Nicotine Affects Murine Aortic Stiffness and Fatigue Response During Supraphysiological Cycling

Nicotine exposure is a major risk factor for several cardiovascular diseases. Although the deleterious effects of nicotine on aortic remodeling processes have been studied to some extent, the biophysical consequences are not fully elucidated. In this investigation, we applied quasi-static and dynamic loading to quantify ways in which exposure to nicotine affects mechanical behavior of murine arterial tissue. Segments of thoracic aortas from C57BL/6 mice exposed to 25 mg/kg/day of subcutaneous nicotine for 28 days were subjected to uniaxial tensile loading in an open-circumferential configuration. Comparing aorta segments from nicotine-treated mice relative to an equal number of control counterparts, stiffness in the circumferential direction was nearly two-fold higher (377 kPa ± 165 kPa vs. 191 kPa ± 65 kPa, n = 5, p = 0.03) at 50% strain. Using a degradative power-law fit to fatigue data at supraphysiological loading, we observed that nicotine-treated aortas exhibited significantly higher peak stress, greater loss of tension, and wider oscillation band than control aortas (p = 0.01 for all three variables). Compared to simple stress relaxation tests, fatigue cycling is shown to be more sensitive and versatile in discerning nicotine-induced changes in mechanical behavior over many cycles. Supraphysiological fatigue cycling thus may have broader potential to reveal subtle changes in vascular mechanics caused by other exogenous toxins or pathological conditions.

Priming anodal transcranial direct current stimulation attenuates short interval intracortical inhibition and increases time to task failure of a constant workload cycling exercise

Transcranial direct current stimulation (tDCS), a non-invasive neuromodulatory technique has been shown to increase the excitability of targeted brain area and influence endurance exercise performance. However, tDCS-mediated interaction between corticospinal excitability, GABAA mediated intracortical inhibition and endurance exercise performance remains understudied. In two separate sessions, twelve subjects performed fatigue cycling exercise (80% peak power output) sustained to task failure in a double-blinded design, following either ten minutes of anodal tDCS (atDCS) or sham. Corticospinal excitability and short interval intracortical inhibition (SICI) were measured at baseline, post neuromodulation and post-exercise using paired-pulse transcranial magnetic stimulation (TMS) in a resting hand muscle. There was a greater a decrease in SICI (P < 0.05) post fatigue cycling with atDCS priming compared to sham. Time to task failure (TTF) was significantly increased following atDCS compared to sham (P < 0.05). These findings suggest that atDCS applied over the motor cortex can augment cycling exercise performance; and this outcome may be mediated via a decrease in the excitability of GABAA inhibitory interneurons.

Reliability of screen-printed conductors and resistors during fatigue cycling on flexible substrate

Scalable printing of conductor and resistor components has revolutionized the field of flexible electronics by enabling a myriad of low cost highly conformable devices. Flexible electronic devices need to exhibit reliable performance under strenuous mechanical deformations to be adopted in applications such as human and asset monitoring. The reliability of the devices is in turn affected by the microstructure of the materials, manufacturing processes, and conditions of use. In this research, the mechanical behavior and microstructural properties of stretchable silver conductor and stretchable carbon conductor inks on flexible substrate are studied. The test vehicles (such as 4- point probe structures are screen printed on thermoplastic polyurethane (TPU) and cured in a convection oven. The quality of the printed traces including the resolution and thickness profile are measured by Confocal Laser Scanning microscope. The microstructure of the sample including particle/nanoparticles morphology is studied by Scanning Electron Microscopy (SEM). The electrical resistance is measured by 4-point probes method and the sheet resistance of the printed samples is calculated. The mechanical and electrical reliability of the samples are investigated by fatigue-cycling and in-situ measuring of the electrical resistance. In terms of electrical conductivity, the silver printed traces show different behavior compared to the carbon printed samples when exposed to fatigue cycling. The electrical resistance of the printed silver trace increases during the fatigue cycling. Higher extension rate along with higher strain magnitude accelerate the rate of increase in the electrical resistance. The relative electrical resistance of the carbon trace initially drops to 0.7 after 40 cycles and remains constant for the rest of the cycles. The extension rate does not considerably change the electrical resistance of carbon trace. The stability in electrical resistance is crucial in applications where electrical shielding is concerned.

