Immobilization of biocatalysts with bombyx mori silk fibroin by several kinds of physical treatment and its application to glucose sensors

Structure of silk I (Bombyx mori silk fibroin before spinning) in the dry and hydrated states studied using 13C solid-state NMR spectroscopy

Nowadays, much attention has been paid to Bombyx mori silk fibroin (SF) by many researchers because of excellent physical properties and biocompatibility. These superior properties originate from the structure of SF and therefore, the structural analysis is a key to clarify the superiority. Here we concentrated on silk I structure (SF structure before spinning). We showed that silk I* (the structure of (GAGAGS)_n which is a main part of SF) is a repeated type II β-turn, neither α-helix nor random coil, from the conformation-dependent ¹³C NMR chemical shift data. This conclusion is different from that obtained using IR by many researchers. Next, the formation of silk I* structure was investigated at molecular level using ¹³C solid-state NMR spectroscopy. Three kinds of ¹³C INEPT, CP/MAS and DD/MAS NMR spectra were observed for SF, [3-¹³C] Ser- and [3-¹³C] Tyr-SF, the crystalline fraction obtained by chymotrypsin treatment of SF and their model peptide with silk I structures in the dry and hydrated states. Especially, the presence of the sequences containing Tyr, (((GX)_m1GY)_m2 where X = A or V) with random coil conformations adjacent to (GAGAGS)_n is an essence to get water-soluble SF and the formation of silk I* structure of (GAGAGS)_n.

Stabilization of Salivary Biomarkers

The use of saliva as a diagnostic biofluid has been increasing in recent years, thanks to the identification and validation of new biomarkers and improvements in test accuracy, sensitivity, and precision that enable the development of new noninvasive and cost-effective devices. However, the lack of standardized methods for sample collection, treatment, and storage contribute to the overall variability and lack of reproducibility across analytical evaluations. Furthermore, the instability of salivary biomarkers after sample collection hinders their translation into commercially available technologies for noninvasive monitoring of saliva in home settings. The present review aims to highlight the status of research on the challenges of collecting and using diagnostic salivary samples, emphasizing the methodologies used to preserve relevant proteins, hormones, genomic, and transcriptomic biomarkers during sample handling and analysis.

Developments of the Electroactive Materials for Non-Enzymatic Glucose Sensing and Their Mechanisms

A comprehensive review of the electroactive materials for non-enzymatic glucose sensing and sensing devices has been performed in this work. A general introduction for glucose sensing, a facile electrochemical technique for glucose detection, and explanations of fundamental mechanisms for the electro-oxidation of glucose via the electrochemical technique are conducted. The glucose sensing materials are classified into five major systems: (1) mono-metallic materials, (2) bi-metallic materials, (3) metallic-oxide compounds, (4) metallic-hydroxide materials, and (5) metal-metal derivatives. The performances of various systems within this decade have been compared and explained in terms of sensitivity, linear regime, the limit of detection (LOD), and detection potentials. Some promising materials and practicable methodologies for the further developments of glucose sensors have been proposed. Firstly, the atomic deposition of alloys is expected to enhance the selectivity, which is considered to be lacking in non-enzymatic glucose sensing. Secondly, by using the modification of the hydrophilicity of the metallic-oxides, a promoted current response from the electro-oxidation of glucose is expected. Lastly, by taking the advantage of the redistribution phenomenon of the oxide particles, the usage of the noble metals is foreseen to be reduced.

Silk Fibroin-Based Materials for Catalyst Immobilization

Silk fibroin is a widely and commercially available natural protein derived from silkworm cocoons. Thanks to its unique amino acid composition and structure, which lead to localized nanoscale pockets with limited but sufficient hydration for protein interaction and stabilization, silk fibroin has been studied in the field of enzyme immobilization. Results of these studies have demonstrated that silk fibroin offers an important platform for covalent and noncovalent immobilization of enzymes through serving as a stabilization matrix/support with high retention of the biological activity of the enzymes of interest. In the hope of providing suggestions for potential future research directions, this review has been written to briefly introduce and summarize key advances in silk fibroin-based materials for immobilization of both enzymes/biocatalysts (including alkaline phosphatase, β-glucosidase, glucose oxidase, lipase, urease, uricase, horseradish peroxidase, catalase, xanthine oxidase, tyrosinase, acetylcholinesterase, neutral protease, α-chymotrypsin, amylase, organophosphorus hydrolase, β-galactosidase, carbonic anhydrase, laccase, zymolyase, phenylalanine ammonia-lyase, thymidine kinase, and several others) and non-enzymatic catalysts (such as Au, Pd, Fe, α-Fe2O3, Fe3O4, TiO2, Pt, ZnO, CuO, Cu2O, Mn3O4, and MnO2).

