the present invention provides a base insulation of soft polyurethane elastomer overcoated with a thin layer of a polyurethane elastomer with a relatively low concentration of ether linkages such as an aliphatic or aromatic polyether urethane having a hardness at least about 90a on the shore a scale or a biostable ether - free polyurethane elastomer . since the environmental stress cracking ( esc ) in such implantable devices is a surface phenomenon , very little of the second elastomer needs to overlay the softer layer . a thin layer , on the order of 0 . 001 inches ( 0 . 0254 mm ), of the is all that is required to impart superior esc and mio resistance to implantable devices . such thicknesses do not appreciably change the overall handling characteristics of the completed device , while increasing the esc resistance . the lamination of the second material layer may be accomplished using any of the existing well known methods including dipping , spraying , and co - extrusion , with co - extrusion being preferred . the layer of second material should generally be in the range of about 0 . 001 inches ( 0 . 0254 mm ) in thickness , although it may be thinner or thicker depending on the application needs . coating of polymeric biomedical devices by dipping , spraying , or co - extrusion techniques are known to those skilled in the art . special care and understanding of polymer biostability are required to provide the optimum product performance . for example , care must be exercised to minimize antioxidant removal ( by thermal or extractive means ), to minimize residual stress in the parts , and to engineer consistent reliable processes . the acceptable second , overlaying material will be a polyurethane elastomer ; either a polyether urethane elastomer having a hardness on the shore a durometer scale of at least about 90a or a substantially ether - free polyurethane elastomer . the elastomer must also be oxidatively and hydrolytically stable and have a toughness in the range of polyurethanes generally . a suitable urethane is pellethane 2363 - 55d or pellethane 2363 - 55de of dow chemical co . of midland , mich . polyurethanes essentially equivalent to pellethane 2363 - 55d are available from other sources such as b . f . goodrich , inc . the pellethane 2363 family of polymers , including 2363 - 80a and 2363 - 55d , are composed of methylene bis isocyanato benzene ( mdi ), butane diol ( bd ) hard segments and polytetramethylene ether oxide ( ptmo ) soft segments . the proportion of hard to soft segments is higher for the harder ( shore 55d ) polymer than for the softer ( shore 80a ) material thereby providing fewer ether linkages which may be subject to in vivo degradation . preferably , the urethane is a substantially ether - free polyurethane since stress cracking appears to have a relation to the ether content of the polymer , with fewer ether linkages being desirable . a polymer without ether linkages may be made by substituting aliphatic , polycarbonate or polydimethylsiloxane groups for the polyether groups of the soft segments . ether - free polyurethanes said to be suitable for in vivo use are disclosed in u . s . pat . no . 4 , 875 , 308 to coury et al . ; published international patent application wo 92 / 04390 ; u . s . pat . no . 5 , 133 , 742 to pinchuk ; and u . s . pat . no . 5 , 109 , 077 to wick which are incorporated herein by reference in their entirety . biostable ether - free polymers include polymedica &# 39 ; s chronoflex al - 80a and chronoflex al - 55d , medtronic , inc .&# 39 ; s family of biostable polyurethanes ( u . s . pat . no . 