Efficient regeneration of rat calvarial defect with gelatin-hydroxyapatite composite cryogel

Enhanced gelatin methacryloyl nanohydroxyapatite hydrogel for high-fidelity 3D printing of bone tissue engineering scaffolds

Patients suffering from large bone defects are in urgent need of suitable bone replacements. Besides biocompatibility, such replacements need to mimic the 3D architecture of bone and match chemical, mechanical and biological properties, ideally promoting ossification. As natural bone mainly contains collagen type I and carbonate hydroxyapatite, a 3D-printable biomaterial consisting of methacrylated gelatin (GelMA) and nanohydroxyapatite (nHAp) would be beneficial to mimic the composition and shape of natural bone. So far, such nanocomposite hydrogels (NCH) suffered from unsatisfactory rheological properties making them unsuitable for extrusion-based 3D printing with high structural fidelity. In this study, we introduce a novel GelMA/nHAp NCH composition, incorporating the rheological modifier carbomer to improve rheological properties and addressing the challenge of calcium cations released from nHAp that hinder GelMA gelation. Leveraging its shear-thinning and self-healing properties, the NCH ink retains its shape and forms cohesive structures after deposition, which can be permanently stabilized by subsequent UV crosslinking. Consequently, the NCH enables the printing of 3D structures with high shape fidelity in all dimensions, including thez-direction, allowing the fabrication of highly macroporous constructs. Both the uncured and the UV crosslinked NCH behave like a viscoelastic solid, withG'>G″ at deformations up to 100-200 %. After UV crosslinking, the NCH can, depending on the GelMA concentration, reach storage moduli of approximately 10 to over 100 kPa and a mean Young's Modulus of about 70 kPa. The printed scaffolds permit not only cell survival but also osteogenic differentiation, highlighting their potential for bone tissue engineering.

Cryogels Composites: Recent Improvement in Bone Tissue Engineering

Autogenous bone grafts have long been considered the optimal choice for bone reconstruction due to their excellent biocompatibility and osteogenic properties. However, their limited availability and associated donor site morbidity have led to exploration of alternative bone substitutes. Cryogels, with their interconnected porosity, shape recovery, and enhanced mass transport capabilities, have emerged as a promising polymer-based solution. By incorporating bioactive glasses and nanofillers, cryogel composites offer bioactivity, cost-efficiency, and easy cell integration. This approach not only enhances bone regeneration but also underscores the broader role of nanotechnology in regenerative medicine. This mini-review discusses the advancement of organic-inorganic composites, focusing on biopolymeric cryogels and inorganic elements for reinforcement. We highlight how cryogels can be integrated into minimally invasive procedures, reducing patient distress and complications, and advanced 3D-printing techniques that enable further customization of these materials to mimic bone tissue architecture, offering potential for patient-specific treatments.

Cryogel Scaffolds for Tissue-Engineering: Advances and Challenges for Effective Bone and Cartilage Regeneration

Critical-sized bone defects and articular cartilage injuries resulting from trauma, osteonecrosis, or age-related degeneration can be often non-healed by physiological repairing mechanisms, thus representing a relevant clinical issue due to a high epidemiological incidence rate. Novel tissue-engineering approaches have been proposed as an alternative to common clinical practices. This cutting-edge technology is based on the combination of three fundamental components, generally referred to as the tissue-engineering triad: autologous or allogenic cells, growth-stimulating factors, and a scaffold. Three-dimensional polymer networks are frequently used as scaffolds to allow cell proliferation and tissue regeneration. In particular, cryogels give promising results for this purpose, thanks to their peculiar properties. Cryogels are indeed characterized by an interconnected porous structure and a typical sponge-like behavior, which facilitate cellular infiltration and ingrowth. Their composition and the fabrication procedure can be appropriately tuned to obtain scaffolds that match the requirements of a specific tissue or organ to be regenerated. These features make cryogels interesting and promising scaffolds for the regeneration of different tissues, including those characterized by very complex mechanical and physical properties, such as bones and joints. In this review, state-of-the-art fabrication and employment of cryogels for supporting effective osteogenic or chondrogenic differentiation to allow for the regeneration of functional tissues is reported. Current progress and challenges for the implementation of this technology in clinical practice are also highlighted.

