Biomaterials for recruiting and activating endogenous stem cells in situ tissue regeneration

Molecular regulators of chemotaxis in human hematopoietic stem cells

Hematopoietic stem cells (HSCs), essential for lifelong blood cell regeneration, are clinically utilized to treat various hematological disorders. These cells originate in the aorta-gonad-mesonephros region, expand in the fetal liver, and mature in the bone marrow. Chemotaxis, involving gradient sensing, polarization, and migration, directs HSCs and is crucial for their homing and mobilization. The molecular regulation of HSC chemotaxis involves chemokines, chemokine receptors, signaling pathways, and cytoskeletal proteins. Recent advances in understanding these regulatory mechanisms have deepened insights into HSC development and hematopoiesis, offering new avenues for therapeutic innovations. Strategies including glucocorticoid receptor activation, modulation of histone acetylation, stimulation of nitric oxide signaling, and interference with m6A RNA modification have shown potential in enhancing CXCR4 expression, thereby improving the chemotactic response and homing capabilities of human HSCs. This review synthesizes current knowledge on the molecular regulation of human HSC chemotaxis and its implications for health and disease.

Recent research progresses of bioengineered biliary stents

Bile duct lesion, including benign (eg. occlusion, cholelithiasis, dilatation, malformation) and malignant (cholangiocarcinoma) diseases, is a frequently encountered challenge in hepatobiliary diseases, which can be repaired by interventional or surgical procedures. A viable cure for bile duct lesions is implantation with biliary stents. Despite the placement achieved by current clinical biliary stents, the creation of functional and readily transplantable biliary stents remains a formidable obstacle. Excellent biocompatibility, stable mechanics, and absorbability are just a few benefits of using bioengineered biliary stents, which can also support and repair damaged bile ducts that drain bile. Additionally, cell sources & organoids derived from the biliary system that are loaded onto scaffolds can encourage bile duct regeneration. Therefore, the implantation of bioengineered biliary stent is considered as an ideal treatment for bile duct lesion, holding a broad potential for clinical applications in future. In this review, we look back on the development of conventional biliary stents, biodegradable biliary stents, and bioengineered biliary stents, highlighting the crucial elements of bioengineered biliary stents in promoting bile duct regeneration. After providing an overview of the various types of cell sources & organoids and fabrication methods utilized for the bioengineering process, we present the in vitro and in vivo applications of bioengineered biliary ducts, along with the latest advances in this exciting field. Finally, we also emphasize the ongoing challenges and future development of bioengineered biliary stents.

Microenvironment-optimized gastrodin-functionalized scaffolds orchestrate asymmetric division of recruited stem cells in endogenous bone regeneration

The regeneration of osteoporotic bone defects remains challenging as the critical stem cell function is impaired by inflammatory microenvironment. Synthetic materials that intrinsically direct osteo-differentiation versus self-renewal of recruited stem cell represent a promising alternative strategy for endogenous bone formation. Therefore, a microenvironmentally optimized polyurethane (PU) /n-HA scaffold to enable sustained delivery of gastrodin is engineered to study its effect on the osteogenic fate of stem cells. It exhibited interconnected porous networks and an elevated sequential gastrodin release pattern to match immune-osteo cascade concurrent with progressive degradation of materials. In a critical-sized femur defect model of osteoporotic rat, 5% gastrodin-PU/n-HA potently promoted neo-bone regeneration by facilitating M2 macrophage polarization and CD146+ host stem cell recruitment to defective site. The implantation time-dependently increased the bone marrow mesenchymal stem cell (BMSC) population, and further culture of BMSCs showed a robust ability of proliferation, migration, and mitochondrial resurgence. Of note, some of cell pairs produced one stemness daughter cell while the other committed to osteogenic lineage in an asymmetric cell division (ACD) manner, and a much more compelling ACD response was triggered when 5% gastrodin-PU/n-HA implanted. Further investigation revealed that one-sided concentrated presentation of aPKC and β-catenin in dividing cells effectively induced asymmetric distribution, which polarized aPKC biased the response of the daughter cells to Wnt signal. The asymmetric cell division in skeletal stem cells (SSCs) was mechanically comparable to BMSCs and also governed by distinct aPKC and β-catenin biases. Concomitantly, delayed bone loss adjacent to the implant partly alleviated development of osteoporosis. In conclusion, our findings provide insight into the regulation of macrophage polarization combined with osteogenic commitment of recruited stem cells in an ACD manner, advancing scaffold design strategy for endogenous bone regeneration.

