On the other hand the properties of
On the other hand, the properties of collagen materials could be efficiently improved by proper chemical modification. The crosslinking agents for collagen generally involve aldehydes (e.g., glutaraldehyde (GA), isocyanates (e.g., hexamethylene diisocyanate (HMDI), photoreactive agents, polyepoxy compounds, inorganic crosslinker , 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC), plant extracts (e.g., genipin , tannic though ) and so on. Genipin and GA are the most commonly used crosslinkers for amino acid crosslinking in the applications of biomedical materials . Thereinto, previous study has been confirmed that the synthetic cross-linkers, such as aldehydes, isocyanates, have been applied to modify collagen-based biomaterials cautiously due to potential cytotoxic effects , although these cross-linkers could significantly promote collagen-based scaffold\'s corresponding physicochemical properties. Meanwhile, in recent years, collagen based materials crosslinked by GA has been found to be easily calcified. Though right now, there are various kinds of chemical crosslinkers, but the suitable and nontoxic crosslinkers are few. For example, EDC and biological enzyme are seen as zero-length crosslinking agents. But the effective crosslinking is limited due to the limited reactive groups in close range. It cannot be used in vivo directly as crosslinking agent and its toxicity cannot be ignored for directly use without post-treatment. Therefore, the use of chemical crosslinkers could be combined with other modified methods or to find nontoxic or low toxicity crosslinkers. Recently, more emphasis is further to be placed upon naturally occurring cross-linkers out of consideration for their desired biocompatibility [32,33]. The basic theory of chemically and physically crosslinking of collagen is shown in Fig. 5 . In recent years, a kind of low toxic chemical crosslinking agents based on the traditional biomass have been reported in application of collagen modification, namely biomass with aldehyde group after oxidation modification, such as dialdehyde carboxymethyl cellulose , dialdehyde cellulose , oxidized starch , dialdehyde alginate (ADA), etc. The biological crosslinking agent could provide potential of functional modification for collagen materials. And the possible interactions between collagen molecule and alginate dialdehyde are shown in Fig. 6. However, more in vitro and in vivo studies should be payed attention to the stability and durability of the crosslinking materials as a function of storage and chronic implant . Besides, collagen based composites with synthetic polymers or natural polymers could also improve the properties of collagen materials in order to meet the different application requirement of materials used in different fields. The mostly used synthetic polymers are polyvinyl alcohol (PVA), polyurethane (PU) , PLA , PLGA , etc. The natural polymer materials with good biological compatibility and biodegradable properties have attracted much attention in recent years. Up to date, these natural polymer materials, including natural carbohydrate polymer materials such as cellulose, chitosan , glycosaminoglycans (chondroitin sulfate, hyaluronic acid (HA), heparin, etc.), or natural protein polymer materials such as silk fibroin , elastin , gelatin  etc., have been applied in collagen based scaffolds for tissue engineering . Click chemistry is a kind of chemical synthesis to acquire various molecular product through small units quickly assembling. It has been applied to natural polymers or synthetic origins in many different architectures including linear (e.g., PEG, polysaccharides, proteins), branched (e.g., dendrimers, graft polymers) and macrocyclic polymers . Oxime click chemistry  has been used to crosslink hydrogels of poly(ethylene glycol)s (PEGs), HA and collagen  that were biodegradable, soft, and supportive of cell adhesion.