7.2.3.2　Hydrogels

Compared with implantable polymer scaffolds，hydrogels are generally softer，can be injected or sprayed conveniently，which have been widely applied in melanoma immunotherapy.Due to their adjustable mechanical and physicochemical properties，hydrogels can control the release of various therapeutic drugs（e.g.，small molecule drugs，large molecule drugs）at different time and space，therefore protect the drugs from degradation by the body.Their cross-linking structures，degradability and controllable drug release property in time and space provide a good foundation for application in delivery of cell and various immunomodulators.Hydrogel materials include synthetic polymer hydrogels（e.g.，PEG），alginate hydrogels，polypeptide hydrogels，natural extract hydrogels（e.g.，hyaluronic acid，chitosan，and collagen）.

On the one hand，hydrogels can be used to deliver immune cytokines such as GM-CSF，TLR3 agonist poly（I：C），tumor antigens，and immune checkpoint inhibitors.For example，Wang et al.constructed an injectable temperature-sensitive PEG-PLGA hydrogel in 2014，which can recruit DCs and macrophages through controlled release of GM-CSF，resulted in improving the immune effect against melanoma.In 2015，Mooney et al.reported a study on a spongy macroporous alginate hydrogel.The hydrogel used tumor cells as antigen，GM-CSF as DCs enhancer and CpG ODN as DCs activator.Local infiltration consisting of conventional DCs and plasmacytoid DCs can be induced followed by effective，persistent and highly specific anti-tumor T cell responses in melanoma models.

On the other hand，hydrogels can be used for the co-delivery of immune cytokines，tumor total antigens or immune checkpoint inhibitors.Wang et al.prepared an injectable and self-assembled polypeptide poly（L-valine）gel loaded with tumor cell lysates as antigens and TLR3 agonist poly（I：C）as immune enhancer，which can induce immune response in CTLs achieving good anti-melanoma effect.Wang et al.self-assembled an injectable PEG-b-poly（L-alanine）hydrogel in aqueous solution using polypeptides.Tumor cell lysate，GM-CSF and immune checkpoint inhibitor（anti-CTLA-4/PD-1 antibody）were co-coated in the hydrogel，which showed a good immunotherapeutic effect on melanoma.This hydrogel combined with immunotherapy could not only significantly increase the activated effector CD8+T cells in the spleen and tumor of immunized mice，but also reduce the proportion of Treg.(https://www.daowen.com)

Hydrogels can be constructed to respond to TME acidity，increased ROS and temperature，etc.，which can control the release of melanoma immunotherapy factors and realize enhanced melanoma immunotherapy.In 2019，Gu et al.constructed an in-situ formed dual response hydrogel for local collaborative delivery of demethylating agent zebularine（Zeb）and aPD-1.aPD-1 was first loaded in pH-sensitive CaCO3 NPs and then encapsulated with Zeb in a ROS-responsive hydrogel to construct the ROS/pH dual response hydrogel.By taking advantage of the acidic and ROS-rich tumor microenvironment within the tumor，the drug release was controlled and the retention time of the drug was prolonged，thus effectively enhancing the anti-melanoma immune response.In addition，in 2018，Gu et al.demonstrated a ROS-responsive injectable polypeptide hydrogel to release aPD-L1 and IDO immunosuppressive enzyme D-1MT，which could effectively reduce the local ROS level and promote the release of immunotherapeutic factors，thereby enhancing the anti-melanoma effect in vivo.Moreover，Gu et al.developed a spray-type bioresponsive fibrin hydrogel containing aCD47 antibody loaded CaCO3 NPs，which can remove H+from surgical wounds and polarize tumor associated macrophages to M1 phenotype（Figure 7-6）.The results showed that the immunotherapeutic fibrin hydrogel could“awaken”the innate and adaptive immunity of the host to inhibit the local recurrence and potential metastatic spread of the tumor after surgery.In addition，Lee et al.designed hydrogels that can intelligent inject levodopa-and poly（e-caprolactone-co-lactide）ester-functionalized hyaluronic acid，which loaded with immunoregulatory factor OVA expressing plasmid（pOVA）and GM-CSF immunoenhancing factors.The hydrogels can effectively eliminated melanoma in mice though recruiting immune cells.

Otherwise，hydrogels can be used to treat melanoma in combination with immunotherapy and other therapies.In 2018，Gu et al.obtained a ROS-degradable hydrogel by cross-linking the unstable ROS-responsive linker with poly（vinyl alcohol）（PVA），while loading with gemcitabine（GEM）and anti-PD-L1 blocking antibody（aPD-L1）.Due to the abundant ROS in the TME，this hydrogel realized the combined chemotherapy and immunity therapy strategy by controlling the release of chemotherapeutic drugs and ICB inhibitors，which enhanced the postoperative ICB and inhibited tumor recurrence.In the same year，Yang et al.used a synthetic amphiphilic polypeptide RADA32（with the sequence of RADARADARADARADA）to self-assemble into a nanofiber hydrogel encapsulated with doxorubicin（DOX）and melittin for a potent chemoimmunotherapy against melanoma through the active regulation of TME.Due to its controlling drug release property and its abilities to regulate innate immune cells，deplete M2-like TAMs，and direct resist cancer and stimulate immune，the hydrogel exhibited potent anti-cancer efficacy for subcutaneous and metastasis tumors.In 2019，Lv et al.designed a poly（ethylene glycol）hydrogel loaded with self cross-linking CpG ODN NPs and photosensitizer IR820 for the combination of photothermal therapy and immunotherapy.The hydrogel can induce melanoma-related antigen release from the killed tumor cells through photothermal killing of melanoma cells，which can effectively stimulate specific anti-tumor immunity，activate the body's systemic anti-tumor immune responses while effectively removing the primary tumor by hyperthermia and achieving a more effective systemic therapeutic effect than single phototherapy.