 
tubing. Air was also fed through the middle of the die to maintain the 
tube shape until the material cooled and established its own integrity. 
The final tube diameter was determined by balancing the extruder rpm, air 
pressure, and take up speed with the die dimensions. The processing 
temperatures used in the fabrication of the tubing were: 
______________________________________ 
Temperature Profile (.degree.F.) 
T1 (feed) T2 T3 T4 (die) 
______________________________________ 
290 300 310 320 
______________________________________ 
The tubing was able to be heat-set into approximately a 1 inch diameter 
curl by looping it around or within a mandril, heating the curled tubing 
to 42.degree.-50.degree. C. (108.degree.--122.degree. F.) and cooling it 
in place. The curl, thus formed, promptly returned to its position when 
straightened. 
The molecular weight of the BaSO.sub.4 filled terpolymer tubing was then 
determined. The weight average-, number average-, and Z-average molecular 
weights, respectively, were 260,000; 152,000; and 442,000. This 
demonstrated that the terpolymers can be processed to retain useful 
properties for applications as a stent. The tubing extrudates were of good 
quality - smooth, homogeneous, tough, and elastic. Preliminary results 
indicated that the tubing embrittled somewhat after 3 weeks in contact 
with aqueous fluids. Although it was still somewhat pliable, ductile 
failure occurred upon handling and bending. At that stage, the M.sub.w, 
M.sub.n, and M.sub.z, respectively, were 27,600; 10,400; and 51,623. The 
polydispersity (M.sub.w /M.sub.n) was 2.65, which is a slight increase 
over the value of 2.0 for the unexposed terpolymer (see Table 7). 
Differential scanning calorimetry indicated substantial hydrolysis and 
degradation. After 6-7 weeks the walls of the tubing appeared much 
thinner. The terpolymer tubing became softer and somewhat fibrous, and 
shredded easily into soft pieces. 
Although the composition comprising the inventive biodegradable, 
biocompatible, resorbable ureteral stent has been disclosed in the context 
of a terpolymer of L(-)lactide, glycolide and epsilon-caprolactone, other 
equivalent compositions are also contemplated as being suitable 
compositions for preparing the stent. 
Thus, it is contemplated that D(-)lactide, the internally optically 
inactive meso-lactide and the optically inactive racemic or D,L-lactide 
can be substituted for the L(-)lactide. It is also contemplated that 
delta-vacero-lactone can be substituted for epilson-caprolactone. 
A discussion of the mechanism of biodegradation of these compounds in the 
form if films is disclosed in Pitt et al "Alphatic Polyesters II. The 
Degradation of Poly (DL-Lactide), Poly (Epilson-Caprolactone), and Their 
Co-Polymers In Vivo", BIOMATERIALS, pages 215-220, (Vol. II, October 1981) 
.