Silk Sericin-Functionalized Bacterial Cellulose as a Potential Wound-Healing Biomaterial

Bacterial cellulose: Biosynthesis, production, and applications

Bacterial cellulose (BC) is a natural polymer produced by the acetic acid producing bacterium and has gathered much interest over the last decade for its biomedical and biotechnological applications. Unlike the plant derived cellulose nanofibres, which require pretreatment to deconstruct the recalcitrant lignocellulosic network, BC are 100% pure, and are extruded by cells as nanofibrils. Moreover, these nanofibrils can be converted to macrofibers that possess excellent material properties, surpassing even the strength of steel, and can be used as substitutes for fossil fuel derived synthetic fibers. The focus of the review is to present the fundamental long-term research on the influence of environmental factors on the organism's BC production capabilities, the production methods that are available for scaling up/scaled-up processes, and its use as a bulk commodity or for biomedical applications.

Bacterial nanocellulose: Present status, biomedical applications and future perspectives

Bacterial nanocellulose (BNC) has emerged as a natural biopolymer of significant importance in diverse technological areas due to its incredible physicochemical and biological characteristics. However, the high capital investments, production cost and lack of well-organized scale-up processes resulting in low BNC production are the major impediments need to be resolved. This review enfolds the three different and important portions of BNC. Firstly, advancement in production technologies of BNC like cell-free extract technology, static intermittent fed batch technology and novel cost-effective substrates that might surmount the barriers associated with BNC production at industrial level. Secondly, as BNC and its composites (with other polymers/nanoparticles) represents the utmost material of preference in current regenerative and diagnostic medicine, therefore recently reported biomedical applications of BNC and functionalized BNC in drug delivery, tissue engineering, antimicrobial wound healing and biosensing are widely been focused here. The third and the most important aspect of this review is an in-depth discussion of various pitfalls associated with BNC production. Recent trends in BNC research to overcome the existing snags that might pave a way for industrial scale production of BNC thereby facilitating its feasible application in various fields are highlighted.

Hydrophobic IR780 loaded sericin nanomicelles for phototherapy with enhanced antitumor efficiency

The near-infrared dye, IR780 iodide, has been utilized in photodynamic therapy (PDT) and photothermal therapy (PTT). However, the hydrophobicity and photosensitivity of IR780 limit its further applications in biomedical fields. Herein, the hydrophilic sericin was modified with hydrophobic cholesterol to form an amphiphilic macromolecular conjugate (Ser-Chol). The tumor-targeting agent, folic acid (FA), was further linked to the conjugate (FA-Ser-Chol). The IR780 could be encapsulated into such amphiphilic macromolecule to form stable micelles (FA-Ser-Chol/IR780) by self-assembly, and the solubility and photo-stability of IR780 were greatly improved. The FA-Ser-Chol/IR780 micelles could be efficiently absorbed by FA-positive gastric cancer cells (BGC-823) through FA receptors, while the uptake micelles showed remarkable PDT and PTT cytotoxicity towards BGC-823 cells under laser irradiation of 808 nm. Therefore, FA-Ser-Chol micelles may serve as a promising IR780 carrier for PDT and PTT therapy.

Facile preparation of a Ca(ii) carboxymethyl cellulose complex with enhanced calcium bioavailability for treatment of osteoporosis

At present, though calcium (Ca) reagents with high calcium contents are widely synthesized, their wide application is limited due to their low absorption rates and poor bioavailability. Here we use a carboxymethyl cellulose (CMC) derivative with high water solubility and biocompatibility as a ligand to bind Ca2+. The resulting CaCMC complex exhibits remarkable solubility and absorbability under both basic and acidic conditions as well as in stomach mimicking and the gastrointestinal tract. Importantly, this Ca reagent shows high in vivo calcium bioavailability. Data from osteoporosis mouse models show that the CaCMC complex is superior to calcium carbonate in the treatment of osteoporosis. Therefore, the resulting CaCMC complex is used as a new, highly effective and desirable Ca supplement for daily life and clinical applications.

