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Abstract: Electrospinning is a versatile method for synthesizing nanofibrous structures from nearly all polymers, offering a solution for the industrial‐scale mass production of nanomaterials in a wide range of applications. However, the continuous non‐woven structure intrinsic to electrospun fibers limits their applications, where a smaller length scale is desired. Here, we present a novel method to synthesize polymeric nanofiber‐fragments based on colloid electrospinning of polymer and sacrificial silica nanoparticles, followed by mechanical fracturing with ultrasonication. The size and hydrophobicity of silica nanoparticles are optimized for their improved integration within the polymer matrix, and the controllability of nanofiber‐fragment length by the amount of silica nanoparticle loading, down to 2 µm in length for poly(vinylidene fluoride‐trifluoroethylene) nanofibers with an average fiber diameter of approximately 100 nm, is shown. The resultant nanofiber‐fragments are shown to maintain their material properties including piezoelectric coefficients and their enhanced injectability for drug delivery application is demonstrated with an animal model.

Continuous quantitative monitoring of the change in mineral content during the bone healing process is crucial for efficient clinical treatment. Current radiography-based modalities, however, pose various technological, medical, and economical challenges such as low sensitivity, radiation exposure risk, and high cost/instrument accessibility. In this regard, an analytical approach utilizing electrochemical impedance spectroscopy (EIS) assisted by machine learning algorithms is developed to quantitatively characterize the physico-electrochemical properties of the bone, in response to the changes in the bone mineral contents. The system is designed and validated following the process of impedance data measurement, equivalent circuit model designing, machine learning algorithm optimization, and data training and testing. Overall, the systematic machine learning-based classification utilizing the combination of EIS measurements and electrical circuit modeling offers a means to accurately monitor the status of the bone healing process.

Due to dissimilarities in genetics and metabolism, current animal models cannot accurately depict human neurological diseases. To develop patient-specific in vitro neural models, a functional material-based technology that offers multi-potent stimuli for enhanced neural tissue development is devised. An electrospun piezoelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) nanofibrous scaffold is systematically optimized to maximize its piezoelectric properties while accommodating the cellular behaviors of neural stem cells. Hydro-acoustic actuation is elegantly utilized to remotely activate the piezoelectric effect of P(VDF-TrFE) scaffolds in a physiologically-safe manner for the generation of cell-relevant electric potentials. This mechano-electrical stimulation, which arose from the deflection of the scaffold and its consequent generation of electric charges on the scaffold surface under hydro-acoustic actuation, induces the multi-phenotypic differentiation of neural stem cells simultaneously toward neuronal, oligodendrocytic, and astrocytic phenotypes. As compared to the traditional biochemically-mediated differentiation, the 3D neuron-glial interface induced by the mechano-electrical stimulation results in enhanced interactions among cellular components, leading to superior neural connectivity and functionality. These results demonstrate the potential of piezoelectric material-based technology for developing functional neural tissues in vitro via effective neural stem cell modulation with multi-faceted regenerative stimuli.

The control over biodistribution and pharmacokinetics is critical to enhance the efficacy and minimize the side effects of therapeutic agents. To address the need for an on-demand drug delivery system for precise control over the release time and the quantity of drugs, we exploited the mechano-responsiveness of piezoelectric poly(vinylidene fluoride-trifluroethylene) (P(VDF-TrFE)) nanofibers for drug delivery applications. The large surface area-to-volume ratio inherent to nanomaterials, together with the transformative piezoelectric properties, allowed us to use the material as an ultrasensitive and mechano-responsive drug delivery platform driven by the direct piezoelectric effect. The intrinsic negative zeta potential of the nanofibers was utilized to electrostatically load cationic drug molecules, where surface potential changes by exogenous mechanical actuation trigger the release of drug molecules. We show that the drug release kinetics of the P(VDF-TrFE) nanofibers depends on the fiber diameter, thus piezoelectric properties. We further demonstrated that the drug release quantity can be tuned by the applied pressure or dose of physiologically safe corporeal shockwaves as a mechanical stimulus in in vitro and ex vivo models. Overall, we demonstrated the utility of piezoelectric electrospun nanofibers for mechano-responsive controlled drug release.

