Light-Emitting Microfibers from Lotus Root for Eco-Friendly Optical Waveguides and Biosensing.

Optical biosensors based on micro/nanofibers are highly valuable for probing and monitoring liquid environments and bioactivity. Most current optical biosensors, however, are still based on glass, semiconductors, or metallic materials, which might not be fully suitable for biologically relevant environments. Here, we introduce biocompatible and flexible microfibers from lotus silk as microenvironmental monitors that exhibit waveguiding of intrinsic fluorescence as well as of coupled light. These features make single-filament monitors excellent building blocks for a variety of sensing functions, including pH probing and detection of bacterial activity. These results pave the way for the development of new and entirely eco-friendly, potentially multiplexed biosensing platforms.

Size-dependent optical forces on dielectric microspheres in hollow core photonic crystal fibers: erratum.

The beam shape coefficients for cylindrical vector modes are of great importance for other researchers to reproduce our results, however they were accidentally reported incorrectly in our recently published manuscript [Opt. Express30(14), 24407 (2022)10.1364/OE.458674]. This erratum reports the correct form for the two expressions. Two typographical errors in auxiliary equations are also reported and two labels in particle time of flight probability density function plots are fixed.

Benchmarking YOLOv5 and YOLOv7 models with DeepSORT for droplet tracking applications.

Tracking droplets in microfluidics is a challenging task. The difficulty arises in choosing a tool to analyze general microfluidic videos to infer physical quantities. The state-of-the-art object detector algorithm You Only Look Once (YOLO) and the object tracking algorithm Simple Online and Realtime Tracking with a Deep Association Metric (DeepSORT) are customizable for droplet identification and tracking. The customization includes training YOLO and DeepSORT networks to identify and track the objects of interest. We trained several YOLOv5 and YOLOv7 models and the DeepSORT network for droplet identification and tracking from microfluidic experimental videos. We compare the performance of the droplet tracking applications with YOLOv5 and YOLOv7 in terms of training time and time to analyze a given video across various hardware configurations. Despite the latest YOLOv7 being 10% faster, the real-time tracking is only achieved by lighter YOLO models on RTX 3070 Ti GPU machine due to additional significant droplet tracking costs arising from the DeepSORT algorithm. This work is a benchmark study for the YOLOv5 and YOLOv7 networks with DeepSORT in terms of the training time and inference time for a custom dataset of microfluidic droplets.

Size-dependent optical forces on dielectric microspheres in hollow core photonic crystal fibers.

Optical forces on microspheres inside hollow core photonic crystal fibers (HC-PCFs) are often predicted using a ray optics model, which constrains its validity based on wavelength and microsphere sizes. Here, we introduce a rigorous treatment of the electromagnetic forces based on the Lorenz-Mie theory, which involves analytical determination of beam shape coefficients for the optical modes of a HC-PCF. The method is more practicable than numerical approaches and, in contrast with ray optics models, it is not limited by system size parameters. Time of flight measurements of microspheres flying through the HC-PCF lead to results consistent with the Lorenz-Mie predictions.

Three-Dimensional Printable Conductive Semi-Interpenetrating Polymer Network Hydrogel for Neural Tissue Applications.

Intrinsically conducting polymers (ICPs) are widely used to fabricate biomaterials; their application in neural tissue engineering, however, is severely limited because of their hydrophobicity and insufficient mechanical properties. For these reasons, soft conductive polymer hydrogels (CPHs) are recently developed, resulting in a water-based system with tissue-like mechanical, biological, and electrical properties. The strategy of incorporating ICPs as a conductive component into CPHs is recently explored by synthesizing the hydrogel around ICP chains, thus forming a semi-interpenetrating polymer network (semi-IPN). In this work, a novel conductive semi-IPN hydrogel is designed and synthesized. The hybrid hydrogel is based on a poly(N-isopropylacrylamide-co-N-isopropylmethacrylamide) hydrogel where polythiophene is introduced as an ICP to provide the system with good electrical properties. The fabrication of the hybrid hydrogel in an aqueous medium is made possible by modifying and synthesizing the monomers of polythiophene to ensure water solubility. The morphological, chemical, thermal, electrical, electrochemical, and mechanical properties of semi-IPNs were fully investigated. Additionally, the biological response of neural progenitor cells and mesenchymal stem cells in contact with the conductive semi-IPN was evaluated in terms of neural differentiation and proliferation. Lastly, the potential of the hydrogel solution as a 3D printing ink was evaluated through the 3D laser printing method. The presented results revealed that the proposed 3D printable conductive semi-IPN system is a good candidate as a scaffold for neural tissue applications.

Nanoscale Elastoplastic Wrinkling of Ultrathin Molecular Films.

