Computational design of drainage systems for vascularized scaffolds

Incidence of superficial left hepatic vein and its usability for graft hepatic vein venoplasty in pediatric liver transplantation

BACKGROUNDS

The anatomy of the left hepatic vein (LHV) is variable; thus, it should be considered for graft hepatic vein (GHV) venoplasty for left lateral section (LLS) and left liver grafts. This study assessed the incidence of superficial LHV (sLHV) branches according to LHV anatomy and its usability for GHV venoplasty in pediatric liver transplantation (LT).

METHODS

This study consisted of three parts: (1) anatomical classification of LHV variations and the incidence of sLHV branches; (2) morphometric simulative analysis of GHV reconstruction and (3) clinical application based on LHV anatomy.

RESULTS

The LHV anatomy of 248 potential LLS graft donors was classified into four types according to the number and location of GHV openings: one single opening (type 1; n = 186 [75.0%]), two large openings (type 2; n = 35 [14.1%]), one large and one small adjacent opening (type 3; n = 14 [5.6%]), and two large widely-separated openings (type 4; n = 13 [5.2%]). An sLHV branch was identified in 87 of 248 (35.1%) donor livers. Morphometric analysis of simulative GHV venoplasty with an sLHV branch increased GHV diameters by 30% in type 1 LLS grafts and 20% in type 2/3 LLS grafts. An analysis of 50 consecutive patients who underwent pediatric LT showed that the 2-year rates of GHV obstruction were 2.0% with LLS grafts and 0% with left liver grafts.

CONCLUSIONS

The GHV orifice can be enlarged through LHV anatomy-based unification venoplasty. Unification venoplasty with an sLHV branch provided sufficient enlargement of the GHV orifice.

In silico analysis of a hierarchical microfluidic vascular network: Detecting the location of angiogenic sprouting

Lack of oxygen is one of the leading causes of failure in engineered tissue. Therefore, angiogenesis will be necessary for the survival of larger tissues in vivo. In addition, a proper lymphatic system that plays an essential role in relieving inflammation and maintaining tissue homeostasis is of great importance for tissue regeneration and repair. Many biomechanical parameters are involved in controlling angiogenesis and capillary network generation, which are challenging to study and control in experimental studies or in vitro. In the present study, using numerical modeling, the effect of various geometric and biomechanical parameters in creating suitable conditions for angiogenesis was investigated. Furthermore, sprouting points were predicted using flow dynamics. For this purpose, a porous scaffold, flow channels with parametric geometry that followed Murray's law, and a drainage channel were considered. Results suggested that the geometry of the microfluidic channels and the characteristics of the vessel wall and scaffold plays a complementary role in determining the transmural pressure. It was found that a twofold increase in the vascular hydraulic conductivity can reduce the minimum transmural pressure by up to 28% and increase the drainage flow rate by 44%. In addition, the minimum magnitude of transmural pressure tends to zero for scaffold's hydraulic conductivity values smaller than 10^-11  m³ s kg^-1 . The results of this study can be used in optimizing the design of the relevant microfluidic systems to induce angiogenesis and avoid leakage in the constructed implantable tissue.

Microfluidic Biomaterials

Since their initial description in 2005, biomaterials that are patterned to contain microfluidic networks ("microfluidic biomaterials") have emerged as promising scaffolds for a variety of tissue engineering and related applications. This class of materials is characterized by the ability to be readily perfused. Transport and exchange of solutes within microfluidic biomaterials is governed by convection within channels and diffusion between channels and the biomaterial bulk. Numerous strategies have been developed for creating microfluidic biomaterials, including micromolding, photopatterning, and 3D printing. In turn, these materials have been used in many applications that benefit from the ability to perfuse a scaffold, including the engineering of blood and lymphatic microvessels, epithelial tubes, and cell-laden tissues. This article reviews the current state of the field and suggests new areas of exploration for this unique class of materials.

Biofabrication strategies for creating microvascular complexity

Design and fabrication of effective biomimetic vasculatures constitutes a relevant and yet unsolved challenge, lying at the heart of tissue repair and regeneration strategies. Even if cell growth is achieved in 3D tissue scaffolds or advanced implants, tissue viability inevitably requires vascularization, as diffusion can only transport nutrients and eliminate debris within a few hundred microns. This engineered vasculature may need to mimic the intricate branching geometry of native microvasculature, referred to herein as vascular complexity, to efficiently deliver blood and recreate critical interactions between the vascular and perivascular cells as well as parenchymal tissues. This review first describes the importance of vascular complexity in labs- and organs-on-chips, the biomechanical and biochemical signals needed to create and maintain a complex vasculature, and the limitations of current 2D, 2.5D, and 3D culture systems in recreating vascular complexity. We then critically review available strategies for design and biofabrication of complex vasculatures in cell culture platforms, labs- and organs-on-chips, and tissue engineering scaffolds, highlighting their advantages and disadvantages. Finally, challenges and future directions are outlined with the hope of inspiring researchers to create the reliable, efficient and sustainable tools needed for design and biofabrication of complex vasculatures.

