Digital reproduction of biopotential waveforms for neurophysiological studies.

A simple biological signal generator capable of reproducing complex biopotential waveforms is described. It is constructed by a combination of digital and analog circuit components and can be used under different experimental conditions, such as in calibration of biomedical instrumentation systems, or simply as a function generator providing voltage outputs of various waveforms. The biopotential waveform to be generated is sampled at a high frequency and the samples are stored sequentially in a programmable read only memory (PROM). The samples are then fed in the same sequence to a digital-to-analog (D/A) converter and the resulting output is amplified and a DC offset is added. External controls are provided to adjust the DC offset, amplitude and repetition rate of the signal generated. The reproduced voltage signals are stable and superior in quality to those produced by conventional biological signal generators.

Stabilization of gas cavitation nuclei by surface-active compounds.

Gas bubbles are the primary agent in producing the pathogenic effects of decompression sickness. Numerous experiments indicate that bubbles originate in water, and probably also in man, as pre-existing gas nuclei. This is surprising considering that gas phases larger than 1 micron should rise to the surface of a standing liquid, whereas smaller ones should dissolve rapidly due to surface tension. Several stabilizing mechanisms have been suggested, and each has been refuted on experimental grounds. In this article, we propose a new model that arises out of a systematic study of the earlier theories. We review these theories and conclude that gas cavitation nuclei must be held intact by surface-active skins that are initially permeable. The first quantitative analysis of bubble formation data from supersaturated gelatin is summarized and leads to the further conclusion that skins can become impermeable if the ambient pressure is increased rapidly by a sufficient amount. Our model owes much to Sirotyuk, who "demonstrated experimentally that stabilization of gas bubbles acting as cavitation nuclei in water is always attributable to the presence of surface-active substances in the water".