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Silk fibroin from the domesticated silkworm Bombyx mori is a naturally occurring biopolymer with charged hydrophilic terminal regions that end-cap a hydrophobic core consisting of repeating sequences of glycine, alanine, and serine residues. Taking inspiration from mussels that produce proteins rich in L-3,4-dihydroxyphenylalanine (DOPA) to adhere to a variety of organic and inorganic surfaces, the silk fibroin was functionalized with catechol groups. Silk fibroin was selected for its high molecular weight, tunable mechanical and degradation properties, aqueous processability, and wide availability. The synthesis of catechol-functionalized silk fibroin polymers containing varying amounts of hydrophilic polyethylene glycol (PEG, 5000 g/mol) side chains was carried out to balance silk hydrophobicity with PEG hydrophilicity. The efficiency of the catechol functionalization reaction did not vary with PEG conjugation over the range studied, although tuning the amount of PEG conjugated was essential for aqueous solubility. Adhesive bonding and cell compatibility of the resulting materials were investigated, where it was found that incorporating as little as 6 wt % PEG prior to catechol functionalization resulted in complete aqueous solubility of the catechol conjugates and increased adhesive strength compared with silk lacking catechol functionalization. Furthermore, PEG-silk fibroin conjugates maintained their ability to form ß-sheet secondary structures, which can be exploited to reduce swelling. Human mesenchymal stem cells (hMSCs) proliferated on the silks, regardless of PEG and catechol conjugation. These materials represent a protein-based approach to catechol-based adhesives, which we envision may find applicability as biodegradable adhesives and sealants. [ABSTRACT FROM AUTHOR]

Preliminary studies have shown that silk fibroin can protect biomacromolecules from thermal degradation, but a deeper understanding of underlying mechanisms needed to fully leverage the stabilizing potential of this matrix has not been realized. In this study, we investigate stabilization of plasma C-reactive protein (CRP), a diagnostic indicator of infection or inflammation, to gain insight into stabilizing mechanisms of silk. We observed that the addition of antiplasticizing excipients that suppress β-relaxation amplitudes in silk matrices resulted in enhanced stability of plasma CRP. These observations are consistent with those made in sugar-glass-based protein-stabilizing matrices and suggest fundamental insight into mechanisms as well as practical strategies to employ with silk protein matrices for enhanced stabilization utility. [ABSTRACT FROM AUTHOR]

Native silk fibers exhibit strength and toughness that rival those of the best synthetic fibers. Despite significant research, further insight is still needed to understand the mechanisms by which silkworms are capable of spinning such tough fibers. Here we propose that π-π and π-OH group interactions of tyrosine side chains provide templating effects, such that the crystal-forming domains are in registration, thereby fostering the self-assembly of the spinning dope. Intrinsic fluorescence measurements, in conjunction with circular dichroism, showed that during self-assembly of regenerated silk solutions, the tyrosine residues were localized in a more hydrophobic local environment, suggesting preferential assembly. In situ Fourier transform infrared spectroscopy indicated that cross-linking of the tyrosine residues resulted in the development of extended β-sheet structure. Additionally, control of cross-link density directly influenced the degree of crystallinity upon drying. Molecular dynamics simulations were performed on silk mimetic peptides in order to more thoroughly understand the role of tyrosines. The results indicated that tyrosine residues tended to transiently colocate in solution due to π-π interactions and hydrogen bonds with adjacent residues and with the peptide backbone. These more stable tyrosine interactions resulted in reduced lateral chain fluctuations and increased incidence of coordinated intrachain association, while introduction of a dityrosine bond directly promoted the formation of β-sheet structures. In total, the experimental and modeling data support a critical role for tyrosine-tyrosine interactions as a key early feature in the self-assembly of regenerated silk protein chains and therefore are important in the robust and unusual mechanical properties ultimately achieved in the process. [ABSTRACT FROM AUTHOR]

