Yong-Ung Lee

3D-Printed Biodegradable Polymeric Vascular Grafts

Congenital heart defect interventions may benefit from the fabrication of patient-specific vascular grafts because of the wide array of anatomies present in children with cardiovascular defects. 3D printing is used to establish a platform for the production of custom vascular grafts, which are biodegradable, mechanically compatible with vascular tissues, and support neotissue formation and growth.

Regenerative implants for cardiovascular tissue engineering.

A fundamental problem that affects the field of cardiovascular surgery is the paucity of autologous tissue available for surgical reconstructive procedures. Although the best results are obtained when an individuals own tissues are used for surgical repair, this is often not possible as a result of pathology of autologous tissues or lack of a compatible replacement source from the body. The use of prosthetics is a popular solution to overcome shortage of autologous tissue, but implantation of these devices comes with an array of additional problems and complications related to biocompatibility. Transplantation offers another option that is widely used but complicated by problems related to rejection and donor organ scarcity. The field of tissue engineering represents a promising new option for replacement surgical procedures. Throughout the years, intensive interdisciplinary, translational research into cardiovascular regenerative implants has been undertaken in an effort to improve surgical outcome and better quality of life for patients with cardiovascular defects. Vascular, valvular, and heart tissue repair are the focus of these efforts. Implants for these neotissues can be divided into 2 groups: biologic and synthetic. These materials are used to facilitate the delivery of cells or drugs to diseased, damaged, or absent tissue. Furthermore, they can function as a tissue-forming device used to enhance the bodys own repair mechanisms. Various preclinical studies and clinical trials using these advances have shown that tissue-engineered materials are a viable option for surgical repair, but require refinement if they are going to reach their clinical potential. With the growth and accomplishments this field has already achieved, meeting those goals in the future should be attainable.

Computational model of the in vivo development of a tissue engineered vein from an implanted polymeric construct

Advances in vascular tissue engineering have been tremendous over the past 15 years, yet there remains a need to optimize current constructs to achieve vessels having true growth potential. Toward this end, it has been suggested that computational models may help hasten this process by enabling time-efficient parametric studies that can reduce the experimental search space. In this paper, we present a first generation computational model for describing the in vivo development of a tissue engineered vein from an implanted polymeric scaffold. The model was motivated by our recent data on the evolution of mechanical properties and microstructural composition over 24 weeks in a mouse inferior vena cava interposition graft. It is shown that these data can be captured well by including both an early inflammatory-mediated and a subsequent mechano-mediated production of extracellular matrix. There remains a pressing need, however, for more data to inform the development of next generation models, particularly the precise transition from the inflammatory to the mechanobiological dominated production of matrix having functional capability.

Biaxial mechanical properties of the inferior vena cava in C57BL/6 and CB-17 SCID/bg mice.

Multiple murine models have proven useful in studying the natural history of neovessel development in the tissue engineering of vascular grafts. Nevertheless, to better understand longitudinal changes in the biomechanics of such neovessels, we must first quantify native tissue structure and properties. In this paper, we present the first biaxial mechanical data for, and nonlinear constitutive modeling of, &QJ;the inferior vena cava from two models used in tissue engineering: wild-type C57BL/6 and immunodeficient CB-17 SCID/bg mice. Results show that inferior vena cava from the latter are significantly stiffer in the circumferential direction, both materially (as assessed by a stored energy function) and structurally (as assessed by the compliance), despite a lower intramural content of fibrillar collagen and similar wall thickness. Quantifying the natural history of neovessel development in different hosts could lead to increased insight into the mechanisms by which cells fashion and maintain extracellular matrix in order to match best the host stiffness while ensuring sufficient vascular integrity.

Fast-degrading bioresorbable arterial vascular graft with high cellular infiltration inhibits calcification of the graft

Objective: Bioresorbable vascular grafts are biologically active grafts that are entirely reconstituted by host‐derived cells through an inflammation‐mediated degradation process. Calcification is a detrimental condition that can severely affect graft performance. Therefore, prevention of calcification is of great importance to the success of bioresorbable arterial vascular grafts. The objective of this study was to test whether fast‐degrading (FD) bioresorbable arterial grafts with high cellular infiltration will inhibit calcification of grafts. Methods: We created two versions of bioresorbable arterial vascular grafts, slow‐degrading (SD) grafts and FD grafts. Both grafts had the same inner layer composed of a 50:50 poly(l‐lactic‐co‐&egr;‐caprolactone) copolymer scaffold. However, the outer layer of SD grafts was composed of poly(l‐lactic acid) nanofiber, whereas the outer layer of FD grafts was composed of a combination of poly(l‐lactic acid) and polyglycolic acid nanofiber. Both grafts were implanted in 8‐ to 10‐week‐old female mice (n = 15 in the SD group, n = 10 in the FD group) as infrarenal aortic interposition conduits. Animals were observed for 8 weeks. Results: von Kossa staining showed calcification in 7 of 12 grafts in the SD group but zero in the FD group (P < .01, χ2 test). The cell number in the outer layer of FD grafts was significantly higher than in the SD grafts (SD, 0.87 ± 0.65 × 103/mm2; FD, 2.65 ± 1.91 × 103/mm2; P = .02). Conclusions: The FD bioresorbable arterial vascular graft with high cellular infiltration into the scaffold inhibited calcification of grafts. Clinical Relevance: Bioresorbable vascular grafts are biologically active grafts that are entirely reconstituted by host‐derived cells throughout the life span of the patient. Calcification of the graft is a detrimental condition that can severely affect graft performance. Therefore, preventing graft calcification is of great importance to success of bioresorbable vascular grafts. We hypothesized that remaining scaffold polymer could affect graft calcification. Therefore, we created fast‐degrading bioresorbable arterial vascular grafts. We report here that no calcification occurred in the fast‐degrading grafts, although calcification occurred in the slow‐degrading grafts. These findings provide further strategies for prevention of calcification after implantation of bioresorbable vascular grafts in the clinical setting.

