Endothelialization of Rationally Microtextured Surfaces with Minimal Cell Seeding Under Flow

The Effects of Biomimetic Surface Topography on Vascular Cells: Implications for Vascular Conduits

Cardiovascular diseases (CVDs) are the leading cause of mortality worldwide and represent a pressing clinical need. Vascular occlusions are the predominant cause of CVD and necessitate surgical interventions such as bypass graft surgery to replace the damaged or obstructed blood vessel with a synthetic conduit. Synthetic small-diameter vascular grafts (sSDVGs) are desired to bypass blood vessels with an inner diameter <6 mm yet have limited use due to unacceptable patency rates. The incorporation of biophysical cues such as topography onto the sSDVG biointerface can be used to mimic the cellular microenvironment and improve outcomes. In this review, the utility of surface topography in sSDVG design is discussed. First, the primary challenges that sSDVGs face and the rationale for utilizing biomimetic topography are introduced. The current literature surrounding the effects of topographical cues on vascular cell behavior in vitro is reviewed, providing insight into which features are optimal for application in sSDVGs. The results of studies that have utilized topographically-enhanced sSDVGs in vivo are evaluated. Current challenges and barriers to clinical translation are discussed. Based on the wealth of evidence detailed here, substrate topography offers enormous potential to improve the outcome of sSDVGs and provide therapeutic solutions for CVDs.

The influence of physical and spatial substrate characteristics on endothelial cells

Cardiovascular diseases are a main cause of death worldwide, leading to a growing demand for medical devices to treat this patient group. Central to the engineering of such devices is a good understanding of the biology and physics of cell-surface interactions. In existing blood-contacting devices, such as vascular grafts, the interaction between blood, cells, and material is one of the main limiting factors for their long-term durability. An improved understanding of the material's chemical- and physical properties as well as its structure all play a role in how endothelial cells interact with the material surface. This review provides an overview of how different surface structures influence endothelial cell responses and what is currently known about the underlying mechanisms that guide this behavior. The structures reviewed include decellularized matrices, electrospun fibers, pillars, pits, and grated surfaces.

Proper Orthogonal Decomposition Analysis Reveals Cell Migration Directionality During Wound Healing

A proper orthogonal decomposition (POD) order reduction method was implemented to reduce the full high dimensional dynamical system associated with a wound healing cell migration assay to a low-dimensional approximation that identified the prevailing cell trajectories. The POD analysis generated POD modes that were representative of the prevalent cell trajectories. The shapes of the POD modes depended on the location of the cells with respect to the wound and exposure to disturbed (DF) or undisturbed (UF) fluid flow where the net flow was in the antegrade direction with a retrograde component or fully antegrade, respectively. For DF and UF, the POD modes of the downstream cells identified trajectories that moved upstream against the flow, while upstream POD modes exhibited sideways cell migrations. In the absence of flow, no major shape differences were observed in the POD modes on either side of the wound. The POD modes also served to reconstruct the cell migration of individual cells. With as few as three modes, the predominant cell migration trajectories were reconstructed, while the level of accuracy increased with the inclusion of more POD modes. The POD order reduction method successfully identified the predominant cell migratory trajectories under static and varying pulsatile fluid flow conditions serving as a first step in the development of artificial intelligence models of cell migration in disease and development.

Bistability of Dielectrically Anisotropic Nematic Crystals and the Adaptation of Endothelial Collectives to Stress Fields

Endothelial monolayers physiologically adapt to flow and flow-induced wall shear stress, attaining ordered configurations in which elongation, orientation, and polarization are coherently organized over many cells. Here, with the flow direction unchanged, a peculiar bi-stable (along the flow direction or perpendicular to it) cell alignment is observed, emerging as a function of the flow intensity alone, while cell polarization is purely instructed by flow directionality. Driven by the experimental findings, the parallelism between endothelia is delineated under a flow field and the transition of dual-frequency nematic liquid crystals under an external oscillatory electric field. The resulting physical model reproduces the two stable configurations and the energy landscape of the corresponding system transitions. In addition, it reveals the existence of a disordered, metastable state emerging upon system perturbation. This intermediate state, experimentally demonstrated in endothelial monolayers, is shown to expose the cellular system to a weakening of cell-to-cell junctions to the detriment of the monolayer integrity. The flow-adaptation of monolayers composed of healthy and senescent endothelia is successfully predicted by the model with adjustable nematic parameters. These results may help to understand the maladaptive response of in vivo endothelial tissues to disturbed hemodynamics and the progressive functional decay of senescent endothelia.

