Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-07-25
2004-04-06
Acquah, Samuel A. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C528S361000, C528S492000, C524S784000
Reexamination Certificate
active
06716932
ABSTRACT:
BACKGROUND
1. Technical Field
The present disclosure relates to bioabsorbable polymeric resins especially for use in manufacturing implantable surgical articles. More particularly, this disclosure relates to bioabsorbable polymers having very little variation in inherent viscosity within any given batch of the polymer.
2. Background of Related Art
Bioabsorbable polymers have been prepared from a variety of monomers including glycolide, lactide, p-dioxanone, &egr;-caprolactone, trimethylene carbonate and others. Absorbable homopolymers and copolymers have been used to fabricate a wide variety of implantable surgical devices such as, for example, clips and staples which are used to seal and/or suture body tissue during a surgical procedure, and to provide hemostasis.
Polymers can be characterized by their molecular weight (M) and degree of polymerization (P). The number average degree of polymerization (P.n.) defines the concentration of separate polymer chains in the polymer. The number average molecular weight M
n
is P.n. times the molecular weight of the repeating units in the chain.
The viscosity of a polymer, &eegr;, is related to the molecular weight or degree of polymerization. Rather than determine the molecular weight of a polymer, viscosity measurements are commonly used in the absorbable polymer industry as a quick indicator of the polymer's molecular weight. Viscosity measurements are made by dissolving the polymer in a given amount of solvent to form a solution and measuring the solution's resistance to flow at a given temperature.
One type of commonly used viscosity measurement is inherent viscosity &eegr;
inh
. Inherent viscosity &eegr;
inh
is defined by the equation
&eegr;
inh
=ln &eegr;
r
/c
wherein &eegr;
r
is the relative viscosity and c is in units of g/100 cc of solution.
Surgical devices made from bioabsorbable homopolymers or copolymers are often very small. The mechanical properties of the polymer and its dimensional stability can be critical in such applications. The mechanical properties of the polymer depend, at least in part, on its molecular weight. Clearly then, it is important to have consistency in molecular weight (as reflected by viscosity measurements) so that such surgical devices can be made having uniform strength and absorption properties.
However, variations in viscosity are frequently observed not only from one batch of polymer to another using the same process, but even within the same batch of polymer. It would be advantageous to minimize variations in molecular weight, as measured by viscosity, in batches of bioabsorbable polymers.
SUMMARY
Absorbable polymers are provided herein which are highly consistent with respect to inherent viscosity. Specifically, polymers in accordance with this disclosure have an inherent viscosity characterized by a standard deviation of about 0.05 or less. Methods for producing a batch of absorbable polymeric resin having an inherent viscosity characterized by a standard deviation of about 0.05 or less are also described herein.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
While the methods herein are described in terms of the production of an absorbable glycolide-lactide copolymer, for which it is particularly preferred, it should be understood that various alternative monomers may be employed herein for the production of absorbable homopolymers or copolymers. Suitable alternative monomers include, but are not limited to, 1,3-dioxan-2-one, 1,4-dioxan-2-one and &egr;-caprolactone.
The standard deviation, &sgr;, of a distribution of elements in a population is defined in accordance with the following mathematical formula:
σ
=
[
(
1
/
N
)
∑
i
=
1
N
(
x
i
-
μ
)
2
]
1
/
2
wherein N is the number of elements in the population and &mgr; is the mean value, or average:
μ
=
(
1
/
N
)
∑
i
=
1
N
x
i
In a normal, or Gaussian, distribution a graphical representation of a random sampling is depicted by a bell shaped curve in which 68.3% of the population falls within the limits defined by &mgr;−&sgr; and &mgr;+&sgr;.
The standard deviation, &sgr;, of the inherent viscosity within a single batch of the polymer as produced according to the method herein, will be no more than about 0.05, preferably no more than about 0.025 and more preferably no more than about 0.01. Thus, if the inherent viscosity is determined by multiple sampling of the batch of polymer, the viscosity readings will exhibit a low variability. That is, the highest and lowest viscosity measurements will be close to the average measurement. Accordingly, the consistency of the polymer produced by the method herein is high.
The term “batch” as used herein refers to the quantity of copolymer retrieved from a polymerization vessel. If polymerization is conducted on a bench scale, a batch may be as little as 0.2 kilograms of copolymer. On a pilot scale, a batch can typically be from about 1 to about 10 kilograms of copolymer. Normally, commercial scale batches contain from about 20 to about 200 kilograms or more of copolymer.
The term “batch viscosity deviation” or “BVD” as used herein refers to the standard deviation of at least ten measurements of inherent viscosity on ten different randomly selected samples from a single batch of copolymer.
Particularly useful absorbable copolymers in accordance with the methods described herein contain from at least about 15% (mole percent) but no more than about 30% glycolide so that fasteners or other implantable surgical devices made from the copolymer is not excessively brittle, exhibits an acceptable absorption profile and is not more than about 20% crystalline. Preferably, the copolymer is substantially amorphous. As used herein, “substantially amorphous” means having 10% or less crystallinity. The copolymer usually should not be more than 20% crystalline so that the fastener will not be more than 10% crystalline. Crystallinity normally decreases with processes that heat the copolymer above melting and then cool rapidly enough to prevent reorientation (e.g., the typical injection molding processes used to fabricate many types of surgical devices). With those processes, the copolymer itself can be of relatively high crystallinity. With processes in which crystallinity is not decreased appreciably, the copolymer must itself be of low crystallinity for the surgical device produced by the process to be substantially amorphous.
The copolymer before being formed into the surgical device should have an inherent viscosity &eegr;
inh
of at least 1.3 when measured in chloroform at 30° C. in a concentration of 0.25 g/dl (grams of copolymer per deciliter of solution). A Ubbelohde Viscometer may be used for measuring the viscosity. Where the surgical device being formed is a fastener, the fastener should have an inherent viscosity &eegr;
inh
of at least 0.9, which corresponds to an average molecular weight of about 90,000. (The process of forming the fastener from the copolymer tends to reduce the inherent viscosity.)
The glass transition temperature when measured by differential scanning calorimetry at 20° C./min should be at least 56° C. for the copolymer before being formed into a surgical device and at least 54° C. and preferably at least 56° C. after the device is formed. (The fastener forming process tends to reduce the glass transition temperature also.)
A Perkin-Elmer Model DSC-2 Differential Scanning Calorimeter can be used to measure glass transition temperature (Tg). Seven to eight mg of the sample are sealed in a aluminum sample pan, which is then placed in the measuring head of the calorimeter. The sample is heated to relieve all stress and orientation, which may cause spurious thermal effects (e.g., heated at a rate of 20° C./ min. to a temperature of 170°-180° C.) and then cooled at 10° C./min. to a temperature below the expected glass transition temperature (typically to 0° C.) The sample is scanned at a heating rate of 20° C./min. through the glass transition. The glass transition temperature is taken as the mid-point of the transition regi
Clift Nelson J.
Elliott Thomas F.
Kennedy John
Acquah Samuel A.
Tyco Healthcare Group LP
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