Recent Advances in Degradable Biomedical Polymers for Prevention, Diagnosis and Treatment of Diseases

Biomedical polymers play a key role in preventing, diagnosing, and treating diseases, showcasing a wide range of applications. Their unique advantages, such as rich source, good biocompatibility, and excellent modifiability, make them ideal biomaterials for drug delivery, biomedical imaging, and tissue engineering. However, conventional biomedical polymers suffer from poor degradation in vivo, increasing the risks of bioaccumulation and potential toxicity. To address these issues, degradable biomedical polymers can serve as an alternative strategy in biomedicine. Degradable biomedical polymers can efficiently relieve bioaccumulation in vivo and effectively reduce patient burden in disease management. This review comprehensively introduces the classification and properties of biomedical polymers and the recent research progress of degradable biomedical polymers in various diseases. Through an in-depth analysis of their classification, properties, and applications, we aim to provide strong guidance for promoting basic research and clinical translation of degradable biomedical polymers.

Chairside live biotherapeutic hydrogel for comprehensive periodontitis therapy

Periodontitis, characterized by microbial dysbiosis and immune dysregulation, destroys tooth-supporting tissues and negatively affects overall health. Current strategies face significant challenges in restoring damaged tissues while halting periodontitis progression. In this study, we introduce a live biotherapeutic product (LBP) in an engineered living hydrogel for comprehensive periodontitis therapy. A dental blue light-responsive hydrogel (LRG) was fabricated to deliver and confine live Lactobacillus rhamnosus GG (LGG) in periodontal pockets, endowing the LRG with sustained antibacterial and immunomodulatory effects. The LRG was engineered through peptide modification to also promote tissue regeneration. Both in vitro and in vivo evaluations confirmed the effectiveness of this integrated therapeutic strategy, which combines antibacterial, anti-inflammatory, and regenerative properties with an underlying immunomodulatory mechanism that involves suppressor of cytokine signaling (SOCS)3 upregulation and the Janus kinase/signal transducer and activator of transcription (JAK-STAT) pathway suppression in macrophages. Demonstrating a new paradigm, this proof of concept highlights the synergistic integration of live organisms and synthetic material engineering in a chairside treatment to address the multifaceted etiology of periodontitis.

A Novel Superparamagnetic-Responsive Hydrogel Facilitates Disc Regeneration by Orchestrating Cell Recruitment, Proliferation, and Differentiation within Hostile Inflammatory Niche

In situ disc regeneration is a meticulously orchestrated process, which involves cell recruitment, proliferation and differentiation within a local inflammatory niche. Thus far, it remains a challenge to establish a multi-staged regulatory framework for coordinating these cellular events, therefore leading to unsatisfactory outcome. This study constructs a super paramagnetically-responsive cellular gel, incorporating superparamagnetic iron oxide nanoparticles (SPIONs) and aptamer-modified palladium-hydrogen nanozymes (PdH-Apt) into a double-network polyacrylamide/hyaluronic acid (PAAm/HA) hydrogel. The Aptamer DB67 within magnetic hydrogel (Mag-gel) showed a high affinity for disialoganglioside (GD2), a specific membrane ligand of nucleus pulposus stem cells (NPSCs), to precisely recruit them to the injury site. The Mag-gel exhibits remarkable sensitivity to a magnetic field (MF), which exerts tunable micro/nano-scale forces on recruited NPSCs and triggers cytoskeletal remodeling, consequently boosting cell expansion in the early stage. By altering the parameters of MF, the mechanical cues within the hydrogel facilitates differentiation of NPSCs into nucleus pulposus cells to restore disc structure in the later stage. Furthermore, the PdH nanozymes within the Mag-gel mitigate the harsh inflammatory microenvironment, favoring cell survival and disc regeneration. This study presents a remote and multi-staged strategy for chronologically regulating endogenous stem cell fate, supporting disc regeneration without invasive procedures.

Stem Cells: Present Understanding and Prospects for Regenerative Dentistry

Regenerative medicine in dentistry focuses on repairing damaged oral tissues using advanced tools like stem cells, biomaterials, and tissue engineering (TE). Mesenchymal stem cells (MSCs) from dental sources, such as dental pulp and periodontal ligament, show significant potential for tissue regeneration due to their proliferative and differentiative abilities. This systematic review, following PRISMA guidelines, evaluated fifteen studies and identified effective strategies for improving dental, periodontal, and bone tissue regeneration through scaffolds, secretomes, and bioengineering methods. Key advancements include the use of dental pulp stem cells (DPSCs) and periodontal ligament stem cells (PDLSCs) to boost cell viability and manage inflammation. Additionally, pharmacological agents like matrine and surface modifications on biomaterials improve stem cell adhesion and promote osteogenic differentiation. By integrating these approaches, regenerative medicine and TE can optimize dental therapies and enhance patient outcomes. This review highlights the potential and challenges in this field, providing a critical assessment of current research and future directions.