In-Situ Monitoring of a Filament Wound Pressure Vessel by the MWCNT Sensor under Hydraulic Fatigue Cycling and Pressurization

As a result of the high specific strength/stiffness to mass ratio, filament wound composite pressure vessels are extensively used to contain gas or fluid under pressure. The ability to in-situ monitor the composite pressure vessels for possible damage is important for high-pressure medium storage industries. This paper describes an in-situ monitoring method to permanently monitor composite pressure vessels for their structural integrity. The sensor is made of a multi-walled carbon nanotube (MWCNT) that can be embedded in the composite skin of the pressure vessels. The sensing ability of the sensor is firstly evaluated in various mechanical tests, and in-situ monitoring experiments of a full-scale composite pressure vessel during hydraulic fatigue cycling and pressurization are performed. The monitoring results of the MWCNT sensor are compared with the strains measured by the strain gauges. The results show that the measured signal by the developed sensor matches the mechanical behavior of the composite laminates under various load conditions. In the hydraulic fatigue test, the relationship between the resistance and the strain is built, and could be used to quantitative monitor the filament wound pressure vessel. The bursting of the pressure vessel can be detected by the sharp increase of the MWCNT sensor resistance. Embedding the MWCNT sensor into the composite pressure vessel is successfully demonstrated as a promising method for structural health monitoring.

Fatigue Cycling of Electrical Interconnects Dispensed on Flexible Substrate

In the presented work, electrical traces were directly printed on 2 mil thick polyimide flexible substrate by a dispenser system using two different silver pastes, SW 1400 paste from Asahi Co. and 125-13 HV paste from Creative Materials Co. The dispenser printing parameters were optimized to achieve the finest possible line width and the printing quality of both materials was investigated. The electrical behavior of the dispensed traces was investigated by monitoring the change in the electrical resistance of the test samples during fatigue cycling at different strains, strain percentage of 1.50%, 2.0%, and 2.5% for different number of cycles up to 1000 cycles. The life time of the dispensed traces versus the applied strain was modeled using Coffin-Manson relation setting 20% change in the initial resistance as the failure criteria. Based on the change in the trace resistance during testing, we concluded that the dispensed SW 1400 silver paste traces were less robust than the dispensed 125-13 HV traces. The finer microstructure, smaller particle size, and shorter inter particles distances of the 125-13 HV silver paste enhanced its durability when subject to fatigue cycling. Moreover, 125-13 HV paste presented better and more uniform printed traces.

Post-Fatigue Creep and Stress Relaxation Response of a Hybrid Polymer Composite

Polymer composites have a characteristic, composition specific visco-elastic property which influences the damage progression during fatigue cycling. While some researchers have studied the time dependent constitutive response of polymer composites during the first cycle of fatigue loading, very few have experimentally investigated the dependence of visco-elastic response of built-up polymer composite materials at various stages of fatigue cycling [1]. Our earlier studies on fatigue response of polymer composites focused primarily on the stiffness degradation as a function of applied cycles of loading, which represents the gross response of the material [2]. While doing such an experiment, complimentary experimental techniques to measure the temperature evolution was attempted through the use of infrared thermal imaging technique, which gave some insight into the change in temperature response as a function of fatigue cycling. However, there was no systematic measurement of creep and stress relaxation response of the composite material as a function of induced fatigue damage. The present paper describes the results of creep and stress-relaxation obtained during uni-axial fatigue loading of a hybrid polymer composite material. For this purpose, a woven carbon fiber mat was chosen as the synthetic fiber and Flax fiber in the unidirectional form was chosen as the natural fiber that is laid between the two layers of woven carbon fiber mat. Epoxy LY 556 and hardener Araldite® was used for building up of composite laminate by hand-lay-up technique. Dog-bone shaped tensile specimens with a gage width of 13 mm and gage length of 57 mm were extracted from the 250 × 250 mm sq. plate laminate of 2.1 mm thickness using a numerical controlled milling machine. The specimens were tested at 35% of their median tensile strengths under fatigue at a positive stress ratio (Pmin/Pmax) of 0.1 in tension-tension loading. Prior to start of fatigue loading, the specimens were held in load control and the strain in the gage length was measured for understanding the creep response over 2500 seconds. For stress-relaxation characterization, the specimens were held in extensometer control over a period of 2500 sec. The creep and stress relaxation tests were carried out after periodic intervals of fatigue cycling. It was observed that in the case of un-impacted specimens, the creep rate is consistent with the stiffness variation, which in turn, is dependent on the number of fatigue cycles - till it showed signs of de-lamination. Thereafter it was governed by the woven synthetic fiber response. Similarly, the stress relaxation response was found to decrease with increasing fatigue cycles. In case of impacted specimens, the local deformation had a prominent role in terms of creep and stress relaxation response.