Evaporation and Rheology Chart the Processability Map for Centrifugal Force Spinning

Pinching dynamics, extensional rheology, and stringiness of saliva substitutes

Saliva substitutes are human-made formulations extensively used in medicine, food, and pharmaceutical research to emulate human saliva's biochemical, tribological, and rheological properties. Even though extensional flows involving saliva are commonly encountered in situations such as swallowing, coughing, sneezing, licking, drooling, gleeking, and blowing spit bubbles, rheological evaluations of saliva and its substitutes in most studies rely on measured values of shear viscosity. Natural saliva possesses stringiness or spinnbarkeit, governed by extensional rheology response, which cannot be evaluated or anticipated from the knowledge of shear rheology response. In this contribution, we comprehensively examine the rheology of twelve commercially available saliva substitutes using torsional rheometry for rate-dependent shear viscosity and dripping-onto-substrate (DoS) protocols for extensional rheology characterization. Even though most formulations are marketed as having suitable rheology, only three displayed measurable viscoelasticity and strain-hardening. Still, these too, failed to emulate the viscosity reduction with the shear rate observed for saliva or match perceived stringiness. Finally, we explore the challenges in creating saliva-like formulations for dysphagia patients and opportunities for using DoS rheometry for diagnostics and designing biomimetic fluids.

Recent Advances in Centrifugal Spinning and Their Applications in Tissue Engineering

Over the last decade, researchers have investigated the potential of nano and microfiber scaffolds to promote wound healing, tissue regeneration, and skin protection. The centrifugal spinning technique is favored over others due to its relatively straightforward mechanism for producing large quantities of fiber. Many polymeric materials have yet to be investigated in search of those with multifunctional properties that would make them attractive in tissue applications. This literature presents the fundamental process of fiber generation, and the effects of fabrication parameters (machine, solution) on the morphologies such as fiber diameter, distribution, alignment, porous features, and mechanical properties. Additionally, a brief discussion is presented on the underlying physics of beaded morphology and continuous fiber formation. Consequently, the study provides an overview of the current advancements in centrifugally spun polymeric fiber-based materials and their morphological features, performance, and characteristics for tissue engineering applications.

Fabrication of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) Fibers Using Centrifugal Fiber Spinning: Structure, Properties and Application Potential

Biobased and biodegradable polyhydroxyalkanoates (PHAs) are currently gaining momentum. Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) polymer has a useful processing window for extrusion and injection molding of packaging, agricultural and fishery applications with required flexibility. Processing PHBHHx into fibers using electrospinning or centrifugal fiber spinning (CFS) can further broaden the application area, although CFS remains rather unexplored. In this study, PHBHHx fibers are centrifugally spun from 4-12 wt.% polymer/chloroform solutions. Beads and beads-on-a-string (BOAS) fibrous structures with an average diameter (ϕ_av) between 0.5 and 1.6 µm form at 4-8 wt.% polymer concentrations, while more continuous fibers (ϕ_av = 3.6-4.6 µm) with few beads form at 10-12 wt.% polymer concentrations. This change is correlated with increased solution viscosity and enhanced mechanical properties of the fiber mats (strength, stiffness and elongation values range between 1.2-9.4 MPa, 11-93 MPa, and 102-188%, respectively), though the crystallinity degree of the fibers remains constant (33.0-34.3%). In addition, PHBHHx fibers are shown to anneal at 160 °C in a hot press into 10-20 µm compact top-layers on PHBHHx film substrates. We conclude that CFS is a promising novel processing technique for the production of PHBHHx fibers with tunable morphology and properties. Subsequent thermal post-processing as a barrier or active substrate top-layer offers new application potential.

Centrifugally spun poly(D,L-lactic acid)-alginate composite microbeads for drug delivery and tissue engineering

This work was based on medium-viscosity alginate as a minor constituent in composites with poly lactic acid (PLA) with the objective to prepare compositional variants through Forcespinning® (FS); for future medical applications. Composites within 0.08-0.25 wt% medium-viscosity alginate were used, at fixed PLA, 6.6 wt%, compared with a study using 0.17-0.48 wt% low-viscosity alginate (same PLA), starting from water-in-oil emulsions, before FS. The presence of alginate is proposed here to influence the high surface tension existing at the emulsion water/oil interface, reducing the total energy at this interface, and/or facilitating the particles in the amphiphilic blend to lie-flat (re-orient) for better fit to the PLA curvature. The study revealed a direct correlation of the inner-phase size (alginate/water ratio), to the change in the morphology and structure of the resultant composites before and after FS. The change in the alginate type, revealed characteristics better suited for medical applications by the medium-viscosity alginate. Composites at alginate- medium-viscosity; ≤0.25 wt%, and low-viscosity; ≤0.48 wt%, had fiber networks interwoven with micro-beads, with characteristics better suited for controlled-release drug delivery applications. Alternatively, each alginate type at 1.1 wt%, composites with PLA at 6.6 wt% could bring about homogenous fibrous materials better suited for wound dressing.