Silk Fibroin: An Emerging Biocompatible Material for Application of Enzymes and Whole Cells in Bioelectronics and Bioanalytical Sciences

Enzymes and whole cells serve as the active biological entities in a myriad of applications including bioprocesses, bioanalytics, and bioelectronics. Conserving the natural activity of these functional biological entities during their prolonged use is one of the major goals for validating their practical applications. Silk fibroin (SF) has emerged as a biocompatible material to interface with enzymes as well as whole cells. These biomaterials can be tailored both physically and chemically to create excellent scaffolds of different forms such as fibers, films, and powder for immobilization and stabilization of enzymes. The secondary structures of the SF-protein can be attuned to generate hydrophobic/hydrophilic pockets suitable to create the biocompatible microenvironments. The fibrous nature of the SF protein with a dominant hydrophobic property may also serve as an excellent support for promoting cellular adhesion and growth. This review compiles and discusses the recent literature on the application of SF as a biocompatible material at the interface of enzymes and cells in various fields, including the emerging area of bioelectronics and bioanalytical sciences.

Characteristics of an Extended Gate Field-Effect Transistor for Glucose Sensing Using an Enzyme-Containing Silk Fibroin Membrane as the Bio-Chemical Component

The characteristics of a glucose sensor based on an ion-sensitive TiO2/Ti extended gate electrode field-effect transistor (EGFET) are reported. A glucose oxidase-containing silk fibroin membrane was immobilized on a TiO2/Ti surface as the bio-sensing component. This EGFET-type biosensor was estimated to be able to detect a glucose concentration as low as 0.001 mg/mL in an aqueous electrolyte, which enables the sensing of glucose in the saliva and sweat. The endurance of this sensor was also examined, and it was found that the retention time of the original sensitivity for repeated use at room temperature was more than 30 days, with a high heat tolerance temperature close to 60 °C.

Incorporation of SPION-casein core-shells into silk-fibroin nanofibers for cardiac tissue engineering

Mimicking the structure of extracellular matrix (ECM) of myocardium is necessary for fabrication of functional cardiac tissue. The superparamagnetic iron oxide nanoparticles (SPIONs, Fe₃ O₄ ), as new generation of magnetic nanoparticles (NPs), are highly intended in biomedical studies. Here, SPION NPs (1 wt%) were synthesized and incorporated into silk-fibroin (SF) electrospun nanofibers to enhance mechanical properties and topography of the scaffolds. Then, the mouse embryonic cardiac cells (ECCs) were seeded on the scaffolds for in vitro studies. The SPION NPs were studied by scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscope (TEM). SF nanofibers were characterized after incorporation of SPIONs by SEM, TEM, water contact angle measurement, and tensile test. Furthermore, cytocompatibility of scaffolds was confirmed by 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. SEM images showed that ECCs attached to the scaffolds with elongated morphologies. Also, the real-time PCR and immunostaining studies approved upregulation of cardiac functional genes in ECCs seeded on the SF/SPION-casein scaffolds including GATA-4, cardiac troponin T, Nkx 2.5, and alpha-myosin heavy chain, compared with the ones in SF. In conclusion, incorporation of core-shells in SF supports cardiac differentiation, while has no negative impact on ECCs' proliferation and self-renewal capacity.

Silk Fibroin as a Functional Biomaterial for Drug and Gene Delivery

Silk is a natural polymer with unique physicochemical and mechanical properties which makes it a desirable biomaterial for biomedical and pharmaceutical applications. Silk fibroin (SF) has been widely used for preparation of drug delivery systems due to its biocompatibility, controllable degradability and tunable drug release properties. SF-based drug delivery systems can encapsulate and stabilize various small molecule drugs as well as large biological drugs such as proteins and DNA to enhance their shelf lives and control the release to enhance their circulation time in the blood and thus the duration of action. Understanding the properties of SF and the potential ways of manipulating its structure to modify its physicochemical and mechanical properties allows for preparation of modulated drug delivery systems with desirable efficacies. This review will discuss the properties of SF material and summarize the recent advances of SF-based drug and gene delivery systems. Furthermore, conjugation of the SF to other biomolecules or polymers for tissue-specific drug delivery will also be discussed.

