the present invention provides polyurethanes and polyurethane / ureas which are particularly suited to rapid prototyping techniques such as fused deposition modelling and therefore have specific characteristics as described in the preamble of this specification . in a preferred form , this invention provides a biocompatible biodegradable polyurethane or polyurethane / urea comprising diisocyanates , polyol of molecular weight 200 - 600 and a conventional chain extender and / or a chain extender having a hydrolysable linking group . isocyanates suitable for preparing polyurethanes and polyurethane / ureas according to the invention include but are not limited to the following : polyols or “ soft segments ” which may be used to prepare the polyurethanes and polyurethane / ureas of the invention are most preferably those having a molecular weight of 200 - 400 . the structure of the polyol in the present invention is preferably : where h and / or k can equal 0 ( as is the case of the dimer , eg , h = 0 , j = 1 and k = 1 ) or are integers as is j and r ″ and r ′″ independently of each other are hydrogen , hydroxy alkyl , aminoalkyl ( both primary and secondary ) or carboxy alkyl and r and r ′ cannot be hydrogen , but can be a linear or branched alkyl , alkenyl , aminoalkyl , alkoxy or aryl . the molecular weight of the entire structure is more preferably 120 to 400 . less preferably the molecular weight can be up to 2000 and much less preferably above 2000 . four examples of suitable soft segments are as follows : poly ( ε - caprolactone ) diol , mw 400 ( from example 1 ): where r is ( ch 2 — ch 2 ), r ′ is ( ch 2 ) 5 , r ″ and r ′″ are both h , and j = 1 and ( h + k )= 2 . 96 ( glycolic acid - ethylene glycol ) dimer ( from example 8 ): where r is ( ch 2 — ch 2 ), r ′ is ( ch 2 ), r ″ and r ′″ are both h , j = 1 and ( h + k )= 1 poly ( ethylene glycol ), mw 400 ( from example 4 ): h = 0 , k = 0 , j =˜ 13 , r is ( ch 2 — ch 2 ), and r ″ and r ′″ are both h poly ( ethylene glycol ) bis ( 3 - aminopropyl ) terminated ( aldrich ): where r is ( ch 2 — ch 2 ), r ″ and r ′″ are both —( ch 2 ) 3 nh 2 , j = 34 and ( h + k )= 0 either or both of r and r ′ can contain nonlinear structures , for example where r ′═( ch 2 chch 3 ) which is lactic acid . however , the r and r ′ should preferably not contain groups such as oh and nh 2 which are likely to cause crosslinking . suitable compounds include but are not limited to the following polyester polyols : pga — poly -( glycolic acid ) diol , where r is typically —( ch 2 ch 2 )— pla — poly -( lactic acid ) diol , where r is typically —( ch 2 ch 2 )— examples of other polyols which may act as soft segments include poly -( 4 - hydroxybutyrate ) diol ( p4hb diol ), poly -( 3 - hydroxybutyrate ) diol ( p3hb diol ), polypropylene glycol and any copolymers of the aforesaid including plga diol , p ( la / cl ) diol , p ( 3hb / 4hb ) diol . chain extenders according to the invention are any low molecular weight molecule having two or more functional groups which when reacted with diisocyanates form a urethane or urea linkage . preferably the chain extender is difunctional and examples of such chain extenders are diols , dithiols , diamines , amino alcohols and dicarboxylic acids . diols are also relatively non - toxic and can be resorbed or excreted from the body upon degradation and examples include ethylene glycol , diethylene glycol , tetraethylene glycol , 1 , 3 - propane diol , 1 , 4 - butane diol , 1 , 6 - hexane diol , ga - eg dimer , la - eg dimer , trimers including a combination of la and / or ga and eg , and oligomeric diols such as dimers and trimers . examples of amines that may be used are butane diamine , ethanolamine , glycine and lysine . incorporated into the hard segment , these chain extenders increase degradation . esters in the hard segment degrade much faster than urethane linkages . the following chain extenders are illustrated : preferred polyurethane and polyurethane / ureas prepared according to the invention may utilise pcl diol , pga diol , pla diol or peg diol and hdi / eg as the hard segment . another preferred polyurethane or polyurethane / urea according to the invention includes a diol of poly ( 4 - hydroxybutyrate ) or copolymers therewith to give an improved range of properties and degradation rates . according to the present invention , the monomeric units of the polyurethanes or polyurethane / ureas of the invention are preferably reacted by bulk polymerisation to form a straight - chain poly -( ester - urethane ) block copolymer . catalysts such as titanium butoxide , tyzor - la , stannous octoate , ferric acetyl acetonate , magnesium methoxide , zinc octoate , manganese 2 - ethyl hexanoate , amine catalyst may , if desired , be used in such polymerisation . the general form of the repeat units in the polymer after polymerisation is : where r 1 is from the diisocyanate e . g . hexamethylene diisocyanate . r 2 is from a low molecular weight diol chain extender e . g . ethylene glycol . r 3 is from a soft segment diol e . g . pcl diol ( mw 400 ). the pronumeral ‘ n ’ represents the average number of repeat units in the hard segment . the pronumeral ‘ p ’ is proportional to the molecular weight of the polymer and includes both the hard segment repeat units and the soft segment . in a preferred embodiment of the invention , the hard segment represents 20 to 100 % by weight of the polyurethane / polyurethane / urea . more preferably the hard segment represents 60 to 70 % by weight . the polyol and chain extender may be the same compound and this corresponds to the embodiment where the hard segment corresponds to 100 % by weight of the polyurethane / polyurethane / urea . it has been found that there must be a reasonably high proportion of hard segment for the materials to have adequate properties to extrude through fdm as well as a reasonably high melt flow index . the following examples are not intended to limit the invention but rather illustrate the nature of the broad invention and its applicability . materials : the pcl diol ( molecular weight 402 . 1 ) from era polymer pty was dried at 90 ° c . for 4 hours under vacuum ( 0 . 1 torr ). ethylene glycol ( aldrich ) was degassed at 90 ° c . under vacuum ( 0 . 1 torr ) for three hours and hdi ( aldrich ) was used as received . a polyurethane composition based on a mixture of pcl diol , eg and hdi was prepared by a one - step bulk polymerisation procedure . stannous octoate ( aldrich ) was kept moisture - free and used as received . a mixture of pcl ( 25 . 000 g ) and eg ( 9 . 696 g ) and stannous octoate ( 0 . 0714 g ) was placed in a 100 ml predried polypropylene beaker , covered with aluminium foil and heated to 70 ° c . under nitrogen in a laboratory oven . hdi ( 36 . 732 g ) was weighed in a separate wet - tared predried polypropylene beaker and added to the pcueg / stannous octoate beaker and stirred manually until gelation occurred ( 90 seconds ), at which time the viscous mixture was poured onto a teflon coated metal tray to cure at 100 ° c . for a period of about 18 hours . the resulting polymer was clear , colourless and tough . a sample of the polymer after curing was compression moulded at 175 ° c . to a 1 mm thick flat sheet for tensile testing . dumbbells punched from the sheet were tensile tested using an instron model 5568 universal testing machine . the mechanical properties of the materials prepared in example 1 were examined and the results are shown in table 1 . the solid polymer sheet was chopped into about 1 cm 3 pieces with clean tin - snips , cooled in liquid nitrogen and ground into a powder using a cryogrinder . the polymer powder was then dried at 100 ° c . under vacuum overnight . the polymer was extruded on a mini - extruder equipped with a 1 . 