Microstructural manipulation of electrospun scaffolds for specific bending stiffness for heart valve tissue engineering

Abstract

Biodegradable thermoplastic elastomers are attractive for application in cardiovascular tissue construct development due to their amenability to a wide range of physical property tuning. For heart valve leaflets, while low flexural stiffness is a key design feature, control of this parameter has been largely neglected in the scaffold literature where electrospinning is being utilized. This study evaluated the effect of processing variables and secondary fiber populations on the microstructure, tensile and bending mechanics of electrospun biodegradable polyurethane scaffolds for heart valve tissue engineering. Scaffolds were fabricated from poly(ester urethane) urea (PEUU) and the deposition mandrel was translated at varying rates in order to modify fiber intersection density. Scaffolds were also fabricated in conjunction with secondary fiber populations designed either for mechanical reinforcement or to be selectively removed following fabrication. It was determined that increasing fiber intersection densities within PEUU scaffolds was associated with lower bending moduli. Further, constructs fabricated with stiff secondary fiber populations had higher bending moduli whereas constructs with secondary fiber populations which were selectively removed had noticeably lower bending moduli. Insights gained from this work will be directly applicable to the fabrication of soft tissue constructs, specifically in the development of cardiac valve tissue constructs.

Fig. 1

(A) Schematic of electrospinning apparatus for two-component scaffolds. PEUU was fed from the same location for every group. The mandrel was rotated and translated along its longitudinal axis at varying speeds. Secondary polymer fibers were introduced through separate nozzles. (B) Image of a polymeric specimen loaded in the bending device (scale bar = 1 cm).

Fig. 2

(A) SEM micrographs of representative scaffolds from each translated group (scale bar = 5 μm) and (B) their corresponding digitized structure. (C) A plot depicting the quantitative relationship between translational speed during fabrication and microstructural elements. (D) High magnification morphology of a fiber intersection.

Fig. 3

(A) The relationship between translational velocity during fabrication (top x axis), fiber intersection density (bottom x axis), and bending modulus (y axis). (B) Uniaxial tensile modulus of electrospun scaffolds fabricated under different translational velocities. (C) Suture retention strengths of scaffolds fabricated under different translational velocities.

Fig. 4

(A) Fluorescent micrograph qualitatively depicting relative distribution of PEUU fibers (green) to PCL fibers (red) in a 75/25 volume flow rate ratio construct. Scale bar = 20 μm. (B) Microstructure of representative PEUU/PCL blended scaffolds. Scale bar = 10 μm. (C) Change in fiber intersection density observed between PEUU/PCL ratios. No other differences in microstructural features were observed.

Fig. 5

(A) Uniaxial tensile mechanical response of constructs containing increasing quantities of PCL fibers. (B) Planar biaxial mechanical properties of PEUU:PCL blended scaffolds groups with different symbols (‡,†,*) are significantly different from one another (p < 0.05).

Fig. 6

Bending modulus of mixed polymer constructs at varying ratios of PEUU:PCL. Groups with different symbols (‡,†) are significantly different from one another, and groups with (*) are significantly different from all other groups (p < 0.05). Solid reference line indicates the bending modulus of native costal cartilage [39]. Dashed reference line indicates the bending modulus of the intact septum [40].

Fig. 7

(A) Representative structural images of PEUU/PEO 75/25 scaffolds as-spun (dry), after 1 s and after 4 h of soaking in water. Scale bar = 10 μm. (B) Fluorescent micrographs of PEUU (green)/PEO (red) blended constructs before (above) and after (below) treatment with water. Scale bar = 20 μm. (C) Difference in normalized fiber intersection density between as-spun 100% PEUU constructs and constructs following removal of PEO fibers; *indicates statistical significance (p < 0.05).

Fig. 8

(A) Tensile modulus of constructs containing varying quantities of PEO following contact with water. (B) Suture retention strength of constructs following PEO fiber removal. (C) Biaxial mechanical response of constructs following PEO fiber removal. Groups with different symbols (‡,†) are significantly different from one another (p < 0.05).

Fig. 9

(A) Qualitative depiction of constructs originally containing varying amounts of PEO placed in a cantilever position following contact with water. (B) Bending modulus of constructs following PEO fiber removal. Reference line indicates the bending modulus of the native pulmonary valve (491 kPa) [30]. *indicates statistical significance (p < 0.05).