Recrystallization improves the mechanical properties of sintered electrospun polycaprolactone.

Abstract

Resorbable electrospun polycaprolactone (PCL) scaffolds for tissue reconstruction can provide physicians with an "off the shelf" product tailored to the patient's specific tissue architecture. However, many tissue-engineering platforms do not possess the necessary long-term mechanical stability needed to properly support tissue development. Sintering has been explored as a means of altering the properties of electrospun PCL. However, crystallinity-driven changes in mechanical properties following thermal treatment have not been previously investigated. PCL nanofibers were produced by electrospinning and subsequently thermally sintered (at 55, 56 and 58°C) to enhance their long-term mechanical integrity in response to representative biological milieux. Scaffolds initially sintered at 56°C displayed 6-fold increases in compressive strength and 3-fold increases in modulus, while displaying 10-fold increases in energy dissipation with increasing sintering temperature. Sintering just below the Tm resulted in amorphization of the 55°C sample as indicated by the 20-fold lower XRD peak intensities. Although crystallinity is suppressed, the polymer chains likely retain chain alignment from electrospinning and are apparently highly susceptible to recrystallization. After only 1d PBS exposure, the 55°C samples recover a substantial fraction of the as-spun crystallinity; 7d of exposure fully restores as-spun peak intensities. The mechanical properties of all three (55, 56, or 58°C) scaffolds displayed peak values of compressive strength and modulus following 7d exposure. In contrast with the current state-of-the-art which assumes that tissue engineering scaffolds only grow weaker following exposure, in these scaffolds maximum values of compressive strength and modulus were observed after 7d of aqueous immersion. This suggests that polymeric recrystallization can be used to increase or optimize mechanical properties in vitro/in vivo. Scaffolds that increase their mechanical integrity during biological exposures constitute a new pathway enabling clinical advances.