Hierarchical Microspheres Constructed from Chitin Nanofibers Penetrated Hydroxyapatite Crystals for Bone Regeneration

Biodegradable chitin nanofiber-alginate dialdehyde hydrogel: An injectable, self-healing scaffold for anti-tumor drug delivery

Injectable hydrogels fabricated from natural polymers have attracted increasing attentions for their potential in biomedical application owing to the biocompatibility and biodegradability. A new class of natural polymer based self-healing hydrogel is constructed through dynamic covalent bonds. The injectable self-healing hydrogels are fabricated by introducing alginate aldehyde to form Schiff base bonds with the chitin nanofibers. These hydrogels demonstrate excellent self-healing properties, injectability, and pH-responsive sol-gel transition behaviors. As a result, they can serve as carriers to allow an effective encapsulation of doxorubicin (DOX) for drug delivery. Furthermore, these hydrogels exhibit excellent biocompatibility and degradability in vitro and in vivo. The sustained release of DOX from the hydrogels effectively suppresses tumor growth in animal models without causing significant systemic toxicity, suggesting their potential application in anti-tumor therapies.

Chitin microspheres: From fabrication to applications

Chitin microspheres (CMs) have attracted increasing attention due to their biocompatibility, uniform size and shape, large surface area, and porous structure. Considerable research efforts have been focused on developing CMs and promoting their applications in various areas. In this context, this review aims to describe the most recent progress in the fabrication and application of CMs. Different routes that can be used to prepare CMs, such as the drip method and the emulsion method, are emphatically introduced. Moreover, the applications of CMs as drug delivery systems, wound dressings, three-dimensional (3D) scaffolds, water purification, and functional supporting materials in the fields of biomedicine, tissue engineering, environmental protection, and energy storage are also highlighted. We hope this review can provide a comprehensive and useful database for further innovation of CMs.

In Situ Construction of Morphologically Different Hydroxyapatite-Mineralized Structures on a Three-Dimensional Bionic Chitin Scaffold

Slow healing at the tendon-bone interface is a prominent factor in the failure of tendon repair surgeries. The development of functional biomaterials with 3D gradient structures is urgently needed to improve tendon-bone integration. The crystalline form of hydroxyapatite (HAP) has a crucial impact on cell behavior, which directly influences protein adsorption, such as bone morphogenetic protein 2, the adhesion, proliferation, and osteogenic differentiation with cells. This work aimed to generate gradient mineral structures in situ by stabilizing calcium and phosphate ions using a polymer-induced liquid precursor process. To regulate the crystalline growth of HAP at the interface of β-chitin, this work made use of the surface properties of the organic matrix found in cuttlefish bone. These techniques allowed us to prepare an organic-inorganic composite gradient scaffold comprising plate-like HAP mineralized in situ on the surface of the scaffold and fibrous HAP in the scaffold's interior. Organic-inorganic composite gradient materials are anticipated for use in tendon-bone healing produced via the in situ construction of gradient-distributed HAP mineralization layers having varying crystalline morphologies on chitin scaffolds that possess a three-dimensional bionic structure.

Hemostasis-osteogenesis integrated Janus carboxymethyl chitin/hydroxyapatite porous membrane for bone defect repair

Barrier membranes with osteogenesis are desirable for promoting bone repair. Janus membrane, which has a bilayered structure with different properties on each side, could meet the osteogenesis/barrier dual functions of guided bone regeneration. In this work, new biodegradable Janus carboxymethyl chitin membrane with asymmetric pore structure was prepared based on thermosensitive carboxymethyl chitin without using any crosslinkers. Nano-hydroxyapatites were cast on single-sided membrane. The obtained carboxymethyl chitin/nano-hydroxyapatite Janus membrane showed dual biofunctions: the dense layer of the Janus membrane could act as a barrier to prevent connective tissue cells from invading the bone defects, while the porous layer (with pore size 100-200 μm) containing nano-hydroxyapatite could guide bone regeneration. After implanted on the rat critical-sized calvarial defect 8 weeks, carboxymethyl chitin/nano-hydroxyapatite membrane showed the most newly formed bone tissue with the highest bone volume/total volume ratio (10.03 ± 1.81 %, analyzed by micro CT), which was significantly better than the commercial collagen membrane GTR® (5.05 ± 0.76 %). Meanwhile, this Janus membrane possessed good hemostatic ability. These results suggest a facile strategy to construct hemostasis-osteogenesis integrated Janus carboxymethyl chitin/hydroxyapatite membrane for guided bone regeneration.

