Segmented polyurethanes based on triblock copolyether as biomedical materials.

Segmented polyurethanes based on triblock copolyether as biomedical materials

Biomedical materials are usually employed in contact with biomedical tissues, organs or bodies, and the compatibility between the materials and the tissues is essential. However, it is recognized to be very difficult to synthesize an exact material having the same physiological, chemical and physical properties as the living tissues. Therefore much more research on biocompatible materials is needed for the progress of biomedical engineering.

Polyurethanes have been known to exhibit excellent mechanical properties [refs. 1-7] and a good thromboresistance [refs. 1-5, 8-12]. These properties are explained in terms of their microphase separated structure [refs. 3-11]. Among biomedical materials, Biomer (Ethicon) is the most well known, and it was used as one of the raw materials to fabricate the implantable artificial hearts [ref. 5]. Biomer is chemically a segmented polyurethaneurea (SPUU) prepared from hydroxyl-terminated poly(oxytetramethylene) (HT-PT), 4,4'-diphenylmethane diisocyanate (MD1) and ethylenediamine (ED). Pellethane (Upjohn Chemical) is a commercially available biomedical-grade segmented polyurethane prepared from HT-PT, MDI and 1,4-buthanediol [ref. 12]. Recently, quite a few polyurethanes were reported to show a good blood compatibility. For example, segmented polyurethaneurea from poly(oxyethylene)-poly (oxypropylene)-poly(oxyethylene) [ref. 13], poly (oxyethylene)-grafted polyurethane [ref. 14] and heparin-bonded polyurethane [ref. 15], to name a few. In many cases, the introduction of poly(oxyethylene) (PEO) units was found to improve the biocompatibility.

In this article we described the properties of polyurethane and polyurethaneurea from triblock copolyether (HT-ETE), the center block of which is poly(oxytetramethylene) (PTHF), and the endblocks are PEO. The chemical structures of our segmented polyurethane, SEU, and polyurethaneurea, SEUU, are shown in figure 1. SEU and SEUU are expected to show better biocompatibility than SPU and SPUU, which are the polyurethane and polyurethaneurea using HT-PT, i.e., poly(oxytetramethylene) glycol as a prepolymer. Morphology and mechanical properties of these polymers are investigated in connection with their antithrom-bogenicity.

Experimental

Materials

The poly(oxytetramethylene) glycol (Sanyo Chemical Industries Ltd., HT-PT, Mn=1830) was dried for several days at 40-50 [degrees] C under vacuum. Ethylene oxide (EO) was distilled several times in the presence of calcium hydride and sealed into glass ampoules under vacuum. Ethylenediamine (ED) and ethylene glycol (EG) were distilled in the presence of sodium and calcium hydride, respectively. N,N'-dimethylacetamide (DMAc) was distilled under a reduced pressure (60 [degrees] C/11 mm Hg). 4,4'diphenylmethane diisocyanate (MDI, Tokyokasei Co., Ltd.) and other reagents were used without further purifications.

Synthesis of HT-ETE

Hydroxyl-terminated PEO-PTHF-PEO triblock copolyether (HT-ETE) was synthesized by anionic polymerization of EO at 45 [degrees] C using potassium salt of HT-PT as an initiator [ref. 16]. The EO polymerization was stopped by 2-propanol solution of hydrochloric acid. The results on characterization of HT-ETE are shown in table 1.

Syntheses of segmented polyurethane and polyurethaneurea

SEU and SEUU were synthesized by the prepolymer method as shown in figure 2. In DMAc, HT-ETE was allowed to react with MDI in the presence of 1,8-diazabicyclo [5.4.0]undecene-7 (DBU) at 30 [degrees] C for 1.5 h, followed by dropwise addition of EG for SEU or ED for SEUU in DMAc at 30 [degrees] C to carry out the chain-extension reaction [refs. 13, 16]. After the addition of chain-extender, temperature was increased to 50 [degrees] C within 30 min. The molar ratio of HT-ETE, MDI and chain-extender was 1/2/1. The polymer was recovered by precipitation in water and dried under vacuum at 50 [degrees] C. The solid polymers were subjected to the Soxlet-extraction by acetone for 48 h before use.

