The main inorganic component of bone is hydroxyapatite (HA) crystals, which are embedded in the extracellular matrix (ECM) of bone. As the organic component of bone tissue, bone ECM is mainly composed of type I collagen fibers and serves as an inductive template for bone repair ^[1][2][87,88]. The mineral hydroxyapatite crystals deposit along the long axis of collagen type I fibers and present a hierarchical deposition within zones between collagen fibrils at the nanoscale ^[3][4][5][89,90,91].

The minerals of bone tissue are hierarchically assembled from the nanoscale ^[3][89]. Before mineralization, the organic phase of bone has been assembled, which can finely regulate crystal nucleation and growth. Needle-like mineral particles coalesce horizontally into platelets, neither inside nor outside the fibers, but form fractal-like hierarchical bone architecture with continuous intersecting fibers ^[3][89]. The mineralized collagen fibers on the microscopic scale are arranged in a complex hierarchical structure. At the macro level, most bones contain helical patterns in their anatomical shapes to increase adaptation to force. At the micro level, the spiral secondary bone itself is formed by concentric slices of mineralized collagen fibers. In terms of scaffold designing, biomimetic approaches, which can simulate molecular structural and biocompatibility with complex natural bone tissue ^[6][7][8][9][92,93,94,95], have gained increasing attention. By exploiting the unique properties of the pure or composite nanocellulose scaffolds, it is possible to improve the properties of the biomimetic materials with controlled and layered structures in nanostructures ^[10][55].

Electrospinning offers clear advantages for the preparation of scaffolds based on nanocellulose, including control over composition, structural design, and functional expansion ^[11][12][96,97]. It is a promising method for producing 3D aerogels in BTE and for mimicking the extracellular matrix (ECM) ^{[13][14][15][16]}[35,98,99,100]. The core–shell structure of electrospinning is composed of PHB/G and PHB/G/Fe₃O₄ compositions, which result in lower melting points compared to pure PHB scaffolds. The resulting hybrid scaffolds have a lower crystallinity and are non-toxic, with the added benefit of high saturation magnetization in the magnetite composite scaffolds, which makes them well-suited for biomedical applications ^[17][101]. In addition, gas foaming is a process that involves introducing inert gas-foaming agents into the polymer phase, generating gas bubbles inside the 2D scaffolds via subsequent chemical reactions to expand the interconnected pores within the scaffolds ^[18][50]. Aerogels can also be prepared using gas foaming technology, which involves reassembling tightly packed 2D electrospinning nanofibers into fluffy 3D scaffolds with high porosity and large pores ^[13][35]. While 3D aerogels produced by gas foaming show great promise in BTE applications, there have been very few studies on fabricating nano cellulose-based aerogels using this technology. Therefore, future research should focus on this area.

Since 自1971, when the first generation of a cellulose-based aerogel with a large specific surface area was fabricated, various studies have been performed to evaluate the toxicity, antibacterial properties, and mechanical properties of nanocellulose aerogels and provide a theoretical basis for their application in BTE ^{[19][20][21][22]}年制造第一代具有大比表面积的纤维素基气凝胶以来，已经进行了各种研究来评估纳米纤维素气凝胶的毒性，抗菌性能和机械性能，并为其在BTE中的应用提供理论基础[83，104，105，106]. Li et al. prepared a CNC-based aerogel by direct ink writing and freeze-drying and proved that the resulting aerogel exhibited dual porous and controllable structures ^[23]. The main disadvantage of 等人通过直接墨水书写和冷冻干燥制备了基于CNC的气凝胶，并证明所得气凝胶表现出双重多孔和可控结构[107]。基于CNC-based aerogels is obviously their brittleness, which would lead to structural damage during cell incorporation and growth. Optimizing the crosslinking method might improve mechanical performance by adding 的气凝胶的主要缺点显然是它们的脆性，这会导致细胞掺入和生长过程中的结构损伤。优化交联方法可能会通过在交联前向纳米纤维素聚合物分散体中添加2.5 wt% polyamide-epichlorohydrin (wt%聚酰胺-环氧氯丙烷（kymene) into the nanocellulose polymer dispersion before cross-linking. This increased the Young’s modulus of the composite aerogel from 7 MPa to 8.94 MPa ^[23]. Epoxypropane exhibits significant cytotoxicity, and its linear structure with bulky side chains limits its degradation compared to other types of chemical crosslinking agents, which restricts its usage in tissue engineering ^[24][25]. In another study, ）来提高机械性能。这使复合气凝胶的杨氏模量从7MPa增加到8.94MPa[107]。环氧丙烷表现出显著的细胞毒性，与其他类型的化学交联剂相比，其具有庞大侧链的线性结构限制了其降解，这限制了其在组织工程中的使用[63，108]。在另一项研究中，Osorio et al. grafted a hydrazide group onto a carboxylic acid group to form a hydrazone linkage on the surface of a 等人将酰肼基团接枝到羧酸基团上，在CNC-based aerogel and proved that the prepared cellulose aerogel presented an excellent flexibility, high porosity, and osteoconductive properties after chemical crosslinking ^[26].基气凝胶表面形成腙键，并证明制备的纤维素气凝胶在化学交联后表现出优异的柔韧性、高孔隙率和骨传导性能[64]。