Ultrathin molecular films deposited on a substrate are ubiquitously used in electronics, photonics, and additive manufacturing methods. The nanoscale surface instability of these systems under uniaxial compression is investigated here by molecular dynamics simulations. We focus on deviations from the homogeneous macroscopic behavior due to the discrete, disordered nature of the deformed system, which might have critical importance for applications. The instability, which develops in the elastoplastic regime above a finite critical strain, leads to the growth of unidimensional wrinkling up to strains as large as 0.5. We highlight both the dominant wavelength and the amplitude of the wavy structure. The wavelength is found to scale geometrically with the film length, λ∝L, up to a compressive strain of ε≃0.4 at least, depending on the film length. The onset and growth of the wrinkling under small compression are quite well described by an extended version of the familiar square-root law in the strain ε observed in macroscopic systems. Under large compression (ε≳0.25), we find that the wrinkling amplitude increases while leaving the cross section nearly constant, offering a novel interpretation of the instability with a large amplitude. The contour length of the film topography is not constant under compression, which is in disagreement with the simple accordion model. These findings might be highly relevant for the design of novel and effective wrinkling and buckling patterns and architectures in flexible platforms for electronics and photonics.

The Heterogeneity of Renal Stem Cells and Their Interaction with Bio- and Nano-materials.

For a long time, the kidney has been considered incapable of regeneration. Instead, in recent years, studies have supported the existence of heterogeneity of renal stem/progenitor cells with the ability to regenerate both glomerular and tubular epithelial cells. Indeed, several studies evidence that renal progenitor cells, releasing chemokines, growth factors, microvesicles, and transcription factors through paracrine mechanisms, can induce tissue regeneration and block pathological processes of the kidney. In this chapter the potentiality of the kidney regenerative processes is considered and reviewed, and the main classes of stem/progenitor cells that might contribute to the renal tissue renewal is analyzed. Moreover, we evaluate the role of biomaterials in the regulation of cellular functions, specifically addressing renal stem/progenitor cells. Materials can be synthesized and tailored in order to recreate a finely structured microenvironment (by nanostructures, nanofibers, bioactive compounds, etc.) with which the cells can interact actively. For instance, by patterning substrates in regions that alternately promote or prevent protein adsorption, cell adhesion and spreading processes can be controlled in space. We illustrate the potentiality of nanotechnologies and engineered biomaterials in affecting and enhancing the behavior of renal stem/progenitor cells. Although there are still many challenges for the translation of novel therapeutics, advances in biomaterials and nanomedicine have the potential to drastically change the clinical and therapeutic landscape, even in combination with stem cell biology.

Nanowire-Intensified Metal-Enhanced Fluorescence in Hybrid Polymer-Plasmonic Electrospun Filaments.

Hybrid polymer-plasmonic nanostructures might combine high enhancement of localized fields from metal nanoparticles with light confinement and long-range transport in subwavelength dielectric structures. Here, the complex behavior of fluorophores coupling to Au nanoparticles within polymer nanowires, which features localized metal-enhanced fluorescence (MEF) with unique characteristics compared to conventional structures, is reported. The intensification effect when the particle is placed in the organic filaments is remarkably higher with respect to thin films of comparable thickness, thus highlighting a specific, nanowire-related enhancement of MEF effects. A dependence on the confinement volume in the dielectric nanowire is also indicated, with MEF significantly increasing upon reduction of the wire diameter. These findings are rationalized by finite element simulations, predicting a position-dependent enhancement of the quantum yield of fluorophores embedded in the fibers. Calculation of the ensemble-averaged fluorescence enhancement unveils the possibility of strongly enhancing the overall emission intensity for structures with size twice the diameter of the embedded metal particles. These new, hybrid fluorescent systems with localized enhanced emission, and the general nanowire-enhanced MEF effects associated to them, are highly relevant for developing nanoscale light-emitting devices with high efficiency and intercoupled through nanofiber networks, highly sensitive optical sensors, and novel laser architectures.

Nanoparticle-doped electrospun fiber random lasers with spatially extended light modes.

Complex assemblies of light-emitting polymer nanofibers with molecular materials exhibiting optical gain can lead to important advance to amorphous photonics and to random laser science and devices. In disordered mats of nanofibers, multiple scattering and waveguiding might interplay to determine localization or spreading of optical modes as well as correlation effects. Here we study electrospun fibers embedding a lasing fluorene-carbazole-fluorene molecule and doped with titania nanoparticles, which exhibit random lasing with sub-nm spectral width and threshold of about 9 mJ cm-2 for the absorbed excitation fluence. We focus on the spatial and spectral behavior of optical modes in the disordered and non-woven networks, finding evidence for the presence of modes with very large spatial extent, up to the 100 µm-scale. These findings suggest emission coupling into integrated nanofiber transmission channels as effective mechanism for enhancing spectral selectivity in random lasers and correlations of light modes in the complex and disordered material.

