**1. Introduction**

Collagen is the main structural protein in the extracellular matrix (ECM), constituting approximately 30% of the whole body protein content in animals [1]. More than 29 different types of collagen have been identified and described. In the human body, most of the collagen is type I [2]. The collagen protein contains triple-helix structures that consist of three almost identical polypeptide chains [3]. Type I collagen is present in bone, skin, dentin, cornea, blood vessels, fibrocartilage, and tendon; it has the unique ability to form fibrils that have high tensile strength and important functions [4,5]. In the past decades, it has been widely used in food manufacturing and the cosmetics industry [6].

Recently, the interest in collagen has become widespread among medicine and tissue engineering because of its predominance in the ECM, excellent biocompatibility, low antigenicity and available methods of isolation from a variety of sources [7,8]. Currently, the major sources of collagen are the tendon or skin of bovine and porcine. However, the prevalence of transferring diseases including foot-and-mouth disease (FMD), bovine spongiform encephalopathy (BSE), and transmissible spongiform encephalopathy (TSE), and the religious barriers of Muslims and Jews [9–11], have limited its application [12]. Therefore, it is essential to find a safety source of collagen for human application. Marine collagen has been isolated and characterized from various marine sources, and can generally be categorized according to source: vertebrates or invertebrates. Vertebrates sources include cat fish [13], silvertip shark [14], salmon [15], yellow tuna [16], and marine mammals such as minke whale [17]. Invertebrates source include jellyfish [18–20], squid [21], and sponges [22–25]. Researchers have demonstrated that similar characteristics exist between marine collagen and mammalian [26–28]. However, some differences exist between collagen extracted from marine sources and collagen extracted from mammals. Compared with the mammalian collagen, marine collagen has lower gelling and melting temperatures, but relatively higher viscosities than equivalent bovine forms [29]. Fish collagens show a similar amino acid distribution to mammalian collagen, with decreased amounts of proline and hydroxyproline, and increased serine, threonine, and in some cases, methionine and hydroxylysine [30]. Compared with mammalian collagen, the difference in the amino acid distribution of fish collagen causes labile cross-links and heat sensitivity [31]. In recent years, marine collagen has been widely used in medicine and tissue engineering fields [32], such as cartilage [33], corneal [34], ligament [35], muscle [36], skin [37], tracheal [38], and vascular [39].

The Nile tilapia (*Oreochromis niloticus*) is a worldwide cultured fish that possesses an important position in China's aquaculture and exports industry [26]. Tilapia skin is a main by-product of its processing, which contains approximately 30% collagen [40]. Our previous study revealed that both acid-soluble collagen (ASC) and pepsin-soluble collagen (PSC) extracted from Nile tilapia skin can be used as raw materials in food and cosmetic preparation [41]. Further, we want to explore whether either is suitable for use in biomedical applications. However, the biocompatibilities of pure collagen extracted from Nile tilapia need be addressed, since the biocompatibilities of fish collagen are profoundly influenced by the molecular composition and arrangement, which is thought to be varied by different extraction methods. In this work, we extract acid-soluble (ASC) and pepsin-soluble (PSC) collagen from Nile tilapia skin, and then describe their physical properties, chemical properties, and biocompatibilities, in order to explore the possibility for applications in biomedical fields.

#### **2. Results and Discussion**