Centrifugally spun alginate-poly(lactic acid) microbeads: A promising carrier for drug delivery and tissue engineering

A facile and high yield centrifugal spinning technique known as Forcespinning® (FS) was used to develop unique microstructures consisting of PLA microbeads along alginate fibers. Morphological variation and structural features appeared in the field-emission scanning electron micrographs for the PLA-alginate composites and dried PLA-alginate films from precursor emulsions at constant PLA and varied alginate contents. Shrunk and deflated microbeads were observed for composites whilst spherical beads were evident for the PLA control. Furthermore, PLA was found surrounding the alginate when the alginate was present at 0.24 wt% or lower, while alginate (mushroom-like structures), were seen protruding through the PLA layer at ≥0.34 wt% alginate. Rheological characterization of the composite emulsions revealed that the filler (alginate) provided shear thinning properties including pseudoplasticity, desirable for printing and other related applications in contrast to the Newtonian flow shown by the PLA control. Along with infra-red spectroscopy, the nanocomposites were further characterized using thermal gravimetry and differential scanning calorimetry featuring reversible events influenced by heat capacity and irreversible kinetic/thermodynamic counterparts. The work provides a comprehensive investigation of biocompatible networks of PLA-alginate microbeads embedded in nano-sized fibers and the prospective application of these microbeads as a drug delivery system.

Temperature-controlled dripping-onto-substrate (DoS) extensional rheometry of polymer micelle solutions

Capillary-driven thinning of a liquid bridge is commonly used to measure the extensional rheology of macromolecular solutions for assessment of material sprayability, printability, and jettability. Methods like dripping-onto-substrate (DoS) rheometry are often preferred to methods like capillary breakup extensional rheometry (CaBER) due to low required sample volume and ability to measure low-viscosity fluids; however, DoS measurements to-date have been limited to ambient temperatures. Here, an environmental control chamber is developed to enable temperature-controlled DoS (TC-DoS) measurements, and the temperature-dependent extensional rheology of a model system of poloxamer 234 (P234) in NaF brine is examined. Spherical P234 micelles at ambient conditions exhibit inertiocapillary (IC) thinning; above the sphere-to-rod transition temperature, the liquid bridge evolves towards viscocapillary (VC) thinning as micelles lengthen and shear viscosity increases. Above 37 °C, wormlike micelle (WLM) formation results in pronounced elastocapillary (EC) thinning, and further WLM growth and entanglement results in three elasticity-dominated flow regimes: EC thinning, beads-on-a-string (BOAS) instability formation, and BOAS thinning. Despite having a substantially larger amphiphile molecular weight and micelle cross-sectional radius than surfactant WLMs, entangled P234 WLMs exhibit similar extensional behavior and achieve comparable maximum Trouton ratios. Comparing DoS measurements of P234 WLMs with prior studies on surfactant WLMs reveals that the maximum Trouton ratio depends on the ratio of shear and extensional relaxation times, a trend undetectable via CaBER due to pre-deformation during the initial step stretch. These findings reveal rich temperature-dependent flow behaviors in polymer micelles and highlight the importance of using a minimally-disruptive method such as TC-DoS when measuring the extensional rheology of microstructured and thermosensitive fluids.

Evaporation-controlled dripping-onto-substrate (DoS) extensional rheology of viscoelastic polymer solutions

Extensional flow properties of polymer solutions in volatile solvents govern many industrially-relevant coating processes, but existing instrumentation lacks the environment necessary to control evaporation. To mitigate evaporation during dripping-onto-substrate (DoS) extensional rheology measurements, we developed a chamber to enclose the sample in an environment saturated with solvent vapor. We validated the evaporation-controlled DoS device by measuring a model high molecular weight polyethylene oxide (PEO) in various organic solvents both inside and outside of the chamber. Evaporation substantially increased the extensional relaxation time \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\lambda _{E}$$\end{document}λE for PEO in volatile solvents like dichloromethane and chloroform. PEO/chloroform solutions displayed an over 20-fold increase in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\lambda _{E}$$\end{document}λE due to the formation of an evaporation-induced surface film; evaporation studies confirmed surface features and skin formation reminiscent of buckling instabilities commonly observed in drying polymer solutions. Finally, the relaxation times of semi-dilute PEO/chloroform solutions were measured with environmental control, where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\lambda _{E}$$\end{document}λE scaled with concentration by the exponent \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$m=0.62$$\end{document}m=0.62. These measurements validate the evaporation-controlled DoS environment, and confirm that chloroform is a good solvent for PEO, with a Flory exponent of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\nu =0.54$$\end{document}ν=0.54. Our results are the first to control evaporation during DoS extensional rheology, and provide guidelines establishing when environmental control is necessary to obtain accurate rheological parameters.