Channels in a porous scaffold: a new player for vascularization

Vascularization is essential for tissue regeneration. Despite extensive efforts in the past decades, sufficient and rapid vascularization remains a major challenge in tissue engineering. Many studies have shown that the addition of channels in a porous scaffold can provide the ability to promote cell growth and rapid vascularization, thus leading to better outcomes in new tissue formation. Large size scaffolds lack perfusable channel networks and negatively impair the survival of transplanted cells and tissue function development, leading to necrotic core formation and the failure of functional tissue formation. Presently, there are many methods to produce channels in porous scaffolds for vascularization. Here, we review the function of channels in porous scaffolds and the approaches to produce those channels.

Design principles for lymphatic drainage of fluid and solutes from collagen scaffolds

In vivo, tissues are drained of excess fluid and macromolecules by the lymphatic vascular system. How to engineer artificial lymphatics that can provide equivalent drainage in biomaterials remains an open question. This study elucidates design principles for engineered lymphatics, by comparing the rates of removal of fluid and solute through type I collagen gels that contain lymphatic vessels or unseeded channels, or through gels without channels. Surprisingly, no difference was found between the fluid drainage rates for gels that contained vessels or bare channels. Moreover, solute drainage rates were greater in collagen gels that contained lymphatic vessels than in those that had bare channels. The enhancement of solute drainage by lymphatic endothelium was more pronounced in longer scaffolds and with smaller solutes. Whole-scaffold imaging revealed that endothelialization aided in solute drainage by impeding solute reflux into the gel without hindering solute entry into the vessel lumen. These results were reproduced by computational models of drainage with a flow-dependent endothelial hydraulic conductivity. This study shows that endothelialization of bare channels does not impede the drainage of fluid from collagen gels and can increase the drainage of macromolecules by preventing solute transport back into the scaffold. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 106-114, 2018.

Proliferative and Differentiation Potential of Multipotent Mesenchymal Stem Cells Cultured on Biocompatible Polymer Scaffolds with Various Physicochemical Characteristics

Biocompatibility of film and fibrous scaffolds from polylactide-based polymers and the relationship between their architecture and the functional characteristics of mesenchymal stem cells were studied. Cell culturing on polylactide-based film and fibrous matrixes did not deteriorate cell morphology and their proliferation and differentiation capacities. The rate of cell proliferation and penetration in microporous 3D matrices with the same porosity parameters and pore size depended on their spatial organization. The above materials can be used as scaffolds for mesenchymal stem cells for creation of tissue engineering implants. The scaffold size and structure should be determined by the defects in the organs in which the regeneration processes have to be stimulated.

3D-printed fluidic networks as vasculature for engineered tissue

Fabrication of vascular networks within engineered tissue remains one of the greatest challenges facing the fields of biomaterials and tissue engineering. Historically, the structural complexity of vascular networks has limited their fabrication in tissues engineered in vitro. Recently, however, key advances have been made in constructing fluidic networks within biomaterials, suggesting a strategy for fabricating the architecture of the vasculature. These techniques build on emerging technologies within the microfluidics community as well as on 3D printing. The freeform fabrication capabilities of 3D printing are allowing investigators to fabricate fluidic networks with complex architecture inside biomaterial matrices. In this review, we examine the most exciting 3D printing-based techniques in this area. We also discuss opportunities for using these techniques to address open questions in vascular biology and biophysics, as well as for engineering therapeutic tissue substitutes in vitro.

Physical and Chemical Signals That Promote Vascularization of Capillary-Scale Channels

Proper vascularization remains critical to the clinical application of engineered tissues. To engineer microvessels in vitro, we and others have delivered endothelial cells through preformed channels into patterned extracellular matrix-based gels. This approach has been limited by the size of endothelial cells in suspension, and results in plugging of channels below ~30 μm in diameter. Here, we examine physical and chemical signals that can augment direct seeding, with the aim of rapidly vascularizing capillary-scale channels. By studying tapered microchannels in type I collagen gels under various conditions, we establish that stiff scaffolds, forward pressure, and elevated cyclic AMP levels promote endothelial stability and that reverse pressure promotes endothelial migration. We applied these results to uniform 20-μm-diameter channels and optimized the magnitudes of pressure, flow, and shear stress to best support endothelial migration and vascular stability. This vascularization strategy is able to form millimeter-long perfusable capillaries within three days. Our results indicate how to manipulate the physical and chemical environment to promote rapid vascularization of capillary-scale channels within type I collagen gels.

