Silk fibroin as an organic polymer for controlled drug delivery

Effect of Glycerol on Structure and Properties of Silk Fibroin/Pearl Powder Blend Films

In the present study, glycerol was used as plasticizer to prepare silk fibroin (SF)/pearl powder (PP) blend films. The effects of amount of glycerol on structure and properties of the films were investigated. The surface morphology was observed with scanning electron microscopy. The structure of films was investigated by X-ray diffraction and thermal analysis. The mechanical properties of the films were measured on a universal testing machine, and the dissolution rate of SF was examined by ultraviolet spectroscopy. The results showed that surface of pure SF films was smooth, but the surface of films containingPP was uneven, particles of PP dispersed in the films. The structure of the film without glycerol was mainly amorphous structure. The structure of the SF in the film was mainly silk I and silk II when the proportion of glycerol added was in the range of 10%- 20%, while the main structure of the SF in the films was silk I when the proportion of glycerol was more than 20%. The dissolution rate of SF in films without glycerol is rather great, while the dissolution rate had a significant decrease by adding glycerol. There was no significant difference in dissolution rates of SF which were all below 1.3% when the proportion of glycerol is 10-40%. The films without glycerol had very small elongation at break. The elongation at break of SF films increased with the increased amount of glycerol. Compared to films without glycerol, there was a significant difference when the proportion of glycerol was greater than 10%. The tensile strength of the films dropped significantly with the increase of glycerol, but there was no significant difference when the proportion of glycerol was greater than 30%. Therefore, the advisable addition percentage of glycerol is 20%.

Preparation of Silk Fibroin/Pearl Powder Blend Films

Silk fibroin (SF) / pearl powder (PP) blend films with glycerol as a crosslinker were prepared by adding PP into SF solution and casting the SF solution onto the polystyrene disks at room temperature. The effects of amount of PP on surface morphology, structure, mechanical properties and water-solubility of films were investigated in this study. The surface morphology was observed with scanning electron microscope. The structure of films was investigated by X-ray diffraction. The dissolution rates of SF in films were measured by ultraviolet spectroscopy and mass loss percentages of the films were examined by weighing method. The elongation at break and the tensile strength of the films were measured on a universal testing machine. The results showed that surface of films without PP was smooth, particles rose increasingly obviously on the surface of films with PP with the increase of amount of PP. The condensed structure of SF in the films without PP was mainly silk I crystals. When the addition of PP was more than 5%, it had obvious effects on the crystal formation of SF in the films. Apart from silk I crystals, silk II crystals formed in the condensed structure of SF containing PP. The mass loss rate of films decreased with the increase of PP and the corresponding dissolution rate of SF was between 0.29% and 0.48% when the proportion of PP was in the range of 0% to 15%. The elongation at break was significantly affected by the addition of PP, but there were no significant effects on the tensile strength. When the proportion of PP was more than 5%, the elongation at break of the films dropped significantly.

Fibroin and fibroin blended three-dimensional scaffolds for rat chondrocyte culture

Background

In our previous study, we successfully developed 3-D scaffolds prepared from silk fibroin (SF), silk fibroin/collagen (SF/C) and silk fibroin/gelatin (SF/G) using a freeze drying technique. The blended construct showed superior mechanical properties to silk fibroin construct. In addition, collagen and gelatin, contain RGD sequences that could facilitate cell attachment and proliferation. Therefore, in this study, the ability of silk fibroin and blended constructs to promote cell adhesion, proliferation and production of extracellular matrix (EMC) were compared.

Methods

Articular chondrocytes were isolated from rat and cultured on the prepared constructs. Then, the cell viability in SF, SF/C and SF/G scaffolds was determined by MTT assay. Cell morphology and distribution were investigated by scanning electron microscopy (SEM) and histological analysis. Moreover, the secretion of extracellular matrix (ECM) by the chondrocytes in 3-D scaffolds was assessed by immunohistochemistry.

Results

Results from MTT assay indicated that the blended SF/C and SF/G scaffolds provided a more favorable environment for chondrocytes attachment and proliferation than that of SF scaffold. In addition, scanning electron micrographs and histological images illustrated higher cell density and distribution in the SF/C and SF/G scaffolds than that in the SF scaffold. Importantly, immunohistochemistry strongly confirmed a greater production of type II collagen and aggrecan, important markers of chondrocytic phenotype, in SF blended scaffolds than that in the SF scaffold.

