electronics and biological cell.

[BPO]

Hi everybody,

I'm a biologist and I would like to try to culture some cells on a
Gold (or other biocompatible metal) rod/needle. I would like to try
some basic electronics measurements, as capacitance, conductance,
etc... and try to relate them to cell growth.

As I'm really bad in electronics, I come up with several basic
questions for you.

1)How can I measure the capacitance of a metal needle ?
2) What sort of instrument (cheap one)do I need ? I expect changes in
the 0.1 pF range.
3) Could I measure these electrical properties by accessing only one
end of the metal needle ?

Thx.

 

[Roger Johansson]

I'm a biologist and I would like to try to culture some cells on a
Gold (or other biocompatible metal) rod/needle. I would like to try
some basic electronics measurements, as capacitance, conductance,
etc... and try to relate them to cell growth.

As I'm really bad in electronics, I come up with several basic
questions for you.

1)How can I measure the capacitance of a metal needle ?

In relation to what? You need to define another body and seek the
capacitance between these two bodies.
Between your needle and the planet earth, for example.

2) What sort of instrument (cheap one)do I need ? I expect changes in
the 0.1 pF range.

Hardly cheap. Such small capacitances are difficult to measure, because
the capacitances of bodies around it will interfere.

One of the common ways to measure small capacitances is to involve it in
an oscillator, so the frequency of the oscillator depends on the
capacitance.

3) Could I measure these electrical properties by accessing only one
end of the metal needle ?

Yes. A piece of metal is to the electrical current like water is to
sound. You can explore a water body with the help of sound, measure
echoes and timings and determine the size of the water body, its length,
etc..

In a similar way can we explore a piece of metal through measuring
reflected waves, capacitance, etc.., even though we only have access to
one end of the metal body.

When measuring small capacitances remember that two inch-long pieces of
isolated wire tightly twisted together have a capacitance of something
like 7pF. It is a trick often used to create a small capacitor out of
wires. I hope you understand that an instrument measuring sub-pF
capacitances would have a giant problem with its own terminals, unless
they were placed at least a meter apart. And the connecting leads will
have even bigger problems as their capacitance will be dependent on their
position.

 

[Ban]

When measuring small capacitances remember that two inch-long pieces
of isolated wire tightly twisted together have a capacitance of
something like 7pF. It is a trick often used to create a small
capacitor out of wires. I hope you understand that an instrument
measuring sub-pF capacitances would have a giant problem with its own
terminals, unless they were placed at least a meter apart. And the
connecting leads will have even bigger problems as their capacitance
will be dependent on their position.

I have the L/C Meter IIB by "Almost All Digital Electronics" which is
sold(as a kit) for a little more than 100 bucks, which has a resolution of
0.01pF. You can position the probes in the right way and then zero the
display. When I move my hand from 20cm to 2 cm closer to the probes, the
capacitance increases about 0.03pF. Amazingly accurate(1%) and sensitive.
when you cross the probes the capacitance increases about 0.95pF. I think it
comes from Australia?

 

[Roger Johansson]

I have the L/C Meter IIB by "Almost All Digital Electronics" which is
sold(as a kit) for a little more than 100 bucks, which has a resolution
of 0.01pF. You can position the probes in the right way and then zero
the display. When I move my hand from 20cm to 2 cm closer to the
probes, the capacitance increases about 0.03pF. Amazingly accurate(1%)
and sensitive. when you cross the probes the capacitance increases
about 0.95pF. I think it comes from Australia?

I found the schematics for this L/Cmeter at:

http://www.aade.com/lcm2binst/LC2Binst.htm

If the 311 is used as an oscillator here, the display component
has to work as a frequency counter.