Neuregulin 1 functionalization of organic fibers for Schwann cell guidance.

The repair of peripheral nerve lesions is a clinical problem where the functional recovery is often far from being satisfactory, although peripheral nerves generally retain good potential for regeneration. Here, we develop a novel scaffold approach based on bioactive fibers of poly(ε-caprolactone) where nanotopographical guidance and neuregulin 1 (NRG1) cues are combined. We interface them with rat primary Schwann cells (SCs), the peripheral glial cells that drive initial regeneration of injured nerves, and found that the combination of NRG1 with parallel nano-fibrous topographies is effective in improving SC growth up to 72 h, alignment to fiber topography, and bipolar differentiation, opening original perspectives for nerve repair applications.

Anisotropic Conjugated Polymer Chain Conformation Tailors the Energy Migration in Nanofibers.

Conjugated polymers are complex multichromophore systems, with emission properties strongly dependent on the electronic energy transfer through active subunits. Although the packing of the conjugated chains in the solid state is known to be a key factor to tailor the electronic energy transfer and the resulting optical properties, most of the current solution-based processing methods do not allow for effectively controlling the molecular order, thus making the full unveiling of energy transfer mechanisms very complex. Here we report on conjugated polymer fibers with tailored internal molecular order, leading to a significant enhancement of the emission quantum yield. Steady state and femtosecond time-resolved polarized spectroscopies evidence that excitation is directed toward those chromophores oriented along the fiber axis, on a typical time scale of picoseconds. These aligned and more extended chromophores, resulting from the high stretching rate and electric field applied during the fiber spinning process, lead to improved emission properties. Conjugated polymer fibers are relevant to develop optoelectronic plastic devices with enhanced and anisotropic properties.

Enhancement of radiative processes in nanofibers with embedded plasmonic nanoparticles.

Efficient manipulation and long-distance transport of single photons is a key component in nanoscale quantum optics. In this Letter, we study the emission properties of an individual light emitter placed into a nanofiber and coupled to a metallic nanoparticle. We find that plasmonic field enhancement together with nanofiber optical confinement uniquely and synergistically contribute to an overall increase of emission rates as well as quantum yields. We predict a quantum yield enhancement up to a factor of 2.5 with respect to free space for an averaged dipole orientation.

Core-Shell Electrospun Fibers Encapsulating Chromophores or Luminescent Proteins for Microscopically Controlled Molecular Release.

Core-shell fibers are emerging as interesting microstructures for the controlled release of drugs, proteins, and complex biological molecules, enabling the fine control of microreservoirs of encapsulated active agents, of the release kinetics, and of the localized delivery. Here we load luminescent molecules and enhanced green fluorescent proteins into the core of fibers realized by coaxial electrospinning. Photoluminescence spectroscopy evidences unaltered molecular emission following encapsulation and release. Moreover, the release kinetics is microscopically investigated by confocal analysis at individual-fiber scale, unveiling different characteristic time scales for diffusional translocation at the core and at the shell. These results are interpreted by a two stage desorption model for the coaxial microstructure, and they are relevant in the design and development of efficient fibrous systems for the delivery of functional biomolecules.

Surface-enhanced Raman spectroscopy in 3D electrospun nanofiber mats coated with gold nanorods.

Nanofibers functionalized by metal nanostructures and particles are exploited as effective flexible substrates for surface-enhanced Raman scattering (SERS) analysis. Their complex three-dimensional structure may provide Raman signals enhanced by orders of magnitude compared to untextured surfaces. Understanding the origin of such improved performances is therefore very important for pushing nanofiber-based analytical technologies to their upper limit. Here, we report on polymer nanofiber mats which can be exploited as substrates for enhancing the Raman spectra of adsorbed probe molecules. The increased surface area and the scattering of light in the nanofibrous system are individually analyzed as mechanisms to enhance Raman scattering. The deposition of gold nanorods on the fibers further amplifies Raman signals due to SERS. This study suggests that Raman signals can be finely tuned in intensity and effectively enhanced in nanofiber mats and arrays by properly tailoring the architecture, composition, and light-scattering properties of the complex networks of filaments.

Three-Dimensional Model for Electrospinning Processes in Controlled Gas Counterflow.

We study the effects of a controlled gas flow on the dynamics of electrified jets in the electrospinning process. The main idea is to model the air drag effects of the gas flow by using a nonlinear Langevin-like approach. The model is employed to investigate the dynamics of electrified polymer jets at different conditions of air drag force, showing that a controlled gas counterflow can lead to a decrease of the average diameter of electrospun fibers, and potentially to an improvement of the quality of electrospun products. We probe the influence of air drag effects on the bending instabilities of the jet and on its angular fluctuations during the process. The insights provided by this study might prove useful for the design of future electrospinning experiments and polymer nanofiber materials.

