Polyurethane Elastomer For
Polymer Division, BMT Wing,
Sree Chitra Tirunai Institute for Medical Sciences & Technology,
Trivandrum, 695 012, India
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.
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.
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
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’
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.
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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