The role of interfacial viscoelasticity in the stabilization of an electrospun jet

Development of highly bacterial filtration efficient and antibacterial cellulose acetate/gum rosin composite nanofibers for facemask application

Cellulose acetate (CA) is a non-toxic, renewable, and biodegradable polymeric material that can be effectively electrospuned into bacterial filtration efficient nanofiber membrane for face mask application. However, its fragile and non-antibacterial nature influenced its scalability. In this context, natural antibacterial gum rosin (GR) additive can be explored. Therefore, the present study aimed to produce a CA/GR composite nanofibers membrane for the finest bacterial filtration, excellent antibacterial moiety, and improved tensile properties for facemask application. Hence, in this work, we have studied the effect of GR concentrations (0-15 g) on the needleless electrospinning behavior and fibers' morphology through rheology, electrical conductivity, and SEM analysis. These analyses revealed that GR significantly affects the fibers' spinning behavior, morphology, and diameter of the produced fibers. Later, ATR-FTIR spectroscopy mapped the functional changes in the produced nanofibers that affirmed the integration of GR with CA polymer. This modification resulted in a 3-fold rise in tensile strength and an 11-fold decline in elongation% in 15 g CA/GR composite nanofibers membrane than the control sample. Furthermore, it has shown 98.79 ± 0.10% bacterial filtration efficiency and ∼ 93 % reduction in Staphylococcus Aureus and Klebsiella Pneumoniae bacterial growth, elucidating a high-efficiency level for potential facemask application.

Electrospun Membranes Based on Quaternized Polysulfones: Rheological Properties-Electrospinning Mechanisms Relationship

Composite membranes based on a polymer mixture solution of quaternized polysulfone (PSFQ), cellulose acetate phthalate (CAP), and polyvinylidene fluoride (PVDF) for biomedical applications were successfully obtained through the electrospinning technique. To ensure the polysulfone membranes' functionality in targeted applications, the selection of electrospinning conditions was essential. Moreover, understanding the geometric characteristics and morphology of fibrous membranes is crucial in designing them to meet the performance standards necessary for future biomedical applications. Thus, the viscosity of the solutions used in the electrospinning process was determined, and the morphology of the electrospun membranes was examined using scanning electron microscopy (SEM). Investigations on the surfaces of electrospun membranes based on water vapor sorption data have demonstrated that their surface properties dictate their biological ability more than their specific surfaces. Furthermore, in order to understand the different macromolecular rearrangements of membrane structures caused by physical interactions between the polymeric chains as well as by the orientation of functional groups during the electrospinning process, Fourier transform infrared (FTIR) spectroscopy was used. The applicability of composite membranes in the biomedical field was established by bacterial adhesion testing on the surface of electrospun membranes using Escherichia coli and Staphylococcus aureus microorganisms. The biological experiments conducted establish a foundation for future applications of these membranes and validate their effectiveness in specific fields.

Role of Rheology in Morphology Development and Advanced Processing of Thermoplastic Polymer Materials: A Review

This review presents fundamental knowledge and recent advances pertaining to research on the role of rheology in polymer processing, highlights the knowledge gap between the function of rheology in various processing operations and the importance of rheology in the development, characterization, and assessment of the morphologies of polymeric materials, and offers ideas for enhancing the processabilities of polymeric materials in advanced processing operations. Rheology plays a crucial role in the morphological evolution of polymer blends and composites, influencing the type of morphology in the case of blends and the quality of dispersion in the cases of both blends and composites. The rheological characteristics of multiphase polymeric materials provide valuable information on the morphologies of these materials, thereby rendering rheology an important tool for morphological assessment. Although rheology extensively affects the processabilities of polymeric materials in all processing operations, this review focuses on the roles of rheology in film blowing, electrospinning, centrifugal jet spinning, and the three-dimensional printing of polymeric materials, which are advanced processing operations that have gained significant research interest. This review offers a comprehensive overview of the fundamentals of morphology development and the aforementioned processing techniques; moreover, it covers all vital aspects related to the tailoring of the rheological characteristics of polymeric materials for achieving superior morphologies and high processabilities of these materials in advanced processing operations. Thus, this article provides a direction for future advancements in polymer processing. Furthermore, the superiority of elongational flow over shear flow in enhancing the quality of dispersion in multiphase polymeric materials and the role of extensional rheology in the advanced processing operations of these materials, which have rarely been discussed in previous reviews, have been critically analyzed in this review. In summary, this article offers new insights into the use of rheology in material and product development during advanced polymer-processing operations.

