Substrate dependance, temperature dependance and temperature sensitivity and resolution of doped-silicon microcantilevers

This thesis aims to characterize microcantilevers with integrated heater-thermometers. This research concentrates on characterization for use in data storage, sensing, surface science, and nano-manufacturing. The rst objective seeks to understand the speci c thermal interactions between a heated microcantilever tip and various substrates. The experiments investigate thermal conductance, thermal time constant, and temperature-dependant adhesion force between and cantilever tip and substrates of silicon, quartz, and polyimide. The second objective is to utilize a heated microcantilever as a heater-thermometer. The experiments investigate the thermal calibration sensitivity and resolution under steady and periodic conditions near room-temperature. The results were compared to the Raman spectroscopy, which measures the local temperature at the cantilever tip

Magnetic field tuning of mechanical properties of ultrasoft PDMS-based magnetorheological elastomers for biological applications

We report tuning of the moduli and surface roughness of magnetorheological elastomers (MREs) by varying applied magnetic field. Ultrasoft MREs are fabricated using a physiologically relevant commercial polymer, SylgardTM 527, and carbonyl iron powder (CIP). We found that the shear storage modulus, Young\u27s modulus, and root-mean-square surface roughness are increased by ∼41×, ∼11×, and ∼11×, respectively, when subjected to a magnetic field strength of 95.5 kA m−1. Single fit parameter equations are presented that capture the tunability of the moduli and surface roughness as a function of CIP volume fraction and magnetic field strength. These magnetic field-induced changes in the mechanical moduli and surface roughness of MREs are key parameters for biological applications

The effect of polymer stiffness on magnetization reversal of magnetorheological elastomers

Ultrasoft magnetorheological elastomers (MREs) offer convenient real-time magnetic field control of mechanical properties that provides a means to mimic mechanical cues and regulators of cells in vitro. Here, we systematically investigate the effect of polymer stiffness on magnetization reversal of MREs using a combination of magnetometry measurements and computational modeling. Poly-dimethylsiloxane- based MREs with Young’s moduli that range over two orders of magnitude were synthesized using commercial polymers SylgardTM 527, Sylgard 184, and carbonyl iron powder. The magnetic hysteresis loops of the softer MREs exhibit a characteristic pinched loop shape with almost zero remanence and loop widening at intermediate fields that monotonically decreases with increasing polymer stiffness. A simple two-dipole model that incorporates magneto-mechanical coupling not only confirms that micrometer-scale particle motion along the applied magnetic field direction plays a defining role in the magnetic hysteresis of ultrasoft MREs but also reproduces the observed loop shapes and widening trends for MREs with varying polymer stiffnesses

Retroviral matrix and lipids, the intimate interaction

Retroviruses are enveloped viruses that assemble on the inner leaflet of cellular membranes. Improving biophysical techniques has recently unveiled many molecular aspects of the interaction between the retroviral structural protein Gag and the cellular membrane lipids. This interaction is driven by the N-terminal matrix domain of the protein, which probably undergoes important structural modifications during this process, and could induce membrane lipid distribution changes as well. This review aims at describing the molecular events occurring during MA-membrane interaction, and pointing out their consequences in terms of viral assembly. The striking conservation of the matrix membrane binding mode among retroviruses indicates that this particular step is most probably a relevant target for antiviral research

Detection of mass, growth rate, and stiffness of single breast cancer cells using micromechanical sensors

