Biomedical Engineering Reference
In-Depth Information
Fig. 11.17
Basic
composition of polyurethanes
with hard and soft segments
O
O
CH
2
CH
2
O
CNH
NHC
n
m
soft segment
hard segment
11.9
Thermoplastic Elastomers
11.9.1
Polyurethane
The basic unit of polyurethane consists of, as most technically used polymers, hard
and soft segments as shown in Fig.
11.17
. They need no special 'tricks' for being
thermoplastic but, when shaped by injection molding, they have the unpleasant
property to adhere to the injection molds; can be overcome but releasing agents any-
way need special attention. Companies do not like to communicate about the content
of their products. PUs are widely applied in the extremely important field of vascu-
lar surgery (see Chaps. 12 and 13). The major brands of commercially available
biocompatible PUs are Biomer
TM
, Cardiothane
TM
, Pellethane
TM
and Tecoflex
TM
.
For the aimed application, Pellethane should be the most suitable (with respect to
the range of available hardnesses).
11.9.2
Thermoplastic Polyolefins
TPOs are a magic mixture of three main components: the thermoplast polypropy-
lene as matrix, dispersed in this matrix an EPDM rubber, thoroughly mixed under
addition of a nonspecified mineral oil. EPDM is dominating the synthetic rub-
ber market. It is a terpolymer consisting of copolymerized ethene [CH
2
D
CH
2
],
CH-CH
3
], and a nonconjugated diene.
6
Santoprene and Sarlink are
two TPOs, respectively, manufactured by Monsanto and DSM, the main difference
being the degree of cross-linking (>95 and <95% respectively).
propene [CH
2
D
Premedical Tests, Assessment of Fatigue Damage, In Vivo Experiments
Premedical testing.
A material which is intrinsically toxic is out of the question for
biomedical use. Therefore, the designers looked first whether or not the TPO and
6
The IUPAC names for ethylene and propylene are ethene and propene.
Nonconjugated
diene
means that the two double bonds are separated by two or more single bonded carbons [CH
2
D
CH-
[-C-C-]
n
-CH
D
CH
2
].
