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.

Research Progress on Injectable Microspheres as New Strategies for the Treatment of Osteoarthritis Through Promotion of Cartilage Repair

Osteoarthritis (OA) is a degenerative disease caused by a variety of factors with joint pain as the main symptom, including fibrosis, chapping, ulcers, and loss of cartilage. Traditional treatment can only delay the progression of OA, and classical delivery system have many side effects. In recent years, microspheres have shown great application prospects in the field of OA treatment. Microspheres can support cells, reproduce the natural tissue microenvironment in vitro and in vivo, and are an efficient delivery system for the release of drugs or biological agents, which can promote cell proliferation, migration, and differentiation. Thus, they have been widely used in cartilage repair and regeneration. In this review, preparation processes, basic materials, and functional characteristics of various microspheres commonly used in OA treatment are systematically reviewed. Then it is introduced surface modification strategies that can improve the biological properties of microspheres and discussed a series of applications of microsphere functionalized scaffolds in OA treatment. Finally, based on bibliometrics research, the research development, future potential, and possible research hotspots of microspheres in the field of OA therapy is systematically and dynamically evaluated. The comprehensive and systematic review will bring new understanding to the field of microsphere treatment of OA.

Structure and properties of sulfopropyl chitins prepared in NaOH/urea aqueous solutions

A series of sulfopropyl chitins (SCs) with the degree of substitution (DS) ranging from 0.11 to 0.40 and high degree of acetylation (DA ≥ 0.82) were homogeneously synthesized by reacting chitin with sodium 3-chloro-2-hydroxypropanesulfonate (SCHPS) in NaOH/urea aqueous solutions under mild conditions. The structure and properties of SCs were characterized with 1H NMR, CP/MAS 13C NMR, FT-IR, XPS, XRD, elemental analysis, GPC, AFM, ζ-potential and rheological measurements. The mild reaction conditions resulted in less N-deacetylation and uniform structures with substitution occurring predominantly at the hydroxyl groups at C6 of the chitin backbone. The DS value for SC soluble in dilute alkali solution is as low as 0.16. SC exhibited good solubility in distilled water when its DS value reached 0.28. Water-soluble SCs self-assembled in water into micelles by the attractive hydrophobic and hydrogen-bonding interactions between polymer chains. The water-insoluble SC-2 with lower DS could thermally form smart hydrogels at body temperature (37 °C) in physiological condition. Moreover, the SCs exhibited good biocompatibility, making them suitable for biomedical applications.

Synthesis and evaluation of water-soluble imidazolium salt chitin with broad-spectrum antimicrobial activity and excellent biocompatibility for infected wound healing

Infections caused by bacteria have long constituted a major threat to human health and the economy. Therefore, there is an urgent need to design broad-spectrum antibacterial materials possessing good biocompatibility to treat such infections. Herein, inspired by the good biocompatibility of chitin and antibacterial properties of imidazolium salts, a polysaccharide-based material, imidazolium salt chitin (IMSC), was homogeneously prepared using a facile method with epichlorohydrin as a chemical crosslinker to combine chitin with imidazole to enhance Staphylococcus aureus (S. aureus)-infected wound healing. The characteristics, antimicrobial properties, and biosafety of IMSC were evaluated. The results demonstrated successful grafting of imidazole onto chitin. Furthermore, IMSC exhibited good water solubility, broad-spectrum antimicrobial activity, hemocompatibility, and biocompatibility. Moreover, IMSC enabled complete healing of S. aureus-infected wound in Sprague-Dawley rats within 15 days of application, thus demonstrating that IMSC could reduce wound inflammation and remarkably accelerate wound healing owing to its efficient antibacterial activity and ability to promote collagen deposition in and around the wound area. Therefore, this study provides a promising and potential therapeutic strategy for infected wound healing by synthesizing a water-soluble and broad-spectrum antimicrobial material exhibiting good biocompatibility.

