Semimetal to semiconductor transition in Bi/TiO2 core/shell nanowires.

We demonstrate the full thermoelectric and structural characterization of individual bismuth-based (Bi-based) core/shell nanowires. The influence of strain on the temperature dependence of the electrical conductivity, the absolute Seebeck coefficient and the thermal conductivity of bismuth/titanium dioxide (Bi/TiO2) nanowires with different diameters is investigated and compared to bismuth (Bi) and bismuth/tellurium (Bi/Te) nanowires and bismuth bulk. Scattering at surfaces, crystal defects and interfaces between the core and the shell reduces the electrical conductivity to less than 5% and the thermal conductivity to less than 25% to 50% of the bulk value at room temperature. On behalf of a compressive strain, Bi/TiO2 core/shell nanowires show a decreasing electrical conductivity with decreasing temperature opposed to that of Bi and Bi/Te nanowires. We find that the compressive strain induced by the TiO2 shell can lead to a band opening of bismuth increasing the absolute Seebeck coefficient by 10% to 30% compared to bulk at room temperature. In the semiconducting state, the activation energy is determined to |41.3 ± 0.2| meV. We show that if the strain exceeds the elastic limit the semimetallic state is recovered due to the lattice relaxation.

Nanometrology: Absolute Seebeck coefficient of individual silver nanowires.

Thermoelectric phenomena can be strongly modified in nanomaterials. The determination of the absolute Seebeck coefficient is a major challenge for metrology with respect to micro- and nanostructures due to the fact that the transport properties of the bulk material are no more valid. Here, we demonstrate a method to determine the absolute Seebeck coefficient S of individual metallic nanowires. For highly pure and single crystalline silver nanowires, we show the influence of nanopatterning on S in the temperature range between 16 K and 300 K. At room temperature, a nanowire diameter below 200 nm suppresses S by 50% compared to the bulk material to less than S = 1 µVK-1, which is attributed to the reduced electron mean free path. The temperature dependence of the absolute Seebeck coefficient depends on size effects. Thermodiffusion and phonon drag are reduced with respect to the bulk material and the ratio of electron-phonon to phonon-phonon interaction is significantly increased.

Advanced platform for the in-plane ZT measurement of thin films.

The characterization of nanostructured samples with at least one restricted dimension like thin films or nanowires is challenging, but important to understand their structure and transport mechanism, and to improve current industrial products and production processes. We report on the 2nd generation of a measurement chip, which allows for a simplified sample preparation process, and the measurement of samples deposited from the liquid phase using techniques like spin coating and drop casting. The new design enables us to apply much higher temperature gradients for the Seebeck coefficient measurement in a shorter time, without influencing the sample holder's temperature distribution. Furthermore, a two membrane correction method for the 3ω thermal conductivity measurement will be presented, which takes the heat loss due to radiation into account and increases the accuracy of the measurement results significantly. Errors caused by different sample compositions, varying sample geometries, and different heat profiles are avoided with the presented measurement method. As a showcase study displaying the validity and accuracy of our platform, we present temperature-dependent measurements of the thermoelectric properties of an 84 nm Bi87Sb13 thin film and a 15 µm PEDOT:PSS thin film.

Highly elastic conductive polymeric MEMS.

Polymeric structures with integrated, functional microelectrical mechanical systems (MEMS) elements are increasingly important in various applications such as biomedical systems or wearable smart devices. These applications require highly flexible and elastic polymers with good conductivity, which can be embedded into a matrix that undergoes large deformations. Conductive polydimethylsiloxane (PDMS) is a suitable candidate but is still challenging to fabricate. Conductivity is achieved by filling a nonconductive PDMS matrix with conductive particles. In this work, we present an approach that uses new mixing techniques to fabricate conductive PDMS with different fillers such as carbon black, silver particles, and multiwalled carbon nanotubes. Additionally, the electrical properties of all three composites are examined under continuous mechanical stress. Furthermore, we present a novel, low-cost, simple three-step molding process that transfers a micro patterned silicon master into a polystyrene (PS) polytetrafluoroethylene (PTFE) replica with improved release features. This PS/PTFE mold is used for subsequent structuring of conductive PDMS with high accuracy. The non sticking characteristics enable the fabrication of delicate structures using a very soft PDMS, which is usually hard to release from conventional molds. Moreover, the process can also be applied to polyurethanes and various other material combinations.

Highly flexible capacitive strain gauge for continuous long-term blood pressure monitoring.

An innovative procedure for measuring blood pressure, with none of the disadvantages of current procedures, is proposed. A highly-flexible capacitive strain gauge has been designed to measure changes in the diameter of a blood vessel; such changes are indicative of blood pressure. The sensor is implanted and wrapped around an arterial blood vessel during the normal course of a surgical procedure. In vivo tests, demonstrating the feasibility of this concept, are reported, along with in vitro tests and notes on sensor design and fabrication. These continuous blood pressure monitoring sensors may be used for a continuous long-term monitoring of blood pressure and pulse. They may also be combined with a real-time nerve stimulation technique or a course of medication to create a closed-loop system for blood-pressure control.

