Biomaterials possess multiple attractive physical and chemical characteristics, including biocompatibility, printability, crosslinking capacity, biodegradability, specificity, controlled release, and bioplasticity (Williams, 2008; Bose et al., 2017). As carrier molecules, biomaterials also provide a specific 3D microenvironment to promote cell growth and function (Mitrousis et al., 2018; Wang et al., 2019). Biodegradability is considered to simulate the natural microenvironments of tissues and organs. Printability is the ability to temporally and spatially deposit biomaterials accurately. Crosslinking is a technique that uses a balancing system, printability, and bioactivity to rapidly mold soft printable materials into 3D structures to facilitate the preparation and printing of scaffolds. Specific formulations of various natural and synthetic biomaterials are vital to form successful 3D scaffold.

Natural materials include collagen (Chang et al., 2011), gelatin (Chang et al., 2020), fibrin (Rabbani et al., 2017), silk fibroin (Qing et al., 2018), alginate (Yang et al., 2018), gellan gum, hyaluronic acid (HA) (Yazdani et al., 2019), chitosan (Kim et al., 2016), Matrigel (Wang et al., 2020b); while synthetic polymer materials include poly caprolactone (PCL) (Zhang et al., 2017), polydimethylsiloxane (PDMS) (McKee et al., 2017), polyethylene oxide (PEO) (Evrova et al., 2016), polylactic acid (PLA) (Gandolfi et al., 2020), poly lactic acid-glycolic acid (PLGA) (Swanson et al., 2020), polyvinyl alcohol (PVA) (Kalachaveedu et al., 2020), polyethylene glycol (PEG), tribasic calcium phosphate (TCP) (Ying et al., 2020). Biomaterials used for 3D bioprinting are called “bio-inks.”

Extrusion-based bioprinting (also called direct ink writing) is a type of 3D fabrication. Bio-inks are discharged from a syringe in continuous filaments that can be layered to create the desired design. The height of the architecture is based on piling several layers of these filaments. There are two types of extrusion-based bioprinting: (1) pneumatic extrusion bioprinting, which uses compressed air to push the bio-ink from a syringe (Lee et al., 2015b); and (2) mechanical extrusion bioprinting, which uses a stepping motor connected to a piston or screw to extrude the bio-ink. The mechanical outlet mechanism is more precise than the pneumatic system and can print semi-solid and solid bio-inks more effectively (Skardal et al., 2010; Fielding et al., 2012). Micro-extrusion is a form of extended extrusion bioprinting in which the diameter of the extrusion syringe is less than 1 mm (Murphy and Atala, 2014).

There are four types of droplet-based bioprinting. Inkjet-based bioprinting utilizes gravity, atmospheric pressure, and fluid mechanics to generate and eject liquid droplets onto a receiving matrix (Liu and Derby, 2019). However, the high pressure may be harmful to cells contained in droplets when ejected through a syringe with a very small diameter.

Electrohydrodynamic jet bioprinting, which utilizes an electric field to drag bio-ink droplets out the syringe, thereby eliminating the need for great pressure (Onses et al., 2015), is suitable for applications that require syringes with small diameters (≤100 μm) and high concentrations of bio-inks (Jayasinghe et al., 2006).

Acoustic droplet ejection bioprinting is an extension of DOD to pattern cells that uses the acoustic radiation force related to the ultrasonic field to shift momentum from the gas–liquid interface to the formation of droplets (Elrod et al., 1989). A specific amount of liquid is ejected when the sound pressure of the ultrasonic field is greater than the surface tension (Demirci and Montesano, 2007).

Laser-assisted bioprinting is a predetermined computer-aided design technology with the use of a scanning mirror system that is focused on the laser-induced forward transfer effect and laser-guided direct writing.

Photocuring-based bioprinting uses liquid light-curable resins in the photopolymerization phase that chemically react to create solid artifacts when exposed to light. Stereolithography (Melchels et al., 2010) and digital light processing (DLP) (Patel et al., 2017; Schmidt and Colombo, 2018) are both photopolymer additive manufacturing techniques.

Embryonic stem cells (ESCs), upon separation from the inner mass of blastocysts, have stable developmental potential and the capacity of prolonged undifferentiated proliferation to form the three primary germ layers (i.e., the ectoderm, mesoderm, and endoderm) (Thomson et al., 1998).