Advanced Silk Fibroin Biomaterials and Application to Small-Diameter Silk Vascular Grafts

As the incidences of cardiovascular diseases have been on the rise in recent years, the need for small-diameter artificial vascular grafts is increasing globally. Although synthetic polymers such as expanded polytetrafluoroethylene or poly(ethylene terephthalate) have been successfully used for artificial vascular grafts ≥6 mm in diameter, they fail at smaller diameters (<6 mm) due to thrombus formation and intimal hyperplasia. Thus, development of vascular grafts for small diameter vessel replacement that are <6 mm in diameter remains a major clinical challenge. Silk fibroin (SF) from Bombyx mori silkworm is well-known as an excellent textile and also has been used as suture material in surgery for more than 2000 years. Many attempts to develop small-diameter SF vascular grafts with <6 mm in diameter have been reported. Here, research and development in small-diameter vascular grafts with SF are reviewed as follows: (1) the heterogeneous structure of SF fiber (Silk II), including the packing arrangements and type II β-turn structure of SF (Silk I*) before spinning; (2) SF modified by transgenic silkworm, which is more suitable for vascular grafts; (3) preparation of small-diameter SF vascular grafts; (4) characterization of SF in the hydrated state, including dynamics of water molecules by nuclear magnetic resonance; and (5) evaluation of the SF grafts by in vivo implantation experiment. According to the findings, SF is a promising material for small-diameter vascular graft development.

Practical insights on enzyme stabilization

Enzymes are efficient catalysts designed by nature to work in physiological environments of living systems. The best operational conditions to access and convert substrates at the industrial level are different from nature and normally extreme. Strategies to isolate enzymes from extremophiles can redefine new operational conditions, however not always solving all industrial requirements. The stability of enzymes is therefore a key issue on the implementation of the catalysts in industrial processes which require the use of extreme environments that can undergo enzyme instability. Strategies for enzyme stabilization have been exhaustively reviewed, however they lack a practical approach. This review intends to compile and describe the most used approaches for enzyme stabilization highlighting case studies in a practical point of view.

13C NMR characterization of hydrated 13C labeled Bombyx mori silk fibroin sponges prepared using glycerin, poly(ethylene glycol diglycidyl ether) and poly(ethylene glycol) as porogens

There is a need to prepare softer and highly flexible Bombyx mori silk fibroin (SF) sponges for the development of biomaterials that are biodegradable and with stiffness that matches sponges and soft tissues. In this paper, we prepared SF sponges using glycerin (Glyc), poly(ethylene)glycol diglycidyl ether (PGDE) and poly(ethylene)glycol (PEG) as porogens. The detailed characterization of the hydrated SF sponges was done using three ¹³C solid state NMR techniques, viz.,¹³C refocused insensitive nuclei enhanced by polarization transfer (r-INEPT) NMR, ¹³C cross polarization/magic angle spinning (CP/MAS) NMR, and ¹³C dipolar decoupled-magic angle spinning (DD/MAS) NMR. These three NMR methods gave respective information on fast motion, slow motion, and both fast and slow motions for the local structure and dynamics of the hydrated SF sponges. There was no significant difference in the r-INEPT spectra of the three hydrated SF sponges. On the other hand, there were significant differences among the ¹³C CP/MAS NMR spectra of the three sponges. The fractions of two kinds of β-sheet structure, two kinds of random coil conformations with mobile and immobile motions, and the Silk I* (type II β-turn) conformation were determined for the Ser residues in the ¹³C DD/MAS NMR spectra. Similarly, the fractions of several conformations were also determined for Tyr, Ala and Gly residues in SF, which showed significant differences among the three hydrated sponges. The relationship between the local structure of these hydrated SF sponges and their mechanical properties was also briefly discussed.

