Polyurethane and polyurea biomaterials for use in medical...

Details Polyurethane and polyurea biomaterials for use in medical... Polyurethane and polyurea biomaterials for use in medical...

2000-06-13

2002-08-20

Gorr, Rachel (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From reactant having at least one -n=c=x group as well as...

C528S085000, C600S372000, C607S119000

Reexamination Certificate

active

06437075

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to medical devices that include polyurethane and polyurea biomaterials, particularly elastomers, containing sacrificial moieties (e.g., sulfur-containing moieties) that preferentially oxidize relative to other moieties in the polymer.
BACKGROUND OF THE INVENTION
The chemistry of polyurethanes and polyureas is extensive and well developed. Typically, polyurethanes and polyureas are made by a process in which a polyisocyanate is reacted with a molecule having at least two hydrogen atoms reactive with the polyisocyanate, such as a polyol or polyamine. The resulting polymer can be further reacted with a chain extender, such as a diol or diamine, for example. The polyol or polyamine can be a polyester, polyether, or polycarbonate polyol or polyamine, for example.
Polyurethanes and polyureas can be tailored to produce a range of products from soft and flexible to hard and rigid. They can be extruded, injection molded, compression molded, and solution spun, for example. Thus, polyurethanes and polyureas, particularly polyurethanes, are important biomedical polymers, and are used in implantable devices such as artificial hearts, cardiovascular catheters, pacemaker lead insulation, etc.
Commercially available polyurethanes used for implantable applications include BIOMER segmented polyurethanes, manufactured by Ethicon, Inc., of Sommerville, N.J., PELLETHANE segmented polyurethanes, sold by Dow Chemical. Midland, Mich., and TECOFLEX segmented polyurethanes sold by Thermedics, Inc., Woburn, Mass. These polyurethanes and others are described in the article “Biomedical Uses of Polyurethanes,” by Coury et al., in
Advances in Urethane Science and Technology
, 9, 130-168, edited by Kurt C. Frisch and Daniel Klempner, Technomic Publishing Co., Lancaster, Pa. (1984). Typically, polyether polyurethanes exhibit more biostability than polyester polyurethanes, and are therefore generally preferred biopolymers.
Polyether polyurethane elastomers, such as PELLETHANE 2363-80A (P80A) and 2363-55D (P55D), which are believed to be prepared from polytetramethylene ether glycol (PTMEG) and methylene bis(diisocyanatobenzene) (MDI) extended with butane diol (BDO), are widely used for implantable cardiac pacing leads. Pacing leads are insulated wires with electrodes that carry stimuli to tissues and biologic signals back to implanted pulse generators. The use of polyether polyurethane elastomers as insulation on such leads has provided significant advantage over silicone rubber, primarily because of the higher tensile strength and elastic modulus of the polyurethanes.
There is some problem, however, with stress cracking of polyether polyurethane insulation, which can cause failure. Polyether polyurethanes are susceptible to oxidation in the body, particularly in areas that are under stress. When oxidized, polyether polyurethane elastomers lose strength and form cracks, which eventually breach the insulation. This, thereby, allows bodily fluids to enter the lead and form a short between the lead wire and the implantable pulse generator (IPG). It is believed that the ether linkages degrade, perhaps due to metal ion catalyzed oxidative attack at stress points in the material.
One approach to solving this problem has been to coat the conductive wire of the lead. Another approach has been to add an antioxidant to the polyurethane. Yet another approach has been to develop new polyurethanes that are more resistant to oxidative attack. Such polyurethanes include only segments that are resistant to metal induced oxidation, such as hydrocarbon- and carbonate-containing segments. For example, polyurethanes that are substantially free of ether and ester linkages have been developed. This includes the segmented aliphatic polyurethanes of U.S. Pat. No. 4,873,308 (Coury et al.). Although such materials produce more stable implantable devices than polyether polyurethanes, there is still a need for biostable polymers, particularly polyurethanes suitable for use as insulation on pacing leads.
SUMMARY OF THE INVENTION
The present invention relates to medical devices comprising a biomaterial formed from a polymer comprising urethane groups, urea groups, or combinations thereof (i.e., polyurethanes, polyureas, or polyurethane-ureas). Preferably, the polymer is a segmented polyurethane. The polymer also includes sacrificial moieties (preferably in the polymer backbone) that preferentially oxidize relative to all other moieties in the polymer. These sacrificial moieties are present upon initial formation of the polymer and oxidize upon contact with the environment or body, for example. Significantly, the oxidation of such sacrificial moieties typically provides improved mechanical properties, such as increased tensile strength and/or increased modulus of elasticity, relative to the polymer prior to oxidation. As used herein, “sacrificial moiety” refers to the atom(s) or functional group that has the lowest oxidation potential within the molecule. Such moieties are the preferential sites for oxidation. Preferably, the sacrificial moiety is a sulfur- or phosphorus-containing moiety, more preferably, a sulfur-containing moiety, which can be oxidized to form a functional group that imparts stronger mechanical properties to the polymer. More preferably, the polymer includes at least about 1.0 weight percent sulfur or phosphorus, and most preferably, at least about 1.2 weight percent sulfur or phosphorus (preferably, sulfur), based on the total weight of the polymer. The polymer is also preferably substantially free of ester linkages and more preferably, substantially free of ester and ether linkages.
Preferably, the biomaterials of the present invention are used in a pacing lead as the insulation. Thus, the present invention provides a medical electrical lead comprising: an elongate insulation sheath formed from a polymer comprising urethane groups, urea groups, or combinations thereof, and sacrificial moieties (preferably in the polymer backbone) that preferentially oxidize relative to all other moieties in the polymer, as described above; an elongated conductor, located within the elongated insulation sheath; an electrode coupled to a distal end of the elongated conductor; and an electrical connector coupled to a proximal end of the elongated conductor.
The present invention also provides a medical device comprising a biomaterial formed from a segmented polymer comprising urethane groups, urea groups, or combinations thereof, wherein the polymer is prepared from isocyanate-containing compounds and compounds of the formula:
Y—R
1
—(—X—R
2
—X—R
1
—)
n
—X—R
2
—Y
wherein Y is either OH or NH
2
, n=0-100 (preferably 0-10), X is S or P—R
3
, R
1
and R
2
are each independently straight, branched, or cyclic aliphatic groups (preferably alkyl groups), and R
3
is an aliphatic, aromatic, or araliphatic group. These sulfur and phosphorus-containing groups are the preferred sacrificial moieties that can be oxidized to functional groups that preferably provide enhanced mechanical properties to the polymer. For example, a sulfur atom or sulfide (—S—) can be oxidized to a sulfoxide or sulfoxy (—S(O)—) group (with an IR peak at about 1030 cm
−1
) and then to a sulfone (—SO
2
—) group (with an IR peak at 1125 cm
−1
upon oxidation). Sulfoxide- and sulfone-containing polymers typically have a higher tensile strength and a higher modulus of elasticity than similar polymers containing sulfide moieties.
Also provided is a medical device comprising a biomaterial formed from a polymer comprising alternating soft and hard segments linked by urethane groups, urea groups, or combinations thereof, wherein: (a) the soft segments are of the formula (—O or —OCNH)—(R
a
—U—R
b
—U)
y
—R
a
—(O— or NHCO—), wherein: (i) each R
a
and R
b
is independently a hydrocarbon moiety that can include linear, branched, cyclic structures, or combinations thereof, having a molecular weight of less than about 4000, wherein at least one R
a
or R
b
noncrystallizing in the polymer at ambient temperature; (ii) each

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