Conclusion

Addition of collagen and gelatin to SF solution not only improved the mechanical properties of the scaffolds but also provided an effective biomaterial constructs for chondrocyte growth and chondrocytic phenotype maintenance. Therefore, SF/C and SF/G showed a great potential as a desirable biomaterial for cartilage tissue engineering.

Effect of silk protein processing on drug delivery from silk films

Sericin removal from the core fibroin protein of silkworm silk is a critical first step in the use of silk for biomaterial-related applications, but degumming can affect silk biomaterial properties, including molecular weight, viscosity, diffusivity and degradation behavior. Increasing the degumming time (10, 30, 60, and 90 min) decreases the average molecular weight of silk protein in solution, silk solution viscosity, and silk film glass-transition temperature, and increases the rate of degradation of a silk film by protease. Model compounds spanning a range of physical-chemical properties generally show an inverse relationship between degumming time and release rate through a varied degumming time silk coating. Degumming provides a useful control point to manipulate silk's material properties.

Electronic and optoelectronic materials and devices inspired by nature

Inorganic semiconductors permeate virtually every sphere of modern human existence. Micro-fabricated memory elements, processors, sensors, circuit elements, lasers, displays, detectors, etc are ubiquitous. However, the dawn of the 21st century has brought with it immense new challenges, and indeed opportunities-some of which require a paradigm shift in the way we think about resource use and disposal, which in turn directly impacts our ongoing relationship with inorganic semiconductors such as silicon and gallium arsenide. Furthermore, advances in fields such as nano-medicine and bioelectronics, and the impending revolution of the 'ubiquitous sensor network', all require new functional materials which are bio-compatible, cheap, have minimal embedded manufacturing energy plus extremely low power consumption, and are mechanically robust and flexible for integration with tissues, building structures, fabrics and all manner of hosts. In this short review article we summarize current progress in creating materials with such properties. We focus primarily on organic and bio-organic electronic and optoelectronic systems derived from or inspired by nature, and outline the complex charge transport and photo-physics which control their behaviour. We also introduce the concept of electrical devices based upon ion or proton flow ('ionics and protonics') and focus particularly on their role as a signal interface with biological systems. Finally, we highlight recent advances in creating working devices, some of which have bio-inspired architectures, and summarize the current issues, challenges and potential solutions. This is a rich new playground for the modern materials physicist.

Antibiotic-Releasing Silk Biomaterials for Infection Prevention and Treatment

Effective treatment of infections in avascular and necrotic tissues can be challenging due to limited penetration into the target tissue and systemic toxicities. Controlled release polymer implants have the potential to achieve the high local concentrations needed while also minimizing systemic exposure. Silk biomaterials possess unique characteristics for antibiotic delivery including biocompatibility, tunable biodegradation, stabilizing effects, water-based processing and diverse material formats. We report on functional release of antibiotics spanning a range of chemical properties from different material formats of silk (films, microspheres, hydrogels, coatings). The release of penicillin and ampicillin from bulk-loaded silk films, drug-loaded silk microspheres suspended in silk hydrogels and bulk-loaded silk hydrogels was investigated and in vivo efficacy of ampicillin-releasing silk hydrogels was demonstrated in a murine infected wound model. Silk sponges with nanofilm coatings were loaded with gentamicin and cefazolin and release was sustained for 5 and 3 days, respectively. The capability of silk antibiotic carriers to sequester, stabilize and then release bioactive antibiotics represents a major advantage over implants and pumps based on liquid drug reservoirs where instability at room or body temperature is limiting. The present studies demonstrate that silk biomaterials represent a novel, customizable antibiotic platform for focal delivery of antibiotics using a range of material formats (injectable to implantable).