In spite of the recent advances in the development of high performing piezoelectric materials, their applications are typically limited to the direct conversion of mechanical impact energy to electrical energy, potentially risking mechanical failures. In this study, we developed piezoelectric poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) nanofibers integrated with SiO2-shelled Fe3O4 magnetic nanoparticles, to utilize magnetic energy to reliably drive the piezoelectric effect. Specifically, we show that the shape of the magnetic nanoparticles exerts a significant effect on the efficiency of the magneto-mechano-electrical energy conversion as magnetic nanorods exhibit approximately 70% enhancement in electric field generation under cyclic magnetic fields as compared to nanospheres. Under an alternating magnetic field of 200 mT, the magnetic nanorod-piezoelectric nanofiber composite generated a peak-to-peak voltage of approximately 30 mVp-p with a superior durability without any performance degradation after over 1 million cycles. This study demonstrates the potential of magnetic-field responsive, piezoelectric-based materials in energy harvesting applications from non-mechanical energy sources.

While electrodeposited antimony telluride thin films with silver contents demonstrated promising thermoelectric properties, their thermal conductivity and the silver content dependence remain unknown. Here, we report the thermal conductivities of Ag3.9Sb33.6Te62.5 and AgSbTe2 thin films with controlled annealing and temperature conditions and demonstrate the impact of silver content on thermal transport. After annealing at 160 °C, the room-temperature thermal conductivity of Ag3.9Sb33.6Te62.5 and AgSbTe2 thin films increases from 0.24 to 1.59 Wm-1 K-1 and from 0.17 to 0.56 Wm-1 K-1, respectively. Using phonon transport models and X-ray diffraction measurements, we attribute the thermal conductivity increases to the crystal growth and explain the thermal conductivity variations with the degree of crystallization. Unlike electrical properties reported in previous studies, the presence of silver contents has little impact on the thermal conductivity of Ag3.9Sb33.6Te62.5 and leads to a strong reduction in the thermal conductivity of AgSbTe2 thin films. By performing transient thermal conductivity measurements at 94 °C, we find the crystallization activation energy of Ag3.9Sb33.6Te62.5 and AgSbTe2 films as 1.14 eV and 1.16 eV, respectively. Their differences reveal the role of silver in inhibiting the nucleation and growth of Sb2Te3 crystals and impeding thermal transport. These findings provide guidance for optimizing doping and annealing conditions of antimony tellurides for near-room-temperature thermoelectric applications.

High-throughput synthesis of Samarium-Cobalt sub-micron fibers with controlled composition and dimension was demonstrated by combining electrospinning and reduction-diffusion processes. The composition of fibers was readily varied (8 < Sm < 20 at.%) by adjusting precursor composition whereas the diameter of fibers was precisely controlled by varying electrospinning parameters (e.g., applied voltage, solution feed rate, temperature, and humidity) to reach single-domain size. X-ray diffraction patterns confirmed that single phase Sm2Co17 fibers were synthesized when the metal precursor ratio (Sm3+/(Sm3++Co2+)) was precisely controlled at 10.6%, whereas mixed phases (i.e., Co-Sm2Co17 or Sm2Co17-Sm2Co7) were observed when the ratio is deviated from the stoichiometric. Magnetic saturation (Ms ) of the synthesized fibers monotonically decreased with an increased in Sm content. In contrast, coercivity (Hci) monotonically increased with an increase in Sm content.

Lead telluride (PbTe) nanofibers were fabricated by galvanic displacement of electrospun cobalt nanofibers where their composition and morphology were altered by adjusting the electrolyte composition and diameter of sacrificial cobalt nanofibers. By employing Co instead of Ni as the sacrificial material, residue-free PbTe nanofibers were synthesized. The Pb content of the PbTe nanofibers was slightly affected by the Pb2+ concentration in the electrolyte, while the average outer diameter increased with Pb2+ concentration. The surface morphology of PbTe nanofibers was strongly dependent on the diameter of sacrificial nanofibers where it altered from smooth to rough surface as the Pb2+ concentration increased. Some of thermoelectric properties [i.e., thermopower (S) and electrical conductivity(σ)] were systematically measured as a function of temperature. Energy barrier height (Eb) was found to be one of the key factors affecting the thermoelectric properties-that is, higher energy barrier heights increased the Seebeck coefficient, but lowered the electrical conductivity.

