Karl-Friedrich Arndt

Application of Sensitive Hydrogels in Flow Control

WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW For any polymer gel, the amount of solvent uptake is dependent upon the chemical nature of the gel and the nature of its environment— solvent composition, temperature, pH, and so on. We discuss the use of different hydrogels based on crosslinked poly(N-isopro- pylacrylamide) and copolymers with basic or acidic groups as materials for flow control. The design of a chemo-mechanical valve is described. The liquid flows directly through a gel actuator, which consists of a cylinder filled with small particles of the sensitive crosslinked polymer. The flow rate as well as the pressure drop is measured in dependence on the solvent properties. The sensitivity of the gels as well as the time behavior of the valve-function is correlated with the dependence of the degree of swelling on the environment and the swelling and shrinking kinetics of the gels. The stimulus must permeate the gel itself before a gel can respond to the stimulus. By NMR-imaging it is possible to follow the transport processes inside the gel in real-time. With the presented experimental arrangement we could show that sensitive polymers can be used for controlling the flow in dependence on temperature, pH and content of organic solvents in water. Furthermore, the synthesis of a photo-crosslinkable sensitive polymer is described, synthesized and suggestions for an application of thin layers of this polymer in micro-system techniques are made. Copyright

Temperature and pH dependent solubility of novel poly(N-isopropylacrylamide)-copolymers

Temperature and pH sensitive polymers were prepared by the copolymerization of N-isopropylacrylamide (NIPAAm) with varying amounts of acrylamide derivatives bearing carboxylic groups attached to spacers with different chain length (C n AAm). Aqueous solutions of the copolymers show lower critical solution temperature behaviour (LCST). The LCSTs of the aqueous solutions of these copolymers decrease with increasing comonomer content and spacer chain length. Through the use of suitable amounts of the monomers it was possible to accurately predict the phase transition temperature and values were obtained between 10 and 32°C. The LCSTs also strongly depend on the pH-value of the solution. An increase of the pH-value leads to a significant increase in LCST due to the formation of a more hydrophilic copolymer. Through altering the pH it is possible to obtain phase transition temperatures between 10 to 50°C. The LCST behaviour was investigated by means of DSC as a standard method which allows the measurement of the phase transition over a wide range of temperatures and pH-values. Comparison of these results with those from other methods (mainly turbidimetric and viscosimetric measurements) shows a two step mechanism for the phase separation. Dependence of the phase transition temperature of aqueous P(NIPAAm co C 2 AAm) copolymer solutions on the pH-value (lines were drawn to make the diagram clearer).

Hyperbranched PEI with Various Oligosaccharide Architectures : Synthesis, Characterization, ATP Complexation, and Cellular Uptake Properties

We present a rapid synthetic method for the development of hyperbranched PEIs decorated with different oligosaccharide architectures as carrier systems (CS) for drugs and bioactive molecules for in vitro and in vivo experiments. Reductive amination of hyperbranched PEI with readily available oligosaccharides results in sugar functionalized PEI cores with oligosaccharide shells of different densities. These core-shell architectures were characterized by NMR spectroscopy, elemental analysis, SLS, DLS, IR, and polyelectrolyte titration experiments. ATP complexation of theses polycations was examined by isothermal titration calorimetry to evaluate the binding energy and ATP/CS complexation ratios under physiological conditions. In vitro experiments showed an enhanced cellular uptake of ATP/CS complexes compared to those of the free ATP molecules. The results arise to initiate further noncovalent complexation studies of pharmacologically relevant molecules that may lead to the development of therapeutics based on this polymeric delivery platform.

Rolled-Up Magnetic Sensor: Nanomembrane Architecture for In-Flow Detection of Magnetic Objects

Detection and analysis of magnetic nanoobjects is a crucial task in modern diagnostic and therapeutic techniques applied to medicine and biology. Accomplishment of this task calls for the development and implementation of electronic elements directly in fluidic channels, which still remains an open and nontrivial issue. Here, we present a novel concept based on rolled-up nanotechnology for fabrication of multifunctional devices, which can be straightforwardly integrated into existing fluidic architectures. We apply strain engineering to roll-up a functional nanomembrane consisting of a magnetic sensor element based on [Py/Cu](30) multilayers, revealing giant magnetoresistance (GMR). The comparison of the sensors characteristics before and after the roll-up process is found to be similar, allowing for a reliable and predictable method to fabricate high-quality ultracompact GMR devices. The performance of the rolled-up magnetic sensor was optimized to achieve high sensitivity to weak magnetic fields. We demonstrate that the rolled-up tube itself can be efficiently used as a fluidic channel, while the integrated magnetic sensor provides an important functionality to detect and respond to a magnetic field. The performance of the rolled-up magnetic sensor for the in-flow detection of ferromagnetic CrO(2) nanoparticles embedded in a biocompatible polymeric hydrogel shell is highlighted.

Hydrogel Sensors and Actuators

General Properties of Hydrogels.- Synthesis of Hydrogels.- Swelling-Related Processes in Hydrogels.- Modelling and Simulation of the Chemo-Electro-Mechanical Behaviour.- Hydrogels for Chemical Sensors.- Hydrogels for Biosensors.- Hydrogels for Actuators.- Polymer Hydrogels to Enable New Medical Therapies.

Hydrogel-based piezoresistive pH sensors: investigations using FT-IR attenuated total reflection spectroscopic imaging.