Analytical measurements

The number average molecular weight was measured by a vapor pressure osmometer (VPO) (Knauer Co., Ltd.) in benzene at 37 [degrees] C. For gel-permeation chromatography (GPC) measurement, HLC-802UR (Tosoh Co., Ltd.) was used to obtain [M.sub.n], [M.sub.w] and the molecular weight distribution ([M.sub.w]/[M.sub.n]). The eluent was THF, and its flow rate was1 ml/min and the column temperature was 40 [degrees] C. The measurement of nuclear magnetic resonance (NMR) was carried out by Varian T-60A Spectrometer using [CDC1.sub.3] solution and tetramethylsilane as an internal standard. The titration of hydroxyl-group content was carried out by the acetylation method. The differential scanning calorimetry (DSC) was measured on a DSC-8230 (Rigaku International Co., Ltd.), and the heating rate was 10 [degrees] C/min. The infrared (ir) spectroscopic measurement was carried out by a Spectrophotometer 215 (Hitachi Ltd.).

Evaluation of antithrombogenicity

The Lee-White clotting test [refs. 16 and 17].

The inside wall of glass test tube (inner diameter, 10 mm) was coated with a sample as homogeneously as possible, and fresh whole human blood (1 ml) was put into the tube. It was kept at 37 [degrees] C, and the clotting was observed to determine the time of coagulation. The ratio of the coagulation time of sample coated tube to that of glass tube was defined as coagulation time index (CTI).

The platelet adhesion test [ref. 16].

Sample film was put in contact with a drop of whole human blood for 1.5 min., then rinsed with buffer solution, and fixed by glutaraldehyde solution. After the replacement of water to methanol, it was dried using the critical-point drying method using [CO.sub.2]. Au (for SPUU and SEUU) or Au/Pt (for SPU and SEU) vapor was deposited on the film surface, which was observed by a Hitachi-Akashi Mini SEM.

Mechanical measurements Tensile tests were carried out by a tensile tester TOM/200D (Shinko Tushin Kogyo Co., Ltd.) on dry or wet films (40-50 mm x 5 mm x ca. 200-500 [micro] m) made by the casting of DMAc solution of the polymers. The evaporation was carried out under a reduced pressure between r.t. and 50 [degrees] C. Wet samples were prepared by soaking the films in normal saline solution at 37 [degrees] C for two weeks or for six months. The temperature dispersions of dynamic modulus and dynamic loss were determined with a dynamic mechanical analyzer RD 1100-AD (Rhesca Co., Ltd.). The sample dimensions were 40 mm x 10 mm x ca. 300 [micro] m. Stress relaxation measurement was conducted in normal saline solution at 37 [degrees] C, the initial elongation being 30% [ref. 18].

Results and discussion

Synthesis of polyurethane

The synthetic conditions and a few results of characterization of segmented polyurethanes and polyurethaneureas are shown in table 2. The reaction conditions were optimized in order to get soluble products. The GPC of polyurethanes was that of soluble fractions in THF, and the molecular weight was that of hydrodynamically equivalent polystyrene. Though the soluble parts were well over 80% for these polyurethanes, the observed molecular weights were not very high. Polyurethaneureas were not soluble in THF.

Antithrombogenicity

Table 3 shows the antithrombogenicity together with some properties of their prepolymers. We already reported that SEUU showed good blood-compatibility both in vitro and in vivo tests [refs. 16, 19]. SEUU-3 prepared from HT-ETE of 33 mol.% EO content was the most antithrombogenic among segmented polyurethaneureas [refs. 16, 19]. Also in the case of segmented polyurethanes, CTI was influenced by EO content, and SEU-6 prepared from HT-ETE of 62 mol.% EO content showed highest CTI value among them.