Most somatic cells can be reprogrammed to pluripotent SCs by cultivation in vitro for a few weeks with retrovirally imported transcription factors, such as c-Myc, Kfl4, Oct3/4, and Sox2. In addition, somatic cells reversed to the embryonic pluripotent state, similar to ESCs, can generate all the cell types of the body (Takahashi and Yamanaka, 2006; Takahashi et al., 2007). On account of this discovery, Dr. Shinya Yamanaka won the 2012 Nobel prize in Physiology or Medicine.

Mesenchymal stem/stromal cells (MSCs), which are relatively easy to isolate and extensively expand, differentiate into various types of cells, mainly chondrocytes, osteoblasts, and adipocytes (Dominici et al., 2006), which exhibit adherent and fibroblast-like characteristics (Friedenstein, 1976). MSCs have been isolated from various tissues, including bone marrow (bone marrow-derived MSCs, BMSCs), adipose tissue (adipose-derived MSCs, ASCs), umbilical cord blood (umbilical cord-derived MSCs, UCMSCs), dental pulp, skeletal muscle, Wharton’s jelly, synovial membrane, and amniotic fluid (Keating, 2012).

Neural stem/progenitor cells (NSC/NPCs) are derived by separation from the adult and fetal brain. A satellite cell is a type of SC that is derived from skeletal muscle.

Stem cell treatment has progressed rapidly over the last two decades and it is now used as a tissue engineering technology in orthopedic surgery for the treatment of bone fractures, cartilage abnormalities, ligament-tendon injuries, and bone defects. For osteoarthritis (OA), Zhou et al. (2018) reported that local injection of ASCs into a rat model could alleviate histologically confirmed OA of the knee. Sasaki et al. (2017) reported that the enhanced proliferation could almost double the abundance of MSCs in vitro. Injection of MSCs cultured with granulocyte-colony stimulating factor was shown to induce regeneration of the hyaline cartilage for repair of trochlear osteochondral defects (Harada et al., 2015). In a clinical study of OA of the Knee, Al-Najar et al. (2017) reported that intraarticular injections of MSCs had significantly improved knee joint function.

Cardiovascular diseases remain the leading cause of death globally. Heart failure, which is characterized by avascular necrosis of cardiomyocytes at the terminal stage of disease, is caused by dilated cardiomyopathy, coronary heart disease, and severe valvular disease (Elgendy et al., 2019). However, since the efficacy of current treatments to reverse heart failure during myocardial infarction (MI) is limited, cardiomyocyte recovery has become a top priority. Various types of SCs, especially iPSCs, cardiac progenitor cells, and cardiosphere-derived cells, have been applied in myocardial repair. iPSCs have superior therapeutic efficacy in myocardial repair and iPSC-derived cardiomyocytes exhibit many of the same features as cardiac cells, including contractility, spontaneous pumping, and cytokine-mediated ion channel expression (Xi et al., 2010). With the use of a large animal model of immunosuppression-associated MI, Ishida et al. (2019) demonstrated that xenogeneic epicardial transplantation of iPSC-derived cardiomyocytes significantly improved cardiac function and resulted in a significant increase in capillary density in ischemic regions. Cell therapy trials (Tarui et al., 2015; Ishigami et al., 2017) revealed that intracoronary administration of cardiosphere-derived cells improved heart failure, somatic development, and quality of life by reverse remodeling.

For wound treatment, local injection of MSCs reduced apoptosis (Öksüz et al., 2013; Abbas et al., 2018) and improve burn wound progression (Öksüz et al., 2013) in animal models. Local treatment also improved survival after burn injury (Caliari-Oliveira et al., 2016). Furthermore, local injection of MSCs was shown to significantly accelerate the wound healing rate (Caliari-Oliveira et al., 2016; Abo-Elkheir et al., 2017; Chen et al., 2017), re-epithelization (Shi et al., 2017), granulation tissue formation (Caliari-Oliveira et al., 2016; Shi et al., 2017), and vascularization/angiogenesis (Caliari-Oliveira et al., 2016; Hosni Ahmed et al., 2017). SCs accelerated the wound healing process by inducing neo-angiogenesis (Atalay et al., 2014; Liu et al., 2014; Lough et al., 2014), collagen deposition (Liu et al., 2014) and granulation tissue formation, in addition to modulating the inflammatory response (Atalay et al., 2014; Liu et al., 2014; Caliari-Oliveira et al., 2016) and reducing the risk of infection. SC therapy was also shown to improve the healing of burn wounds and the immune response.

