HA is a natural mucopolysaccharide acidic polymer and a main component of the extracellular matrix. It is widely distributed on the market as a hydrogel carrier (Kettenberger et al., 2017) owing to its excellent biocompatibility, high amenability, and multifunctional properties (Qiu et al., 2021), which make it an optimal candidate for tissue engineering as well. Ossipov et al. (2021) reported an injectable and self-healing HA hydrogel that independently released two different drugs in response to acidic and thiol-containing microenvironments. Except for the hydrogel carrier itself, the binding sites available on each individual component must also be considered when designing hydrogels. For instance, the BP prodrug is conjugated to HA via a self-immolative disulfide linker, that is, stable in the blood plasma and cleavable in the cytoplasm; the resulting HA-linked BP ligands reversibly bind Ca²⁺ ions and form coordination hydrogels (Ossipov et al., 2021).

Chitosan, a linear and semi-crystalline polysaccharide, is a direct derivative of chitin, which is the second most abundant natural polymer after cellulose (Friedman and Juneja, 2010; Gong et al., 2022). Chitosan has excellent biocompatibility, biodegradability, adsorption capacity, anti-bacterial properties, and thermosensitive properties (Cancian et al., 2016; Nafee et al., 2018). It can be easily produced through acylation, alkylation, and carboxylation reactions (Shariatinia, 2019). However, chitosan-based hydrogels often fail to meet the mechanical strength requirements of bone fillers. The poor mechanical strength of chitosan-based hydrogels can be compensated by incorporating several types of nanofillers, i.e., carbon nanotubes and cellulose nanocrystals, which is particularly important for treating managing osteoporotic vertebral compression fractures (Cancian et al., 2016; Ghavimi et al., 2019; Vitale et al., 2022).

Gelatin is a type of protein obtained by the partial hydrolysis of collagen; both have similar homologies. Collagen has a rod-like triple-helical structure, which is partially separated and broken when it is partially hydrolyzed to make gelatin (Veis and Cohen, 1960). Collagen-based hydrogels formed under physiological conditions using genipin as a cross-linker exhibited markedly improved mechanical properties, higher gel content, lower swelling ratio, and tunable degradation behaviors against collagenase (Ma et al., 2016). In addition, Ning et al. (2019) reported that the methacrylated gelatin-based hydrogel prolonged the release of the adjuvant (>10 days).

Alginate, derived from algae, is a linear copolymer composed of beta-d-mannuronic acid and C-5-epimer alpha-l-guluronic acid. An alginate solution gels when exposed to Ca²⁺ and other bivalent cations as these cations strongly bind to the G residues of the alginate molecule. Therefore, the calcium alginate gel is considered a three-dimensional network of molecules with cross-links between the G residues of other long-chain molecules through the action of Ca²⁺ (Bjerkan et al., 2004). The drug-release kinetics from hydrogels are commonly controlled by the network properties and drug–network interactions (Ossipov et al., 2021). The alginate gel sometimes degrades too rapidly under acidic conditions, such as the area around the osteoporotic bones. Thus, it has been reported that adding amorphous CaP powders positively affected dissociation rate (Ito et al., 2012; Ito and Otsuka, 2013), owing to the pH buffering mechanism inside the gel, thereby allowing for a controlled drug release.

Some commercially available products, such as Poloxamer 407 and butane-diisocyanate, can be directly used as hydrogels to carry adjuvants and are mainly used to study the function of targeted drugs (Neuerburg et al., 2019; Chen et al., 2021). Other commonly used synthetic materials used to produce hydrogels are reported below.

PEG, also known as a macrogol, is a type of nontoxic and water-soluble polymer with unique hydrophilicity and electrical neutrality. It consists of chemically active hydroxyl groups at both ends, thereby promoting the conjugation with other functional groups (Alper and Pashankar, 2013; Shi et al., 2021). It can rapidly form biocompatible gels and is easily injectable; therefore, it is considered an ideal hydrogel carrier (Li et al., 2020).

PLGA is a biodegradable polymeric compound formed by polymerizing lactic acid and glycolic acid, which are by-products of human metabolic pathways. Therefore, it is nontoxic, except in those individuals suffering from lactose deficiency. The co-polymerization between PLGA and the carried drug may prolong the release time; this confirms that the release of the drugs from the carrier depends on the degradation of the hydrogel and the decomposition of the drug complex (Liu et al., 2017; Peng et al., 2017). Additionally, PLGA can co-polymerize with PEG to form new polymers for hydrogel-based delivery systems (Liu et al., 2017).

