Study of synthesis of nano-hydroxyapatite using a silk fibroin template

Enhanced formation of bioactive and strong silk-bioglass hybrid materials through organic-inorganic mutual molecular nucleation induction and templating

Materials based on silk fibroin (SF) are important for many biomedical applications due to their excellent biocompatibility and tunable biodegradability. However, the insufficient mechanical strength and low bioactivity of these materials have limited their applications. For silk hydrogels, slow gelation is also a crucial problem. In this work, a simple approach is developed to address these challenging problems all at once. By mixing SF solution with bioglass (BG) sol, instant gelation of silk is induced, the storage modulus of the hydrogel and the compressive modulus of the aerogel are significantly enhanced. The formation of a complex of SF and tetraethyl orthosilicate (TEOS), either through hydrogen bonding or TEOS condensation on SF, facilitated the aggregation of SF and, on the other hand, created active sites for the condensation of TEOS and BG formation on the surface of silk nanofibrils. The resultant hybrid gels have much higher capacity for biomineralization, indicating their higher bioactivity, compared with the pristine silk gels. This organic (SF)-inorganic (BG) mutual nucleation induction and templating can be used for a general approach to produce bioactive silk materials of various formats not limited to gels and may also inspire the formation of other functional protein-BG hybrid materials.

Biomimetic Inorganic Nanoparticle-Loaded Silk Fibroin-Based Coating with Enhanced Antibacterial and Osteogenic Abilities

Poor osseointegration and infection are the main reasons leading to the failure of hard tissue implants; especially, in recent years, the failure rate has been increasing every year owing to the continuously increasing conditions such as injury, trauma, diseases, or infections. Therefore, the development of a biomimetic surface coating of bone tissues with antibacterial function is an effective means to improve bone healing and inhibit bacterial infection. Mimicking the natural bone, in this study, we have designed a silk fibroin (collagen-like structure)-based coating inlaid with nanohydroxyapatite (nHA) and silver nanoparticles (AgNPs) for promoting antibacterial ability and osteogenesis, especially focusing on the bone mimetic structure for enhancing bone health. Observing the morphology and size of the composite nanoparticles by transmission electron microscope (TEM), nHA provided nucleation sites for the formation of AgNPs, forming an nHA/AgNP complex with a size of about 100-200 nm. Characterization of the nHA/Ag-loaded silk fibroin biomimetic coating showed an increased surface roughness with good density and compact performances. The silk fibroin-based coating loaded with uniformly distributed AgNPs and nHA could effectively inhibit the adhesion of Staphylococcus aureus on the surface and, at the same time, quickly kill planktonic bacteria, indicating their good antibacterial ability. In vitro cell experiments revealed that the biomimetic silk fibroin-based coating was beneficial to the adhesion, spreading, and proliferation of osteoblasts (MC3T3-E1). In addition, by characterizing LDH and ROS, it was found that the nHA/Ag complex could significantly reduce the cytotoxicity of AgNPs, and the osteoblasts on the coating surface maintained the structure intact.

Osteogenic potential and properties of injectable silk fibroin/tetracalcium phosphate/monetite composite powder biocement systems

The powdered cement tetracalcium phosphate/monetite/silk fibroin composite (CFIB) was prepared by simple mechanical milling of tetracalcium phosphate/monetite powder mixture with fibrous soluble silk fibroin (SF). The powder composite cement mixtures contained 5 and 10 wt % of SF and 2% NaH₂ PO₄ solution with 0.1% genipin was used as a liquid component. The setting time of CFIB cement increased with addition of SF from 5 to 25 min in fully injectable cement with 10 wt % of SF. The compressive strength of hardened composites was reduced to 14 MPa which is close to strength of cancellous bone. The 8% of SF from origin amount in CFIB composites was only desorbed from cements after 7 days soaking in simulated body fluid (SBF). It was found almost full transformation of calcium phosphate components in composite to rod-like nanohydroxyapatite after hardening of CFIB cements in SBF. The SF in hardened cements was present in fine globular form after dissolution, actively affected the fluidity of pastes, morphology of hydroxyapatite particles, and microstructure. The excellent cell proliferation and a high over expression of osteogenic gene markers in MSCs were confirmed after the long-time cultivation in CFIB10 cement extract. Injectable CFIB10 cements have appropriate properties for utilization in bone defect treatments with possible positive effect on healing process.

