Characterizing and imaging gross and real finger contacts under dynamic loading

We describe an instrument intended to study finger contacts under tangential dynamic loading. This type of loading is relevant to the natural conditions when touch is used to discriminate and identify the properties of the surfaces of objects — it is also crucial during object manipulation. The system comprises a high performance tribometer able to accurately record in vivo the components of the interfacial forces when a finger interacts with arbitrary surfaces which is combined with a high-speed, high-definition imaging apparatus. Broadband skin excitation reproducing the dynamic contact loads previously identified can be effected while imaging the contact through a transparent window, thus closely approximating the condition when the skin interacts with a non-transparent surface during sliding. As a preliminary example of the type of phenomenon that can be identified with this apparatus, we show that traction in the range from 10 to 1000 Hz tends to decrease faster with excitation frequency for dry fingers than for moist fingers

Tribological interactions of the finger pad and tactile displays

This thesis summarise the results of an investigation of the tribological interactions of the human finger pad with different surfaces and tactile displays. In the wide range of analyses of the mechanical properties of the finger pad, an attempt has been made to explain the nature of the interactions based on critical material parameters and experimental data. The experimental data are presented together with detailed modelling of the contact mechanics of the finger pad compressed against a smooth flat surface. Based on the model and the experimental data, it was possible to account of the loading behaviour of a finger pad and derive the Young’s modulus of the fingerprint ridges. The frictional measurements of a finger pad against smooth flat surfaces are consistent with an occlusion mechanism that is governed by first order kinetics. In contrast, measurements against a rough surface demonstrated that the friction is unaffected by occlusion since Coulombic slip was exhibited. The thesis includes an investigation of critical parameters such as the contact area. It has been shown that four characteristic length scales, rather than just two as previously assumed, are required to describe the contact mechanics of the finger pad. In addition, there are two characteristic times respectively associated with the growth rates of junctions formed by the finger pad ridges and of the real area of contact. These length and time scales are important in understanding how the Archardian-Hertzian transition drives both the large increase of friction and the reduction of the areal load index during persisting finger contacts with impermeable surfaces. Established and novel models were evaluated with statistically meaningful experiments for phenomena such as lateral displacement, electrostatic forces and squeeze-film that have advanced applications

The Trapeziometacarpal Joint: Tissue Characterization and Surgical Techniques for Treatment of Osteoarthritis

The trapeziometacarpal (TMC) joint is one of the most important joints in the human body. It provides the thumb with the ability to cross over the palm of the hand, thus enabling motions of pinch and grip essential in performing routine daily activities. In the case of repeated use of this joint, the articular cartilage may wear through a progressive joint disease known as osteoarthritis (OA). This disease is characterized by pain at the base of the thumb, decreased range of motion, thumb instability, and decreased grip and pinch strength leading to impairment in vocational activities, significantly affecting quality of life. Much of the research surrounding the TMC joint has focused on development of non-surgical and surgical options for treatment of early and late stage OA. Unfortunately, the extent of research on characterizing the biophysical properties of the TMC joint and surrounding tissue is limited. The following research will seek to identify the ligamentous structures hypothesized to act as primary stabilizers of the TMC joint through advanced, high-resolution motion analyses. Mechanical properties of the primary ligamentous stabilizers will be obtained through uniaxial tensile testing of ligamentous tissue. This tissue will be further characterized through histology, staining for identification of the presence and orientation of essential proteins which may serve to support the argument for primary stabilizing tissue. Using results from the tissue characterization studies, two techniques are presented for the treatment of early and late stage TMC joint osteoarthris, which are designed to maintain and/or regain stability of this joint. The final section introduces a methodology for development of patient-specific computational finite element models of the hand and thumb. Input properties of these models are based on computed tomography data and outputs from the motion analysis and mechanical testing studies

Self-organization of cell shape and movement

This thesis presents work toward understanding the spatial organization of key molecules during cell morphogenesis and migration. Cell migration is essential for many processes including developmental morphogenesis, axon guidance, and immune responses. Chemotaxis, or directed migration guided by chemical cues, requires the spatial and temporal coordination of a multitude of molecules that pattern the force-generating actin cytoskeleton to build plasma membrane protrusions and power cell motility. This work focuses on identifying novel chemotaxis effectors, dissecting their molecular signaling logic, and exploring how key molecules spatially organize to enable the large-scale, self-organization of cell shape and movement. In the first project, we identified and characterized a novel signaling effector of neutrophil chemotaxis (Chapter 2). From a mass spectrometry pulldown screen, we identified Homer3 as a Gαi2 interacting protein. With biochemical and cell biology techniques, we report that Homer3 is necessary for efficient chemotaxis by regulating the polarized spatial organization, rather than the magnitude and kinetics, of key signaling molecules. Overall, our work characterized how Homer3 functions as a scaffold to spatially organize polarity signaling and actin assembly.In the second project, we studied the spatial organization of the WAVE complex, which is a key effector of cell shape and migration across eukaryotes (Chapter 3). Using quantitative, live-cell super-resolution microscopy, we discovered how the WAVE complex spatially assembles into nanometer scale ring structures at sites of saddle membrane curvature in the absence of actin polymerization. This geometric association for the WAVE complex could explain emergent cell behaviors, such as expanding and self-straightening lamellipodia as well as the ability of endothelial cells to recognize and seal transcellular holes. In the third project, I describe my pilot work using nanotopography to physically manipulate cell geometry to assay curvature sensation (Chapter 4). The interdisciplinary nature of this experiment, which spans nano-engineering, cell biology, and high-resolution microscopy, highlights a combination of expertise that will undoubtedly unveil exciting insights

Modeling and simulation in tribology across scales: An overview

This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and micro-scales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions

Doctor of Philosophy

dissertationThis dissertation describes the use of cortical surface potentials, recorded with dense grids of microelectrodes, for brain-computer interfaces (BCIs). The work presented herein is an in-depth treatment of a broad and interdisciplinary topic, covering issues from electronics to electrodes, signals, and applications. Within the scope of this dissertation are several significant contributions. First, this work was the first to demonstrate that speech and arm movements could be decoded from surface local field potentials (LFPs) recorded in human subjects. Using surface LFPs recorded over face-motor cortex and Wernickes area, 150 trials comprising vocalized articulations of ten different words were classified on a trial-by-trial basis with 86% accuracy. Surface LFPs recorded over the hand and arm area of motor cortex were used to decode continuous hand movements, with correlation of 0.54 between the actual and predicted position over 70 seconds of movement. Second, this work is the first to make a detailed comparison of cortical field potentials recorded intracortically with microelectrodes and at the cortical surface with both micro- and macroelectrodes. Whereas coherence in macroelectrocorticography (ECoG) decayed to half its maximum at 5.1 mm separation in high frequencies, spatial constants of micro-ECoG signals were 530-700 ?m-much closer to the 110-160 ?m calculated for intracortical field potentials than to the macro-ECoG. These findings confirm that cortical surface potentials contain millimeter-scale dynamics. Moreover, these fine spatiotemporal features were important for the performance of speech and arm movement decoding. In addition to contributions in the areas of signals and applications, this dissertation includes a full characterization of the microelectrodes as well as collaborative work in which a custom, low-power microcontroller, with features optimized for biomedical implants, was taped out, fabricated in 65 nm CMOS technology, and tested. A new instruction was implemented in this microcontroller which reduced energy consumption when moving large amounts of data into memory by as much as 44%. This dissertation represents a comprehensive investigation of surface LFPs as an interfacing medium between man and machine. The nature of this work, in both the breadth of topics and depth of interdisciplinary effort, demonstrates an important and developing branch of engineering