Tunable two-step shape and dimensional changes with temperature of a PNIPAM/CNC hydrogel.

PNIPAM (poly(N-isopropylacrylamide)), a well-studied thermo-responsive polymer, undergoes conformational transition around 32 °C. On the other hand, cellulose nanocrystals (CNCs), as a promising and biocompatible material, has rarely been introduced to the PNIPAM-based fibrous hydrogel system. CNCs' impact on the temperature responsive behaviors of hydrogels, either in single layer or bilayer hydrogel systems, is yet to be investigated. In this work, stable well dispersed PNIPAM/CNC suspensions (with various CNC proportions) are prepared and electrospun into nanofiber membranes. The corresponding hydrogels are then obtained via UV-induced crosslinking. CNCs are found to exert a significant constraint effect on hydrogel swelling when it exceeded 5 wt% but a negligible effect on contraction. The difference between hydrogels with various CNC proportions regarding their temperature responsive behaviors is utilized to fabricate bilayer hydrogels. These bilayer samples are capable of generating 3D geometries when they come into contact with water for the first time via anisotropic swelling between the two layers and changing their dimension reversibly in the following swelling and contraction. In addition, these geometries are found to be highly tunable via the finely tuned thickness ratio between the two layers. This promising feature would significantly extend the application of these materials in tissue engineering where a controllable geometry of the culture substrate is of great importance.

Understanding the Effect of Conformational Rigidity on Rheological Behavior and Formation of Polysaccharide-Based Hybrid Hydrogels.

The importance of conformational rigidity on macroscopic rheological properties was revealed using two model polysaccharides, namely, xanthan gum and hyaluronic acid. Xanthan gum has a rigid tertiary conformation due to its ordered double-helical structure, and the interactions between the tertiary structures result in the formation of a network/quaternary structure. In comparison, hyaluronic acid possesses a relatively flexible tertiary conformation due to its secondary random coil structure. Xanthan gum exhibits a much stronger shear thinning and more solidlike behavior compared to hyaluronic acid, owing to its network/quaternary structure. The rigid tertiary structure and the presence of a network/quaternary structure also endow xanthan gum with better resistance against environmental changes (e.g., salt and/or urea addition, temperature change) compared to hyaluronic acid. The network/quaternary structure allows xanthan gum to form gels with chitosan via electrostatic interactions when using the vapor-induced gelation technique, which is not possible for hyaluronic acid due to its flexible tertiary conformation under similar conditions.

Orienting Cellulose Nanocrystal Functionalities Tunes the Wettability of Water-Cast Films.

Cellulose nanocrystal (CNC)-based materials display apparently erratic wetting behaviors with contact angle (CA) variations as large as 30° from sample to sample. This work hypothesizes that it is the orientation of CNC amphiphilic functionalities at the interface with air that causes the variability in CA. By exploiting relationships with the Hansen solubility parameter theory, a set of surface tension parameters is proposed for both the polar and the non-polar surfaces of cellulose Iß nanocrystals. These coefficients elucidate the wettability of CNC materials by establishing a correlation between the wetting properties of the air/sample interface and its chemical composition in terms of non-polar moieties. Advancing/receding CA experiments suggest that, while spin-coating CNC suspensions yield purely polar films, oven-casting them produces amphiphilic surfaces. We proposed a mechanism where the state of dispersion (individual or agglomerated) in which CNCs reach the air/water interface during casting is the determining factor: while individual nanocrystals find it more stable to orient their non-polar surfaces toward the interface, the aspect ratio of CNC agglomerates favors an orientation of their polar surfaces. This represents the first compelling evidence of CNC orientation at an interface and can be applied to Pickering emulsions and nanocomposites and to the production of CNC materials with tuned wettability.

Microbiological and real-time mechanical analysis of Bacillus licheniformis and Pseudomonas fluorescens dual-species biofilm.

