H. Jang

Effect of the Abrasive Size on the Friction Effectiveness and Instability of Brake Friction Materials: A Case Study with Zircon

AbstractnThe friction-induced vibration triggered at the sliding interface between the gray iron disk and brake friction material was studied by changing the size of the zircon particles in the friction material. The friction tests were performed using a reduced brake dynamometer and the friction characteristics of the friction materials containing zircon particles with sizes of 3, 50, and 100xa0μm were analyzed. Our results show that the properties of the sliding surface were strongly affected by the entrenchment of the abrasive particles in the friction layers during sliding. The friction effectiveness was inversely proportional to the size of the abrasive, while friction instability was pronounced when smaller zircon particles were used. The smaller zircon particles produced larger plateaus on the sliding surface with low contact stiffness. However, the contact plateaus with the low contact stiffness showed higher amplitudes of the friction oscillations, suggesting a surface with low stiffness also can produce high propensity of friction instability during sliding. Based on the friction stability diagram and surface properties, such as contact stiffness and surface roughness, it was suggested that the static coefficient of friction, which was changed as a function of dwell time, was crucial to understand the cause of friction-induced force oscillations and propensity of friction instability of brake friction materials.n

Frequency dependence of CA3 spike phase response arising from h-current properties

The phase of firing of hippocampal neurons during theta oscillations encodes spatial information. Moreover, the spike phase response to synaptic inputs in individual cells depends on the expression of the hyperpolarization-activated mixed cation current (Ih), which differs between CA3 and CA1 pyramidal neurons. Here, we compared the phase response of these two cell types, as well as their intrinsic membrane properties. We found that both CA3 and CA1 pyramidal neurons show a voltage sag in response to negative current steps but that this voltage sag is significantly smaller in CA3 cells. Moreover, CA3 pyramidal neurons have less prominent resonance properties compared to CA1 pyramidal neurons. This is consistent with differential expression of Ih by the two cell types. Despite their distinct intrinsic membrane properties, both CA3 and CA1 pyramidal neurons displayed bidirectional spike phase control by excitatory conductance inputs during theta oscillations. In particular, excitatory inputs delivered at the descending phase of a dynamic clamp-induced membrane potential oscillation delayed the subsequent spike by nearly 50 mrad. The effect was shown to be mediated by Ih and was counteracted by increasing inhibitory conductance driving the membrane potential oscillation. Using our experimental data to feed a computational model, we showed that differences in Ih between CA3 and CA1 pyramidal neurons could predict frequency-dependent differences in phase response properties between these cell types. We confirmed experimentally such frequency-dependent spike phase control in CA3 neurons. Therefore, a decrease in theta frequency, which is observed in intact animals during novelty, might switch the CA3 spike phase response from unidirectional to bidirectional and thereby promote encoding of the new context.

Nanoscale Friction Characteristics of a Contact Junction with a Field-Induced Water Meniscus

The effect of the field-induced water meniscus on the friction level and oscillation was studied using an atomic force microscopy tip sliding on a highly ordered pyrolytic graphite (HOPG) surface. The results showed that the friction characteristics were significantly affected by the bias voltage, humidity, and sliding velocity. The friction level increased when the bias voltage increased beyond a threshold value (Vth). The stick–slip-type friction oscillation was observed at low velocity and periodicity, and intensity of the friction oscillation was diminished at high velocity. The velocity dependence of the friction oscillation indicated that the mechanical properties of the field-induced water meniscus were comparable to the solid state due to field-induced ordering of water molecules in the meniscus. The static friction and amplitude of the friction oscillation during scanning were inversely proportional to the scan velocity, suggesting that the hydrophobic HOPG surface turned hydrophilic due to the electrical field at the meniscus and demonstrating the possibility of manipulating the friction using an electrical bias, which can be useful for functionalizing nanoscale devices.