7 mm die at 180 ° c . and 40 rpm . the polymer was taken off by a belt conveyor and cooled at ambient temperature in air without water bath . the filament was spooled and kept under nitrogen in a moisture - free environment for at least one week prior to use . the polymer filament was fed though the fdm apparatus and a small lattice was made to show that the material was suitable for fdm . the scaffolds were characterised by light microscopy and sem and were shown to have very good precision and weld . it has been shown to work with a number of commercially available nozzle diameters . the operating envelope temperature inside the machine was 25 ° c . and the heating zone was set at 168 ° c . sem micrographs and optical microscopy of fdm scaffolds are shown in fig1 - 6 . preparation of 12tm1 ( a softer material than example 1 , 60 % hard segment , 40 % pcl diol 400 ) materials : the pcl diol ( molecular weight 402 . 1 ) from era polymer pty was dried at 90 ° c . for 4 hours under vacuum ( 0 . 1 torr ). ethylene glycol ( aldrich ) was degassed at 90 ° c ./ 0 . 1 torr for 3 hours and hdi ( aldrich ) was used as received . a polyurethane composition based on a mixture of pcl diol , eg and hdi was prepared by a one - step bulk polymerisation procedure . stannous octoate ( aldrich ) was kept moisture - free and used as received . a mixture of pcl ( 40 . 0 g ) and eg ( 11 . 663 g ) and stannous octoate ( 0 . 100 g ) was placed in a 100 ml predried polypropylene beaker , covered with aluminium foil and heated to 70 ° c . under nitrogen in a laboratory oven . hdi ( 48 . 337 g ) was weighed in a separate wet - tared predried polypropylene beaker , covered and then added to the pcueg / stannous octoate beaker and stirred manually until gelation occurred ( 90 seconds ). the viscous mixture was poured onto a teflon coated metal tray to cure at 70 ° c . for a period of about 18 hours . the resulting polymer was clear , colourless and tough . a sample of the polymer after curing was compression moulded at 170 ° c . to a 1 mm thick flat sheet for tensile testing . dumbbells punched from the sheet were tensile tested using an instron model 5568 universal testing machine . the mechanical properties of the materials prepared in example 2 were examined and the results are shown in table 1 . the solid polymer sheet was chopped into about 1 cm 3 pieces with clean tin - snips , cooled in liquid nitrogen and ground into powder using a cryogrinder . the polymer powder was then dried at 70 ° c . under vacuum overnight . the polymer was extruded on the mini - extruder equipped with a 1 . 7 mm die at 175 ° c . and 35 - 40 rpm . the polymer was taken off on a rotating shaft and cooled at ambient temperature in air without water bath . the filament was spooled and kept under nitrogen in a moisture - free environment for at least one week prior to use . the polymer filament was fed though the fdm apparatus and a small lattice was made to show that the material was suitable for fdm . preparation of 12tm6 ( a harder material than example 1 , 70 % hard segment , 30 % pcl diol 400 ) materials : the pcl diol ( molecular weight 402 . 1 ) from era polymer pty was dried at 90 ° c . for 4 hours under vacuum ( 0 . 1 torr ). ethylene glycol ( aldrich ) was degassed at 90 ° c ./ 0 . 1 torr for 3 hours and hdl ( aldrich ) was used as received . a polyurethane composition based on a mixture of pcl diol , eg and hdi was prepared by a one - step bulk polymerisation procedure . stannous octoate ( aldrich ) was kept moisture - free and used as received . a mixture of pcl ( 21 . 0 g ) and eg ( 10 . 840 g ) and stannous octoate ( 0 . 070 g ) was placed in a 100 ml predried polypropylene beaker , covered with aluminium foil and heated to 70 ° c . under nitrogen in a laboratory oven . hdi ( 38 . 160 g ) was weighed in a separate predried polypropylene beaker and added to the pcl / eg / stannous octoate beaker and stirred until gelation occurred ( 60 seconds ), at which time the viscous mixture was poured onto a teflon coated metal tray to cure at 100 ° c . for a period of about 18 hours . the resulting polymer was clear , colourless and tough . a sample of the polymer after curing was compression moulded at 175 ° c . to a 1 mm thick flat sheet for tensile testing . dumbbells punched from the sheet were tensile tested using an instron model 5568 universal testing machine . the mechanical properties of the materials prepared in example 3 were examined and the results are shown in table 1 . the solid polymer sheet was chopped into about 1 cm 3 pieces with clean tin - snips , cooled in liquid nitrogen and ground into powder using a cryogrinder . the polymer powder was then dried at 70 ° c . under vacuum overnight . the polymer was extruded on the mini - extruder equipped with a 1 . 