Crosslinking of collagen scaffolds promotes blood and lymphatic vascular stability

The low stiffness of reconstituted collagen hydrogels has limited their use as scaffolds for engineering implantable tissues. Although chemical crosslinking has been used to stiffen collagen and protect it against enzymatic degradation in vivo, it remains unclear how crosslinking alters the vascularization of collagen hydrogels. In this study, we examine how the crosslinking agents genipin and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide alter vascular stability and function in microfluidic type I collagen gels in vitro. Under moderate perfusion (∼10 dyn/cm(2) shear stress), tubes of blood endothelial cells (ECs) exhibited indistinguishable stability and barrier function in untreated and crosslinked scaffolds. Surprisingly, under low perfusion (∼5 dyn/cm(2) shear stress) or nearly zero transmural pressure, microvessels in crosslinked scaffolds remained stable, while those in untreated gels rapidly delaminated and became poorly perfused. Similarly, tubes of lymphatic ECs under intermittent flow were more stable in crosslinked gels than in untreated ones. These effects correlated well with the degree of mechanical stiffening, as predicted by analysis of fracture energies at the cell-scaffold interface. This work demonstrates that crosslinking of collagen scaffolds does not hinder normal EC physiology; instead, crosslinked scaffolds promote vascular stability. Thus, routine crosslinking of scaffolds may assist in vascularization of engineered tissues.

Artificial lymphatic drainage systems for vascularized microfluidic scaffolds

The formation of a stably perfused microvasculature continues to be a major challenge in tissue engineering. Previous work has suggested the importance of a sufficiently large transmural pressure in maintaining vascular stability and perfusion. Here we show that a system of empty channels that provides a drainage function analogous to that of lymphatic microvasculature in vivo can stabilize vascular adhesion and maintain perfusion rate in dense, hydraulically resistive fibrin scaffolds in vitro. In the absence of drainage, endothelial delamination increased as scaffold density increased from 6 to 30 mg/mL and scaffold hydraulic conductivity decreased by a factor of 20. Single drainage channels exerted only localized vascular stabilization, the extent of which depended on the distance between vessel and drainage as well as scaffold density. Computational modeling of these experiments yielded an estimate of 0.40-1.36 cm H2O for the minimum transmural pressure required for vascular stability. We further designed and constructed fibrin patches (0.8 × 0.9 cm(2)) that were perfused by a parallel array of vessels and drained by an orthogonal array of drainage channels; only with the drainage did the vessels display long-term stability and perfusion. This work underscores the importance of drainage in vascularization, especially when a dense, hydraulically resistive scaffold is used.

Carbon nanotube interaction with extracellular matrix proteins producing scaffolds for tissue engineering

In recent years, significant progress has been made in organ transplantation, surgical reconstruction, and the use of artificial prostheses to treat the loss or failure of an organ or bone tissue. In recent years, considerable attention has been given to carbon nanotubes and collagen composite materials and their applications in the field of tissue engineering due to their minimal foreign-body reactions, an intrinsic antibacterial nature, biocompatibility, biodegradability, and the ability to be molded into various geometries and forms such as porous structures, suitable for cell ingrowth, proliferation, and differentiation. Recently, grafted collagen and some other natural and synthetic polymers with carbon nanotubes have been incorporated to increase the mechanical strength of these composites. Carbon nanotube composites are thus emerging as potential materials for artificial bone and bone regeneration in tissue engineering.

Plasma expanders stabilize human microvessels in microfluidic scaffolds

Plasma expanders such as dextran and hydroxyethyl starch (HES) are important components of solutions designed to maintain vascular volume in the clinical setting and to preserve organs ex vivo before transplantation. Here, we show that these polymers also exert stabilizing effects on engineered microvessels in microfluidic type I collagen and fibrin scaffolds. Standard growth media, which did not contain dextran or HES, led to severe leakage, vascular collapse, and catastrophic failure of perfusion. Remarkably, vessels that were provided with 3% dextran or 5% HES had few focal leaks, maintained adhesion to the scaffold, and were typically viable and patent for at least 2 weeks. We found that the junctional marker VE-cadherin localized to a wide band in the presence of plasma expanders, but only at concentrations that also stabilized vessels. In conjunction with a previous computational model (Wong et al., Biomaterials 2010;31:4706-4714), our results suggest that plasma expanders stabilize microvessels via physical mechanisms that enhance VE-cadherin localization at junctions and thereby limit vascular leakiness.