In natural habitats, bacterial species often coexist in biofilms. They interact in synergetic or antagonistic ways and their interactions can influence the biofilm development and properties. Still, very little is known about how the coexistence of multiple organisms impact the multispecies biofilm properties. In this study, we examined the behaviour of a dual-species biofilm at the air-liquid interface composed by two environmental bacteria: Bacillus licheniformis and a phenazine mutant of Pseudomonas fluorescens. Study of the planktonic and biofilm growths for each species revealed that P. fluorescens grew faster than B. licheniformis and no bactericidal effect from P. fluorescens was detected, suggesting that the growth kinetics could be the main factor in the dual-species biofilm composition. To validate this hypothesis, the single- and dual-species biofilm were characterized by biomass quantification, microscopy and rheology. Bacterial counts and microscale architecture analysis showed that both bacterial populations coexist in the mature pellicle, with a dominance of P. fluorescens. Real-time measurement of the dual-species biofilms' viscoelastic (i.e. mechanical) properties using interfacial rheology confirmed that P. fluorescens was the main contributor of the biofilm properties. Evaluation of the dual-species pellicle viscoelasticity at longer time revealed that the biofilm, after reaching a first equilibrium, created a stronger and more cohesive network. Interfacial rheology proves to be a unique quantitative technique, which combined with microscale imaging, contributes to the understanding of the time-dependent properties within a polymicrobial community at various stages of biofilm development. This work demonstrates the importance of growth kinetics in the bacteria competition for the interface in a model dual-species biofilm.

Antibacterial Activity of Neat Chitosan Powder and Flakes.

This study investigates the antibacterial activity of neat chitosan powder and flakes against three different bacterial species, Escherichia coli, Listeria innocua and Staphylococcus aureus, which are frequent causes of food spoilage. The effect of chitosan concentration and purity, as well as the influence of temperature, ionic strength (salt) and impact of a solid physical support in the medium are examined. Results show that the antibacterial activity of neat chitosan: (i) requires partial solubilisation; (ii) can be promoted by environmental factors such as adequate temperature range, ionic strength and the presence of a solid physical support that may facilitate the attachment of bacteria; (iii) depends on bacterial species, with a sensitivity order of E. coli > L. innocua > S. aureus; and (iv) increases with chitosan concentration, up to a critical point above which this effect decreases. The latter may be due to remaining proteins in chitosan acting as nutrients for bacteria therefore limiting its antibacterial activity. These results on the direct use of chitosan powder and flakes as potential antimicrobial agents for food protection at pH values lower than the chitosan pKa (6.2-6.7) are promising.

Mechanism of Action of Electrospun Chitosan-Based Nanofibers against Meat Spoilage and Pathogenic Bacteria.

This study investigates the antibacterial mechanism of action of electrospun chitosan-based nanofibers (CNFs), against Escherichia coli, Salmonella enterica serovar Typhimurium, Staphylococcus aureus and Listeria innocua, bacteria frequently involved in food contamination and spoilage. CNFs were prepared by electrospinning of chitosan and poly(ethylene oxide) (PEO) blends. The in vitro antibacterial activity of CNFs was evaluated and the susceptibility/resistance of the selected bacteria toward CNFs was examined. Strain susceptibility was evaluated in terms of bacterial type, cell surface hydrophobicity, and charge density, as well as pathogenicity. The efficiency of CNFs on the preservation and shelf life extension of fresh red meat was also assessed. Our results demonstrate that the antibacterial action of CNFs depends on the protonation of their amino groups, regardless of bacterial type and their mechanism of action was bactericidal rather than bacteriostatic. Results also indicate that bacterial susceptibility was not Gram-dependent but strain-dependent, with non-virulent bacteria showing higher susceptibility at a reduction rate of 99.9%. The susceptibility order was: E. coli > L. innocua > S. aureus > S. Typhimurium. Finally, an extension of one week of the shelf life of fresh meat was successfully achieved. These results are promising and of great utility for the potential use of CNFs as bioactive food packaging materials in the food industry, and more specifically in meat quality preservation.

Chitosan-Based Conventional and Pickering Emulsions with Long-Term Stability.

