VTechWorks Repository :: Browsing by Author "Grant, John Wallace"

Browsing by Author "Grant, John Wallace"

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Activation of bacterial channel MscL in mechanically stimulated droplet interface bilayers
Najem, Joseph S.; Dunlap, Myles D.; Rowe, Ian D.; Freeman, Eric C.; Grant, John Wallace; Sukharev, Sergei; Leo, Donald J. (Springer Nature, 2015-09-08)
MscL, a stretch-activated channel, saves bacteria experiencing hypo-osmotic shocks from lysis. Its high conductance and controllable activation makes it a strong candidate to serve as a transducer in stimuli-responsive biomolecular materials. Droplet interface bilayers (DIBs), flexible insulating scaffolds for such materials, can be used as a new platform for incorporation and activation of MscL. Here, we report the first reconstitution and activation of the low-threshold V23T mutant of MscL in a DIB as a response to axial compressions of the droplets. Gating occurs near maximum compression of both droplets where tension in the membrane is maximal. The observed 0.1-3 nS conductance levels correspond to the V23T-MscL sub-conductive and fully open states recorded in native bacterial membranes or liposomes. Geometrical analysis of droplets during compression indicates that both contact angle and total area of the water-oil interfaces contribute to the generation of tension in the bilayer. The measured expansion of the interfaces by 2.5% is predicted to generate a 4-6 mN/m tension in the bilayer, just sufficient for gating. This work clarifies the principles of interconversion between bulk and surface forces in the DIB, facilitates the measurements of fundamental membrane properties, and improves our understanding of MscL response to membrane tension.
Analysis of a bonded joint using bulk adhesive properties
Osiroff, Talia (Virginia Tech, 1988-12-05)
Adhesives and adhesively bonded structures are being considered as a viable alternative to conventional fastening methods. In order to gain wider acceptance, it is essential to address the issue of the mechanical characterization of adhesive materials and its implementation in the design of bonded joints. While measuring the in-situ properties of the adhesive in a joint is a difficult task, characterizing its bulk properties is a relatively simpler undertaking. The objective of this study was to propose and verify an experimental procedure that would allow the analytical prediction of the viscoelastic behaviour of a bonded joint, using bulk adhesive properties. The Arcan joint geometry was chosen because of the simple state of stress within the adhesive.
Analysis of Vestibular Hair Cell Bundle Mechanics Using Finite Element Modeling
Silber, Joseph Allan (Virginia Tech, 2002-11-18)
The vestibular system of vertebrates consists of the utricle, saccule, and the semicircular canals. Head movement causes deformation of hair cell bundles in these organs, which translate this mechanical stimulus into an electrical response sent to the nervous system. This study consisted of two sections, both utilizing a Fortran-based finite element program to study hair cell bundle response. In the first part, the effects of variations in geometry and material properties on bundle mechanical response were studied. Six real cells from the red eared slider turtle utricle were modeled and their response to a gradually increased point load was analyzed. Bundle stiffness and tip link tension distributions were the primary data examined. The cells fell into two groups based on stiffness. All cells exhibited an increase in stiffness as the applied load was increased, but cells in the stiffer group showed a greater increase. Tip link tensions in the compliant group were approximately 3 times as high as those in the stiffer group. Cells in the stiffer group were larger, with more cilia, and also had a higher stereocilia/kinocilium height ratio than the cells in the other group. The stereocilia/kinocilium height ratio was the most important geometric factor in influencing bundle stiffness. Modeling a bundle as just its middle row of stereocilia resulted in some decrease in stiffness, but more significantly, a stiffness that was virtually constant as applied load increased. Tip link tension distributions showed serial behavior in the core rows of stereocilia and parallel behavior in the outer rows; this trend intensified if the tip link elastic modulus was increased. It was demonstrated that full three-dimensional modeling of bundles is critical for obtaining complete and accurate results. In the second part of the study, tip link ion gates were modeled. Sufficient tension in a tip link caused that link's ion gate to open, increasing the length of the link and causing its tension to decrease or the link to go slack. The two parameters that were varied were tip link elastic modulus and tip link gating distance d (change in length of the link). Bundle stiffness drops of up to 25% were obtained, but only when tip links went slack after gate opening; tip link slackening was dependent on tip link gating distance. Higher tip link modulus resulted in higher stiffness drops. Variable tip link modulus and tip link pre-tensioning were modeled. Variable tip link modulus resulted in increased bundle stiffness, especially under high applied loads, and in some cases, resulted in greater bundle stiffness drops when ion gates opened. Tip link pre-tensioning had no noticeable effect on bundle response. No evidence against inclusion of pre-tensioning or variable tip link elastic modulus was found.
