Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
2000-12-13
2003-09-30
Moore, Margaret G. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C528S028000, C428S423100, C428S904000, C428S015000
Reexamination Certificate
active
06627724
ABSTRACT:
The present invention relates to polysiloxane-containing polyurethane elastomeric compositions having improved properties which make them useful for a variety of applications, in particular the manufacture of medical devices, articles or implants which contact living tissues or bodily fluids.
Polyurethane elastomers represent an important class of segmented copolymers with excellent mechanical properties including high tensile strength, good tear and abrasion resistance and a relatively good stability in biological environments. Accordingly, polyurethanes are widely used in medical implants such as cardiac pacemakers, catheters, implantable prostheses, cardiac assist devices, heart valves and vascular grafts. The excellent mechanical properties of segmented polyurethanes are attributed to their two phase morphology derived from microphase separation of soft and hard segments. In polyurethanes used for long term medical implants, the soft segments are typically formed from a polyether macrodiol such as polytetramethylene oxide (PTMO) whereas the hard segments are derived from a diisocyanate such as 4,4′-methylenediphenyl diisocyanate (MDI) and a diol chain extender such as 1,4-butanediol.
Although PTMO-based polyurethanes are the materials of choice for a wide variety of medical implants, in some cases the polyurethanes degrade causing malfunction or failure of the implant. Degradation is usually apparent in terms of surface or deep cracking, stiffening, erosion or the deterioration of mechanical properties such as flexural strength
1
. The mechanisms responsible for such degradations include environmental stress cracking, the generation of cracks and crazes produced by the medium acting on the polyurethane at certain stress levels and metal ion induced oxidation. It is generally accepted that ether linkages in the PTMO-based elastomers are the most vulnerable sites for initiation of degradations
2
. Efforts have been made to overcome this problem by developing polyurethanes that are not exclusively based on PTMO such as those disclosed in Australian Patent No. 657267, U.S. Pat. No. 4,875,308 (Courey et al), U.S. Pat. No. 5,133,742 (Pinchuck) and U.S. Pat. No. 5,109,077 (Wick). Nevertheless, the combination of degradation resistance, mechanical characteristics, processability and clarity is suboptimal for certain applications. For example, there has been a long felt need for polyurethanes combining low durometer hardness, high flexibility, good processability and high resistance to degradation within the pacing industry for the insulation of leads. With the polyurethanes described in the aforementioned patents, there appears to be a lower limit to flexibility and Shore hardness, below which degradation resistance and/or mechanical properties are adversely affected.
Polycarbonate macrodiols have also been used as reactive ingredients in the synthesis of block and segmented copolymer systems, in particular high performance polyurethanes. Processes for preparing polycarbonate macrodiols based on a range of bishydroxy alkylene compounds are disclosed in JP 62,241,920 (Toa Gosei Chemical Industry Co. Ltd.), JP 64,01,726 (Dainippon Ink and Chemicals, Inc.), JP 62,187,725 (Daicel Chemical Industries, Ltd.), DE 3,717,060 (Bayer A. G.), U.S. Pat. No. 4,105,641 (Bayer Aktiengesellschaft), U.S. Pat. No. 4,131,731 (Beatrice Foods Company) and U.S. Pat. No. 5,171,830 (Arco Chemical Technology).
Polysiloxane-based materials, especially polydimethyl siloxane (PDMS) exhibit characteristics such as low glass transition temperatures, good thermal, oxidative and hydrolytic stabilities, low surface energy, good haemocompatibility and low toxicity. They also display an improved ability to be bonded to silicone components, by such procedures as gluing, solvent welding, coextrusion or comolding. For these reasons PDMS has been used in biomedical applications
3,4
. However, PDMS-based polymers generally have limitations and do not exhibit the necessary combination of tear resistance, abrasion resistance and tensile properties for many types of implants intended for long term use. It would be desirable for polymers to be available with the stability and biological properties of PDMS, but the strength, abrasion resistance, processability and other physical properties of polyurethanes. Polyurethanes incorporating PDMS would appear to fulfil this need, but to date, despite much experimentation, no compositions have been produced with the optimal combination of physical and biological properties.
Previous attempts to incorporate PDMS into polyurethanes have not been very successful
5
. Speckhard et al
6
have indicated that as a result of the large differences in solubility parameters between polysiloxane and hard segments, PDMS-based polyurethanes are likely to be highly phase separated materials having poor mechanical properties. As a consequence of a large difference in polarity between hard and soft segments, it is anticipated that premature phase separation will occur during synthesis leading to compositional heterogenity and a low molecular weight. This is borne out by experiment and typically the tensile strength and elongation at break of PDMS-based polyurethanes is about 7 MPa and 200%, respectively
6
.
Several techniques have been reported in the literature to improve mechanical properties of PDMS-based polyurethanes with the primary focus being increasing the interfacial adhesion between soft PDMS phase and hard segments. These include mixing with certain polyethers or polyesters
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, (b) introduction of polar functionality to PDMS (c) use of copolymers of PDMS and polyethers as soft segments and (d) hard segment modifications. Only marginal improvements in mechanical properties have been observed using these techniques.
A requirement accordingly exists to develop methods for incorporating polysiloxane segments as part of the polyurethane structure to yield materials with good mechanical properties. The current demand for materials with improved biocompatibility and stability warrants development of polysiloxane-containing polyurethanes, especially those that are resistant to degradation when implanted for long periods of time.
According to the present invention there is provided a material having improved mechanical properties, clarity, processability and/or degradation resistance comprising a polyurethane elastomeric composition which includes a soft segment derived from a polysiloxane macrodiol and a polyether and/or polycarbonate macrodiol.
It will be appreciated that more than one polysiloxane macrodiol and polyether and/or polycarbonate macrodiol may be present in the polyurethane elastomeric composition.
The present invention also provides use of the polyurethane elastomeric composition defined above as a material having improved mechanical properties, clarity, processability and/or degradation resistance.
The present invention further provides the polyurethane elastomeric composition defined above when used as a material having improved mechanical properties, clarity, processability and/or degradation resistance.
The mechanical properties which are improved include tensile strength, tear strength, abrasion resistance, Durometer hardness, flexural modulus and related measures of flexibility.
The improved resistance to degradation includes resistance to free radical, oxidative, enzymatic and/or hydrolytic processes and to degradation when implanted as a biomaterial.
The improved processability includes ease of processing by casting such as solvent casting and by thermal means such as extrusion and injection molding, for example, low tackiness after extrusion and relative freedom from gels.
There is also provided a degradation resistant material which comprises the polyurethane elastomeric composition defined above.
The polyurethane elastomeric composition of the present invention shows significantly improved degradation resistance over the softer grades disclosed in U.S. Pat. No. 4,875,308 and the commercially available polyurethane Pellethane 2363-80A (Registered Trade Mark). It als
Gunatillake Pathiraja Arachchillage
McCarthy Simon John
Meijs Gordon Francis
Aortech Biomaterials PTY LTD
Moore Margaret G.
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