High-throughput production of kilogram-scale nanofibers by Kármán vortex solution blow spinning

The interaction between gas flow and liquid flow, governed by fluid dynamic principles, is of substantial importance in both fundamental science and practical applications. For instance, a precisely designed gas shearing on liquid solution may lead to efficacious production of advanced nanomaterials. Here, we devised a needleless Kármán vortex solution blow spinning system that uses a roll-to-roll nylon thread to deliver spinning solution, coupled with vertically blowing airflow to draw high-quality nanofibers with large throughput. A wide variety of nanofibers including polymers, carbon, ceramics, and composites with tunable diameters were fabricated at ultrahigh rates. The system can be further upgraded from single thread to multiple parallel threads and to the meshes, boosting the production of nanofibers to kilogram scale without compromising their quality.

Interfacial properties of milk proteins: A review

The interfacial properties of dairy proteins are of great interest to the food industry. Food manufacturing involves various environmental conditions and multiple processes that significantly alter the structure and colloidal stability of food materials. The effects of concentration, pH, heat treatment, addition of salts etc., have considerable influence on the surface activity of proteins and the mechanical properties of the interfacial protein films. Studies to date have established some understanding of the links between environmental and processing related parameters and their impacts on interfacial behavior. Improvement in knowledge may allow better design of interfacial protein structures for different food applications. This review examines the effects of environmental and processing conditions on the interfacial properties of dairy proteins with emphasis on interfacial tension dynamics, dilatational and surface shear rheological properties. The most commonly used surface analytical techniques along with relevant methods are also addressed.

Fabrication and Properties of Electrospun Collagen Tubular Scaffold Crosslinked by Physical and Chemical Treatments

Tissue engineered scaffold was regarded as a promising approach instead of the autograft. In this study, small diameter electrospun collagen tubular scaffold with random continuous smooth nanofibers was successfully fabricated. However, the dissolution of collagen in concentrated aqueous (conc. aq.) acetic acid caused to the serious denaturation of collagen. A novel method ammonia treatment here was adopted which recovered the collagen triple helix structure according to the analysis of IR spectra. Further dehydrothermal (DHT) and glutaraldehyde (GTA) treatments were applied to introduce the crosslinks to improve the properties of collagen tube. The nanofibrous structure of collagen tube in a wet state was preserved by the crosslinking treatments. Swelling ratio and weight loss decreased by at least two times compared to those of the untreated collagen tube. Moreover, tensile strength was significantly enhanced by DHT treatment (about 0.0076 cN/dTex) and by GTA treatment (about 0.075 cN/dTex). In addition, the surface of crosslinked collagen tube kept the hydrophilic property. These results suggest that DHT and GTA treatments can be utilized to improve the properties of electrospun collagen tube which could become a suitable candidate for tissue engineered scaffold.

Flow field-based data analysis in interfacial shear rheometry

Developments in interfacial shear rheometers have considerably improved the quality of experimental data. However, data analysis in interfacial shear rheometry is still an active field of research and development due to the intrinsic complexity introduced by the unavoidable contact of the interface with, at least, one supporting bulk subphase. Nonlinear velocity profiles, both at the interface and the bulk phases, pervade the system dynamical behavior in the most usual experimental geometries, particularly in the case of soft interfaces. Such flow configurations demand data analysis schemes based on the explicit calculation of the flow field in both the interface and the bulk phases. Such procedures are progressively becoming popular in this context. In this review, we discuss the most recent advances in interfacial shear rheology data analysis techniques. We extensively review some recently proposed flow field-based data analysis schemes for the three most common interfacial shear rheometer geometries (magnetic needle, double wall-ring, and bicone), showing under what circumstances the calculation of the flow field is mandatory for a proper analysis of the experimental data. All cases are discussed starting at the appropriate hydrodynamical models and using the equation of motion of the probe to set up an iterative procedure to compute the value of the complex Boussinesq number and, from it, the complex interfacial viscosity or, equivalently, the complex interfacial modulus. Moreover, two examples of further extensions of such techniques are proposed, concerning the micro-button interfacial shear rheometer and the potential application of interfacial rheometry instruments, together with adapted flow field-based data analysis techniques, for bulk rheometry, particularly in the case of soft samples.