Suppression of Low-Frequency Electronic Noise in Polymer Nanowire Field-Effect Transistors.

The authors report on the reduction of low-frequency noise in semiconductor polymer nanowires with respect to thin-films made of the same organic material. Flicker noise is experimentally investigated in polymer nanowires in the range of 10-10(5) Hz by means of field-effect transistor architectures. The noise in the devices is well described by the Hooge empirical model and exhibits an average Hooge constant, which describes the current power spectral density of fluctuations, suppressed by 1-2 orders of magnitude compared to thin-film devices. To explain the Hooge constant reduction, a resistor network model is developed, in which the organic semiconducting nanostructures or films are depicted through a two-dimensional network of resistors with a square-lattice structure, accounting for the different anisotropy and degree of structural disorder of the active nanowires and films. Results from modeling agree well with experimental findings. These results support enhanced structural order through size-confinement in organic nanostructures as effective route to improve the noise performance in polymer electronic devices.

Ratiometric Organic Fibers for Localized and Reversible Ion Sensing with Micrometer-Scale Spatial Resolution.

A fundamental issue in biomedical and environmental sciences is the development of sensitive and robust sensors able to probe the analyte of interest, under physiological and pathological conditions or in environmental samples, and with very high spatial resolution. In this work, novel hybrid organic fibers that can effectively report the analyte concentration within the local microenvironment are reported. The nanostructured and flexible wires are prepared by embedding fluorescent pH sensors based on seminaphtho-rhodafluor-1-dextran conjugate. By adjusting capsule/polymer ratio and spinning conditions, the diameter of the fibers and the alignment of the reporting capsules are both tuned. The hybrid wires display excellent stability, high sensitivity, as well as reversible response, and their operation relies on effective diffusional kinetic coupling of the sensing regions and the embedding polymer matrix. These devices are believed to be a powerful new sensing platform for clinical diagnostics, bioassays and environmental monitoring.

Nanofibers: Ratiometric Organic Fibers for Localized and Reversible Ion Sensing with Micrometer-Scale Spatial Resolution (Small 48/2015).

On page 6417, L. L. del Mercato, D. Pisignano, and co-workers report a new type of 3D nanostructured pH-sensing organic fiber with embedded ratiometric fluorescent capsules. Upon proton-induced switching, the fibers undergo optical changes that are recorded by fluorescence detectors and correlated to the analyte concentration. The developed electrospinning fabrication approach is facile and versatile and enables the creation of sensitive and highly robust pH-sensing 3D scaffolds for environmental monitoring and biomedical applications, including tissue engineering and wound healing.

Sub-ms dynamics of the instability onset of electrospinning.

Electrospun polymer jets are imaged for the first time at an ultra-high rate of 10,000 frames per second, investigating the process dynamics, and the instability propagation velocity and displacement in space. The polymer concentration, applied voltage bias and needle-collector distance are systematically varied, and their influence on the instability propagation velocity and on the jet angular fluctuations is analyzed. This allows us to unveil the instability formation and cycling behavior, and its exponential growth at the onset, exhibiting radial growth rates of the order of 10(3) s(-1). Allowing the conformation and evolution of polymeric solutions to be studied in depth, high-speed imaging at the sub-ms scale shows significant potential for improving the fundamental knowledge of electrified jets, leading to finely controllable bending and solution stretching in electrospinning, and consequently better designed nanofiber morphologies and structures.

Organic nanofibers embedding stimuli-responsive threaded molecular components.

While most of the studies on molecular machines have been performed in solution, interfacing these supramolecular systems with solid-state nanostructures and materials is very important in view of their utilization in sensing components working by chemical and photonic actuation. Host polymeric materials, and particularly polymer nanofibers, enable the manipulation of the functional molecules constituting molecular machines and provide a way to induce and control the supramolecular organization. Here, we present electrospun nanocomposites embedding a self-assembling rotaxane-type system that is responsive to both optical (UV-vis light) and chemical (acid/base) stimuli. The system includes a molecular axle comprised of a dibenzylammonium recognition site and two azobenzene end groups and a dibenzo[24]crown-8 molecular ring. The dethreading and rethreading of the molecular components in nanofibers induced by exposure to base and acid vapors, as well as the photoisomerization of the azobenzene end groups, occur in a similar manner to what observed in solution. Importantly, however, the nanoscale mechanical function following external chemical stimuli induces a measurable variation of the macroscopic mechanical properties of nanofibers aligned in arrays, whose Young's modulus is significantly enhanced upon dethreading of the axles from the rings. These composite nanosystems show therefore great potential for application in chemical sensors, photonic actuators, and environmentally responsive materials.