Fabrication and Optimization of Stable, Optically Transparent, and Reusable pH-Responsive Silk Membranes

The fabrication of silk-based membranes that are stable, optically transparent and reusable is yet to be achieved. To address this bottleneck we have developed a method to produce transparent chromogenic silk patches that are optically responsive to pH. The patches were produced by blending regenerated silk fibroin (RSF), Laponite RD (nano clay) and the organic dyes neutral red and Thionine acetate. The Laponite RD played a central role in the patch mechanical integrity and prevention of dye leaching. The process was optimized using a factorial design to maximize the patch response to pH by UV absorbance and fluorescence emission. New patches of the optimized protocol, made from solutions containing 125 μM neutral red or 250 μM of Thionine and 15 mg/mL silk, were further tested for operational stability over several cycles of pH altering. Stability, performance, and reusability were achieved over the tested cycles. The approach could be extended to other reporting molecules or enzymes able to bind to Laponite.

Trilayered sulfated silk fibroin vascular grafts enhanced with braided silk tube

The scaffold component is a major barrier to the development of a clinically useful small-diameter tissue-engineered vascular graft. Scaffold requirements include matching the mechanical and structural properties with those of native vessels and optimizing the microenvironment for cell integration, adhesion, and growth. Trilayered sulfated silk fibroin graft was developed to mimic native tissue structure and function. Physical properties and cell studies were assessed to evaluate the viability of their usage in small-diameter tissue-engineered vascular grafts. Compared with previously fabricated silk fibroin vascular grafts, these trilayered grafts provided comparable water permeability, tensile strength, burst pressure, as well as suture retention strength, to saphenous veins for vascular grafts. In addition, the in vitro results showed good cytocompatibility of the trilayered grafts. These physical and cellular outcomes indicate potential utility of these trilayered sulfated silk fibroin grafts for small-diameter vascular grafts.

Development of Small-Diameter Vascular Grafts Based on Silk Fibroin Fibers from Bombyx mori for Vascular Regeneration

In the field of surgical revascularization, the need for functional small-diameter (1.5-4.0 mm in diameter) vascular grafts is increasing. Several synthetic biomaterials have been tested for this purpose, but in many cases they cause thrombosis. In this study, we report the development of small-diameter vascular grafts made from silk fibroin fibers from the domestic silkworm Bombyx mori or recombinant silk fibroin fibers from a transgenic silkworm. The vascular grafts were prepared by braiding, flattening and winding the silk fibers twice onto a cylindrical polymer tube followed by coating with an aqueous silk fibroin solution. The grafts, which are 1.5 mm in inner diameter and 10 mm in length, were implanted into rat abdominal aorta. An excellent patency (ca. 85%, n= 27) at 12 months after grafting with wild-type silk fibers was obtained. Endothelial cells and smooth muscle cells migrated into the silk fibroin graft early after implantation, and became organized into an endothelium and a media-like smooth muscle layer.

Silk-based stabilization of biomacromolecules

Silk fibroin is a high molecular weight amphiphilic protein that self-assembles into robust biomaterials with remarkable properties including stabilization of biologicals and tunable release kinetics correlated to processing conditions. Cells, antibiotics,monoclonal antibodies and peptides, among other biologics, have been encapsulated in silk using various processing approaches and material formats. The mechanistic basis for the entrapment and stabilization features, along with insights into the modulation of release of the entrained compounds from silks will be reviewed with a focus on stabilization of bioactive molecules.

Introduction of VEGF or RGD sequences improves revascularization properties of Bombyx mori silk fibroin produced by transgenic silkworm

Transgenic silk fibroins incorporated the VEGF and RGD were prepared. The VEGF SF showed lower platelet adhesion than the RGD SF and WT SF. An in vivo implantation study supported these in vitro results.

Silk-based biomaterials for sustained drug delivery

Silk presents a rare combination of desirable properties for sustained drug delivery, including aqueous-based purification and processing options without chemical cross-linkers, compatibility with common sterilization methods, controllable and surface-mediated biodegradation into non-inflammatory by-products, biocompatibility, utility in drug stabilization, and robust mechanical properties. A versatile silk-based toolkit is currently available for sustained drug delivery formulations of small molecule through macromolecular drugs, with a promise to mitigate several drawbacks associated with other degradable sustained delivery technologies in the market. Silk-based formulations utilize silk's well-defined nano- through microscale structural hierarchy, stimuli-responsive self-assembly pathways and crystal polymorphism, as well as sequence and genetic modification options towards targeted pharmaceutical outcomes. Furthermore, by manipulating the interactions between silk and drug molecules, near-zero order sustained release may be achieved through diffusion- and degradation-based release mechanisms. Because of these desirable properties, there has been increasing industrial interest in silk-based drug delivery systems currently at various stages of the developmental pipeline from pre-clinical to FDA-approved products. Here, we discuss the unique aspects of silk technology as a sustained drug delivery platform and highlight the current state of the art in silk-based drug delivery. We also offer a potential early development pathway for silk-based sustained delivery products.