High-toughness natural polymer nonwoven preforms inspired by silkworm cocoon structure

As the interest in environmentally friendly materials and concerns regarding depletion of petroleum resources has increased, the research on natural polymers is being actively pursued. Among the various materials based on natural polymeric resources, the interest in using natural fibers in bio-composites has grown due to their lightweight, non-toxicity, low cost, and abundance. However, the lack of interfacial adhesion between filaments and poor water resistance make the use of natural fiber-based polymer composites less attractive. To overcome these drawbacks, formaldehyde-based synthetic binders have been used. However, this requires an additional synthesis of the binder, and potential toxicity problems exist. In this work, robust and rigid natural polymer nonwoven preforms were prepared by mixing jute fibers with silk sericin (SS). SS was employed as a natural facile binder and the strong binding between jute fibers and SS resulted in remarkable enhancements in tensile strength, elongation, and toughness, which increased up to 539.1, 385.7, and 1943.8%, respectively, compared with the pristine jute nonwoven. In addition, the dense and rigid structure obtained through SS coating ensured the structural stability of the nonwoven preforms in moisture environments. Silkworm cocoon-structured natural polymer nonwoven preforms with excellent mechanical strength and higher physical stability may have more potential utilization in the composite material fields.

Silk sericin-enhanced microstructured bacterial cellulose as tissue engineering scaffold towards prospective gut repair

As a first step towards the production of functional cell sheets applicable for the regeneration of gut muscle layer, microstructured bacterial cellulose (mBC) was assessed for its ability to support the growth of enteric nervous system (ENS) and gut smooth muscle cells (SMCs). To improve the cellular response, mBC was modified with silk sericin (SS) which has renowned abilities in supporting tissue regeneration. While SS did not impair the line structures imparted to BC by PDMS templates, similarly to the patterns, it affected its physical properties, ultimately leading to variations in the behavior of cells cultured onto these substrates. Enabled by the stripes on mBC, both SMCs and ENS cells were aligned in vitro, presenting the in vivo-like morphology essential for peristalsis and gut function. Interestingly, cell growth and differentiation remarkably enhanced upon SS addition to the samples, indicating the promise of the mBC-SS constructs as biomaterial not only for gut engineering, but also for tissues where cellular alignment is required for function, namely the heart, blood vessels, and similars.

Development of gelatin/bacterial cellulose composite sponges as potential natural wound dressings

A novel BG composite sponge comprising of bacterial cellulose (BC) and gelatin has been synthesized using glutaraldehyde as the cross-linker by a facile method. The morphology, chemical composition and structures of the novel sponges were characterized by SEM, EDS and FTIR spectroscopy. The fabricated BG sponges have regular honeycomb-like structure with uniform pore distribution and large surface area. They have very high porosity of 94%-95% and great swelling property ranging from 3000 to 3150%. Moreover, the released rate of the model drug ampicillin (AP) from the composite sponges depends on the initial addition of AP that the diffusional constant (n) determined using Korsmeyer-Peppas model lies between 0.45 and 0.89, indicating the AP release from BG composite sponges follows non-Fickian diffusion. More interestingly, antibacterial activity of BG sponges was investigated by diffusion disk method against E.coli, C. albicans and S. aureus. The results demonstrated that the obtained BG sponges exhibit excellent antibacterial activity, thus making them have great potentials in various antibacterial applications, especially in the wound dressings.

Versatile Application of Nanocellulose: From Industry to Skin Tissue Engineering and Wound Healing

Nanocellulose is cellulose in the form of nanostructures, i.e., features not exceeding 100 nm at least in one dimension. These nanostructures include nanofibrils, found in bacterial cellulose; nanofibers, present particularly in electrospun matrices; and nanowhiskers, nanocrystals, nanorods, and nanoballs. These structures can be further assembled into bigger two-dimensional (2D) and three-dimensional (3D) nano-, micro-, and macro-structures, such as nanoplatelets, membranes, films, microparticles, and porous macroscopic matrices. There are four main sources of nanocellulose: bacteria (Gluconacetobacter), plants (trees, shrubs, herbs), algae (Cladophora), and animals (Tunicata). Nanocellulose has emerged for a wide range of industrial, technology, and biomedical applications, namely for adsorption, ultrafiltration, packaging, conservation of historical artifacts, thermal insulation and fire retardation, energy extraction and storage, acoustics, sensorics, controlled drug delivery, and particularly for tissue engineering. Nanocellulose is promising for use in scaffolds for engineering of blood vessels, neural tissue, bone, cartilage, liver, adipose tissue, urethra and dura mater, for repairing connective tissue and congenital heart defects, and for constructing contact lenses and protective barriers. This review is focused on applications of nanocellulose in skin tissue engineering and wound healing as a scaffold for cell growth, for delivering cells into wounds, and as a material for advanced wound dressings coupled with drug delivery, transparency and sensorics. Potential cytotoxicity and immunogenicity of nanocellulose are also discussed.