Deep eutectic solvent-assisted fabrication of bioinspired 3D carbon-calcium phosphate scaffolds for bone tissue engineering

Tissue engineering is a burgeoning field focused on repairing damaged tissues through the combination of bodily cells with highly porous scaffold biomaterials, which serve as templates for tissue regeneration, thus facilitating the growth of new tissue. Carbon materials, constituting an emerging class of superior materials, are currently experiencing remarkable scientific and technological advancements. Consequently, the development of novel 3D carbon-based composite materials has become significant for biomedicine. There is an urgent need for the development of hybrids that will combine the unique bioactivity of ceramics with the performance of carbonaceous materials. Considering these requirements, herein, we propose a straightforward method of producing a 3D carbon-based scaffold that resembles the structural features of spongin, even on the nanometric level of their hierarchical organization. The modification of spongin with calcium phosphate was achieved in a deep eutectic solvent (choline chloride : urea, 1 : 2). The holistic characterization of the scaffolds confirms their remarkable structural features (i.e., porosity, connectivity), along with the biocompatibility of α-tricalcium phosphate (α-TCP), rendering them a promising candidate for stem cell-based tissue-engineering. Culturing human bone marrow mesenchymal stem cells (hMSC) on the surface of the biomimetic scaffold further verifies its growth-facilitating properties, promoting the differentiation of these cells in the osteogenesis direction. ALP activity was significantly higher in osteogenic medium compared to proliferation, indicating the differentiation of hMSC towards osteoblasts. However, no significant difference between C and C-αTCP in the same medium type was observed.

Medical applications of biopolymer nanofibers

A wide array of biomedical applications, extending from the fabrication of implant materials to targeted drug delivery, can be attributed to polymers. The utilization of chemical monomers to form polymers, such as polypropylene, polystyrene, and polyethylene, can provide high mechanical stability to them and they can be utilized for diverse electronic or thermal applications. However, certain chemical-based synthetic polymers are toxic to humans, animals, plants, and microbial cells. Thus, biopolymers have been introduced as an alternative to make them utilizable for biomedical applications. Even though biopolymers possess beneficial biomedical applications, they are not stable in biological fluids and exhibit toxicity in certain cases. Recent advances in nanotechnology have expanded its applicational significance in various domains, especially in the evolution of biopolymers to transform them into nanoparticles for numerous biomedical applications. In particular, biopolymers are fabricated as nanofibers to enhance their biological properties and to be utilized for exclusive biomedical applications. The aim of this review is to present an overview of various biopolymer nanofibers and their distinct synthesis approaches. In addition, the medical applications of biopolymer nanofibers, including antimicrobial agents, drug delivery systems, biosensor production, tissue engineering, and implant fabrication, are also discussed.

Facile and Scalable Synthesis and Self-Assembly of Chitosan Tartaric Sodium

Chitosan-based nanostructures have been widely applied in biomineralization and biosensors owing to its polycationic properties. The creation of chitosan nanostructures with controllable morphology is highly desirable, but has met with limited success yet. Here, we report that nanostructured chitosan tartaric sodium (CS-TA-Na) is simply synthesized in large amounts from chitosan tartaric ester (CS-TA) hydrolyzed by NaOH solution, while the CS-TA is obtained by dehydration-caused crystallization. The structures and self-assembly properties of CS-TA-Na are carefully characterized by Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (1H-NMR), X-ray diffraction (XRD), differential scanning calorimeter (DSC), transmission electron microscopy (TEM), a scanning electron microscope (SEM) and a polarizing optical microscope (POM). As a result, the acquired nanostructured CS-TA-Na, which is dispersed in an aqueous solution 20-50 nm in length and 10-15 nm in width, shows both the features of carboxyl and amino functional groups. Moreover, morphology regulation of the CS-TA-Na nanostructures can be easily achieved by adjusting the solvent evaporation temperature. When the evaporation temperature is increased from 4 °C to 60 °C, CS-TA-Na nanorods and nanosheets are obtained on the substrates, respectively. As far as we know, this is the first report on using a simple solvent evaporation method to prepare CS-TA-Na nanocrystals with controllable morphologies.