Failure Analysis of TEVG’s II: Late Failure and Entering the Regeneration Pathway

Tissue-engineered vascular grafts (TEVGs) are a promising alternative to treat vascular disease under complex hemodynamic conditions. However, despite efforts from the tissue engineering and regenerative medicine fields, the interactions between the material and the biological and hemodynamic environment are still to be understood, and optimization of the rational design of vascular grafts is an open challenge. This is of special importance as TEVGs not only have to overcome the surgical requirements upon implantation, they also need to withhold the inflammatory response and sustain remodeling of the tissue. This work aims to analyze and evaluate the bio-molecular interactions and hemodynamic phenomena between blood components, cells and materials that have been reported to be related to the failure of the TEVGs during the regeneration process once the initial stages of preimplantation have been resolved, in order to tailor and refine the needed criteria for the optimal design of TEVGs.

A preliminary study on polycaprolactone and gelatin-based bilayered tubular scaffolds with hierarchical pore size constructed from nano and microfibers for vascular tissue engineering

Due to the insufficient endothelialization and the poor colonization of smooth muscle cells (SMCs), small-diameter vascular tissue engineering is still challenging. An ideal vascular graft is expected to effectively support the formation of endothelial monolayer and the colonization of SMCs. In this study, we proposed a bilayered scaffold with hierarchical pore size constructed from nano and microfibers by electrospinning PCL-PEG-PCL (PCE) and a blend of PCE and gelatin (PCEG) sequentially. The structural features of nano and microfibers were tuned by the concentration of PCE and the proportion of PCE/gelatin in electrospun solution respectively. The results demonstrated the best nanofiber morphology and relatively high mechanical properties were achieved in 18% PCE (w/v) (PCE18) and PCE and gelatin with a weight ratio of 7:3 (P7G3) at a concentration of 18% (w/v) electrospun membranes. The in vitro co-culturing studies of cells and membranes indicated all the PCE membranes supported the proliferation and spreading of endothelial cells and the further endothelialization of the membranous surface, while PCEG membranes facilitated the migration inward of SMCs. Taking the porosity and mechanical properties into consideration, PCE18 and P7G3 were chosen to construct the inner and outer layers of the bilayered scaffold with hierarchical pore size respectively. The circumferential ring test demonstrated that the bilayered scaffold has good mechanical property both in dry and wet state. After cells were co-cultured with this bilayered scaffold for 7 days, the results manifested a continuous endothelial monolayer has formed on the luminal surface and the SMCs have started to colonized from outer layers, indicating the vast potential of this bilayered scaffold in vascular remodeling and regeneration.

A free-form patterning method enabling endothelialization under dynamic flow

Endothelialization strategies aim at protecting the surface of cardiovascular devices upon their interaction with blood by the generation and maintenance of a mature monolayer of endothelial cells. Rational engineering of the surface micro-topography at the luminal interface provides a powerful access point to support the survival of a living endothelium under the challenging hemodynamic conditions created by the implant deployment and function. Surface structuring protocols must however be adapted to the complex, non-planar architecture of the target device precluding the use of standard lithographic approaches. Here, a novel patterning method, harnessing the condensation and evaporation of water droplets on a curing liquid elastomer, is developed to introduce arrays of microscale wells on the surface of a biocompatible silicon layer. The resulting topographies support the in vitro generation of mature human endothelia and their maintenance under dynamic changes of flow direction or magnitude, greatly outperforming identical, but flat substrates. The structuring approach is additionally demonstrated on non-planar interfaces yielding comparable topographies. The intrinsically free-form patterning is therefore compatible with a complete and stable endothelialization of complex luminal interfaces in cardiovascular implants.