Bombyx mori(BM) silk fibroin iscomposed of two different subunits: heavy chain and light chain fibroinlinked by a covalent disulfide bond. Current methods of separatingthe two silk fractions is complicated and produces inadequate quantitiesof the isolated components for the study of the individual light andheavy chain silks with respect to new materials. We report a simplemethod of separating silk fractions using formic acid. The formicacid treatment partially releases predominately the light chain fragment(soluble fraction) and then the soluble fraction and insoluble fractionscan be converted into new materials. The regenerated original (total)silk fibroin and the separated fractions (soluble vs insoluble) haddifferent molecular weights and showed distinctive pH stabilitiesagainst aggregation/precipitation based on particle charging. Allsilk fractions could be electrospun to give fiber mats with viscosityof the regenerated fractions being the controlling factor for successfulelectrospinning. The silk fractions could be mixed to give blendswith different proportions of the two fractions to modify the diameterand uniformity of the electrospun fibers formed. The soluble fractioncontaining the light chain was able to modify the viscosity by thinningthe insoluble fraction containing heavy chain fragments, perhaps analogousto its role in natural fiber formation where the light chain providesincreased mobility and the heavy chain producing shear thickeningeffects. The simplicity of this new separation method should enableaccess to these different silk protein fractions and accelerate theidentification of methods, modifications, and potential applicationsof these materials in biomedical and industrial applications. [ABSTRACT FROM AUTHOR]

An inkjet printing approach is presentedfor the facile fabricationof microscopic arrays of biocompatible silk “nests”capable of hosting live cells for prospective biosensors. The patterningof silk fibroin nests were constructed by the layer-by-layer (LbL)assembly of silk polyelectrolytes chemically modified with poly-(l-lysine) and poly-(l-glutamic acid) side chains. Theinkjet-printed silk circular regions with a characteristic “nest”shape had diameters of 70–100 μm and a thickness severalhundred nanometers were stabilized by ionic pairing and by the formationof the silk II crystalline secondary structure. These “locked-in”silk nests remained anchored to the substrate during incubation incell growth media to provide a biotemplated platform for printing-in,immobilization, encapsulation and growth of cells. The process ofinkjet-assisted printing is versatile and can be applied on any typeof substrate, including rigid and flexible, with scalability and facileformation. [ABSTRACT FROM AUTHOR]

Derived from Bombyxmoricocoons,regenerated silk fibroin (RSF) exhibits excellent biocompatibility,high toughness, and tailorable biodegradability. Additionally, RSFmaterials are flexible, optically clear, easily patterned with nanoscalefeatures, and may be doped with a variety bioactive species. Thisunique combination of properties has led to increased interest inthe use of RSF in sustainable and biocompatible electronic devices.In order to explore the applicability of this biopolymer to the developmentof future bioelectronics, the dielectric breakdown strength (Ebd) of RSF thin films was quantified as a functionof protein conformation. The application of processing conditionsthat increased β-sheet content (as determined by FTIR analysis)and produced films in the silk II structure resulted in RSF materialswith improved Ebdwith values reachingup to 400 V/μm. [ABSTRACT FROM AUTHOR]

The recent increase in the developmentof polymer-based technologicaldevices has necessitated a surge in associated fabrication strategies.An adequate understanding of the interfacial properties in such systemsis required to meet the challenges inherent to these applications,ranging from electronics to biomedicine. In this work, we utilizethermal reflow to investigate the interfacial properties of multilayersilk fibroin film constructs. We demonstrate that reflow can be utilizedto control the water content, glass transition, and β sheetcrystallinity of such constructs, leading to control of the mechanicalproperties at the interface. Such analysis may lend insight into theinterfacial properties of similar semicrystalline biopolymers, increasingthe number of fabrication options for the development of devices atthe biological–technological nexus. [ABSTRACT FROM AUTHOR]