Silk fibroin/chitosan/TGF-β1-loaded microsphere scaffolds for cartilage reparation

BACKGROUND

Transforming growth factor-β1 (TGF-β1) plays an important role in chondrocyte growth and the synthesis of extracellular matrix (ECM). Due to the rapid metabolism, controlled release systems for TGF-β1 have attracted increasing interest recently.

OBJECTIVE

In this study, a silk fibroin (SF)/chitosan (CS) scaffold incorporated with TGF-β1-loaded microspheres (MSs) was created for cartilage reparation.

METHOD

The optimal proportion of the SF/CS composite scaffold was determined by evaluating their micromorphology and the proliferation rate of fibroblasts on the surface. Then, SF/CS/TGF-β1-loaded MS scaffolds were prepared by the adsorption method. TGF-β1 release capacity, degradation patterns, cytocompatibility and in vivo implantation were evaluted.

RESULTS

The SF/CS/TGF-β1-loaded MS scaffold showed good TGF-β1 release over more than 16 days, which could sequentially stimulate chondrocyte synthetic activity. In vitro cell proliferation experiments showed the SF/CS/TGF-β1-loaded MS scaffold could promote chondrocytes adhesion, growth, proliferation and maintained the cellular morphology. An in vivo study demonstrated that a low inflammatory response was observed in rats and that the materials exhibited good biocompatibility.

CONCLUSION

the results indicated that our SF/CS/TGF-β1-loaded MS scaffold constitute a promising therapeutic option for cartilage reparation.

Rapid regeneration of enamel-like-oriented inorganic crystals by using rotary evaporation

Enamel, the hardest tissue in the human body, has excellent mechanical properties, mainly due to its highly ordered spatial structure. Fabricating enamel-like structure is still a challenge today. In this work, a simple and highly efficient method was introduced, using the silk fibroin as a template to regulate calcium- and phosphate- supersaturated solution to regenerate enamel-like hydroxyapatite crystals on various substrates (enamel, dentin, titanium, and polyethylene) under rotary evaporation. The enamel-like zinc oxide nanorod array structure was also successfully synthesized using the aforementioned method. This strategy provides a new approach to design and fabricate mineral crystals with particular orientation coatings for materials.

Hydroxyapatite/silk fibroin composite biomimetic scaffold for dental pulp repair

Dental pulp repair is a difficult clinical problem. In the present study, the authors aimed to mimic the extracellular matrix of dental pulp tissue structurally and compositionally. Nanofibrous silk fibroin (SF) scaffolds containing hydroxyapatite (HAp) nanoparticles were fabricated by using the freeze-drying approach. Rod-shaped HAp was successfully embedded in the composite scaffold, the diameter of which was about 100–200 nm as shown by transmission electron microscopy analysis. The three-dimensional microstructure of the composite scaffold prepared in various ratios of HAp to SF was observed by scanning electron microscopy and the pore size of the optimal scaffold was about 30–120 μm. Meanwhile, the hemocompatibility of the composite scaffolds was evaluated based on their impact on the clotting function by way of activated partial thromboplastin time, prothrombin time and thromboelastographic assays. The scaffolds possessed a low hemolysis rate of red blood cells. Furthermore, cell culture tests using dental pulp stem cells found that the scaffolds had good biocompatibility. There biomimetic HAp/SF composite scaffolds may serve as a promising biomaterial for dental pulp repair.

Fabrication and characterization of strontium-hydroxyapatite/silk fibroin biocomposite nanospheres for bone-tissue engineering applications

Osteoinductive bone filling biomaterials are in high demand for effective bone defect reconstruction. In this study, we aimed to design both organic and inorganic substances containing strontium-doped hydroxyapatite/silk fibroin (SrHA/SF) biocomposite nanospheres as an osteoinductive bone defect-filling biomaterial. SrHA/SF nanospheres were prepared with different concentration of Sr using ultrasonic coprecipitation method. The nanospheres were characterized using XRD, FTIR, SEM, TEM, ICP-AES and TGA. Solid and dense SrHA/SF nanospheres with 500-700 nm size and rough surfaces were synthesized successfully. Higher crystallinity and HA/SF phase were observed with the increase in Sr-concentration. The doping of different concentration of Sr did not affect the size and surface characteristics of the nanospheres. ICP-AES data showed that Sr/Ca ratio in SrHA/SF is very close to the nominal value. Nanospheres with higher concentration of Sr did not negatively affect the biocompatibility, but enhanced viability of mesenchymal stem cells (MSCs). Moreover, SrHA/SF nanospheres showed higher osteogenic differentiation potential compared to HA/SF nanospheres as indicated by the results from ALP staining, ALP activity, and Runx2, Alp, Col-1 and Opn gene expression assay in MSCs culture. Our findings suggest this novel design of biocompatible and osteoinductive SrHA/SF biocomposite nanospheres as a potential bone defect-filling biomaterial for bone regenerative applications.