Chitosan-based conventional and Pickering oil-in-water (O/W) emulsions with very fine droplet size (volume average diameter, dv, as low as 1.7 µm) and long-term stability (up to 5 months) were ultrasonically generated at different pH values without the addition of any surfactant or cross-linking agent. The ultrasonication treatment was found to break and disperse chitosan agglomerates effectively (particularly at pH ≥ 4.5) and also reduce the chitosan molecular weight, benefiting its emulsification properties. The emulsion stability and emulsion type could be controlled by chitosan solution pH. Increasing pH from 3.5 to 5.5 led to the formation of conventional emulsions with decreasing droplet size (dv from 14 to 2.1 µm) and increasing emulsion stability (from a few days to 2 months). These results can be explained by the increase of dynamic interfacial pressure, which results from the conformation transition of chitosan molecules from an extended state to a more flexible structure as pH increases. At pH = 6.5 (the acid dissociation constant (pKa) of chitosan), the chitosan molecules self-assembled into well-dispersed nanoparticles (dv = 82.1 nm) with the assistance of ultrasonication, which resulted in a Pickering emulsion with the smallest droplet size (dv = 1.7 µm) and highest long-term stability (up to 5 months) because of the presence of chitosan solid nanoparticles at the oil/water interface. The key originality of this study is the elucidation of the role of pH in the formation of conventional and Pickering chitosan-based O/W emulsions with the assistance of ultrasonication. Our results suggest that chitosan possesses great potential to be used as an effective pH-controlled emulsifier and stabilizer without the need of other additives.

Properties of polylactide inks for solvent-cast printing of three-dimensional freeform microstructures.

Solvent-cast printing is a highly versatile microfabrication technique that can be used to construct various geometries such as filaments, towers, scaffolds, and freeform circular spirals by the robotic deposition of a polymer solution ink onto a moving stage. In this work, we have performed a comprehensive characterization of the solvent-cast printing process using polylactide (PLA) solutions by analyzing the flow behavior of the solutions, the solvent evaporation kinetics, and the effect of process-related parameters on the crystallization of the extruded filaments. Rotational rheometry at low to moderate shear rates showed a nearly Newtonian behavior of the PLA solutions, while capillary flow analysis based on process-related data indicated shear thinning at high shear rates. Solvent vaporization tests suggested that the internal diffusion of the solvent through the filaments controlled the solvent removal of the extrudates. Different kinds of three-dimensional (3D) structures including a layer-by-layer tower, nine-layer scaffold, and freeform spiral were fabricated, and a processing map was given to show the proper ranges of process-related parameters (i.e., polymer content, applied pressure, nozzle diameter, and robot velocity) for the different geometries. The results of differential scanning calorimetry revealed that slow solvent evaporation could increase the ability of PLA to complete its crystallization process during the filament drying stage. The method developed here offers a new perspective for manufacturing complex structures from polymer solutions and provides guidelines to optimize the various parameters for 3D geometry fabrication.

Hierarchical structure and physicochemical properties of plasticized chitosan.

Plasticized chitosan with hierarchical structure, including multiple length scale structural units, was prepared by a "melt"-based method, that is, thermomechanical mixing, as opposed to the usual casting-evaporation procedure. Chitosan was successfully plasticized by thermomechanical mixing in the presence of concentrated lactic acid and glycerol using a batch mixer. Different plasticization formulations were compared in this study, in which concentrated lactic acid was used as protonation agent as well as plasticizer. The microstructure of thermomechanically plasticized chitosan was investigated by X-ray diffraction, scanning electron microscopy, and optical microscopy. With increasing amount of additional plasticizers (glycerol or water), the crystallinity of the plasticized chitosan decreased from 63.7% for the original chitosan powder to almost zero for the sample plasticized with additional water. Salt linkage between lactic acid molecules and amino side chains of chitosan was confirmed by FTIR spectroscopy: the lactic acid molecules expanded the space between the chitosan molecules of the crystalline phase. In the presence of other plasticizers (glycerol and water), various levels of structural units including an amorphous phase, nanofibrils, nanofibril clusters, and microfibers were produced under mechanical shear and thermal energy and identified for the first time. The thermal and thermomechanical properties of the plasticized chitosan were measured by thermogravimetric analysis, differential scanning calorimetric, and DMA. These properties were correlated with the different levels of microstructure, including multiple structural units.