Boucherie et al. (2013) reported that transient neuroepithelium, which was generated by ESCs cultured together with a Matrigel extracellular matrix, had induced conversion into retinal progenitors in 5 days. The retinal progenitors had differentiated into Crx1-expressing photoreceptor precursors after just 10 days and then attained rod photoreceptor identity within 4 weeks. Huo et al. (2010) reported that most MSCs injected to the subretinal space of rats with retinosis remained in the cones and retinal pigment epithelium (RPE), and differentiated into retinal pigment epithelial and photoreceptor cells. Further, pan-cytokeratin and rhodopsin were expressed in engrafted MSCs. Tucker et al. (2011) reported that transplantation of iPSCs into immune-compromised mice with retinal degeneration formed teratomas containing all three germ layers and gradually exhibited normal retinal physiological characteristics in response to the delivery of neurotransmitters.

The SCs can restrict secondary injury, reduce inflammation, and secrete paracrine factors to protect surviving neurons. SCs also facilitate axon regeneration, and differentiate into new neurons to replace injured neurons in spinal cord injury (SCI) (Leng et al., 2019; Zhang et al., 2019; Feng et al., 2021). The results of a phase I clinical trial conducted by Mendonça et al. (Mendonça et al., 2014) showed that administration of BMSCs improved behavioral scores, electromyography, and somatosensory evoked potentials. A long-term follow-up study revealed that administration of BMSCs improved upper-limb motor capacity, electrophysiological function, and quality of life in three patients with grade B impairment (Park et al., 2012). Lower doses of ASCs were used to increase the cell migration rate and infarct volume of permanent occlusion in rat models of stroke during functional rehabilitation (Grudzenski et al., 2017).

Mesenchymal stem cells are often used in bone regeneration engineering. For cartilage formation: Xia et al. (2018) reported that pericellular coating with collagen I promoted the adhesion of MSCs to cartilage slices and direct intra-articular injection of MSCs enhanced homing and retention in cartilage defects. Intercellular associations were also stimulated by pericellular coating with collagen I, which promoted the expression of aggrecan, N-cadherin, and collagen II. The increased homing rate was related to intercellular contact. Sawatjui et al. (2015) reported that a silk fibroin-gelatin-chondroitin sulfate-HA scaffold outperformed a single silk fibroin scaffold in vitro. The hybrid scaffold might serve as a supporting system as well as a cartilage modeling environment for chondrogenesis by facilitating cartilage regeneration. This hybrid scaffold can potentially improve chondrogenesis by inducing proliferation and chondrogenic differentiation of MSCs. Chang et al. (2020) reported that honeycomb-like gelatin scaffolds can promote the survival of UCMSCs and chondrogenic differentiation to form hyaline-like cartilage. Zhang et al. (2017) reported that MSCs in a PCL scaffold improved fibrocartilage regeneration and mechanical efficiency, providing a practical alternative to shield the articular cartilage from injury following meniscectomy. Zorzi et al. (2015) reported that as compared to the control group, ASCs seeded on collagen/chitosan scaffolds improved the healing and thickness of chondral lesions in an adult ovine model.

Further clinical studies conducted by Gobbi et al. reported that an HA-based active bone marrow concentrate scaffold for management of small or significant lesions, one or multiple lesions in one or two compartments, and subsequent lesion therapy resulted in reasonably stable long-term results in cartilage repair (Gobbi and Whyte, 2016) and knee full-thickness cartilage injury (Gobbi and Whyte, 2019).

Moreover, Tseng et al. (2020) reported that the grafting of a layer of the anti-oxidative reagent N-acetylcysteine and extracellular matrix-like type I collagen increased the adhesion capabilities of rat ASCs and the proliferation of SCs in vitro. In addition, dynamic compression stimulus accelerated the differentiation of rat ASCs into the chondrogenic lineage within a relatively brief period.