Solvent Mediating the in Situ Self-Assembly of Polysaccharides for 3D Printing Biomimetic Tissue Scaffolds

Intensively studied 3D printing technology is frequently hindered by the effective printable ink preparation method. Herein, we propose an elegant and gentle solvent consumption strategy to slowly disrupt the thermodynamic stability of the biopolymer (polysaccharide: cellulose, chitin, and chitosan) solution to slightly induce the molecule chains to in situ self-assemble into nanostructures for regulating the rheological properties, eventually achieving the acceptable printability. The polysaccharides are dissolved in the alkali/urea solvent. The weak Lewis acid fumed silica (as solvent mediator) is used to (i) slowly and partially consume the alkali/urea solvent to induce the polysaccharide chains to self-assemble into nanofibers to form a percolating network in a limited scale without leading to gelation and (ii) act as the support to increase the solution modulus, for achieving superior printability and scaffold design flexibility. As a demonstration, the resulting polysaccharide scaffolds with biomimetic nanofibrous structures exhibit superior performances in both the cell-free and cell-loaded bone tissue engineering strategies, showing the potential in tissue engineering. Moreover, the fumed silica could be completely removed by alkali treatment without defecting the nanofibrous structure, showing the potential in various applications. We anticipate our solvent-mediated 3D printing ink preparation concept could be used to fabricate other polymeric facile inks and for widespread applications in diverse fields.

Polyphenol-mediated chitin self-assembly for constructing a fully naturally resourced hydrogel with high strength and toughness

Natural polymeric hydrogels are expected to serve as potential structural biomaterials, but, most of them are usually soft and fragile. Herein, a polyphenol-mediated self-assembly (PMS) strategy was developed to significantly enhance the chitin hydrogel strength and toughness at the same time, which is distinctive from the rigid-soft double-network energy-dissipation approaches. A polyphenol (tannic acid, TA as a model compound) was introduced to compete with the chitin chains self-assembly for simultaneously forming the weak chitin-TA and strong chitin-chitin networks. High-density noncovalent crosslinking involving hydrogen bonding and ionic and hydrophobic interactions endowed the PMS hydrogels with a high modulus and strength. The relatively weaker chitin-TA crosslinking acted as the sacrificial bonds to dissipate the energy, leading to the high toughness. The mechanical properties of the PMS chitin hydrogels depended on the TA concentration and ethanol aqueous coagulation, which mainly contributed to the hydrophobic and hydrophilic interactions formation, respectively. The fully naturally robust chitin-TA hydrogels exhibited considerable antibacterial properties, stomach acid solubility, and excellent biocompatibility and degradability, enabling their potential in food, biomedical, and sustainable applications.

Effects of nanofibers on mesenchymal stem cells: environmental factors affecting cell adhesion and osteogenic differentiation and their mechanisms

Nanofibers can mimic natural tissue structure by creating a more suitable environment for cells to grow, prompting a wide application of nanofiber materials. In this review, we include relevant studies and characterize the effect of nanofibers on mesenchymal stem cells, as well as factors that affect cell adhesion and osteogenic differentiation. We hypothesize that the process of bone regeneration in vitro is similar to bone formation and healing in vivo, and the closer nanofibers or nanofibrous scaffolds are to natural bone tissue, the better the bone regeneration process will be. In general, cells cultured on nanofibers have a similar gene expression pattern and osteogenic behavior as cells induced by osteogenic supplements in vitro. Genes involved in cell adhesion (focal adhesion kinase (FAK)), cytoskeletal organization, and osteogenic pathways (transforming growth factor-β (TGF-β)/bone morphogenic protein (BMP), mitogen-activated protein kinase (MAPK), and Wnt) are upregulated successively. Cell adhesion and osteogenesis may be influenced by several factors. Nanofibers possess certain physical properties including favorable hydrophilicity, porosity, and swelling properties that promote cell adhesion and growth. Moreover, nanofiber stiffness plays a vital role in cell fate, as cell recruitment for osteogenesis tends to be better on stiffer scaffolds, with associated signaling pathways of integrin and Yes-associated protein (YAP)/transcriptional co-activator with PDZ-binding motif (TAZ). Also, hierarchically aligned nanofibers, as well as their combination with functional additives (growth factors, HA particles, etc.), contribute to osteogenesis and bone regeneration. In summary, previous studies have indicated that upon sensing the stiffness of the nanofibrous environment as well as its other characteristics, stem cells change their shape and tension accordingly, regulating downstream pathways followed by adhesion to nanofibers to contribute to osteogenesis. However, additional experiments are needed to identify major signaling pathways in the bone regeneration process, and also to fully investigate its supportive role in fabricating or designing the optimum tissue-mimicking nanofibrous scaffolds.