Solvent-cast three-dimensional printing of multifunctional microsystems.

The solvent-cast direct-write fabrication of microstructures is shown using a thermoplastic polymer solution ink. The method employs the robotically controlled microextrusion of a filament combined with a rapid solvent evaporation. Upon drying, the increased rigidity of the extruded filament enables the creation of complex freeform 3D shapes.

Characterization of heat-treated chitosan cast films and their antimicrobial activity on the growth of natural flora of pasteurized milk.

This research aimed to evaluate the physicochemical and biocidal properties of chitosan films obtained through the solvent casting method using two different molecular weights, and thermally treated for an extended time (3 weeks) at 70 °C under vacuum condition (RH 0 %). The effect of storage time (for 30 and 180 days) under ambient conditions (23 °C and RH 40 %) on the properties of heat-treated cast films and their biocidal effectiveness was also assessed. FTIR-ATR, TGA and XRD of resulting films were analyzed to explore the dependency of antibacterial performance on the alteration in molecular and chemical structure. The results demonstrated that the solubility of treated films at 70 °C was proportionally reduced, resulting from the reduction of protonated amines and an increase in crystallinity. Likewise, increasing storage time led to a significant lowering in the solubilization of cast films. It was found that the solubilized fraction of chitosan cast films is the active fraction with the biocide behavior that can act against bacteria. In addition, the effectiveness of migrated chitosan was examined against the natural flora of pasteurized milk, such as Paenibacillus and Pseudomonas fluorescens. The results showed that cast films obtained from chitosan with lower molecular weight caused a reduction in the total count of viable cells without a significant effect on the properties of milk.

Core-shell structured PEO-chitosan nanofibers by coaxial electrospinning.

Core-shell structured PEO-chitosan nanofibers have been produced using a coaxial electrospinning setup. PEO and chitosan solutions, both in an aqueous acetic acid solvent, were used as the inner (core) and outer (shell) layer, respectively. Uniform-sized defect-free nanofibers of 150-190 nm diameter were produced. In addition, hollow nanofibers could be obtained subsequent to PEO washing of the membranes. The core-shell nanostructure and existence of chitosan on the shell layer were confirmed by TEM images obtained before and after washing the PEO content with water. The presence of chitosan on the surface of the composite nanofibers was further supported by XPS studies. The chitosan and PEO compositions in the nanofibrous mats were determined by TGA analysis, which were similar to their ratio in the feed solutions. The local compositional homogeneity of the membranes and the efficiency of the washing step to remove PEO were also verified by FTIR. In addition, DSC and XRD were used to characterize the crystalline structure and morphology of the co-electrospun nonwoven mats. The prepared coaxial nanofibers (hollow and solid) have several potential applications due to the presence of chitosan on their outer surfaces.

Morphological and Rheological Properties of PLA, PBAT, and PLA/PBAT Blend Nanocomposites Containing CNCs.

Morphological and rheological properties of poly(lactic acid), PLA (semicrystalline and amorphous), and poly(butylene adipate-co-terephthalate), PBAT, and their blends (75 wt%/25 wt%; PLA/PBAT) were investigated in the presence of cellulose nanocrystals (CNCs) prepared from solution casting followed by melt mixing. For the solution casting step, the CNCs were either incorporated into the matrix, the dispersed phase, or both. The dispersion and distribution of the CNCs in the neat polymers and localization in their blends were analyzed via scanning electron microscopy (SEM) and atomic force microscopy (AFM). The highly dispersed CNCs in the solution cast nanocomposites were agglomerated after melt mixing. In the blends with 1 wt% CNCs, the nanoparticles were mostly localized on the surface of the PBAT droplets irrespective of their initial localization. The rheological behavior of the single polymer matrix nanocomposites and their blends was determined in dynamic and transient shear flow in the molten state. Upon melt mixing the complex viscosity and storage modulus of the solution cast nanocomposites decreased markedly due to re-agglomeration of the CNCs. Under shearing at 0.1 s-1, a significant droplet coalescence was observed in the neat blends, but was prevented by the presence of the CNCs at the interface in the blend nanocomposites.

