PhD, Universite Laval
Cardiovascular diseases are the leading cause of deaths of the 21st century. The most common cause of these diseases is related to the narrowing of blood vessels that reduces or even block the blood flow to downstream tissues. The treatments for this condition range from lifestyle changes to pharmaceutical and surgical interventions. Among the surgical procedures, vascular bypass grafting is a good alternative for patients requiring long term revascularization solutions. It consists in the use of autologous or synthetic vascular grafts to create a new passage for the blood flow. The limitations of autologous grafts and the low performance of synthetic grafts, especially for small diameter vessels, motivated the investigation for alternative vascular conduits based on tissue engineering techniques. Engineered vascular grafts are composed by cells and natural or synthetic biomaterials (scaffolds) that together aims to mimic a native tissue. However, achieving the proper mechanical properties without compromissing the biological similarity is a challenge in vascular tissue engineering. One of the recurrent difference of these models with respect to native blood vessels is cell density, usually between 0.5 to 1 million cells/mL vs approximately 200 million cells/mL. In this work, the main component of the vessel wall - collagen type I - was mixed as a gel with human umbilical smooth muscle cells and molded in a tubular shape. The influence of cell seeding density was evaluated on the static maturation of these constructs in terms of expression of some key extracellular proteins by the cells, compaction rate and mechanical properties.
Abstract: The acquisition of new thorough knowledge on the interactions existing between vascular cells would represent a step forward in the engineering of vascular tissues. In this light, herein we designed a physiological-like tri-culture in vitro vascular wall model using a planar cellularized collagen gel as the scaffold. The model can be obtained in 24 h and features multi-layered hierarchical organization composed of a fibroblast-containing adventitia-like layer, a media-like layer populated by smooth muscle cells and an intima-like endothelial cell monolayer. After 7 days of static culture, the compaction of the collagen matrix by the vascular cells was achieved, and the deposition of the vascular extracellular matrix components fibronectin, fibrillin-1 and tropoelastin was observed. The blood-compatible functionality of the endothelial cell monolayer was demonstrated by a blood clotting assay: after 7 days of maturation, clotting was prevented on the endothelialized constructs (more than 80% free hemoglobin maintained after 60 min of blood contact) but not at all on non-endothelialized ones (less than 20% free hemoglobin). In addition, western blotting results suggested that in the tri-culture model the loss of smooth muscle cell phenotype was delayed compared to what was observed in the mono-culture model, finally resulting in a behaviour more similar to the in vivo conditions. Overall, our findings indicate that this in vitro model has the potential to be used as an advanced system to examine vascular cell behavioural interactions, as well as for drug testing and the investigation of physiological and pathological processes.
Pub.: 05 Dec '16, Pinned: 20 Aug '17
Abstract: Vascular tissue engineering focuses on the replacement of diseased small-diameter blood vessels with a diameter less than 6 mm for which adequate substitutes still do not exist. One approach to vascular tissue engineering is to culture vascular cells on a scaffold in a bioreactor. The bioreactor establishes pseudophysiological conditions for culture (medium culture, 37°C, mechanical stimulation). Collagen gels are widely used as scaffolds for tissue regeneration due to their biological properties; however, they exhibit low mechanical properties. Mechanical characterization of these scaffolds requires establishing the conditions of testing in regard to the conditions set in the bioreactor. The effects of different parameters used during mechanical testing on the collagen gels were evaluated in terms of mechanical and viscoelastic properties. Thus, a factorial experiment was adopted, and three relevant factors were considered: temperature (23°C or 37°C), hydration (aqueous saline solution or air), and mechanical preconditioning (with or without). Statistical analyses showed significant effects of these factors on the mechanical properties which were assessed by tensile tests as well as stress relaxation tests. The last tests provide a more consistent understanding of the gels' viscoelastic properties. Therefore, performing mechanical analyses on hydrogels requires setting an adequate environment in terms of temperature and aqueous saline solution as well as choosing the adequate test.
Pub.: 31 Jul '12, Pinned: 20 Aug '17
Abstract: Synthetic materials are known to initiate clinical complications such as inflammation, stenosis, and infections when implanted as vascular substitutes. Collagen has been extensively used for a wide range of biomedical applications and is considered a valid alternative to synthetic materials due to its inherent biocompatibility (i.e., low antigenicity, inflammation, and cytotoxic responses). However, the limited mechanical properties and the related low hand-ability of collagen gels have hampered their use as scaffold materials for vascular tissue engineering. Therefore, the rationale behind this work was first to engineer cellularized collagen gels into a tubular-shaped geometry and second to enhance smooth muscle cells driven reorganization of collagen matrix to obtain tissues stiff enough to be handled. The strategy described here is based on the direct assembling of collagen and smooth muscle cells (construct) in a 3D cylindrical geometry with the use of a molding technique. This process requires a maturation period, during which the constructs are cultured in a bioreactor under static conditions (without applied external dynamic mechanical constraints) for 1 or 2 weeks. The "static bioreactor" provides a monitored and controlled sterile environment (pH, temperature, gas exchange, nutrient supply and waste removal) to the constructs. During culture period, thickness measurements were performed to evaluate the cells-driven remodeling of the collagen matrix, and glucose consumption and lactate production rates were measured to monitor the cells metabolic activity. Finally, mechanical and viscoelastic properties were assessed for the resulting tubular constructs. To this end, specific protocols and a focused know-how (manipulation, gripping, working in hydrated environment, and so on) were developed to characterize the engineered tissues.
Pub.: 02 Jul '15, Pinned: 20 Aug '17
Abstract: In the past decades, vascular tissue engineering has made great strides towards bringing engineered vascular tissues to the clinics and, in parallel, obtaining in-lab tools for basic research. Herein, we propose the design of a novel dual-mode bioreactor, useful for the fabrication (construct mode) and in vitro stimulation (culture mode) of collagen-based tubular constructs. Collagen-based gels laden with smooth muscle cells (SMCs) were molded directly within the bioreactor culture chamber. Based on a systematic characterization of the bioreactor culture mode, constructs were subjected to 10% cyclic strain at 0.5 Hz for 5 days. The effects of cyclic stimulation on matrix re-arrangement and biomechanical/viscoelastic properties were examined and compared vs. statically cultured constructs. A thorough comparison of cell response in terms of cell localization and expression of contractile phenotypic markers was carried out as well. We found that cyclic stimulation promoted cell-driven collagen matrix bi-axial compaction, enhancing the mechanical strength of strained samples with respect to static controls. Moreover, cyclic strain positively affected SMC behavior: cells maintained their contractile phenotype and spread uniformly throughout the whole wall thickness. Conversely, static culture induced a noticeable polarization of cell distribution to the outer rim of the constructs and a sharp reduction in total cell density. Overall, coupling the use of a novel dual-mode bioreactor with engineered collagen-gel-based tubular constructs demonstrated to be an interesting technology to investigate the modulation of cell and tissue behavior under controlled mechanically conditioned in vitro maturation.
Pub.: 23 Feb '17, Pinned: 20 Aug '17
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