Opportunities of Bacterial Cellulose to Treat Epithelial Tissues

In this mini-review, we highlight the potential of the biopolymer bacterial cellulose to treat damaged epithelial tissues. Epithelial tissues are cell sheets that delimitate both the external body surfaces and the internal cavities and organs. Epithelia serve as physical protection to underlying organs, regulate the diffusion of molecules and ions, secrete substances and filtrate body fluids, among other vital functions. Because of their continuous exposure to environmental stressors, damage to epithelial tissues is highly prevalent. Here, we first compare the properties of bacterial cellulose to the current gold standard, collagen, and then we examine the use of bacterial cellulose patches to heal specific epithelial tissues; the outer skin, the ocular surface, the oral mucosa and other epithelial surfaces. Special emphasis is made on the dermis since, to date, this is the most widespread medical use of bacterial cellulose. It is important to note that some epithelial tissues represent only the outermost layer of more complex structures such as the skin or the cornea. In these situations, depending on the penetration of the lesion, bacterial cellulose might also be involved in the regeneration of, for instance, inner connective tissue.

Polysaccharides for tissue engineering: Current landscape and future prospects

Biological studies on the importance of carbohydrate moieties in tissue engineering have incited a growing interest in the application of polysaccharides as scaffolds over the past two decades. This review provides a perspective of the recent approaches in developing polysaccharide scaffolds, with a focus on their chemical modification, structural versatility, and biological applicability. The current major limitations are assessed, including structural reproducibility, the narrow scope of polysaccharide modifications being applied, and the effective replication of the extracellular environment. Areas with opportunities for further development are addressed with an emphasis on the application of rationally designed polysaccharides and their importance in elucidating the molecular interactions necessary to properly design tissue engineering materials.

Three-Dimensionally Printed Silk-Sericin-Based Hydrogel Scaffold: A Promising Visualized Dressing Material for Real-Time Monitoring of Wounds

A wound dressing which can be convenient for real-time monitoring of wounds is particularly attractive and user-friendly. In this study, a nature-originated silk-sericin-based (SS-based) transparent hydrogel scaffold was prepared and evaluated for the visualization of wound care. The scaffold was fabricated from a hybrid interpenetrating-network (IPN) hydrogel composed of SS and methacrylic-anhydride-modified gelatin (GelMA) by 3D printing. The scaffold transformed into a highly transparent hydrogel upon swelling in PBS, and thus, anything underneath could be easily read. The scaffold had a high degree of swelling and presented a regularly macroporous structure with pores around 400 μm × 400 μm, which can help maintain the moist and apinoid environment for wound healing. Meanwhile, the scaffolds were conducive to adhesion and proliferation of L929 cells. A coculture of HaCaT and HSF cells on the scaffold showed centralized proliferation of the two cells in distributed layers, respectively, denoting a promising comfortable environment for re-epithelialization. Moreover, in vivo studies demonstrated that the scaffold showed no excessive inflammatory reaction. In short, this work presented an SS-based transparent hydrogel scaffold with steerable physical properties and excellent biocompatibility through 3D printing, pioneering promising applications in the visualization of wound care and drug delivery.

Fabrication of the FGF1-functionalized sericin hydrogels with cell proliferation activity for biomedical application using genetically engineered Bombyx mori (B. mori) silk

Sericin, as the major component of Bombyx mori silk, is a useful biomaterial for tissue engineering due to its hydrophilicity, biocompatibility and biodegradability. Here, we report the fabrication of a human acidic fibroblast growth factor (FGF1)-functionalized sericin hydrogel using a transgenic silkworm spun silk with FGF1 incorporated in its sericin layer. Sericin, together with FGF1, were simultaneously extracted from the silk fiber and then exposed to cold-induced hydrogel formation without additional crosslinking. The fabricated FGF1 sericin hydrogels demonstrated injectability, useful mechanical properties and a porous microstructure, which contributed to cell adhesion and survival. In addition, FGF1 achieved long-term storage in the sericin hydrogels over a wide range of temperatures. Further, the sericin-FGF1 demonstrated sustained release to promote cell proliferation and wound healing. Furthermore, cellular inflammatory responses showed that the FGF1 sericin hydrogels exhibited biocompatibility and no immunogenicity. This study revealed the successful exploration of FGF1-functionalized sericin hydrogels as a new protein-based biomaterial to expand applications of FGF1 and sericin in tissue and medical engineering. Further, we demonstrated a strategy for the predesign of exogenous protein-functionalized sericin hydrogels through genetically modifying silk fibers as sources for their cost effective production at a large scale.