7 mm die at 175 ° c . and 40 rpm . the polymer was taken off on a rotating shaft and cooled at ambient temperature in air without water bath . the filament was spooled and kept under nitrogen in a moisture - free environment for at least one week prior to use . the polymer filament was fed though the fdm apparatus and a small lattice was made to show that the material was suitable for fdm . materials : the peg diol ( molecular weight 394 . 7 ) from aldrich was dried at 90 ° c . for 4 hours under vacuum ( 0 . 1 torr ). ethylene glycol ( aldrich ) was degassed at 90 ° c ./ 0 . 1 torr for three hours and hdi ( aldrich ) was used as received . a polyurethane composition based on a mixture of pcl diol , eg and hdi was prepared by a one - step bulk polymerisation procedure . stannous octoate ( aldrich ) was kept moisture - free and used as received . a mixture of peg ( 20 . 000 g ) and eg ( 7 . 715 g ) and stannous octoate ( 0 . 0571 g ) was placed in a 100 ml predried polypropylene beaker , covered with aluminium foil and heated to 70 ° c . under nitrogen in a laboratory oven . hdi ( 29 . 428 g ) was weighed in a separate predried polypropylene beaker , and added to the peg / eg / stannous octoate beaker and stirred until gelation occurred ( 150 seconds ), when the viscous mixture was poured onto a teflon coated metal tray to cure at 70 ° c . for a period of about 18 hours . the resulting polymer was clear , colourless and tough . a sample of the polymer after curing was compression moulded at 150 ° c . to a 1 mm thick flat sheet for tensile testing . dumbbells punched from the sheet were tensile tested using an instron model 4032 universal testing machine . the solid polymer sheet was chopped into about 1 cm 3 pieces with clean tin - snips , cooled in liquid nitrogen and ground into powder using a cryogrinder . the polymer powder was then dried at 100 ° c . under vacuum overnight . the polymer was extruded on the mini - extruder equipped with a 1 . 7 mm die at 150 ° c . and 40 rpm . the polymer was taken off by a belt conveyor and cooled at ambient temperature in air without water bath . the filament was spooled and kept under nitrogen in a moisture - free environment for at least one week prior to use . the polymer filament was fed though the fdm apparatus and a small lattice was made to show that the material was suitable for fdm . the scaffolds were characterised by light microscopy and sem and were shown to have very good precision and weld . it has been shown to work with a number of commercially available nozzle diameters . the operating envelope temperature inside the machine was 25 ° c . and the heating zone was set at 168 ° c . materials : the peg diol ( molecular weight 394 . 7 ) from aldrich was dried at 90 ° c . for 4 hours under vacuum ( 0 . 1 torr ). ethylene glycol ( aldrich ) was degassed at 90 ° c ./ 0 . 1 torr for 3 hours . methyl ester of lysine diisocyanate mldi ( kyowa hakko kogyo co . ltd ) was used as received . a polyurethane composition based on a mixture of pcl diol , eg and hdi was prepared by a one - step bulk polymerisation procedure . stannous octoate ( aldrich ) was kept moisture - free and used as received . a mixture of peg ( 12 . 814 g ) and eg ( 16 . 380 g ) and stannous octoate ( 0 . 0992 g ) was placed in a 100 ml predried polypropylene beaker , covered with aluminium foil and heated to 70 ° c . under nitrogen in a laboratory oven . mldi ( 70 . 