Wolf-Joachim Fischer

Photopatterning of thermally sensitive hydrogels useful for microactuators

Abstract Hydrogels based on PNIPAAm [poly-( N -isopropylacrylamide)] are an attractive working material for microactuators because of their swelling with thermally induced phase transitions leading to a very high volume expansion. By the employment of co-polymerization with a chromophore, a specially designed polymer has been developed that can be photocrosslinked to a gel and by this photopatterned. The focal point of this work is the preparation and characterization of mircopatterns using this co-polymer. The efficiency of the photochemical reaction is quite good and patterning results down to 20 μm spaces have been achieved. The swelling of dot-like patterned hydrogel films on Si/SiO 2 substrates in water occurs with similar temperature dependency and a swelling ratio of 3 to 3.5 in the same order of magnitude compared to bulk PNIPAAm-based gels, but the transition time of 2 to 8 s measured at the gelous dots is much shorter than that of macroscopic objects, making the material interesting for microactuator performance. The transition temperature decreases from 32 to 34°C at pure PNIPAAm to 18 to 20°C at the photopatterned gels of the copolymer.

Studies on phase transformations of Cu-phthalocyanine thin films

Copper phthalocyanine (CuPc) thin films were obtained by a sublimation technique on Si wafer substrates maintained at room temperature. As-deposited CuPc films with less than 0.1 μm thickness crystallize primarily in the α-form with a preferential orientation of the crystallites in the [2 0 0] direction. The effect of randomizing of the orientation of the CuPc crystallites is observed as the film thickness increases, whereas the preference in appearance of the α-form remains. The changes in phase composition, structure, morphology and surface chemical composition of as-deposited CuPc thin films due to different heat treatment conditions were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS).

Microfabrication of thermoelectric generators on flexible foil substrates as a power source for autonomous microsystems

A flexible thermoelectric generator with overall dimensions of 16×20×0.05 mm has been fabricated using a unique low-cost procedure as part of our ongoing research efforts on developing economical and reliable energy sources for autonomous microsystems. The generator consists of a multiplicity of micro Sb-Bi thermocouple strips embedded in a 50 µm thick flexible epoxy film and is capable of generating a voltage of 0.25 V at a temperature difference of 30 K. Its fabrication involves only a few steps, such as foil lithography, electroplating, embedding and wet chemical etching. The exposure of aqueous photoresist on flexible foil substrates is carried out using a specially constructed mask aligner by Karl Suss GmbH. The thermocouple strips, with a cross section of 40×10 µm and a length of 20 mm, are electroplated galvanostatically from their associated acidic electrolytes into the patterned AZ 111XFS template on a 50 µm thick copper foil. After a top-over embedment of the electroplated structures with an epoxy film, the original copper substrate is removed completely by wet chemical etching, leaving the generator module to be embedded in the epoxy film. This process is proven to be cost-effective, easily manageable and highly reliable. It is now being applied to our current fabrication of more efficient thermoelectric generators based on n- and p-type Bi2Te3 compound materials.

Biologically Inspired Omniphobic Surfaces by Reverse Imprint Lithography

Springtail skin morphology is translated into robust omniphobic polymer membranes by reverse imprint lithography. The combination of overhanging cross-sections and their arrangement in a self-supporting comblike pattern are crucial for mechanically stable coatings that can be even applied to curved surfaces.

Tungsten-oxide thin films as novel materials with high sensitivity and selectivity to NO2, O3 and H2S. Part I: Preparation and microstructural characterization of the tungsten-oxide thin films

Tungsten-oxide thin films are promising materials for use in highly effective gas-sensing devices for NO2, ozone and H2S detection in ambient air. In this work tungsten-oxide thin films were obtained by electron-beam deposition and annealed in the temperature range 350–800 °C for 1–3 h. The structure, morphology and phase composition of the as-deposited and annealed films were characterized by X-ray diffraction, SEM and AFM. The changes of phase composition and the microstructure in dependence of the annealing conditions are described in detail. The direction of the phase transformations for different annealing conditions is influenced by the very high macrostresses that appear as an additional, independent thermodynamic factor. During annealing at 350–400 °C for 1–3 h and at 800 °C for 1 h predominantly semiconductor phases are formed, whereas the thin films annealed at 500–600 °C for 1–3 h and 800 °C for 2 h consist mainly of phases with more pronounced metallic properties. The processes of realignment of crystal structures during solid-phase transformation lead not only to the growth of new crystallites with a preferential orientation but also to a change in the direction of preferred growth with increasing annealing temperature and time. The films can be divided into two main groups: compact (as-deposited and annealed at 350–500 °C for 1–3 h) and porous (annealed at 600–800 °C for 1–3 h) layers. The gas-sensing properties of these films and the correlation between microstructure and sensing properties will be described in the second part of this paper.