Evaluation of Chemo- and Photo-toxicity of a Live Fluorescent Dye for Cell Analysis

Live cell imaging is used to track the dynamic adaptation of cell size and motility to various external factors. Bright-field configuration can be used for these experiments; however, the analysis can be challenging and difficult to automate. In this direction, a superior alternative is represented by the use of live cell dyes, which provide intense fluorescence from subcellular structures of living cells. Yet, the potential chemo- and photo-toxicity of the fluorophores poses the necessity of an accurate protocol optimization to avoid artefacts. Toxicity studies generally focus on cell proliferation and apoptosis, neglecting the cellular activities under investigation. Here, we present the case of SYTO 13 in combination with primary endothelial cells. The optimization of the staining procedure is tested comparing cell proliferation and motility rate. In addition, the combined effect of staining and fluorescent illumination, reporting for photochemical toxicity, is evaluated. We demonstrate that while cell viability and proliferation are mainly unaffected by the staining and imagining protocols, a significant reduction of the motility rate is induced both by the chemical dye alone and in combination with fluorescent illumination. The general implications for this procedure are discussed.

Topography-driven alterations in endothelial cell phenotype and contact guidance

Understanding how endothelial cell phenotype is affected by topography could improve the design of new tools for tissue engineering as many tissue engineering approaches make use of topography-mediated cell stimulation. Therefore, we cultured human pulmonary microvascular endothelial cells (ECs) on a directional topographical gradient to screen the EC vascular-like network formation and alignment response to nano to microsized topographies. The cell response was evaluated by microscopy. We found that ECs formed unstable vascular-like networks that aggregated in the smaller topographies and flat parts whereas ECs themselves aligned on the larger topographies. Subsequently, we designed a mixed topography where we could explore the network formation and proliferative properties of these ECs by live imaging for three days. Vascular-like network formation continued to be unstable on the topography and were only produced on the flat areas and a fibronectin coating did not improve the network stability. However, an instructive adipose tissue-derived stromal cell (ASC) coating provided the correct environment to sustain the vascular-like networks, which were still affected by the topography underneath. It was concluded that large microsized topographies inhibit vascular endothelial network formation but not proliferation and flat and nano/microsized topographies allow formation of early networks that can be stabilized by using an ASC instructive layer.

Lipoconstruct surface topography grating size influences vascularization onset in the dorsal skinfold chamber model

After skin tissue injury or pathological removal, vascularization timing is paramount in graft survival. As full thickness skin grafts often fail to become perfused over larger surfaces, split-thickness grafts are preferred and can be used together with biomaterials, which themselves are non-angiogenic. One way of promoting vascular ingrowth is to "pre-vascularize" an engineered substitute by introducing endothelial cells (ECs). Since it has been previously demonstrated that surface structured biomaterials have an effect on wound healing, skin regeneration, and fibrosis reduction, we proposed that a microvascular-rich lipoconstruct with anisotropic topographical cues could be a clinically translatable vascularization approach. Murine lipofragments were formed with three polydimethylsiloxane molds (flat, 5 µm, and 50 µm parallel gratings) and implanted into the dorsal skinfold chamber of male C57BL/6 mice. Vascular ingrowth was observed through intravital microscopy over 21 days and further assessed by histology and protein identification. Our investigation revealed that topographical feature size influenced the commencement of neovascular ingrowth, with 5 µm gratings exhibiting early construct perfusion at 3 days post-operation, and 50 µm being delayed until day 5. We therefore postulate that surface structured lipoconstructs may serve as an easily obtained and produced construct suitable for providing soft tissue and ECs to tissue defects. STATEMENT OF SIGNIFICANCE: Skin graft failures due to inadequate or uneven perfusion frequently occur and can be even more complicated in deep, difficult to heal wounds, or bone coverage. In complex injuries, biomaterials are often used to cover bone structures with a standard split thickness graft; however, perfusion can take up to 3 weeks. Thus, any means to promote faster and uniform vascularization could significantly reduce healing time, as well as lower patient down-time. As pre-vascularized constructs have reported success in research, we created a cost-efficient, translatable method with no additional laboratory time as adipose tissue can be harvested and used immediately. We further used surface topography as an aspect to modulate construct perfusion, which has been reported for the first time here.

