Artificial lymphatic drainage systems for vascularized microfluidic scaffolds

Abstract

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 mg/mL to 30 mg/mL and scaffold hydraulic conductivity decreased by a factor of twenty. 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 by 0.9 cm2) 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.

Vascular Development and Morphogenesis in Biomaterials

“…This resistance lies in series with the resistance across the vessel wall; thus, in scaffolds of higher resistance, the pressure gradients in the scaffold are higher, the pressure differences across the vessel wall (i.e., P TM ) are lower, and the vessel is less stable (Truslow et al 2009). Indeed, we have experimentally found in fibrin gels of various densities that vascular stability is anticorrelated with scaffold permeability K (Figure 6.2b) (Wong et al 2013). Vessels in fibrin gels with K above ~10 −9 cm 4 / dyn ⋅ s are stable, but not in gels with lower permeabilities.…”

“…In resistive scaffolds, the introduction of empty channels to directly lower the pressure within the scaffold should increase P TM and hence enhance vascular stability. These artificial lymphatic-like structures serve to drain fluid from the scaffold at a small, but measurable, rate (Wong et al 2013). By placing such drainage channels perpendicular to the vessel axis, we have shown that drainage effectively stabilizes the portions of the vessel that lie sufficiently close to the drainage channel (within ~1 mm, for a drainto-vessel distance of ~0.5 mm in 30 mg/mL fibrin gels) (Figure 6.4b) (Wong et al 2013).…”

“…Once the channels have formed, their vascularization is straightforward albeit technically demanding. Below, we restrict the discussion to the vascularization of single channels (Chrobak et al 2006) with and without accompanying drainage channels (Wong et al 2013) because these simple geometries have allowed routine mathematical analyses of the fluid and solid mechanics of the vessels and scaffolds. For step-by-step experimental procedures, we refer the reader to published protocols Tien 2009, 2011).…”

“…This method precisely controls the lumenal (i.e., intravascular) pressure and its axial gradient, and can reliably maintain steady flow for weeks, but only if the endothelium remains adherent to the scaffold. Recently, we have developed methods to control the fluid pressure of the scaffold outside the vessel (i.e., the extravascular or interstitial fluid pressure) (Wong et al 2013). This work was inspired by the design and function of the lymphatic system in vivo, in which lowpressure endothelial tubes drain excess fluid from the tissue space and thus maintain the interstitial pressure at near-atmospheric pressures (Schmid-Schönbein 1990).…”

“…The endothelium can gradually delaminate from the microfluidic channel over the course of a week ( Figure 6.2b); flow rates will likewise decrease, and often substantially. To understand the origin of this effect, and to discover signals that can promote vascular stability, we have manipulated the hydrodynamic stresses across the vessel wall directly (through variation of lumenal pressure P lumen and interstitial pressure P scaffold ) or indirectly (through changes in transvascular fluid flow via alterations in endothelial permeability) (Price et al 2010, Leung et al 2012, 2013.…”