Poly-ε-caprolactone scaffold and reduced in vitro cell culture:beneficial effect on compaction and improved valvular tissue formation

Tissue-engineered heart valves (TEHVs), based on polyglycolic acid (PGA) scaffolds coated with poly-4-hydroxybutyrate (P4HB), have shown promising in vivo results in terms of tissue formation. However, a major drawback of these TEHVs is compaction and retraction of the leaflets, causing regurgitation. To overcome this problem, the aim of this study was to investigate: (a) the use of the slowly degrading poly-e-caprolactone (PCL) scaffold for prolonged mechanical integrity; and (b) the use of lower passage cells for enhanced tissue formation. Passage 3, 5 and 7 (P3, P5 and P7) human and ovine vascular-derived cells were seeded onto both PGA-P4HB and PCL scaffold strips. After 4 weeks of culture, compaction, tissue formation, mechanical properties and cell phenotypes were compared. TEHVs were cultured to observe retraction of the leaflets in the native-like geometry. After culture, tissues based on PGA-P4HB scaffold showed 50-60% compaction, while PCL-based tissues showed compaction of 0-10%. Tissue formation, stiffness and strength were increased with decreasing passage number; however, this did not influence compaction. Ovine PCL-based tissues did render less strong tissues compared to PGA-P4HB-based tissues. No differences in cell phenotype between the scaffold materials, species or cell passage numbers were observed. This study shows that PCL scaffolds may serve as alternative scaffold materials for human TEHVs with minimal compaction and without compromising tissue composition and properties, while further optimization of ovine TEHVs is needed. Reducing cell expansion time will result in faster generation of TEHVs, providing more rapid treatment for patients

Intermittent straining accelerates the development of tissue properties in engineered heart valve tissue

Tissue-engineered heart valves lack sufficient amounts of functionally organized structures and consequently do not meet in vivo mechanical demands. To optimize tissue architecture and hence improve mechanical properties, various in vitro mechanical conditioning protocols have been proposed, of which intermittent straining is most promising in terms of tissue properties. We hypothesize that this is due to an improved collagen matrix synthesis, maturation, and organization, triggered by periodic straining of cells. To test this hypothesis, we studied the effect of intermittent versus constrained conditioning with time (2–4 weeks), using a novel model system of human heart valve tissue. Temporal variations in collagen production, cross-link density, and mechanical properties were quantified in engineered heart valve tissue, cyclically strained for 3-h periods, alternated with 3-h periods rest. In addition, an innovative method for vital collagen imaging was used to monitor collagen organization. Intermittent straining resulted in increased collagen production, cross-link densities, collagen organization, and mechanical properties at faster rates, as compared to constrained controls, leading to stronger tissues in shorter culture periods. This is of utmost importance for heart valve tissue engineering, where insufficient mechanical properties are currently the main limiting factor

Tissue-Engineered Heart Valves

A tissue engineered heart valve (TEHV) could serve as a living, implantable valve replacement that would grow and adapt with the patient. A TEHV consists of relevant cells seeded on or entrapped in a scaffold material which is designed to degrade as the cells produce their own extracellular matrix components. Because the valve consists of living tissue, it can grow and remodel as a patient ages, making it an especially attractive replacement option for pediatric and young adult patients. To date, using various cell sources, scaffold materials, and/or in vitro culture protocols, several laboratories have produced TEHVs with the appropriate geometry and near-native mechanical properties. TEHVs implanted in the pulmonary position in sheep in our laboratory have shown promising short-term functionality but fail to maintain good performance after several months in vivo. Upcoming TEHV research will focus on optimization of TEHV components and in vitro culture conditions in order to improve long-term function post-implant, with the hope of performing human implants in the future