Cancer is an intricate disease that stems from a number of different mutations in a cell. These mutations often control the cellular growth and proliferation, a hallmark of cancer, and give rise to many altered biophysical properties. There exists a complex relationship between the behavior of a cell, its physical properties, and its surrounding environment. Knowledge gleaned from cellular biomechanics can lead to an improved understanding of disease progression and provide methods to target it. There are many studies that look at biophysical changes on a large population level, though there is much information that is lost by treating populations as homogeneous in properties and cell cycle phase. Biophysical studies on individual cells can link mechanics with function through coordination with the cell cycle, which is a fundamental physiological process that is crucial for understanding cellular physiology and metabolism. Development of more precise, reliable, and versatile measurement techniques will provide a greater understanding the physical properties of a cell and how they affect its behavior. Microelectromechanical systems (MEMS) technology can provide tools for manipulating, processing, and analyzing single cells, thus enabling detailed analyses of their biophysical properties. Growth is a vital element of the cell cycle, and cell mass homeostasis ensures that the cell mass and cell cycle transitions are coordinately linked. An accurate measurement of growth throughout the cell cycle is fundamental to understanding mechanisms of cellular proliferation in cancer. Growth can be identified through many ways; however, cell mass has been unexplored until the recent development of cantilever-type MEMS devices for mass sensing through resonant frequency shift. Measuring the dependency of growth rate on cellular mass may help explain the coordination and regulation of the cell cycle. However, MEMS mass sensing devices still require further development and characterization in order to reliably investigate long-term cell growth over the duration of the cell cycle. This dissertation focuses on the use of MEMS resonant pedestal sensors for measuring the mass and growth rate of single cancer cells. This work included characterization and improvement of the sensors to address current challenges in the measurement of long-term growth rate. The MEMS resonant pedestal sensors were first used to measure physical properties of biomaterials, including the micromechanical properties of hydrogels through verification of stiffness effect on mass measurements. Before studying live cells, modifications to the fabrication process were introduced to improve cell capture and retention. These include integration of an on-chip microfluidic system for delivery of fluids during mass measurements and the micro-patterning of sensor surfaces for select functionalization and passivation. These modifications enable long-term measurement of the changes in mass of normal and cancerous cells over time. This is the first investigation of the differences in growth rate between normal and cancer cells using MEMS resonant sensors

Optomechanical measurement of the stiffness of single adherent cells

Recent advances in mechanobiology have accumulated strong evidence showing close correlations between the physiological conditions and mechanical properties of cells. In this paper, a novel optomechanical technique to characterize the stiffness of single adherent cells attached on a substrate is reported. The oscillation in a cell\u27s height on a vertically vibrating reflective substrate is measured with a laser Doppler vibrometer as apparent changes in the phase of the measured velocity. This apparent phase shift and the height oscillation are shown to be affected by the mechanical properties of human colorectal adenocarcinoma cells (HT-29). The reported optomechanical technique can provide high-throughput stiffness measurement of single adherent cells over time with minimal perturbation

Biodegradable Monocrystalline Silicon Photovoltaic Microcells As Power Supplies For Transient Biomedical Implants

Bioresorbable electronic materials serve as foundations for implantable devices that provide active diagnostic or therapeutic function over a timeframe matched to a biological process, and then disappear within the body to avoid secondary surgical extraction. Approaches to power supply in these physically transient systems are critically important. This paper describes a fully biodegradable, monocrystalline silicon photovoltaic (PV) platform based on microscale cells (microcells) designed to operate at wavelengths with long penetration depths in biological tissues (red and near infrared wavelengths), such that external illumination can provide realistic levels of power. Systematic characterization and theoretical simulations of operation under porcine skin and fat establish a foundational understanding of these systems and their scalability. In vivo studies of a representative platform capable of generating ≈60 µW of electrical power under 4 mm of porcine skin and fat illustrate an ability to operate blue light-emitting diodes (LEDs) as subdermal implants in rats for 3 d. Here, the PV system fully resorbs after 4 months. Histological analysis reveals that the degradation process introduces no inflammatory responses in the surrounding tissues. The results suggest the potential for using silicon photovoltaic microcells as bioresorbable power supplies for various transient biomedical implants

Hydrodynamic loading and viscous damping of patterned perforations on microfabricated resonant structures

We examined the hydrodynamic loading of vertically resonating microfabricated plates immersed in liquids with different viscosities. The planar structures were patterned with focused ion beam, perforating various shapes with identical area but varying perimeters. The hydrodynamic loading of various geometries was characterized from resonant frequency and quality factor. In water, the damping increased linearly with the perimeter at 45.4 × 10 -3 Ns/m 2, until the perforation\u27s radius was 123% ± 13% of the depth of penetration of fluid\u27s oscillation. The added mass effect decreased with perforations and recovered to the level of un-perforated structures when the perforation\u27s radius became smaller than the depth of penetration. © 2012 American Institute of Physics