M Marc Simonet

Multi-scale mechanical characterization of scaffolds for heart valve tissue engineering.

Electrospinning is a promising technology to produce scaffolds for cardiovascular tissue engineering. Each electrospun scaffold is characterized by a complex micro-scale structure that is responsible for its macroscopic mechanical behavior. In this study, we focus on the development and the validation of a computational micro-scale model that takes into account the structural features of the electrospun material, and is suitable for studying the multi-scale scaffold mechanics. We show that the computational tool developed is able to describe and predict the mechanical behavior of electrospun scaffolds characterized by different microstructures. Moreover, we explore the global mechanical properties of valve-shaped scaffolds with different microstructural features, and compare the deformation of these scaffolds when submitted to diastolic pressures with a tissue engineered and a native valve. It is shown that a pronounced degree of anisotropy is necessary to reproduce the deformation patterns observed in the native heart valve.

Differential Response of Endothelial and Endothelial Colony Forming Cells on Electrospun Scaffolds with Distinct Microfiber Diameters

Electrospun scaffolds for in situ tissue engineering can be prepared with different fiber diameters to influence cell recruitment, adhesion, and differentiation. For cardiovascular applications, we investigated the impact of different fiber diameters (2, 5, 8, and 11 μm) in electrospun poly(ε-caprolactone) scaffolds on endothelial colony forming cells (ECFCs) in comparison to mature endothelial cells (HUVECs). In 2D cultures and on 2 μm fiber scaffolds, ECFC morphology and phenotype resemble those of HUVECs. When cultured on scaffolds with 5-11 μm fibers, a different behavior was detected. HUVECs developed a cytoskeleton organized circumferentially around the fibers, with collagen alignment in the same direction. ECFCs, instead, aligned the cytoskeleton along the scaffold fiber axis and deposited a homogeneous layer of collagen over the fibers; moreover, a subpopulation of ECFCs gained the αSMA marker. These results showed that ECFCs do not behave like mature endothelial cells in a 3D fibrous environment.

Development of Non-Cell Adhesive Vascular Grafts Using Supramolecular Building Blocks

Cell-free approaches to in situ tissue engineering require materials that are mechanically stable and are able to control cell-adhesive behavior upon implantation. Here, the development of mechanically stable grafts with non-cell adhesive properties via a mix-and-match approach using ureido-pyrimidinone (UPy)-modified supramolecular polymers is reported. Cell adhesion is prevented in vitro through mixing of end-functionalized or chain-extended UPy-polycaprolactone (UPy-PCL or CE-UPy-PCL, respectively) with end-functionalized UPy-poly(ethylene glycol) (UPy-PEG) at a ratio of 90:10. Further characterization reveals intimate mixing behavior of UPy-PCL with UPy-PEG, but poor mechanical properties, whereas CE-UPy-PCL scaffolds are mechanically stable. As a proof-of-concept for the use of non-cell adhesive supramolecular materials in vivo, electrospun vascular scaffolds are applied in an aortic interposition rat model, showing reduced cell infiltration in the presence of only 10% of UPy-PEG. Together, these results provide the first steps toward advanced supramolecular biomaterials for in situ vascular tissue engineering with control over selective cell capturing.

Tailoring the void space and mechanical properties in electrospun scaffolds towards physiological ranges

Electrospinning has proven to be a promising method to produce scaffolds for tissue engineering despite the frequently encountered limitations in 3-dimensional tissue formation due to a lack of cell infiltration. To fully unlock the potential of electrospun scaffolds for tissue engineering, the void space within the fibrous network needs to be increased substantially and in a controlled manner. Low-temperature electrospinning (LTE) increases the fiber to fiber distance by embedding ice particles as void spacers during fiber deposition. Scaffold porosities up to 99.5% can be reached and in line with the increase in void space, the mechanical properties of the scaffolds shift towards the range for native biological tissue. While both the physiological mechanical properties and high porosity were promising for tissue engineering applications, control of the porosity in three dimensions was still limited when using LTE methods. Based on a range of LTE spun scaffolds made of poly(lactic acid) and poly(ε-caprolactone), we found that changing the ratio between the rate of ice crystal formation and polymer fiber deposition only had a small effect on the 3D-porosity of the final scaffold architecture. Varying the fiber stiffness, however, offers considerable control over the scaffold void space.

Heart valve tissue regeneration

Abstract: Creating a functional valve equivalent to the appropriate tissue structure and mechanical properties of the heart is a key issue in the area of heart valve tissue engineering. The design and production of three-dimensional scaffolds with both microscopically and macroscopically optimised material properties to regulate tissue development and ultimately valve functioning is believed to be critical. Electrospinning offers many ways of developing such scaffolds. In this chapter we aim to provide an overview of the various requirements for these scaffolds as well as presenting ways of achieving them using electrospinning.

Ex vivo proof-of-concept of end-to-end scaffold-enhanced laser-assisted vascular anastomosis of porcine arteries.