Mechanical loading on cell-free polymer composite scaffold enhances in situ regeneration of fully functional Achilles tendon in a rabbit model

Traditional tendon engineering using cell-loaded scaffold has limited application potential due to the need of autologous cells. We hypothesize that potent mechanical loading can efficiently induce in situ Achilles tendon regeneration in a rabbit model by using a cell-free porous composite scaffold. In this study, melt-spinning was used to fabricate PGA (polyglycolic acid) and PLA (polylactic acid) filament fibers as well as non-woven PGA fibers. The PLA/PGA (4:2) filament fibers were further braided into a hybrid yarn，which was knitted into a PLA/PGA tubular mesh with potent mechanical property for sustaining natural tendon strain. The results showed that a complete cross-section of Achilles tendon created a model of full mechanical loading on the bridging scaffold, which could efficiently induce in situ tendon regeneration by promoting host cell infiltration, matrix production and tissue remodeling. Histologically, mechanical loading assisted in forming parallel aligned collagen fibers and tenocytes in a fashion similar to those of native tendon. Transmission electron microscope further demonstrated that mechanical strain induced collagen fibril development by increasing fibril diameter and forming bipolar structure, which resulted in enhanced mechanical properties. Interestingly, the synergistic effect between mechanical loading and hyaluronic acid modification was also observed on the induced tenogenic differentiation of infiltrated host fibroblasts. In conclusion, potent mechanical loading is the key inductive microenvironment for in situ tendon regeneration for this polymer-based composite scaffold with proper matrix modification, which may serve as a universal scaffold product for tendon regeneration.

Advances and applications of biomimetic biomaterials for endogenous skin regeneration

Endogenous regeneration is becoming an increasingly important strategy for wound healing as it facilitates skin's own regenerative potential for self-healing, thereby avoiding the risks of immune rejection and exogenous infection. However, currently applied biomaterials for inducing endogenous skin regeneration are simplistic in their structure and function, lacking the ability to accurately mimic the intricate tissue structure and regulate the disordered microenvironment. Novel biomimetic biomaterials with precise structure, chemical composition, and biophysical properties offer a promising avenue for achieving perfect endogenous skin regeneration. Here, we outline the recent advances in biomimetic materials induced endogenous skin regeneration from the aspects of structural and functional mimicry, physiological process regulation, and biophysical property design. Furthermore, novel techniques including in situ reprograming, flexible electronic skin, artificial intelligence, single-cell sequencing, and spatial transcriptomics, which have potential to contribute to the development of biomimetic biomaterials are highlighted. Finally, the prospects and challenges of further research and application of biomimetic biomaterials are discussed. This review provides reference to address the clinical problems of rapid and high-quality skin regeneration.

Emerging Strategies in Mesenchymal Stem Cell-Based Cardiovascular Therapeutics

Cardiovascular diseases continue to challenge global health, demanding innovative therapeutic solutions. This review delves into the transformative role of mesenchymal stem cells (MSCs) in advancing cardiovascular therapeutics. Beginning with a historical perspective, we trace the development of stem cell research related to cardiovascular diseases, highlighting foundational therapeutic approaches and the evolution of cell-based treatments. Recognizing the inherent challenges of MSC-based cardiovascular therapeutics, which range from understanding the pro-reparative activity of MSCs to tailoring patient-specific treatments, we emphasize the need to refine the pro-regenerative capacity of these cells. Crucially, our focus then shifts to the strategies of the fourth generation of cell-based therapies: leveraging the secretomic prowess of MSCs, particularly the role of extracellular vesicles; integrating biocompatible scaffolds and artificial sheets to amplify MSCs' potential; adopting three-dimensional ex vivo propagation tailored to specific tissue niches; harnessing the promise of genetic modifications for targeted tissue repair; and institutionalizing good manufacturing practice protocols to ensure therapeutic safety and efficacy. We conclude with reflections on these advancements, envisaging a future landscape redefined by MSCs in cardiovascular regeneration. This review offers both a consolidation of our current understanding and a view toward imminent therapeutic horizons.

Injectable "Homing-Like" Bioactive Short-Fibers for Endometrial Repair and Efficient Live Births

The prevalence of infertility caused by endometrial defects is steadily increasing, posing a significant challenge to women's reproductive health. In this study, injectable "homing-like" bioactive decellularized extracellular matrix short-fibers (DEFs) of porcine skin origin are innovatively designed for endometrial and fertility restoration. The DEFs can effectively bind to endometrial cells through noncovalent dipole interactions and release bioactive growth factors in situ. In vitro, the DEFs effectively attracted endometrial cells through the "homing-like" effect, enabling cell adhesion, spreading, and proliferation on their surface. Furthermore, the DEFs effectively facilitated the proliferation and angiogenesis of human primary endometrial stromal cells (HESCs) and human umbilical vein endothelial cells (HUVECs), and inhibited fibrosis of pretreated HESCs. In vivo, the DEFs significantly accelerated endometrial restoration, angiogenesis, and receptivity. Notably, the deposition of endometrial collagen decreased from 41.19 ± 2.16% to 14.15 ± 1.70% with DEFs treatment. Most importantly, in endometrium-injured rats, the use of DEFs increased the live birth rate from 30% to an impressive 90%, and the number and development of live births close to normal rats. The injectable "homing-like" bioactive DEFs system can achieve efficient live births and intrauterine injection of DEFs provides a new promising clinical strategy for endometrial factor infertility.