Besides, McKee et al. (2017) reported that increased mRNA expression of RhoA in differentiated ESCs implanted into PDMS scaffolds as a result of compression-induced stress. When the ESCs in PDMS scaffolds were subjected to compression-induced stress and treated with an RhoA inhibitor, the chondro-inductive effect of RhoA was downregulated along with the transcription and translation of early markers of chondrogenesis.

Scaffold-based SC therapy with the use of MSCs, iPSCs, and ASCs has been applied for the regeneration of cardiomyocytes following MI.

Wang et al. (2020c) reported that injection with a collagen scaffold improved the durability of transplanted UCMSCs, implying greater angiogenesis and cardiomyocyte viability after MI.

Firoozi et al. (2020) confirmed cardiac-compatible properties of a self-assembling peptide (SAP) hydrogel with mild gelation, injectability, repair ability, and possible sequence alteration. An SAP hydrogel composite with a self-assembling gel-forming core sequence (RADA) was modified with a SDKP motif to facilitate pro-angiogenic and anti-fibrotic behaviors for use as a cardioprotective scaffold. With the addition of RADA-SDKP, injection of the compound hydrogel restored cardiac function following acute MI and gradually improved clinical outcomes.

Rabbani et al. (2017) reported that the combination of Wharton jelly-derived MSCs with a novel compound containing PEG, HA, and chitosan for heart regeneration. The cell/scaffold complex improved defect size and cardiac function, while promoting neoangiogenesis and cardiomyogenesis. Also, Rojas et al. (2015) reported that a fibrinogen biomatrix improved the retention of cardiac iPSCs and sustained improved cellular refill and function of infarcted myocardium. Therefore, fibrinogen could be considered as an ideal natural intramyocardial scaffold in the future.

Gelmi et al. (2016) reported that a conductive polymer polypyrrole-coated PLGA scaffold provided a microenvironment, including electrical and mechanical stimulation, which could promote proliferation, differentiation, and survival of transplanted SCs by controlling the microenvironment and mimicking the structural architecture of the heart. The hybrid scaffold is used to stimulate the viability and mediate the differentiation of iPSCs. Khan et al. (2015) stated that the morphology and roles of iPSC-derived cardiomyocytes cultured under an anisotropic atmosphere provided through an aligned nanofiber patch changed more than when cultured on a flat surface. These cells more closely resembled natural cardiac tissue and, thus, were suitable for cardiac implantation.

Besides, Bai et al. (2019) reported that an extracellular matrix (ECM) hydrogel greatly promoted the proliferation of SCs derived from brown adipose tissue and cardiomyogenic differentiation. The combination of SCs derived from brown adipose tissue and an ECM hydrogel retained cardiac activity and chamber geometry.

Oltolina et al. (2015) reported that spheroids were aggregated by seeding cardiac progenitor cells onto methylcellulose hydrogel-coated microwells. When spheroids were inserted into the heart walls and cardiotoxin-injured myocardium of female mice, the cells from the spheroids exhibited the very same engraftment capacity. Jamaiyar et al. (2017) reported that induced vascular progenitor cells (iVPCs) in a micro-bundle scaffold achieved greater engraftment, survival, and retention in the myocardium. Treatment with iVPCs and polymer micro-bundles enhanced the viability of cardiomyocytes, heart efficiency, and valve density, and reduced infarction size as determined by echocardiography.

The use of MSCs, iPSCs, and ASCs in combination with different types of 3D scaffolds achieved more significant effects than traditional therapies, while improving cardiomyocyte survival and improving infarcted tissues to varying degrees.

Kalachaveedu et al. (2020) reported that an electrospun nanofibrous Guar gum/PVA-based scaffold matrix, which incorporated four traditional medicinal plant extracts, was characterized by good water absorption and thermal stability. The scaffold outperformed skin in terms of elastic modulus, fiber spinnability, and tensile strength. Integrating a mat of herbal nanofibers and MSCs achieved complete skin repair with minimal scarring. Han et al. (2019) reported that activation of the Wnt signaling pathway promoted wound healing for diabetics by regulating the proliferation and differentiation of UCMSCs in a collagen-chitosan acellular dermal matrix scaffold. Shou et al. (2018) reported that the defensive use of a hydrogel composed of 3D chitin nanofibers increased the viability of BMSCs and acted as a functional scaffold that enhanced the regenerative capacity of BMSCs to facilitate wound healing.

