Jonathan S. Colton

Role of material microstructure in plate stiffness with relevance to microcantilever sensors

This work examines the effect of microstructure upon microcantilever bending stiffness. An existing beam theory model, based upon an isotropic Hookes law constitutive relationship, is compared to a model based upon a micropolar elasticity constitutive model. The micropolar approach introduces a bending stiffness relation which is a function of any two independent elastic constants of the Hookes law model (e.g., the elastic modulus and the Poissons ratio), and an additional material constant (called γ). A consequence of the additional material constant is the prediction of an increased bending stiffness as the cantilever thickness decreases—a stiffening due to the material microstructure which becomes measurable at micron-order thicknesses. Polypropylene microcantilevers, which have a non-homogeneous microstructure due to their semi-crystalline nature, were fabricated via injection molding. A nanoindenter was used to measure their stiffness. The nanoindenter-determined stiffness values, which include the effect of the additional micropolar material constant, are compared to stiffness values obtained from beam theory. The nanoindenter stiffness values are seen to be at least four times larger than the beam theory stiffness predictions. This stiffening effect has relevance in future MEMS applications which employ materials with non-homogeneous microstructures instead of the conventional MEMS materials (e.g., silicon, silicon nitride), which have a very uniform microstructure.

Microcantilevers: Sensing Chemical Interactions via Mechanical Motion

2.4. Temperature Effects 527 2.4.1. Effect on Material Properties 528 2.4.2. Effect on Geometry 528 3. Detection Schemes 528 3.1. Optical Lever 528 3.2. Interferometer 529 3.3. Piezoresistive 529 3.4. Capacitive 529 4. Design, Materials, and Fabrication 529 4.1. Design Considerations 530 4.2. Fabrication of Silicon-based Cantilevers 530 4.2.1. Film Deposition 530 4.2.2. Photolithography 530 4.2.3. Etching 530 4.2.4. Doping 530 4.3. Fabrication of Polymeric Cantilevers 531 5. Chemical Selectivity 531 6. Chemical Applications 533 6.1. Volatile Organics 533 6.2. Chemical Warfare Agents 534 6.3. Explosives 534 6.4. Toxic Metal Ions 534 7. Biological Applications 534 7.1. Cells 534 7.2. Viruses 534 7.3. Antigen−Antibody Interactions 535 7.4. DNA Hybridization 536 7.5. Enzymes 537 8. Recommendations for Future Work 538 8.1. Guidelines for Reporting Sensor Performance 538 8.2. Experimental Design Considerations 538 8.3. Fruitful Areas for Further Research 539 8.3.1. More Selective Coatings 539 8.3.2. Increased Sensitivity and Faster Response 539

Influence of surface stress on the resonance behavior of microcantilevers

This work presents a model to predict the effect of surface stresses on the ith-mode bending resonant frequency of microcantilevers and its experimental validation. With this model, one can calculate the surface stress acting upon the microcantilever solely by measuring resonant frequencies whereas previously one needed to measure the deflection. Resonant frequency measurement has distinct advantages in terms of ease and accuracy of measurement.

Characterization of microcantilevers solely by frequency response acquisition

A method is presented to determine the geometry of tipless microcantilevers by measuring the resonance frequencies of at least one of their bending, lateral and torsional resonance modes, and having knowledge of the beam’s elastic modulus, Poisson’s ratio and density. Once the geometry is known, the beam’s stiffness and mass can be calculated. Measurement of multiple modes allows for multiple estimates of cantilever geometry. Multiple data points from the experimental results show that this approach yields dimensional values accurate to roughly 2.5% as compared to SEM-determined length, width and thickness. Stiffness values determined with this new technique are roughly 4.7% and 6.5% less than two existing characterization methods (i.e., Sader’s method and Euler–Bernoulli beam theory predictions), and roughly 16% greater than Hutter and Bechhoefer’s stiffness determination method.