The results of platelet adhesion revealed the contribution of EO units to antithrombogenicity. A lot of adhered platelets were found on the surfaces of SPUU-2 and SPU-2 after the contact with blood, and many of them suffered deformations. On the other hand, the surfaces of SEUUs and SEUs displayed far fewer numbers of adhered platelets, showing low interaction with the thrombocyte. The hydrated surfaces of SEUU and SEU showed the less protein adsorption and platelet adhesion than SPUU and SPU [ref. 20].

By these evaluation methods the antithrombogenicities of SEUU and SEU seemed to be almost identical, and both were superior to SPUU or SPU. These results indicate that the presence of hydrophilic PEO segments has much improved the thromboresistance of segmented polyurethane and polyurethaneurea.

Microphase separation

Estimations of microphase separation in the polymers were carried out using polymer films prepared from 4 wt.% or 10 wt.% solution.

The ir spectra of SPUU and SEUU are shown in figure 3. The absorptions assignable to the hydrogen bonded groups were observed: N-H at [3340cm.sup.-1], C=O (urethane) at [1714cm.sup.-1], C=O (urea) at [1635cm.sup.-1]. The other peaks due to C=O in nonhydrogen bonded urethane groups at [1734cm.sup.-1], aliphatic ether linkage at [1110cm.sup.-1] and C=C stretching in benzene ring (in MDI units) at [1600cm.sup.-1] also confirmed the assumed structures shown in figure 1. C=O (urea) at [1635cm.sup.-1] of SEUU was more clearly observed than that of SPUU. In the case of SPU and SEU, C=O stretching (urea) was absent, but the other features were almost the same.

The results of DSC measurements are shown in figure 4. In segmented polyurethaneurea, glass transition (a) and melting (b) of soft segments and melting of hard segments (c) were clearly observed. In segmented polyurethanes the phase transition of soft and hard segments was also observed, but melting point of the hard segments was lower than those of corresponding segmented polyurethaneureas. It was due to the absence of urea linkage, i.e., the aggregation of hard segments was less tight than the corresponding segmented polyurethaneurea. The glass transition temperature of the soft segment matrix of SPU-2 was higher than that of SPUU-2, which is also interpreted by the less microphase separation in SPU-2.

Figure 5 depicts the temperature dispersion of dynamic modulus and dynamic loss of segmented polyurethanes and polyurethaneureas. Glass transitions and meltings of hard segments were clearly observed in all polymers. Among segmented polyurethaneurea, very broad peak of the loss was recognized around 100 [degrees] C and 150 [degrees] C in SPUU-2 whereas not in SEUU-3. This phenomenon is interpretable in terms of the degree of microphase separation between hard and soft segments, i.e., much better separation in SEUU-3 than in SPUU-2. This difference may be one of the reasons of the better antithrombogenicity of SEUU-3 than SPUU-2.

For segmented polyurethanes, glass transitions of SPU-2 and SEU-3 were also clearly observed, but these polymers flowed above 80 [degrees] C and SPU-2 did not show the rubbery plateau region. Hence dynamic moduli of SPU-2 and SEU-3 depended much on temperature. These results indicate that hard segment domain formation in segmented polyurethane was not so clear as in segmented polyurethaneurea.

Mechanical properties

Tensile stress-strain curves of segmented polyurethanes and polyurethaneureas are shown in figure 6. In both dry and wet states, the stress at 100% elongation ([M.sub.100]) and the tensile strength at break ([T.sub.B]) were decreased with the increase of EO content. On the contrary, elongation increased with EO content. In both polymers, the larger the EO content, the more flexible they became.

The tensile behavior was analyzed by the Mooney-Rivlin equation [refs. 21-22]. (1) [Delta]/([Alpha] - [Alpha.sup.-2]) = [2C.sub.1] + [2C.sub.2]/[Alpha] where [Delta] is stress, [Alpha] is extension ratio, [C.sub.1] and [C.sub.2] are constants. [C.sub.1] is expressed as (2) [2C.sub.1] = v kT where v is network-chain density and k is the Boltzmann constant. The plot followed by equation (1) is shown in figure 7. At the intermediate extension region the plots followed equation (1).