Lee et al. (2020) reported that the ASC-derived spheroids were tightly entrapped by electrospun alginate nanofibers and alginate strut, which then released numerous factors related to angiogenesis and wound healing in a coordinated manner. The scaffold loaded with spheroids facilitated the formation of capillary-like structures in umbilical vein epithelial cells.

In summary, SC therapy, including MSCs and ASCs, combined with a 3D scaffold can better promote wound healing and reduce scarring (Figure 2). All 3D scaffolds based on a single-component scaffold adopt the form of the composite and exhibit better physical properties, such as spinnability and tensile strength, as well as biological properties, such as low skin toxicity.

Haghighat et al. (2020) reported that the use of β-carotene as a differentiation medium and an alginate-based scaffold to induce the differentiation of ciliary epithelium-derived MSCs into advanced retinal cells. Holan et al. (2015) reported that the combination of MSCs and a nanofiber scaffold improved healing via enhanced corneal thickness, re-epithelialization, and blood vessel formation, while inhibiting local inflammation. Besides, Yazdani et al. (2019) reported that the use of an HA hydrogel crosslinked with PEG diacrylate. For ocular restoration, the optimized HA scaffold improved the morphology of sheets of oral mucosal epithelial cells, cell metabolism, and the expression of genes associated with adherence and stemness, while reducing cellular damage. M’Barek et al. (2017) reported that an amniotic membrane scaffold produced from ESC-derived RPE cells on amniotic membrane sheets. As compared to the injection of ESC-RPE cells in suspension, transplantation of sheets of ESC-RPE cells prevented the death of photoreceptor cells and increased vision despite retinal degeneration.

In summary, SC therapy combined with polysaccharide-based 3D scaffolds significantly improved eye reconstruction.

Li et al. (2018a) reported that the use of an HA scaffold modified with adhesive peptides for the treatment of SCI. MSCs with recombinant brain-derived neurotrophic factor exhibited improved cell survival and sustained gene expression in vitro. MSCs also effectively improved the integrity of spinal tissue, alleviated inflammation, and inhibited glial scar formation.

Caron et al. (2016) reported that a new agarose/carbomer-based hydrogel optimized the viability, density, and delivery of paracrine factors of MSCs. In a mouse model of SCI, MSCs combined with a hydrogel scaffold greatly modulated the pro-inflammatory environment, improved the numbers of M2 macrophages, and facilitated regeneration of the original environment (Shevela et al., 2019). Kim et al. (2016) reported that as compared with intralesional injection, transplantation of scaffold-based MSCs achieved a greater implantation success rate and locomotor ability in acute SCI, especially with the use of a chitosan scaffold, followed by a PLGA scaffold. Besides, Wang et al. (2020b) reported that Matrigel efficiently supported the survival and differentiation of NSCs. As compared with rats treated with phosphate-buffered saline (sham control) or with Matrigel transplants, NSCs in Matrigel improved the behavioral recovery and the expression levels of neuronal and reactive astrocyte marker in a rat model of SCI. Zweckberger et al. (2016) reported that SAP-treated animals had more living NPCs and the differentiation capability of oligodendrocytes and neurons had increased. The combined treatment with SAP and NPCs improved the reservation and behavioral outcomes of the corticospinal tract. Qing et al. (2018) reported that a novel heterostructure scaffold composed of electrospun silk nanofibers layered on graphene paper, which had high biocompatibility and conductivity, had effectively induced oriented growth and improved differentiation of neuroblastoma cells.

For stroke treatment, Moshayedi et al. (2016) reported that as a platform to promote the adhesion of structural motifs and release of growth factors, an HA-based self-polymerizing hydrogel promoted differential maturation of NPCs in the stroke cavity and improved the survival rate of NPCs, which can be tracked by magnetic resonance imaging (Guo et al., 2019; Wang et al., 2020).

