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As a functional material, silk has been widely used in tissue engineering, drug release, and tissue regeneration. Increasing subtle control of silk hierarchical structures and thus specific functional performance is required for these applications but remains a challenge. Here, we report a novel silk nanofiber solution achieved through tuning solvent systems used to generate the material. Unlike the β-sheet rich silk nanofibers reported previously, these new silk nanofibers are mainly composed of amorphous structures and maintain a solution state in aqueous environments. The amorphous silk nanofibers are stable enough for storage and also metastable, making them easy to use in the further fabrication of materials through various processes. Silk scaffolds, hydrogels, and films were prepared from these silk nanofiber solutions. These silk materials from amorphous nanofiber solutions show different properties and tunable performance features. Therefore, these amorphous silk nanofibers are suitable units or building blocks for designing silk-based materials. [ABSTRACT FROM AUTHOR]

The design and developmentof future molecular photonic/electronicsystems pose the challenge of integrating functional molecular buildingblocks in a controlled, tunable, and reproducible manner. The modularnature and fidelity of the biosynthesis method provides a unique chemistryapproach to one-pot synthesis of environmental factor-responsive chimericproteins capable of energy conversion between the desired forms. Inthis work, facile tuning of dynamic thermal response in plasmonicnanoparticles was facilitated by genetic engineering of the structure,size, and self-assembly of the shell silk-elastin-like protein polymers(SELPs). Recombinant DNA techniques were implemented to synthesizea new family of SELPs, S4E8Gs, with amino acid repeats of [(GVGVP)4(GGGVP)(GVGVP)3(GAGAGS)4] and tunablemolecular weight. The temperature-reversible conformational switchingbetween the hydrophilic random coils and the hydrophobic β-turnsin the elastin blocks were programmed to between 50 and 60 °Cby site-specific glycine mutation, as confirmed by variable-temperatureproton NMR and circular dichroism (CD) spectroscopy, to trigger thenanoparticle aggregation. The dynamic self-aggregation/disaggregationof the Au-SELPs nanoparticles was regulated in size and pattern bythe β-sheet-forming, thermally stable silk blocks, as revealedby transmission electron microscopy (TEM) and dynamic light scattering(DLS). The thermally reversible, shell dimension dependent, interparticleplasmon coupling was investigated by both variable-temperature UV–visspectroscopy and finite-difference time-domain (FDTD)-based simulations.Good agreement between the calculated and measured spectra sheds lighton design and synthesis of responsive plasmonic nanostructures byindependently tuning the refractive index and size of the SELPs throughgenetic engineering. [ABSTRACT FROM AUTHOR]

Ceramicscaffolds such as biphasic calcium phosphate (BCP) havebeen widely studied and used for bone regeneration, but their brittlenessand low mechanical strength are major drawbacks. We report the firstsystematic study on the effect of silk coating in improving the mechanicaland biological properties of BCP scaffolds, including (1) optimizationof the silk coating process by investigating multiple coatings, and(2) in vitro evaluation of the osteogenic response of human mesenchymalstem cells (hMSCs) on the coated scaffolds. Our results show thatmultiple silk coatings on BCP ceramic scaffolds can achieve a significantcoating effect to approach the mechanical properties of native bonetissue and positively influence osteogenesis by hMSCs over an extendedperiod. The silk coating method developed in this study representsa simple yet effective means of reinforcement that can be appliedto other types of ceramic scaffolds with similar microstructure toimprove osteogenic outcomes. [ABSTRACT FROM AUTHOR]

Substrate mechanical properties have remarkable influenceson cellbehavior and tissue regeneration. Although salt-leached silk scaffoldshave been used in tissue engineering, applications in softer tissueregeneration can be encumbered with excessive stiffness. In the presentstudy, silk-bound water interactions were regulated by controllingprocessing to allow the preparation of salt-leached porous scaffoldswith tunable mechanical properties. Increasing silk-bound water interactionsresulted in reduced silk II (β-sheet crystal) formation duringsalt-leaching, which resulted in a modulus decrease in the scaffolds.The microstructures as well as degradation behavior were also changed,implying that this water control and salt-leaching approach can beused to achieve tunable mechanical properties. Considering the utilityof silk in various fields of biomedicine, the results point to a newapproach to generate silk scaffolds with controllable properties tobetter mimic soft tissues by combining scaffold preparation methodsand silk self-assembly in aqueous solutions. [ABSTRACT FROM AUTHOR]

The aim of this study was to understand the structureand biodegradationrelationships of silk particles intended for targeted biomedical applications.Such a study is also useful in understanding structural remodellingof silk debris that may be generated from silk-based implants. Ultrafinesilk particles were prepared using a combination of efficient wet-millingand spray-drying processes with no addition of chemicals other thanthose used in degumming. Milling reduced the intermolecular stackingforces within the β-sheet crystallites without changing theintramolecular binding energy. Because of the rough morphology andthe ultrafine size of the particles, degradation of silk particlesby protease XIV was increased by about 3-fold compared to silk fibers.Upon biodegradation, the thermal degradation temperature of silk increased,which was attributed to the formation of tight aggregates by the hydrolyzedresidual macromolecules. A model of the biodegradation mechanism ofsilk particles was developed based on the experimental data. The modelexplains the process of disintegration of β-sheets, supportedby quantitative secondary structural analysis and microscopic images. [ABSTRACT FROM AUTHOR]