Integrated Piezoelectric/Conductive Composite Cryogel Creates Electroactive Microenvironment for Enhanced Bone Regeneration

Natural bone tissue possesses inherent electrophysiological characteristics, displaying conductivity and piezoelectricity simultaneously; hence, the reconstruction of local electrical microenvironment at defect site provides an effective strategy to enhance osteogenesis. Herein, a composite cryogel-type scaffold (referred to as Gel-PD-CMBT) is developed for bone regeneration, utilizing gelatin (Gel) in combination with a conductive poly(ethylene dioxythiophene)/polystyrene sulfonate matrix and Ca/Mn co-doped barium titanate (CMBT) nanofibers as the piezoelectric filler. The incorporation of these components results in the formation of an integrated piezoelectric/conductive network within the scaffold, facilitating charge migration and yielding a conductivity of 0.59 S cm-1 . This conductive scaffold creates a promising electroactive microenvironment, which is capable of up-regulating biological responses. Furthermore, the interconnected porous structure of the Gel-PD-CMBT scaffold not only provides mechanical stability but also offered ample space for cellular and tissue ingrowth. This Gel-PD-CMBT scaffold demonstrates a greater capacity to promote cellular osteogenic differentiation in vitro and neo-bone formation in vivo. In summary, the Gel-PD-CMBT scaffold, with its integrated piezoelectricity and conductivity, effectively restores the local electroactive microenvironment, offering an ideal platform for the regeneration of electrophysiological bone tissue.

Tissue-Engineered Injectable Gelatin-Methacryloyl Hydrogel-Based Adjunctive Therapy for Intervertebral Disc Degeneration

Gelatin-methacryloyl (GelMA) hydrogels are photosensitive with good biocompatibility and adjustable mechanical properties. The GelMA hydrogel composite system is a prospective therapeutic material based on a tissue engineering platform for treating intervertebral disc (IVD) degeneration (IVDD). The potential application value of the GelMA hydrogel composite system in the treatment of IVDD mainly includes three aspects: first, optimization of the current clinical treatment methods, including conservative treatment and surgical treatment; second, regeneration of IVD cells to reverse or repair IVDD; and finally, IVDD instead of injury plays a biomechanical role. In this paper, we summarized and analyzed the preparation of GelMA hydrogels and their excellent biological characteristics as carriers and comprehensively demonstrated the research status and prospects of GelMA hydrogel composite systems in IVDD treatment. In addition, the challenges facing the application of GelMA hydrogel composite systems and the progress of research on new hydrogels modified by GelMA hydrogels are presented. Hopefully, this study will provide theoretical guidance for the future application of GelMA hydrogel composite systems in IVDD.

Standard in vitro evaluations of engineered bone substitutes are not sufficient to predict in vivo preclinical model outcomes

