New Generation Polyurethane Elastomer For
Biomedical Applications

Muthu Jayabalam
Polymer Division, BMT Wing,
Sree Chitra Tirunai Institute for Medical Sciences & Technology,
Trivandrum, 695 012, India

Abstract

The reported experimental and clinical failures of poly ether urethane elastomers in long term biomedical applications are due to degradation by hydrolysis, oxidation and mechano-chemical degradation. A new generation poly urethane elastomer having outstanding biodurability and biocompatibility is required for the development of blood compatible devices. A poly urethane urea was synthesized using aliphatic diisocyanate (SMDI), ether less hydrocarbon polyol (hydroxy terminated poly butadiene) and diamine (hexamethylene diamine). Environmental stress corrosion resistance test on this elastomer reveals appreciable stability in hydrolytic and oxidative media. Accelerated flexural fatigue test also reveals flexing endurance. The test suggest the possible use of this poly urethane-urea for the development of cardiac assist devices.

Introduction

Polyurethane elastomers have been extensively in biomedical applications. Polyether urethanes have proved sufficient biocompatibility and bio-durability for short term use in a variety of blood contacting applications such as experimental artifical hearts, left ventricle assist devices (1) and blood pumps (2). However the long term biostability of polyurethane is a great concern because of failure of polyurethane-coated leads of pacemaker during long-term implantations (3). Polyurethane used as vital components in biomechanically sensitive blood contacting devices have such as pump membrances of blood pumps and flexible leaflets of artificial hearts and left ventricular assist devices have to undergo repeated cyclic flexion (4). Therefore it is essential to develop low elastic modulus poly-membranes to flex more freely without producing adverse changes in blood flow (5). In addition to the requirement of low elastic modulus, biodurability of the polyurethane is the essential requirement. Biodegradation induced by hydrolytic and oxidative mechanism leads to catastrophic failure of the device. Therefore it is essential to develop polyurethane with minimal or nil chemical groups which are prone to degradation. Aromatic polyurethane based on diphenyl methane diisocyanate (MDI) undergo thermal and thernohydrolytic degradation producing methylene diailine (MDA), a suspected carcinogen (6,7). Therefore polyurethanes based on cycle aliphatic diisocyanate were prepared for biomedical applications by the investigators (8-14). The present paper deals with the development of a new generation aliphatic polyurethane-urea for blood contact applications.


Experimental

A polyurethane-urea coded as HFL 18 PU was synthesized using methylene bis (p-cyclohexy) diisocyanate (SMDI), hydroxy terminated (HTPBD) and 1,6- hexamethylene diamine (HAD). The exact formulation is not given due to proprietary reasons. The polymer was prepared by two step solution polymerisation method, casting on silicone-coated glass plates, curing in air oven and purification by soxhlet extraction using absolute alcohol. The purified polymer was subjected to evaluation. The polymer was characterized for density, crosslink density and molecular weight between crosslinks. The density was determined by sink-float method using water-ethanol mixtures of varying specific gravity. The crosslink density and molecular weight between cross links was determined by using swelling value as per the method published elsewhere (11, 15). The surface properties of the polymer were determined using a goniometer. Clean polymer sample was used for this study. Water contact angle was noted using a water droplet carefully placed on the polymer sample (sessile drop). The interfacial free energy was calculated using standard tables (16). ATR-IR spectral studies was carried out on the clean polyurethane-urea sheets using a Perkin-Elmer spectro photometer. The tensile properties was determined as per ASTM standard procedure D 812 using rectangular specimen. As Instron Automated Materials Testing System (IX) 1.09 was used. The crosshead speed was 100mm/min and sample rate was 10pt/sec. The shore ‘A’ hardness was determined for all polymers. Polymer sheets were filed together to a thickness of 6mm and used as sample for testing the hardness. The thermogravimetric analysis of the polymer was carried out under nitrogen. The sample was heated at the rate of 100C/min. About 10 mg of the sample was used. A Dupont 2000 TGA unit was used. The resistance to environmental stress corrosion cracking (ESC) was investigated using stressed sample in Ringer’s solution and phosphate buffered saline at 50 0C for 2 days. The ASTM standard D 1695 procedure was followed. The visible changes in the polymer was noted and reported as passed or failed. Accelerated flexural fatigue was carried out using elastomer strips (5 0x5 0x1cm). The frequency of flexing was at 1425 cycles per min. under load 216g. A single phase induction motor with speed of 1425 rpm was used. The test was carried out as per ASTM standard D 671.

Results and Discussion

The polymer formed by two step condensation of SMDI and HTPED and SMDI and HAD is composed of soft segment of HTPBD and hard segment of SMDI-HAD reaction units. Ultimately SMDI and HTPBD react to give urethane linkages while SMDI and HAD react to give urea linkages in the polyurethane-urea elastomer. The formation of polyurethane urea is established by using FTIR spectral analysis. The spectrum indicates peaks for urea carbonyl (hydrogen bonded) at 1632. 22cm-1. The spectrum doesn’t indicates peak for free urea carbonyl than in the case of free groups indicating the strong hydrogen bonding interactions (ordered). The urea linkages are involved in extensive 3 dimensional hydrogen bonding.

The polymer exhibits high degree of hydrogen bonding which could result to long range ordering as reported elsewhere (17). Such long range ordering leads to the appearance of spherulites and virtually corsslinked state (17,18). Moreover, the polymer is insoluble in known solvents such as dimenthly accetaminde, dimethyl formamide and tetrahydrofuran which are generally used for dissolution of linear thermoplastic polyurethane. In these solvents the present polymer only swell. The data of swelling studies and determination of crosslink density and molecular weight between crosslinks are given in Table 1. The studies reveal high degree of virtual corsslinking through hydrogen bonding resulting a thermosetting-like character (18)

The surface properties reveal that polymer is hydrophobic in nature (Table-1). The mechanical properties (Table-2) indicate that the polymer is a soft elastomer with low elastic modulus (tensile stress at 100% strain) and shore ‘A’ hardness.

The TGA studies reveal appreciable thermal stability. (Figure 2). The polymer undergoes two step degradation, the fist at around 400 0C and the second around 500 0C. The higher thermal stability reveals that the polyol segment is protected by the three dimensional crosslinked network. Therefore this elastomer can be sterilized by autoclave sterlization.

The resistance to environmental stress cracking in the Ringer’s solution and phosphate buffered saline was observed. There was not no visible crack and while solid appearance around the crack. The polyurethane ureas are adequately stable in the environmental-stress-cracking environmental. The accelerated flexural fatigue test also reveals appreciable flexing endurance of over 100 million cycles without any visible cracks on the surface.

The conclusion the present polyurethane-urea is found to have low elastic modulus, hydrophobicity, and resistance to hydrolytic degradation and environmental stress corrosion cracking. These favorable characteristic of these polymers are due to the presence of aliphatic hydrocarbon polymer are more promising for use in dynamic blood contact applications.

Acknowledgement

The author acknowledges Department of Biotechnology, New Delhi for the research grant under which this work was carried out. The author thanks Dr. P. Ramesh, Mr. Willey Paul and Mrs. Radha for their help in the evaluation of the polymers.

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