4 , 873 , 308 ) and akzo / enka &# 39 ; s pur series of polyurethanes . these materials are coatable over the preferred lead insulator material , pellethane 2363 - 80a , by methods such as solution coating or coextrusion . the esc and mio reduction of the invention may be achieved with many implantable medical devices . such medical devices can include insulator sheaths of cardiac pacemaker leads , artificial heart diaphragms , artificial heart valve leaflets , sewing cuffs and the like . however , the preferred use of the invention is to provide improved resistance to degradation in critical lead insulation applications . in a typical lead and lead insulator assembly , an elastomeric polyurethane insulator is the outer element through which coiled conductors pass . the configuration can include separate , mutually insulated coils in which the multiple coils are carried in separate insulator passages in coaxial or side - by side arrangement or multi - polar coiled conductors having individually insulated coil wires which pass through an outer insulator sheath of polyurethane elastomer . such a lead system is disclosed in greater detail in u . s . pat . no . 5 , 040 , 544 issued to lessar et al . which is incorporated herein by reference in its entirety . in such lead systems , the polyurethane lead insulator is essentially an extruded piece of tubing of the desired shape and size required to carry the conductors . an outside diameter of the insulator is typically in the range of about 0 . 020 &# 34 ; to 0 . 090 &# 34 ; with a wall thickness typically in the range of about 0 . 005 &# 34 ; to 0 . 010 &# 34 ;. in the following examples , implanted tubing samples such as those used for pacing lead insulators were provided with materials and treatments intended to address the issues of esc and mio . the effect of using an overlaying material of differing mechanical properties can be easily calculated for a pacing lead insulator . for a composite tube having an inner , base material and an overlaying , outer material , the following formula can be applied to determine its stiffness : ## equ1 ## where e 1 is the elastic modulus of the base material , e 2 is the elastic modulus of the overlaying material , d 1 is the inside diameter of the base material , d 2 is the outside diameter of the base material and the inside diameter of the overlaying material , and d 3 is the outside diameter of the overlaying material . therefore , if one wished to provide a pacing lead insulator with a base material of pellethane 80a and with an overlaying layer of pellethane 55d , the relative overall stiffness of the lead insulator would be as set forth in table 1 . table 1__________________________________________________________________________ stiffness of co - extruded pacing lead insulation base coatingcoating elastic elastic inside outside outsidethickness modulus modulus diameter diameter diameter stiffness ( ei )( in ) e . sub . 1 e . sub . 2 d . sub . 1 ( in ) d . sub . 2 ( in ) d . sub . 3 ( in ) composite__________________________________________________________________________all p80 a 3 , 400 0 . 073 0 . 093 -- 7 . 70 . 0005 3 , 400 10 , 200 0 . 073 0 . 0925 0 . 093 8 . 30 . 001 3 , 400 10 , 200 0 . 073 0 . 092 0 . 093 8 . 80 . 0015 3 , 400 10 , 200 0 . 073 0 . 0915 0 . 093 9 . 30 . 002 3 , 400 10 , 200 0 . 073 0 . 091 0 . 093 9 . 9all p55d 10 , 200 0 . 073 0 . 093 -- 23__________________________________________________________________________ it is therefore apparent that a co - extruded coating with a stiffer material causes little change in the stiffness of the lead insulator . a lead insulator of pellethane 55d would be roughly three times as stiff as a lead insulator of pellethane 80a and yet a coextruded lead insulator with a 0 . 001 inch coating of pellethane 55d over a base of pellethane 80a can be expected to have a stiffness only about 14 % greater than the stiffness of the insulator made with pellethane 80a alone . five different tubings were fabricated and implanted in rabbits to study esc resistance . the tubings had a 0 . 