Integrated GelMA and interleukin 8-loaded liposome composite scaffold for endogenous BMSCs recruitment in bone repair

Bone repair strategies, based on endogenous stem cell recruitment, can effectively avoid immune rejection and the low utilization of exogenous stem cells. Endogenous stem cells can be recruited to the implantation site by loading chemokines onto bone tissue-engineered scaffolds. However, challenges such as unstable chemokine activity and easy inactivation after implantation remain significant. In the present study, composite fiber scaffolds ((IL8@LIP)-GelMA) consisting of Interleukin 8 (IL8) -loaded liposomes and GelMA were constructed by electrospinning and photocrosslinking, and its ability to recruit bone marrow-derived mesenchymal stem cells (BMSCs) and immunomodulatory effect was investigated. Compared to GelMA loaded directly with IL8, scaffolds of (IL8@LIP)-GelMA demonstrated superior protection of IL8 activity, ensuring a slow and continuous release. Both in vivo and in vitro experiments demonstrated that the (IL8@LIP)-GelMA scaffolds effectively recruited BMSCs to the desired sites. Additionally, the (IL8@LIP)-GelMA scaffolds exhibited the capacity to recruit more macrophages to the implantation site. Importantly, they promoted the polarization of macrophages toward the M2 anti-inflammatory phenotype, facilitating the transition from the inflammatory stage to the tissue repair stage. Therefore, (IL8@LIP)-GelMA scaffolds show great potential for cell-free tissue engineering applications and provide insights into the loading mode of growth factors in scaffolds.

Elastic porous microspheres/extracellular matrix hydrogel injectable composites releasing dual bio-factors enable tissue regeneration

Injectable biomaterials have garnered increasing attention for their potential and beneficial applications in minimally invasive surgical procedures and tissue regeneration. Extracellular matrix (ECM) hydrogels and porous synthetic polymer microspheres can be prepared for injectable administration to achieve in situ tissue regeneration. However, the rapid degradation of ECM hydrogels and the poor injectability and biological inertness of most polymeric microspheres limit their pro-regenerative capabilities. Here, we develop a biomaterial system consisting of elastic porous poly(l-lactide-co-ε-caprolactone) (PLCL) microspheres mixed with ECM hydrogels as injectable composites with interleukin-4 (IL-4) and insulin-like growth factor-1 (IGF-1) dual-release functionality. The developed multifunctional composites have favorable injectability and biocompatibility, and regulate the behavior of macrophages and myogenic cells following injection into muscle tissue. The elicited promotive effects on tissue regeneration are evidenced by enhanced neomusle formation, vascularization, and neuralization at 2-months post-implantation in a male rat model of volumetric muscle loss. Our developed system provides a promising strategy for engineering bioactive injectable composites that demonstrates desirable properties for clinical use and holds translational potential for application as a minimally invasive and pro-regenerative implant material in multiple types of surgical procedures.

Bioactive materials for in vivo sweat gland regeneration

Loss of sweat glands (SwGs) commonly associated with extensive skin defects is a leading cause of hyperthermia and heat stroke. In vivo tissue engineering possesses the potential to take use of the body natural ability to regenerate SwGs, making it more conducive to clinical translation. Despite recent advances in regenerative medicine, reconstructing SwG tissue with the same structure and function as native tissue remains challenging. Elucidating the SwG generation mechanism and developing biomaterials for in vivo tissue engineering is essential for understanding and developing in vivo SwG regenerative strategies. Here, we outline the cell biology associated with functional wound healing and the characteristics of bioactive materials. We critically summarize the recent progress in bioactive material-based cell modulation approaches for in vivo SwG regeneration, including the recruitment of endogenous cells to the skin lesion for SwG regeneration and in vivo cellular reprogramming for SwG regeneration. We discussed the re-establishment of microenvironment via bioactive material-mediated regulators. Besides, we offer promising perspectives for directing in situ SwG regeneration via bioactive material-based cell-free strategy, which is a simple and effective approach to regenerate SwG tissue with both fidelity of structure and function. Finally, we discuss the opportunities and challenges of in vivo SwG regeneration in detail. The molecular mechanisms and cell fate modulation of in vivo SwG regeneration will provide further insights into the regeneration of patient-specific SwGs and the development of potential intervention strategies for gland-derived diseases.

