Liquid crystalline elastomers as stimuli-responsive microactuators

Liquid crystalline elastomers (LCEs) are known to perform a reversible change of shape upon the phase transition from the semi-ordered liquid crystalline state to the chaotic isotropic state. This unique behavior of these “artificial muscles” arises from the self-organizing properties of liquid crystals (mesogens) in combination with the entropy-elasticity of the slightly crosslinked elastomer network. In this work, micrometer-sized LCE actuators are fabricated in a microfluidic setup. The microtubular shear flow provides for a uniform orientation of the mesogens during the crosslinking, a perquisite for obtaining actuating LCE samples. The scope of this work was to design different actuator geometries and to broaden the applicability of the microfluidic device for different types of liquid crystalline mesogens, ranging from side-chain to main-chain systems, as well as monomer and polymer precursors. For example, the thiol-ene “click” mechanism was used for the polymerization and crosslinking of main-chain LCE actuators. The main focus was, however, placed on acrylate monomers and polymers with LC side chains. A LC polymer precursor, comprising mesogenic and crosslinkable side-chains was synthesized. Used in combination with an LC monomer, the polymeric crosslinker promoted a stable LC phase, which allowed the mixture to be isothermally handled in the microfluidic reactor. If processed without the additional LC components, the polymer precursor yielded actuating fibers. A suitable co-flowing continuous phase facilitates the formation of a liquid jet and lowers the tendency for drop formation. By modification of the microfluidic device, it was further possible to prepare core-shell particles, comprised of an LCE shell and filled with an isotropic liquid. In analogy to the heart, a hollow muscle, the elastomer shell expels the inner liquid core upon its contraction. The feasibility of the core-shell particles as micropumps was demonstrated. In general, the synthesized LCE microactuators may be utilized as active components in micromechanical and lab-on-chip systems.

Detektion und Synthese eines liganden-gesteuerten Ionenkanals im mikrofluidischen Analysesystem

Membranproteine spielen eine wichtige Rolle bei physiologischen Prozessen wie Signalweiterleitung oder Immunreaktion. Deshalb stehen sie im Fokus der pharmakologischen Wirkstoffentwicklung und es besteht großes Interesse, Membranproteinbasierte Biosensoren zu entwickeln, die sich z.B. als Screening-Plattformen eignen. Allerdings stellt die Handhabung von Membranproteinen wegen ihrer amphiphilen Struktur eine große Herausforderung dar. Membranproteine werden meist in Zellkultur oder in bakteriellen Expressionssystemen synthetisiert. Diese Verfahren liefern aber oft nur eine geringe Ausbeute und erlauben wenig Kontrolle über die Expressionsbedingungen. Als alternativer Ansatz bietet sich stattdessen die in vitro Synthese von Proteinen an, die in einer zellfreien Umgebung stattfindet. Ziel der vorliegenden Arbeit war die Etablierung eines miniaturisierten Analysesystems, das Aktivitätsmessungen an in vitro synthetisierten Ionenkanälen erlaubt. Dafür wurde ein Labon- Chip entwickelt, der elektrochemische und optische Nachweismethoden in parallelen Anätzen ermöglicht. Als amphiphile Umgebung für die Inkorporation von Membranproteinen wurden vier verschieden biomimetische Membranaufbauten hinsichtlich ihrer Dichtigkeit und ihrer Reproduzierbarkeit untersucht. Als Methode fanden insbesondere die Impedanzspektroskpie und die Oberflächenplasmonen-Resonanzspektroskopie Anwendung. Die peptide cushioned Bilyer Lipid Membranes (pcBLM) eignete sich dabei am besten für Untersuchungen an Membranproteinen. Zur Detektion der Ionenkanalaktivität wurde eine neue Messmethode etabliert, die auf der Messung der Impedanz bei fester Frequenz basiert und u.a. eine Aussage über die Änderung des Membranwiderstandes bei Aktivierung erlaubt. Am Beispiel des nicotinischen Acetylcholinrezeptors (nAchR) konnte gezeigt werden, dass sich die Aktivität von Ionenkanälen mit dem entwickelten Chip-System nachweisen ließ. Die Spezifität der Methode konnte durch verschiedene Kontrollen wie die Zugabe eines nicht-aktivierenden Liganden oder Inhibition des Rezeptors nachgewiesen werden. Weiterhin konnte die in vitro Synthese des Ionenkanals a7 nAchR durch Radioaktivmarkierung nachgewiesen werden. Die Inkorporation des Rezeptors in die biomimetischen Membranen wurde mit Immunodetektion und elektrochemischen Methoden untersucht. Es zeigte sich, dass die funktionelle Inkorporation des a7 nAchR davon abhing, welcher biomimetische Membranaufbau verwendet wurde.

Template-assisted patterning of functional polymers

In this thesis, anodic aluminum oxide (AAO) membranes, which provide well-aligned uniform mesoscopic pores with adjustable pore parameters, were fabricated and successfully utilized as templates for the fabrication of functional organic nanowires, nanorods and the respective well-ordered arrays. The template-assisted patterning technique was successfully applied for the realization of different objectives:rnHigh-density and well-ordered arrays of hole-conducting nanorods composed of cross-linked triphenylamine (TPA) and tetraphenylbenzidine (TPD) derivatives on conductive substrates like ITO/glass have been successfully fabricated. By applying a freeze-drying technique to remove the aqueous medium after the wet-chemical etching of the template, aggregation and collapsing of the rods was prevented and macroscopic areas of perfectly freestanding nanorods were feasible. Based on the hole-conducting nanorod arrays and their subsequent embedding into an electron-conducting polymer matrix via spin-coating, a novel routine concept for the fabrication of well-ordered all-organic bulk heterojunction for organic photovoltaic applications was successfully demonstrated. The increased donor/acceptor interface of the fabricated devices resulted in a remarkable increase of the photoluminescence quenching compared to a planar bilayer morphology. Further, the fundamental working principle of the templating approach for the solution-based all-organic photovoltaic device was demonstrated for the first time.rnFurthermore, in order to broaden the applicability of patterned surfaces, which are feasible via the template-based patterning of functional materials, AAO with hierarchically branched pores were fabricated and utilized as templates. By pursuing the common templating process hierarchically polymeric replicas, which show remarkable similarities with interesting biostructures, like the surface of the lotus leaf and the feet of a gecko, were successfully prepared.rnIn contrast to the direct infiltration of organic functional materials, a novel route for the fabrication of functional nanowires via post-modification of reactive nanowires was established. Therefore, reactive nanowires based on cross-linked pentafluorophenylesters were fabricated by utilizing AAO templates. The post-modification with fluorescent dyes was demonstrated. Furthermore, reactive wires were converted into well-dispersed poly(N-isopropylacrylamide) (PNIPAM) hydrogels, which exhibit a thermal-responsive reversible phase transition. The reversible thermal-responsible swelling of the PNIPAM nanowires exhibited a more than 50 % extended length than in the collapsed PNIPAM state. rnLast but not least, the shape-anisotropic pores of AAO were utilized to uniformly align the mesogens of a nematic liquid crystalline elastomer. Liquid crystalline nanowires with a narrow size distribution and uniform orientation of the liquid crystalline material were fabricated. It was shown that during the transition from the nematic to the isotropic phase the rod’s length shortened by roughly 40 percent. As such these liquid crystalline elastomeric nanowires may find application, as wire-shaped nanoactuators in various fields of research, like lab-on-chip systems, micro fluidics and biomimetics.rn