Engineered channels enhance cellular density in perfused scaffolds

Scaffold-based tissue engineering provides cells with an engineered matrix to enhance and direct cell attachment, proliferation and differentiation. One critical limitation to current tissue engineering approaches is the inability to create densely populated constructs thicker than a few 100 μm. We hypothesized that development of porous, channeled scaffolds would increase cell density and uniformity of their spatial distribution through scaffold channel perfusion. Patterned polyurethane sheets were fabricated using a sprayed phase separation technique and laminated together to form 1.5 mm thick channeled scaffolds. Hydraulic permeability testing confirmed the presence of functional channels throughout the multilaminate construct. A continuous flow bioreactor was used to perfuse the construct with medium during the culture period. Cross-sectional cell densities and spatial uniformities were measured in channeled and nonchanneled scaffolds under different seeding and culture conditions. Channeled scaffolds were found to have higher densities of human mesenchymal stem cells than nonchanneled samples. Perfused scaffolds had more uniform spatial distribution of cells within the scaffold compared to statically cultured scaffolds. In conclusion, we have shown the channeled scaffolds to be a promising approach toward creating thick tissue-engineered constructs.

Perfusion systems that minimize vascular volume fraction in engineered tissues

This study determines the optimal vascular designs for perfusing engineered tissues. Here, "optimal" describes a geometry that minimizes vascular volume fraction (the fractional volume of a tissue that is occupied by vessels) while maintaining oxygen concentration above a set threshold throughout the tissue. Computational modeling showed that optimal geometries depended on parameters that affected vascular fluid transport and oxygen consumption. Approximate analytical expressions predicted optima that agreed well with the results of modeling. Our results suggest one basis for comparing the effectiveness of designs for microvascular tissue engineering.

Effect of mechanical factors on the function of engineered human blood microvessels in microfluidic collagen gels

This work examines how mechanical signals affect the barrier function and stability of engineered human microvessels in microfluidic type I collagen gels. Constructs that were exposed to chronic low flow displayed high permeabilities to bovine serum albumin and 10 kDa dextran, numerous focal leaks, low size selectivity, and short lifespan of less than one week. Higher flows promoted barrier function and increased longevity; at the highest flows, the barrier function rivaled that observed in vivo, and all vessels survived to day 14. By studying the physiology of microvessels of different geometries, we established that shear stress and transmural pressure were the dominant mechanical signals that regulated barrier function and vascular stability, respectively. In microvessels that were exposed to high flow, elevation of intracellular cyclic AMP further increased the selectivity of the barrier and strongly suppressed cell proliferation. Computational models that incorporated stress dependence successfully predicted vascular phenotype. Our results indicate that the mechanical microenvironment plays a major role in the functionality and stability of engineered human microvessels in microfluidic collagen gels.

Morphometric and simulation analyses of right hepatic vein reconstruction in adult living donor liver transplantation using right lobe grafts

The incidence of clinically significant right hepatic vein (RHV) stenosis after adult living donor liver transplantation has been higher than expected. In this study, an assessment of the risk factors for the development of RHV stenosis in this context was undertaken. Hepatic anatomy, surgical techniques, and the incidence of RHV stenosis 1 year after transplantation were evaluated retrospectively in 225 recipients of right lobe grafts. These patients underwent independent RHV reconstruction, which was facilitated by the application of computed tomography morphometry and computational simulation analyses. Three types of preparation of the orifice of the graft RHV and 7 types of preparation for venoplasty of the recipient RHV were used. The frequency of high, middle, and low sites of RHV insertion into the inferior vena cava (IVC) was 56.0%, 36.4%, and 7.6%, respectively, for donors, and 26.7%, 58.7%, and 14.7%, respectively, for recipients. Nine patients (4%) developed RHV stenosis of early onset that required stent insertion during the first 2 postoperative weeks; in 12 patients (5.3%), RHV stenosis of delayed onset occurred. Inappropriate matching of RHV sites of insertion correlated with the incidence of stenosis of early onset (P = 0.039). Technical refinements to avoid adverse consequences of inappropriate ventrodorsal matching of RHV sites of insertion include making the recipient RHV orifice wide and enlarging the recipient IVC by a customized incision and patch venoplasty after anatomical assessment of the RHV and IVC of the graft and recipient.

The role of cyclic AMP in normalizing the function of engineered human blood microvessels in microfluidic collagen gels

Nearly all engineered tissues must eventually be vascularized to survive. To this end, we and others have recently developed methods to synthesize extracellular matrix-based scaffolds that contain open microfluidic networks. These scaffolds serve as templates for the formation of endothelial tubes that can be perfused; whether such microvascular structures are stable and/or functional is largely unknown. Here, we show that compounds that elevate intracellular concentrations of the second messenger cyclic AMP (cAMP) strongly normalize the phenotype of engineered human microvessels in microfluidic type I collagen gels. Cyclic AMP-elevating agents promoted vascular stability and barrier function, and reduced cellular turnover. Under conditions that induced the highest levels of cAMP, the physiology of engineered microvessels in vitro quantitatively mirrored that of native vessels in vivo. Computational analysis indicated that cAMP stabilized vessels partly via its enhancement of barrier function.