Electron transport and bulk-like behavior of Wiedemann-Franz law for sub-7 nm-thin iridium films on silkworm silk

For ultrathin metallic films, either supported or free-standing, the inside nanocrystalline nature significantly reduces the electron and thermal transport. Quantum mechanical reflection of electrons at the grain boundary reduces the electrical conductivity further than the thermal conductivity, leading to a Lorenz number in the order of 7.0 × 10(-8) W Ω K(-2), much higher than that of the bulk counterpart. We report on a finding that for ultrathin (0.6-6.3 nm) iridium films coated on degummed silkworm silk fibroin, the electron transport is around 100-200% higher than that of the same film on glass fiber, even though the grain size of Ir film on silkworm silk is smaller than that on glass fiber. At the same time, the thermal conductivity of the Ir film is smaller or close to that of the film on glass fiber. Its Lorenz number is found close to that of bulk crystalline Ir despite the nanocrystalline structure in the Ir films. This is similar to the behavior of metallic glasses. Our study of gold films on silkworm silk reveals the same trend of change as compared to that on glass fiber. Electron hopping and tunneling in silkworm silk is speculated to be responsible for the observed electron transport. The finding points out that silk could provide a better substrate for flexible electronics with significantly faster electron transport.

Thermally induced increase in energy transport capacity of silkworm silks

This work reports on the first study of thermally induced effect on energy transport in single filaments of silkworm (Bombyx mori) fibroin degummed mild (type 1), moderate (type 2), to strong (type 3). After heat treatment from 140 to 220°C, the thermal diffusivity of silk fibroin type 1, 2, and 3 increases up to 37.9, 20.9, and 21.5%, respectively. Our detailed scanning electron microscopy study confirms that the sample diameter change is almost negligible before and after heat treatment. Raman analysis is performed on the original and heat-treated (at 147°C) samples. After heat treatment at 147°C, the Raman peaks at 1081, 1230, and 1665 cm(-1) become stronger and narrower, indicating structural transformation from amorphous to crystalline. A structure model composed of amorphous, crystalline, and laterally ordered regions is proposed to explain the structural change by heat treatment. Owing to the close packing of more adjacent laterally ordered regions, the number and size of the crystalline regions of Bombyx mori silk fibroin increase by heat treatment. This structure change gives the observed significant thermal diffusivity increase by heat treatment.

Isolation and biochemical characterization of a glucose dehydrogenase from a hay infusion metagenome

Glucose hydrolyzing enzymes are essential to determine blood glucose level. A high-throughput screening approach was established to identify NAD(P)-dependent glucose dehydrogenases for the application in test stripes and the respective blood glucose meters. In the current report a glucose hydrolyzing enzyme, derived from a metagenomic library by expressing recombinant DNA fragments isolated from hay infusion, was characterized. The recombinant clone showing activity on glucose as substrate exhibited an open reading frame of 987 bp encoding for a peptide of 328 amino acids. The isolated enzyme showed typical sequence motifs of short-chain-dehydrogenases using NAD(P) as a co-factor and had a sequence similarity between 33 and 35% to characterized glucose dehydrogenases from different Bacillus species. The identified glucose dehydrogenase gene was expressed in E. coli, purified and subsequently characterized. The enzyme, belonging to the superfamily of short-chain dehydrogenases, shows a broad substrate range with a high affinity to glucose, xylose and glucose-6-phosphate. Due to its ability to be strongly associated with its cofactor NAD(P), the enzyme is able to directly transfer electrons from glucose oxidation to external electron acceptors by regenerating the cofactor while being still associated to the protein.

