Tissue-engineered heart valves for minimally invasive surgery

Abstract

The quest for the ideal heart valve replacement is ongoing as current substitutes, such as mechanical and bioprosthetic valves, lack the capability for regeneration and growth. This lack of regeneration and growth represents a substantial limitation for long-term durability especially in children and young adults. Guided tissue regeneration and tissue-engineering of valve replacements are proposed to overcome these limitations, as described in the first chapter. Additionally, minimally invasive valve replacement procedures are rapidly evolving as an alternative treatment option for patients with valvular heart disease. With respect to the obvious potential advantages of combining both technologies, the aim of this thesis was to investigate the feasibility and functionality of tissue-engineered heart valves (TEHV) for minimally invasive surgical implantation. The second chapter describes the proof of concept of minimally invasive surgical implantation of TEHV, based on autologous cells and rapid degrading synthetic scaffolds, in the ovine model. To enable minimally invasive implantation of the TEHV, the scaffolds were integrated into self-expandable stents and crimped in order to fit the delivery device for trans-apical implantation. In-vitro simulation of this crimping procedure revealed no detectable structural damage to the TEHV leaflets. Subsequently, the stented TEHV were trans-apically implanted as pulmonary valve replacements in an ovine model. Feasibility, safety, and proof of principle of the trans-apical implantation of TEHV were demonstrated. Valve performance was satisfactory, but leaflet mobility was hampered by thickening and retraction of the leaflets. To evaluate whether this in-vivo thickening was due to the substantial tissue deformations associated with the crimping procedure, the outcome of trans-apically and surgically implanted TEHV were directly compared as described in the third chapter. It was concluded that crimping had no adverse effect on the integrity or the functional outcome of TEHV, as all explanted valves demonstrated comparable layered tissue formation and retraction of the leaflets independent of the implantation method used. Tissue thickening of the living TEHV in-vivo was not primarily caused or enhanced by the crimping procedure, but rather may represent a phase of tissue remodeling. Retraction induced leaflet shrinkage is an undesired phenomenon and is hypothesized to be mainly cell mediated. Therefore, decellularization of the TEHV before implantation was proposed in the fourth chapter. In-vitro analyses demonstrated that decellularization of TEHV does not alter the collagen structure or tissue strength. Further, decellularization improved in-vitro valve functionality due to reduced retraction of the leaflets. Decellularization of in-vitro grown TEHV provides largely available off-the-shelf homologous scaffolds suitable for reseeding with autologous cells and trans-apical valve delivery. Preliminary results of trans-apical implantation of such decellularized TEHV in sheep demonstrated the principal feasibility of these homologous valve replacements for minimally invasive surgical procedures, as described in the fifth chapter. Decellularized TEHV demonstrated functionality and complete host repopulation in-vivo without thickening after 8 weeks. Longer follow-up periods demonstrated mild thickening, progressive valvular insufficiency, and a reduced leaflet size after 16 and 24 weeks. Remodeling of the in-vivo repopulated decellularized leaflets was demonstrated by histology and by an increasing degree of anisotropic mechanical behavior over time. These results demonstrate that the use of decellularized TEHV as valve replacements is a promising approach, but valvular insufficiency still prevents long-term functioning. Reseeding these valves prior to implantation or optimizing valve geometry represent future strategies. In conclusion, the results presented in this thesis demonstrate that trans-apical implantation of TEHV is feasible. The observed in-vitro and in-vivo retraction and in-vivo thickening of living TEHV was reduced by decellularization of the engineered tissues. Moreover, it was demonstrated that decellularized TEHV can be reseeded in-vitro or become rapidly repopulated with endogenous cells in-vivo. Although it remains a challenge to overcome the in-vivo reduction of leaflet size on the long term, the rapid cell infiltration capacity and the unrestricted and off-the-shelf availability of these decellularized engineered valves enable their potentially application as superior alternative to today’s bioprostheses