Assessing the Cardiovagal Baroreflex
Behnam, Abrahm John (Virginia Tech, 2007-02-02)
Abrupt decreases and increases in systolic arterial blood pressure produce baroreflex mediated shortening and lengthening, respectively, of the R-R interval. This phenomenon, otherwise known as the cardivagal baroreflex, is best described by the sigmoid relationship between R-R interval length and systolic blood pressure. The linear portion of this relationship is used to derive the slope or gain of the cardiovagal baroreflex. Importantly, lower levels of cardiovagal baroreflex have been associated with poor orthostatic tolerance and an increased cardiovascular disease-related mortality. The most commonly used and accepted technique to assess cardiovagal barorelex gain is the modified Oxford techinique. Bolus injections of sodium nitroprusside followed by phenylephrine HCL are used to decrease and raise blood pressure ~15 mmHg, respectively. The baroreflex control of the cardiac vagal outflow can then be assessed by the relation of the R-R interval to systolic blood pressure. However, the modified Oxford technique does not always reveal the nonlinear nature of baroreflex relations. The reasons for this has been unclear. Thus, analysis of baroreflex gain when nonlinearities are not revealed is problematic. Five classifications of baroreflex trials have been identified: acceptable, threshold-heavy, saturation-heavy, linear-heavy, and random trials. A new method of gain estimation was developed that combines the strengths of the current methods of gain estimation with the knowledge of the classifications of baroreflex trials. Using this method, cardiovagal baroreflex gain assessment can be maximized if threshold-heavy, saturation-heavy, and random trials are filtered out of the analysis and the manual method is used to estimate gain on the remaining trials. In addition, a link seems to exist between the variability of delta and the variability in baroreflex gain between different subjects.
A beam test for adhesives
Fior, Valerie F. (Virginia Tech, 1988-07-15)
The strength of materials solution for a new bonded cantilever beam test specimen to determine adhesive shear properties is reviewed and discussed. A parametric analysis for the adhesive shear stress and for the end deflection reveals the specimen dimensions required for reliable bonded adhesive shear properties determination. Recommendations are provided for conducting reproducible tests. A pure and quasi-uniform shear test for stiff adhesives is proposed. Analytical solutions are compared with Finite Element solutions from VISTA and NOVA for the stresses in the adhesive. It appears that the assumption of pure shear is nearly valid even for very stiff and/or very thick adhesives. In order to increase the end point deformations for stiff adhesives, a modified specimen is proposed. Three-dimensional effects through the thickness of the adhesive layer are studied with the program ABAQUS. Experiments were performed using the two methods derived from theory and good correlation between theory and experiment were obtained with some restrictions. For both methods, experimental results underlined the need for defining proper specimen geometry prior to testing. Simple numerical codes are proposed to facilitate this purpose.
Bi-layered viscoelastic model for a step change in velocity and a constant acceleration stimulus for the human otolith organs
Coggins, M. Denise (Virginia Tech, 1996-10-05)
The otolith organs are commonly modeled as a system consisting of three distinct elements, a viscous endolymph fluid in contact with a rigid otoconial layer that is attached to the skull by a viscoelastic gel layer. However, in this model the gel layer is considered as a bi-layered viscoelastic solid and is modeled as a simple Kelvin-Voigt material. The governing differential equations of motion are derived and nondimensionalized yielding - three non-dimensional parameters: nondimensional viscosity, nondimensional elasticity and nondimensional density. These non-dimensional parameters are derived from experimental research. The shear stresses acting at the interface of the viscoelastic bi-layered gel are nondimensionalized and equated. The governing differential equations are then solved using finite difference techniques on a digital computer for a step-change in velocity and a constant acceleration stimulus. The results indicate that the inclusion of a viscoleastic bi-layered gel is essential for the model to produce greater otoconial layer deflections that are consistent with physiologic displacements. Future mathematical modeling of the otolith organs should include the effects of a viscoelastic bi-layered gel, as this is a major contributor to system damping and response and increased otoconial layer deflections.