THE NUCLEATION OF MICROCELLULAR FOAMS IN SEMI CRYSTALLINE THERMOPLASTICS

ABSTRACT The nucleation of microcellular foams in amorphous thermoplastics has been performed by supersaturation with gas at an elevated temperature. Pressure and temperature are then carefully reduced in the vicinity of the glass transition temperature of the material. The result is a foam structure with cells on the order of 10 microns. This material exhibits greatly increased impact strength, as well as thermal and electrical insulation properties as compared to conventional foams. A new process has been developed to produce microcellular foams in semi-crystalline polymers. The process operates in the vicinity of the polymers melting point, as opposed to the glass transition point. This is due the low solubility of the gases in and the rigidity of the crystalline phase. Microcellular foams have been produced successfully in polypropylene. The effects of various additives have been investigated experimentally. The theory developed for the amorphous materials has been compared to these new experimental ...

Assessment of percolation and homogeneity in ABS/carbon black composites by electrical measurements

In this study, composites consisting of an insulating poly(acrylonitrile-co-butadiene-co-styrene) polymer matrix and a conducting carbon black (CB) additive were produced by twin-screw extrusion. Both direct current and alternating current electrical measurements were used to evaluate the electrical properties of the composite and to assess whether sufficient mixing was achieved. Electrical measurement results and scanning electron micrographs show that once-extruded composites had a porous structure and poor conductivity while twice-extruded composites were much more homogeneous and had higher conductivity. The percolation threshold of the twice-extruded poly(acrylonitrile-co-butadiene-co-styrene)/CB composites was found to be between 8 and 10% CB. Electrical measurements provided a feedback loop for improving processing of the composite material.

Chemical sensing with micromolded plastic microcantilevers

This paper describes microcantilever sensors produced via injection molding. The injection mold design is novel in that it employs one floating and one fixed mold half, hence only necessitating high flatness on two surfaces (e.g., the mating surfaces of the mold), whereas the remainder of the mold can be machined to only moderate tolerances. The mold holds a sub-100 nanometer flatness error over the entire mold mating surfaces, needed to produce micro- and nanoscale parts. Micrometer-scale cantilevers are produced and characterized as a test case. Microcantilevers are fabricated from three different polymeric materials and have exceptional repeatability as evidenced by their measured first-mode bending resonant frequencies. As a precursor to biological sensing, gold-thiol chemical sensing results obtained with the injection-molded cantilevers are also presented and show values that agree with the literature. As a whole, this work shows that the polymeric microcantilever parts fabricated via injection molding are mechanical and functional equivalents to their silicon-type counterparts, and are cheaper and easier to manufacture. [1483].

Injection moulding of high aspect ratio micron-scale thickness polymeric microcantilevers

Tipless thermoplastic microcantilevers suitable for chemical and biological sensing applications were fabricated by injection moulding. Their stiffnesses and resonant frequencies were each determined by two techniques. Polystyrene beams produced by this method exhibited stiffnesses ranging from 0.01 to 10 N m−1, making them feasible for biosensing applications. The approach proved repeatable with low standard deviations on the parameters measured on 22 microcantilever beams (stiffness and first-mode resonant frequency) made from the same mould. The variations were much lower than those of similar, commercially available, silicon-type beams. The polymeric microcantilevers were shown to be of at least equal calibre to commercially available microcantilevers.

Production and characterization of polymer microcantilevers

This work describes the production of microcantilever beams via a solvent casting technique. The beams produced had dimensions of roughly 500 by 50 by 2 μm (length, width, and thickness, respectively). A subset of the beams produced were characterized and were shown to have comparable dynamic mechanical behavior as that of existing ceramic and photopolymer microcantilevers.

Fabrication and analysis of plastic hypodermic needles

Plastic hypodermic needles may help reduce illness and disease due to unsterile re-use, as they may be more easily disabled and disposed of as compared to metal ones. This paper presents the fabrication of plastic hypodermic needles using micro-injection moulding and the analyses of their buckling behaviour. As a needle cannula is a thin-walled column (here 0.7 mm outer diameter with a 0.15 mm wall thickness), it is vulnerable to buckling. The buckling behaviour is characterized by numerical simulation and experiments, which are compared to the penetration forces for rubber skin mimic and human skin.