Design of an implantable active microport system for patient specific drug release.

We present a novel concept of an implantable active microport based on micro technology that incorporates a high-resolution volumetric dosing unit and a drug reservoir into the space of a conventional subcutaneous port. The controlled release of small drug volumes from such an "active microport" is crucial e.g. for innovative methods in cancer treatment or pain therapy. Our microport system delivers a flow rate in the range of 10-1,000 mul/h and enables a patient-specific release profile. The core of our device is a two-stage piezoelectric micropump. It features a backpressure-independent volumetric dosing capability i.e. a stable flow rate is ensured up to a backpressure of 30 kPa. The stroke volume and hence the resolution of the mircopump is voltage controlled and can be preset between 10 and 200 nl. A miniaturized high-performance electronic control unit enables freely programmable dosing profiles. This electronic circuit is optimized for both energy consumption and weight which are both essential for a portable device. The data of an implemented pressure sensor are used to permanently monitor the dosing process and to detect a potential catheter occlusion. A polyurethane soft lithography process is introduced for the fabrication of the prototype. Therewith, a compact multilayer system has been developed which measures only 50 x 35 x 25 mm(3).

[German Artificial Sphincter System--GASSII: first in vivo evaluation of a novel and highly integrated sphincter prosthesis for therapy of major fecale incontinence]. / German Artificial Sphincter System-GASS II: Erste in vivo Evaluation eines neuen hochintegrativen Neosphinkters zur Therapie der hochgradigen Stuhlinkontinenz.

The German Artificial Sphincter System GASS consists of a support ring which includes a fluid reservoir on the outer side and an occlusive cuff on the inner side. The cuffs are designed as polyurethane hollow bodies with a pre-determined inflation volume and are connected to an integrated piezo micropump/valve unit. To evaluate the threshold of continence, the GASS was placed around the anorectal junction via a perineal approach in one mini pig. The novel cuff design reduces the occlusion pressure and allows low compression volumes. Low operating pressures indicate a minor risk of ischemia injury of the bowel. The operation time is estimated at about 6 days with no recharging of the battery. The novel remote controlled GASS is a highly integrated prosthesis for placement around the anal canal or lower rectum and is effective in restoring continence for liquids and solids in vitro and in vivo.

Protein detection with a novel ISFET-based zeta potential analyzer.

This publication presents a novel ISFET-based measurement concept for the determination of the zeta potential, which is known to be an efficient method for the detection of protein accumulations onto surfaces. The basic set-up consists of two monolithically integrated ISFET sensors arranged in a serial flow configuration together with a precoated fused silica capillary, which provides the reactive surface for the protein detection. In comparison with the state of the art, this novel biosensor system is characterized by a small size, an extremely low reagent consumption, a simple fluidic concept, a short analysis time, and a very effective noise suppression due to the differential ISFET set-up. In the following, an overview is given over the theoretical background of the measurement principle. In order to get deeper insight into the theoretical background of the measurement principle, a simulation model was developed which is based on the site-binding theory and takes into account the different proton dissociation equilibria of the surface groups as well as the influence of monovalent electrolyte ions. A quasi-Newton iteration after Broyden was used for the numerical solution of the formulated equation system. For an experimental confirmation of the simulation results, the calculated zeta potential vs. pH curves were compared with measured data for various modifications of the fused silica capillaries (in untreated state, after a hydrothermal activation, and after the deposition of several silanes) and it could be shown, that the chosen physical model represents a satifactory theoretical basis for the description of the occuring surface effects. Measurements before and after a covalent coupling of the model analyte lysozyme were performed in order to demonstrate the feasibility of an immunosensor based on the measurement of the streaming potential and showed a significant shift of the zeta potential vs. pH curves.

A quartz crystal biosensor for measurement in liquids.

The detection of anti-human immunodeficiency virus (HIV) antibodies by means of synthetic HIV peptide immobilized on a piezoelectric quartz sensor is demonstrated. The measurement set-up consists of an oscillator circuit, a suitably modified AT-cut thickness-shear-mode quartz crystal with gold electrodes, which is housed in a special reaction vessel, and a computer-controlled frequency counter for the registration of the measured frequency values. The quartz crystal is adapted for a steady operation in liquids at a frequency of 20 MHz. In phosphate-buffered saline solution the oscillator reaches a stability of about 0.5 Hz within a few seconds, of about 2 Hz within 10 min and about 30 Hz within 1 h. The frequency shift due to the adsorption of various proteins to the uncoated sensor surface has been investigated. It can be shown that a stable adsorptive binding of proteins to an oscillating gold surface is feasible and can be used for the immobilization of a receptor layer (e.g. HIV peptide). Specific binding of the anti-HIV monoclonal antibody to the HIV peptide immobilized on the quartz sensor is demonstrated. Control experiments show, however, additional unspecific binding. According to the experiments, the Sauerbrey formula gives a sufficiently accurate value for the decrease of the resonant frequency due to adsorption or binding of macromolecular proteins on the quartz crystal surface.