Biodynamic Analysis of Human Torso Stability using Finite Time Lyapunov Exponents
Tanaka, Martin L. (Virginia Tech, 2008-03-25)
Low back pain is a common medical problem around the world afflicting 80% of the population some time in their life. Low back injury can result from a loss of torso stability causing excessive strain in soft tissue. This investigation seeks to apply existing methods to new applications and to develop new methods to assess torso stability. First, the time series averaged finite time Lyapunov exponent is calculated from data obtained during seated stability experiments. The Lyapunov exponent is found to increase with increasing task difficulty. Second, a new metric for evaluating torso stability is introduced, the threshold of stability. This parameter is defined as the maximum task difficulty in which dynamic stability can be maintained for the test duration. The threshold of stability effectively differentiates torso stability at two levels of visual feedback. Third, the state space distribution of the finite time Lyapunov exponent (FTLE) field is evaluated for deterministic and stochastic systems. Two new methods are developed to generate the FTLE field from time series data. Using these methods, Lagrangian coherent structures (LCS) are found for an inverted pendulum, the Acrobot, and planar wobble chair models. The LCS are ridges in the FTLE field that separate two inherently different types of motion when applied to rigid-body dynamic systems. As a result, LCS can be used to identify the boundaries of the basin of stability. Finally, these new methods are used to find the basin of stability from time series data collected from torso stability experiments. The LCS and basins of stability provide a richer understanding into the system dynamics when compared to existing methods. By gaining a better understanding of torso stability, it is hoped this knowledge can be used to prevent low back injury and pain in the future. These new methods may also be useful in evaluating other biodynamic systems such as standing postural sway, knee stability, or hip stability as well as time series applications outside the area of biomechanics.
A Biomechanical Cadaver Study to Determine the Effectiveness of the Lateral Graft Technique and Isometric Suture Placement for Extracapsular Stabilization of the Cranial Cruciate Ligament Deficient Stifle in the Dog
Harper, Tisha Adele Maria (Virginia Tech, 2003-03-18)
Objective – 1) To determine whether a graft of fascia lata and part of the patellar ligament, used in an extracapsular fashion from the tibial crest to the femorofabellar ligament, would eliminate abnormal cranial drawer motion in the cranial cruciate ligament (CrCL) deficient stifle 2) To determine if two new tibial suture anchor points would enhance biomechanical function of the lateral fabellar-tibial suture (FTS). Study Design – Experimental. Animals – 28 canine cadaver hind limbs. Methods – Stifles were mounted in a jig that allowed tibial rotation during loading and were tested between loads of â 65 to 80 N in caudal and cranial drawer respectively. Stifles were tested with the CrCL intact followed by one of four stabilization techniques after CrCL transection: lateral graft technique (LGT) and three FTS with different tibial anchor points. Results – Differences in cranial drawer motion (displacement) and stiffness between the LGT and standard FTS were not significant in two data sets, when compared to the intact CrCL. The FTS with the anchor point in the tibial crest showed the least displacement of all stabilization methods. Differences in stiffness were not significant between the stabilization techniques. Conclusions – Stability provided by the LGT is comparable to that of the standard FTS for the CrCL-deficient stifle in the cadaver. Altering the tibial anchor points for the FTS did not improve stiffness or result in a further decrease in cranial drawer motion. Clinical Relevance – The LGT could be used for the treatment of acute and chronic CrCL ruptures in the dog. A clinical study is recommended.