GABAA receptor-mediated feedforward and feedback inhibition differentially modulate hippocampal spike timing-dependent plasticity

Synaptic plasticity is believed to play an important role in hippocampal learning and memory. The precise and relative timing of pre- and postsynaptic activity has been shown to determine the sign and amplitude of hippocampal synaptic plasticity through spike timing-dependent plasticity (STDP). While most studies on STDP have mainly focused on excitatory synapses, neural networks are composed not only of excitatory synapses, but also of inhibitory synapses. Interneurons are known to make inhibitory synaptic connections with hippocampal CA1 pyramidal neurons through feedforward and feedback inhibitory networks. However, the roles of different inhibitory network structures on STDP remain unknown. Using a simplified hippocampal network model with a deterministic Ca(2+) dynamics-dependent STDP model, we show that feedforward and feedback inhibitory networks differentially modulate STDP. Moreover, inhibitory synaptic weight and synaptic location influenced the STDP profile. Taken together, our results provide a computational role of inhibitory network in STDP and in memory processing of hippocampal circuits.

GABAA receptor-mediated feedforward and feedback inhibition differentially modulate the gain and the neural code transformation in hippocampal CA1 pyramidal cells

Diverse variety of hippocampal interneurons exists in the CA1 area, which provides either feedforward (FF) or feedback (FB) inhibition to CA1 pyramidal cell (PC). However, how the two different inhibitory network architectures modulate the computational mode of CA1 PC is unknown. By investigating the CA3 PC rate-driven input-output function of CA1 PC using inxa0vitro electrophysiology, inxa0vitro-simulation of inhibitory network, and in silico computational modeling, we demonstrated for the first time that GABAA receptor-mediated FF and FB inhibition differentially modulate the gain, the spike precision, the neural code transformation and the information capacity of CA1 PC. Recruitment of FF inhibition buffered the CA1 PC spikes to theta-frequency regardless of the input frequency, abolishing the gain and making CA1 PC insensitive to its inputs. Instead, temporal variability of the CA1 PC spikes was increased, promoting the rate-to-temporal code transformation to enhance the information capacity of CA1 PC. In contrast, the recruitment of FB inhibition sub-linearly transformed the input rate to spike output rate with high gain and low spike temporal variability, promoting the rate-to-rate code transformation. These results suggest that GABAA receptor-mediated FF and FB inhibitory circuits could serve as network mechanisms for differentially modulating the gain of CA1 PC, allowing CA1 PC to switch between different computational modes using rate and temporal codes ad hoc. Such switch will allow CA1 PC to efficiently respond to spatio-temporally dynamic inputs and expand its computational capacity during different behavioral and neuromodulatory states inxa0vivo.

Current Trends in Memory Implantation and Rehabilitation

Hippocampus is believed to be the brain region critical for memory storage and recall. Damage to the hippocampus by lesions or neurodegenerative diseases such as Alzheimer’s disease could lead to memory deficits. However, there is yet no treatment method available. Direct deep-brain stimulation (DBS) of the hippocampus has been attempted in an effort to find a treatment method for memory dysfunction and Alzheimer’s disease in the last few decades but with limited success. Recently, a novel approach has been developed where an implantation of a very large scale integration (VLSI) microchip containing a biomimetic computational model could act as an artificial bridge to replace the damaged hippocampal circuit in vivo. Here, we discuss the memory implantation techniques; from the conventional DBS method to the current memory implantation technology using an artificial neural microchip. Furthermore, we propose future directions towards the development of a physiologically realistic memory implantation chip design that could enhance the performance of the memory implant and be used for the treatment of memory-related neurodegenerative diseases.

The Wear Mechanism of a Polyphenylene Sulfide (PPS) Composite Mixed with Ethylene Butyl Acrylate (EBA)

Friction and wear of polyphenylene sulfide (PPS) mixed with ethylene butyl acrylate (EBA) were studied using a block-on-ring tribotester. Results showed that the friction coefficient decreases with an increase in the EBA content due to the reduced surface contact area occupied by the PPS. The PPS/EBA composite showed higher threshold temperatures for viscoelastic behavior and the wear rate was significantly decreased after mixing with EBA due to a radical reduction in transfer films. The wear mechanism of the composite changed from an adhesive wear mode to an abrasive wear mode upon the addition of EBA, which supports the wear debris morphology and the wear map suggested by Lancaster (Proc Inst Mech Eng 183:98–106, 1968).