Alexander Bietsch

Bio: Alexander Bietsch is an academic researcher from IBM. The author has contributed to research in topics: Substrate (printing) & Microcontact printing. The author has an hindex of 23, co-authored 49 publications receiving 4381 citations. Previous affiliations of Alexander Bietsch include University of Jena & University of Basel.

Current-driven insulator–conductor transition and nonvolatile memory in chromium-doped SrTiO3 single crystals

Abstract: Materials showing reversible resistive switching are attractive for today’s semiconductor technology with its wide interest in nonvolatile random-access memories. In doped SrTiO3 single crystals, we found a dc-current-induced reversible insulator–conductor transition with resistance changes of up to five orders of magnitude. This conducting state allows extremely reproducible switching between different impedance states by current pulses with a performance required for nonvolatile memories. The results indicate a type of charge-induced bulk electronic change as a prerequisite for the memory effect, scaling down to nanometer-range electrode sizes in thin films.

Printing meets lithography: soft approaches to high-resolution patterning

Abstract: We are developing a high-resolution printing technique based on transferring a pattern from an elastomeric stamp to a solid substrate by conformal contact. This is an attempt to enhance the accuracy of classical printing to a precision comparable with optical lithography, creating a low-cost, large-area, high-resolution patterning process. First, we introduce the components of this technique, called soft lithography, and review its evolution. Topics described in detail are the stamp material, stamp architecture, pattern design rules, and printing tools. The accuracy of the prints made by thin patterned elastomeric layers supported on a stiff and flexible backplane is then assessed, and defects are characterized using a new electrical metrology approach. This is followed by a discussion of various printing processes used in our laboratory: 1) thiol printing for high-resolution patterns of noble metals that may also be used as sacrificial masks; 2) confined contact processing with liquids in cavities or channels to chemically convert a substrate or deposit layers of materials or biomolecules; 3) printing of catalysts to mediate patterned deposition of metals; and 4) structured, light-guiding stamps for transferring high-resolution patterns into photoresists. Finally, we compare classical and high-resolution printing approaches, and describe their potential for emerging micro-and nano-scale patterning technologies.

Microfluidic Networks for Chemical Patterning of Substrates: Design and Application to Bioassays

Abstract: This article describes the design, function, and application of simple microfluidic networks as conduits for the patterned delivery of chemical reactants onto a substrate. It demonstrates how such networks, made in an elastomer, allow simultaneous delivery of functionally distinct molecules onto targeted regions of a surface (Delamarche, E. et al. Science 1997, 276, 779−781). Microfluidic networks generally consume less than microliter quantities of solution and are thus well suited for use when the required reagents are scarce or precious, as often occurs in experiments and technologies that place biochemicals on solid planar substrates. We illustrate some of the particular challenges of doing chemistry inside the narrow confines of capillaries defined by fluidic networks, in addition to the advantages attendant to this approach, in the context of forming patterned arrays of different, and functional, immunoglobulins useful in highly localized biological assays.

Conformal contact and pattern stability of stamps used for soft lithography

Abstract: Patterning in soft lithography techniques such as microcontact printing or light-coupling mask lithography is mediated by surface topographical patterns of elastomeric stamps: intimate contact with the substrate is achieved locally at the protruding areas, whereas a gap remains between the substrate and recessed zones. This principle challenges the properties of the stamp, especially when printing high-resolution or extreme aspect-ratio patterns with high accuracy. On the one hand, the stamp must be soft enough to enable conformal contact with the substrate, which means that it must adapt elastically without leaving voids created by the natural roughness of the substrate. On the other hand, a precise definition of micropatterns requires a rigid material. In this article, we analyze the conditions of elasticity, roughness, and energy of adhesion to establish conformal contact between an elastomer and the target surface. Furthermore, we address questions of replication accuracy and evaluate local elastic deformation induced by normal forces using model calculations for simple pattern geometries. Pressure applied during contact leads to a sagging or collapse of the unsupported areas. We discuss implications on both material and pattern design that allow spontaneous propagation of conformal contact while inhibiting the spreading of collapse.

Rapid and label-free nanomechanical detection of biomarker transcripts in human RNA

Abstract: The availability of entire genome sequences has triggered the development of microarrays for clinical diagnostics that measure the expression levels of specific genes. Methods that involve labelling can achieve picomolar detection sensitivity, but they are costly, labour-intensive and time-consuming. Moreover, target amplification or biochemical labelling can influence the original signal. We have improved the biosensitivity of label-free cantilever-array sensors by orders of magnitude to detect mRNA biomarker candidates in total cellular RNA. Differential gene expression of the gene 1-8U, a potential marker for cancer progression or viral infections, has been observed in a complex background. The measurements provide results within minutes at the picomolar level without target amplification, and are sensitive to base mismatches. This qualifies the technology as a rapid method to validate biomarkers that reveal disease risk, disease progression or therapy response. We foreseee cantilever arrays being used as a tool to evaluate treatment response efficacy for personalized medical diagnostics.

Rapid prototyping of microfluidic systems in poly(dimethylsiloxane)

Abstract: This paper describes a procedure that makes it possible to design and fabricate (including sealing) microfluidic systems in an elastomeric materialpoly(dimethylsiloxane) (PDMS)in less than 24 h. A network of microfluidic channels (with width >20 μm) is designed in a CAD program. This design is converted into a transparency by a high-resolution printer; this transparency is used as a mask in photolithography to create a master in positive relief photoresist. PDMS cast against the master yields a polymeric replica containing a network of channels. The surface of this replica, and that of a flat slab of PDMS, are oxidized in an oxygen plasma. These oxidized surfaces seal tightly and irreversibly when brought into conformal contact. Oxidized PDMS also seals irreversibly to other materials used in microfluidic systems, such as glass, silicon, silicon oxide, and oxidized polystyrene; a number of substrates for devices are, therefore, practical options. Oxidation of the PDMS has the additional advantage that it ...

Nanoionics-based resistive switching memories

Abstract: Many metal–insulator–metal systems show electrically induced resistive switching effects and have therefore been proposed as the basis for future non-volatile memories. They combine the advantages of Flash and DRAM (dynamic random access memories) while avoiding their drawbacks, and they might be highly scalable. Here we propose a coarse-grained classification into primarily thermal, electrical or ion-migration-induced switching mechanisms. The ion-migration effects are coupled to redox processes which cause the change in resistance. They are subdivided into cation-migration cells, based on the electrochemical growth and dissolution of metallic filaments, and anion-migration cells, typically realized with transition metal oxides as the insulator, in which electronically conducting paths of sub-oxides are formed and removed by local redox processes. From this insight, we take a brief look into molecular switching systems. Finally, we discuss chip architecture and scaling issues.

Fabrication of microfluidic systems in poly(dimethylsiloxane)

Abstract: Microfluidic devices are finding increasing application as analytical systems, biomedical devices, tools for chemistry and biochemistry, and systems for fundamental research. Conventional methods of fabricating microfluidic devices have centered on etching in glass and silicon. Fabrication of microfluidic devices in poly(dimethylsiloxane) (PDMS) by soft lithography provides faster, less expensive routes than these conventional methods to devices that handle aqueous solutions. These soft-lithographic methods are based on rapid prototyping and replica molding and are more accessible to chemists and biologists working under benchtop conditions than are the microelectronics-derived methods because, in soft lithography, devices do not need to be fabricated in a cleanroom. This paper describes devices fabricated in PDMS for separations, patterning of biological and nonbiological material, and components for integrated systems.