Electrospinning and electrospraying technologies for food applications

Electrospinning and electrospraying are versatile techniques for the production of nano- to micro-scale fibers and particles. Over the past 2 decades, significant progresses have been made to advance the fundamental understandings of these electrohydrodynamic processes. Researchers have investigated different polymeric and non-polymeric substrates for producing submicron electrospun/electrosprayed materials of unique morphologies and physicochemical properties. This chapter provides an overview on the basic principles of electrospinning and electrospraying, highlighting the effects of key processing and solution parameters. Electrohydrodynamic phenomena of edible substrates, including polysaccharides (xanthan, alginate, starch, cyclodextrin, pullulan, dextran, modified celluloses, and chitosan), proteins (zein, what gluten, whey protein, soy protein, gelatin, etc.), and phospholipids are reviewed. Selected examples are presented on how ultrafine fibers and particles derived from these substrates are being exploited for food and nutraceutical applications. Finally, the challenges and opportunities of the electrostatic methods are discussed.

Pullulan-alginate fibers produced using free surface electrospinning

Pullulan-alginate ultrafine fibers, with and without CaCl₂, were electrospun from aqueous polymer solutions using a free-surface electrospinning method, without the use of synthetic spinning aid polymer. Aqueous pullulan solution (10%, w/w) could be electrospun into beaded fibers of 110 nm in diameter with a board diameter distribution. By contrast, continuous and smooth fibers were formed when 0.8 to 1.6% (w/w) alginate was added to the 10% (w/w) pullulan solutions, producing smaller fibers ranging from 87 to 57 nm in diameter. The positive effect of alginate can be attributed to the increase in polymer chain entanglement, as well as enhanced hydrogen bonding interaction between pullulan and alginate. The addition of trace amount of CaCl₂ (up to 0.045%, w/w) resulted in smooth and ultrafine fibers that were significantly smaller in diameter and greater thermal stability than those prepared without the addition of CaCl_2. The production of typical electrospun fibers involves the use of undesirable organic solvents and/or non-food grade synthetic spinning aid polymer. The water-based edible biopolymer systems presented in this study can be useful for the preparation of nano-scale fibers that are more conducive for food, nutraceutical, and pharmaceutical applications.

Biomaterials in Mechano-oncology: Means to Tune Materials to Study Cancer

ECM stiffness is emerging as a prognostic marker of tumor aggression or potential for relapse. However, conflicting reports muddle the question of whether increasing or decreasing stiffness is associated with aggressive disease. This chapter discusses this controversy in more detail, but the fact that tumor stiffening plays a key role in cancer progression and in regulating cancer cell behaviors is clear. The impact of having in vitro biomaterial systems that could capture this stiffening during tumor evolution is very high. These cell culture platforms could help reveal the mechanistic underpinnings of this evolution, find new therapeutic targets to inhibit the cross talk between tumor development and ECM stiffening, and serve as better, more physiologically relevant platforms for drug screening.

Influence of Polymer Relaxation Time on the Electrospinning Process: Numerical Investigation

“Electrospinnability”, or the ease with which a solution can be used to obtain bead-free uniform fibers, depends on a large number of parameters, including solution properties, process parameters and ambient conditions. In this study, the effect of the polymer relaxation time on electrospinning of dilute polymer solutions is investigated numerically. It is shown that elastic stresses (ES) increase exponentially with the Deborah number (De). For each polymer concentration there exists a critical De below which the ES are insufficient to overcome capillary stresses (CS) and lead to the formation of beaded fibers. However, above the critical De, there is a higher probability of the ES overcoming the CS and leading to the formation of uniform fibers. This analysis suggests the possibility of improved electrospinnability even with dilute polymer solutions, provided the relaxation time is sufficiently large. It is also found that changes in the drag coefficient due to change in the polymer conformation and self-concentration of polymer molecules would become significant for the electrospinning of polymer solutions with a longer relaxation time and high conductivity.

Complex Coacervation: Chemically Stable Fibers Electrospun from Aqueous Polyelectrolyte Solutions

In this study, we provide the first demonstration that aqueous complex coacervates can be electrospun into chemically robust polyelectrolyte complex (PEC) fiber mats. PECs form due to electrostatic complexation between oppositely charged polymers. Here, we exploit the ability of salt to plasticize PECs, thus enabling the electrospinning of solid fibers. Electrospinning solutions were composed of a pair of strong polyelectrolytes, poly(4-styrenesulfonic acid, sodium salt) and poly(diallyldimethylammonium chloride) using potassium bromide as the plasticizing salt. We systematically investigated the effect of salt concentration and electrospinning apparatus parameters on fiber formation. Electrospun PEC fiber mats were stable over a wide range of pH values, ionic strength conditions, and many organic solvents. This study demonstrates that the electrospinning of aqueous complex coacervates can generate chemically robust, free-standing PEC fiber mats while circumventing the reliance on organic solvents, the challenge of working with entangled polyelectrolytes in solution, and the need to chemically cross-link the as-spun fibers. These PEC fiber mats hold potential in applications where environmentally benign fiber mats are imperative, such as tissue engineering scaffolds and water purification technologies.

