**1. Introduction**

Attempts to develop synthetic vascular grafts for the replacement of diseased vascular sec‐ tions have been an area of active research over the past decades [1]. However, thrombosis formation as a result of platelet adhesion to the luminal surface of synthetic graft and reste‐ nosis caused by host inflammatory remain a challenge, especially for small-diameter (<6 mm) graft replacement [2,3]. Therefore, the haemocompatibility of the biomaterial used in the graft is a prerequisite for clinical success. As the result, various strategies have been de‐ veloped to improve the blood compatibility of biomaterial surfaces, including the surface immobilization of anti-coagulants such as heparin [4] and sulfated silk fibroin [5], the incor‐ poration of polyethylene oxide or negatively charged side chains [6,7], and surface passiva‐ tion with protein layers, such as albumin [8]. Despite the efficacy of these approaches in preventing acute thrombogenesis, concerns remain on the drug elution lifespan, with possi‐ ble consequence of late thrombosis [9]. To avoid undesirable blood-material interaction, the seeding of autologous endothelial cells (ECs) onto the luminal surface of the graft is consid‐ ered to be an ideal approach to increase the patency of synthetic grafts [10]. Many studies have indicated that endothelial cells release factors that regulate thrombogenesis and plate‐ let activation [11], while delayed or absent stent endothelialization has been implicated in late thrombosis and adverse clinical outcomes [13]. Thus, rapid endothelialization of vascu‐ lar grafts is of great importance for blood-contacting vessels for long-term patency.

Due to its slow degradation rates in vivo (2-4 years) [14], good mechanical strength, and bio‐ compatibility with vascular cell types [15,16], polycaprolactone (PCL) is currently being ex‐

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tensively investigated as scaffolds for vascular tissue engineering applications [17-21]. However, the intrinsic hydrophobicity and poor cytocompatibility of PCL substrates lead to poor affinity for cell adhesion, thereby restricting their applications as blood-contacting de‐ vices. Consequently, surface modification of PCL is necessary to improve cell adhesion and proliferation. Functional polymer brushes containing reactive hydroxyl (-OH), carboxyl (- COOH) or amine (-NH2) groups have been successfully grafted onto the PCL surfaces using γ-ray irradiated, ozone or photo-induced polymerization grafting to introduce hydrophilici‐ ty [9,16,22-24]. These flexible reactive groups on the polymer brushes are well-suited to con‐ jugate bioactive macromolecules for improved cytocompatibility. However, γ-ray irradiated, ozone or photo-induced polymerization grafting of polymer brushes has several limitations, including low density of grafting due to steric hindrance, uncontrollable graft yield of polymer brushes, and undesired formation of a covalent bond between reactive groups on the polymer brushes and the surface [25]. Hence, an alternative grafting approach that allows control over brush density, polydispersity and composition is desired.

One such alternative is the use of surface-initiated atom transfer radical polymerization (ATRP) approach to covalently graft polymer brushes in a tunable and controllable manner [26]. This approach allows the preparation of well-defined dense polymer brushes contain‐ ing reactive pendant groups (e.g. -OH, -COOH, or epoxide groups), and provides highly re‐ active binding sites for functional biomolecules.[27] As a result, surface-initiated ATRP provides a promising approach to fabricate PCL substrates with well-defined polymer brushes of controlled length and density, as well as tunable grafting density of biomacromo‐ lecules. However, to the best of our knowledge, only few studies have been devoted to mod‐ ifying biodegradable polyester polymers using surface-initiated ATRP to improve their cytocompatibility or blood compatibility [27,28]. Also, the functionality of the attached cells was not thoroughly investigated in those studies.

As such, the aim of the current study is to utilize the surface-initiated ATRP method to tailor PCL substrates with dense functional P(GMA) brushes and high-density immobilized gela‐ tin to improve their properties for cell attachment and proliferation. Each functionalization step was ascertained by XPS, AFM and water contact angle measurements. The cytocompat‐ ibility of the functionalized PCL substrates was evaluated using human umbilical vein en‐ dothelial cells (HUVECs) and the effect of different surface properties on the regulation of the thrombogenicity of the attached cells was also investigated.
