Stable Electrospinning of Core-Functionalized Coaxial Fibers Enabled by the Minimum-Energy Interface Given by Partial Core–Sheath Miscibility

Abstract

Core–sheath electrospinning is a powerful tool for producing composite fibers with one or multiple encapsulated functional materials, but many material combinations are difficult or even impossible to spin together. We show that the key to success is to ensure a well-defined core–sheath interface while also maintaining a constant and minimal interfacial energy across this interface. Using a thermotropic liquid crystal as a model functional core and polyacrylic acid or styrene-butadiene-styrene block copolymer as a sheath polymer, we study the effects of using water, ethanol, or tetrahydrofuran as polymer solvent. We find that the ideal core and sheath materials are partially miscible, with their phase diagram exhibiting an inner miscibility gap. Complete immiscibility yields a relatively high interfacial tension that causes core breakup, even preventing the core from entering the fiber-producing jet, whereas the lack of a well-defined interface in the case of complete miscibility eliminates the core–sheath morphology, and it turns the core into a coagulation bath for the sheath solution, causing premature gelation in the Taylor cone. Moreover, to minimize Marangoni flows in the Taylor cone due to local interfacial tension variations, a small amount of the sheath solvent should be added to the core prior to spinning. Our findings resolve a long-standing confusion regarding guidelines for selecting core and sheath fluids in core–sheath electrospinning. These discoveries can be applied to many other material combinations than those studied here, enabling new functional composites of large interest and application potential.

Multiresponsive Cylindrically Symmetric Cholesteric Liquid Crystal Elastomer Fibers Templated by Tubular Confinement

“…[ 42 ] By adapting these methods to spin a dissolved polymer as sheath that will form a tube directly around a CLCE precursor core upon sheath solvent removal, our CLCE fibers could be made without length restriction. This is not a trivial route, however, as it is critical to balance minimal core–sheath interfacial tension against partial immiscibility, [ 43 ] and also to avoid that the presence of the core reduces spinnability of the sheath. [ 44 ] We are currently exploring these avenues and hope to report on the results in the near future.…”

Continuous Flow Microfluidic Production of Arbitrarily Long Tubular Liquid Crystal Elastomer Peristaltic Pump Actuators

“…With a common solvent diffusing from one phase to its neighbor, the interface is blurred out, corresponding to γ → 0 and thus stabilizing the interface, as recently demonstrated during coaxial electrospinning of immiscible liquids stabilized by a common co-solvent. [43] The miscibility of acetic acid and DCM has the additional benefit that the extraction of DCM from the LCO solution into the surrounding aqueous phases, required to recover the nematic phase of the LCO, should be promoted. As shown in Video S2, Supporting Information, and the top row of Figure 3 (showing still frames from the video), the length of the coaxial jet is significantly extended by the presence of acetic acid, the first signs of undulating interfaces seen only toward the end of the capillary device, ≈5 cm from the origin of the jet.…”

Cellulosic Nanofibers Utilizing a Silicone Elastomeric Core to Form Stretchable Paper

“…[19][20][21] Various soft bioelectronics platforms for wearable and implantable devices have demonstrated continuous monitoring and stimulation of physiological information. Examples include noninvasive 3D microfluidic biosensors that detect biomarkers in sweat or saliva, [22][23][24][25][26] patterned microelectronics such as LED displays on stretchable rubber, [27] an inkjet-printed transistor array on a silicone elastomer, [16] and breathable porous biosensors for free gas exchange. [17,18] To take advantage of paper-based electronics and soft bioelectronics, a single fiber structure has been designed for building units of stretchable paper consisting of silicone elastomers and cellulose.…”

“…We hypothesize that coaxial electrospinning can employ a core-sheath nanofiber structure of an elastomeric core to a cellulose polymeric sheath. By altering electrospinning processing parameters, [19,20] fiber morphology, [21] coresheath structure, [22] and other physical properties can be modified to allow for both elastomer and cellulose polymer to coexist into a single fiber creating a stretchable paper. [23] Herein, a nanofibrous substrate integrating a silicone elastomer and cellulose-based polymer to create a stretchable paper.…”

“…[ 19–21 ] Various soft bioelectronics platforms for wearable and implantable devices have demonstrated continuous monitoring and stimulation of physiological information. Examples include noninvasive 3D microfluidic biosensors that detect biomarkers in sweat or saliva, [ 22–26 ] patterned microelectronics such as LED displays on stretchable rubber, [ 27 ] an inkjet‐printed transistor array on a silicone elastomer, [ 16 ] and breathable porous biosensors for free gas exchange. [ 17,18 ]…”