Biomechanics of hair cell kinocilia: experimental measurement of kinocilium shaft stiffness and base rotational stiffness with Euler-Bernoulli and Timoshenko beam analysis
Spoon, Corrie E.; Grant, John Wallace (Company of Biologists, 2011-03-01)
Vestibular hair cell bundles in the inner ear contain a single kinocilium composed of a 9+2 microtubule structure. Kinocilia play a crucial role in transmitting movement of the overlying mass, otoconial membrane or cupula to the mechanotransducing portion of the hair cell bundle. Little is known regarding the mechanical deformation properties of the kinocilium. Using a force-deflection technique, we measured two important mechanical properties of kinocilia in the utricle of a turtle, Trachemys (Pseudemys) scripta elegans. First, we measured the stiffness of kinocilia with different heights. These kinocilia were assumed to be homogenous cylindrical rods and were modeled as both isotropic Euler-Bernoulli beams and transversely isotropic Timoshenko beams. Two mechanical properties of the kinocilia were derived from the beam analysis: flexural rigidity (El) and shear rigidity (kGA). The Timoshenko model produced a better fit to the experimental data, predicting El=10,400 pN mu m(2) and kGA=247 pN. Assuming a homogenous rod, the shear modulus (G=1.9 kPa) was four orders of magnitude less than Young's modulus (E=14.1 MPa), indicating that significant shear deformation occurs within deflected kinocilia. When analyzed as an Euler-Bernoulli beam, which neglects translational shear, El increased linearly with kinocilium height, giving underestimates of El for shorter kinocilia. Second, we measured the rotational stiffness of the kinocilium insertion (kappa) into the hair cell's apical surface. Following BAPTA treatment to break the kinocilial links, the kinocilia remained upright, and kappa was measured as 177 +/- 47 pN mu m rad(-1). The mechanical parameters we quantified are important for understanding how forces arising from head movement are transduced and encoded by hair cells.
Biped robot with a vestibular system
Huang, Chuen-Chane (Virginia Tech, 1991-12-01)
The kinematics and dynamics of two legged or biped walking is considered. The resulting governing equations include actuator torques for a robot and muscle generated torques for a human. These torques are those necessary at each joint of a leg, including the foot, for a successful stride. The equations are developed from a consistent set variables with respect to a single inertial reference frame. This single reference frame approach has not been used by previous investigators. Control of the joint torques makes biped walking an extraordinary complex problem from a dynamics and control viewpoint. The control scheme that is developed incorporates the use of the direction of gravity as an important element in the overall control. The inclusion of gravity in biped robot walking has not previously been properly considered in other works. A way is described to separate gravity and acceleration which are measured by an accelerometer which is on the robot. This system incorporates the use of angular motion sensing of the robot segment that contains the linear accelerometers. This system was formulated based on human motion sensing and what probably is present in the human central nervous system for processing these signals.
Canine mandibular osteotomy model: the effects of fixation on bone healing and nerve regeneration
Kern, Douglas A. (Virginia Tech, 1993-04-09)
Osteotomies made between premolar 3 and premolar 4 in the body of the mandible in canine cadaver hemimandibles (n = 48) were stabilized with five interdental fixation apparatuses in a preliminary biomechanical study. Testing in bending determined ultimate strength, stiffness, and yield strength of the interdental fixation apparatuses. Erich arch bar supplemented with acrylic had significantly (P < 0.05) greater ultimate strength, stiffness, and yield strength than Stout loop supplemented with acrylic, Acrylic, Stout loop, and Erich arch bar alone. Due to the combined superior biomechanical strength of Erich arch bar supplemented with acrylic, it was utilized as the interdental fixation apparatus for the in vivo study. Bilateral osteotomies made between premolar 3 and premolar 4 in the body of the mandible were stabilized with monocortical bone plate (n = 6), interdental (n = 6), and external skeletal fixation (n = 6). None of the dogs showed clinical evidence of pain or discomfort associated with the fixation devices or the development of neuromas. Radiographic signs of bone healing were observed at all osteotomy sites by 16 weeks. Histologic evaluation of bone healing of the mandible with monocortical bone plate, interdental fixation, and external skeletal fixator was not significantly different (P > 0.05) at 8 and 16 weeks postoperatively. The inferior alveolar nerves were evaluated electrophysiologically pre-operatively and at 4, 8, and 16 weeks postoperatively. Nerves were histologically evaluated at 4, 8, and 16 weeks after injury. Nerve function disappeared immediately postoperatively and returned in 64% (24 of 36) by 4 weeks, in 78% (28 of36) by 8 weeks, and 83% (30 of 36) by 16 weeks. Neuromas developed in 100% (36 of 36) of the nerves. Using a transverse osteotomy model, results indicate that the type of bone and nerve healing does not significantly differ between fixation groups tested. Therefore, a simpler and more economical fixation device, Erich arch bar-acrylic, should be suitable to repair selected mandibular fractures in the dog.
