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
2000-12-18
2003-06-24
Woodward, Ana (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...
C424S406000, C424S426000, C525S413000, C525S415000, C623S023720, C623S023730
Reexamination Certificate
active
06583232
ABSTRACT:
The present invention relates to bioresorbable polymeric compositions for use in the manufacture of medical devices, methods of making said compositions, medical devices made from said compositions and methods of treatment of the human or animal body involving such devices.
There is a need for surgical repair devices such as sutures, bone plates, interference screws, tissue fasteners, staples and other tissue and fracture fixation devices which are bioresorbable. Reference herein to a material being “bioresorbable” means that it breaks down over a finite period of time due to the chemical/biological action of the body. Preferably, complete resorption occurs within about 5 years, More preferably within about 3 years. This breakdown is at a rate allowing the repair device to maintain sufficient integrity while the soft tissue or bone heals: surgical repair devices formed of materials which are resorbed too quickly may fail when compressive, tensile or flexural loads are placed on them before the tissue or bone has fully healed. Advantages of using bioresorbable materials over non-bioresorbable materials, e.g. metals, are that they encourage tissue repair and further surgery is not required to remove them. In addition, there is the issue of stress-shielding: tissues like bone tend to grow well in regions where there is a prevalence of high stress. If the stress is reduced or removed, because, for example, an implant is bearing all the load, then the tissue may tend to recede around it resulting in loosening over the longer term. Implanted bioresorbable materials do not tend to give rise to adverse effects due to stress-shielding.
It is known to use certain bioresorbable polymeric materials, like polyglycolic acid (PGA) and polylactic acid (PLA), for manufacturing surgical devices. These have the disadvantage, however, that they are brittle.
In addition, it is known to form blends of these materials and others, e.g. polycaprolactone (PCL), polytrimethylene carbonate (PTMC) and polydioxanone (PDO), to achieve desired physical attributes, like melting point and mechanical properties. It can prove difficult, however, to achieve the desired bioresorption rate of such materials in vivo.
Reference is also made to U.S. Pat. No. 5,475,063, which teaches a blend of a bioresorbable random copolymer and another bioresorbable polymer. This is not only the stated aim of this document, but in all the examples manufacture of the copolymer is by non-sequential addition of the components, i.e. a random copolymer will result.
It is an object of the present invention to provide polymers, particularly for medical applications, with desirable mechanical and resorption properties.
According to a first aspect of the invention, a bioresobable polymeric composition is presented comprising a blend comprising a first bioresorbable polymer and a second, bioresorbable polymer, wherein the first bioresorbable polymer is a block copolymer.
Polymer blends are usually classified as either miscible or immiscible. Many combinations of polymers form immiscible blends, this being determined by a delicate balance of entropic and enthalpic forces of the blended polymers. The compatibility of two polymers in a mobile phase depends mainly on the forces acting between the various groups in the chains of the same material as well as between the groups in the chains of the two different materials. In non-polar or weakly polar polymers the physical forces acting are principally dispersion forces. Therefore, when two non-polar polymers in a mobile isotropic liquid state are mixed together in a blend, phase separation into a dispersed and a continuous phase usually occurs. This phase separation is referred to herein as “macrophase separation”.
The physical behaviour of block copolymers is related to solid state morphology. Block copolymers sometimes exhibit phase separation which typically gives rise to a continuous phase consisting of one block type in a continuous matrix consisting of a second block type. In many applications, the dispersed phase consists of hard domains which are crystalline or glassy and amorphous, the matrix being soft and rubber-like. This phase separation is referred to herein as “microphase separation”. For more details regarding phase separation in block copolymers, reference is made to D. C. Allport and W. H. Janes, “Block Copolymers”, Applied Science Publishers Ltd., London, 1973.
In a blend of a block copolymer and another polymer, it is possible to have microphase separation within the block copolymer itself and macrophase separation between the copolymer and the other polymer. Reference herein to microphase-separated copolymer implies that the dimensions of the domains are in the size range of less than or equal to 500 nm. Reference herein to macrophase separation implies domain sizes (i.e. domains of dispersed and continuous phases) in the size range of greater than or equal to 1 micron, unless a compatibiliser has been added, in which case the dimensions of the domains domain sizes are larger than 500 nm.
A blend of this type gives a large number of variables which may be altered, to allow the rate of bioresorption and desired mechanical properties to be precisely tailored to desired levels: not only may the second bioresorbable polymer and the at least two types of block of the first bioresorbable polymer be varied, but either polymer may form the dispersed or the continuous phase, providing even more scope for variation of the properties of the material.
As stated, the first bioresorbable polymer may form the dispersed phase or the continuous phase. Preferably, the first bioresorbable polymer forms the dispersed phase and the second bioresorbable polymer forms the continuous phase.
Advantageously, there is also microphase separation within the first bioresorbable polymer. This allows the possibility of selecting a block of the copolymer which resorbs relatively quickly, generating porosity and allowing tissue ingrowth. It also allows the possibility of having a further block of the copolymer which modifies the blend properties (e.g. toughens it).
Advantageously, the second bioresorbable polymer and each of the types of block of the first bioresorbable polymer have different resorption rates. This allows porosity to be generated by resorption in certain parts of the blend, but, at the same time, structural integrity to be maintained while this is occurring.
Most preferably, at least one of the types of block of the first bioresorbable polymer is selected to have a higher rate of resorption than both the other type(s) of block of said first bioresorbable polymer and the second bioresorbable polymer.
The first resorbable polymer is a copolymer, for example a diblock (i.e. AB), triblock (i.e. ABA) or multiblock (e.g. ABC or segmented) block copolymer.
The bioresorbable repeating units of the first bioresorbable polymer may be selected from saturated or unsaturated esters, including orthoesters, carbonates, anhydrides, amides, ethers, or saccharides.
Advantageously, the repeating units of the first bioresorbable polymer are derived from cyclic monomers capable of undergoing ring opening followed by polymerisation. Preferred cyclic monomers are cyclic esters and carbonates, like lactide (LA), glycolide (GA), caprolactone (CL), p-dioxanone (p-DO) and trimethylene carbonate (TMC). The ring opening reaction has the advantage that it may produce higher molecular weight polymers which may have superior mechanical and degradation properties. In addition, polyesters and polycarbonates have the advantage that they degrade in vivo to produce non-toxic by-products like carbon dioxide and water.
More preferably, the block copolymers comprise GA and/or TMC. Most preferably, the block copolymer is PGA-PTMC-PGA, which will also be referred to herein as Polyglyconate B and, in one form, as MAXON B™. The PGA blocks degrade relatively rapidly in vivo to give porosity and allow tissue ingrowth, while the PTMC blocks provide rubber-toughening which helps maintain the structural integrity of the blended material.
Acc
Larson & Taylor PLC
Smith & Nephew Plc
Woodward Ana
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