Understanding the optimal conditions required for bone healing can have a substantial impact to target the problem of non-unions and large bone defects. The combination of bioactive factors, regenerative progenitor cells and biomaterials to form a tissue engineered (TE) complex is a promising solution but translation to the clinic has been slow. We hypothesized the typical material testing algorithm used is insufficient and leads to materials being mischaracterized as promising. In the first part of this study, human bone marrow - derived mesenchymal stromal cells (hBM-MSCs) were embedded in three commonly used biomaterials (hyaluronic acid methacrylate, gelatin methacrylate and fibrin) and combined with relevant bioactive osteogenesis factors (dexamethasone microparticles and polyphosphate nanoparticles) to form a TE construct that underwent in vitro osteogenic differentiation for 28 days. Gene expression of relevant transcription factors and osteogenic markers, and von Kossa staining were performed. In the second and third part of this study, the same combination of TE constructs were implanted subcutaneously (cell containing) in T cell-deficient athymic Crl:NIH-Foxn1^rnu rats for 8 weeks or cell free in an immunocompetent New Zealand white rabbit calvarial model for 6 weeks, respectively. Osteogenic performance was investigated via MicroCT imaging and histology staining. The in vitro study showed enhanced upregulation of relevant genes and significant mineral deposition within the three biomaterials, generally considered as a positive result. Subcutaneous implantation indicates none to minor ectopic bone formation. No enhanced calvarial bone healing was detected in implanted biomaterials compared to the empty defect. The reasons for the poor correlation of in vitro and in vivo outcomes are unclear and needs further investigation. This study highlights the discrepancy between in vitro and in vivo outcomes, demonstrating that in vitro data should be interpreted with extreme caution. In vitro models with higher complexity are necessary to increase value for translational studies. STATEMENT OF SIGNIFICANCE: Preclinical testing of newly developed biomaterials is a crucial element of the development cycle. Despite this, there is still significant discrepancy between in vitro and in vivo test results. Within this study we investigate multiple combinations of materials and osteogenic stimulants and demonstrate a poor correlation between the in vitro and in vivo data. We propose rationale for why this may be the case and suggest a modified testing algorithm.

Prognostic and therapeutic potential of microRNAs for fracture healing processes and non-union fractures: A systematic review

BACKGROUND

Approximately 10% of all bone fractures result in delayed fracture healing or non-union; thus, the identification of biomarkers and prognostic factors is of great clinical interest. MicroRNAs (miRNAs) are known to be involved in the regulation of the bone healing process and may serve as functional markers for fracture healing.

AIMS AND METHODS

This systematic review aimed to identify common miRNAs involved in fracture healing or non-union fractures using a qualitative approach. A systematic literature search was performed with the keywords 'miRNA and fracture healing' and 'miRNA and non-union fracture'. Any original article investigating miRNAs in fracture healing or non-union fractures was screened. Eventually, 82 studies were included in the qualitative analysis for 'miRNA and fracture healing', while 19 were selected for the 'miRNA and fracture non-union' category.

RESULTS AND CONCLUSIONS

Out of 151 miRNAs, miR-21, miR-140 and miR-214 were the most investigated miRNAs in fracture healing in general. miR-31-5p, miR-221 and miR-451-5p were identified to be regulated specifically in non-union fractures. Large heterogeneity was detected between studies investigating the role of miRNAs in fracture healing or non-union in terms of patient population, sample types and models used. Nonetheless, our approach identified some miRNAs with the potential to serve as biomarkers for non-union fractures, including miR-31-5p, miR-221 and miR-451-5p. We provide a discussion of involved pathways and suggest on alignment of future research in the field.

A biomimetic piezoelectric scaffold with sustained Mg2+ release promotes neurogenic and angiogenic differentiation for enhanced bone regeneration

Natural bone is a composite tissue made of organic and inorganic components, showing piezoelectricity. Whitlockite (WH), which is a natural magnesium-containing calcium phosphate, has attracted great attention in bone formation recently due to its unique piezoelectric property after sintering treatment and sustained release of magnesium ion (Mg²⁺). Herein, a composite scaffold (denoted as PWH scaffold) composed of piezoelectric WH (PWH) and poly(ε-caprolactone) (PCL) was 3D printed to meet the physiological demands for the regeneration of neuro-vascularized bone tissue, namely, providing endogenous electric field at the defect site. The sustained release of Mg²⁺ from the PWH scaffold, displaying multiple biological activities, and thus exhibits a strong synergistic effect with the piezoelectricity on inhibiting osteoclast activation, promoting the neurogenic, angiogenic, and osteogenic differentiation of bone marrow mesenchymal stromal cells (BMSCs) in vitro. In a rat calvarial defect model, this PWH scaffold is remarkably conducive to efficient neo-bone formation with rich neurogenic and angiogenic expressions. Overall, this study presents the first example of biomimetic piezoelectric scaffold with sustained Mg²⁺ release for promoting the regeneration of neuro-vascularized bone tissue in vivo, which offers new insights for regenerative medicine.