Due to the existence of hydrogen bonds, nanocellulose can not only be self-assembled itself, but also assembled with other polymer materials由于氢键的存在，纳米纤维素不仅可以自己自组装，还可以与其他高分子材料组装。单独使用纤维素的气凝胶具有亲水性和骨传导不良的缺点[109，110]。为了克服这些缺点并保持其固有的优越性，纳米纤维素气凝胶的复合制备方法越来越受到关注，因为通过将纤维素与不同的有机和无机化合物相结合来调整骨支架的力学性能，生物降解性，生物活性和优越的生物学性能[111，112]. The aerogels with cellulose alone presented the disadvantages of hydrophilicity and poor osteoconduction ^[27][28]. In order to overcome these drawbacks and preserve their inherent superiorities, the composite fabrication methods of nanocellulose aerogels have received more and more attention, as the mechanical properties, biodegradability, bioactivity, and superior biological properties of bone scaffolds are adjusted by combining cellulose with different organic and inorganic compounds ^[29][30]. The contents regarding the combination of nanocellulose with other materials are summarized as follows (Table 1).关于纳米纤维素与其他材料组合的内容总结如下（表2）。

As an environmentally friendly biomaterial, hydroxyapatite (羟基磷灰石（HA) has excellent biocompatibility and constitutes the inorganic phase of bone, which can release various bone conduction ions to the surrounding environment ^[9][45][46]. ）作为一种环境友好型生物材料，具有优异的生物相容性，构成骨骼的无机相，可以向周围环境释放各种骨传导离子[95，122，123]。然而，However, the disadvantages of HA, such as low in vivo absorption, low crack resistance, and poor bone irritation, limit its clinical application ^[47]. The addition of 的缺点，如体内吸收率低、抗裂性低、骨刺激差等，限制了其临床应用[124]。在纤维素基气凝胶中添加HA to cellulose可以增强构建有机-based aerogels can enhance the mechanical properties of building organic-inorganic aerogels. Huang et al. attached an in-situ HA coating of about 10 nm to the CNC surface and then crosslinked it with polymethyl vinyl ether malonic acid (无机气凝胶的机械性能。Huang等人将约10nm的原位HA涂层附着在CNC表面，然后与聚甲基乙烯基醚丙二酸（PMVEMA) and polyethylene glycol (PEG) to enhance the mechanical properties of the composite. Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) show esterification between CNC, HAP, ）和聚乙二醇（PEG）交联，以增强复合材料的力学性能。傅里叶变换红外光谱（FTIR）和核磁共振（NMR）显示CNC、HAP、PMVEMA and PEG. The results show that the attachment of HA increases the compressive strength of the obtained stent to 和PEG之间发生酯化反应。结果表明，HA的附着使所得支架的抗压强度提高到41.8 MPa, which provides broad potential for the development of BTE stent ^[31]. ，这为BTE支架的发展提供了广阔的潜力[113]。Cheng et al. also fabricated an 等人还制造了一种HA-BC aerogel and mineralized it in situ by embedding the aerogel in CaCl.气凝胶，并通过将气凝胶嵌入CaCl中进行原位矿化。₂and和 K₂High crude oil高原油₄Solution.解决 The results show that composite aerogels with excellent biocompatibility enhance the mechanical properties and mimic the structure of natural bone ^[32]. The above studies prove that the incorporation of 方案。结果表明，具有优异生物相容性的复合气凝胶增强了力学性能，并仿生了天然骨骼的结构[102]。上述研究证明，将HA into nanocellulose aerogels can not only improve its mechanical properties, but also serve as a template for biomimetic mineralization. In addition, due to the urgent need for biomimetic theory, advanced preparation techniques such as 掺入纳米纤维素气凝胶中不仅可以提高其力学性能，而且可以作为仿生矿化的模板。此外，由于仿生理论的迫切需求，可以使用3D printing can be used to orderly deposit HA layers onto collagen fibers to simulate the microstructure of natural bone tissue.打印等先进的制备技术将HA层有序沉积到胶原纤维上，以模拟天然骨组织的微观结构。

