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_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ `` a device without an oscillator either does nt do anything or expects to be driven by something else ( which probably contains an oscillator ) .
'' + horowitz and hill , the art of electronics , 1980 . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _    the plasmodium of _ physarum polycephalum _ ( order _ physarales _ , class _ myxomecetes _ , subclass _ myxogastromycetidae _ ) is a single cell , visible with the naked eye , with many diploid nuclei .
the plasmodium feeds on bacteria and microscopic food particles by endocytosis . when placed in an environment with distributed sources of nutrients the plasmodium forms a network of protoplasmic tubes connecting the food sources .
the topology of the plasmodium s protoplasmic network optimises the plasmodium s harvesting of nutrient resource from the scattered sources of nutrients and makes more efficient the transport of intra- cellular components  @xcite . in @xcite we have shown how to construct specialised and general purpose massively - parallel amorphous computers from the plasmodium ( slime mould ) of _ p. polycephalum _ that are capable of solving problems of computational geometry , graph - theory and logic .
plasmodium s foraging behaviour can be interpreted as a computation  @xcite : data are represented by spatial of attractants and repellents , and results are represented by structure of protoplasmic network  @xcite .
plasmodium can solve computational problems with natural parallelism , e.g. related to shortest path  @xcite and hierarchies of planar proximity graphs  @xcite , computation of plane tessellations  @xcite , execution of logical computing schemes  @xcite , and natural implementation of spatial logic and process algebra  @xcite .    in the framework of our `` physarum chip '' eu project
@xcite we aim to experimentally implement a working prototype of a physarum based general purpose computer .
this computer will combine self - growing computing circuits made of a living slime mould with conventional electronic components .
data and control inputs to the physarum chip will be implemented via chemical , mechanical and optical means . aiming to develop a component base of future physarum computers we designed physarum tactile sensor  @xcite and undertook foundational studies towards fabrication of slime mould chemical sensors ( physarum nose )  @xcite , physarum memristive devices  @xcite and insulated physarum wires  @xcite .
a future physarum chip will be a hybrid living - electronic computing device .
being a bio - electronic device the chip will need components to generate waveforms and a clock , the source of regularly spaced pulses , implemented as oscillators .
we experimentally demonstrate that it is possible to implement an electronic oscillator  a device which converts direct current to alternating current signal  with living slime _
polycephalum_. experimental setup is outlined in sect .
[ methods ] .
section  [ results ] presents experimental results on physarum resistance oscillations , input to output potential transfer function and current dynamics in physarum oscillators .
advantages and limitations of living physarum oscillators are discussed in sect .
[ discussion ] .
a scheme of experimental setup is shown in fig .
[ scheme ] .
two blobs of agar 2 ml each ( fig .
[ scheme]b ) were placed on electrodes ( fig .
[ scheme]c ) stuck to a bottom of a plastic petri dish ( 9  cm ) .
distance between proximal sites of electrodes is 10  mm in all experiments .
physarum was inoculated on one agar blob .
we waited till physarum colonised the first blob , where it was inoculated , and propagated towards and colonised the second blob .
when second blob is colonised , two blobs of agar , both colonised by physarum ( fig .
[ scheme]a ) , became connected by a single protoplasmic tube ( fig .
[ scheme]d ) . in each experiment a resistance , potential and current were measured during 10  min with four wires using fluke 8846a precision voltmeter , test current 1@xmath00.0013 @xmath1a .
direct current potential was applied using gw instek gps-1850d laboratory dc power supply .
dynamics of resistance of the blobs connected by a single protoplasmic tube was measured in 22 experiments ; dynamics of electrical potential difference between blobs and current for direct current potential applied in a range of 2  v to 15  v was measured in 6 experiments for each value of potential difference applied .
resistance of two agar blobs colonised by physarum and connected with each other by a silver wire exhibits quasi - chaotic oscillations ( fig .  [ exampleresistance]a ) with a wide range of dominating frequencies ( fig .
[ fftsamples]a ) .
when a physarum propagates from one agar blob to another blob it connects two blobs with a single protoplasmic tube ( fig .
[ scheme]b ) .
average resistance of a 10  mm protoplasmic tube is 3  mohm , standard deviation 0.715  mohm .
resistance of the protoplasmic tube exhibits oscillatory behaviour ( fig .
[ exampleresistance]bc ) with highly pronounced dominating frequency ( fig .
[ fftsamples]bc ) .
average dominating frequency is 0.0137606  hz , median dominating frequency is 0.0126953  hz , standard deviation is 0.0033492429  hz . the resistance oscillations have average amplitude 0.59  mohm , standard deviation 0.256  mohm ; minimum amplitude of resistance oscillations observed was 0.11  mohm and maximum amplitude 1  mohm .
oscillation in resistance observed are due to peristaltic contractions of the protoplasmic tube  @xcite , see details in sect .
[ discussion ] .
when we apply a potential to a protoplasmic tube oscillations of the tube s resistance result in oscillation of the output potential as shown in fig .
[ outputvoltages ] .
values of average output potential ; and frequencies , periods and amplitudes of potential oscillations are given in tab .