Outer-inner dual reinforced micro/nano hierarchical scaffolds for promoting osteogenesis

Biomimetic scaffolds have been extensively studied for guiding osteogenesis through structural cues. Inspired by the natural bone growth process, we have employed a hierarchical outer-inner dual reinforcing strategy, which relies on the interfacial ionic bond interaction between amine/calcium and carboxyl groups, to build a nanofiber/particle dual strengthened hierarchical silk fibroin scaffold. This scaffold can provide an applicable form of osteogenic structural cue and mimic the natural bone forming process. Owing to the active interaction between compositions located in the outer pore space and the inner pore wall, the scaffold has over 4 times improvement in the mechanical properties, followed by a significant alteration of the cell-scaffold interaction pattern, demonstrated by over 2 times elevation in the spreading area and enhanced osteogenic activity potentially involving the activities of integrin, vinculin and Yes-associated protein (YAP). The in vivo performance of the scaffold identified the inherent osteogenic effect of the structural cue, which promotes rapid and uniform regeneration. Overall, the hierarchical scaffold is promising in promoting uniform bone regeneration through its specific structural cue endowed by its micro-nano construction.

Design, Fabrication, and Function of Silk-Based Nanomaterials

Animal silks are built from pure protein components and their mechanical performance, such as strength and toughness, often exceed most engineered materials. The secret to this success is their unique nanoarchitectures that are formed through the hierarchical self-assembly of silk proteins. This natural material fabrication process in sharp contrast to the production of artificial silk materials, which usually are directly constructed as bulk structures from silk fibroin (SF) molecular. In recent years, with the aim of understanding and building better silk materials, a variety of fabrication strategies have been designed to control nanostructures of silks or to create functional materials from silk nanoscale building blocks. These emerging fabrication strategies offer an opportunity to tailor the structure of SF at the nanoscale and provide a promising route to produce structurally and functionally optimized silk nanomaterials. Here, we review the critical roles of silk nanoarchitectures on property and function of natural silk fibers, outline the strategies of utilization of these silk nanobuilding blocks, and we provide a critical summary of state of the art in the field to create silk nanoarchitectures and to generate silk-based nanocomponents. Further, such insights suggest templates to consider for other materials systems.

Silk scaffolds in bone tissue engineering: An overview

Bone tissue plays multiple roles in our day-to-day functionality. The frequency of accidental bone damage and disorder is increasing worldwide. Moreover, as the world population continues to grow, the percentage of the elderly population continues to grow, which results in an increased number of bone degenerative diseases. This increased elderly population pushes the need for artificial bone implants that specifically employ biocompatible materials. A vast body of literature is available on the use of silk in bone tissue engineering. The current work presents an overview of this literature from materials and fabrication perspective. As silk is an easy-to-process biopolymer; this allows silk-based biomaterials to be molded into diverse forms and architectures, which further affects the degradability. This makes silk-based scaffolds suitable for treating a variety of bone reconstruction and regeneration objectives. Silk surfaces offer active sites that aid the mineralization and/or bonding of bioactive molecules that facilitate bone regeneration. Silk has also been blended with a variety of polymers and minerals to enhance its advantageous properties or introduce new ones. Several successful works, both in vitro and in vivo, have been reported using silk-based scaffolds to regenerate bone tissues or other parts of the skeletal system such as cartilage and ligament. A growing trend is observed toward the use of mineralized and nanofibrous scaffolds along with the development of technology that allows to control scaffold architecture, its biodegradability and the sustained releasing property of scaffolds. Further development of silk-based scaffolds for bone tissue engineering, taking them up to and beyond the stage of human trials, is hoped to be achieved in the near future through a cross-disciplinary coalition of tissue engineers, material scientists and manufacturing engineers.