For glioblastoma, Sheets et al. (2020) reported that gelatin matrices (GEMs) significantly supported the viability, persistence, and efficacy of seeded NSCs. Delivery with a scaffold of GEMs enabled therapeutic cells to persist in an immunologically active post-surgical environment, while maintaining stemness and the ability to target tumors. In a mouse model, GEM-NSCs significantly reduced the residual tumor volume. Moore et al. (2020) designed composite gelatin-electrospun scaffolds with two degradation profiles due to different ratios of cyclic to acyclic acetals (fast and slow). NSC implantation efficiency, persistence, and long-term survival were all improved by the fast and slow degrading scaffolds, respectively, and scaffold degradation had little effect on the persistence of NSCs.

This section described 3D scaffold-based SC therapy for treatment of neurological diseases, including SCI, stroke, and glioblastoma (Figure 3). SC therapy with MSCs or NSCs is advantageous for nerve repair by maintaining the survival of implanted SCs and enhancing neuron proliferation and differentiation.

Zhang et al. (2018a) reported that MSC-derived exosomes are able to repair osteochondral defects via an organized multi-faceted response that includes increased migration, proliferation, and matrix synthesis, decreased apoptosis, and modulated immunoreaction. Cui et al. (2016) reported that exosomes derived from mineralized osteoblasts significantly influenced miRNA profiles in recipient bone marrow cells, thereby promoting differentiation into osteoblasts. Axin1 expression was inhibited by changes in miRNA profiles, while β-catenin expression was increased and activated the Wnt signaling pathway.

Arslan et al. (2013) reported that the administration of MSC-derived exosomes decreased the infarct size and increased cardiac function in a mouse model of myocardial ischemia-reperfusion injury. After revascularization, exosome treatment greatly decreased neutrophil and macrophage infiltration, implying that exosomes have an anti-inflammation effect. Bian et al. (2014) found that hypoxic BMSC-derived exosomes were fully accepted by umbilical vein endothelial cells, resulting in increased proliferation and migration. Exosome administration decreased infarct duration, restored cardiac activity, and induced angiogenesis in the infarcted area of a rat model of acute MI. Yu et al. (2015) reported that BMSC-derived exosomes overexpressing the transcription factor GATA-binding protein 4 released more miRNA-19a. In a rat model of acute MI, exosome administration conveyed an anti-apoptosis effect under hypoxic conditions in vitro, while restoring cardiac activity and reducing infarct volume. MiR-19a was shown to be active in exosome cardioprotection by downregulating phosphatase and tensin homolog expression and activating the AKT signaling pathway.

Exosomes are becoming more widely used in regenerative medicine due to anti-inflammatory properties and the ability to promote angiogenesis through proliferative and migratory phenotypes, wound healing, and anti-aging properties.

Exosomes facilitate a major change of the recipient macrophages to the anti-inflammatory phenotype during inflammation (Regenerative et al., 2017). Exosomes have the potential to inhibit the immune response by regulating the recruitment, differentiation, and proliferation of B lymphocyte, as well as converting activated T lymphocytes to the T-regulatory phenotype (Nosbaum et al., 2015; Monguió-Tortajada et al., 2017). According to numerous reports, exosomes also have immunomodulatory effects through particular miRNAs, such as miRNA-21, miRNA-181c, and miRNA-146a (Ti et al., 2015, 2016), and various receptor signaling pathways (Li et al., 2016).

Exosomes, which promote neoangiogenesis, tissue regeneration, collagen deposition, re-epithelialization, and wound healing during proliferation (Midwood et al., 2004), can be preserved and transport their RNA and protein cargos to recipient cells to regulate migration and proliferation. Exosomes can also promote the synthesis and expression levels of types I and III collagen and elastin during the remodeling process (Zhang et al., 2015). In addition, exosomes suppress scar formation by regulating collagen production at various stages of the wound healing process (Kou et al., 2018).

Yu et al. (2016) investigated that intravitreal administration of MSC-derived exosomes inhibited inflammation, limited the extent of damage, reduced apoptosis, and improved visual function via downregulation of MCP-1. Zhang et al. (2018b) concluded that injection of MSC-derived exosomes into the eye after a normal pars plana vitrectomy could increase the anatomical and functional effects of macular holes, which is difficult to treat during the initial surgery. Some studies Trichonas et al. (2010); Nakazawa et al. (2011), and Xie et al. (2017) have found that MSC-derived exosomes induce the production of inflammatory cytokines and increase autophagy, thereby increasing the survival of photoreceptors. In a rat model of retinal detachment, exosomes were reported to inhibit the induction of TNF-α and subsequently suppress the inflammatory response and cell death following retinal damage by decreasing autophagy.