[C.sub.1] values did not indicate a definite tendency, but [C.sub.2] changed with EO content among segmented polyurethanes and polyurethaneureas as shown in figure 8. The figure also shows that [C.sub.2] of the polyurethaneurea is larger than [C.sub.2] of polyurethane. [C.sub.2] is known to change by the swelling [ref. 23], and not only by the crosslink density [ref. 24]. In our results, [C.sub.2] of the wet sample (swollen in water) is smaller than that of the dry sample. Consequently, the decrease of [C.sub.2] with EO content is at least partly due to the larger swelling by water. Krigbaum et al claimed that [C.sub.2] did not vanish even under equilibrium conditions, and the internal energy made a large contribution to the [C.sub.2] term [ref. 25] and Ciferri suggested in his private communication to Krigbaum that [C.sub.2] may arise mainly from temporary crosslinks [ref. 25]. Therefore, another factor which may explain the behavior in figure 8 is the structure of the polymer. We estimate that [C.sub.2] was affected by the secondary bond formation between hard and soft segments which were orginated from hydrogen bond. The higher the EO content and hence the moleculer weight was, the smaller was the number of NH group per a unit volume. So figure 8 also shows that [C.sub.2] changes with hydrogen bond density.

Figure 9 shows the stress relaxation curves. The initial stretch was set at 30% elongation, which was a usual deformation for the biological soft tissues. The figure clearly demonstrates the effect of PEO units, i.e., the introduction of PEO units in SPUU and SPU decreased the stress relaxation. The relaxation strength is defined by (3) Relaxation strength (%) = ([Delta.sub.0] - [Delta.sub.t]) / [Delta.sub.0]) x 100 where [Delta.sub.0] is initial (maximum) stress and [Delta.sub.t] is stress at time t (in s) after the stretch [ref. 18]. The values at t = 5 min are listed in table 4. The more the EO content, the smaller the relaxation strength. The reported results on the blood vessels of rabbits whose tissues suffered from heredohyperlipoidemia exhibited larger relaxation strength [ref. 18]. It can be said that the introduction of PEO units in the soft segments improved the stress relaxation properties since those of SEUU and SEU are less than the relaxation strength of SPUU and SPU. Relaxation strength of segmented polyurethaneurea was smaller than that of segmented polyurethane. This difference was caused by the stronger interaction of the urea linkage to result in the tighter hard segment domains.

Stress relaxation curves are usually expressed by empirical formula [ref. 26]. (4) [Delta.sub.t]/[Delta.sub.o] = a - b log t where a and b are constant. The relaxation curves at the initial stages (up to 5 hours or so) obeyed equation (4). The value of b changed with EO contents (table 4). These results of stress relaxation showed that SEUU is superior to SPUU, SPU and SEU in elastic properties.

Long-term hydrolytic stability

PEO units are more hydrophilic than PTHF units, and the polyurethane whose soft segments were consisting of only PEO units was reported to be susceptible to hydrolysis [ref. 27]. We therefore studied the effect of six months soaking in normal saline solution at 37 [degrees] C on the tensile properties. The results showed the higher the EO units content, the higher was the swelling, but the tensile properties did not change by the six months soaking. These results indicate that the urethane or urethaneurea from polyethers shows much better hydrolytic stability than that from polyesters, and hard segments from MDI-ED or MDI-EG are very stable even in aqueous conditions for six months.

Conclusion Novel ABA type triblock copolyether, i.e., PEO-PTHF-PEO was prepared, and segmented polyurethane and polyurethaneurea were synthesized from the copolyether. The effect of the introduction of hydrophilic PEO units was found to be great, that is, the antithrombogenicity of segmented polyurethane and polyurethaneurea was much improved. In addition to better biocompatibility, tensile and stress relaxation tests showed that those having PEO segments displayed excellent mechanical properties. The morphology of them was investigated by DSC and dynamic mechanical measurements, and microphase separation was observed in all polymers. However, those having PEO units showed clearer phase separation than those without PEO units. The above mentioned improvements are estimated to be due to the better microphase separation. [Table 1 to 4 Omitted] [Figure 1 to 9 Omitted]

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