Silkworms and spiders generate fibers that exhibit highstrengthand extensibility. The underlying mechanisms involved in processingsilk proteins into fiber form remain incompletely understood, resultingin the failure to fully recapitulate the remarkable properties ofnative fibers in vitro from regenerated silk solutions. In the presentstudy, the extensibility and high strength of regenerated silks wereachieved by mimicking the natural spinning process. Conformationaltransitions inside micelles, followed by aggregation of micelles andtheir stabilization as they relate to the metastable structure ofsilk are described. Subsequently, the mechanisms to control the formationof nanofibrous structures were elucidated. The results clarify thatthe self-assembly of silk in aqueous solution is a thermodynamicallydriven process where kinetics also play a key role. Four key factors,molecular mobility, charge, hydrophilic interactions, and concentrationunderlie the process. Adjusting these factors can balance nanostructureand conformational composition, and be used to achieve silk-basedmaterials with properties comparable to native fibers. These mechanismssuggest new directions to design silk-based multifunctional materials. [ABSTRACT FROM AUTHOR]

Silk fibroin (SF) nanofibers, formed through electrospinning,haveattractive utility in regenerative medicine due to the biocompatibility,mechanical properties, and tailorable degradability. The mechanismof SF electrospun nanofiber formation was studied to gain new insightinto the formation and control of nanofibers. SF electrospinning solutionswith different nanostructures (nanospheres or nanofilaments) wereprepared by controlling the drying process during the preparationof regenerated SF films. Compared to SF nanospheres in solution, SFnanofilaments had better spinnability with lower viscosity when theconcentration of silk protein was below 10%, indicating a criticalrole for SF morphology, and in particular, nanostructures, for theformation of electrospun fibers. More interesting, the diameter ofelectrospun fibers gradually increased from 50 to 300 nm as the concentrationof SF nanofilaments in the solution increased from 6 to 12%, implyingsize control by simply adjusting SF nanostructure and concentration.Aside from process parameters investigated in previous studies, suchas SF concentration, viscosity, and electrical potential, the presentmechanism emphasizes significant influence of SF nanostructure onspinnability and diameter control of SF electrospun fibers, providinga controllable option for the preparation of silk-based electrospunscaffolds for biomaterials, drug delivery, and tissue engineeringneeds. [ABSTRACT FROM AUTHOR]

Novel protein chimeras constituted of “silk”anda silica-binding peptide (KSLSRHDHIHHH) were synthesized by geneticor chemical approaches and their influence on silica-silk based chimeracomposite formation evaluated. Genetic chimeras were constructed from6 or 15 repeats of the 32 amino acid consensus sequence of Nephila clavipesspider silk ([SGRGGLGGQG AGAAAAAGGA GQGGYGGLGSQG]n) to which one silica binding peptide wasfused at the N terminus. For the chemical chimera, 28 equiv of thesilica binding peptide were chemically coupled to natural Bombyx morisilk after modification of tyrosine groups bydiazonium coupling and EDC/NHS activation of all acid groups. Aftersilica formation under mild, biomaterial-compatible conditions, theeffect of peptide addition on the properties of the silk and chimericsilk-silica composite materials was explored. The composite biomaterialproperties could be related to the extent of silica condensation andto the higher number of silica binding sites in the chemical chimeraas compared with the genetically derived variants. In all cases, thestructure of the protein/chimera in solution dictated the type ofcomposite structure that formed with the silica deposition processhaving little effect on the secondary structural composition of thesilk-based materials. Similarly to our study of genetic silk basedchimeras containing the R5 peptide (SSKKSGSYSGSKGSKRRIL), the roleof the chimeras (genetic and chemical) used in the present study residedmore in aggregation and scaffolding than in the catalysis of condensation.The variables of peptide identity, silk construct (number of consensusrepeats or silk source), and approach to synthesis (genetic or chemical)can be used to “tune” the properties of the compositematerials formed and is a general approach that can be used to preparea range of materials for biomedical and sensor-based applications. [ABSTRACT FROM AUTHOR]

We report on an application of silk as an ultrathin redox-active template for controllable, one-step synthesis of gold nanoparticles via control over silk secondary structure. We found that both silk I and silk II molecular layers can facilitate gold nanoparticle formation at ambient conditions, indicating that tyrosine groups are available for metal ion reduction in both forms of silk. We suggest that the presence of β-sheets in silk II facilitates tyrosine ordering thereby resulting in well-dispersed, uniform nanoparticles with diameters of less than 6 nm. In addition, the mineralization does not result in transformation of the silk I secondary structure to silk II. In fact, the silk I structure is stabilized from further transformation into silk II even upon drying. These results are critical for developing a better understanding of silk interfacial behavior and offer an opportunity to design a new class of nanocomposites that combine the beneficial features of silk with those of the nanoparticles. [ABSTRACT FROM AUTHOR]