Monitoring pluripotent stem cell behaviors (self-renewal and differentiation to specific lineages/phenotypes) is critical for a fundamental understanding of stem cell biology and their translational applications. In this study, a multi-modal stem cell monitoring system was developed to quantitatively characterize physico-electrochemical changes of the cells in real time, in relation to cellular activities during self-renewal or lineage-specific differentiation, in a non-destructive, label-free manner. The system was validated by measuring physical (mass) and electrochemical (impedance) changes in human induced pluripotent stem cells undergoing self-renewal, or subjected to mesendodermal or ectodermal differentiation, and correlating them to morphological (size, shape) and biochemical changes (gene/protein expression). An equivalent circuit model was used to further dissect the electrochemical (resistive and capacitive) contributions of distinctive cellular features. Overall, the combination of the physico-electrochemical measurements and electrical circuit modeling collectively offers a means to longitudinally quantify the states of stem cell self-renewal and differentiation.

Piezoelectricity-based energy harvesting from wasted mechanical energies has garnered an increasing attention as a clean energy source. Especially, flexible organic piezoelectric materials provide an opportunity to exploit their uses in mechanically challenging areas where brittle inorganic counterparts have mechanical limitations. In this regard, electrospinning has shown its advantages of producing poly(vinylidene fluoride) (PVDF)-based nanofibrous structures without the necessity of a secondary processing to induce/increase piezoelectric properties. However, the effects of electrospun fiber dimension, one of the main morphological parameters in electrospun fibers, on piezoelectricity have not been fully understood. In this study, two dependent design of experiments (DOEs) were utilized to systematically control the dimensions of electrospun poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) to produce nanofibers having their diameter ranging from 1000 to sub-100 nm. Such a dimensional reduction resulted in the increase of piezoelectric responsible electroactive phase content and the degree of crystallinity. These changes in crystal structure led to approximately 2-fold increase in piezoelectric constant as compared to typical P(VDF-TrFE) thin films. More substantially, the dimensional reduction also increased the Young's modulus of the nanofibers up to approximately 80-fold. The increases in piezoelectric constant and Young's modulus collectively enhanced piezoelectric performance, resulting in the exponential increase in electric output of nanofiber mats when the fiber diameters were reduced from 860 nm down to 90 nm. Taken together, the results suggest a new strategy to improve the piezoelectric performance of electrospun P(VDF-TrFE) via optimization of their electromechanical and mechanical properties.

Essential properties for a Point of Use (POU) water filter include maintaining high removal capacity and rate, with excellent mechanical properties to withstand pressure drop. Herein, mechanically robust tri-composite polyamide 6/iron oxide nanoparticles/tetra-n-butylammonium bromide (PA6/α-Fe[sub 2]O[sub 3]/TBAB) nanofiber composite membranes were electrospun for phosphate (P) remediation, where the diameter and composition were tuned by controlling solution compositions and electrospinning conditions. Tri-composite composition and morphology affect phosphate uptake where the adsorption capacity followed Langmuir isotherm whereas the adsorption kinetics followed pseudo second order behavior. Mechanical properties (i.e., Young's Modulus (E) and toughness) were significantly influenced by the composition and morphology of the tricomposite, as well. Although additional TBAB and iron oxide decreased toughness, there are optimum composition ranges which resulted in maximum Young's Modulus. Of the synthesized nanofiber membranes, PA6/α-Fe2O3/TBAB nanofibers with 17% α-Fe2O3 and 2% TBAB showed excellent phosphate uptake capacity [i.e., 8.9 mg/g (52 mg of P/g of α-Fe2O3)] while it is bendable, stretchable, and able to plastically deform without fracturing (i.e., Young's modulus of 2.06 x 108 Pa and Toughness of 1.35 x 106 J m-3). With concerns over the impact of P on water resources and the long-term availability of limited P resources, this tri-composite membrane is well suited for applications in both wastewater treatment and resource recovery. [ABSTRACT FROM AUTHOR]