073 inch ( 0 . 185cm ) id ( inside diameter ) by a 0 . 093 inch ( 0 . 236 cm ) od ( outside diameter ). the test material strands consisted of five polysulfone dumbbell shaped mandrels ( each approximately 0 . 165 cm diameter by 1 . 27 cm long ). each dumbbell supported a sample of strained ( 400 %) or unstrained ( 0 %) test or control tubing . 2 - 0 ticron suture was used to sustain the strain of these samples . five individual samples were tied together to form a strand . each strand was identified with an attached colored glass bead . there was a total of 130 samples , with 10 samples for each condition explanted at 12 weeks . tubing samples were formed by co - extruding either pellethane 2363 - 55d ( p55d ) tubing or pellethane 2363 - 55de ( p55de ) over pellethane 2363 - 80a ( p80a ). the test tubings were then compared to the esc resistance of positive and negative control samples , respectively , p80a tubing and p55d tubing . control tubing conditions and test tubing conditions are given below . c p80a / p55de , wall thickness 0 . 0045 inches ( 0 . 011 cm )/ 0 . 002 inches ( 0 . 005 cm ). 0 % elongation . p80a / p55de , wall thickness 0 . 0045 inches ( 0 . 011 cm )/ 0 . 002 inches ( 0 . 005 cm ). 400 % elongation . d p80a / p55d , wall thickness 0 . 0045 inches ( 0 . 011 cm )/ 0 . 002 inches ( 0 . 005 cm ). 0 % elongation . p80a / p55d , wall thickness 0 . 0045 inches ( 0 . 011 cm )/ 0 . 002 inches ( 0 . 005 cm ). 400 % elongation . e p80a / p55d , wall thickness 0 . 007 inches ( 0 . 018 cm )/ 0 . 001 inches ( 0 . 003 cm ). 0 % elongation . p80a / p55d , wall thickness 0 . 007 inches ( 0 . 018 cm )/ 0 . 001 inches ( 0 . 003 cm ). 400 % elongation . the test material strands were implanted subcutaneously in rabbits and removed at 12 weeks . the samples were examined at 30 × to 70 × magnification for esc and defects . the samples were then rated for environmental stress cracking with results reported as a fraction , x / y . the definitions of x and y are : 4 = cracks greater than 50 % of the tubing wall but not to 100 % table 2__________________________________________________________________________esc resistance of co - extruded tubing p80a / p55d and p80a / p55ide at 12weeksrabbit #% strain condition a condition b condition c condition d condition e__________________________________________________________________________311 0 % 1 / 1 0 / 0 0 / 0 0 / 0 0 / 0 0 % 1 / 3 0 / 0 0 / 0 0 / 0 0 / 0400 % 1 / 3 0 / 0 0 / 0 0 / 0 0 / 0400 % 1 / 3 0 / 0 0 / 0 0 / 0 0 / 0318 0 % 1 / 2 0 / 0 0 / 0 0 / 0 0 / 0 0 % 1 / 2 0 / 0 0 / 0 0 / 0 0 / 0400 % 2 / 1 0 / 0 0 / 0 0 / 0 0 / 0400 % 1 / 2 0 / 0 0 / 0 0 / 0 0 / 0320 0 % 0 / 0 0 / 0 0 / 0 0 / 0 0 / 0 0 % 1 / 2 0 / 0 0 / 0 0 / 0 0 / 0400 % 2 / 1 0 / 0 0 / 0 0 / 0 0 / 0400 % 2 / 1 0 / 0 0 / 0 0 / 0 0 / 0321 0 % 0 / 0 0 / 0 0 / 0 0 / 0 0 / 0 0 % 0 / 0 0 / 0 0 / 0 0 / 0 0 / 0400 % 2 / 1 0 / 0 0 / 0 0 / 0 0 / 0400 % 0 / 0 0 / 0 0 / 0 0 / 0 0 / 0325 0 % 0 / 0 0 / 0 0 / 0 0 / 0 0 / 0 0 % 0 / 0 0 / 0 0 / 0 0 / 0 0 / 0400 % 1 / 2 0 / 0 0 / 0 0 / 0 0 / 0400 % 0 / 0 0 / 0 0 / 0 0 / 0 0 / 0__________________________________________________________________________ conditions : a = p80a controls , stress relieved b = p55d controls c = p80a / p55de d = p80a / p55d e = p80a / p55d no esc was observed on any of the co - extruded tubing samples . no esc was found on the pellethane 55d control ( negative ) samples , whereas four pellethane 80a control ( positive ) samples had areas of shallow esc . pellethane 2363 - 80a tubing was coated with a solution of enka polyurethane pur 981 , medtronic biostable polyurethane , and alternatively , pellethane 2363 - 55d . a 12 % stock solution of the coating polymer in dmac was cut to a 2 % solution and used to dip coat cut segments of pellethane 80a tubing . a 2 % solution of