Challenges and strategies for in situ endothelialization and long-term lumen patency of vascular grafts

Vascular diseases are the most prevalent cause of ischemic necrosis of tissue and organ, which even result in dysfunction and death. Vascular regeneration or artificial vascular graft, as the conventional treatment modality, has received keen attentions. However, small-diameter (diameter < 4 mm) vascular grafts have a high risk of thrombosis and intimal hyperplasia (IH), which makes long-term lumen patency challengeable. Endothelial cells (ECs) form the inner endothelium layer, and are crucial for anti-coagulation and thrombogenesis. Thus, promoting in situ endothelialization in vascular graft remodeling takes top priority, which requires recruitment of endothelia progenitor cells (EPCs), migration, adhesion, proliferation and activation of EPCs and ECs. Chemotaxis aimed at ligands on EPC surface can be utilized for EPC homing, while nanofibrous structure, biocompatible surface and cell-capturing molecules on graft surface can be applied for cell adhesion. Moreover, cell orientation can be regulated by topography of scaffold, and cell bioactivity can be modulated by growth factors and therapeutic genes. Additionally, surface modification can also reduce thrombogenesis, and some drug release can inhibit IH. Considering the influence of macrophages on ECs and smooth muscle cells (SMCs), scaffolds loaded with drugs that can promote M2 polarization are alternative strategies. In conclusion, the advanced strategies for enhanced long-term lumen patency of vascular grafts are summarized in this review. Strategies for recruitment of EPCs, adhesion, proliferation and activation of EPCs and ECs, anti-thrombogenesis, anti-IH, and immunomodulation are discussed. Ideal vascular grafts with appropriate surface modification, loading and fabrication strategies are required in further studies.

Self-Assembled Polyprodrug Amphiphile for Subcutaneous Xenograft Tumor Inhibition with Prolonged Acting Time In Vivo

Polymeric drug delivery system termed as "polyprodrug amphiphile" poly(2-methylacryloyloxyethyl phosphorylcholine)-b-poly(10-hydroxy-camptothecin methacrylate (pMPC-b-pHCPT) is developed for the prolonged-acting cancer therapy. It is obtained by two-step reversible addition-fragmentation chain transfer polymerization of zwitterionic monomer MPC and an esterase-responsive polymerizable prodrug methacrylic anhydride-CPT, respectively. This diblock polymer is composed of both antifouling (pMPC) and bioactive (pHCPT) segments and the drug is designed as a building block to construct the polymer skeleton directly. Due to its distinct amphiphilicity, the polymer can self-assemble into micelles with different dynamic sizes by facilely tuning the ratio of MPC/HCPT under physiological conditions. The outer pMPC shell is superhydrophilic to form dense hydrate layer preventing the nanosystem from unwanted nonspecific protein adsorption, which is the main lead cause of the rapid clearance of nanoparticles in vivo, thus facilitating the accumulation of drugs in tumor sites via enhanced permeability and retention effect. The configuration of the polyprodrug amphiphile is confirmed by several measurements. The resistance to albumin adsorption, prolonged plasma retention time, accumulation in tumor sites, and anticancer activity of the micelles is also investigated in vitro and in vivo. This novel amphiphile can be expected as a promising agent for the passive targeted prolonged-acting cancer therapy.

Surface modification and endothelialization of biomaterials as potential scaffolds for vascular tissue engineering applications

This review highlights the recent developments of surface modification and endothelialization of biomaterials in vascular tissue engineering applications.

Elastic, double-layered poly (l-lactide-co-ϵ-caprolactone) scaffold for long-term vascular reconstruction

Synthetic vessel grafts have been used as vascular substitutes for cardiovascular bypass procedures. In this study, we developed a novel tubular double-layered poly(l-lactide-co-ϵ-caprolactone) scaffold that did not require pretreatment with cell seeding by promoting autologous tissue regeneration by inducing the proliferation and differentiation of endothelial and smooth muscle progenitor cells after implantation. The patency and mechanical properties were maintained for one year after implantation, although 95% of the poly(l-lactide-co-ϵ-caprolactone) scaffolds had degraded. After this period, there was a lining of endothelial cells, an accumulation of collagen and elastin, and the development of neovascularization inside the poly(l-lactide-co-ϵ-caprolactone).

Small-diameter tissue engineered vascular graft made of electrospun PCL/lecithin blend

In this study, natural lecithin was incorporated into cholesterol-poly(ε-caprolactone) (Chol-PCL) by solution blending in order to modify the performance of the hydrophobic and bio-inert PCL. The fibrous Chol-PCL/lecithin membranes were fabricated by electrospinning, and the surface morphology and properties were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, static water contact angle, and mechanical tensile testing. The blood compatibility of the scaffolds was evaluated by in vitro hemolysis assay. The cytocompatibility of the scaffolds was investigated by cell adhesion and proliferation using bone-marrow mesenchymal stem cells (MSCs). Subcutaneous implantation was also performed to evaluate the in vivo inflammatory reaction. The tubular tissue-engineered vascular graft (TEVG) was further constructed by rolling cell sheet comprising fibrous membrane and MSCs. Furthermore, endothelial cells (ECs) were seeded onto the lumen of the graft with the aim to form vascular endothelium. The preliminary results indicate that electrospun Chol-PCL/lecithin scaffolds show improved hemocompatibility and cytocompatibility compared with neat Chol-PCL, and combining the Chol-PCL/lecithin fibrous scaffold with MSCs and ECs with well controlled distribution is a promising strategy for constructing TEVGs.

Electrospun poly (lactic-co-glycolic acid)/ multiwalled carbon nanotubes composite scaffolds for guided bone tissue regeneration

A series of poly (lactic-co-glycolic acid) (PLGA)/multiwalled carbon nanotubes (MWNTs) composite scaffolds was prepared by electrospinning for tissue engineering applications. The effect of MWNTs content on the structure and properties of composite scaffolds was characterized by scanning electron microscopy (SEM), X-ray diffractrometry (XRD), thermogravimetric analysis (TGA), and universal testing machine (UTM). The attachment ability and viability of rat bone marrow-derived mesenchymal stem cells (BMSCs) in the presence of the scaffolds were also investigated. Morphological characterization showed that the addition of different amounts of MWNTs increased the average fiber diameter; for instance, from 717 nm (neat PLGA) to 2193 nm at 1.0% MWNTs. Thermal characterization showed that the incorporation of MWNTs into the PLGA matrix increased the thermal stability of the composite scaffolds. The analysis of mechanical properties of the PLGA/MWNTs composites revealed great improvement over pure PLGA scaffold. The attachment and proliferation of BMSCs were significantly increased in the PLGA/MWNTs scaffolds compared with the PLGA control. Therefore, the PLGA/MWNTs composite scaffolds fabricated by electrospinning may be potentially useful in tissue engineering applications, particularly as scaffolds for bone tissue regeneration.