00 g ) was measured in a separate predried polypropylene beaker and added to the beaker containing mixture of peg / eg / stannous octoate and stirred until gelation occurred (˜ 300 seconds ), at which time the viscous mixture was poured onto a teflon coated metal tray to cure at 70 ° c . for a period of about 18 hours . the resulting polymer was clear , slightly golden in colour and tough . a sample of the polymer after curing was compression moulded at 175 ° c . to a 1 mm thick flat sheet for tensile testing . materials : the eg ( aldrich ) was degassed at 90 ° c ./ 0 . 1 torr for three hours . mldi ( kyowa hakko kogyo co . ltd ) was used as received . a polyurethane composition based on a 1 to 1 ratio of mldi and eg was prepared by a one - step bulk polymerisation procedure . stannous octoate ( aldrich ) was kept moisture free and used as received . eg ( 22 . 000 g ) and stannous octoate ( 0 . 0972 g ) were weighed into a 100 ml predried polypropylene beaker , covered with aluminium foil and heated to 70 ° c . under nitrogen in a laboratory oven . mldi ( 75 . 214 g ) was measured in a separate predried polypropylene beaker , covered with aluminium foil and also heated under nitrogen at 70 ° c . before being added to the eg / stannous octoate and stirred until gelation occurred (˜ 700 sec ), at which time the viscous mixture was poured onto a teflon coated metal tray to cure at 70 ° c . overnight for a period of about 18 hours . the resulting polymer was clear , golden in colour , very hard and brittle . the melt flow index of the material prepared was measured to be 136 g / 10 min with a 2 . 16 kg load . preparation of 12tm19 illustrating shape memory effects ( 100 % hard segment using mld1 and 2 - ethyl - 1 , 3 - hexanediol ) materials : the 2 - ethyl - 1 , 3 - hexanediol ( fluka ) was degassed at 90 ° c ./ 0 . 1 torr for 3 hours . mldi ( kyowa hakko kogyo co . ltd ) was used as received . a polyurethane composition based on a 1 to 1 ratio of mldi and 2 - ethyl - 1 , 3 - hexanediol was prepared by a one - step bulk polymerisation procedure . stannous octoate ( aldrich ) was kept moisture free and used as received . 2 - ethyl - 1 , 3 - hexanediol ( 8 . 269 g ) and stannous octoate ( 0 . 021 g ) were weighed into a 100 ml predried polypropylene beaker , covered with aluminium foil and heated to 70 ° c . under nitrogen in a laboratory oven . mldi ( 12 . 000 g ) was measured in a separate predried polypropylene beaker , covered with aluminium foil and also heated under nitrogen at 70 ° c . before being added to the 2 - ethyl - 1 , 3 - hexanediol / stannous octoate and stirred until gelation occurred (˜ 30 min ), at which time the viscous mixture was poured onto a teflon coated metal tray to cure at 70 ° c . overnight for a period of about 18 hours . the resulting polymer was clear , golden in colour , very hard and brittle . dsc was taken on a mettler dsc 30 and showed the tg to be ˜ 30 ° c . when left at room temperature it was hard and brittle but it reversibly softened in the hand and became elastic . 22 . 19 g of glycolic acid ( ga ) ( sigma ) was heated at 200 ° c . under nitrogen outgassing in a round bottomed flask equipped with a stillhead sidearm and condenser to collect the water runoff . after 18 hours the nitrogen was stopped and vacuum applied ( 0 . 1 torr ), by which stage the ga had polymerised to a white solid ( pga ). dry ethylene glycol ( eg ) ( aldrich ) ( 106 g ) was added to the pga in an approximate ratio of 5 : 1 in order to transesterify the polymer . this was refluxed for a period of 8 hours in total and was followed by gpc until there were three major products : eg , eg - ga and some eg - ga - ga . the eg was removed under vacuum and heat and the resulting chain extender was used to make a polyurethane ( 16tm7 ). preparation of a polyurethane using a hydrolysable chain extender ( 16tm7 from example 8 ) materials : the 15tm7 ( ga - eg diol chain extender ) was degassed at 90 ° c ./ 0 . 1 torr for three hours , as was the pcl diol ( mw400 ). hdi ( aldrich ) was used as received . a polyurethane composition based on an 80 % hard segment composition was prepared by a one - step bulk polymerisation procedure . stannous octoate ( aldrich ) was kept moisture free and used as received . 