The uterus, which is composed of the perimetrium, myometrium, and endometrium, is an important female reproductive organ that is essential for embryo implantation and development [99]. Caesarean section, severe infection, and serious injuries may cause the aberrant activation of fibrosis and expression of estrogen receptor alpha, ultimately leading to scar formation and uterine adhesions, which severely affects women’s health [100]. The effects of common treatments, including hysteroscopy adhesiolysis and placement of physical barriers, remain poor in severe cases [101]. Additional therapies for preventing adhesions as well as uterine regeneration and reconstruction involve the administration of therapeutic substances including but not limited to drugs, growth factors, chemokines, and exosomes [20,100,102,103]. However, repeated administration of some therapeutic substances was required due to their poor solubility, high diffusibility, and rapid clearance which resulted in very low drug concentration at the site of injury in the uterus. [104]. As such, suitable therapeutic agent-based delivery systems are urgently in need for the treatment of some uterine disorders or injuries to improve the half-life of the substances, delivery efficacy, and therapeutic effect.

β-estradiol (E2) is an endogenous hormone that has antifibrosis, adhesion prevention, and endometrial regeneration effects [102]. In 2020, a system capable of delivering E2 with a 21-day release profile consistent with the female menstrual cycle was developed by dispersing E2-loaded PLGA microspheres in the amniotic extracellular matrix [105]. In another study, an E2-loaded nanoparticulate decellularized uterus embedded with aloe/poloxamer hydrogel (E2@uECMNPs/AP) was used to treat intrauterine adhesion (IUA) in a rat model [106]. The E2@uECMNPs/AP group exhibited significantly enhanced morphological recovery and less uterine fibrosis than the IUA group, the E2 alone group, the commercially available E2 gel group, and the AP hydrogel group, indicating that this delivery system was capable of effectively promoting endometrial regeneration and preventing re-adhesion [106]. Moreover, considering that sex hormones are key factors to regulate the menstrual cycle phases of endometrium, enzyme expression and hormone metabolism, polymorphisms that exist with these enzymes could also be taken into consideration while designing hormone delivery systems to ensure drug efficacy [107]. Other than estrogen, regenerative therapies delivering bioactive agents such as vitamin C and curcumin have also demonstrated promising results in the treatment of uterine disorders or injuries [101,108].

Several uterine regeneration-related growth factors or chemokines, such as keratinocyte growth factor, bFGF, VEGF, and stromal cell-derived factor-1 (SDF-1) have also been used in delivery-based systems to obtain a controlled spatiotemporal release profile and restore the anatomy and function of the uterus [109,110,111,112,113]. For example, Wenbo et al. designed a chitosan–heparin delivery system for controlling the release of SDF-1α to an injured rat endometrium [112]. In this study, the number of glands, thickness of the endometrium, and level of fibrosis did not differ between the SDF-1α-treated and the normal control groups after 7 days [112]. In another study, SDF-1α carried by a silk fibroin-bacterial cellulose membrane was employed to treat full-thickness uterine injury [100]. This SDF-1α-loaded delivery system showed promising effects on arteriogenesis, mature endometrium formation, and pregnancy outcome [100]. Further, Jiang et al. successfully constructed a collagen-targeting bFGF delivery system by fusing a collagen-binding domain peptide to the N-terminal of bFGF [114]. Administration of this bFGF delivery system around the scarred endometrium of 18 uterine-infertile women every 4 weeks improved endometrial thickness, scarring of the endometrial area, vascular density, and menstrual blood volume [103]. Notably, three of the 18 bFGF-treated patients achieved pregnancy over 20 gestational weeks [103]. Liu et al. loaded stem cell secretome, which contained a great variety of regeneration-related growth factors, on a crosslinked hyaluronic acid hydrogel to facilitate the sustained release of growth factors and support the morphological and functional recovery of the uterus [115]. This approach demonstrated great potential for clinical translation because of its long shelf-life and superior safety profile.

