Vascularization by Julia Tomaszewski
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Introduction Vascularization is the spontaneous development of vascular networks within an engineered tissue once being implanted and interacting with a host’s tissue . This spontaneous capillary formation may take up to several weeks, depending on the thickness of this tissue. This is because the vascular interactions from the host are only able to penetrate several tenths of a centimeter through the tissue per day . This is the main challenge of vascularization: the inability to provide an immediate blood supply to engineered vascular tissues implanted into a host. Methods to improve the mass transfer characteristics of this tissue are currently being developed which would allow rapid oxygen and nutrient diffusion throughout the engineered tissue. Capillaries are typically 100-200 µm away from the cells where easy transfer of oxygen and nutrients to the cells occurs . Tissues such as skin and cartilage are relatively simple to construct and implant into a patient because they are thin and can obtain oxygen and nutrients from the patient fairly easily, but thicker tissues, such as the heart, brain, and liver, require additional methods for diffusion and transfer of oxygen and nutrients . Prevascularization techniques have the most potential of increasing the thickness of vascular tissues [1,2,4], meaning the vascular tissue is constructed in vitro and then implanted into the patient.
- Cells are able to diffuse blood in vitro, but once implanted into host, they lose this ability. In vivo methods already have the host blood flow.
Motivation  Strategies of Vascularization
Scaffolds One of the easiest designed strategies of vascularization is the use of engineered scaffolds to implant tissues . This is because, in addition to simple transplantation of the desired tissues, they may also contain additional elements or factors to promote diffusion and transport within the tissue. Scaffolds may be used to transport valuable growth factors into the host that promote angiogenesis in the engineered tissue. Growth factors such as vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and platelet-derived growth factor (PDGF) may be used to produce this angiogenic effect . With the addition of these growth factors in the scaffold, the implanted tissue is able to integrate into the host with increased stability while also increasing the rate of blood flow throughout the tissue, improving the viability of the cells . When multiple growth factors are combined in a scaffold, the formation of blood vessels is increased. For example, when VEGF and PDGF are coupled in a scaffold, VEGF is released rapidly while PDGF is released slowly and more uniformly  because of their varying stimulation levels in the blood vessel . Growth factors show great promise for vascularization, but growth factors added in excess may be toxic to the tissue and blood vessels. This is because growth factors added in excess have proved to cause leaky vessels and may even cause hypotension . The design of the scaffold may also be engineered to increase vascularization. Pore construction of the scaffold is essential for vessel diffusion. A pore size greater 250 µm has shown to increase the rate of vessel diffusion of the tissue . In addition to pore size, pores must also be connected in a way to permit cell mobility to promote vessel transmission . Difficulty persists in pore construction because of the randomness of formation . In vivo Systems To produce vascular tissues up to 1 mm in thickness, cell sheet technology can be used. This is a multi-step process requiring multiple surgeries, but once these tissues are implanted into a host, blood vessels are able to immediately integrate throughout it . First, an 80 µm thick cell sheet tissue is implanted directly onto a healthy site in the host . Blood is supplied to this implanted tissue by the host . A study conducted in rats required 1-3 days for the tissues to form vascular networks . With the successful network organization, additional cell sheet layers can then be added to this tissue construct on the rat over time . The development of a thick cell sheet structure can take multiple weeks because of the slow vessel diffusion limitation . Once the thick tissue has completely vascularized, it is then removed from this healthy site of the host and transplanted onto the damaged site. Once implanted, this tissue is immediately able to integrate its capillary network with the host capillary network throughout the entire engineered tissue , which is the main advantage of this strategy.