Drug loading and release on tumor cells using silk fibroin-albumin nanoparticles as carriers

Polymeric and biodegradable nanoparticles are frequently used in drug delivery systems. In this study silk fibroin-albumin blended nanoparticles were prepared using the desolvation method without any surfactant. These nanoparticles are easily internalized by the cells, reside within perinuclear spaces and act as carriers for delivery of the model drug methotrexate. Methotrexate loaded nanoparticles have better encapsulation efficiency, drug loading ability and less toxicity. The in vitro release behavior of methotrexate from the nanoparticles suggests that about 85% of the drug gets released after 12 days. The encapsulation and loading of a drug would depend on factors such as size, charge and hydrophobicity, which affect drug release. MTT assay and conjugation of particles with FITC demonstrate that the silk fibroin-albumin nanoparticles do not affect the viability and biocompatibility of cells. This blended nanoparticle, therefore, could be a promising nanocarrier for the delivery of drugs and other bioactive molecules.

Bioengineered silk proteins to control cell and tissue functions

Silks are defined as protein polymers that are spun into fibers by some lepidoptera larvae such as silkworms, spiders, scorpions, mites, and flies. Silk proteins are usually produced within specialized glands in these animals after biosynthesis in epithelial cells that line the glands, followed by secretion into the lumen of the gland prior to spinning into fibers.The most comprehensively characterized silks are from the domesticated silkworm (Bombyx mori) and from some spiders (Nephila clavipes and Araneus diadematus). Silkworm silk has been used commercially as biomedical sutures for decades and in textile production for centuries. Because of their impressive mechanical properties, silk proteins provide an important set of material options in the fields of controlled drug release, and for biomaterials and scaffolds for tissue engineering. Silkworm silk from B. mori consists primarily of two protein components, fibroin, the structural protein of silk fibers, and sericins, the water-soluble glue-like proteins that bind the fibroin fibers together. Silk fibroin consists of heavy and light chain polypeptides linked by a disulfide bond. Fibroin is the protein of interest for biomedical materials and it has to be purified/extracted from the silkworm cocoon by removal of the sericin. Characteristics of silks, including biodegradability, biocompatibility, controllable degradation rates, and versatility to generate different material formats from gels to fibers and sponges, have attracted interest in the field of biomaterials. Cell culture and tissue formation using silk-based biomaterials have been pursued, where appropriate cell adhesion, proliferation, and differentiation on or in silk biomaterials support the regeneration of tissues. The relative ease with which silk proteins can be processed into a variety of material morphologies, versatile chemical functionalization options, processing in water or solvent, and the related biological features of biocompatibility and enzymatic degradability make these proteins interesting candidates for biomedical applications.

Silk constructs for delivery of musculoskeletal therapeutics

Silk fibroin (SF) is a biopolymer with distinguishing features from many other bio- as well as synthetic polymers. From a biomechanical and drug delivery perspective, SF combines remarkable versatility for scaffolding (solid implants, hydrogels, threads, solutions), with advanced mechanical properties and good stabilization and controlled delivery of entrapped protein and small molecule drugs, respectively. It is this combination of mechanical and pharmaceutical features which renders SF so exciting for biomedical applications. This pattern along with the versatility of this biopolymer has been translated into progress for musculoskeletal applications. We review the use and potential of silk fibroin for systemic and localized delivery of therapeutics in diseases affecting the musculoskeletal system. We also present future directions for this biopolymer as well as the necessary research and development steps for their achievement.

Doxorubicin-loaded silk films: drug-silk interactions and in vivo performance in human orthotopic breast cancer

Breast cancer is the most common of all malignant diseases in women. Systemic chemotherapy provides low clinical benefit for locoregional control of the disease, while localised chemotherapy may provide a therapeutic advantage. In this study, doxorubicin-loaded silk films were directly applied to tumours. Affinity binding studies demonstrated that the adsorption of doxorubicin onto silk was partially dependent on crystallinity. By manipulating silk crystallinity, or β-sheet content, the doxorubicin release rate could be controlled ranging from immediate release to prolonged release over >4 weeks. The therapeutic impact of doxorubicin-loaded silk films on primary tumour growth and metastasis was assessed in mice using a humanised orthotopic breast cancer model (adenocarcinoma). Both soluble and stabilised silk films loaded with doxorubicin had a significantly greater primary tumour response than the equivalent dose of doxorubicin administered intravenously in the absence of the silk film carrier. In addition to reducing primary tumour growth, stabilised silk films loaded with doxorubicin also reduced metastatic spread and autopsy indicated that these films were not associated with any local or systemic toxicities. Collectively, these results suggest that the future use of this approach for localised chemotherapy is promising.