The strong swelling ability of the pH-responsive poly(acrylic acid)/poly(vinyl alcohol) (PAA/PVA) hydrogel makes the development of a new type of sensor possible, which combines piezoresistive-responsive elements as mechanoelectrical transducers and the phase transition behavior of hydrogels as a chemomechanical transducer. The sensor consists of a pH-responsive PAA/PVA hydrogel and a standard pressure sensor chip. However, a time-dependent sensor output voltage mirrors only the physical swelling process of the hydrogel but not the corresponding chemical reactions. Therefore, an investigation of the swelling behavior of this hydrogel is essential for the optimization of sensor design. In this work, Fourier transform infrared (FT-IR) spectroscopic imaging was used to study the swelling of the hydrogel under in situ conditions. In particular, laterally and time-resolved FT-IR images were obtained in the attenuated total reflection mode and the entire data set of more than 80,000 FT-IR spectra was evaluated by principal component analysis (PCA). The first and third principal components (PCs) indicate the swelling process. Molecular changes within the carboxyl groups were observed in the second and fourth PC and identified as key processes for the swelling behavior. It was found that time-dependent molecular changes are similar to the electrical sensor output signal. The results of the FT-IR spectroscopic images render an improved chemical sensor possible and demonstrate that in situ FT-IR imaging is a powerful method for the characterization of molecular processes within chemical-sensitive materials.

Gelation mechanism of poly(N-isopropylacrylamide)-clay nanocomposite hydrogels synthesized by photopolymerization.

The gelation process of poly-(N-isopropylacrylamide)-clay nanocomposite hydrogels (PNIPAAm-clay NC gels) was investigated by dynamic and static light scattering (DLS and SLS), as well as by fluorescence correlation spectroscopy (FCS). The photopolymerization method chosen for the radical polymerizing system ensured that, when the irradiation is removed, the reaction stopped immediately. Experiments showed that shortly before the gelation threshold is reached, no changes in the DLS autocorrelation functions appear, while the monomer conversion can be observed by 1H NMR spectroscopy. These results correspond to the formation of microparticles, in which the PNIPAAm chains are closely attached to the clay platelets. During the further polymerization process, clay clusters are developed before the sol-gel threshold is reached. FCS measurements were performed to obtain information on the motion of the clay platelets inside the NC gel. The DLS method gives only an average of the motions in the gel. In a time window between 10 micros and 1 s, the clay sheets labeled with Rhodamine B show no characteristic motions.

Characterization methods for radiation crosslinked poly(vinyl methyl ether) hydrogels

Abstract The paper reviews recent results of radiation crosslinking of poly(vinyl methyl ether) (PVME). It will give an overview of possible characterization methods for both, soluble and crosslinked PVME. The irradiation of aqueous low concentrated PVME solutions with γ-rays of low doses results in structural changes of PVME molecules. We are able to monitor changes in the chemical structure by spectroscopic methods (IR, NMR) as well as the changes of molecular parameters (e.g. molecular weight, molecular weight distribution, branches) by classical methods for polymer characterization (size exclusion chromatography with diverse detector systems, SLS, viscosimetry). The characterization of the network parameters (crosslinking density νc, molecular weight of the network chains Mc) of PVME bulkgels crosslinked by irradiation at high dose values by classical methods (swelling and compression measurements) provides incorrect results because of the high porosity of the gels. PVME microgel particles can be prepared by irradiation of a phase separated diluted aqueous PVME solution above their lower critical solution temperature. These microgels with decreased dimensions were characterized by SLS, DLS and field emission scanning electron microscopy.

Photocrosslinking of thin films of temperature‐sensitive polymers

The use of polymeric materials with temperature-dependent degrees of swelling (especially polymers that exhibit lower critical solution temperature (LCST) behaviour in aqueous solutions) in microsystems requires the preparation and patterning of layers in the µm range. Copolymers based on N-isopropylacrylamide were modi-fied with a stilbazolium salt chromophore to yield photocrosslinkable temperature-sensitive polymers. The chromophore and the polymers were characterized by UV, IR, 1H-NMR (nuclear magnetic resonance) and 13C-NMR spectra. The resulting polymers showed LCST behaviour, which was measured by differential scanning calorimetry. The photocrosslinking properties were studied by UV irradiation of the thin films and measurement of the changes in the UV absorption spectra. By irradiation of thin films through a mask it was possible to obtain patterned networks in the µm range (20 µm space width and ≥50 µm line width). The resulting patterned networks showed temperature-dependent swelling properties in aqueous media. Copyright

Anomalous Diffusion in Thermoresponsive Polymer–Clay Composite Hydrogels Probed by Wide-Field Fluorescence Microscopy

Thermoresponsive materials exhibit an enormous potential for tissue engineering, separation systems, and drug delivery. We investigated the diffusion of laponite clay nanoparticles, which serve as physical cross-linkers to achieve improved material properties in poly(N-isopropylacrylamide) (PNIPAM)-clay composite hydrogels close to the gel point. The networks are formed through physical interactions between PNIPAM chains and clay nanoparticles after these two components are mixed. In contrast to previous studies, a covalent labeling strategy was chosen to minimize the amount of free dyes in solution. Single-particle tracking of the labeled clay nanoparticles showed that their diffusion is anomalous at all temperatures used in this study, reflecting the viscoelastic behavior as a cross-linker. Stepwise heating from 24 to 38 °C resulted in a slight increase of the diffusion coefficient and the anomality parameter α up to the volume phase transition temperature of ca. 31 °C, which was followed by a significant drop of both parameters, reflecting strongly hindered motion of the collapsed nanoparticle aggregates.