An Optimally Energized Cardiac Pacemaker

Batteries that power implanted cardiac pacemakers deteriorate because of energy drain and permeation of body fluids. They must, therefore, be replaced periodically, most often by means of an operation. In this paper, the stimulating waveform that draws the least amount of energy from the battery and also delivers the minimum energy to a heart cell is derived. It is shown that a 31-percent reduction in energy (with respect to that required by the standard rectangular pulse) can be achieved if the pulse duration is about twice the cell time constant. Moreover, it is demonstrated that the required energy is insensitive to pulse-duration variations (due to component changes). An easily synthesized suboptimal approximation is developed and requires only slightly more energy than the optimal pulse.     

Chemical Control of Friction: Mixed Lubricant Monolayers

Controlling frictional behavior in nanoscale sheared systems can be made possible when the relationship between the macroscopic frictional response and the microscopic properties of the sheared systems is established. Here, a new approach is proposed for tuning the frictional response and obtaining desirable frictional properties. This tuning is achieved through shear-induced phase transitions in a mixed lubricant monolayer consisting of a base solvent and an additive. The interaction between the solvent and additive molecules and their relative concentrations are shown to be the major parameters in determining the magnitude of the friction force and the nature of the response (stick–slip or sliding).     

Optimal Energy Delivery by a Brain Pacemaker for Cerebellar Control of After-Discharge

In recent years, electrical stimulation of the cerebellum has been used successfully to control epileptic seizure activity in man. This paper deals with the minimization of the energy transferred from a brain pacemaker to the cerebellum. Reducing this energy may reduce considerably the damage done to the brain tissue, and will probably prolong the life of an implanted power source used to energize such a pacer. Using an electrical model for stainless steel electrodes imbedded in brain tissue, it is shown theoretically that an exponentially increasing stimulating pulse can reduce the energy for inhibition of after-discharge activity by at least 30 percent compared to the corresponding quantity obtained for a rectangular waveform. Preliminary in vivo tests of this theory have been conducted in cats. Electrically induced after-discharge activity was obtained by stimulating the sensorymotor cortex. Three different waveshapes (rectangular, triangular, and exponentially increasing) were applied to the cerebellum, and the minimum energy required to inhibit the elicited after-discharge was found for various pulsewidths, repetition rates, and train lengths. The preliminary data indicate that nonrectangular waveforms require less energy than the rectangular waveshape to achieve threshold inhibition of after-discharge activity.     

Following Single Molecules by Force Spectroscopy

Dynamic force spectroscopy of single molecules, in which an adhesion bond is driven away from equilibrium by a spring pulled with velocity V, is described by a model that predicts the distribution of rupture forces (mean and variance), all amenable to experimental tests. The distribution has a pronounced asymmetry, which has recently been observed experimentally. The mean rupture force follows a (lnV)^2/3 dependence on the pulling velocity and differs from earlier predictions. Interestingly, at low pulling velocities a rebinding process is observed whose signature is an intermittent behavior of the spring force that delays the rupture. Based on the rupture mechanism, we propose a new “pick-up-and-put-down” method to manipulate individual molecules with scanning probes. We demonstrate that the number of molecules picked up by the tip and deposited at a different location can be controlled by adjusting the pulling velocity of the tip and the distance of closest approach of the tip to the surface.