PU vascular grafts

Our group also developed rapamycin (RM)-eluting stents using electrospun PU vascular grafts that could effectively suppress local smooth muscle cell proliferation. We observed that the release kinetics was characteristic of a Fickian diffusion for at least 77 days in vitro. RM-PU fibers generafed via powder blending showed the highest encapsulation efficiency. 

The most used materials to manufacture artificial grafts are polyethylene tere-phthalate (PET, Dacron ), polytetrafluoroethylene (PTFE), and PUs. The first PU vascular graft was a polyester-based PU (Vascugraft ). These prostheses showed good biocompatibility but a poor chemical stability in Also polyether-based PUs... 

The development of soft-tissue engineering needs bioresorbable materials exhibiting elastomeric properties. Elastomeric polyurethane (PU) vascular grafts can withstand the action of stress and load and undergo an elastic recovery with little or no hysteresis. In recent years, biocompatible and biodegradable segmented polyurethanes (SPUs) have been studied for applications in the tissue engineering field. 

Approaches to develop vascular grafts from bioresorbable PUs are ongoing, and while many laboratory studies have been performed there are hardly any bioresorbable PUs available in the market. Table 15.3 sunamarizes the mechanical performance of some selected bioresorbable PU vascular grafts. 

Medical PUs are another subset of PU elastomers. Segmented PUs were first suggested for use in a biomedical application in 1967. ° Early work with PU elastomers showed that these materials could be used for implants without causing a large, unwanted inflammatory response. The first medical devices made of PUs, however, were found to be susceptible to hydrolysis and degraded faster than desired. ° From that time, new biostable materials have been developed for use as pacemaker leads, catheters, vascular grafts. 

The compliance and longevity of a potential vascular graft material can influence the retention of mechanical integrity, Studies in the polyurethane/poly-Z-lactide (PU/PLLA) system have shown that a decrease in the percentage of polyurethane causes a decrease in compliance and an increase in aneurismal degeneration. These studies further showed that high mechanical compliance of the graft at implantation stimulates elastin forma-... 

Poly(ether-urethanes) (PU) BIOSPAN Polymer Technology Croup, Inc. Heart valves, vascular grafts, and other blood-contacting devices... 

Surprisingly, the use of cocultures with degradable PUs in cardiac TE systems has been limited when vascular grafts are excluded from the research. A study by Parrag et al. used murine-derived embryonic stem cells (mESCs) and mouse embryonic fibroblasts (MEEs) to TE cardiomyocyte-derived tissues. mESCs are pluripotent cells that require proper cues to differentiate into specific cell types. In the study, Parrag et al. showed that both the coculture of mESCs with MEEs and the use of aligned microfi-brous PU scaffolds provided the cues necessary to induce mESC differentiation to a functioning cardiomyocyte phenotype [104]. 

More recently, the Santerre lab used the coculture of primary cells with monocytes/ MDMs to promote a wound-heaUng milieu to encourage cell attachment, infiltration, and proliferation on D-PHl PU films and scaffolds for TE vascular graft applications [67,91,105-107]. In the context of blood vessel TE, it was found that by coculturing monocytes with ECs on D-PHI films that the ECs attached better and spread out more while displaying more EC functional markers than EC monocultures (CD31) [91 ]. VSMCs benefited from coculture with monocytes on both film and porous scaffold forms of D-PHI [67,106,107], On porous scaffolds, the monocyte coculture helped VSMCs migrate within the pores and increased deposition of extracellular matrix (ECM) proteins [107]. A recent study found that monocyte-conditioned medium could also promote VSMC attachment to... 

Therefore, electrospun RM-containing PU fibers can serve as effective drug carriers for the local suppression of cell proliferation and could be used as RM-eluting scaffolds for vascular grafts [101,102]. 

Aliphatic polyesters, copolyesters, and natural macromolecules (mainly collagen, elastin, fibrin, chitosan, and hyaluronic acid (HA)), as well as their blends, have also been used to prepare polymeric-based vascular grafts with tuned mechanical properties. Couet et al. reported a comprehensive overview of materials that have been explored as scaffolds for vascular tissue engineering [59]. The analysis of the mechanical behavior of non-PU-based vascular grafts is beyond the scope of this chapter. 

Beyond the biocompatibility presented by medical grade and novel synthesized bioresorbable PUs, they lack the bioactivity of natural biopolymers such as collagen and elastin. In response to this challenge, many authors have blended PUs with natural polymers to improve the biocompatibility and bioactivity of vascular grafts. Table 15.4 summarizes the mechanical performance of some selected vascular grafts based on PU blends. 

Guo F, Wang N, Wang L, Hou L, Ma L, Liu J, et al. An electrospun strong PCL/PU composite vascular graft with mechanical anisotropy and cyclic stability. J Mater Chem A 2015 3 4782-7. 

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