15tm7 ( 30 . 73 g ) and pcl diol ( mw402 . 099 ) ( 20 . 05 g ) and stannous octoate ( 0 . 100 g ) were weighed into a 100 ml predried polypropylene beaker , covered with aluminium foil and heated to 70 ° c . under nitrogen in . a laboratory oven . hdi ( 49 . 47 g ) was measured in a separate predried polypropylene beaker , covered with aluminium foil and also heated under nitrogen at 70 ° c . before being added to the pcl diol / 15tm7 / stannous octoate mixture and stirred until gelation occurred when the viscous mixture was poured onto a teflon coated metal tray to cure at 70 ° c . overnight for a period of about 18 hours . the resulting polymer was slightly cloudy , hard but flexible . table 2 melt flow index of various materials the melt flow index of various materials according to the present invention was calculated , along with the readily available commercial materials : acrylonitrile butadiene styrene ( abs ), polyamide and investment casting wax ( icw ). in order to be suitable for fdm , the materials of the present invention preferably should have a mfi which is similar or higher than that of the commercial samples , without significant degradation of the material . mfi ( g / 10 min ), 2 . 16 kg material temperature (° c .) weight abs 270 8 . 5 polyamide 140 75 icw 73 9 . 5 14tm3 - 1 160 7 . 64 12tm4 - 6 165 10 . 43 16tm9 160 136 it will be appreciated that the scope of the invention is not limited to the specific examples described herein but extends to the general inventive concepts defined . none of the examples should be considered limiting . this example illustrates the cell compatibility of scaffolds fabricated using polymers prepared according to the invention . polymers were prepared according to the procedure disclosed in example 1 and 3d scaffolds were fabricated using the procedure described in example 1a . three dimensional scaffolds similar to those shown in fig1 to 3 were seeded with primary ovine fibroblasts explanted from the aortic heart - valve leaflet . the cells were grown in dulbecco &# 39 ; s modified eagle &# 39 ; s medium ( dmem ) in static culture for a period of nine weeks . the temperature was 37 ° c . and incubator contained 5 % co 2 ( g ). the dmem was replaced every five days . at the end of the nine weeks the scaffolds were cross linked using glutaraldehyde and then dehydrated progressively through ethanol and dried . sem micrographs and optical microscopy of the cell - seeded fdm scaffolds are shown in fig7 - 11 . this example illustrates the preparation of polyurethanes by varying the weight percentage of hard segment , the molecular weight of the soft segment polyol and the type of polyol . the quantities of the diisocyanate , polyol and the chain extender used are summarised in table 3 . the following example illustrates the procedure used in making sample with code tm1 - 9 in table 3 . other materials in the table were prepared accruing the same one - step polymerisation procedure . materials : the pcl diol ( molecular weight 1000 ) from era polymer pty ltd was dried at 90 ° c . for four hours under vacuum ( 0 . 1 torr ). ethylene glycol ( aldrich ) was distilled and degassed at 90 ° c . under vacuum ( 0 . 1 torr ) for three hours . ethyl - ldi was distilled before use . stannous octoate ( aldrich ) was used as received . a polyurethane composition based on a mixture of pcl diol , eg and eldi was prepared by a one - step bulk polymerisation procedure . a mixture of pcl diol ( 20 . 000 g ) and eg ( 3 . 336 g ) and stannous octoate ( 0 . 040 g ) were placed in a 100 ml predried glass beaker , covered with aluminium foil and heated to 70 ° c . under nitrogen in a laboratory oven . eldi ( 16 . 