Silk materials--a road to sustainable high technology

This review addresses the use of silk protein as a sustainable material in optics and photonics, electronics and optoelectronic applications. These options represent additional developments for this technology platform that compound the broad utility and impact of this material for medical needs that have been recently described in the literature. The favorable properties of the material certainly make a favorable case for the use of silk, yet serve as a broad inspiration to further develop biological foundries for both the synthesis and processing of Nature's materials for technological applications.

Silk film culture system for in vitro analysis and biomaterial design

Silk films are promising protein-based biomaterials that can be fabricated with high fidelity and economically within a research laboratory environment ^1,2 . These materials are desirable because they possess highly controllable dimensional and material characteristics, are biocompatible and promote cell adhesion, can be modified through topographic patterning or by chemically altering the surface, and can be used as a depot for biologically active molecules for drug delivery related applications ^3-8 . In addition, silk films are relatively straightforward to custom design, can be designed to dissolve within minutes or degrade over years in vitro or in vivo, and are produce with the added benefit of being transparent in nature and therefore highly suitable for imaging applications ^9-13. The culture system methodology presented here represents a scalable approach for rapid assessments of cell-silk film surface interactions. Of particular interest is the use of surface patterned silk films to study differences in cell proliferation and responses of cells for alignment ^12,14 . The seeded cultures were cultured on both micro-patterned and flat silk film substrates, and then assessed through time-lapse phase-contrast imaging, scanning electron microscopy, and biochemical assessment of metabolic activity and nucleic acid content. In summary, the silk film in vitro culture system offers a customizable experimental setup suitable to the study of cell-surface interactions on a biomaterial substrate, which can then be optimized and then translated to in vivo models. Observations using the culture system presented here are currently being used to aid in applications ranging from basic cell interactions to medical device design, and thus are relevant to a broad range of biomedical fields.

Design and characterization of a silk-fibroin-based drug delivery platform using naproxen as a model drug

The objective of this proof-of-concept study was to develop a platform for controlled drug delivery based on silk fibroin (SF) and to explore the feasibility of using SF in oral drug delivery. The SF-containing matrixes were prepared via spray-drying and film casting, and the release profile of the model drug naproxen sodium was evaluated. Attenuated total reflectance Fourier transform infrared spectroscopy (FTIR) has been used to observe conformational changes in SF- and drug-containing compositions. SF-based films, spray-dried microparticles, and matrixes loaded with naproxen were prepared. Both FTIR spectra and in vitro dissolution data demonstrated that SF β-sheet conformation regulates the release profile of naproxen. The controlled release characteristics of the SF-containing compositions were evaluated as a function of SF concentration, temperature, and exposure to dehydrating solvents. The results suggest that SF may be an attractive polymer for use in controlled drug delivery systems.

Review physical and chemical aspects of stabilization of compounds in silk

The challenge of stabilization of small molecules and proteins has received considerable interest. The biological activity of small molecules can be lost as a consequence of chemical modifications, while protein activity may be lost due to chemical or structural degradation, such as a change in macromolecular conformation or aggregation. In these cases, stabilization is required to preserve therapeutic and bioactivity efficacy and safety. In addition to use in therapeutic applications, strategies to stabilize small molecules and proteins also have applications in industrial processes, diagnostics, and consumer products like food and cosmetics. Traditionally, therapeutic drug formulation efforts have focused on maintaining stability during product preparation and storage. However, with growing interest in the fields of encapsulation, tissue engineering, and controlled release drug delivery systems, new stabilization challenges are being addressed; the compounds or protein of interest must be stabilized during: (1) fabrication of the protein or small molecule-loaded carrier, (2) device storage, and (3) for the duration of intended release needs in vitro or in vivo. We review common mechanisms of compound degradation for small molecules and proteins during biomaterial preparation (including tissue engineering scaffolds and drug delivery systems), storage, and in vivo implantation. We also review the physical and chemical aspects of polymer-based stabilization approaches, with a particular focus on the stabilizing properties of silk fibroin biomaterials.