生物活性玻璃（Bioactive glass (BG) can release calcium and phosphate into the surrounding environment, resulting in HA deposition on the surface of biomaterial after in vivo transplantation. Bone substitutes based on bioactive glasses have been widely used in BTE. Kamel et al. prepared a nanofibrillated cellulose aerogel loaded with strontiumborate-based bioactive ceramic particles and rosuvastatin to process the extraction socket ^[48]. The results showed that composite aerogels exhibited excellent mechanical properties, promoted the proliferation of G）可以将钙和磷酸盐释放到周围环境中，导致生物活性玻璃在体内移植后HA沉积在生物材料表面。基于生物活性眼镜的骨骼替代品已在BTE中得到广泛应用。Kamel等人制备了一种负载有硼酸锶基生物活性陶瓷颗粒和瑞舒伐他汀的纳米原纤化纤维素气凝胶，以处理提取插座[34]。结果表明，复合气凝胶表现出优异的力学性能，促进了MG-63 cells, and were promising materials for preserving dental sockets.细胞的增殖，是保存牙槽的有前景的材料。

The organic phase of natural bone tissue, as a layered skeleton, plays a vital role in biomineralization. 天然骨组织的有机相作为分层骨架，在生物矿化中起着至关重要的作用。常用的生物聚合物包括壳聚糖（Commonly used biopolymers include chitosan (CS), collagen, cellulose, etc. All of these have proven to be suitable platforms for mimicking the inorganic phase of bone and as an alternative to manufacturing composite scaffolds similar in structure and composition to natural bone ^[49]. As the main component of the organic phase of bone tissue, collagen (S）、胶原蛋白、纤维素等。所有这些都已被证明是模仿骨无机相的合适平台，并可替代制造结构和成分与天然骨相似的复合支架[125]。作为骨组织有机相的主要成分，天然骨中的胶原蛋白（Col) in the natural bone can serve as a template for biomineralization, controlling the orientation and shape of ）可以作为生物矿化的模板，通过提供成核位点来控制HA crystals by providing nucleation sites. Then, in vivo, biomineralization processes occur and lead to the nucleation and growth of HA nanocrystals along the Colfiber axial direction ^[50]. In recent years, many studies have focused on the preparation of cellulose biomimetic scaffolds. For example, 晶体的方向和形状。然后，体内生物矿化过程发生并导致HA纳米晶体沿Col纤维轴向成核和生长[126]。近年来，许多研究集中在纤维素仿生支架的制备上。例如，He et al. prepared biomimetic collagen等人通过胶原蛋白-羧甲基纤维素的生物分子模板制备了仿生胶原蛋白-carboxymethylcellulose/hydroxyapatite scaffolds from a biomolecular template of collagen-carboxymethylcellulose, and the scaffolds had good biocompatibility. By controlling the ratio of collagen to carboxymethyl cellulose in the template, the bone inductance, bone conductivity, and mechanical strength of the composite can be changed and adjusted according to the requirements of BTE ^[51]. 羧甲基纤维素/羟基磷灰石支架，且支架具有良好的生物相容性。通过控制模板中胶原蛋白与羧甲基纤维素的比例，可以根据BTE的要求改变和调整复合材料的骨电感率、骨传导性和机械强度[127]。Xu et al. prepared nano cellulose等人通过在纤维素气凝胶中添加胶原蛋白和HA来制备纳米纤维素-collagen (COL)-nanohydroxyapatite (n-HA) organic-inorganic hybrid aerogels by adding collagen and HA to cellulose aerogels and found that composite aerogels exhibit porous 3D structures with high compressive strength, excellent osteogenic and angiogenic abilities in vitro and in vivo ^[42]胶原蛋白（COL）-纳米羟基磷灰石（n-HA）有机-无机杂化气凝胶，发现复合气凝胶在体外和体内均表现出具有高抗压强度、出色的成骨和血管生成能力的多孔3D结构[119]. Based on the above studies, it can be concluded with certainty, Organic-inorganic hybrid materials based on 基于上述研究，可以肯定地得出结论，基于Col and nanocellulose combined can construct multi-level biomimetic scaffolds from macro to micro, which have great potential for bone defect repair.和纳米纤维素结合的有机-无机杂化材料可以构建从宏观到微观的多级仿生支架，具有巨大的骨缺损修复潜力。