Development of a porous layer-by-layer microsphere with branched aliphatic hydrocarbon porogens

Porous polymer microspheres are employed in biotherapeutics, tissue engineering, and regenerative medicine. Porosity dictates cargo carriage and release that are aligned with the polymer physicochemical properties. These include material tuning, biodegradation, and cargo encapsulation. How uniformity of pore size affects therapeutic delivery remains an area of active investigation. Herein, we characterize six branched aliphatic hydrocarbon-based porogen(s) produced to create pores in single and multilayered microspheres. The porogens are composed of biocompatible polycaprolactone, poly(lactic-co-glycolic acid), and polylactic acid polymers within porous multilayered microspheres. These serve as controlled effective drug and vaccine delivery platforms.

Functional microspheres for tissue regeneration

As a new type of injectable biomaterials, functional microspheres have attracted increasing attention in tissue regeneration because they possess some advantageous properties compared to other biomaterials, including hydrogels. A variety of bio-inspired microspheres with unique structures and properties have been developed as cellular carriers and drug delivery vehicles in recent years. In this review, we provide a comprehensive summary of the progress of functional and biodegradable microspheres that have been used for tissue regeneration over the last two decades. First, we briefly introduce the biomaterials and general methods for microsphere fabrication. Next, we focus on the newly developed technologies for preparing functional microspheres, including macroporous microspheres, nanofibrous microspheres, hollow microspheres, core-shell structured microspheres, and surface-modified functional microspheres. After that, we discuss the application of functional microspheres for tissue regeneration, specifically for bone, cartilage, dental, neural, cardiac, and skin tissue regeneration. Last, we present our perspectives and future directions of functional microspheres as injectable carriers for the future advancement of tissue regeneration.

Contributions of Women in Recent Research on Biopolymer Science

Nowadays, biopolymers are playing a fundamental role in our society because of the environmental issues and concerns associated with synthetic polymers. The aim of this Special Issue entitled ‘Women in Polymer Science and Technology: Biopolymers’ is highlighting the work designed and developed by women on biopolymer science and technology. In this context, this short review aims to provide an introduction to this Special Issue by highlighting some recent contributions of women around the world on the particular topic of biopolymer science and technology during the last 20 years. In the first place, it highlights a selection of important works performed on a number of well-studied natural polymers, namely, agar, chitin, chitosan, cellulose, and collagen. Secondly, it gives an insight into the discovery of new polysaccharides and enzymes that have a role in their synthesis and in their degradation. These contributions will be paving the way for the next generation of female and male scientists on this topic.

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.

Naturally Formed Chitinous Skeleton Isolated from the Marine Demosponge Aplysina fistularis as a 3D Scaffold for Tissue Engineering

Tissue engineering (TE) is a field of regenerative medicine that has been experiencing a special boom in recent years. Among various materials used as components of 3D scaffolds, naturally formed chitinous materials seem to be especially attractive because of their abundance, non-toxic and eco-friendly character. In this study, chitinous skeleton isolated from the marine sponge Aplysina fistularis (phylum: Porifera) was used for the first time as a support for the cultivation of murine fibroblasts (Balb/3T3), human dermal fibroblasts (NHDF), human keratinocyte (HaCaT), and human neuronal (SH-SY5Y) cells. Characterization techniques such as ATR FTIR, TGA, and μCT, clearly indicate that an interconnected macro-porous, thermostable, pure α-chitin scaffold was obtained after alkali–acid treatment of air-dried marine sponge. The biocompatibility of the naturally formed chitin scaffolds was confirmed by cell attachment and proliferation determined by various microscopic methods (e.g., SEM, TEM, digital microscopy) and specific staining. Our observations show that fibroblasts and keratinocytes form clusters on scaffolds that resemble a skin structure, including the occurrence of desmosomes in keratinocyte cells. The results obtained here suggest that the chitinous scaffold from the marine sponge A. fistularis is a promising biomaterial for future research about tissues regeneration.