Cyclic Adenosine Monophosphate-Enhanced Calvarial Regeneration by Bone Marrow-Derived Mesenchymal Stem Cells on a Hydroxyapatite/Gelatin Scaffold

Cyclic adenosine monophosphate (cAMP) plays a significant role in inducing new bone formation by mediating various signal pathways. However, cAMP, combined with biomaterials, is rarely investigated to reconstruct calvarial defects. In this study, cAMP was loaded into a hydroxyapatite (HA)/gelatin (Gel) construct and implanted into critical skull defects in rats to evaluate the potential for enhancing skull regeneration. The physiochemical characteristics, the biocompatibility of Gel and HA/Gel scaffolds, and the regenerated bone tissue were assessed. The resulting HA/Gel scaffolds possessed a 3D interconnected porous structure with extensively distributed HA crystals and favorable physiochemical properties. Rat bone marrow-derived mesenchymal stem cells (rBMSCs) within the HA/Gel scaffold showed greater biocompatibility. Compared with the Gel and HA/Gel groups, the cAMP-HA/Gel group revealed the highest bone density, more mature mineralized tissue, and more favorable integration between the new bone and inherent bone as analyzed by cone beam computed tomography and hematoxylin & eosin and Masson staining, respectively. Collectively, our study verified HA/Gel scaffolds as a prospective biomimetic treatment with biocompatibility and the therapeutic potential of cAMP in promoting new bone growth of a skull, which indicates its promise as a growth factor for bone tissue engineering.

Fabrication and characterization of biodegradable KH560 crosslinked chitin hydrogels with high toughness and good biocompatibility

Chitin hydrogels have multiple advantages of nontoxicity, biocompatibility, biodegradability, and three-dimensional hydrophilic polymer network structure similar to the macromolecular biological tissue. However, the mechanical strength of chitin hydrogels is relatively weak. Construction of chitin hydrogels with high mechanical strength and good biocompatibility is essential for the successful applications in biomedical field. Herein, we developed double crosslinked chitin hydrogels by dissolving chitin in KOH/urea aqueous solution with freezing-thawing process, then using KH560 as cross-linking agent and coagulating in ethanol solution at low temperature. The obtained chitin/ KH560 (CK) hydrogels displayed good transparency and toughness with compressed nanofibrous network and porous structure woven with chitin nanofibers. Moreover, the optimal CK hydrogels exhibited excellent mechanical properties (σ_b = 1.92 ± 0.21 Mpa; ε_b = 71 ± 5 %), high swelling ratio, excellent blood compatibility, biocompatibility and biodegradability, which fulfill the requirements of biomedical materials and showing potential applications in biomedicine.

Insight into Morphology Change of Chitin Microspheres using Tertiary Butyl Alcohol/H2 O Binary System Freeze-Drying Method

The morphology of materials usually plays a significant role in their applications; the mechanical properties of the materials and characteristics such as specific surface area, surface energy, adsorbability, and wettability are dependent on the morphology. This study is focused on studying the effects of different tertiary butyl alcohol (TBA) aqueous solutions on the freeze-dried morphologies of chitin microspheres (CMs). By constructing a TBA/H₂ O phase diagram, the underlying mechanisms of morphology change are explored. It is found that by freeze drying the CMs with 20 and 100 wt% TBA, a fine nanofiber weaved pore structure can be obtained. Away from these two ratios, the nanofibers are oppressed by the large crystals formed during the precool process or bind together due to the existence of water in the secondary drying stage, poor morphology and pore characteristics appearing. Moreover, the 20 wt% TBA freeze-drying route is conducive to split the CMs and other polysaccharide (PS) microspheres. The split method is also helpful for exploring the internal structure of the microspheres. Therefore, this study makes it possible to simplify the morphology control of CMs, which helps in the characterization of porous PS-based microspheres.