Chitosan (CS), with a structural similarity with glycosaminoglycan, has excellent osteoconduction ability ^[52][53][128,129]. In a study, high-pressure homogenization and freeze-drying technologies were utilized to fabricate CNF-based and chitosan-based composite aerogels. The results showed the CNF aerogels exhibited the highest porosity, lowest density, and worst mechanical properties. However, adding chitosan into CNF can not only significantly improve the mechanical properties but also reduce the water absorption of the composite aerogels ^[37][117]. In another study, Matinfar et al. prepared biphasic and triphasic calcium phosphate fiber-reinforced CS- carboxymethyl cellulose (CMC) porous scaffolds by a freeze-drying method ^[34][114]. The broad band observed in the chitosan spectrum between 3367–3449 cm⁻¹ corresponds to the stretching vibration of N–H and O–H groups. In addition, the CMC powder spectrum exhibited distinctive bands at 1602 cm⁻¹, 1424 cm⁻¹, and 1330 cm⁻¹, which are characteristic of carboxyl, methyl, and hydroxyl groups, respectively. Furthermore, a band at 1057 cm⁻¹, attributed to the stretching vibrations of -CH2OH, was also observed. The biphasic fiber was composed of HA and triclinic apatite, and the triphasic fiber was composed of HA, β-tricalcium phosphate, and calcium pyrophosphate. Thus, the incorporation of the organic phase into the CS–CMC aerogels further enhanced their mechanical properties and effectively solved the above-mentioned problem.

Polyvinyl alcohol (PVA) is also a favorable biopolymer. With insufficient mechanical strength, which is significantly lower than natural bone, PVA alone is not suitable to be fabricated into BTE substitutes. Incorporation of PVA into the nanocrystalline cellulose scaffolds could also solve this problem and tailor their biological performance. Zhou et al. synthesized a PVA/CNFs/gelatin hybrid aerogel by the utilization of gelatin as the crosslinking agent. The modulus of the PVA/CNFs/gelatin aerogels is 1.65 MPa, significantly higher than those of the pure CNF and PVA/CNF aerogels ^[27][109]. Cataldi et al. combined nanocrystalline cellulose with PVA to fabricate a composite scaffold with enhanced tensile stress, contributed by the involvement of the nanocrystalline cellulose. However, the incorporation of an excessive amount of nanocrystalline cellulose also led to the agglomeration of nanoparticles and decreased the tensile stress of the composite scaffold ^[54][130]. Liu et al. prepared CNFs/PVA/montmorillonite aerogels and investigated the effects of crosslinkers (borax and glutaraldehyde) on the formation of the interface bonding and porous network. The results proved that glutaraldehyde crosslinking resulted in larger and looser pores of the composite aerogels as compared with those prepared by the borax crosslinking method ^[55][131]. Therefore, adding nanocellulose would increase the mechanical performance of the composite scaffolds, whereas incorporation of PVA enhances their biocompatibility.

Silk fibroin (SF), with favorable biocompatibility and noncarcinogenic ability, is extracted from silkworm cocoons and has the ability to promote preosteoblasts proliferation and MSCs osteogenic differentiation, demonstrating favorable advantages in bone regeneration ^[56][57][132,133]. However, its short absorption times and low mechanical properties limited the application of SF in the BTE field due to the high requirements for bone substitutes and the relatively long healing process of bone tissues. After the combination of SF and nanocellulose materials with relatively longer absorption periods and higher mechanical properties than SF, SF/nanocellulose composites exhibit the advantages of both SF (good biocompatibility, easy degradation, and excellent osteoinductive ability) and nanocellulose (remarkable mechanical strengths and long absorption time), making them great prospects for functional applications in BTE. Chen et al. prepared mineralized self-assembled silk fibroin (SF) –cellulose composite aerogels with an interpenetrating network by freeze-drying. In situ mineralization was then performed to control the nucleation and growth of n–HA crystals onto the surface of the composite aerogels ^[36][116]. After the mineralization of HA, the zeta potentials of the cellulose aerogel and SF/nanocellulose composite decreased from −11.1 mV and −26.3 mV to −6.3 mV and −4.1 mV, respectively. These zeta potentials are close to the −5.8 mV of n–HA. The results show that mineralized SF–cellulose composite aerogels have a good microstructure such as ideal cancellous bone, moderately adjusted compressive strength, and high degradative rate in vitro. In addition, it can also promote the proliferation of human embryonic kidney cells (HEK293T) which has potential in BTE ^[36][116]. Although only a few studies have focused on SF–cellulose-based aerogels and their application in BTE fields, there is still an attractive potential for nanocellulose-based aerogels in repairing bone defects.