Microfluidic extensional rheometry using stagnation point flow

Characterization of the extensional rheometry of fluids with complex microstructures is of great relevance to the optimization of a wide range of industrial applications and for understanding various natural processes, biological functions, and diseases. However, quantitative measurement of the extensional properties of complex fluids has proven elusive to researchers, particularly in the case of low viscosity, weakly elastic fluids. For some time, microfluidic platforms have been recognized as having the potential to fill this gap and various approaches have been proposed. This review begins with a general discussion of extensional viscosity and the requirements of an extensional rheometer, before various types of extensional rheometers (particularly those of microfluidic design) are critically discussed. A specific focus is placed on microfluidic stagnation point extensional flows generated by cross-slot type devices, for which some important developments have been reported during the last 10 years. Additional emphasis is placed on measurements made on relevant biological fluids. Finally, the operating limits of the cross-slot extensional rheometer (chiefly imposed by the onset of elastic and inertial flow instabilities) are discussed.

The electrospinning behavior of poly(vinyl alcohol) in DMSO–water binary solvent mixtures

The fiber diameters obtained from PVA–DMSO–water ternary system are the result of the interplay between the solvent–solvent and solvent–polymer interactions.

Dynamic behavior of interfaces: modeling with nonequilibrium thermodynamics

In multiphase systems the transfer of mass, heat, and momentum, both along and across phase interfaces, has an important impact on the overall dynamics of the system. Familiar examples are the effects of surface diffusion on foam drainage (Marangoni effect), or the effect of surface elasticities on the deformation of vesicles or red blood cells in an arterial flow. In this paper we will review recent work on modeling transfer processes associated with interfaces in the context of nonequilibrium thermodynamics (NET). The focus will be on NET frameworks employing the Gibbs dividing surface model, in which the interface is modeled as a two-dimensional plane. This plane has excess variables associated with it, such as a surface mass density, a surface momentum density, a surface energy density, and a surface entropy density. We will review a number of NET frameworks which can be used to derive balance equations and constitutive models for the time rate of change of these excess variables, as a result of in-plane (tangential) transfer processes, and exchange with the adjoining bulk phases. These balance equations must be solved together with mass, momentum, and energy balances for the bulk phases, and a set of boundary conditions coupling the set of bulk and interface equations. This entire set of equations constitutes a comprehensive continuum model for a multiphase system, and allows us to examine the role of the interfacial dynamics on the overall dynamics of the system. With respect to the constitutive equations we will focus primarily on equations for the surface extra stress tensor.

Creep anomaly in electrospun fibers made of globular proteins

The anomalous responses of electrospun nanofibers and film fabricated of unfolded bovine serum albumin (BSA) under constant stress (creep) is observed. In contrast to typical creep behavior of viscoelastic materials demonstrating (after immediate elastic response) a time-dependent elongation, in case of low applied stresses (<1 MPa) the immediate elastic response of BSA samples is followed by gradual contraction up to 2%. Under higher stresses (2-6 MPa) the contraction phase changes into elongation; and in case of stresses above 7 MPa only elongation was observed, with no initial contraction. The anomalous creep behavior was not observed when the BSA samples were subjected to additional creep cycles independently on the stress level. The above anomaly, which was not observed before either for viscoelastic solids or for polymers, is related to specific protein features, namely, to the ability to fold. We hypothesize that the phenomenon is caused by folding of BSA macromolecules into dry molten globule states, feasible after cross-linked bonds break up, resulting from the applied external force.

Extensional flow of hyaluronic acid solutions in an optimized microfluidic cross-slot device

We utilize a recently developed microfluidic device, the Optimized Shape Cross-slot Extensional Rheometer (OSCER), to study the elongational flow behavior and rheological properties of hyaluronic acid (HA) solutions representative of the synovial fluid (SF) found in the knee joint. The OSCER geometry is a stagnation point device that imposes a planar extensional flow with a homogenous extension rate over a significant length of the inlet and outlet channel axes. Due to the compressive nature of the flow generated along the inlet channels, and the planar elongational flow along the outlet channels, the flow field in the OSCER device can also be considered as representative of the flow field that arises between compressing articular cartilage layers of the knee joints during running or jumping movements. Full-field birefringence microscopy measurements demonstrate a high degree of localized macromolecular orientation along streamlines passing close to the stagnation point of the OSCER device, while micro-particle image velocimetry is used to quantify the flow kinematics. The stress-optical rule is used to assess the local extensional viscosity in the elongating fluid elements as a function of the measured deformation rate. The large limiting values of the dimensionless Trouton ratio, Tr ∼ O(50), demonstrate that these fluids are highly extensional-thickening, providing a clear mechanism for the load-dampening properties of SF. The results also indicate the potential for utilizing the OSCER in screening of physiological SF samples, which will lead to improved understanding of, and therapies for, disease progression in arthritis sufferers.