STATEMENT OF SIGNIFICANCE

The state-of-art of silk biomaterials in bone tissue engineering, covering their wide applications as cell scaffolding matrices to micro-nano carriers for delivering bone growth factors and therapeutic molecules to diseased or damaged sites to facilitate bone regeneration, is emphasized here. The review rationalizes that the choice of silk protein as a biomaterial is not only because of its natural polymeric nature, mechanical robustness, flexibility and wide range of cell compatibility but also because of its ability to template the growth of hydroxyapatite, the chief inorganic component of bone mineral matrix, resulting in improved osteointegration. The discussion extends to the role of inorganic ions such as Si and Ca as matrix components in combination with silk to influence bone regrowth. The effect of ions or growth factor-loaded vehicle incorporation into regenerative matrix, nanotopography is also considered.

Bioinspired and Biomimetic AgNPs/Gentamicin-Embedded Silk Fibroin Coatings for Robust Antibacterial and Osteogenetic Applications

With the progressively increasing demand for orthopedic Ti implants, the balance between two primary complications restricting implant applications is needed to be solved: the lack of bone tissue integration and biomedical device-associated infections (BAI), where emergence of multiresistance bacteria make it worse. Notably, a combination of silver nanoparticles (AgNPs) and a kind of antibiotic can synergistically inhibit bacterial growth, where a low concentration of AgNPs has been confirmed to promote the proliferation and osteogenesis of osteoblasts. In this work, we built AgNPs/gentamicin (Gen)-embedded silk fibroin (SF)-based biomimetic coatings on orthopedic titanium by a facile dipping-drying circular process and with the assistance of polydopamine (PD). Ag⁺ was reduced to AgNPs by SF under ultraviolet (UV) irradiation, and then they were detected by transmission electron microscope (TEM) images and UV-visible (UV-vis) analyses. Intriguingly, the addition of Gen highly improved the reduction efficiency of Ag⁺. The antibacterial efficiency of SF-based coatings was examined by challenging them with pathogenic Staphylococcus aureus (S. aureus) bacteria which produced biofilms, and consequently, we found that low concentration loading, durable release of Ag⁺ (28 days), and 10-fold improvement of antibacterial efficiency were achieved for our novel AgNPs- and Gen-embeded silk fibroin coatings. In bacteria and a cells cocultured system, AgNPs/Gen-embedded coatings strongly inhibited adhesion and proliferation of S. aureus, simultaneously improving cell adhesion and growth. To investigate cytocompatibility and osteogenic potential, different coatings were cultured with MC3T3 cells; AgNPs/Gen-embedded coatings showed generally acceptable biocompatibility (cell adhesion, proliferation, and viability) and accelerated osteoblast maturation (alkaline phosphatase production, matrix secretion, and calcification). Expectantly, this novel biofunctional coating will have promising applications in orthopedic and dental titanium implants thanks to its excellently antibacterial, biocompatible, and osteogenic activities.

Fabrication, characterization, and in vitro study of zinc substituted hydroxyapatite/silk fibroin composite coatings on titanium for biomedical applications

Zinc substituted hydroxyapatite/silk fibroin composite coatings were deposited on titanium substrates at room temperature by electrophoretic deposition. Microscopic characterization of the synthesized composite nanoparticles revealed that the particle size ranged 50-200 nm, which increased a little after zinc substitution. The obtained coatings maintained the phase of hydroxyapatite and they could induce fast apatite formation in simulated body fluid, indicating high bone activity. The cell culturing results showed that the biomimetic hydroxyapatite coatings could regulate adhesion, spreading, and proliferation of osteoblastic cells. Furthermore, the biological behavior of the zinc substituted hydroxyapatite coatings was found to be better than the bare titanium without coatings and hydroxyapatite coatings without zinc, increasing MC3T1-E1 cell differentiation in alkaline phosphatase expression.