Robust and responsive silk ionomer microcapsules

We demonstrate the assembly of extremely robust and pH-responsive thin shell LbL microcapsules from silk fibroin counterparts modified with poly(lysine) and poly(glutamic) acid, which are based on biocompatible silk ionomer materials in contrast with usually exploited synthetic polyelectrolytes. The microcapsules are extremely stable in an unusually wide pH range from 1.5 to 12.0 and show a remarkable degree of reversible swelling/deswelling response in dimensions, as exposed to extreme acidic and basic conditions. These changes are accompanied by reversible variations in shell permeability that can be utilized for pH-controlled loading and unloading of large macromolecules. Finally, we confirmed that these shells can be utilized to encapsulate yeast cells with a viability rate much higher than that for traditional synthetic polyelectrolytes.

Effect of β-sheet crystalline content on mass transfer in silk films

The material properties of silk are favorable for drug delivery due to the ability to control material structure and morphology under ambient, aqueous processing conditions. Mass transport of compounds with varying physical-chemical characteristics was studied in silk fibroin films with control of β-sheet crystalline content. Two compounds, vitamin B12 and fluorescein isothiocynate (FITC) labeled lysozyme were studied in a diffusion apparatus to determine transport through silk films. The films exhibited size exclusion phenomenon with permeability coefficients with contrasting trends with increases in β-sheet crystallinity. The size exclusion phenomenon observed with the two model compounds was characterized by contrasting trends in permeability coefficients of the films as a function of β-sheet crystallinity. The diffusivity of the compounds was examined in the context of free volume theory. Apart from the β-sheet crystallinity, size of the compound and its interactions with silk influenced mass transfer. Diffusivity of vitamin B12 was modeled to define a power law relationship with β-sheet crystallinity. The results of the study demonstrate that diffusion of therapeutic agents though silk fibroin films can be directed to match a desired rate by modulating secondary structure of the silk proteins.

Materials fabrication from Bombyx mori silk fibroin

Silk fibroin, derived from Bombyx mori cocoons, is a widely used and studied protein polymer for biomaterial applications. Silk fibroin has remarkable mechanical properties when formed into different materials, demonstrates biocompatibility, has controllable degradation rates from hours to years and can be chemically modified to alter surface properties or to immobilize growth factors. A variety of aqueous or organic solvent-processing methods can be used to generate silk biomaterials for a range of applications. In this protocol, we include methods to extract silk from B. mori cocoons to fabricate hydrogels, tubes, sponges, composites, fibers, microspheres and thin films. These materials can be used directly as biomaterials for implants, as scaffolding in tissue engineering and in vitro disease models, as well as for drug delivery.

Silk fibroin and polyethylene glycol-based biocompatible tissue adhesives

Tissue sealants have emerged in recent years as strong candidates for hemostasis. A variety of formulations are currently commercially available and though they satisfy many of the markets' needs there are still key aspects of each that need improvement. Here we present a new class of blends, based on silk fibroin and chemically active polyethylene glycols (PEGs) with strong adhesive properties. These materials are cytocompatible, crosslink within seconds via chemical reaction between thiols and maleimides present on the constituent PEGs and have the potential to further stabilize through β-sheet formation by silk. Based on the silk concentration in the final formulation, the adhesive properties of these materials are comparable or better than the current leading PEG-based sealant. In addition, the silk-PEG based materials show decreased swelling and longer degradation times. Such properties would make them suitable for applications for which the current sealants are contraindicated.

Silk fibroin biomaterials for controlled release drug delivery

INTRODUCTION

Given the benefits of polymer drug delivery implants over traditional periodic systemic administration, the development of biomaterial systems with the necessary properties (biocompatibility, degradation, stabilization, controllability) is paramount. Silk fibroin represents a promising, naturally derived polymer for local, controlled, sustained drug release from fully degrading implants and the polymer can be processed into a broad array of material formats.

AREAS COVERED

This review provides an overview of silk biomaterials for drug delivery, especially those that can function as long-term depots. Fundamentals of structure and assembly, processing options, control points and specific examples of implantable silk drug delivery systems (sponges, films) and injectable systems (microspheres, hydrogels) from the 1990s and onwards are reviewed.

EXPERT OPINION

Owing to its unique material properties, stabilization effects and tight controllability, silk fibroin is a promising biomaterial for implantable and injectable drug delivery applications. Many promising control points have been identified, and characterization of the relationships between silk processing and/or material properties and the resulting drug loading and release kinetics will ultimately enhance the overall utility of this unique biomaterial. The ever-expanding biomaterial 'tool kit' that silk provides will eventually allow the simultaneous optimization of implant structure, material properties and drug release behavior that is needed to maximize the cost-efficiency, convenience, efficacy and safety of many new and existing therapeutics, especially those that cannot be delivered by means of traditional administration approaches.