Recent Advances in Macroporous Hydrogels for Cell Behavior and Tissue Engineering

Hydrogels have been extensively used as scaffolds in tissue engineering for cell adhesion, proliferation, migration, and differentiation because of their high-water content and biocompatibility similarity to the extracellular matrix. However, submicron or nanosized pore networks within hydrogels severely limit cell survival and tissue regeneration. In recent years, the application of macroporous hydrogels in tissue engineering has received considerable attention. The macroporous structure not only facilitates nutrient transportation and metabolite discharge but also provides more space for cell behavior and tissue formation. Several strategies for creating and functionalizing macroporous hydrogels have been reported. This review began with an overview of the advantages and challenges of macroporous hydrogels in the regulation of cellular behavior. In addition, advanced methods for the preparation of macroporous hydrogels to modulate cellular behavior were discussed. Finally, future research in related fields was discussed.

Progress in Gelatin as Biomaterial for Tissue Engineering

Tissue engineering has become a medical alternative in this society with an ever-increasing lifespan. Advances in the areas of technology and biomaterials have facilitated the use of engineered constructs for medical issues. This review discusses on-going concerns and the latest developments in a widely employed biomaterial in the field of tissue engineering: gelatin. Emerging techniques including 3D bioprinting and gelatin functionalization have demonstrated better mimicking of native tissue by reinforcing gelatin-based systems, among others. This breakthrough facilitates, on the one hand, the manufacturing process when it comes to practicality and cost-effectiveness, which plays a key role in the transition towards clinical application. On the other hand, it can be concluded that gelatin could be considered as one of the promising biomaterials in future trends, in which the focus might be on the detection and diagnosis of diseases rather than treatment.

Composite Scaffolds from Gelatin and Bone Meal Powder for Tissue Engineering

Bone tissue engineering offers versatile solutions to broaden clinical options for treating skeletal injuries. However, the variety of robust bone implants and substitutes remains largely uninvestigated. The advancements in hydrogel scaffolds composed of natural polymeric materials and osteoinductive microparticles have shown to be promising solutions in this field. In this study, gelatin methacrylate (GelMA) hydrogels containing bone meal powder (BP) particles were investigated for their osteoinductive capacity. As natural source of the bone mineral, we expect that BP improves the scaffold's ability to induce mineralization. We characterized the physical properties of GelMA hydrogels containing various BP concentrations (0, 0.5, 5, and 50 mg/mL). The in vitro cellular studies revealed enhanced mechanical performance and the potential to promote the differentiation of pre-osteoblast cells. The in vivo studies demonstrated both promising biocompatibility and biodegradation properties. Overall, the biological and physical properties of this biomaterial is tunable based on BP concentration in GelMA scaffolds. The findings of this study offer a new composite scaffold for bone tissue engineering.

An Overview on Collagen and Gelatin-Based Cryogels: Fabrication, Classification, Properties and Biomedical Applications

Decades of research into cryogels have resulted in the development of many types of cryogels for various applications. Collagen and gelatin possess nontoxicity, intrinsic gel-forming ability and physicochemical properties, and excellent biocompatibility and biodegradability, making them very desirable candidates for the fabrication of cryogels. Collagen-based cryogels (CBCs) and gelatin-based cryogels (GBCs) have been successfully applied as three-dimensional substrates for cell culture and have shown promise for biomedical use. A key point in the development of CBCs and GBCs is the quantitative and precise characterization of their properties and their correlation with preparation process and parameters, enabling these cryogels to be tuned to match engineering requirements. Great efforts have been devoted to fabricating these types of cryogels and exploring their potential biomedical application. However, to the best of our knowledge, no comprehensive overviews focused on CBCs and GBCs have been reported currently. In this review, we attempt to provide insight into the recent advances on such kinds of cryogels, including their fabrication methods and structural properties, as well as potential biomedical applications.

