Xue-Feng Yuan

Electrospun silk fibroin fiber diameter influences in vitro dermal fibroblast behavior and promotes healing of ex vivo wound models.

Replicating the nanostructured components of extracellular matrix is a target for dermal tissue engineering and regenerative medicine. Electrospinning Bombyx mori silk fibroin (BMSF) allows the production of nano- to microscale fibrous scaffolds. For BMSF electrospun scaffolds to be successful, understanding and optimizing the cellular response to material morphology is essential. Primary human dermal fibroblast response to nine variants of BMSF scaffolds composed of nano- to microscale fibers ranging from ~250 to ~1200 nm was assessed in vitro with regard to cell proliferation, viability, cellular morphology, and gene expression. BMSF support of epithelial migration was then assessed through utilization of a novel ex vivo human skin wound healing model. Scaffolds composed of the smallest diameter fibers, ~250 -300 nm, supported cell proliferation significantly more than fibers with diameters approximately 1 μm (p < 0.001). Cell morphology was observed to depart from a stellate morphology with numerous cell -fiber interactions to an elongated, fiber-aligned morphology with interaction predominately with single fibers. The expressions of extracellular matrix genes, collagen types I and III (p < 0.001), and proliferation markers, proliferating cell nuclear antigen (p < 0.001), increased with decreasing fiber diameter. The re-epithelialization of ex vivo wound models was significantly improved with the addition of BMSF electrospun scaffolds, with migratory keratinocytes incorporated into scaffolds. BMSF scaffolds with nanofibrous architectures enhanced proliferation in comparison to microfibrous scaffolds and provided an effective template for migratory keratinocytes during re-epithelialization. The results may aid in the development of effective BMSF electrospun scaffolds for wound healing applications

Adult stem cells in tissue engineering.

Tissue engineering is a rapidly evolving field of research that has yet to fulfil its promise in the translation and potential application of adult stem cells in clinical practice. Recently, it has become apparent that specific adult stem cells are capable of transdifferentiation. The successful application of adult stem cells is thought to be central in creating truly biomimetic tissue. Although still most widely utilized, research suggests that in the future, bone marrow-derived stem cells may no longer be considered the most suitable candidates for use in tissue engineering. Independent studies have successfully engineered a range of tissues in vitro and in vivo using hair follicle- and adipose-derived stem cells. Owing to their potency, relative abundance and noninvasive extraction, these populations may be the most promising studied to date. This review aims to discuss these candidate adult stem cell populations in an attempt to assess the most promising avenues of research.

Interfacial rheology of natural silk fibroin at air/water and oil/water interfaces.

The interfacial viscoelastic behavior of natural silk fibroin at both the air/water and oil/water interfaces is reported. This natural multiblock copolymer is found to be strongly amphiphilic and forms stable films at these interfaces. The result is an interfacial layer that is rheologically complex with strong surface elastic moduli that are only slightly frequency-dependent. The kinetics of surface viscoelastic evolution are reported as functions of time for various concentrations of the spread films. Films deposited by Langmuir-Blodgett deposition were studied by scanning electron microscopy (SEM) to reveal a fibrous structure at the interface. The production of stable O/W emulsions by silk fibroin further confirms the generation of the elastic films at the oil/water interfaces.

Structure and rheology of aqueous micellar solutions and gels formed from an associative poly(oxybutylene)-poly(oxyethylene)-poly(oxybutylene) triblock copolymer

The structure and shear flow behaviour of aqueous micellar solutions and gels formed by an amphiphilic poly(oxybutylene)-poly(oxyethylene)-poly(oxybutylene) triblock copolymer with a lengthy hydrophilic poly(oxyethylene) block has been investigated by rheology, small angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS). SANS revealed that bridging of chains between micelles introduces, in the micellar solution, an attractive long-range component which can be described through a potential of interaction corresponding to sticky soft spheres. The strength of the attractive interaction increases with increasing concentration. Rheology showed that the dependence of the storage modulus with temperature can be explained as a function of the micellar bridging, micellisation and phase morphology. SAXS studies showed that the orientation adopted by the system in the gel phase under shear is similar to that previously observed by us for the gel phase of a poly(oxyethylene)-poly(oxybutylene) diblock copolymer with a long poly(oxyethylene) chain, suggesting that the micellar corona/core length ratio and not the architecture of the block copolymer influences the alignment of the gel phase under shear.