Precious metal recovery by selective adsorption using biosorbents

Silk sericin and chitosan biosorbents are low cost and highly efficient biosorbents derived from waste biomass. Both biosorbents displayed good capacity and excellent selectivity for gold adsorption. Silk sericin and chitosan adsorbed respectively 1 and 3.3mmolg(-1) of gold and have K(d) values of 450 and 34,000, respectively. Experimental evidence showed that gold adsorbed on the amide groups of the silk sericin, while gold and copper adsorbed on the amino groups of chitosan via charge-interactions and complexation. Binary (Au-Cu), five (Au-Co-Ni-Cu-Zn) and six (Au-Pd-Co-Ni-Cu-Zn) component separations consistently showed that silk sericin has better selectivity (Sel(Au)>2.4) than chitosan. It is possible to recover gold at 99.5% purity by silk sericin and 90% if the solution contained palladium.

Immobilisation of bifenthrin for termite control

BACKGROUND

Termites are worldwide pests causing considerable damage to agriculture, forestry and buildings. While various approaches have been tried to eliminate termite populations, the relevant toxicants are associated with certain risks to the environment and human health.

RESULTS

In this study, to combine the merits of effective chemical control by bifenthrin and a drug photoimmobilisation technique, silk fibroin was used as a carrier to embed bifenthrin, which was then photoactively immobilised by ultraviolet treatment on the surface of wood (cellulose). The immobilised bifenthrin embedded in the photoactive silk fibroin was characterised by Fourier transform infrared spectroscopy (FTIR), ultraviolet absorption spectroscopy (UV), fluorescence measurement and CHN analysis. The surface structures and biological activity were examined by scanning electron microscopy (SEM), atomic force microscopy (AFM), electron spectroscopy for chemical analysis (ESCA) and bioassays respectively.

CONCLUSIONS

The results indicate that the embedded and immobilised bifenthrin has been very well protected from free release and has a long-term stability allowing slow release with a high efficiency against termites at a low dose of 1.25 µg cm(-2). This study provides a novel and environmentally benign technique for termite control by photoimmobilising silk-fibroin-embedded bifenthrin on the surface of materials that are otherwise easily attacked by termites.

Mechanisms of controlled release from silk fibroin films

The controlled release of fluorescein-iso-thio-cyanate (FITC)-labeled dextrans from methanol-treated and untreated silk fibroin films was modeled to characterize the release kinetics and mechanisms. Silk films were prepared with FITC-dextrans of various molecular weights (4, 10, 20, 40 kDa). Methanol treatment was used to promote crystallinity. The release data were assessed with two different models, an empirical exponential equation commonly fit to release data and a mechanism-based semiempirical model derived from Fickian diffusion through a porous film. The FITC-dextran release kinetics were evaluated as a function of molecular weight and compared between the untreated- and methanol-treated films. For the empirical model, the estimated values of the model parameters decreased with the molecular weight of the analyte and showed no significant difference between untreated- and methanol-treated films. For the diffusion-based model, the estimated diffusion coefficient was smaller for the methanol-treated films than for the untreated films. Also, the diffusion coefficient was observed to decrease linearly with increasing molecular weight of the analyte. The percent of FITC-dextran loading entrapped and not released was less for the methanol-treated films than for untreated films and linearly increased with molecular weight. A linear regression was fit to the relationship between molecular weight and the percent of entrapped FITC-dextran particles. Using these defined linear relationships, we present an updated version of the diffusion model for simulating release of FITC-dextran of varied molecular weights from methanol-treated and untreated silk films.

Protein-based block copolymers

Advances in genetic engineering have led to the synthesis of protein-based block copolymers with control of chemistry and molecular weight, resulting in unique physical and biological properties. The benefits from incorporating peptide blocks into copolymer designs arise from the fundamental properties of proteins to adopt ordered conformations and to undergo self-assembly, providing control over structure formation at various length scales when compared to conventional block copolymers. This review covers the synthesis, structure, assembly, properties, and applications of protein-based block copolymers.