STATEMENT OF SIGNIFICANCE

Sericin from the Bombyx mori silk, is regarded as a desirable biomaterial for tissue engineering due to its hydrophilicity, biocompatibility and biodegradability. Genetically engineering the sericin with functional exogenous proteins would enhance its biofunctions and further expand its application in tissue engineering. In this study, we demonstrated a method to fabricate a human acidic fibroblast growth factor (FGF1)-functionalized sericin hydrogel using a transgenic silkworm spun silk with FGF1 incorporated in its sericin layer. The fabricated FGF1 sericin hydrogels demonstrated injectability, porous microstructure, biocompatibility and no immunogenicity which contributed to cell adhesion and survival. Remarkably, FGF1 could achieve a long-term stability in the sericin hydrogels over a wide range of temperatures and sustained release to promote cell proliferation and wound healing. This study revealed the successful exploration of FGF1-functionalized sericin hydrogels as a new protein-based biomaterial in tissue and medical engineering application, and provided a strategy for the predesign of exogenous protein-functionalized sericin hydrogels through genetically modifying silk fibers as sources for their cost effective production at a large scale.

Preparation and characterization of gelatin/sericin/carboxymethyl chitosan medical tissue glue

Background:

The development and application of medical glue has been continuously expanding and advancing. However, there are few glues that combine low-cost with excellent biocompatibility.

Methods:

We have prepared a medical tissue glue using a gelatin (Gel), sericin (SS) and carboxymethyl chitosan (CMCS) blend solution, cross-linked with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC). The combination’s characteristics and microstructure morphology were observed by Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM). Bond strength tests were used to measure the bond strength of the glue. To assay blood compatibility, a hemolytic test, dynamic coagulation test and platelet adherence test were also investigated. Further, the cellular behavior of L-929 and a systemic acute toxicity test on the Gel/SS/CMCS tissue glue were also investigated by MTT and H&E staining.

Results:

Characterization analysis showed that there was stable binding between raw materials, forming an amide bond with homogeneous holes. The bond strength of the tissue glue reached 2.50 ± 0.04 N in 10 minutes, slightly higher than the alpha-cyanoacrylate biological glue (2.25 ± 0.05 N). Blood compatibility tests revealed that the glue had outstanding blood compatibility. Further, cytotoxicity test and systemic acute toxicity test both showed that the glue was without cytotoxicity and not toxic to the body.

Conclusions:

The Gel/SS/CMCS tissue glue we prepared at low cost had excellent biocompatibility and structural characteristics. It could be a better candidate for tissue engineering in biomedical applications applied in clinical practice to promote skin wound healing and to further reduce the formation of skin wound scars.

A transparent wound dressing based on bacterial cellulose whisker and poly(2-hydroxyethyl methacrylate)

The current study was aimed to develop a transparent wound dressing comprised of bacterial cellulose (BC) and poly (2-hydroxyethyl methacrylate) (PHEMA) hydrogel coated with silver (Ag) nanoparticles. Briefly, different concentrations of BC whiskers (BCWs) were added into the HEMA solution to form PHEMA/BCWs hydrogel with volume ratio of monomer HEMA and BCWs as 7:3 and 1:1. The addition of BCWs into PHEMA matrix improved its equilibrium water content and light transmittance about 20%-40% and 10%, respectively. The Young's modulus for PHEMA was found to be 0.72MPa, which was improved to 0.57MPa and 0.50MPa for PHEMA/BCWs 7:3 and PHEMA/BCWs 1:1, respectively. Further, immersion of PHEMA/BCWs hydrogel in the AgNO₃ and NaBH₄ solutions bestowed it with antibacterial property and produced inhibition zones of 0.5±0.15cm and 0.25±0.15cm against Escherichia coli and Staphylococcus aureus, respectively. Similarly, PHEMA/BCWs prepared with 0.001M AgNO₃ and 0.001M NaBH₄ solutions showed 99% and 90% reduction in colony forming unit (CFU) for E. coli and S. aureus, respectively, after 24h. The PHEMA/BCWs/Ag hydrogel facilitated the growth of NIH3T3 fibroblast, showing their low toxicity. These results demonstrate the suitability of PHEMA/BCWs/Ag hydrogel for its application as potential transparent wound dressing material for skin repair.