Introduction of Nature's Complexity in Engineered Blood-compatible Biomaterials

Biomaterials with excellent blood-compatibility are needed for applications in vascular replacement therapies, such as vascular grafts, heart valves and stents, and in extracorporeal devices such as hemodialysis machines and blood-storage bags. The modification of materials that are being used for blood-contacting devices has advanced from passive surface modifications to the design of more complex, smart biomaterials that respond to relevant stimuli from blood to counteract coagulation. Logically, the main source of inspiration for the design of new biomaterials has been the endogenous endothelium. Endothelial regulation of hemostasis is complex and involves a delicate interplay of structural components and feedback mechanisms. Thus, challenges to develop new strategies for blood-compatible biomaterials now lie in incorporating true feedback controlled mechanisms that can regulate blood compatibility in a dynamic way. Here, supramolecular material systems are highlighted as they provide a promising platform to introduce dynamic reciprocity, due to their inherent dynamic nature.

Advancements in mechanical circulatory support for patients in acute and chronic heart failure

Cardiogenic shock (CS) continues to have high mortality and morbidity despite advances in pharmacological, mechanical, and reperfusion approaches to treatment. When CS is refractory to medical therapy, percutaneous mechanical circulatory support (MCS) should be considered. Acute MCS devices, ranging from intra-aortic balloon pumps (IABPs) to percutaneous temporary ventricular assist devices (VAD) to extracorporeal membrane oxygenation (ECMO), can aid, restore, or maintain appropriate tissue perfusion before the development of irreversible end-organ damage. Technology has improved patient survival to recovery from CS, but in patients whom cardiac recovery does not occur, acute MCS can be effectively utilized as a bridge to long-term MCS devices and/or heart transplantation. Heart transplantation has been limited by donor heart availability, leading to a greater role of left ventricular assist device (LVAD) support. In patients with biventricular failure that are ineligible for LVAD implantation, further advancements in the total artificial heart (TAH) may allow for improved survival compared to medical therapy alone. In this review, we discuss the current state of acute and durable MCS, ongoing advances in LVADs and TAH devices, improved methods of durable MCS implantation and patient selection, and future MCS developments in this dynamic field that may allow for optimization of HF treatment.

Facile endothelium protection from TNF-α inflammatory insult with surface topography

Adverse events triggered by the direct contact between blood and synthetic materials constitute a sincere shortcoming of cardiovascular implant technology. A well-connected autologous endothelium, generated through the process of endothelialization, impedes such interaction and endows the implant luminal interface with optimal protection. The endothelialization of artificial substrates is the result of a complex interplay between endothelial cells (ECs), surface topography, and flow-generated wall shear stress (WSS). This is however tainted by the pro-inflammatory signaling, typical of cardiovascular patients, which compromises endothelial integrity and survival. Here, we challenge human endothelial monolayers with the pro-inflammatory factor TNF-α under realistic WSS conditions. In these experimental settings we demonstrate that the simple contact between ECs and an optimized surface geometry can inhibit NF-kB activation downstream of TNF-α yielding increased stability of VE-Cadherin mediated cell-to-cell junctions and of focal adhesions. Therefore the here-presented topographic modification can be implemented on a range of artificial substrates enabling their endothelialization under supra-physiological flow and in the presence of pro-inflammatory insults. These new findings constitute an important step toward achieving the full hemocompatibility of cardiovascular implants.

A Novel Bioreactor System for the Assessment of Endothelialization on Deformable Surfaces

The generation of a living protective layer at the luminal surface of cardiovascular devices, composed of an autologous functional endothelium, represents the ideal solution to life-threatening, implant-related complications in cardiovascular patients. The initial evaluation of engineering strategies fostering endothelial cell adhesion and proliferation as well as the long-term tissue homeostasis requires in vitro testing in environmental model systems able to recapitulate the hemodynamic conditions experienced at the blood-to-device interface of implants as well as the substrate deformation. Here, we introduce the design and validation of a novel bioreactor system which enables the long-term conditioning of human endothelial cells interacting with artificial materials under dynamic combinations of flow-generated wall shear stress and wall deformation. The wall shear stress and wall deformation values obtained encompass both the physiological and supraphysiological range. They are determined through separate actuation systems which are controlled based on validated computational models. In addition, we demonstrate the good optical conductivity of the system permitting online monitoring of cell activities through live-cell imaging as well as standard biochemical post-processing. Altogether, the bioreactor system defines an unprecedented testing hub for potential strategies toward the endothelialization or re-endothelialization of target substrates.