The Polymeric Matrix Composition of Vibrio cholerae Biofilms Modulate Resistance to Silver Nanoparticles Prepared by Hydrothermal Synthesis.

Biofilms represent the dominant microbial lifestyle in nature. These complex microbial communities in which bacteria are embedded in a self-produced protective polymeric extracellular matrix, display an enhanced resistance to antimicrobials and thus represent a major health challenge. Although nanoparticles have proven to be effective against bacteria, the interactions between nanoparticles and the polymeric biofilm matrix are still unclear. In this work, silver nanoparticles (AgNPs) were used on mature biofilms formed by the pathogen Vibrio cholerae, and their effects on the biofilm microstructure were evaluated. Bacteria cells within mature biofilms showed an increased tolerance to AgNPs, with their elimination requiring a concentration nine times higher than planktonic cells. Mutant strains not able to form a pellicle biofilm were four times more susceptible to AgNPs than the wild-type strain forming a strong biofilm. Moreover, electron microscopy analysis revealed that AgNPs interacted with the extracellular matrix components and disrupted its microstructure. Finally, two major proteins, Bap1 and RbmA, appeared to mediate the biofilm bacterial resistance to AgNPs. This work highlights the role of the polymeric biofilm matrix composition in resistance to AgNPs. It underlines how crucial it is to understand and characterize the interactions between nanoparticles and the biofilm matrix, in order to design appropriate metallic nanoparticles efficient against bacterial biofilms.

Rheological insights on the evolution of sonicated cellulose nanocrystal dispersions.

Cellulose nanocrystals (CNCs) are promising biomaterials, but their tendency to agglomerate when dried limits their use in several applications. Ultrasonication is commonly used to disperse CNCs in water, bringing enough energy to the suspension to break agglomerates. While the optimized parameters for sonication are now well defined for small volumes of low concentration CNC suspensions, a deeper understanding of the influence of the dispersing process is needed to work with larger volumes, at higher concentrations. Herein, rheology is used to define the distribution and dispersion states upon ultrasonication of a 3.2 wt% CNC suspension. After considering the importance of the measurement sampling volume, the behavior of a more concentrated suspension (6.4 wt%) is examined and compared with a never-dried suspension of the same concentration to validate the dispersion state.

Evidence-based guidelines for the ultrasonic dispersion of cellulose nanocrystals.

Nanoparticles possess unique, size-driven properties. However, they can be challenging to use as they easily agglomerate - their high surface area-to-volume ratio induces strong interparticle forces, generating agglomerates that are difficult to break. This issue prevails in organic particles as well, such as cellulose nanocrystals (CNCs); when in their dried form, strong hydrogen bonding enhances agglomeration. Ultrasonication is widely applied to prepare CNC suspensions, but the methodology employed is non-standardized and typically under-reported, and process efficiency is unknown. This limits the ability to adapt dispersion protocols at industrial scales. Herein, numerical simulations are used in conjunction with validation experiments to define and optimize key parameters for ultrasonic dispersion of CNCs, allowing an operating window to be inferred.

Self-assembly behaviors of colloidal cellulose nanocrystals: A tale of stabilization mechanisms.

HYPOTHESIS: In solvent casting, colloidal nanocrystal self-assembly patterns are controlled by a mix of cohesive and repulsive interactions that promote destabilization-induced self-assembly (DISA) or evaporation-induced self-assembly (EISA). Tuning the strength and nature of the stabilization mechanisms may allow repulsive interactions to govern self-assembly during the casting of colloidal cellulose nanocrystal (CNC) suspensions. EXPERIMENTS: We propose a tool to classify the level of electrostatic and solvation-induced stabilizations based on two solvent parameters only: dielectric constant, ε, and chemical affinity for CNCs, in terms of Hansen Solubility Parameters, Ra. These criteria are applied to study CNC self-assembly in solvent casting experiments in various media and binary mixtures. FINDINGS: In solvent casting of suspensions stabilized through a combination of electrostatic and solvation effects, the primarily governing mechanism is EISA, which leads to the formation of chiral nematic domains and optically active thin films. In electrostatically-stabilized suspensions, EISA and DISA are in competition and casting may yield anything from a continuous film to a powder. In other suspensions, DISA prevails and evaporation yields a powder of CNC agglomerates. By classifying media according to their stabilization mechanisms, this work establishes that the behavior of CNC suspensions in solvent casting may be predicted from solvent parameters only.

