Among them, collagen, mainly found in bone (type Ⅰ), cartilage (type Ⅱ) and blood vessel walls (type Ⅲ), is the main protein component of natural bone and has an amino acid sequence that facilitates cell adhesion [
123,
124]. Besides, due to its animal origin, this protein is non-cytotoxic, biocompatible and biodegradable, and these advantages make it a promising candidate for BTES [
125]. Moreover, it also has abundant mineral deposition sites in its structure, which are well induced for calcium deposition in bone tissues [
126]. Attractively, collagen can be processed into a variety of forms such as membranes, fibers or foams and applied to various tissues such as bone, cartilage, heart, ligaments and nerves depending on the needs, which are necessary and important for clinical applications [
127–
131]. However, its disadvantages such as low elasticity and mechanical strength and poor stability
in vivo make it difficult to prepare pure collagen as a scaffold material alone, and it is usually compounded with other materials to further modulate mechanical and degradation properties. Qian's group designed and prepared a composite hydrogel composed of collagen and nano-HA (
Fig. 6a) [
132]. This composite gel tissue engineering scaffold material showed good biocompatibility and induced regeneration and healing of bone tissue well after implantation into the body (
Figs. 6b and
c). SF is a natural polymeric fibrous protein extracted from silk [
133]. SF can self-assemble to form a fibrous structure with excellent biocompatibility and certain mechanical properties, which is very conducive to cell attachment and growth, and is a suitable biomaterial for bone repair and regeneration [
134–
136]. Currently, SF has been prepared as thin films, hydrogels, porous materials, sponges or composite scaffolds and used in different tissue engineering fields [
137–
140]. For instance, Jo
et al. evaluated the possibility of SF/HAp/sodium alginate composites as bone fillers [
141]. The results confirmed that the rate of defective bone healing was significantly higher in the SF-based scaffold-filled than in the blank group. Moreover, the degraded scaffold was not surrounded by immunogenic reactions or giant cells, suggesting that the scaffold can successfully complete bone tissue repair without causing inflammation or rejection reactions (
Fig. 6d). CS, a linear polysaccharide derived from chitin by deacetylation, has a semi-crystalline structure and is the main component of the exoskeleton of crustaceans [
142]. CS is a cationic polymer that can interact electrostatically with negatively charged biomolecules and cell membranes, having a strong adsorption capacity to biological tissues [
143]. In addition, CS can hydrolyze the bacterial cell wall by binding to phosphopeptidic acid in the bacterial peptidoglycan layer through non-covalent bonding, thus achieving satisfactory bactericidal and antibacterial properties [
144–
146]. Moreover, its good solubility has led to the development of various forms of bone and cartilage tissue engineering materials such as films, particles and hydrogels [
147]. Bikendra
et al. [
148] fabricated high-strength hydrogel scaffolds by hybridizing regenerated cellulose (rCL) nanofibers with CS hydrogels. The hybrid materials significantly improved the mechanical properties of the gel (
Fig. 6e). The porous and high-strength rCL/CS scaffold enhanced the process of accelerated proliferation and biomineralization of pro-osteoblasts (
Fig. 6f). Hyaluronic acid (Ha) is widely distributed in the tissue interstitium of vertebrates, and has good application prospects as an excellent tissue engineering material. For example, Wu
et al. [
149] prepared a new scaffold by introducing polyphosphate (PolyP) as an osteoinductive effect signaling molecule into cross-linked Ha hydrogels. The scaffold can enhance the osteogenic effect of preosteoblasts. Taken together, natural polymers have excellent biological properties and demonstrate innate advantages in the field of bone tissue engineering. However, these polymers have inherent drawbacks, such as poor mechanical properties and unregulated degradation rates.