Preparation of 3D fibroin/chitosan blend porous scaffold for tissue engineering via a simplified method

In this work, we developed a simple and flexible method to manufacture a 3D porous scaffold based on the blend of regenerated silk fibroin (RSF) and chitosan (CS). No crosslinker or other toxic reagents were used in this method. The pores of resulted 3D scaffolds were connected with each other, and their sizes could be easily controlled by the concentration of the mixed solution. Compared with pure RSF scaffolds, the water absorptivities of these RSF/CS blend scaffolds with significantly enhanced mechanical properties were greatly increased. The results of MTT and RT-PCR tests indicated that the chondrocytes grew very well in these blend RSF/CS porous scaffolds. This suggested that the RSF/CS blend scaffold prepared by this new method could be a promising candidate for applications in tissue engineering.

Preparation and Characterization of Silk Fibroin Microspheres

An improved method of water-in-oil-in-water (w1/o/w2) multi-emulsion was developed to prepare silk fibroin (SF) microspheres that had improved drug-loading amount and better drug-loading efficiency. SF and dexamethasone sodium phosphate (DSP) were used as one water phase(w1), liquid paraffin was used as oil phase and isopropanol was used as the other water phase(w2). In the preparation process, isopropanol was also used to induce the crystallization of SF. The SF microsphere was mainly composed of silk II proteins. The drug-loading amount of SF microsphere ranged from 28.45 µg/mg to 79.21 µg/mg, and the drug-loading efficiency varied from 30.80% to 87.16%. The spherical morphology of the microsphere was observed by SEM. The particle diameter was measured by the Laser particle sizer and their average sizes varied from 7.41 µm to 66.10 µm. The drug releasing profile of the microspheres was measured in vitro. The burst release was significant when isopropanol-to-SF ratio was lower than 3:1，although the drug-loading amount and the drug-loading efficiency was high; It showed a higher drug-loading amount and efficiency, as well as an obvious continuous release effect when the ratio was higher than 4:1; It also showed a certain continuous release effect when the ratio was 4:1.

Incorporation of proteinase inhibitors into silk-based delivery devices for enhanced control of degradation and drug release

Controlling the rate of silk degradation is critical to its potential use in biomedical applications, including drug delivery and tissue engineering. The effect of protease concentration on accelerating degradation, and the use of ethylenediamine tetraacetic acid (EDTA) on reducing rates of degradation and on drug release from silk-based drug carriers was studied. Increased rates of proteolysis resulted in increased dye release from silk carriers, while EDTA release from the silk carriers inhibited proteolysis. The sustained release of EDTA from silk carriers in combination with the release of the small molecule anti-convulsant adenosine was investigated in vitro. This combination of factors resulted in delayed release of adenosine by inhibiting proteolytic activity. These results introduce a promising strategy to control drug delivery through the regulation of silk degradation rate, achieved via manipulation of local proteolytic activity. This ability to modulate enzyme function could be applicable to a range of silk biomaterial formats as well as other biodegradable polymers where enzymatic functions control biomaterial degradation and drug release rates.

pH-Sensitive ionomeric particles obtained via chemical conjugation of silk with poly(amino acid)s

Silk-fibroin-based biomaterials have been widely utilized for a range of biomaterial-related systems. For all these previously reported systems, the β-sheet forming feature of the silk was the key stabilizing element of the final material structure. Herein, we describe a different strategy, based on the engineering of silk-based ionomers that can yield stable colloidal composites or particle suspensions through electrostatic interactions. These silk-based ionomers were obtained by carbodiimide-mediated coupling of silk fibroin with polylysine hydrobromide and polyglutamic acid sodium salts, respectively. Colloidal composites could be obtained by mixing the ionomeric pair at high concentration (i.e., 25% w/v), while combining them at lower concentrations (i.e., 5% w/v) yielded particle suspensions. The assembly of the ionomers was driven by electrostatic interactions, pH-dependent, and reversible. The network assembly appeared to be polarized, with the interacting poly(amino acid) chains clustered to the core of the particles and the silk backbone oriented outward. In agreement with this assembly mode, doxorubicin, a hydrophilic antitumor drug, could be released at a slow rate, in a pH-dependent manner, indicating that the inside of the ionomeric particles was mainly hydrophilic in nature.