Characterization of adhesively bonded joints using bulk adhesive properties
Kon, Haruhiko (Virginia Tech, 1991-10-05)
Though using bulk adhesive properties to predict adhesively bonded joint response has yet to be proven infallible, based upon the success of previous works, this effort attempts to shed some light on the stresses present in a typical automotive bonded joint. Adhesive material properties obtained in previous works were used in a finite element analysis of a simulated automotive joint to predict the stresses in that joint. The automotive joint analyzed was a simplified representation of a joint provided by General Motors. The specifications included the rate or stiffness of the joint and the materials to be used. The basic design of the joint is a rectangular solid section steel frame to which an SMC panel is bonded using Ashland Chemical urethane based adhesives. Due to computer time constraints and problem complexity, a complete analysis including a time dependent, viscoelastic analysis was not possible. The linear elastic case analyzed gave important insight into the magnitudes of stresses to be expected in a typical joint. It was found that for an applied load to produce a 1 degree deflection in the steel frame, the stresses in the adhesive were below 20% of the ultimate tensile strength of the adhesive. This low stress state is significant because the adhesive behaves as a linear viscoelastic material in that range, making further analysis less complicated and time consuming.
A Comparative In Vitro Study of the Flow Characteristics Distal to Mechanical and Natural Mitral Valves
Mace, Amber (Virginia Tech, 2002-12-16)
Mechanical heart valve (MHV) flows are characterized by high shear stress, regions of recirculation, and high levels of turbulent fluctuations. It is well known that these flow conditions are hostile to blood constituents, which could lead to thromboembolism. In the ongoing effort to reduce long-term complications and morbidity, it is imperative that we better understand the flow characteristics of the natural valve as well as that of the mechanical valve. In this study, we overcome many of the limitations imposed by other measurement techniques by employing a powerful, high-speed Time-Resolved Digital Particle Image Velocimetry (TRDPIV) system to map the flow field. We compare the flows downstream from a St. Jude Medical bileaflet MHV, a porcine mitral valve (MV), and a combination of both valves to simulate the technique of chordal preservation. Instantaneous velocity fields and vorticity maps are presented, which provide detailed information about the development of the flow. Time-averaged velocity, vorticity, and turbulent kinetic energy measurements are also discussed. Asynchronous leaflet behavior was observed in all cases involving the mechanical valve. Extensive vortex formation and propagation are present distal to the MHV, which leads to high levels of jet dispersion. The porcine mitral jet exhibits lateral oscillatory behavior, but it does not disperse like the MHV. In the MHV/porcine combination system, the native tissue limits vortex propagation and jet dispersion. The results presented provide insight on the hemodynamic characteristics of natural and MHVs, reveal the detrimental character of asynchronous leaflet opening, document the mechanism of vortex formation and interaction distal to the valve, and illustrate the importance of chordal preservation. These results may improve MHV replacement clinical practice and/or motivate and aid the design of MHVs that better mimic natural mitral flow patterns.
Composite circular braid mechanics
Hopper, Robert Huston (Virginia Tech, 1991-08-15)
Braided composites find many diverse applications in modern technology and tailoring the mechanical properties of these structures has become increasingly important. This thesis will examine one class of circular braids encompassing an elastic core. By hypothesizing four modes of operation and incorporating primary influences, the mechanical response of the composite is predicted based on its initial parameters and material properties. The ability to model the yarns that constitute the braid as nonlinear materials enables the simulated response to span finite deformations. A scheme for nondimensionalizing the model parameters and governing equations for each mode of operation is also proposed and implemented. In an effort to validate the assumptions underlying the model's formulation, a series of experimental trials are documented that verify the fundamental braid mechanics. A wide variety of analytical cases are also introduced to investigate the influences of various model parameters. Possible extensions for the existing model are also noted.
