Matthias M. L. Arras

Electrospinning of aligned fibers with adjustable orientation using auxiliary electrodes

Abstract A conventional electrospinning setup was upgraded by two turnable plate-like auxiliary high-voltage electrodes that allowed aligned fiber deposition in adjustable directions. Fiber morphology was analyzed by scanning electron microscopy and attenuated total reflection Fourier transform infrared spectroscopy (FTIR-ATR). The auxiliary electric field constrained the jet bending instability and the fiber deposition became controllable. At target speeds of 0.9 m s−1 90% of the fibers had aligned within 2°, whereas the angular spread was 70° without the use of auxiliary electrodes. It was even possible to orient fibers perpendicular to the rotational direction of the target. The fiber diameter became smaller and its distribution narrower, while according to the FTIR-ATR measurement the molecular orientation of the polymer was unaltered. This study comprehensively documents the feasibility of directed fiber deposition and offers an easy upgrade to existing electrospinning setups.

Biomimetic 3D hydroxyapatite architectures with interconnected pores based on electrospun biaxially orientated PCL nanofibers

We report here a facile strategy to fabricate three-dimensional (3D) hydroxyapatite (HA) architectures with well-defined long continuous interconnected pores by using electrospinning and biomimetic mineralization. To this end, a polymeric nanofiber (NF) scaffold with well-defined architecture was fabricated by electrospinning, and bone morphogenetic protein 2 (BMP2) was then adsorbed onto the chemically modified NFs through bio-conjugation. The 3D nanoporous HA architecture was finally fabricated by biomimetic mineralization of the NF–BMP2 hybrid in simulated body fluids and subsequent dissolution of NFs in hexafluoroisopropanol. The formation of NF–BMP2 hybrid was identified by confocal laser scanning microscopy analysis. The crystal structure of HA crystals formed on NFs was examined by X-ray diffraction. The chemical composition and interconnected porous structure of the created 3D HA architectures were measured by X-ray photoelectron spectroscopy, focused ion beam scanning electron microscopy, and transmission electron microscopy, respectively. This bottom-up strategy based on electrospinning and biomimetic mineralization opens up a new way to prepare diverse porous HA-based hybrid materials and shows great potential in drug delivery, gene transfer and tissue engineering.

Electrodynamic control of the nanofiber alignment during electrospinning

A technique is presented to electrospin straight and aligned fibers on a stationary featureless target. Two parallel rotatable plate-like auxiliary electrodes were applied with a time-varying square wave potential. Thereby, the electrospinning jet was periodically deflected between the electrodes, which led to an aligned fiber-deposition. Straight fibers were deposited at a potential difference of 11 kV and a switching frequency of 40 Hz between the auxiliary electrodes. With this setup, freely adjustable orientations can be achieved regardless of the targets design or its movement.

Enveloping Self-Assembly of Carbon Nanotubes at Copolymer Micelle Cores

Carbon nanotubes (CNTs) and their polymer nanocomposites are interesting materials for future applications, for example in optics or electronics. Research faces two major challenges with these outstanding nanofillers: control over dispersion and spatial arrangement within the nanocomposite, both required to achieve optimal structure and properties of CNT-based nanocomposites. We report on novel self-assembled multiwall CNT (MWCNT)/block copolymer (BCP) nanostructures realized by patterning MWCNTs with amphilphilic diblock copolymer micelles. A high molecular weight poly(styrene)-b-poly(2-vinylpyridine) BCP which forms large micelles (250 nm) was chosen to facilitate the templating by reducing the bending energy induced in the MWCNTs. We tested the hypothesis that it is possible to use an amphiphilic BCP as a dispersing agent and its spherical micelles as a template at the same time without modification of the CNTs. In thin films of the MWCNT/BCP micelles, highly separated MWCNTs were repeatedly observed which enveloped the core of the BCP micelles, i.e., the unfunctionalized MWCNTs segregated to the interface between the two BCP phases. Depending on the size of the MWCNTs, ring-like (split-ring) or network forming structures were obtained. The MWCNT templating mechanism, i.e., the segregation to the interface, is explained by the interfacial tension within the BCP interface and the chain entropy. The reported new complex nanocomposite has potential to be applied for example as cost-effective split-ring resonators for metamaterials or for conductive polymer films with an extremely low percolation threshold.

An advanced geometrical model for laminated woven fabrics using Lamé exponents with enhanced accuracy

Three-dimensional stiffness tensors of laminated woven fabrics used in high-performance composites need precise prediction. To enhance the accuracy in three-dimensional stiffness tensor prediction, the fabric’s architecture must be precisely modeled. We tested the hypotheses that: (i) an advanced geometrical model describes the meso-level structure of different fabrics with a precision higher than established models, (ii) the deviation between predicted and experimentally determined mean fiber-volume fraction (cf) of laminates is below 5%. Laminates of different cf and fabrics were manufactured by resin transfer molding. The laminates’ meso-level structure was determined by analyzing scanning electron microscopy images. The prediction of the laminates’ cf was improved by up to 5.1 vol% ( 11 . 0 %) compared to established models. The effect of the advanced geometrical model on the prediction of the laminate’s in-plane stiffness was shown by applying a simple mechanical model. Applying an advanced geometrical model may lead to more accurate simulations of parts for example in automotive and aircraft.

Steering of an electrospinning jet by dynamic manipulation of the electric field.

Fiber alignment approaches in general rely at least on a specially structured or moving target. This study’s objective was to electrospin aligned fibers on a stationary featureless target. To exploit the controllability of the charged jet by an auxiliary electric field, two rotatable plate like auxiliary electrodes were applied with a time-varying square wave potential. Thereby, the electrospinning jet was periodically deflected between the electrodes, which led to an aligned fiber deposition. The straightness of the fibers in dependence of the auxiliary electric field was investigated by scanning electron microscopy and compared to a moving target alignment technique. Straight fibers were deposited at a potential difference of 11 kV and a switching frequency of 40 Hz between the auxiliary electrodes. With this setup, freely adjustable orientations can be achieved regardless of the targets design or its movement.

Optofluidic Chip System with Integrated Fluidically Controllable Optics

This paper describes an optofluidic approach for fiber coupling and flexible beam shaping in the central plane of all-glass microfluidic devices. Special microchannels with half circular sidewalls have been integrated into the device in order to create micro lens systems. Focal length of each of these micro-lenses can be controlled by the refractive index of the fluid inside the channel. That way adaptive, microfluidically controllable lens systems can be realized for use in beam shaping and light-section creation. A prototyped chip with five parallel fluidic channels with different distances between them and with free adjustable and combinational fluidic refraction indices inside has been prepared and investigated. A total of six fiber channels on each side are used for light input and output. The developed chip concept has a great potential for application in small biochemical analysis units like lab-on-a-chip or μ-total analysis systems. Simulations of light distribution inside the chip in dependence on fluid refraction indices as well as geometrical parameters are in accordance with realized transmission measurements.