Rheology of human blood plasma: viscoelastic versus Newtonian behavior

We investigate the rheological characteristics of human blood plasma in shear and elongational flows. While we can confirm a Newtonian behavior in shear flow within experimental resolution, we find a viscoelastic behavior of blood plasma in the pure extensional flow of a capillary breakup rheometer. The influence of the viscoelasticity of blood plasma on capillary blood flow is tested in a microfluidic device with a contraction-expansion geometry. Differential pressure measurements revealed that the plasma has a pronounced flow resistance compared to that of pure water. Supplementary measurements indicate that the viscoelasticity of the plasma might even lead to viscoelastic instabilities under certain conditions. Our findings show that the viscoelastic properties of plasma should not be ignored in future studies on blood flow.

Resemblance of electrospun collagen nanofibers to their native structure

Electrospinning is a promising method to mimic the native structure of the extracellular matrix. Collagen is the material of choice, since it is a natural fibrous structural protein. It is an open question how much the spinning process preserves or alters the native structure of collagen. There are conflicting results in the literature, mainly due to the different solvent systems in use and due to the fact that gelatin is employed as a reference state for the completely unfolded state of collagen in calculations. Here we used circular dichroism (CD) and Fourier-transform infrared spectroscopy (FTIR) to investigate the structure of regenerated collagen samples and scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to illuminate the electrospun nanofibers. Collagen is mostly composed of folded and unfolded structures with different ratios, depending on the applied temperature. Therefore, CD spectra were acquired as a temperature series during thermal denaturation of native calf skin collagen type I and used as a reference basis to extract the degree of collagen folding in the regenerated electrospun samples. We discussed three different approaches to determine the folded fraction of collagen, based on CD spectra of collagen from 185 to 260 nm, since it would not be sufficient to obtain simply the fraction of folded structure θ from the ellipticity at a single wavelength of 221.5 nm. We demonstrated that collagen almost completely unfolded in fluorinated solvents and partially preserved its folded structure θ in HAc/EtOH. However, during the spinning process it refolded and the PP-II fraction increased. Nevertheless, it did not exceed 42% as deduced from the different secondary structure evaluation methods, discussed here. PP-II fractions in electrospun collagen nanofibers were almost same, being independent from the initial solvent systems which were used to solubilize the collagen for electrospinning process.

Electrospinning covalently cross-linking biocompatible hydrogelators

Many hydrogel materials of interest are homogeneous on the micrometer scale. Electrospinning, the formation of sub-micrometer to micrometer diameter fibers by a jet of fluid formed under an electric field, is one process being explored to create rich microstructures. However, electrospinning a hydrogel system as it reacts requires an understanding of the gelation kinetics and corresponding rheology near the liquid-solid transition. In this study, we correlate the structure of electrospun fibers of a covalently cross-linked hydrogelator with the corresponding gelation transition and kinetics. Polyethylene oxide (PEO) is used as a carrier polymer in a chemically cross-linking poly(ethylene glycol)-high molecular weight heparin (PEG-HMWH) hydrogel. Using measurements of gelation kinetics from multiple particle tracking microrheology (MPT), we correlate the material rheology with the the formation of stable fibers. An equilibrated, cross-linked hydrogel is then spun and the PEO is dissolved. In both cases, microstructural features of the electrospun fibers are retained, confirming the covalent nature of the network. The ability to spin fibers of a cross-linking hydrogel system ultimately enables the engineering of materials and microstructural length scales suitable for biological applications.

Interfacial rheology: an overview of measuring techniques and its role in dispersions and electrospinning

Interfacial rheological properties have yet to be thoroughly explored. Only recently, methods have been introduced that provide sufficient sensitivity to reliably determine viscoelastic interfacial properties. In general, interfacial rheology describes the relationship between the deformation of an interface and the stresses exerted on it. Due to the variety in deformations of the interfacial layer (shear and expansions or compressions), the field of interfacial rheology is divided into the subcategories of shear and dilatational rheology. While shear rheology is primarily linked to the long-term stability of dispersions, dilatational rheology provides information regarding short-term stability. Interfacial rheological characteristics become relevant in systems with large interfacial areas, such as emulsions and foams, and in processes that lead to a large increase in the interfacial area, such as electrospinning of nanofibers.