pellethane 80a containing blue dextran in dmac and a 2 % solution of mdx silicone in hexane were also used to dip coat cut segments of pellethane 80a tubing . the p80a tubing segments were cleaned in isopropyl alcohol and then dipped and withdrawn smoothly from a cylinder containing the overcoat polymer . the dipped tubing was allowed to drip several seconds and then hung in a 40 ° c . forced air oven with the circulation off . after 10 to 20 minutes the air and heat were turned on for at least one hour between each coat . a total of 4 coats was applied . finally , the coated tubes were dried overnight at 40 ° c . in the oven with heat and air circulation on . the mdx tubing samples were dried for an additional 2 days . pellethane 80a tubing segments were also surface - grafted with an acrylamide solution . clean p80a tubing segments were placed into a 40 % acrylamide in di water solution containing ceric ion for 25 minutes . the ceric ion causes the graft copolymerization of acrylamide on the surface of the tubing . following surface - grafting the samples were rinsed thoroughly in di water . segments of pellethane 80a / 55de coextruded tubing and pellethane 80a tubing were also tested in this study for esc resistance . the tubing specimens in this study were tested and analyzed for esc according to the procedures described in example 1 , the results of which are set forth in table 3 shown below . table 3__________________________________________________________________________biostability study of surface treatmentson pellethane 2363 - 80a tubing strained 400 % rabbit # conditions a b c d e f g h__________________________________________________________________________104 0 / 0 0 / 0 0 / 0 1 / 2 0 / 0 0 / 0 0 / 0 0 / 00 / 0 0 / 0 0 / 0 2 / 3 0 / 0 0 / 0 0 / 0 0 / 0105 5 / 1 5 / 3 5 / 4 5 / 4 5 / 1 0 / 0 0 / 0 2 / 20 / 0 0 / 0 0 / 0 4 / 1 5 / 1 0 / 0 0 / 0 5 / 1107 5 / 1 5 / 2 5 / 2 5 / 3 0 / 0 0 / 0 0 / 0 0 / 01 / 1 2 / 2 5 / 1 5 / 4 5 / 1 0 / 0 0 / 0 5 / 2108 5 / 1 5 / 1 4 / 2 5 / 3 0 / 0 0 / 0 0 / 0 1 / 20 / 0 0 / 0 0 / 0 1 / 2 5 / 1 0 / 0 0 / 0 1 / 3109 1 / 2 5 / 1 5 / 1 5 / 1 0 / 0 0 / 0 0 / 0 5 / 30 / 0 4 / 1 5 / 4 5 / 4 5 / 1 0 / 0 0 / 0 4 / 1110 0 / 0 5 / 2 5 / 1 5 / 3 5 / 1 0 / 0 0 / 0 0 / 00 / 0 0 / 0 0 / 0 0 / 0 5 / 1 0 / 0 0 / 0 1 / 5__________________________________________________________________________ conditions : a . enka b . biostable c . mdx silicone d . acrylamide grafted e . blue dextran / p80a f . p55d g . p55de coextruded p80a h . pellethane 236380a ( control ) no esc was observed on the p55d dip coated and p55de coextruded samples . all other tubing specimens had varying amounts of esc . this completes the description of the preferred and alternate embodiments of the invention . those skilled in the art may recognize other equivalents to the specific embodiment described herein which equivalents are intended to be encompassed by the claims attached hereto . while this invention may be embodied in many different forms , described in detail herein specific preferred embodiments of the invention . the present disclosure is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiments illustrated .
the present invention provides resistance to degradation from environmental stress cracking and metal ion induced oxidation for implantable , flexible pacing lead insulators having a body of polyether polyurethane elastomer material . a thin layer of a second polyurethane elastomer is applied as an overcoat to the lead insulator body . the second polyurethane is more resistant to esc and mio than the elastomer comprising the body of the insulator . because esc and mio are surface phenomina , only a thin layer of the second material is required and the mechanical properties of the base material will determine the overall mechanical properties of the lead insulator .