Flexible polymer humidity sensor fabricated by inkjet printing

In this paper a humidity sensor concept is presented which is fully fabricated by inkjet printing. In the first step conductive finger electrodes are inkjet printed on flexible polyimide substrate with a silver neodecanoate based ink. Afterwards a humidity sensitive layer is printed upon using a polymer particle based ink. Two approaches are presented for minimizing the line width of the electrode fingers. The humidity sensitivities of the sensor capacitances are investigated. The results are promising and demonstrate, that inkjet printed sensors are potentially suited for label applications. The achieved sensitivity, repeatability and hysteresis are in acceptable ranges.

Multicolor generation using silicon nanodisk absorber

A multicolored matrix that spans the visible range was demonstrated by using silicon nanodisk arrays. A nanostructured silicon substrate, which featured periodic silicon nanodisk arrays of various diameters, inter-nanodisk distances, and heights, was fabricated using electron-beam lithography and reactive ion etching. These silicon nanodisks were able to support HE1m leaky modes, which depended on the diameter of the nanodisks, resulting in wavelength-dependent reflection spectra. The resonant wavelength redshifted linearly with the increasing nanodisk diameter. The output color lay in the visible range and was observed to be tunable when varying the diameter, interdistance, and height. The results of finite-difference time-domain simulations exhibited close agreement with the observed optical properties of the periodic silicon nanodisk arrays.

Passive Wireless Resonant Galfenol Sensor for Osteosynthesis Plate Bending Measurement

The healing process of bone fractures can be monitored by a measurement of the osteosynthesis plate bending. For this purpose a wireless magnetostrictive bending sensor is proposed. A planar rectangular coil on top of a magnetostrictive Galfenol ( alloy) layer forms an electrical resonant circuit. The sensor is manufactured in thin film technology. Coil turns were electrodeposited by pattern plating. Sensors with overall dimensions of (13 2 0.5) mm were manufactured with varied turn numbers showing self-resonance frequencies from 5 to 50 MHz. Examined sensors possess linear frequency-force characteristic up to 6 N with frequency shifts of 6 kHz . In order to obtain the sensor resonance frequency wirelessly, several measurement techniques were employed using inductively coupled coils. Frequency domain measurements have been carried out by employing a network analyzer with a single detection coil and a lock-in amplifier with separate coils for excitation and detection. In time domain measurements, two coils for transmission and reception are used. In the transmit case a short sine pulse excites the sensor and afterwards its decaying response signal is received. To determine the resonance frequency, a frequency counting, Fourier or wavelet technique can be used. By integrating additional cores of high permeability into sensor and detection coil, measurement ranges can be increased.

Determination of the wine preservative sulphur dioxide with cyclic voltammetry using inkjet printed electrodes.

During winemaking sulphur dioxide is added to prevent undesirable reactions. However, concerns over the harmful effects of sulphites have led to legal limits being placed upon such additives. There is thus a need for simple and selective determinations of sulphur dioxide in wine, especially during winemaking. The simultaneous detection of polyphenols and sulphur dioxide, using cyclic voltammetry at inert electrodes is challenging due to close oxidation potentials. In the present study, inkjet printed electrodes were developed with a suitable voltammetric signal on which the polyphenol oxidation is suppressed and the oxidation peak height for sulphur dioxide corresponds linearly to the concentration. Different types of working electrodes were printed. Electrodes consisting of gold nanoparticles mixed with silver showed the highest sensitivity towards sulphur dioxide. Low cost production of the sensor elements and ultra fast determination of sulphur dioxide by cyclic voltammetry makes this technique very promising for the wine industry.

Improvement of sputtered Galfenol thin films for sensor applications

Galfenol Fe83Ga17 films are sputtered on Si wafers without, and with Ti or Ti/Cu metallic seed layers in order to obtain a magnetoelastic layer which is sensitive to bending deformations of the compound structure. The layer thicknesses range from 100?nm to 5? ?m. Layer morphology, texture, and the Villari effect are examined. The texture of the Galfenol films is strongly influenced by the seed layer. No low-index texture components are found for films directly deposited on Si and SiO2. On Ti, a (111) texture is formed on layers with more than 1000?nm thickness. A favorable (110) fiber texture is formed on Ti/Cu. Deforming the bimorphs (Si + layer system) by 0.012%, the Villari effect is detected due to the change in relative permeability. The maximum change occurs for Galfenol films with a thickness of 1? ?m on a Ti/Cu buffer layer. The films open a route to the incorporation of magnetoelastic films into integrated magnetoelastic sensor devices.