OBJECTIVE The low welding strength of laser-assisted vascular anastomosis (LAVA) has hampered the clinical application of LAVA as an alternative to suture anastomosis. To improve welding strength, LAVA in combination with solder and polymeric scaffolds (ssLAVA) has been optimized in vitro. Currently, ssLAVA requires proof-of-concept in a physiologically representative ex vivo model before advancing to in vivo studies. This study therefore investigated the feasibility of ex vivo ssLAVA in medium-sized porcine arteries. METHODS Scaffolds composed of poly(ε-caprolactone) (PCL) or poly(lactic-co-glycolic acid) (PLGA) were impregnated with semisolid solder and placed over coapted aortic segments. ssLAVA was performed with a 670-nm diode laser. In the first substudy, the optimum number of laser spots was determined by bursting pressure analysis. The second substudy investigated the resilience of the welds in a Langendorf-type pulsatile pressure setup, monitoring the number of failed vessels. The type of failure (cohesive vs adhesive) was confirmed by electron microscopy, and thermal damage was assessed histologically. The third substudy compared breaking strength of aortic repairs made with PLGA and semisolid genipin solder (ssLAVR) to repairs made with BioGlue. RESULTS ssLAVA with 11 lasing spots and PLGA scaffold yielded the highest bursting pressure (923 ± 56 mm Hg vs 703 ± 96 mm Hg with PCL ssLAVA; P = .0002) and exhibited the fewest failures (20% vs 70% for PCL ssLAVA; P = .0218). The two failed PLGA ssLAVA arteries leaked at 19 and 22 hours, whereas the seven failed PCL ssLAVA arteries burst between 12 and 23 hours. PLGA anastomoses broke adhesively, whereas PCL welds failed cohesively. Both modalities exhibited full-thickness thermal damage. Repairs with PLGA scaffold yielded higher breaking strength than BioGlue repairs (323 ± 28 N/cm(2) vs 25 ± 4 N/cm(2), respectively; P = .0003). CONCLUSIONS PLGA ssLAVA yields greater anastomotic strength and fewer anastomotic failures than PCL ssLAVA. Aortic repairs with BioGlue were inferior to those produced with PLGA ssLAVR. The results demonstrate the feasibility of ssLAVA/R as an alternative method to suture anastomosis or tissue sealant. Further studies should focus on reducing thermal damage.

Biodegradable polymer scaffold, semi‐solid solder, and single‐spot lasing for increasing solder‐tissue bonding in suture‐free laser‐assisted vascular repair

We recently showed the fortifying effect of poly‐caprolactone (PCL) scaffold in liquid solder‐mediated laser‐assisted vascular repair (ssLAVR) of porcine carotid arteries, yielding a mean ± SD leaking point pressure of 488 ± 111 mmHg. Despite supraphysiological pressures, the frequency of adhesive failures was indicative of weak bonding at the solder‐tissue interface. As a result, this study aimed to improve adhesive bonding by using a semi‐solid solder and single‐spot vs. scanning irradiation. In the first experiment, in vitro ssLAVR (n = 30) was performed on porcine abdominal aorta strips using a PCL scaffold with a liquid or semi‐solid solder and a 670‐nm diode laser for dual‐pass scanning. In the second experiment, the scanning method was compared to single‐spot lasing. The third experiment investigated the stability of the welds following hydration under quasi‐physiological conditions. The welding strength was defined by acute breaking strength (BS). Solder‐tissue bonding was examined by scanning electron microscopy and histological analysis was performed for thermal damage analysis. Altering solder viscosity from liquid to semi‐solid solder increased the BS from 78 ± 22 N/cm2 to 131 ± 38 N/cm2. Compared to scanning ssLAVR, single‐spot lasing improved adhesive bonding to a BS of 257 ± 62 N/cm2 and showed fewer structural defects at the solder‐tissue interface but more pronounced thermal damage. The improvement in adhesive bonding was associated with constantly stronger welds during two weeks of hydration. Semi‐solid solder and single‐spot lasing increased welding strength by reducing solder leakage and improving adhesive bonding, respectively. The improvement in adhesive bonding was associated with enhanced weld stability during hydration. Copyright

Mechanics of Electrospun Scaffolds: An Application to Heart Valve Tissue Engineering

In the last decade electrospinning has shown its potential of being a feasible technique to manufacture scaffolds for tissue engineering [1]. Previous studies observed that, on a micrometer scale, the topology of the scaffold plays a fundamental role in the spreading and the differentiation of the cells [2], and in the growth of neo-extracellular matrix. On a tissue scale (in the order of cm) the stiffness of the construct enables the possibility of applying mechanical cues for the development of a functional engineered tissue [3]. Studies on scaffold mechanics based on volume-averaging theory succeeded in demonstrating that the arrangement of the micro-scale scaffold components influences the macro-scale mechanical behavior [4].Copyright