Computational and Experimental Modeling of the Bioheat Transfer Process of Perfusion in Tissue Applied to Burn Wounds
Al-Khwaji, Abdusalam (Virginia Tech, 2013-04-29)
A new mathematical model has been developed along with a new parameter estimation routine using surface temperature and heat flux measurements to estimate blood perfusion and thermal resistance in living tissue. Dynamic thermal measurements collected at the surface of the sensor before and after imposing a dynamic thermal cooling event are used with the model to estimate the blood perfusion, thermal resistance and core temperature. The Green\'s function based analytical solution does not require calculation of the whole tissue temperature distribution, which was not the case for the previous models. The result from the new model was proved to have better and more consistent results than previous models. The new model was validated to solve one of the unsolved biomedical problems which is the ability of detecting burn severity. The method was tested with a phantom perfusion system. The results matched known blood perfusion and thermal resistance values. The method was also tested with burns on animal models. Inflammation effects associated with the burns were studied using a newly developed term called the Burn Factor. This correlated with the severity of imposed burns. This work consists of three journal papers. The first paper introduces the mathematical model and its validation with finite-difference solutions. The second paper validates the physical aspects of the usage of the model with thermal measurement in detecting simulated burned layers and the associated perfusion. The third paper demonstrates the ability of the model to use thermal measurements to detect different burn severity of an animal model and to study the healing process.
Computational Stress and Deformation Analysis of Mammary Prosthesis
Potter, Tavis L. (Virginia Tech, 2003-02-14)
A linear and non-linear material model for the breast implants was developed through axial tension testing, while linear and non-linear breast tissue models were assumed based on smooth muscle. These material models were to develop axisymmetric finite element models to determine the stresses in the implant walls under tissue loading. The non-linear material models were used to more accurately model the complex nature of the implant stresses. After analysis it was found that the implants were under compressive loading which meant that local buckling in the implant might be possible. For accurate stress prediction in the implant walls and to fully characterize implant buckling a more accurate non-linear breast tissue material model needs to be developed. Having this material model would allow for a full three-dimensional finite element model can be developed. With the development of a three-dimensional FEA model the implant buckling and implant stresses could be fully characterized. Ultimately allowing for accurate implant stress estimation and fatigue life calculation using the Palmgren-Miner rule, S-N curves, and an external load spectra.
A Computational Study into the Effect of Structure and Orientation of the Red Ear Slider Turtle Utricle on Hair Bundle Stimulus
Davis, Julian Ly (Virginia Tech, 2007-11-30)
The vestibular system consists of several organs that contribute to ones sense of balance. One set of organs, otoconial organs, have been shown to respond to linear acceleration (1949). Hair bundles (and hair cells), which are the mechano-electric transducers found within otoconial organs, respond to displacement of the overlying otoconial membrane (OM). Structure, position and orientation of the OM within the head may influence the stimulus of hair bundles by changing the deformation characteristics of the OM. Therefore, studying the deformation characteristics of the OM with finite element models presents a unique advantage: the ability to study how different variables may influence the deformation of the OM. Previous OM models have ignored complicated OM geometry in favor of single degree of freedom (De Vries 1951)or distributed parameter models (Grant et al. 1984; Grant and Cotton 1990; Grant et al. 1994). Additionally, OMs have been modeled considering three dimensional geometry (Benser et al. 1993; Kondrachuk 2000; 2001a), however OM layer thicknesses were assumed to be constant. Further, little research has investigated the effect of position and orientation of otoconial organs on the deformation of the OM (Curthoys et al. 1999), due to natural movement of the head. The effect of structure, position and orientation of the utricle of a red ear slider turtle on the stimulation of hair bundles in the OM is investigated here. Using confocal images, a finite element model of the utricle OM is constructed considering its full 3D geometry and varying OM layer thickness. How specific geometric variables, which are missing from other OM models, effect the deformation of the utricle OM is studied. Next, since hair bundles are part of the structure of the OM, their contribution to the deformation of the utricular OM is quantified. Then, using computed tomography of a turtle head and high speed video of turtle feeding strikes, acceleration at the utricle during natural motion is estimated. Finally, the effects of orientation of the utricle in the head on the stimulus of hair bundles within the organ is investigated. In summary, a model and methods are developed through which deformation of the turtle utricle OM through natural movements of the head may be studied. Variables that may contribute to utricle OM deformation are investigated. Utricle OM geometry, hair bundles, position and orientation all play a role in utricle OM deflection and therefore hair bundle stimulus. Their effects are quantified and their roles are discussed in this dissertation.