In-situ wound healing by SDF-1-mimic peptide-loaded click crosslinked hyaluronic acid scaffold

Endogenous stem cell-based in-situ tissue regeneration has recently gained considerable attention. In this study, we investigated the potential of a chemokine, SDF-1-mimic peptide (SMP), to promote endogenous stem cell-based in-situ wound healing. Our approach involved the development of a click crosslinked hyaluronic acid scaffold loaded with SMP (Cx-HA + SMP) to release SMP in a wound site. The Cx-HA scaffold maintained its structural integrity throughout the wound healing process and also captured endogenous stem cells. Gradual SMP release from the Cx-HA + SMP scaffold established a concentration gradient at the wound site. In animal wound experiments, Cx-HA + SMP exhibited faster wound contraction compared to Cx-HA + SDF-1. Additionally, Cx-HA + SMP resulted in approximately 1.2-1.6 times higher collagen formation compared to Cx-HA + SDF-1. SMP released from the Cx-HA + SMP scaffold promoted endogenous stem cell migration to the wound site 1.5 times more effectively than Cx-HA + SDF-1. Moreover, compared to Cx-HA + SDF-1, Cx-HA + SMP exhibited higher expression of CXCR4 and CD31, as well as the positive markers CD29 and CD44 for endogenous stem cells. The endogenous stem cells that migrated through Cx-HA + SMP regenerated into wound skin with minimal scar granule formation, similar to the normal tissue. In conclusion, SMP peptide offers greater convenience, while efficiently attracting migrating endogenous stem cells compared to the SDF protein. Our findings suggest that Cx-HA + SMP scaffolds hold promise as a strategy to enhance endogenous stem cell-based in-situ wound healing.

Malate-Based Biodegradable Scaffolds Activate Cellular Energetic Metabolism for Accelerated Wound Healing

The latest advancements in cellular bioenergetics have revealed the potential of transferring chemical energy to biological energy for therapeutic applications. Despite efforts, a three-dimensional (3D) scaffold that can induce long-term bioenergetic effects and facilitate tissue regeneration remains a big challenge. Herein, the cellular energetic metabolism promotion ability of l-malate, an important intermediate of the tricarboxylic acid (TCA) cycle, was proved, and a series of bioenergetic porous scaffolds were fabricated by synthesizing poly(diol l-malate) (PDoM) prepolymers via a facial one-pot polycondensation of l-malic acid and aliphatic diols, followed by scaffold fabrication and thermal-cross-linking. The degradation products of the developed PDoM scaffolds can regulate the metabolic microenvironment by entering mitochondria and participating in the TCA cycle to elevate intracellular adenosine triphosphate (ATP) levels, thus promoting the cellular biosynthesis, including the production of collagen type I (Col1a1), fibronectin 1 (Fn1), and actin alpha 2 (Acta2/α-Sma). The porous PDoM scaffold was demonstrated to support the growth of the cocultured mesenchymal stem cells (MSCs) and promote their secretion of bioactive molecules [such as vascular endothelial growth factor (VEGF), transforming growth factor-β1 (TGF-β1), and basic fibroblast growth factor (bFGF)], and this stem cells-laden scaffold architecture was proved to accelerate wound healing in a critical full-thickness skin defect model on rats.

Mechanical Signal-Tailored Hydrogel Microspheres Recruit and Train Stem Cells for Precise Differentiation

The aberrant mechanical microenvironment in degenerated tissues induces misdirection of cell fate, making it challenging to achieve efficient endogenous regeneration. Herein, a hydrogel microsphere-based synthetic niche with integrated cell recruitment and targeted cell differentiation properties via mechanotransduction is constructed . Through the incorporation of microfluidics and photo-polymerization strategies, fibronectin (Fn) modified methacrylated gelatin (GelMA) microspheres are prepared with the independently tunable elastic modulus (1-10Kpa) and ligand density (2 and 10 µg mL-1 ), allowing a wide range of cytoskeleton modulation to trigger the corresponding mechanobiological signaling. The combination of the soft matrix (2Kpa) and low ligand density (2 µg mL-1 ) can support the nucleus pulposus (NP)-like differentiation of intervertebral disc (IVD) progenitor/stem cells by translocating Yes-associated protein (YAP), without the addition of inducible biochemical factors. Meanwhile, platelet-derived growth factor-BB (PDGF-BB) is loaded onto Fn-GelMA microspheres (PDGF@Fn-GelMA) via the heparin-binding domain of Fn to initiate endogenous cell recruitment. In in vivo experiments, hydrogel microsphere-niche maintained the IVD structure and stimulated matrix synthesis. Overall, this synthetic niche with cell recruiting and mechanical training capabilities offered a promising strategy for endogenous tissue regeneration.