Xin et al. (2013) found that co-administration of BMSC-derived exosomes significantly boosted neurological regeneration and induced neurogenesis and angiogenesis in the ischemic zone after ligating the middle cerebral artery in a rat model of stroke. Doeppner et al. (2015) reported that following ligation of the middle cerebral artery, administration of BMSCs and BMSC-derived exosomes improved neurological function and stimulated angiogenesis and neurogenesis to the same degree in a mouse model of focal cerebral ischemia.

In recent years, 3D scaffold-based exosome therapy has shown great potential in the regeneration of bone cartilage (Figure 1). Narayanan et al. (2016) reported that osteogenic MSC-derived exosomes can be combined with ECM proteins, such as collagen I and fibronectin, to promote the differentiation of MSCs into osteocytes. Yang et al. (2020b) reported that the combination of UCMSC-derived exosomes and HAP-embedded crosslinked HA-alginate hydrogel in situ significantly enhanced bone regeneration of preosteoblasts. Several studies Qi et al. (2016); Zhang et al. (2016), and Ying et al. (2020) reported that the combination of iPSC- or MSC-derived exosomes and a β-TCP scaffold promoted angiogenesis and osteogenesis. Three studies Sanchez et al. (2020); Wang et al. (2020d), and Zha et al. (2021) reported that the combination of exosomes and a PCL scaffold significantly enhanced osteogenic differentiation of MSCs. Gandolfi et al. (2020) reported that the mineral-doped PLA porous scaffolds enriched with MSC-derived exosomes increased the osteogenic commitment of MSCs. Li et al. (2018b) and Swanson et al. (2020) reported that different scaffolds based on biodegradable PLGA, in which exosomes facilitated osteogenic differentiation, promoted mineralization by recruiting endogenous cells to bone defects.

Many types of 3D scaffolds can promote osteogenesis. With the exception of common gels, polysaccharides and related complexes, some scaffolds are composed of metals, decellularized ECM, and or related complexes. Scaffolds of different materials can support the survival of SCs and promote osteogenic differentiation and osteogenesis. The combination of 3D scaffolds and exosomes provides various novel treatment methods for orthopedic injuries and promotes clinical transformation.

Several studies have investigated the efficacy of 3D scaffold-based exosome therapy for skin regeneration (Figure 2). Yang et al. (2020a) reported that the combination of a Pluronic F-127 hydrogel and UCMSC-derived exosomes significantly accelerated wound closure, enhanced regeneration of granulation tissue, and promoted wound healing for diabetics. Wang et al. (2020a) confirmed that a biocompatible 3D porous self-healing methylcellulose-chitosan hydrogel loaded with placental MSC-derived exosomes facilitated wound healing by synergistically inducing angiogenesis and inhibiting apoptosis. Shafei et al. (2020) reported that a alginate-based hydrogel loaded with ASC-derived exosomes improved wound closing, vessel development, and collagen synthesis. Nooshabadi et al. (2020) reported that a chitosan hydrogel scaffold containing exosomes was effective for wound closure and promoted a high degree of re-epithelialization.

Several studies have assessed the effectiveness of 3D scaffold-based exosome therapy for treatment of nerve injuries (Figure 3). Li et al. (2020) reported that topical transplantation of MSC-derived exosomes fixed in a peptide-modified hydrogel provided exosome-encapsulated ECM for repair of damaged nerve tissues and induced comprehensive mitigation of the microenvironment. The implanted exosomes exhibited better retention and continuous release. By mitigating oxidation and inflammation, the exosome-loaded hydrogel significantly elicited nerve repair. Hsu et al. (2020) reported that UCMSC-derived exosomes induced neurite outgrowth and protected neurons from formic acid in vitro. In vivo, an alginate scaffold with exosomes exhibited anti-toxicity, anti-inflammation, and pro-neurotrophy activities in a ligation model of spinal nerve pain.