Small-diameter silk vascular grafts (3 mm diameter) with a double-raschel knitted silk tube coated with silk fibroin sponge

Small-diameter (less than 6 mm in diameter) vascular grafts are highly desirable due to the large demand for surgical revascularization; however, there are no available artificial grafts. Vascular grafts of 1.5 mm diameter prepared by our group with silk fibroin fiber have been proved to be excellent grafts with remarkably high patency and remodeling, based on rat implantation experiment (Enomoto et al., 2010). In this study, a silk fibroin vascular graft with 3 mm diameter which can be used for the coronary arteries or lower extremity arteries is prepared with a double-raschel knitted Bombyx mori silk fiber tube coated with B. mori silk fibroin sponge. Here the silk sponge is prepared from an aqueous solution of the silk fibroin and poly(ethylene) glycol diglycidyl ether as porogen. Sufficient strength, proper elasticity, and protection from loose ends in the implantation process are obtained for the silk fibroin graft; low water permeability and relatively large compliance are also attained. These excellent physical properties make silk fibroin grafts suitable to be implanted in a canine model.

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.

Macro/microporous silk fibroin scaffolds with potential for articular cartilage and meniscus tissue engineering applications

This study describes the developmental physicochemical properties of silk fibroin scaffolds derived from high-concentration aqueous silk fibroin solutions. The silk fibroin scaffolds were prepared with different initial concentrations (8, 10, 12 and 16%, in wt.%) and obtained by combining the salt-leaching and freeze-drying methodologies. The results indicated that the antiparallel β-pleated sheet (silk-II) conformation was present in the silk fibroin scaffolds. All the scaffolds possessed a macro/microporous structure. Homogeneous porosity distribution was achieved in all the groups of samples. As the silk fibroin concentration increased from 8 to 16%, the mean porosity decreased from 90.8±0.9 to 79.8±0.3% and the mean interconnectivity decreased from 97.4±0.5 to 92.3±1.3%. The mechanical properties of the scaffolds exhibited concentration dependence. The dry state compressive modulus increased from 0.81±0.29 to 15.14±1.70 MPa and the wet state dynamic storage modulus increased by around 20- to 30-fold at each testing frequency when the silk fibroin concentration increased from 8 to 16%. The water uptake ratio decreased with increasing silk fibroin concentration. The scaffolds present favorable stability as their structure integrity, morphology and mechanical properties were maintained after in vitro degradation for 30 days. Based on these results, the scaffolds developed in this study are proposed to be suitable for use in meniscus and cartilage tissue-engineered scaffolding.

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.

Formation of Silk Monolayers

Cast silk membranes exhibit useful properties. However, there is limited control over the molecular architecture in these structures. The Langmuir-Blodgett technique can enhance the control of the membrane structure and allow improved control over membrane properties. We have formed natural silk monolayers using the Langmuir technique. Silk fibroin, regenerated from Bombvx mori cocoons, formed stable monolayers evident from pressure/area isotherms. Multilayers of the silk fibroin monolayers were deposited on a number of substrates and characterized. Transmission Electron Microscopy (TEM) and ellipsometry data provide basic information about the physical characteristics of the monolayer. Preliminary analysis of electron diffraction data of the monolayer indicate polycrystalline structure, consistent with the known structure of silk. Infrared spectrometric analysis of the monolayer using Attenuated Total Reflectance (ATR) gave wavenumbers for Amide I, II, III and V bands which compare with the silk II conformation reported for cast silk membranes.

Stabilization of enzymes in silk films

Material systems are needed that promote stabilization of entrained molecules, such as enzymes or therapeutic proteins, without destroying their activity. We demonstrate that the unique structure of silk fibroin protein, when assembled into the solid state, establishes an environment that is conducive to the stabilization of entrained proteins. Enzymes (glucose oxidase, lipase, and horseradish peroxidase) entrapped in these films over 10 months retained significant activity, even when stored at 37 degrees C, and in the case of glucose oxidase did not lose any activity. Further, the mode of processing of the silk protein into the films could be correlated to the stability of the enzymes. The relationship between processing and stability offers a large suite of conditions within which to optimize such stabilization processes. Overall, the techniques reported here result in materials that stabilize enzymes to an extent, without the need for cryoprotectants, emulsifiers, covalent immobilization, or other treatments. Further, these systems are amenable to optical applications and characterization, environmental distribution without refrigeration, are ingestible, and offer potential use in vivo, because silk materials are biocompatible and FDA approved, degradable with proteases, and currently used in biomedical devices.