LIPUS-S/B@NPs regulates the release of SDF-1 and BMP-2 to promote stem cell recruitment-osteogenesis for periodontal bone regeneration

Purpose: Poly (lactic-co-glycolic acid)-based nanoparticles (PLGA NPs) have been widely used as the carrier for sustainable drug delivery. However, the drug release from the NPs was usually incomplete and uncontrollable. Herein, a low intensity pulsed ultrasound (LIPUS) assisted SDF-1/BMP-2@nanoparticles (S/B@NPs) system was fabricated to facilitate stem cell recruitment-osteogenesis for periodontal bone regeneration. Methods: In this work, S/B@NPs were prepared with double-emulsion synthesis method. Then the S/B release profile from NPs was evaluated with or without low intensity pulsed ultrasound treatment. Afterwards, the stem cell recruiting and osteoinductive capacities of LIPUS-S/B@NPs were detected with human periodontal ligament cells (hPDLCs) in vitro and in a rat periodontal bone defect model. Results: The results indicated that S/B@NPs were successfully prepared and LIPUS could effectively regulate the release of S/B and increase their final releasing amount. Moreover, LIPUS-S/B@NPs system significantly promoted hPDLCs migrating and osteogenesis in vitro and recruiting rBMSCs to the rat periodontal defect and facilitated bone regeneration in vivo. Conclusion: Our LIPUS assisted S/B@NPs system can effectively facilitate stem cell recruitment and periodontal bone regeneration. Considering its reliable safety and therapeutic effect on bone fracture, LIPUS, as an adjuvant therapy, holds great potential in the regulation of drug delivery systems for bone healing.

Strategies for in situ tissue engineering of vascularized bone regeneration (Review)

Numerous physiological processes occur following bone fracture, including inflammatory cell recruitment, vascularization, and callus formation and remodeling. In particular circumstances, such as critical bone defects or osteonecrosis, the regenerative microenvironment is compromised, rendering endogenous stem/progenitor cells incapable of fully manifesting their reparative potential. Consequently, external interventions, such as grafting or augmentation, are frequently necessary. In situ bone tissue engineering (iBTE) employs cell-free scaffolds that possess microenvironmental cues, which, upon implantation, redirect the behavior of endogenous stem/progenitor cells towards a pro-regenerative inflammatory response and reestablish angiogenesis-osteogenesis coupling. This process ultimately results in vascularized bone regeneration (VBR). In this context, a comprehensive review of the current techniques and modalities in VBR-targeted iBTE technology is provided.

Functional hydrogels for the repair and regeneration of tissue defects

Tissue defects can be accompanied by functional impairments that affect the health and quality of life of patients. Hydrogels are three-dimensional (3D) hydrophilic polymer networks that can be used as bionic functional tissues to fill or repair damaged tissue as a promising therapeutic strategy in the field of tissue engineering and regenerative medicine. This paper summarises and discusses four outstanding advantages of hydrogels and their applications and advances in the repair and regeneration of tissue defects. First, hydrogels have physicochemical properties similar to the extracellular matrix of natural tissues, providing a good microenvironment for cell proliferation, migration and differentiation. Second, hydrogels have excellent shape adaptation and tissue adhesion properties, allowing them to be applied to a wide range of irregularly shaped tissue defects and to adhere well to the defect for sustained and efficient repair function. Third, the hydrogel is an intelligent delivery system capable of releasing therapeutic agents on demand. Hydrogels are capable of delivering therapeutic reagents and releasing therapeutic substances with temporal and spatial precision depending on the site and state of the defect. Fourth, hydrogels are self-healing and can maintain their integrity when damaged. We then describe the application and research progress of functional hydrogels in the repair and regeneration of defects in bone, cartilage, skin, muscle and nerve tissues. Finally, we discuss the challenges faced by hydrogels in the field of tissue regeneration and provide an outlook on their future trends.

Targeting Endogenous Reactive Oxygen Species Removal and Regulating Regenerative Microenvironment at Annulus Fibrosus Defects Promote Tissue Repair

The excessive reactive oxygen species (ROS) level, inflammation, and weak tissue regeneration ability after annulus fibrosus (AF) injury constitute an unfavorable microenvironment for AF repair. AF integrity is crucial for preventing disc herniation after discectomy; however, there is no effective way to repair the AF. Herein, a composite hydrogel integrating properties of antioxidant, anti-inflammation, and recruitment of AF cells is developed through adding mesoporous silica nanoparticles modified by ceria and transforming growth factor β3 (TGF-β3) to the hydrogels. The nanoparticle loaded gelatin methacrylate/hyaluronic acid methacrylate composite hydrogels eliminate ROS and induce anti-inflammatory M2 type macrophage polarization. The released TGF-β3 not only plays a role in recruiting AF cells but is also responsible for promoting extracellular matrix secretion. The composite hydrogels can be solidified in situ in the defect area to effectively repair AF in rats. The strategies targeting endogenous ROS removal and improving the regenerative microenvironment by the nanoparticle-loaded composite hydrogels have potential applications in AF repair and intervertebral disc herniation prevention.