Biocompatible Chitin Hydrogel Incorporated with PEDOT Nanoparticles for Peripheral Nerve Repair

The nerve guidance conduit (NGC) is a promising clinical strategy for regenerating the critical-sized peripheral nerve injury. In this study, the polysaccharide chitin is used to fabricate the hydrogel film for inducing the impaired sciatic nerve regeneration through incorporating the conductive poly(3,4-ethylenedioxythiophene) nanoparticles (PEDOT NPs) and modifying with cell adhesive tetrapeptide Cys-Arg-Gly-Asp (CRGD) (ChT-PEDOT-p). The partial deacetylation process of chitin for exposing the amino groups is performed to (i) improve the electrostatic interaction between chitin and the negatively charged PEDOT for enhancing the composite hydrogel strength and (ii) offer the active sites for peptide modification. The as-prepared hydrogel remarkably promotes the in vitro RSC-96 cell adhesion and proliferation, as well as the Schwann cell activity-related gene S100, NF-200, and myelin basic protein (MBP) expression. Function of gastrocnemius muscle and thickness of myelinated axon in chitin/PEDOT groups are analogous to the autograft in 10 mm rat sciatic nerve defect. Immunofluorescence, immunohistochemistry, western blotting, and toluidine blue staining analyses on the regenerated sciatic nerve explain that the attachment and proliferation enhancement of Schwann cells and angiogenesis are the vital factors for the chitin/PEDOT composite to facilitate the nerve regeneration. This work provides an applicable chitin-based NGC material for accelerating the peripheral nerve restoration.

Green Fabrication of Chitin/Chitosan Composite Hydrogels and Their Potential Applications

Chitin is the second most abundant natural polysaccharide with biocompatibility and bioactivity. Aqueous KOH/urea solution is reported for rapid dissolution of chitin, therefore providing a greener and more efficient avenue to fabricate chitin-based functional materials. Chitosan is the most important derivative of chitin with the acetylation degree lower than 60%. Herein, novel chitin/chitosan composite hydrogels are fabricated from the green and highly efficient KOH/urea aqueous system for the first time. Both chitin and chitosan are dissolved in aqueous KOH/urea solutions, then cross-linked by epichlorohydrin to form bulk chitin/chitosan composite hydrogels (CCGEL). The structural, thermal, mechanical, and swelling properties of CCGEL are thoroughly studied. The cell studies show that NIH-3T3 cells self-assemble to form regular 3D multicellular spheroids on the CCGEL samples with high viability. L929 cells proliferate and intend to form cell aggregates, and the size of the cell aggregates becomes greater with the increase of chitosan loading. Additionally, the CCGEL samples exhibit antibacterial activities. Thus, this pioneering work has provided crucial information for novel chitin/chitosan composite materials constructed via the direct dissolution of chitin and chitosan in aqueous KOH/urea solutions, and presented their potential applications in the cell culture and antibacterial fields.

Electrodynamic assisted self-assembled fibrous hydrogel microcapsules: a novel 3D in vitro platform for assessment of nanoparticle toxicity