The ovaries, which are composed of the outer layer, capsule, cortex, and medulla, are paired organs that produce oocytes and reproductive hormones [107]. Age, disease, chemotherapy, or radiotherapy may cause various ovarian diseases, such as primary ovarian insufficiency, ovariectomy, and polycystic ovary syndrome [116]. These diseases can result in hormonal imbalances, metabolic syndrome, infertility, and genital atrophy, which threatens the health of women [99,117]. Current treatments generally include hormone replacement therapy and ovary transplantation. Growth factors that dominate follicular survival, latency, activation, and maturation, such as growth and differentiation factor-9 (GDF-9), bone morphogenetic protein-4 (BMP-4), VEGF, platelet derived growth factor (PDGF-ββ), and anti-mullerian hormone, are promising bioactive agents for delivery-based regenerative therapy to restore ovarian function [118,119,120]. For example, VEGF-loaded fibrin-heparin-binding peptide hydrogel was engineered to promote ovarian graft survival in a bilateral ovariectomy mouse model [119]. This delivery system prolonged VEGF activity and release, thereby, promoting angiogenesis, enhancing engraftment, and improving the function of the transplanted ovarian tissue [119]. Additionally, growth factors can be combined for better therapeutic effect. For example, a macroporous alginate scaffold loaded with BMP-4, GDF-9, VEGF, and PDGF-ββ was shown to restore the ovarian function of ovariectomized mice [120]. This regenerative system helped support the follicle reach to antral size, retained hormone-secreting function, and resumed cyclic vaginal appearance, thus, leading to the restoration of ovarian function in vivo [120].

In addition, some living cells, as autologous constant sources of active biomolecules, have been used in delivery-based systems to treat ovarian dysfunction [121,122,123,124]. Green et al. designed a delivery system encapsulating adipose-derived stem cells (ADSCs) in crosslinked alginate beads [125]. The alginate/ADSCs group displayed higher follicle survival and better follicle growth, antrum formation, and oocyte maturation than the follicles cultured alone group, suggesting that cytokine excretion by ADSCs may play an important role in the maturation of early-stage follicles [125]. In 2021, a layer-by-layer form of follicle spheroids composed of autologous ovary cells, gelatin, and/or Matrigel was applied in the treatment of ovarian endocrine function loss [123]. This system significantly restored the endocrine function of ovariectomized rats and induced fewer side effects than in rats treated with synthetic hormones [123]. In another study, bone marrow-derived mesenchymal stem cells (BMSCs) were used to deliver the regulatory factors necessary for estrogen production [124]. The BMSCs-containing system effectively promoted stable and long-term estrogen secretion, regulated pituitary hormones, and improved physiological outcomes in an ovariectomized rat model [124].

The cervix, which is composed of ectocervix, the cervical transformation zone, and the endocervix, plays an important role in uterine growth and fetal development, the dysfunction and abnormality of which may cause premature birth [126]. Cervix cancer is a class of common cervix diseases usually treated by transabdominal hysterectomy. Regenerative therapies may provide promising strategies to reconstruct the structure and preserve the reproductive and physiological function of this kind of cervical defects. Zhao et al. engineered an 3D-printed cervix-like implant with drug release function, which was loaded with anti-human papillomavirus protein under negative pressure [127]. Validation of the quantitative loading and release capacity of this engineered delivery system, which inhibited dissociated virus near the cervix, suggested a promising functional tissue implant for patients whose cervixes have been resected due to human papilloma virus -induced cancer [127].

The vagina is an elastic muscular tube composed of vaginal mucosa, an intermediate muscle layer, and the adventitia. Among them, the vaginal mucosa, whose glycogen content can be adjusted depending on estrogen levels and undergoes cyclical changes, serves as a barrier to the entry of pathogens [128]. Its barrier function is enhanced by the acidic microenvironment created by Lactobacilli colonized in the vagina [129]. In this regard, to avoid potential dysbiosis, the vaginal microbiome should be taken into account when designing delivery systems [107]. Vagina regenerative medicine focuses on the treatment of acquired vaginal trauma, deformity, and agenesis caused by trauma, surgical operation, or disease at birth. In 2021, umbilical cord mesenchymal stem cell-loaded small intestinal submucosa was bioengineered to reconstruct a damaged vagina in a rhesus monkey [130]. The in vitro study demonstrated that this regenerative system produced several bioactive substances, such as elastin, hepatocyte growth factor, insulin-like growth factor, and VEGF [130]. Three months after transplantation, the engineered vagina showed enhanced ECM reorganization, large muscle bundle formation, angiogenesis, and mechanical properties, thus, suggesting a new approach for the repair of vaginal injury [130].