Composites of Bacterial Cellulose and Small Molecule-Decorated Gold Nanoparticles for Treating Gram-Negative Bacteria-Infected Wounds

Bacterial infections, especially multidrug-resistant bacterial infections, are an increasingly serious problem in the field of wound healing. Herein, bacterial cellulose (BC) decorated by 4,6-diamino-2-pyrimidinethiol (DAPT)-modified gold nanoparticles (Au-DAPT NPs) is presented as a dressing (BC-Au-DAPT nanocomposites) for treating bacterially infected wounds. BC-Au-DAPT nanocomposites have better efficacy (measured in terms of reduced minimum inhibition concentration) than most of the antibiotics (cefazolin/sulfamethoxazole) against Gram-negative bacteria, while maintaining excellent physicochemical properties including water uptake capability, mechanical strain, and biocompatibility. On Escherichia coli- or Pseudomonas aeruginosa-infected full-thickness skin wounds on rats, the BC-Au-DAPT nanocomposites inhibit bacterial growth and promote wound repair. Thus, the BC-Au-DAPT nanocomposite system is a promising platform for treating superbug-infected wounds.

Electrospinning of PVA/sericin nanofiber and the effect on epithelial-mesenchymal transition of A549 cells

This research aims to investigate the cell-nanomaterial interaction between epithelial-mesenchymal transition of A549 cell and electrospinning nanofibers composed of polyvinyl alcohol (PVA)/silk sericin (SS). The electrospinning of regenerated nanofiber was performed with water as a spinning solvent and glutaraldehyde as a chemical cross-linker. Solution concentration, applied voltage and spin distances as well as other parameters were optimized to generate fine nanofibers with smooth surface in good homogeneity. From the scanning electron microscopy (SEM) analysis, the nanofibers had an average diameter of 200nm. Epithelial-mesenchymal transition (EMT) is a process by which epithelial cells lose their cell polarity to become mesenchymal stem cells. This transition is affected by multiple biochemical and physical factors in cell metabolism cascade. Herein, we investigate the biophysical effect on A549 EMT by culturing cells on nanofibrous mats with different topography and composition. The cell viability was evaluated by biochemical assay and its morphology was observed with SEM. The results demonstrate that cells appropriately attached to the surface of the nanofibrous mats with extended morphology by their filopodia. Gene expression analysis was conducted by real-time PCR using multiple markers for detecting EMT: N-cadherin (NCad), Vimentin (Vim), Fibronectin (Fib) and Matrix metallopeptidase (MMP9). An increasing expression pattern was observed on NCad, Vim, Fib, with respect to a negative control as cell cultured on polystyrene dish. This result indicates the 200nm PVA/SS nanofibers may induce A549 cells to process epithelial-mesenchymal transition during the culturing.

Strategies for cost-effective and enhanced production of bacterial cellulose

Bacterial cellulose (BC) has received substantial attention because of its high purity, mechanical strength, crystallinity, liquid-absorbing capabilities, biocompatibility, and biodegradability etc. These properties allow BC to be used in various fields, especially in industries producing medical, electronic, and food products etc. A major discrepancy associated with BC is its high production cost, usually much higher than the plant cellulose. To address this limitations, researchers have developed several strategies for enhanced production of BC including the designing of advanced reactors and utilization of various carbon sources. Another promising approach is the production of BC from waste materials such as food, industrial, agricultural, and brewery wastes etc. which not only reduces the overall BC production cost but is also environment-friendly. Besides, exploration of novel and efficient BC producing microbial strains provides impressive boost to the BC production processes. To this end, development of genetically engineered microbial strains has proven useful for enhanced BC production. In this review, we have summarized major efforts to enhance BC production in order to make it a cost-effective biopolymer. This review can be of interest to researchers investigating strategies for enhanced BC production, as well as companies exploring pilot projects to scale up BC production for industrial applications.