Page 136 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

CHAPTER 6
6.5 Discussion
Selection of appropriate materials and valve design is pivotal for long-term functionality of sca olds suitable for in-situ heart valve tissue engineering. The mechanical properties and durability of the materials are crucial. The material should be flexible, have a low Young’s modulus to match native valve leaflet properties as close as possible, but still has to be sti enough to avoid prolapse during diastole. The latter can also be avoided by adaptations to the valve design, e.g. by selection of larger coaptation areas.
It is vital that the material of choice maintains its durability and functionality a er sterilization, since the sterilization processes may a ect the properties of the polymers. Heat and ethylene oxide are widely used sterilization methods. For PCL and PCL2kU4Un, ethylene oxide can function as a plasticizer16, which makes this method unsuitable for our electrospun sca olds. Also heat-related processes are inappropriate here since they reach temperatures above the melting point of the used polymer. Due to its wide acceptance in the clinic, γ-sterilization was the sterilization method of choice in this study. The e ect of γ-sterilization can be best seen in the ~20% drop of the molecular weight caused by chain scissions in both the PCL sca olds and PCL2kU4Un sca old material. These chain scissions reduced the elongation at ultimate tensile strength for both
materials, but the remaining elongation is still high enough for the dedicated purpose. The Young’s modulus and tensile strength were not a ected by the γ-sterilization process.
To test durability within a reasonable timespan, fatigue testing of electrospun PCL and PCL2kU4Un was performed at 10 Hz, a frequency about 5 times higher than the heart rate of an active person. Increasing the frequency results in faster measurements, but generally also in a decrease of the measured fatigue resistance.24,25 This behavior was also confirmed in this study on the γ-sterilized PCL sca olds. The di erence can be explained by the faster straining rate at 10 Hz, which induces more stress to the material that can lead to a faster breaking of the sca old. Despites the sterilized PCL sca olds fail faster with 10 Hz, to mirror the in-vivo behavior of 1 to 3 Hz, the 10 Hz protocol is acceptable to be able to perform measurements in a lesser amount of time. This when taken in consideration, that for the non-sterilized PCL sca olds was no di erence, and, as also in the case of the sterilized samples, a higher measurement generally leads to a reduced fatigue resistance.
Whereas the sterilization had only a minor e ect on the mechanical properties deducted from a tensile test, it dramatically changed the fatigue resistance. Electrospun PCL samples lasted for up to
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