Dysfunction in the male reproductive system is the main cause of male infertility, which heavily affects male reproductive health [131,132,133,134]. Fortunately, regenerative therapy offers an opportunity for these patients to recover and even become biological fathers. Recently, an alginate matrix containing necrosis inhibitor nanoparticles (NECINH-NPS) was used to encapsulate testicular tissue fragments and achieve better reproductive outcomes [135]. After orthotopic auto transplantation, germ cell survival and testicular tissue integrity were found to be significantly improved [135]. Further, calcium-alginate hydrogels encapsulated with VEGF-nanoparticles, PDGF-nanoparticles, and NECINH-NPS have been used to optimize the angiogenesis of transplanted testicular tissue [136]. An in vivo study confirmed that the combined delivery of VEGF and PDGF nanoparticles effectively promoted vascular maturity [126]. In another study, curcumin-loaded iron oxide particles (CIOP) were utilized to treat scrotal hyperthermia-induced azoospermia [134]. The CIOP-treated group exhibited increased testes volumes and seminiferous tubule lengths, as well as improved sperm parameters, stereological parameters, and serum testosterone levels as compared with the scrotal hyperthermia group [134]. Ghorbani et al. incorporated multi-walled carbon nanotubes into poly(L-lactic acid) fibers to deliver naringenin for the regeneration of impaired spermatogenesis [137]. The beneficial role of naringenin, an effective antioxidant, in spermatogenesis was demonstrated in vitro by measuring the reduction in ROS generation in spermatogonial stem cells treated with different concentrations of naringenin [137].

In addition to fertility restoration, regenerative therapy has effectively treated other andrology-related diseases, such as erectile dysfunction and testosterone deficiency [131,133]. For example, ADSCs incubated with NanoShuttle (biocompatible magnetic nanoparticles) were employed to improve cavernous nerve injury-induced erectile dysfunction in a rat model [138]. ADSCs survival was improved under the protection of NanoShuttle and cells remained in the corpus cavernosum for at least 3 days [138]. Additionally, the smooth muscle, endothelium, and nerve tissue were significantly improved in the NanoShuttle/ADSCs group as compared with those in the cavernous nerve injury group without treatment, resulting in favorable erectile dysfunction treatment effects [138].

Regenerative therapy is necessary following various irreversible physiological disorders of the urinary system, including hypospadias, urethral stricture, end-stage bladder disease, and urinary incontinence [128,139].

To increase therapeutic efficacy, growth factors (e.g., insulin-like growth factor 1 (IGF-1), transforming growth factor beta-1 (TGF-β), VEGF, etc.) have been loaded on various delivery-based systems to obtain desired properties [140,141,142]. Yan et al. demonstrated the superiority of IGF-1-loaded alginate-poly-L-ornithine-gelatin (A-PLO-G) microbeads over empty A-PLO-G microbeads to regenerate an external urethral sphincter defect in a stress urinary incontinence (SUI) rat model [142]. The IGF-1-loaded A-PLO-G group showed enhanced regeneration effects, such as well-organized skeletal muscle fibers and vascular development as compared with the group treated with saline or empty A-PLO-G microbeads [142]. Based on these findings, the researchers suggested that promoting skeletal myogenesis and revascularization by periurethral administration of IGF-1-loaded A-PLO-G microbeads may facilitate recovery from SUI [142]. Moreover, Ardeshirylajimi et al. studied the efficacy of a poly (vinylidene fluoride) (PVDF) electrospun fiber scaffold loaded with TGF-β/chitosan nanoparticles for the functional recovery of reconstructed bladders [141]. This delivery system exhibited a long-term sustained release profile, and the bioactive agent-treated group exhibited significantly increased ADSCs proliferation and smooth muscle cells differentiation as compared with groups treated with PVDF and tissue culture polystyrene, suggesting promise for ladder tissue engineering applications [141].

Further, a canonical downstream signaling pathway of TGF-β, Wnt/β-catenin, plays a crucial role in the tissue repair process. For example, ICG-001, a well-established Wnt signaling inhibitor, was loaded on a collagen/poly(L-lactideco-caprolactone) nanoyarn with a core-shell structure to treat urethral defects in a canine model[143]. This ICG-001-delivery system induced decreased fibroblast proliferation, suppressed fibrotic protein expression, and restored a fully functional urethra within 12 weeks [143].

Cell-based delivery systems are another promising approach for urinary tissue regeneration. In one clinical study, autologous buccal epithelial cells encapsulated in thermos-responsive hydrogel were implanted at the stricture site of six male patients with bulbar urethral stricture [144]. All patients recovered with healthy mucosa at the urethrotomy site and voided wells [144]. Although two patients reported recurrence within two years, this delivery system may be beneficial for the treatment of urethral stricture [144]. Wang et al. developed a bladder patch capable of localized non-invasive delivery of transplanted cells in vivo by utilizing ADSCs-labeled ultrasmall SPIONs and porous polyglycolic acid scaffolds [145]. This delivery system not only promoted the regeneration of bladder tissues (urothelium, smooth muscle, neural cells, and blood vessels) but also restored bladder function with augmented capacity, which holds great therapeutic promise for bladder injuries [145]. Furthermore, cell- or tissue-derived EVs and ECM, which contain multiple active biomolecules, can confer desirable biochemical properties to delivery systems, thereby, achieving urinary regeneration effects [21,146,147].