CHAp-PAA is the term used to refer to a supramolecular hydrogel composed of nano-hydroxyapatite, sodium carbonate, and polyacrylic acid (PAA) (Zhao et al., 2020); owing to the high mineralization, such hydrogel can be used as a scaffold to treat bone defects in osteoporotic individuals. Because of the biomineral composition, the hydrogel can mimic the chemical composition and structural characteristics of natural bones, while achieving mechanical stability, biocompatibility, and osteogenesis without delivering any additional therapeutic agents or stem cells (Zhao et al., 2020).

CMT/HEMA (ratio of 1:10) is a hydrogel with a surface that promotes the adhesion of bone precursor cells and efficient growth of bone tissues (Sanyasi et al., 2014). It is highly compatible with bone cells (RAW264.7) and sensitive to neuronal (Neuro2a) and human umbilical vein endothelial (HUVEC) cells (Sanyasi et al., 2014).

To treat osteoporosis successfully, it is important to inhibit the formation, differentiation, and resorption functions of osteoclasts. Osteoclasts are a special type of terminally differentiated cell deriving from a family of mononuclear macrophages in the blood. They can be fused by their mononuclear progenitor cells in various ways to form a multinuclear giant cell, where the RANKL and macrophage colony-stimulating factor (M-CSF) play a crucial role (Rachner et al., 2011). Tumor necrosis factor superfamily 11 (TNFSF11), the gene encoding RANKL, is abundantly expressed by osteoblasts, bone marrow stromal cells, and T and B lymphocytes (Rachner et al., 2011). As a homotrimeric type II transmembrane protein, RANKL can be released from the cell membrane upon decomposition of several extracellular proteases, including disintegrin and metalloprotease (Nagy and Penninger, 2015). RANK and OPG are the two main receptors of RANKL; such receptors are also known as tumor necrosis factor receptor superfamily member 11A (TNFRSF11A) and TNFRSF11B, respectively (Nagy and Penninger, 2015).

When secreted RANKL binds to the membrane-binding receptor RANK on the precursor of osteoclasts, it causes the RANK receptor to polymerize into a trimer that recruits several junction molecules, including tumor necrosis factor receptor-associated factor 6 (TRAF6) (Armstrong et al., 2002). The recruitment of TRAF6 leads to the activation of a variety of signaling pathway cascades, including inhibitor of nuclear factor κB kinase (IKK), mitogen-activated protein kinase (MAPK) family, and cellular Src kinase (c-Src), which enable osteoclasts to differentiate, survive, polarize, and have absorptive activity. The MAPK pathway is composed of three types of molecules: MAPK (including ERK, JNK, and p38), MAPK kinase (MAPKK or MEK), and MAPKK kinase (MAPKKK or MEKK) (Lee et al., 2018). The activation of MAPKs induces the nuclear translocation of c-Fos and c-Jun, while nuclear factor-κB (NF-κB), derived from the IKK pathway, up-regulates c-Fos in the nucleus upon nuclear translocation. The c-Src activates the anti-apoptotic program through protein kinase B. The nuclear factor of activated T-cells cytoplasmic 1 (NFATc1) is a critical transcription factor for osteoclast differentiation. Upon the initial activation through NF-κB and NFATc2, it is then upregulated by the action of c-Jun, p38, c-Fos, calcineurin, and calcium ions (Zhao et al., 2010). Finally, the up-regulated c-Fos and NFATc1 synergistically promote the expression of osteoclast-specific genes, such as tartrate-resistant acid phosphatase (TRAP), cathepsin K (CtsK), matrix metalloproteinase-9 (MMP-9), and vesicle-type ATPase (V-ATPase) V0 domain d2 subunit (Teitelbaum, 2000; Boyle et al., 2003; Nagy and Penninger, 2015). Bone degradation includes polarization, acidification, and protein breakdown; during these steps, the ruffled bone edge formed by osteoclasts plays an important role. In fact, it secretes protons (H⁺) into the bone resorption space through the V-ATPase and proteases such as TRAP, CtsK, and MMP-9; while the former degrade the bone minerals, the latter deteriorate the organic bone components (Georgess et al., 2014) (Figure 3).

The binding between RANKL and RANK, and the following signaling cascade play an important role in osteoclast differentiation and survival. The inhibition of these pathways has become a feasible target for the systematic or local treatment of osteoporosis (Matsumoto and Endo, 2021). Targeting extracellular pathways, OPG-loaded composite hydrogels capable of controlling the release of OPG can inhibit the binding of RANKL and RANK; therefore, the osteoclastic activation is reduced, while promoting bone regrowth and osseointegration in osteoporotic defects (Wang X. et al., 2021). Alendronate, which is widely used to alleviate osteoporosis by inhibiting osteoclasts, is one of the most common drugs loaded on hydrogels (Posadowska et al., 2015; Nafee et al., 2018; Li et al., 2020; Gilarska et al., 2021; Jiang et al., 2022); such a complex would likely inhibit the rate-limiting step in the cholesterol biosynthesis pathway, essential for osteoclast function (Fisher et al., 1999). In addition, it was observed that the loading of zoledronate does not affect its action; in fact, the inhibition of the degradation of the mineralized hydrogel and the resorption of the peri-implant bone are effectively carried out by the loaded and unbound zoledronate (Kettenberger et al., 2017).