Lanthanides-Substituted Hydroxyapatite/Aloe vera Composite Coated Titanium Plate for Bone Tissue Regeneration

PURPOSE

To develop the surface-treated metal implant with highly encouraged positive properties, including high anti-corrosiveness, bio-activeness and bio-compatibleness for orthopedic applications.

METHODS

In this work, the surface of commercially pure titanium (Ti) metal was treated with bio-compatible polydopamine (PD) by merely immersing the Ti plate in PD solution. The composite of trivalent lanthanide minerals (La3+, Ce3+ and Gd3+)-substituted hydroxyapatite (MHAP) with Aloe vera (AV) gel was prepared and coated on the PD-Ti plate by electrophoretic deposition (EPD) method. The choice of trivalent lanthanide ions is based on their bio-compatible nature and bone-seeking properties. The formation of the PD layer, composites, and composite coatings on Ti plate and PD-Ti surface was confirmed by FT-IR, XRD, SEM and HR-TEM observations. In-vitro assessments such as osteoblasts like MG-63 cell viability, alkaline phosphatase activity and mineralization ability of the MHAP/AV composite were tested, and the composite-coated plate was implanted into a rat bone defect model for in-vivo bone regeneration studies.

RESULTS

The coating ability of the MHAP/AV composite was highly preferred to PD-treated Ti plate than an untreated Ti plate due to the metal absorption ability of PD. This was confirmed by SEM analysis. The in-vitro and in-vivo studies show the better osteogenic ability of MHAP/AV composite at 14th day and 4th week of an experimental period, respectively.

CONCLUSION

The osteoblast ability of the fabricated device without producing any adverse effect in the rat model recommends that the fabricated device would serve as a better platform on the hard tissue regeneration for load-bearing applications of orthopedics.

Chitin and its derivatives: Structural properties and biomedical applications

Chitin, a polysaccharide that occurs abundantly in nature after cellulose, has attracted the interest of the scientific community due to its plenty of availability and low cost. Mostly, it is derived from the exoskeleton of insects and marine crustaceans. Often, it is insoluble in common solvents that limit its applications but its deacetylated product, named chitosan is found to be soluble in protonated aqueous medium and used widely in various biomedical fields. Indeed, the existence of the primary amino group on the backbone of chitosan provides it an important feature to modify it chemically into other derivatives easily. In the present review, we present the structural properties of chitin, and its derivatives and highlighted their biomedical implications including, tissue engineering, drug delivery, diagnosis, molecular imaging, antimicrobial activity, and wound healing. We further discussed the limitations and prospects of this versatile natural polysaccharide.

Applications of chitin and chitosan nanofibers in bone regenerative engineering

Promoting bone regeneration and repairing defects are urgent and critical challenges in orthopedic clinical practice. Research on bone substitute biomaterials is essential for improving the treatment strategies for bone regeneration. Chitin and its derivative, chitosan, are among the most abundant natural biomaterials and widely found in the shells of crustaceans. Chitin and chitosan are non-toxic, antibacterial, biocompatible, degradable, and have attracted significant attention in bone substitute biomaterials. Chitin/chitosan nanofibers and nanostructured scaffolds have large surface area to volume ratios and high porosities. These scaffolds can be fabricated by electrospinning, thermally induced phase separation and self-assembly, and are widely used in biomedical applications such as biological scaffolds, drug delivery, bacterial inhibition, and wound dressing. Recently, some chitin/chitosan-based nanofibrous scaffolds have been found structurally similar to bone's extracellular matrix and can assist in bone regeneration. This review outlines the biomedical applications and biological properties of chitin/chitosan-based nanofibrous scaffolds in bone tissue engineering.

Tethering peptides onto biomimetic and injectable nanofiber microspheres to direct cellular response

Biomimetic and injectable nanofiber microspheres (NMs) could be ideal candidate for minimally invasive tissue repair. Herein, we report a facile approach to fabricate peptide-tethered NMs by combining electrospinning, electrospraying, and surface conjugation techniques. The composition and size of NMs can be tuned by varying the processing parameters during the fabrication. Further, bone morphogenic protein-2 (BMP-2) and vascular endothelial growth factor (VEGF) mimicking peptides have been successfully tethered onto poly(ε-caprolactone) (PCL):gelatin:(gelatin-methacryloyl) (GelMA)(1:0.5:0.5) NMs through photocrosslinking of the methacrylic group in GelMA and octenyl alanine (OCTAL) in the modified peptides. The BMP-2-OCTAL peptide-tethered NMs significantly promote osteogenic differentiation of bone marrow-derived stem cells (BMSCs). Moreover, human umbilical vein endothelial cells (HUVECs) seeded on VEGF mimicking peptide QK-OCTAL-tethered NMs significantly up-regulated vascular-specific proteins, leading to microvascularization. The strategy developed in this work holds great potential in developing a biomimetic and injectable carrier to efficiently direct cellular response (Osteogenesis and Angiogenesis) for tissue repair.