Nanoparticle (NP) toxicity assessment is a critical step in assessing the health impacts of NP exposure to both consumers and occupational workers. In vitro assessment models comprising cells cultured in a two-dimensional tissue culture plate (2D-TCP) are an efficient and cost-effective choice for estimating the safety risks of NPs. However, in vitro culture of cells in 2D-TCPs distorts cell–integrin and cell–cell interactions and is not able to replicate an in vivo phenotype. Three-dimensional (3D) in vitro platforms provide a unique alternative to bridge the gap between traditional 2D in vitro and in vivo models. In this study, novel microcapsules of alginate hydrogel incorporated with natural polymeric nanofibers (chitin nanofibrils) and synthetic polymeric nanofibers poly(lactide-co-glycolide) are designed as a 3D in vitro platform. This study demonstrates for the first time that electrodynamic assisted self-assembled fibrous 3D hydrogel (3D-SAF hydrogel) microcapsules with a size in the range of 300–500 μm in diameter with a Young's modulus of 12.7–42 kPa can be obtained by varying the amount of nanofibers in the hydrogel precursor solutions. The 3D-SAF microcapsules were found to mimic the in vivo cellular microenvironment for cells to grow, as evaluated using A549 cells. Higher cellular spreading and prolonged proliferation of A549 cells were observed in 3D-SAF microcapsules compared to control microcapsules without the nanofibers. The 3D-SAF microcapsule integrated well plate was used to assess the toxicity of model NPs, e.g., Al₂O₃ and ZnO. The toxicity levels of the model NPs were found to be dependent on the chemistry of the NPs and their physical agglomeration in the test media. Our results demonstrate that 3D-SAF microcapsules with an in vivo mimicking microenvironment can be developed as a physiologically relevant platform for high-throughput toxicity screening of NPs or pharmaceutical drugs.

Electrohydrodynamic-assisted fabrication of novel nano-net-nanofibrous 3D-SAF hydrogel microcapsules leads to them having tunable mechanical and cell adhesive properties that are applicable to diverse biomedical fields.

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.

Engineering Biomimetic Nanofiber Microspheres with Tailored Size, Predesigned Structure, and Desired Composition via Gas Bubble-Mediated Coaxial Electrospray

Minimally invasive therapies avoiding surgical complexities evoke great interest in developing injectable biomedical devices. Herein, a versatile approach is reported for engineering injectable and biomimetic nanofiber microspheres (NMs) with tunable sizes, predesigned structures, and desired compositions via gas bubble-mediated coaxial electrospraying. The sizes and structures of NMs are controlled by adjusting processing parameters including air flow rate, applied voltage, distance, and spinneret configuration in the coaxial setup. Importantly, unlike the self-assembly method, this technique can be used to fabricate NMs from any material feasible for electrospinning or other nanofiber fabrication techniques. To demonstrate the versatility, open porous NMs are successfully fabricated that consist of various short nanofibers made of poly(ε-caprolactone), poly(lactic-co-glycolic acid), gelatin, methacrylated gelatin, bioglass, and magneto-responsive polymer composites. Open porous NMs support human neural progenitor cell growth in 3D with a larger number and more neurites than nonporous NMs. Additionally, highly open porous NMs show faster cell infiltration and host tissue integration than nonporous NMs after subcutaneous injection to rats. Such a novel class of NMs holds great potential for many biomedical applications such as tissue filling, cell and drug delivery, and minimally invasive tissue regeneration.

3D Chitin Scaffolds of Marine Demosponge Origin for Biomimetic Mollusk Hemolymph-Associated Biomineralization Ex-Vivo

Structure-based tissue engineering requires large-scale 3D cell/tissue manufacture technologies, to produce biologically active scaffolds. Special attention is currently paid to naturally pre-designed scaffolds found in skeletons of marine sponges, which represent a renewable resource of biomaterials. Here, an innovative approach to the production of mineralized scaffolds of natural origin is proposed. For the first time, a method to obtain calcium carbonate deposition ex vivo, using living mollusks hemolymph and a marine-sponge-derived template, is specifically described. For this purpose, the marine sponge Aplysin aarcheri and the terrestrial snail Cornu aspersum were selected as appropriate 3D chitinous scaffold and as hemolymph donor, respectively. The formation of calcium-based phase on the surface of chitinous matrix after its immersion into hemolymph was confirmed by Alizarin Red staining. A direct role of mollusks hemocytes is proposed in the creation of fine-tuned microenvironment necessary for calcification ex vivo. The X-ray diffraction pattern of the sample showed a high CaCO3 amorphous content. Raman spectroscopy evidenced also a crystalline component, with spectra corresponding to biogenic calcite. This study resulted in the development of a new biomimetic product based on ex vivo synthetized ACC and calcite tightly bound to the surface of 3D sponge chitin structure.