[ statisticstable ] .
average output potential and average amplitude of output potential oscillations grow linearly with increase of an input potential ( fig .
[ relations]ab ) .
frequency of oscillations remains almost constant ( fig .
[ relations]c ) , physarum oscillator produces the same frequency oscillations at 2  v and 15  v applied potential .
a ratio of average amplitude of output potential oscillations to average output potential decreases by a power low with increase of input potential ( fig .
[ relations]d ) .
examples of oscillations of output current for for 5 , 10 and 14  v input potential applied as shown in fig .
[ current ] and values of current are given in tab .
[ tabcurrent ] .
stability , accuracy , adjustability and ability to produce accurate waveforms are amongst key desirable features of an ideal electronic oscillator  @xcite .    to test physarum oscillators stability we recorded output behaviour of oscillators for 30  min ,
examples are shown in fig .
[ outputvoltageslongrun ] . in such long runs oscillations of an output potential persisted and frequency of oscillations was stable .
we observed drifts of the output potential baseline and , sometimes sudden yet short - living , changes in amplitude of oscillations , e.g. between 300th and 500th seconds in example fig .
[ outputvoltageslongrun]a , 1100th and 1500th seconds in example fig .
[ outputvoltageslongrun]b , and 1100th and 1500th seconds in example fig .
[ outputvoltageslongrun]c .
frequency of oscillations is _ stable _ during over 70% time of physarum oscillator functioning .
a drift of the electrical potential baseline occurs more likely due to unequal growth of physarum on agar blobs .
dynamically changing difference in physarum body mass on reference and recording electrodes leads to a corresponding changes in resistance of the system and subsequent drift of the background potential .
this drift is often in a range exceeding the electrical potential oscillation amplitude and therefore must be dealt with . immediate solution would be to involve auxiliary components to independently measure background resistance or potential , calculate and adjust the baseline potential  @xcite .
further studies will concern how to control growth of physarum mass on the electrodes .
this could possibly be done by illuminating blobs with a strong light every time increase of mass is detected .
physarum exhibits photo - avoidance and therefore its grows will be limited in the illuminated areas .
protoplasmic tubes of the slime mould exhibit periodic propagations of calcium wave along the tubes .
a calcium ion flux through membrane triggers oscillators responsible for dynamic of contractile activity  @xcite .
the calcium waves are reflected in oscillations of external membrane potential of physarum and periodic reversing of cytoplasmic flow in the tubes  @xcite .
average dominating frequency of resistance oscillation is 0.0137606  hz , i.e. period of oscillations is c. 73  sec .
this is consistent with our previous findings on periods of electrical potential oscillation and reversing of cytoplasmic flow .
thus , average period of electrical potential oscillation recorded in our experiments is 67  sec  @xcite , 97  sec  @xcite , 103  sec  @xcite , and 115  sec  @xcite ; average 95  sec over four above sets of data  @xcite .
cytoplasmic flow in tubes reverses with period 54  sec  @xcite .
further experiments might deal with establishing an exact link between oscillations of a protoplasmic tube s resistance and peristaltic oscillations of the tube  @xcite .
frequency of electrical potential oscillations of physarum can be modified by tactile  @xcite and chemical  @xcite stimuli , by coloured illumination of a protoplasmic tube  @xcite , and by loading physarum with functional nano - particles  @xcite .
the correlation between resistance frequency and frequency of electrical potential oscillations allows to speculate that frequency of resistance oscillation can be also tuned by chemical , optical and tactile control inputs .
thus , we believe the physarum electrical oscillator is _ adjustable_.
applicability of physarum oscillators in conventional electronic circuits is limited to none .
physarum oscillators produce a very low frequency waveforms and therefore can only be used in computing devices where speed of information processing is not critical .
thus a potential application domain of physarum oscillators is in self - growing biological computing devices and hybrid bio - silicon devices , and amorphous bio - inspired robots  @xcite .
another promising application would be in disposable bio - sensors and bio - circuits : living physarum oscillator can produce stable waveforms for up to 5 - 7 days ; such time frame is sufficient to make reliable measurements and to perform non - time consuming computations .
a single protoplasmic tube of an acellular slime mould _ p. polycephalum _ can be employed as a living electronic oscillator which produces stable , accurate , and , in principle , adjustable waveforms .
this is because the tube exhibits periodic peristaltic contractions which lead to oscillations of the tube s resistance .
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we construct electronic oscillator from acellular slime mould _ 
 physarum polycephalum_. the slime mould oscillator is made of two electrodes connected by a protoplasmic tube of the living slime mould . 
 a protoplasmic tube has an average resistance of 3  mohm . 
 the tube s resistance is changing over time due to peristaltic contractile activity of the tube . 
 the resistance of the protoplasmic tube oscillates with average period of 73  sec and average amplitude of 0.6  mohm . 
 we present experimental laboratory results on dynamics of physarum oscillator under direct current voltage up to 15  v and speculate that slime mould _ 
 p. polycephalum _ can be employed as a living electrical oscillator in biological and hybrid circuits .    _ 
 keywords : oscillator , slime mould , bioelectronics _