Eggshell Based Nano-Engineered Hydroxyapatite and Poly(lactic) Acid Electrospun Fibers as Potential Tissue Scaffold

Nanocomposite electrospun fibers were fabricated from poly(lactic) acid (PLA) and needle-like hydroxyapatite nanoparticles made from eggshells. The X-ray diffraction spectrum and the scanning electron micrograph showed that the hydroxyapatite particles are highly crystalline and are needle-liked in shape with diameters between 10 and 20 nm and lengths ranging from 100 to 200 nm. The microstructural, thermal, and mechanical properties of the electrospun fibers were characterized using scanning electron microscope (SEM), thermogravimetric analysis (TGA), dynamic scanning calorimetry (DSC), and tensile testing techniques. The SEM study showed that both pristine and PLA/EnHA fibers surfaces exhibited numerous pores and rough edges suitable for cell attachment. The presence of the rod-liked EnHA particles was found to increase thermal and mechanical properties of PLA fibers relative to pristine PLA fibers. The confocal optical images showed that osteoblast cells were found to attach on dense pristine PLA and PLA/HA-10 wt% fibers after 48 hours of incubation. The stained confocal optical images indicated the secretion of cytoplasmic extension linking adjoining nuclei after 96 hours of incubation. These findings showed that eggshell based nanohydroxyapatite and poly(lactic acid) fibers could be potential scaffold for tissue regeneration.

Microparticles in Contact with Cells: From Carriers to Multifunctional Tissue Modulators

For several decades microparticles have been exclusively and extensively explored as spherical drug delivery vehicles and large-scale cell expansion carriers. More recently, microparticulate structures gained interest in broader bioengineering fields, integrating myriad strategies that include bottom-up tissue engineering, 3D bioprinting, and the development of tissue/disease models. The concept of bulk spherical micrometric particles as adequate supports for cell cultivation has been challenged, and systems with finely tuned geometric designs and (bio)chemical/physical features are current key players in impacting technologies. Herein, we critically review the state of the art and future trends of biomaterial microparticles in contact with cells and tissues, excluding internalization studies, and with emphasis on innovative particle design and applications.

Injectable PLGA microspheres with tunable magnesium ion release for promoting bone regeneration

Magnesium ions (Mg²⁺) are bioactive and proven to promote bone tissue regeneration, in which the enhancement efficiency is closely related to Mg²⁺ concentrations. Currently, there are no well-established bone tissue engineering scaffolds that can precisely control Mg²⁺ release, although this capability could have a marked impact in bone regeneration. Leveraging the power of biodegradable microspheres to control the release of bioactive factors, we developed lactone-based biodegradable microspheres that served as both injectable scaffolds and Mg²⁺ release system for bone regeneration. The biodegradable microsphere (PMg) was prepared from poly(lactide-co-glycolide) (PLGA) microspheres co-embedded with MgO and MgCO₃ at a fixed total loading amount (20 wt%) with different weight ratios (1:0; 3:1; 1:1; 1:3; 0:1). The PMg microspheres demonstrated controlled release of Mg²⁺ by tuning the MgO/MgCO₃ ratios. Specifically, faster release with higher initial concentrations of Mg²⁺ were detected at higher MgO fractions, while long-term sustained release with lower concentrations of Mg²⁺ was obtained at higher MgCO₃ fractions. All prepared PMg microspheres were non-cytotoxic. Furthermore, they promoted attachment, proliferation, osteogenic differentiation, especially, cell migration of bone marrow mesenchymal stromal cells (BMSCs). Among these microspheres, PMg-III microspheres (MgO/MgCO₃ in 1:1) exhibited the strongest promotion of mineral depositions and osteogenic differentiation of BMSCs. PMg-III microspheres were injected into the critical-sized calvarial defect of a rat model, resulting in significant bone regeneration when compared to the control group filled with PLGA microspheres. In the PMg-III group, the new bone volume fraction (BV/TV) and bone mineral density (BMD) reached 32.9 ± 5.6% and 325.7 ± 20.2 mg/cm³, respectively, which were much higher than the values 8.1 ± 2.5% (BV/TV) and 124 ± 35.8 mg/cm³ (BMD) in the PLGA group. These findings indicated that bioresorbable microspheres possessing controlled Mg²⁺ release features were efficient in treating bone defects and promising for future in vivo applications. STATEMENT OF SIGNIFICANCE: Magnesium plays pivotal roles in regulating osteogenesis, which exhibits concentration-dependent behaviors. However, no generally accepted controlled-release system is reported to correlate Mg²⁺ concentration with efficient bone regeneration. Biodegradable microspheres with injectability are excellent cell carriers for tissue engineering, moreover, good delivery systems for bioactive factors. By co-embedding magnesium compounds (MgO, MgCO₃) with different dissolution rates in various ratios, tunable release of Mg²⁺ from the microspheres was readily achieved. Accordingly, significant promotion in bone defect regeneration is achieved with microspheres displaying proper sustained release of Mg²⁺. The developed strategy may serve as valuable guidelines for bone tissue engineering scaffold design, which allows precise control on the release of bioactive metal ions like Mg²⁺ toward potential clinical translation.

