Adam Winkleman

A magnetic trap for living cells suspended in a paramagnetic buffer

This manuscript describes the fabrication and use of a three-dimensional magnetic trap for diamagnetic objects in an aqueous solution of paramagnetic ions; this trap uses permanent magnets. It demonstrates trapping of polystyrene spheres, and of various types of living cells: mouse fibroblast (NIH-3T3), yeast (Saccharomyces cerevisiae), and algae (Chlamydomonas reinhardtii). For a 40mM solution of gadolinium (III) diethylenetriaminepentaacetic acid (Gd·DTPA) in aqueous buffer, the smallest cell (particle) that could be trapped had a radius of ∼2.5μm. The trapped particle and location of the magnetic trap can be translated in three dimensions by independent manipulation of the permanent magnets. This letter a1so characterizes the biocompatibility of the trapping solution.

Patterning micron-sized features in a cross-linked poly(acrylic acid) film by a wet etching process

This paper describes a photolithographic method to create sub-micron-scale patterns of cation-cross-linked poly(acrylic acid) (CCL-PAA). PAA can be cross-linked with a wide range of metal cations-including, but not limited to, Ag, Ca, Pd, Al, La, and Ti. Upon patterning a positive photoresist (diazonaphthoquinone-novolac resin) on a film of CCL-PAA, the exposed regions of CCL-PAA were etched by either an aqueous NaOH or EDTA solution. The initial cross-linking cation could be exchanged for a second cation that could not be patterned photolithographically. We used these patterned films of CCL-PAA i) to host and template the reduction of metallic cations to metallic nanoparticles, and ii) to fabricate porous, low- dielectric substrates.

Templated Self-Assembly Over Patterned Electrodes by an Applied Electric Field: Geometric Constraints and Diversity of Materials

This paper expands the scope and usefulness of a process to assemble dry micrometer-sized particles into arrays over a templated electrode by a high-voltage dc bias. Using the predictions from a theoretical model for the process of assembly, the experimental scope and limitations of this technique were explored and related to the predictions of the model. The range of bead size that can be assembled (20-750 mum) and the effects of changing the ratio of the size of the features in the templated electrode to the size of the particles being assembled were experimentally determined and compared to the theory. It was also demonstrated that: 1) the assembled spheres can be made of materials that are either dielectrics (glass and polystyrene), semiconductors (silicon), or conductors (copper); 2) the material for the electrode can either be gold, silver, copper, or amorphous silicon; and 3) the dielectric substrate only needs to be able to support the applied voltage without breaking down. The experimental results, in general, were predicted and supported by the model.

Mechanics of a process to assemble microspheres on a patterned electrode

A process has been demonstrated recently to assemble microspheres on a patterned electrode under the influence of an applied voltage. Here we examine the mechanics of this process, and describe both the conditions under which excess microspheres jump off the electrode when the voltage is applied, and the forces that attract the remaining microspheres to the desired positions. A quantitative mechanistic understanding of this process rationalizes experimental observations, provides scaling relations, and suggests modifications of the process.