Conventionalfabrication techniques and structures employed inthe design of silk fibroin (SF) based porous materials provide onlylimited control over pore size and require several processing stages.In this study, it is shown that, by utilizing electrohydrodynamicbubbling, not only can new hollow spherical structures of SF be formedin a single step by means of bubbles, but the resulting bubbles canserve as pore generators when dehydrated. The bubble characteristicscan be controlled through simple adjustments to the processing parameters.Bubbles with diameters in the range of 240–1000 μm werefabricated in controlled fashion. FT-IR characterization confirmedthat the rate of air infused during processing enhanced β-sheetpacking in SF at higher flow rates. Dynamic mechanical analysis alsodemonstrated a correlation between air flow rate and film tensilestrength. Results indicate that electrohydrodynamically generatedSF and their composite bubbles can be employed as new tools to generateporous structures in a controlled manner with a range of potentialapplications in biocoatings and tissue engineering scaffolds. [ABSTRACT FROM AUTHOR]

We studied the pH-responsive behavior of layer-by-layer(LbL) microcapsulesfabricated from silk fibroin chemically modified with different polyamino acid side chains: cationic (silk-poly l-lysine, SF-PL)or anionic (silk-poly-l-glutamic acid, SF-PG). We observedthat stable ultrathin shell microcapsules can be assembled with adramatic increase in swelling, thickness, and microroughness at extremelyacidic (pH < 2.5) and basic (pH > 11.0) conditions without noticeabledisintegration. These changes are accompanied by dramatic changesin shell permeability with a 2 orders of magnitude increase in thediffusion coefficient. Moreover, the silk ionomer shells undergo remarkablesoftening with a drop in Young’s modulus by more than 1 orderof magnitude due to the swelling, stretching, and increase in materialporosity. The ability to control permeability and mechanical propertiesover a wide range for thesilk-based microcapsules, with distinguishing stability under harshenvironmental conditions, provides an important system for controlledloading and release and applications in bioengineering. [ABSTRACT FROM AUTHOR]

Organophosphates are some of the most acutely toxic compoundssynthesizedon an industrial scale, and organophosphorus hydrolase (OPH) has theability to hydrolyze and inactivate a number of these chemicals. However,OPH activity is vulnerable to harsh environmental conditions thatwould accompany its practical utility in the field; a limitation thatcan also be extended to conditions required for incorporation of OPHinto useful materials. Here we present evidence that entrapment ofOPH in silk fibroin leads to stabilization of OPH activity under avariety of conditions that would otherwise reduce free enzyme activity,such as elevated temperature, UV light exposure and the presence ofdetergent. Silk fibroin entrapment of OPH also allowed for its dispersalinto a polyurethane-based coating that retained organophosphate hydrolysisactivity after formulation, application and drying. Together, thedata presented here demonstrate the utility of silk fibroin entrapmentfor the protection of OPH activity under a variety of environmentalconditions. [ABSTRACT FROM AUTHOR]

Although natural silk fibers have excellent strengthand flexibility,the regenerated silk materials generally become brittle in the drystate. How to reconstruct the flexibility for silk fibroin has bewilderedscientists for many years. In the present study, the flexible regeneratedsilk fibroin films were achieved by simulating the natural formingand spinning process. Silk fibroin films composed of silk I structurewere first prepared by slow drying process. Then, the silk fibroinfilms were stretched in the wet state, following the structural transitionfrom silk I to silk II. The difference between the flexible film anddifferent brittle regenerated films was investigated to reveal thecritical factors in regulating the flexibility of regenerated silkmaterials. Compared with the methanol-treated silk films, althoughhaving similar silk II structure and water content, the flexible silkfilms contained more bound water rather than free water, implyingthe great influence of bound water on the flexibility. Then, furtherstudies revealed that the distribution of bound water was also a criticalfactor in improving silk flexibility in the dry state, which couldbe regulated by the nanoassembly of silk fibroin. Importantly, theresults further elucidate the relation between mechanical propertiesand silk fibroin structures, pointing to a new mode of generatingnew types of silk materials with enhanced mechanical properties inthe dry state, which would facilitate the fabrication and applicationof regenerated silk fibroin materials in different fields. [ABSTRACT FROM AUTHOR]