## 1 Introduction  
Regenerative medicine is an increasingly important issue in the biomedicine field, which emphasizes the usage of healthy cells to replace necrotic ones to repair functionless tissues or organs [1]. Tissue engineering, aiming to generate biological tissues from the cells, has become one of the most popular research topics in regenerative medicine [2]. The engineered tissues should connect with the patient blood circulation system through the network-like capillaries so that the organ transplant acquires oxygen and nutrient supplies and discharges metabolic wastes [3]. The capillary formation has two processes: vasculogenesis and angiogenesis. Vasulogenesis is one of the characteristics of early embryonic development, which refers to dispersed endothelial cell aggregation to form cords in an avascular environment and the network construction with the center-hollowed tubes [4]. Angiogenesis is for capillary-existing sites, where the endothelial cells that make up the capillaries develop new networks through sprouting, common in self-healing at the wound sites [5]. The way to achieve in vitro capillaries culturing is similar to the vasculogenesis process. A capillary-likenetwork will gradually develop from scattered endothelial cells via cell migration and morphological variation [6]. Many biocompatible materials have been tried as the substrate for culturing endothelial cells. A phenomenon of biogel remodeling occurs during the capillary network formation, in which the endothelial cells exert traction force and thereby change the base material distribution [7; 8; 9]. Scholars have used biogel composed of ingredients such as a mixture of type I collagen and gelatin to mimic the environment of burns and scalds denatured [10]. Researchers have also adopted poly (methyl acrylate) (PMA) or poly (ethyl acrylate) (PEA) 3D printing scaffolds with a fibronectin layer on the surface to study the attachment structure and the function of promoting cell attachment [11; 12]. Fibronectin is a protein dimer consisting of two nearly identical polypeptide chains. Modules III9-10 in fibronectin correspond to the integrin-binding site, where the synergy sequence PHSRN of III9 has a role in modulating binding with \(\alpha\beta\beta 1\) integrin, and the RGD sequence of III10 is the site of cell attachment via \(\alpha\beta\beta 1\) and \(\alpha\
u\beta 3\) integrins [13]. Modules III12-14 correspond to the heparin-binding areas. They can bind several growth factors, including platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), and fibroblast growth factor (FGF) families [14; 15]. Because growth factors can be attached in advance and released gradually, fibronectin is designed as a chemical signal carrier and regarded as a potential material for in vivo experiments [16; 17]. Vascular endothelial growth factor has been widely used in cultivation of capillaries. VEGF can be categorized into types A to D. Type A activates the phosphorylation pathway of VEGF-receptor 2 (VEGFR2), which is essential to the process of blood vessel formation [18; 19; 20; 21]. There are four most notable VEGF-A isoforms found in humans, composed of 121, 145, 165, and 189 amino acid sequences. Except for 121, the other three isoforms have their protein structures located in exons 6a and 7, providing the binding ability to fibronectin with different binding degrees [22]. VEGF-A145 and A189 can strongly bind both the cell surface and extracellular matrix [23; 24; 25], and VEGF-A165 is the most abundantly expressed one and the most potent initiator of vasculogenesis [26]. The effect of VEGF on vasculogenesis is the key to cultivate a capillary network in the engineered tissue. In vivo, VEGF could be soluble in the medium or protein-bound [22; 27; 28]; however, how the soluble and bound forms of VEGF regulate the network forming is still not completely clear.