Characterization of amino acids in silk sericin protein fromGonometa rufobrunnae by MEKC with phenyl isothiocyanate derivatization

An MEKC method was developed for the separation and characterization of phenyl-isothiocyanate (PITC)-labeled amino acids derived from Gonometa rufobrunnae silkworm after microdialysis sample cleanup. The influence of the buffer and SDS concentration on the resolution of the amino acids was investigated. A buffer system consisting of 25 mM phosphate, 10 mM borate buffer at pH 9.00, and 70 mM SDS showed the best results, with 13 PITC-amino acid derivatives being resolved out of 15 possible amino acids that were under study. Microdialysis sampling demonstrated its efficiency as a sample cleanup technique. Sericin protein from G. rufobrunnae was found to be characterized by at least 11 positively identified amino acids. These included His, Tyr, Ser, Ala, Phe, Lys, Gly, Arg, Cys, Glu, and Asp. Leu/Met and Val/Thr were coeluting pairs and hence could not be positively confirmed.

Production and characterization of a silk-like hybrid protein, based on the polyalanine region of Samia cynthia ricini silk fibroin and a cell adhesive region derived from fibronectin

There are a variety of silkworms and silk fibroins produced by them. Silks have many inherent suitable properties for biomaterials. In this paper, a novel silk-like hybrid protein, [DGG(A)(12)GGAASTGRGDSPAAS](5), which consists of polyalanine region of silk fibroin from a wild silkworm, Samia cynthia ricini, and cell adhesive region including Arg-Gly-Asp (RGD) sequence, derived from fibronectin, was designed and produced. The genes encoding the hybrid protein were constructed and expressed in Escherichia coli. The main conformation of the polyalanine region, that is, either alpha-helix or beta-sheet, could be easily controlled by treatment with different acidic solvents, trifluoroacetic acid or formic acid, respectively. This structural change was monitored with 13C CP/MAS NMR. Higher cell adhesive and growth activities of the hybrid protein compared with those of collagen were obtained.

Structural characterization and artificial fiber formation of Bombyx mori silk fibroin in hexafluoro-iso-propanol solvent system

High-resolution solution (13)C-NMR and CD studies of Bombyx mori silk fibroin revealed the presence of an ordered secondary structure 3(10)-helix, in hexafluoro-iso-propanol (HFIP). The solid-state structure of the silk fibroin film prepared by drying it gently from the HFIP solution still keep the structure, 3(10)-helix, which was studied with high-resolution solid state (13)C-NMR. The structural transition from the 3(10)-helix to silk II structure, heterogeneous structure including antiparallel beta-sheet, occurred during the artificial spinning from the HFIP solution. The wide-angle x-ray diffraction and differential scanning calorimetry thermograms of the artificial spinning fiber after postspinning treatments were observed together with the stress-strain curves. The results emphasize that the molecular structures, controlled morphology, and mechanical properties of the protein-based synthetic polymers can be modulated for enhancing biocompatibility.

Effect of the chemical modification of the arginyl residue in Bombyx mori silk fibroin on the attachment and growth of fibroblast cells

We prepared matrices of Bombyx mori silk fibroin (SF) with different degrees of modification of arginyl residues by reaction between 1,2-cyclohexanedione (CHD) and SF. Two kinds of SF, namely native SF (NSF), obtained from the silk gland of silkworm larvae, and regenerated SF (RSF), prepared from cocoons of the same silkworm, were used in this study because their amino acid compositions were slightly different from each other. The attachment and growth of mouse fibroblast (L-929) cells on the matrices of the NSF and RSF, in which half or almost all of the arginyl residues were modified (NSF50, RSF50, NSF100, and RSF100), were studied using a cell culture method. Both NSF50 and NSF100 exhibited higher cell attachment than did the unmodified NSF. While the cell growth on NSF50 was not significantly different from that on NSF and NSF100, the growth on NSF100 was higher than that on NSF. The cells attached to NSF50 and NSF100 were extensively spread out and their filopodia were visible by SEM. The cell attachment and growth on RSF were comparable to those on NSF100. Although RSF50 exhibited almost the same cell attachment as did the unmodified RSF, RSF100 exhibited a lower cell attachment than did the unmodified RSF and RSF50. There were no significant differences in the cell growth among RSF series. The cells attached to RSF50 and RSF100 aggregated to form masses, and their filopodia could not be found. The relationship of cell attachment to the basicity of the substrate is considered because the modification of the positively charged arginyl residue changed the basicity of the substrate and the cell attachment on the substrate.