One-Pot Synthesis of Double-Network PEG/Collagen Hydrogel for Enhanced Adipogenic Differentiation and Retrieval of Adipose-Derived Stem Cells

Hydrogels are widely used in stem cell therapy due to their extensive tunability and resemblance to the extracellular matrix (ECM), which has a three-dimensional (3D) structure. These features enable various applications that enhance stem cell maintenance and function. However, fast and simple hydrogel fabrication methods are desirable for stem cells for efficient encapsulation and to reduce adverse effects on the cells. In this study, we present a one-pot double-crosslinked hydrogel consisting of polyethylene glycol (PEG) and collagen, which can be prepared without the multi-step sequential synthesis of each network, by using bio-orthogonal chemistry. To enhance the adipogenic differentiation efficiency of adipose-derived stem cells (ADSCs), we added degradable components within the hydrogel to regulate matrix stiffness through cell-mediated degradation. Bio-orthogonal reactions used for hydrogel gelation allow rapid gel formation for efficient cell encapsulation without toxic by-products. Furthermore, the hybrid network of synthetic (PEG) and natural (collagen) components demonstrated adequate mechanical strength and higher cell adhesiveness. Therefore, ADSCs grown within this hybrid hydrogel proliferated and functioned better than those grown in the single-crosslinked hydrogel. The degradable elements further improved adipogenesis in ADSCs with dynamic changes in modulus during culture and enabled the retrieval of differentiated cells for potential future applications.

Microneedles for in situ tissue regeneration

Tissue injury is a common clinical problem, which may cause great burden on patients' life. It is important to develop functional scaffolds to promote tissue repair and regeneration. Due to their unique composition and structure, microneedles have attracted extensive attention in various tissues regeneration, including skin wound, corneal injury, myocardial infarction, endometrial injury, and spinal cord injury et al. Microneedles with micro-needle structure can effectively penetrate the barriers of necrotic tissue or biofilm, therefore improving the bioavailability of drugs. The use of microneedles to deliver bioactive molecules, mesenchymal stem cells, and growth factors in situ allows for targeted tissue and better spatial distribution. At the same time, microneedles can also provide mechanical support or directional traction for tissue, thus accelerating tissue repair. This review summarized the research progress of microneedles for in situ tissue regeneration over the past decade. At the same time, the shortcomings of existing researches, future research direction and clinical application prospect were also discussed.

Thermosensitive hydrogel for cartilage regeneration via synergistic delivery of SDF-1α like polypeptides and kartogenin

Regeneration of injured articular cartilage is limited by low early-stage recruitment of stem cells and insufficient chondrogenic differentiation. Hydrogels are widely used to repair cartilage because they have excellent mechanical and biological properties. In this study, a dual drug-loaded thermosensitive hydroxypropyl chitin hydrogel (HPCH) system was prepared to release stromal-derived factor-1α-like polypeptides (SDFP) and kartogenin (KGN) for stem-cell recruitment and chondrogenic differentiation. The hydrogel had a network structure that promoted cell growth and nutrient exchange. Moreover, it was temperature sensitive and suitable for filling irregular defects. The system showed good biocompatibility in vitro and promoted stem-cell recruitment and chondrogenic differentiation. Furthermore, it reduced chondrocyte catabolism under inflammatory conditions. Animal experiments demonstrated that the dual-drug hydrogel systems can promote the regeneration of articular cartilage in rats. This study confirmed that an HPCH system loaded with KGN and SDFP could effectively repair articular cartilage defects and represents a viable treatment strategy.

Biodegradable silk fibroin scaffold doped with mineralized collagen induces bone regeneration in rat cranial defects

Compared with most nondegradable or slowly degradable bone repair materials, bioactive biodegradable porous scaffolds with certain mechanical strengths can promote the regeneration of both new bone and vasculature while the cavity created by their degradation can be replaced by the infiltration of new bone tissue. Mineralized collagen (MC) is the basic structural unit of bone tissue, and silk fibroin (SF) is a natural polymer with adjustable degradation rates and superior mechanical properties. In this study, a three-dimensional porous biomimetic composite scaffold with a two-component SF-MC system was constructed based on the advantages of both materials. The spherical mineral agglomerates of the MC were uniformly distributed on the surface and inside the SF skeleton, which ensured good mechanical properties while regulating the degradation rate of the scaffold. Second, the SF-MC scaffold had good osteogenic induction of bone marrow mesenchymal stem cells (BMSCs) and preosteoblasts (MC3T3-E1) and also promoted the proliferation of MC3T3-E1 cells. Finally, in vivo 5 mm cranial defect repair experiments confirmed that the SF-MC scaffold stimulated vascular regeneration and promoted new bone regeneration in vivo by means of in situ regeneration. Overall, we believe that this low-cost biomimetic biodegradable SF-MC scaffold with many advantages has some clinical translation prospects.