665 g ) was weighed in a separate wet - tared predried polypropylene beaker and added to the pcl / eg / stannous octoate beaker , covered with aluminium foil and heated to 70 ° c . under nitrogen in a laboratory oven . eldi was then added to the pcl / eg / stannous octoate beaker and stirred manually until gelation occurred at which time the viscous mixture was poured onto teflon coated metal tray to cure at 100 ° c . for a period of about 18 hours . the resulting polymer was clear , colourless and rubbery . the molecular weight of the polymer was determined by gel permeation chromatography and the results reported in table 3 are relative to polystyrene standards . the polymers tm1 - 9 , tm1 - 11 , tm1 - 14 , tm1 - 15 and tm1 - 16 were dissolved separately in tetrahydrofuran to make 5 %, 10 % and 20 % solutions . the solutions were used to coat stainless steel coupons by dip - coating and by spin coating ( spin coater : model ws - 400b - 6npp / lite , laurell technologies corporation ). the coatings adhered well to the stainless steel showing their suitability for coating metallic surfaces . these polymers were also soluble in solvents such as chloroform , dichloromethane , dimethyl formamide and dimethyl acetamide . the following example illustrates the preparation of strands , fibres and tubes using a reactive extruder ( prism model ) polyurethanes were produced on a prism 16 mm twin screw extruder of l / d = 26 : 1 via liquid feed of the diisocyanate , polyester polyol , ethylene glycol and catalyst . methyl ester lysine diisocyanate ( m - ldi ), polycaprolactone diol gmw ˜ 426 ( era 2043 ), chain extender ethylene glycol , and catalyst stannous 2 ethyl hexanoate were used as reagents to prepare polyurethanes with hard segment weight percentage of 65 and 95 %. the ratio of isocyanate to hydroxyl was kept at 1 : 1 and the catalyst loading was 0 . 1 wt %. the throughput rate was ˜ 2 g / min and the reaction was controlled via extruder screw speed ( for mixing control ) and via the temperature settings across the 6 individual barrel sections and the dies . materials based on 95 and 65 % hard segment produced good tubes and filaments . a cross - linked polyurethane was produced using this technique by replacing 40 % of the ethylene glycol with trimethylol propane in the 65 % hard segment polyurethane formulation . a cross linked polyurethane material was produced following a one - step procedure as described below . a mixture of pre - dried ( degassed ) macrodiol peg ( 2 . 5 g , mw 394 . 75 ), ethylene glycol ( 18 . 77 g ), trimethylol propane ( 1 . 50 g , 40 mol % of eg ) and catalyst dibutyltin dilaurate ( 0 . 1 wt %) were weighed in a polypropylene beaker . the polymer mixture was then degassed at 70 ° c . for about an hour under a vacuum of 1 torr at 70 ° c . eldi ( 7 . 10 g ) was weighed in a syringe and added to the polyol mixture and stirred rapidly for about 3 minutes and then poured into a teflon - coated metal pan and pressed under a nominal load of 8 tonn for 2 hours at 100 ° c . followed by further curing in a nitrogen - circulating oven 16 hours . the polymer showed maximum tensile stress ( 34 ± 3 mpa ), youngs modulus ( 1 . 0 + 0 . 2 mpa ) and elongation at break 156 ± 32 %). a mixture of pre - dried ( degassed ) macrodiol peg ( 10 . 0 g , mw 394 . 75 ); ethylene glycol ( 7 . 17 g ) and catalyst dibutyltin dilaurate ( 0 . 1 wt %) was weighed in a polypropylene beaker . the polymer mixture was then degassed at 70 ° c . for about an hour under a vacuum of 1 torr at 70 ° c . eldi ( 32 . 82 g ) was weighed in a syringe and added to the polyol mixture and stirred rapidly for about 3 minutes and then poured into a teflon - coated metal pan and pressed under a nominal load of 8 tonne for 2 hours at 100 ° c . followed by further curing in a nitrogen - circulating oven 16 hours . gpc showed molecular weight ( mp ) 112 , 000 and had maximum tensile stress ( 10 ± 0 . 5 mpa ), young &# 39 ; s modulus ( 3 . 7 + 0 . 4 mpa ) and elongation at break 301 ± 6 %).
this invention relates to biocompatible , biodegradable thermoplastic polyurethane or polyurethane / ureas comprising isocyanate , polyol and a conventional chain extender and / or a chain extender having a hydrolysable linking group and their use in tissue engineering and repair applications , particularly as stents and stent coating .