Tropoelastin inhibits intimal hyperplasia of mouse bioresorbable arterial vascular grafts

Neointimal hyperplasia, which results from the activation, proliferation and migration of vascular smooth muscle cells (SMCs), is a detrimental condition for vascular stents or vascular grafts that leads to stenosis. Preventing neointimal hyperplasia of vascular grafts is critically important for the success of arterial vascular grafts. We hypothesized that tropoelastin seeding onto the luminal surface of the graft would prevent neointimal hyperplasia through suppressing neointimal smooth muscle cell proliferation. In this study, we investigated the efficacy of tropoelastin seeding in preventing neointimal hyperplasia of bioresorbable arterial vascular grafts. Poly (glycolic acid) (PGA) fiber mesh coated with poly (l-lactic-co-ε-caprolactone) (PLCL) scaffolds reinforced by poly (l-lactic acid) (PLA) nano-fibers were prepared as bioresorbable arterial grafts. Tropoelastin was then seeded onto the luminal surface of the grafts. Tropoelastin significantly reduced the thickness of the intimal layer. This effect was mainly due to a substantial reduction the number of cells that stained positive for SMC (α-SMA) and PCNA in the vessel walls. Mature elastin and collagen type I and III were unchanged with tropoelastin treatment. This study demonstrates that tropoelastin seeding is beneficial in preventing SMC proliferation and neointimal hyperplasia in bioresorbable arterial vascular grafts. STATEMENT OF SIGNIFICANCE Small resorbable vascular grafts can block due to the over-proliferation of smooth muscle cells in neointimal hyperplasia. We show here that the proliferation of these cells is restricted in this type of graft. This is achieved with a simple dip, non-covalent coating of tropoelastin. It is in principle amendable to other grafts and is therefore an attractive process. This study is particularly significant because: (1) it shows that smooth muscle cell proliferation can be reduced while still accommodating the growth of endothelial cells, (2) small vascular grafts with an internal diameter of less than 1mm are amenable to this process, and (3) this process works for resorbable grafts.

Computational Growth and Remodeling Model for Evolving Tissue Engineered Vascular Grafts in the Venous Circulation

The field of vascular tissue engineering continues to advance rapidly, yet there is a pressing need to understand better the time course of polymer degradation and the sequence of cell-mediated matrix deposition and organization. Mounting evidence suggests that cells respond to mechanical perturbations through a process of growth and remodeling (G&R) to establish, maintain, and restore a preferred state of homeostatic stress. Previous computational models utilizing G&R approaches have captured arterial responses to diverse changes in mechanical loading [1, 8, 9]. Recently, a G&R framework was also introduced to account for the kinetics of polymer degradation as well as synthesis and degradation of neotissue constituents [5]. Niklason et al. demonstrated that models of G&R can predict both evolving tissue composition and mechanical behavior after extended periods of in vitro culture of polymer-based tissue-engineered vascular grafts (TEVGs), thus providing insights into the timecourse of neotissue formation and polymer removal. Moreover, they suggest that models of G&R can be powerful tools for the future refinement and optimization of scaffold designs. Nevertheless, such computational models have not yet been developed for examining the formation of neotissue following the implantation of a polymeric TEVG in vivo.Copyright

Angiotensin II receptor I blockade prevents stenosis of tissue engineered vascular grafts

We previously developed a tissue‐engineered vascular graft (TEVG) made by seeding autologous cells onto a biodegradable tubular scaffold, in an attempt to create a living vascular graft with growth potential for use in children undergoing congenital heart surgery. Results of our clinical trial showed that the TEVG possesses growth capacity but that its widespread clinical use is not yet advisable due to the high incidence of TEVG stenosis. In animal models, TEVG stenosis is caused by increased monocytic cell recruitment and its classic (“M1”) activation. Here, we report on the source and regulation of these monocytes. TEVGs were implanted in wild‐type, CCR2 knockout (Ccr2−/−), splenectomized, and spleen graft recipient mice. We found that bone marrow—derived Ly6C+hi monocytes released from sequestration by the spleen are the source of mononuclear cells infiltrating the TEVG during the acute phase of neovessel formation. Furthermore, short‐term administration of losartan (0.6 g/L, 2 wk), an angiotensin II type 1 receptor antagonist, significantly reduced the macrophage populations (Ly6C+/−/F480+) in the scaffolds and improved long‐term patency in TEVGs. Notably, the combined effect of bone marrow‐derived mononuclear cell seeding with short‐term losartan treatment completely prevented the development of TEVG stenosis. Our results provide support for pharmacologic treatment with losartan as a strategy to modulate monocyte infiltration into the grafts and thus prevent TEVG stenosis.—Ruiz‐Rosado, J. D. D., Lee, Y.‐U., Mahler, N., Yi, T., Robledo‐Avila, F.,Martinez‐Saucedo,D.,Lee,A.Y.,Shoji,T., Heuer, E.,Yates,A.R., Pober, J. S., Shinoka,T., Partida‐Sanchez, S., Breuer, C. K. Angiotensin II receptor I blockade prevents stenosis of tissue engineered vascular grafts. FASEB J. 32, 6822‐6832 (2018). www.fasebj.org