Hierarchical microspheres with macropores fabricated from chitin as 3D cell culture

Hierarchical chitin nanofiber microspheres with open macropores was prepared to be used as scaffold for 3D cell culture.

Biopolymer nanofibrils: structure, modeling, preparation, and applications

Biopolymer nanofibrils exhibit exceptional mechanical properties with a unique combination of strength and toughness, while also presenting biological functions that interact with the surrounding environment. These features of biopolymer nanofibrils profit from their hierarchical structures that spun angstrom to hundreds of nanometer scales. To maintain these unique structural features and to directly utilize these natural supramolecular assemblies, a variety of new methods have been developed to produce biopolymer nanofibrils. In particular, cellulose nanofibrils (CNFs), chitin nanofibrils (ChNFs), silk nanofibrils (SNFs) and collagen nanofibrils (CoNFs), as the four most abundant biopolymer nanofibrils on earth, have been the focus of research in recent years due to their renewable features, wide availability, low-cost, biocompatibility, and biodegradability. A series of top-down and bottom-up strategies have been accessed to exfoliate and regenerate these nanofibrils for versatile advanced applications. In this review, we first summarize the structures of biopolymer nanofibrils in nature and outline their related computational models with the aim of disclosing fundamental structure-property relationships in biological materials. Then, we discuss the underlying methods used for the preparation of CNFs, ChNFs, SNF and CoNFs, and discuss emerging applications for these biopolymer nanofibrils.

Electrospraying Electrospun Nanofiber Segments into Injectable Microspheres for Potential Cell Delivery

Nanofiber microspheres have attracted a lot of attention for biomedical applications because of their injectable and biomimetic properties. Herein, we report for the first time a new method for fabrication of nanofiber microspheres by combining electrospinning and electrospraying and explore their potential applications for cell therapy. Electrospraying of aqueous dispersions of electrospun nanofiber segments with desired length obtained by either cryocutting or homogenization into liquid nitrogen followed by freeze-drying and thermal treatment can form nanofiber microspheres. The microsphere size can be controlled by varying the applied voltage during the electrospray process. A variety of morphologies were achieved including solid, nanofiber, porous and nanofiber microspheres, and hollow nanofiber microspheres. Furthermore, a broad range of polymer and inorganic bioactive glass nanofiber-based nanofiber microspheres could be fabricated by electrospraying of their short nanofiber dispersions, indicating a comprehensive applicability of this method. A higher cell carrier efficiency of nanofiber microspheres as compared to solid microspheres was demonstrated with rat bone marrow-derived mesenchymal stem cells, along with the formation of microtissue-like structures in situ, when injected into microchannel devices. Also, mouse embryonic stem cells underwent neural differentiation on the nanofiber microspheres, indicated by positive staining of β-III-tubulin and neurite outgrowth. Taken together, we developed a new method for generating nanofiber microspheres that are injectable and have improved viability and maintenance of stem cells for potential application in cell therapy.