Investigate the Effect of Thawing Process on the Self-Assembly of Silk Protein for Tissue Applications

Biological self-assembly is a process in which building blocks autonomously organize to form stable supermolecules of higher order and complexity through domination of weak, noncovalent interactions. For silk protein, the effect of high incubating temperature on the induction of secondary structure and self-assembly was well investigated. However, the effect of freezing and thawing on silk solution has not been studied. The present work aimed to investigate a new all-aqueous process to form 3D porous silk fibroin matrices using a freezing-assisted self-assembly method. This study proposes an experimental investigation and optimization of environmental parameters for the self-assembly process such as freezing temperature, thawing process, and concentration of silk solution. The optical images demonstrated the possibility and potential of -80ST48 treatment to initialize the self-assembly of silk fibroin as well as controllably fabricate a porous scaffold. Moreover, the micrograph images illustrate the assembly of silk protein chain in 7 days under the treatment of -80ST48 process. The surface morphology characterization proved that this method could control the pore size of porous scaffolds by control of the concentration of silk solution. The animal test showed the support of silk scaffold for cell adhesion and proliferation, as well as the cell migration process in the 3D implantable scaffold.

Biomimetic synthesis of hybrid hydroxyapatite nanoparticles using nanogel template for controlled release of bovine serum albumin

A biomimetic method was used to prepare hybrid hydroxyapatite (HAP) nanoparticles with chitosan/polyacrylic acid (CS-PAA) nanogel. The morphology, structure, crystallinity, thermal properties and biocompatibility of the obtained hybrid nanogel-HAP nanoparticles have been characterized. In addition, bovine serum albumin (BSA) was used as a model protein to study the loading and release behaviors of the hybrid nanogel-HAP nanoparticles. The results indicated that the obtained HAP nanoparticles were agglomerated and the nanogel could regulate the formation of HAP. When the nanogel concentration decreased, different HAP crystal shapes and agglomerate structures were obtained. The loading amount of BSA reached 67.6 mg/g for the hybrid nanoparticles when the mineral content was 90.4%, which decreased when the nanogel concentration increased. The release profile of BSA was sustained in neutral buffer. Meanwhile, an initial burst release was found at pH 4.5 due to the desorption of BSA from the surface, followed by a slow release. The hemolysis percentage of the hybrid nanoparticles was close to the negative control, and these particles were non-toxic to bone marrow stromal stem cells. The results suggest that these hybrid nanogel-HAP nanoparticles are promising candidate materials for biocompatible drug delivery systems.

Size of silk fibroin β-sheet domains affected by Ca2+

Addition of bioactive glass or other Ca²⁺ source to fibroin changes scaffold degradation and the mechanical and protein secondary structure properties due to the reduction in the size of β-sheet domains.

Synthesis of water-dispersible silicon-containing hydroxyapatite nanoparticles with adjustable degradation rates and their applications as pH-responsive drug carriers

An environmentally friendly method was developed to synthesize water-dispersible Si-HAp nanoparticles with adjustable degradation rates, high loading capacities for anticancer drugs, and sustained and pH-dependent drug release properties.

A novel approach to fabricate silk nanofibers containing hydroxyapatite nanoparticles using a three-way stopcock connector

Electrospinning technique is commonly used to produce micro- and/or nanofibers, which utilizes electrical forces to produce polymeric fibers with diameters ranging from several micrometers down to few nanometers. Desirably, electrospun materials provide highly porous structure and appropriate pore size for initial cell attachment and proliferation and thereby enable the exchange of nutrients. Composite nanofibers consisting of silk and hydroxyapatite nanoparticles (HAp) (NPs) had been considered as an excellent choice due to their efficient biocompatibility and bone-mimicking properties. To prepare these nanofiber composites, it requires the use of acidic solutions which have serious consequences on the nature of both silk and HAp NPs. It is ideal to create these nanofibers using aqueous solutions in which the physicochemical nature of both materials can be retained. However, to create those nanofibers is often difficult to obtain because of the fact that aqueous solutions of silk and HAp NPs can precipitate before they can be ejected into fibers during the electrospinning process. In this work, we had successfully used a three-way stopcock connector to mix the two different solutions, and very shortly, this solution is ejected out to form nanofibers due to electric fields. Different blend ratios consisting HAp NPs had been electrospun into nanofibers. The physicochemical aspects of fabricated nanofiber had been characterized by different state of techniques like that of FE-SEM, EDS, TEM, TEM-EDS, TGA, FT-IR, and XRD. These characterization techniques revealed that HAp NPs can be easily introduced in silk nanofibers using a stopcock connector, and this method favorably preserves the intact nature of silk fibroin and HAp NPs. Moreover, nanofibers obtained by this strategy were tested for cell toxicity and cell attachment studies using NIH 3 T3 fibroblasts which indicated non-toxic behavior and good attachment of cells upon incubation in the presence of nanofibers.