Hartmut Krüger

Fabrication of 2D protein microstructures and 3D polymer–protein hybrid microstructures by two-photon polymerization

Two-photon polymerization (TPP) offers the possibility of creating artificial cell scaffolds composed of micro- and nanostructures with spatial resolutions of less than 1 µm. For use in tissue engineering, the identification of a TPP-processable polymer that provides biocompatibility, biofunctionality and appropriate mechanical properties is a difficult task. ECM proteins such as collagen or fibronectin, which could mimic native tissues best, often lack the mechanical stability. Hence, by generating polymer-protein hybrid structures, the beneficial properties of proteins can be combined with the advantageous characteristics of polymers, such as sufficient mechanical stability. This study describes three steps toward facilitated application of TPP for biomaterial generation. (1) The efficiency of a low-cost ps-laser source is compared to a fs-laser source by testing several materials. A novel photoinitiator for polymerization with a ps-laser source is synthesized and proved to enable increased fabrication throughput. (2) The fabrication of 3D-microstructures with both systems and the fabrication of polymer-protein hybrid structures are demonstrated. (3) The tissue engineering capabilities of TPP are demonstrated by creating cross-linked gelatin microstructures, which clearly forced porcine chondrocytes to adapt their cell morphology.

Soft Polymers for Building up Small and Smallest Blood Supplying Systems by Stereolithography

Synthesis of a homologous series of photo-polymerizable α,ω-polytetrahydrofuranether-diacrylate (PTHF-DA) resins is described with characterization by NMR, GPC, DSC, soaking and rheometrical measurements. The curing speeds of the resins are determined under UV light exposure. Young’s modulus and tensile strength of fully cured resins show flexible to soft material attributes dependent on the molar mass of the used linear PTHF-diacrylates. Structuring the materials by stereo lithography (SL) and multiphoton polymerization (MPP) leads to tubes and bifurcated tube systems with a diameter smaller than 2 mm aimed at small to smallest supplying systems with capillary dimensions. WST-1 biocompatibility tests ofm polymer extracts show nontoxic characteristics of the adapted polymers after a washing process. Some polymers show shape memory effect (SME).

A versatile method for enhancement of electromechanical sensitivity of silicone elastomers

Dielectric elastomer actuators (DEAs) draw their function from their dielectric and mechanical properties. The paper describes the fabrication and various properties of molecularly grafted silicone elastomer films. This was achieved by addition of high-dipole molecular co-substituents to off-the-shelf silicone elastomer kits, Elastosil RT 625 and Sylgard 184 by Wacker and Dow Corning, respectively. Strong push–pull dipoles were chemically grafted to both polymer networks during a one step film formation process. All manufactured films were characterized using 13C-NMR and FT-IR spectroscopy, confirming a successful attachment of the dipoles to the silicone network. Differential scanning calorimetry (DSC) results showed that grafted dipoles were distributed homogeneously throughout the material avoiding the formation of nano-scale aggregates. The permittivity increased with the amount of dipole at all frequencies, while the Youngs modulus and electrical breakdown strength were reduced. Actuation strain measurements in the pure shear configuration independently confirmed the increase in electromechanical sensitivity. The ability to enhance electromechanical properties of off-the-shelf materials could strongly expand the range of actuator properties available to researchers and end-users.

Matrix stiffness dependent electro-mechanical response of dipole grafted silicones

The properties of dielectric elastomer actuators can be optimized by modifying the dielectric or mechanical properties of the dielectric elastomer. This paper presents the simultaneous control of both dielectric and mechanical properties, in a silicone elastomer network comprising cross-linker, chains and grafted molecular dipoles. Chains with two different molecular weights were each combined with varying amounts of grafted dipole. Chemical and physical characterization showed that networks with stoichiometric control of cross-linking density and permittivity were obtained, and that longer chain lengths resulted in higher electrical field response due to the reduction in cross-linking density and correspondingly in mechanical stiffness. Both actuation sensitivities were enhanced by 6.3 and 4.6 times for the short and long chain matrix material, respectively.

Multimodal polymer networks: design and characterisation of nanoheterogeneous PU elastomers

Nanoheterogeneous polymer networks based on polyurethanes (PU) were synthesised by utilisation of the competition between reactivity, dynamics and phase separation processes. Oligomeric α,ω-dihydroxyoxolanes (PTHF), 4,4′-methylene-bisphenylisocyanate (MDI), and trimethylolpropane (TMP) were chosen as reaction components. In dependence of the mole ratio of these components up to four phases concerning their crosslinking density were formed. By a partly substitution of incompatible oligomers for PTHF the network structure was specifically changed including the formation of additional phases. The characterisation of the nanophases was carried out by investigation of the dynamics as reflected by mechanical relaxation spectroscopy. Selected mechanical properties of the samples were measured and discussed using the deduced morphological model.