Time-dependent non-linear dynamics of polymer solutions in microfluidic contraction flow—a numerical study on the role of elongational viscosity

A generalised form of the finitely extensible non-linear elastic (FENE) model for modelling non-linear flow of semi-dilute polymer solutions is proposed. It accounts for conformation-dependent polymer elasticity and predicts shear-thinning shear viscosity, non-linear elongational viscosity and first and second normal stress differences. The rheometric material functions predicted by the model are critically compared with the results of the linear Phan–Thien–Tanner model. The predictabilities of these constitutive models under benchmark flow problems are evaluated by time-dependent simulations, using finite volume method based on a CFD simulation toolbox. The effects of the model parameters, the inertia and the contraction ratio are numerically studied. The modified FENE model qualitatively captures the non-linear flow phenomena of polymer solution in the high elasticity number (

Rheology and Electrospinning of Regenerated Bombyx mori Silk Fibroin Aqueous Solutions

\mathrm {El}

Mixed micelles of block copolymers of ethylene oxide and 1,2-butylene oxide. Solutions and gels of triblock BEB plus diblock EB copolymers studied by light scattering and rheology

) flow regime observed in experiments. The results show that an accurate growth function of the elongational viscosity is the key to the prediction of the time-dependent highly asymmetric flow patterns.

Why does shear banding behave like first-order phase transitions? Derivation of a potential from a mechanical constitutive model

Bombyx mori silk fibroin (BMSF) has received considerable research interest as a potential biomaterial owing to its excellent mechanical properties and benign, versatile material fabrication options, including electrospinning. Despite this, characterizations of regenerated BMSF aqueous solutions and electrospun materials resulting from them are still very limited in the literature. This report details the rheological characterization of regenerated aqueous BMSF solutions under shear and elongational deformation. Well-characterized regenerated BMSF solutions were then systematically electrospun over a range of concentrations and process parameters to determine their effects on electrospinning processing windows and fiber morphology. BMSF solutions could not be electrospun successfully if BMSF concentration was below 20 wt % or the relaxation time measured using the CaBER rheometer was below 0.001 s. Electrospun BMSF fiber diameter was found to increase with solution concentration when stable electrospinning was achieved. An upper threshold of 30 wt % BMSF solution was identified for the formation of fibers with a circular cross section. Adding small amount of high molecular weight poly(ethylene oxide) was an effective rheological modifier that greatly improved the electrospinnability of BMSF solutions. Electrospinning BMSF-PEO solutions over a range of parameters significantly altered the fiber products. Increasing voltage from 0.5 to 1 kV/cm was found to decrease fiber diameter by approximately 50% (p < 0.001). Flow rate was found to have a significant effect on fiber diameter, which decreased with spinneret height. The results presented here provide valuable guidance in the production of BMSF electrospun materials with specific properties for tissue engineering and regenerative medicine.

Aqueous micellar solutions of a triblock copolymer of ethylene oxide and 1,2-butylene oxide, B12E114B12. Scaling the viscoelasticity of fluid and gel

Because of the hydrophobic end blocks, micelles of triblock BEB copolymers (E denotes an oxyethylene unit, B an oxybutylene unit) formed in aqueous solution show effects attributable to intermicellar bridging. The formation of mixed micelles with a diblock EB copolymer reduces but does not eliminate the effect. Light scattering methods applied to dilute solutions have been used to show that this is the case for mixed micelles of B12E114B12 and E43B11 (the subscripts denote block lengths). This work focuses on 20 wt% aqueous solutions and on the effect of mixing on properties relating to the gel state, i.e. dynamic storage modulus (G′) and yield strength (σy). There are two contributory factors determining the gel properties: micelle bridging and micelle packing, with just the latter operative for a diblock gel. Increasing the proportion of diblock copolymer in the mixture reduces the contribution from micelle bridging. Considering 20 wt% solutions at 5 °C, the diblock copolymer solution does not gel at this temperature and values of G′ and σy fall away monotonically to zero. For 20 wt% solutions at 25 °C, the diblock copolymer solution does gel at this temperature and values of G′ and σy pass through minima as the system is changed from 100% triblock to 100% diblock copolymer.

Chain Segregation in Block Copolymers

Numerous numerical and experimental evidence suggest that shear banding behavior looks like first-order phase transitions. In this paper, we demonstrate that this correspondence is actually established in the so-called non-local diffusive Johnson-Segalman model (the DJS model), a typical mechanical constitutive model that has been widely used for describing shear banding phenomena. In the neighborhood of the critical point, we apply the reduction procedure based on the center manifold theory to the governing equations of the DJS model. As a result, we obtain a time evolution equation of the flow field that is equivalent to the time-dependent Ginzburg-Landau (TDGL) equations for modeling thermodynamic first-order phase transitions. This result, for the first time, provides a mathematical proof that there is an analogy between the mechanical instability and thermodynamic phase transition at least in the vicinity of the critical point of the shear banding of DJS model. Within this framework, we can clearly distinguish the metastable branch in the stress-strain rate curve around the shear banding region from the globally stable branch. A simple extension of this analysis to a class of more general constitutive models is also discussed. Numerical simulations for the original DJS model and the reduced TDGL equation is performed to confirm the range of validity of our reduction theory.