First 12-month follow-up of bioresorbable synthetic heart valves implanted in the aortic position in sheep

Background

Replacement of diseased heart valves with the currently available valve prostheses has serious drawbacks. The use of bioresorbable synthetic “in situ tissue engineered” heart valve prostheses has been proposed to overcome the limitations of traditional heart valve prostheses. Such bioresorbable synthetic heart valve prostheses have been successfully tested as pulmonary valve replacements in preclinical studies, but data on aortic valve replacement is lacking. Here, we present the first in-vivo study on the long-term functionality of bioresorbable synthetic heart valves in the high-pressure circulation.

Methods

Approval for the animal studies was obtained by the Amsterdam University Medical Centres Animal Care Ethics Committee (AVD1180020197705) and are in agreement with the current Dutch law on animal experiments (WOD). We surgically implanted bioresorbable synthetic aortic valve prostheses in 20 female Swifter sheep in orthotopic position. The scheduled follow-up times were 1, 3, 6, 9 and 12 months.

Results

Fifteen sheep (75%) recovered well from the surgical valve implantation procedure and were included in this analysis. No sheep died due to valve failure. All valves remained free from active infectious endocarditis, thrombotic complications, and pathological calcification. A total of 10 valves (67%) were intact, thin and pliable and remained free from leaflet thickening, retraction and degradation up to 12 months after implantation. In most valves (67%), the scaffold remained sparsely- or unpopulated by cells during 12 months follow up. In some valves (33%), colonization of the valve scaffold was observed, however this was most often associated with partial degradation of the leaflet and leaflet thickening. In general, the degradation of the scaffold fibres was limited throughout the follow-up period.

Conclusion

We are the first to evaluate in vivo bioresorbable synthetic heart valves in aortic position. While valves remained functional, the study also serves as a starting point to further optimize scaffold and neotissue development for heart valve replacement in the high-pressure environment.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): 1. Cardiovasculair Onderzoek Nederland (grant number CVON2012-01)2. Netherlands Organization for Scientific Research (024.003.013)

The degradation and performance of electrospun supramolecular vascular scaffolds examined upon in vitro enzymatic exposure

To maintain functionality during in situ vascular regeneration, the rate of implant degradation should be closely balanced by neo-tissue formation. It is unknown, however, how the implant's functionality is affected by the degradation of the polymers it is composed of. We therefore examined the macro- and microscopic features as well as the mechanical performance of vascular scaffolds upon in vitro enzymatic degradation. Three candidate biomaterials with supramolecularly interacting bis-urea (BU) hard blocks ('slow-degrading' polycarbonate-BU (PC-BU), 'intermediate-degrading' polycarbonate-ester-BU (PC(e)-BU), and 'fast-degrading' polycaprolactone-ester-BU (PCL-BU)) were synthesized and electrospun into microporous scaffolds. These materials possess a sequence-controlled macromolecular structure, so their susceptibility to degradation is tunable by controlling the nature of the polymer backbone. The scaffolds were incubated in lipase and monitored for changes in physical, chemical, and mechanical properties. Remarkably, comparing PC-BU to PC(e)-BU, we observed that small changes in macromolecular structure led to significant differences in degradation kinetics. All three scaffold types degraded via surface erosion, which was accompanied by fiber swelling for PC-BU scaffolds, and some bulk degradation and a collapsing network for PCL-BU scaffolds. For the PC-BU and PC(e)-BU scaffolds this resulted in retention of mechanical properties, whereas for the PCL-BU scaffolds this resulted in stiffening. Our in vitro study demonstrates that vascular scaffolds, electrospun from sequence-controlled supramolecular materials with varying ester contents, not only display different susceptibilities to degradation, but also degrade via different mechanisms. STATEMENT OF SIGNIFICANCE: One of the key elements to successfully engineer vascular tissues in situ, is to balance the rate of implant degradation and neo-tissue formation. Due to their tunable properties, supramolecular polymers can be customized into attractive biomaterials for vascular tissue engineering. Here, we have exploited this tunability and prepared a set of polymers with different susceptibility to degradation. The polymers, which were electrospun into microporous scaffolds, displayed not only different susceptibilities to degradation, but also obeyed different degradation mechanisms. This study illustrates how the class of supramolecular polymers continues to represent a promising group of materials for tissue engineering approaches.

Supramolecular surface functionalization via catechols for the improvement of cell-material interactions

Optimization of cell-material interactions is crucial for the success of synthetic biomaterials in guiding tissue regeneration. To do so, catechol chemistry is often used to introduce adhesiveness into biomaterials. Here, a supramolecular approach based on ureido-pyrimidinone (UPy) modified polymers is combined with catechol chemistry in order to achieve improved cellular adhesion onto supramolecular biomaterials. UPy-modified hydrophobic polymers with non-cell adhesive properties are developed that can be bioactivated via a modular approach using UPy-modified catechols. It is shown that successful formulation of the UPy-catechol additive with the UPy-polymer results in surfaces that induce cardiomyocyte progenitor cell adhesion, cell spreading, and preservation of cardiac specific extracellular matrix production. Hence, by functionalizing supramolecular surfaces with catechol functionalities, an adhesive supramolecular biomaterial is developed that allows for the possibility to contribute to biomaterial-based regeneration.

Material dependent differences in inflammatory gene expression by giant cells during the foreign body reaction

Multinucleated giant cells (GCs) are often observed in the foreign body reaction against implanted materials. The in vivo function of GCs in this inflammatory process remains to be elucidated. GCs degrade collagen implants in rats and may also orchestrate the inflammatory process via the expression and secretion of modulators, such as cytokines and chemokines. In this study, we show that the gene expression of PMN chemoattractants, CXCL1/KC and CXCL2/MIP-2, is high in GCs micro-dissected from explanted Dacron, cross-linked collagen (HDSC), and bioactive ureido-pyrimidinone functionalized oligocaprolactone (bioactive PCLdiUPy). Conversely, the gene expression levels of TGFbeta and pro-angiogenic mediators VEGF and FGF were found to be low in these GCs as compared with the expression levels in total explants. GCs in bioactive PCLdiUPy displayed high cytokine and angiogenic mediator expression compared with GCs isolated from the two other studied materials, whereas chemokine gene expression in GCs isolated form HDSC was low. Thus, GCs adopt their expression profile in response to the material that is encountered.