Membrane, artificial oscillations

The oscillation of membrane current or membrane potential is well-known to occur in biomembranes of neurons and heart cells, and a great number of experimental and theoretical studies on oscillations in biomembranes as well as artificial membranes [1,2] have been carried out from the viewpoint of their biological importance. The oscillation in the membrane system is also related to the sensing and signal transmission of taste and olfaction. Artificial oscillation systems with high sensitivity and selectivity have been pursued in order to develop new sensors [3-8]. 

The oscillations observed with artificial membranes, such as thick liquid membranes, lipid-doped filter, or bilayer lipid membranes indicate that the oscillation can occur even in the absence of the channel protein. The oscillations at artificial membranes are expected to provide fundamental information useful in elucidating the oscillation processes in living membrane systems. Since the oscillations may be attributed to the coupling occurring among interfacial charge transfer, interfacial adsorption, mass transfer, and chemical reactions, the processes are presumed to be simpler than the oscillation in biomembranes. Even in artificial oscillation systems, elementary reactions for the oscillation which have been verified experimentally are very few. 

The mechanisms of the oscillations in biomembranes have been explained based on the gating of membrane protein called ion channel , and enormous efforts have been made to elucidate the gating process, mainly by reconstitution of channel proteins into bilayer membranes [34—36]. However, oscillations observed with artificial membranes such as thick liquid membranes, lipid-doped filter or BLM suggest that the oscillations... 

The data showed that, with the membrane voltage held constant, the current across the membrane displays nonlinear characteristics during stimulation. These findings seem to be crucial in explanation of the phenomenon of electrical behavior of the artificial proteinoid cell, as the negative resistance is a necessary characteristic for the generation of the oscillations observed. 

Microfabrication of self-oscillating gel has also been attempted by lithography for application to a ciliary motion actuator (artificial cilia) [30]. The gel membrane with microprojection array on the surface was fabricated by utilizing X-lay lithography (LIGA). With the propagation of chemical wave, the microprojection array exhibits dynamic rhythmic motion hke ciha. The actuator may also serve as a microconveyor. 

In the hormone delivery oscillator considered below, neither imposed electric fields or currents, nor pH or salt gradients are used, since they are difficult to impose in a physiological environment Instead, feedback interactions between an enzyme-catalyzed chemical reaction and transport across the membrane are key. Previous efforts coupling enzymes and membrane transport in artificial membranes, leading to oscillations, will now be reviewed. 

Friboulet, A. and Thomas, D. (1982) Electrical excitability of artificial enzyme membranes. III. Hysteresis and oscillations observed with immobilized acetylcholinesterase membranes. Biophys. Chem., 16, 153-157. 

---