Enhancing Tissue Adhesion and Osteoblastic Differentiation of MC3T3-E1 Cells on Poly(aryl ether ketone) by Chemically Anchored Hydroxyapatite Nanocomposite Hydrogel Coating

Tissue adhesion to bone implant and osteoblastic differentiation are the key factors to achieve poly(aryl ether ketone) (PAEK) implant osseointegration. However, physical interaction of implant with tissue and hydroxyapatite coating suffers from slow implant tissue integration and lack of long-term stability. In this study, a novel poly(phthalazinone ether sulfone ketone) containing allyl groups (APPBAESK) is coated onto PPBESK sheet for reacting with the allyl groups of the hydrogel coating to enhance its stability. N-Succinimidyl (NHS)-ester activated group and nano-hydroxyapatite (nano-HA) are introduced into the hydrogel synthesized from gelatin methacrylate (GelMA) and acrylic acid to construct a nanocomposite hydrogel coating on PPBESK which is a promising PAEK implant material. The hydrophilicity of the PPBESK sheet is improved by the hydrogel coating. The chemical components of the nanocomposite hydrogel coating are confirmed by X-ray photoelectron spectroscope, Attenuated total reflection infrared, and X-ray powder diffraction. The tissue shear adhesion strength of the hydrogel coating toward pig skin is enhanced due to the synergism of NHS-ester activated group and nano-HA. The osteogenic differentiation of MC3T3-E1 preosteoblasts is promoted by nano-HA in nanocomposite hydrogel coating. Therefore, the bifunctional nanocomposite hydrogel coating provides a great application prospect in the surface modification of PAEK implants in bone tissue engineering.

Transforming Growth Factor-β3/Recombinant Human-like Collagen/Chitosan Freeze-Dried Sponge Primed With Human Periodontal Ligament Stem Cells Promotes Bone Regeneration in Calvarial Defect Rats

Patients with a skull defect are at risk of developing cerebrospinal fluid leakage and ascending bacterial meningitis at >10% per year. However, treatment with stem cells has brought great hope to large-area cranial defects. Having found that transforming growth factor (TGF)-β3 can promote the osteogenic differentiation of human periodontal ligament stem cells (hPDLSCs), we designed a hybrid TGF-β3/recombinant human-like collagen recombinant human collagen/chitosan (CS) freeze-dried sponge (TRFS) loading hPDLSCs (TRFS-h) to repair skull defects in rats. CFS with 2% CS was selected based on the swelling degree, water absorption, and moisture retention. The CS freeze-dried sponge (CFS) formed a porous three-dimensional structure, as observed by scanning electron microscopy. In addition, cytotoxicity experiments and calcein-AM/PI staining showed that TRFS had a good cellular compatibility and could be degraded completely at 90 days in the implantation site. Furthermore, bone healing was evaluated using micro-computed tomography in rat skull defect models. The bone volume and bone volume fraction were higher in TRFS loaded with hPDLSCs (TRFS-h) group than in the controls (p < 0.01, vs. CFS or TRFS alone). The immunohistochemical results indicated that the expression of Runx2, BMP-2, and collagen-1 (COL Ⅰ) in cells surrounding bone defects in the experimental group was higher than those in the other groups (p < 0.01, vs. CFS or TRFS alone). Taken together, hPDLSCs could proliferate and undergo osteogenic differentiation in TRFS (p < 0.05), and TRFS-h accelerated bone repair in calvarial defect rats. Our research revealed that hPDLSCs could function as seeded cells for skull injury, and their osteogenic differentiation could be accelerated by TGF-β3. This represents an effective therapeutic strategy for restoring traumatic defects of the skull.