Mechanical and microstructural insights of Vibrio cholerae and Escherichia coli dual-species biofilm at the air-liquid interface.

Biofilm is the dominant microbial form found in nature, in which bacterial species are embedded in a self-produced extracellular matrix (ECM). These complex microbial communities are responsible for several infections when they involve multispecies pathogenic bacteria. In previous studies, interfacial rheology proved to be a unique quantitative technique to follow in real-time the biofilm formation at the air-liquid interface. In this work, we studied a model system composed of two bacteria pathogenic capable of forming a pellicle biofilm, V. cholerae and E. coli. We used an integrated approach by combining a real-time quantitative analysis of the biofilm rheological properties, with the investigation of major matrix components and the pellicle microstructure. The results highlight the competition for the interface between the two species, driven by the biofilm formation growth rate. In the dual-species biofilm, the viscoelastic properties were dominated by V. cholera, which formed a mature biofilm 18 h faster than E. coli. The microstructure of the dual-species biofilm revealed a similar morphology to V. cholerae alone when both bacteria were initially added at the same amount. The analysis of some major ECM components showed that E. coli was not able to produce curli in the presence of V. cholerae, unless enough time was given for E. coli to colonize the air-liquid interface first. E. coli secreted phosphoethanolamine (pEtN) cellulose in the dual-species biofilm, but did not form a filamentous structure. Our pathogenic model system demonstrated the importance of the biofilm growth rate for multispecies biofilm composition at the air-liquid interface.

The structural amphiphilicity of cellulose nanocrystals characterized from their cohesion parameters.

Cellulose nanocrystals (CNCs), usually considered as isotropically polar nanoparticles, are sheet-like crystalline assemblies of cellulose chains. Here, we link the anisotropy of the CNC structure to an amphiphilic behavior in suspension. The Hansen solubility parameters (HSP: δD;δP;δH) of wood-based H2SO4-hydrolyzed CNCs were measured from sedimentation tests in a wide set of 59 solvents and binary mixtures. Two sets of cohesion parameters corresponding to a polar surface (18.1; 20.4; 15.3) ±â€¯(0.5; 0.5; 0.4) MPa1/2 and to a mildly non-polar one (17.4; 4.8; 6.5) ±â€¯(0.3; 0.5; 0.6) MPa1/2 were determined, with respective solubility radii of 7.8 and 2.1 MPa1/2. The polar sphere is thought to correspond to the (110) & (11¯0) surfaces of cellulose Iß nanocrystals, while the smaller non-polar sphere is coherent with the exposure of (200) surfaces. The HSP graph provides new insights on the amphiphilic nature of CNCs and a mapping of their chemical affinity for solvents and polymer matrices.

Poly (lactic acid) blends: Processing, properties and applications.

Poly (lactic acid) or polylactide (PLA) is a commercial biobased, biodegradable, biocompatible, compostable and non-toxic polymer that has competitive material and processing costs and desirable mechanical properties. Thereby, it can be considered favorably for biomedical applications and as the most promising substitute for petroleum-based polymers in a wide range of commodity and engineering applications. However, PLA has some significant shortcomings such as low melt strength, slow crystallization rate, poor processability, high brittleness, low toughness, and low service temperature, which limit its applications. To overcome these limitations, blending PLA with other polymers is an inexpensive approach that could also tailor the final properties of PLA-based products. During the last two decades, researchers investigated the synthesis, processing, properties, and development of various PLA-based blend systems including miscible blends of poly l-lactide (PLLA) and poly d-lactide (PDLA), which generate stereocomplex crystals, binary immiscible/miscible blends of PLA with other thermoplastics, multifunctional ternary blends using a third polymer or fillers such as nanoparticles, as well as PLA-based blend foam systems. This article reviews all these investigations and compares the syntheses/processing-morphology-properties interrelationships in PLA-based blends developed so far for various applications.