Polymer multilayer systems for electronic applications

Abstract For polymer electronic applications, it is important to use multilayer structures with conjugated polymers. For this reason, modified polymer structures were synthesized. The cyclovoltammetric measurements allows us to estimate the HOMO and LUMO energy levels of the synthesized materials. This knowledge is necessary for the development of efficient components like polymer light emitting diodes, polymer diodes and transistors. The characterization of such devices by current–voltage and brightness–voltage measurements will be described.

Simultaneous determination of functionality type and molar mass distribution of oligo (1,3,6-trioxocane)s by supercritical fluid chromatography

Abstract Benzyloxy terminated oligo (1,3,6-trioxocane)s are separated into components of single molar mass and functionality by supercritical fluid chromatography. The assignment and quantification of the components is described. It is shown that molar mass and functionality type distribution may be determined simultaneously from one chromatogram.

Blood-Vessel Mimicking Structures by Stereolithographic Fabrication of Small Porous Tubes Using Cytocompatible Polyacrylate Elastomers, Biofunctionalization and Endothelialization

Blood vessel reconstruction is still an elusive goal for the development of in vitro models as well as artificial vascular grafts. In this study, we used a novel photo-curable cytocompatible polyacrylate material (PA) for freeform generation of synthetic vessels. We applied stereolithography for the fabrication of arbitrary 3D tubular structures with total dimensions in the centimeter range, 300 µm wall thickness, inner diameters of 1 to 2 mm and defined pores with a constant diameter of approximately 100 µm or 200 µm. We established a rinsing protocol to remove remaining cytotoxic substances from the photo-cured PA and applied thio-modified heparin and RGDC-peptides to functionalize the PA surface for enhanced endothelial cell adhesion. A rotating seeding procedure was introduced to ensure homogenous endothelial monolayer formation at the inner luminal tube wall. We showed that endothelial cells stayed viable and adherent and aligned along the medium flow under fluid-flow conditions comparable to native capillaries. The combined technology approach comprising of freeform additive manufacturing (AM), biomimetic design, cytocompatible materials which are applicable to AM, and biofunctionalization of AM constructs has been introduced as BioRap® technology by the authors.

DEA material enhancement with dipole grafted PDMS networks

Silicone elastomers are highly suitable for application in the field of dielectric elastomer actuators (DEA) due to their unique material properties (e.g. low glass temperature, thermal stability, large capability of chemical tailoring). The elastomer forming Polydimethysiloxane (PDMS) employed for this study consists of chains with vinyl termination and is cross linked via hydrosilylation to a cross linking molecule in the presence of platinum catalyst. Here, dipole molecules (N-Allyl-N-methyl-4-nitroaniline) were specifically synthesized such that they could chemically graft to the silicone network. The most prominent advantage of this approach is the achievement of a homogeneous distribution of dipoles in the PDMS matrix and a suppression of phase separation due to the grafting to the junction points of the rubber network. Several films with dipole contents ν ranging from 0 %wt up to 10.9 %wt were prepared. The films were investigated to determine their mechanical (tensile testing), dielectric (dielectric relaxation spectroscopy) and electrical (electrical breakdown) properties. This new approach for composites on the molecular level leads to homogeneous films with enhanced material properties for DEA applications. An increase in permittivity from 3.3 to 6.0, a decrease in electrical breakdown from 130 V/μm to 50 V/μm and a lowering of the mechanical stiffness from 1700 kPa to 300 kPa was observed.

Hole-transporting side-chain polystyrenes based on TCTA with tuned glass transition and optimized electronic properties

The development of crosslinkable materials for the fabrication of solution processable OLEDs presents challenges, especially regarding the adjustment of the glass transition (Tg), which has a significant influence on crosslinking kinetics and device life-time. Crosslinkable hole transport materials based on poly(N,N-bis(4-(9H-carbazol-9-yl)phenyl)-4-vinylaniline) (poly-TCTA) with covalently attached plasticizers for Tg control and azide functionalities for azide-alkyne crosslinking are presented. These polymers have an optimal Tg of around 150 °C and show superior crosslinking performances and solution resistibilities. Incorporation of electron-pushing alkoxides to the pendant groups combines the Tg adjustment approach with a systematic tuning of the HOMO level from −5.7 to −5.3 eV. All presented polymers have good charge transport and injection properties and are ideal for applications in phosphorescent OLEDs due to their high triplet energies (>2.8 eV). The new crosslinkable poly-TCTA-based materials are applied as hole-transport layers (HTLs) in fully solution-processed OLEDs. An improvement of the device performance is demonstrated for OLEDs with additional crosslinked HTL.