Design and Characterization of Biomimetic Artificial Hair Cells in an Artificial Cochlear Environment
Travis, Jeffrey Philip (Virginia Tech, 2014-03-11)
This research details the creation and characterization of a new biomimetic artificial inner hair cell sensor in an artificial cochlear environment. Designed to mimic the fluid flows around the inner hair cells of the human cochlea, the artificial cochlear environment produces controlled, linear sinusoidal fluid flows with frequencies between 25 and 400 Hz. The lipid bilayer-based artificial inner hair cell generates current through changes in the bilayer's capacitance. This capacitance change occurs as the sensor's artificial stereocilium transfers the force in the fluid flow to the bilayer. Frequency tuning tests are performed to characterize the artificial inner hair cell's response to a linear chirp signal from 1 to 400 Hz. The artificial inner hair cell's response peaks at a resonant frequency of approximately 83 Hz throughout most of the tests. Modelling the artificial stereocilium as a pinned free beam with a rotational spring at the pinned end yields a rotational spring stiffness of 177*10^-6 Nm/rad. Results with 0 mV potential applied across the bilayer indicate that current generation at 0 mV likely comes from other sources besides the bilayer. Increasing the voltage potential increases the broadband power output of the system, with an approximately linear relationship. A final test keeps the fluid flow frequency constant and varies the fluid velocity and applied voltage potential. Manipulation of the applied voltage potential results in a fluid velocity to RMS current relationship reminiscent of the variable sensitivity of the human cochlea.
Design and Testing of a Hydrogel-Based Droplet Interface Lipid Bilayer Array System
Edgerton, Alexander James (Virginia Tech, 2015-10-12)
The research presented in this thesis includes the development of designs, materials, and fabrication processes and the results of characterization experiments for a meso-scale hydrogel-based lipid bilayer array system. Two design concepts are investigated as methods for forming Droplet Interface Bilayer (DIB) arrays. Both concepts use a base of patterned silver with Ag/AgCl electrodes patterned onto a flat polymer substrate. In one technique, photopolymerizable hydrogel is cured through a mask to form an array of individual hydrogels on top of the patterned electrodes. The other technique introduces a second type of polymer substrate that physically supports an array of hydrogels using a set of microchannels. This second substrate is fitted onto the first to contact the hydrogels to the electrodes. The hydrogels are used to support and shape droplets of water containing phospholipids, which self-assemble at the surface of the droplet when submerged in oil. Two opposing substrates can then be pushed together, and a bilayer will form at the point where each pair of monolayers come into contact. The photopatterning technique is used to produce small arrays of hydrogels on top of a simple electrode pattern. Systems utilizing the microchannel substrate are used to create mesoscale hydrogel arrays of up to 3x3 that maintained a low resistance (~50-150 kΩ) connection to the substrate. Up to three bilayers are formed simultaneously and verified through visual observation and by recording the current response behavior. Arrays of varying sizes and dimensions and with different electrode patterns can be produced quickly and inexpensively using basic laboratory techniques. The designs and fabrication processes for both types of arrays are created with an eye toward future development of similar systems at the microscale.
Design, Fabrication, and Validation of Membrane-Based Sensors
Garrison, Kevin Lee (Virginia Tech, 2012-06-04)
Hair cell structures are one of the most common forms of sensing elements found in nature. In humans, approximately 16,000 auditory hair cells can be found in the cochlea of the ear. Each hair cell contains a stereocilia, which is the primary structure for sound transduction. This study looks to develop and characterize a bilayer lipid membrane (BLM) operated artificial hair cell sensor that resembles the stereocilia of the human ear. To develop this sensor, a flexible substrate with internal compartments for hosting the biomolecules and mating cap are constructed and experimentally characterized. The regulated attachment method (RAM) is used to form bilayers within the sealed device. Capacitance measurements of the encapsulated bilayer show that the sealing cap slightly compresses the bottom insert and reduces the size of the enclosed bilayer. Single channel measurements of alamethicin peptides further verify that the encapsulated device can be used to detect the gating activity of transmembrane proteins in the membrane. The flexible substrate was incorporated into a low-noise, portable test fixture. The response of the sensor and tip velocity of the hair were measured with respect to an impulse input on the test fixture and several frequency response functions (FRFs) were created. The FRF between the sensor and the tip velocity was used to show that the hair vibration was transmitted to the bilayer for certain hair lengths. The transfer function between the sensor and the input was used to show the effect of membrane potential on sensor response.

Browsing by Author "Grant, John Wallace"

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