Absorption rate governs cell transduction in dry macroporous scaffolds

Developing the next generation of cellular therapies will depend on fast, versatile, and efficient cellular reprogramming. Novel biomaterials will play a central role in this process by providing scaffolding and bioactive signals that shape cell fate and function. Previously, our lab reported that dry macroporous alginate scaffolds mediate retroviral transduction of primary T cells with efficiencies that rival the gold-standard clinical spinoculation procedures, which involve centrifugation on Retronectin-coated plates. This scaffold transduction required the scaffolds to be both macroporous and dry. Transduction by dry, macroporous scaffolds, termed "Drydux transduction," provides a fast and inexpensive method for transducing cells for cellular therapy, including for the production of CAR T cells. In this study, we investigate the mechanism of action by which Drydux transduction works through exploring the impact of pore size, stiffness, viral concentration, and absorption speed on transduction efficiency. We report that Drydux scaffolds with macropores ranging from 50-230 μm and with Young's moduli ranging from 25-620 kPa all effectively transduce primary T cells, suggesting that these parameters are not central to the mechanism of action, but also demonstrating that Drydux scaffolds can be tuned without losing functionality. Increasing viral concentrations led to significantly higher transduction efficiencies, demonstrating that increased cell-virus interaction is necessary for optimal transduction. Finally, we discovered that the rate with which the cell-virus solution is absorbed into the scaffold is closely correlated to viral transduction efficiency, with faster absorption producing significantly higher transduction. A computational model of liquid flow through porous media validates this finding by showing that increased fluid flow substantially increases collisions between virus particles and cells in a porous scaffold. Taken together, we conclude that the rate of liquid flow through the scaffolds, rather than pore size or stiffness, serves as a central regulator for efficient Drydux transduction.

Novel coatings for the continuous repair of human bone defects

Bone defects are the second most common tissue grafts after blood. However, bone grafts face several problems, such as bone scaffolds, which have low bioactivity and are prone to corrosion. Much of the current research on bone scaffolds is focused on the mechanical aspects such as structure and strength. Surface modification of the bone scaffold is carried out in terms of the mechanical structure or structural design of the bone scaffold with reference to a bionic structure. However, with the development of mechanical designs, materials science, and medicine, many studies have reported that promoting bone growth by modifying the structure of the scaffold or coating is not possible. Therefore, the application of a bioactive coating to the surface of the bone scaffold is particularly important to generate a synergistic effect between the structure and active coating. In this article, we present several perspectives to improve the bioactivity of bone scaffolds, including corrosion resistance, loading of bioactive coatings or drugs on bone scaffolds, improved adhesion to the surface of the bone scaffolds, immune response modulation, and drawing on bionic structures during manufacturing.

Stem cell homing in periodontal tissue regeneration

The destruction of periodontal tissue is a crucial problem faced by oral diseases, such as periodontitis and tooth avulsion. However, regenerating periodontal tissue is a huge clinical challenge because of the structural complexity and the poor self-healing capability of periodontal tissue. Tissue engineering has led to advances in periodontal regeneration, however, the source of exogenous seed cells is still a major obstacle. With the improvement of in situ tissue engineering and the exploration of stem cell niches, the homing of endogenous stem cells may bring promising treatment strategies in the future. In recent years, the applications of endogenous cell homing have been widely reported in clinical tissue repair, periodontal regeneration, and cell therapy prospects. Stimulating strategies have also been widely studied, such as the combination of cytokines and chemokines, and the implantation of tissue-engineered scaffolds. In the future, more research needs to be done to improve the efficiency of endogenous cell homing and expand the range of clinical applications.

Biomaterials for Tissue Engineering Applications and Current Updates in the Field: A Comprehensive Review

Tissue engineering has emerged as an interesting field nowadays; it focuses on accelerating the auto-healing mechanism of tissues rather than organ transplantation. It involves implanting an In Vitro cultured initiative tissue or a scaffold loaded with tissue regenerating ingredients at the damaged area. Both techniques are based on the use of biodegradable, biocompatible polymers as scaffolding materials which are either derived from natural (e.g. alginates, celluloses, and zein) or synthetic sources (e.g. PLGA, PCL, and PLA). This review discusses in detail the recent applications of different biomaterials in tissue engineering highlighting the targeted tissues besides the in vitro and in vivo key findings. As well, smart biomaterials (e.g. chitosan) are fascinating candidates in the field as they are capable of elucidating a chemical or physical transformation as response to external stimuli (e.g. temperature, pH, magnetic or electric fields). Recent trends in tissue engineering are summarized in this review highlighting the use of stem cells, 3D printing techniques, and the most recent 4D printing approach which relies on the use of smart biomaterials to produce a dynamic scaffold resembling the natural tissue. Furthermore, the application of advanced tissue engineering techniques provides hope for the researchers to recognize COVID-19/host interaction, also, it presents a promising solution to rejuvenate the destroyed lung tissues.