During osteoporosis development, bone marrow mesenchymal stem cells (MSCs) promote the depletion of osteoblasts while increasing the amount of adipocytes, thereby resulting in a slower bone formation rate and improved marrow fat accumulation (Moerman et al., 2004; Li et al., 2015). The specific differentiation direction is precisely regulated by factors in the signaling pathways, transcription factors, and microRNAs. Among numerous studies of signaling pathways, the wingless and int-1 (Wnt) classes and BMP represent two critical signaling pathways (Hu et al., 2018).

The canonical Wnt signaling, also called Wnt/β-catenin, is essential for determining the fate of osteoblast cells. It binds a seven-transmembrane-spanning frizzled protein (Frz) receptor with the low-density lipoprotein receptor-related protein (LRP) 5/6 co-receptor to prevent the phosphorylation and degradation of β-catenin (Kim et al., 2013). Then, β-catenin translocates into the nucleus to promote osteogenesis while inhibiting adipogenesis, regulated by MSCs (Etheridge et al., 2004; Shen et al., 2011; Yuan et al., 2016). Moreover, this function may be achieved by inducing the expression of runt-related transcription factor-2 (runx-2) and osterix, and inhibiting peroxisome proliferation-activated receptor γ (PPARγ) (Bennett et al., 2005; Kang et al., 2007). BMP is the collective name for a series of transforming growth factor-β (TGF-β) family members and operates through either canonical or non-canonical BMP signaling (Chen et al., 2012) upon binding to bone morphogenetic protein receptor I (BMPR-I) and BMPR-II. The canonical BMP signaling induces phosphorylation of Smad1/5/8, which translocates into the nucleus upon the formation of complexes with Smad4; the non-canonical BMP signaling occurs mainly through the p38 MAPK pathway (Chen et al., 2012). Both signaling can regulate the target gene expression of runx-2, osterix, and PPARγ, showing dual roles in inducing osteogenic and adipogenic differentiation of MSCs (Kang et al., 2009). Several studies (Wang et al., 1993; Gori et al., 1999) indicate that a high BMP-2 concentration accelerates osteoblast differentiation, while adipocyte formation is promoted at low concentrations and in the presence of BMP-4 (Tang et al., 2004) (Figure 4).

Since a high concentration of BMP-2 promotes osseointegration, it is one of the most common drugs loaded in hydrogel scaffolds (Lee et al., 2015; Segredo-Morales et al., 2018; García-García et al., 2019; Bai et al., 2020; García-García et al., 2020; Wang et al., 2021b; van Houdt et al., 2021). BMP-7 or BMP-6 loaded hydrogels have been used in local treatment, are potent stimulators of osteogenesis, and can reduce the risk of further osteoporosis-associated secondary fractures (Pelled et al., 2016; Neuerburg et al., 2019). Alternatively, Rosuvastatin is a popular drug that promotes osteogenic differentiation of MSCs in the model of osteoporosis by the Wnt/β-catenin signaling (Wang et al., 2019; Akbari et al., 2020). Simvastatin can also promote osteogenesis, and the underlying mechanism appears to involve a higher expression of BMP-2 (Ito et al., 2012; Fu et al., 2016; Tan et al., 2016; Zhu et al., 2021). In addition, carrying Si or Sr ions has been shown to promote osteogenic differentiation, with the former being controlled by upregulating the expression of the osteogenesis-related genes (Peng et al., 2017; Yilmaz et al., 2021). Hydrogels can also be directly loaded with MSCs, showing potential as a supplement or alternative to the current therapies proposed (Kim et al., 2018; Chang et al., 2019). Adipose-derived stem cells are also able to promote osteogenesis and inhibit adipogenesis of osteoporotic MSCs through activation of the BMP-2/BMP receptor-type IB signal pathway in the local delivery system (Jiang et al., 2014; Ye et al., 2014). Interestingly, it was observed that the treatment of drug-loaded hydrogels by extracorporeal shockwaves can promote bone formation by upregulating the alkaline phosphatase activity, mineralization, and expression of runx-2, type-I collagen, osteocalcin, and osteopontin (Chen et al., 2021).