App. Note 6 - Bio Medical Electronics
With the steady increase in the level of sophistication of biomedical
electronic equipment there is a greater reliance on plug in modules and
the use of microprocessor control in that equipment. While the reliability
of the individual components such a IC's, transistors, resistors, and capacitors
has improved, the connectors used in the equipment still represent the
weakest link in the design. The use of plug-in cards and/or modules does
allow defective cards and/or modules to be replaced quickly but at the
expense of introducing additional sources of unreliability-the connectors.
Where microprocessor control is used, a single marginal contact among
the hundreds often employed can cause the system to crash and often it
is next to impossible to find out which contact is responsible.
The cost of biomedical electronics has risen very quickly and with a
general tightening of hospital budgets the service of the equipment is
often done under some pressure. This combined with the increasing average
age of equipment has made it increasingly difficult for the personnel required
to keep the instruments on line
Because greater demands are being placed on complex biomedical equipment
even connector-caused problems such as intermittent faults, or lack of
reliability can be intolerable and well as being difficult to isolate and
As is the case with other electronic applications what was needed was
a non-toxic material that could be applied to connectors without concern
about bridging between adjacent pins, a material that would actively enhance
the conductivity of each pin and socket, and which would improve the performance
of the connector by reducing microphonics or noise generated within the
contact means. Ideally, the material should be easy to use and apply, should
require no exotic dispensers and when used on connectors made, for example,
of plated free-machining brass should show no tendency to cross-link under
the action of the sulfur in the brass. This effect is called "varnishing"
and is not uncommon among the connector treatments employing a vegetable
oil such as a modified palm oil.
What are Stabilant 22, 22A, 22E, and 22S?
Stabilant 22 is a unique, initially non-conductive block
polymer that has the property of becoming conductive when used in
the tight confines of a contact, while staying non-conductive between adjacent
contacts. The film of Stabilant 22 has the viscosity of a medium
oil and has reasonable lubricating properties making it ideal in slide
or rotary switches not to mention a good preventative of bent pins when
inserting IC's in their sockets.
To put it simply, Stabilant 22 will impart the electrical qualities
of a soldered joint without creating an actual bond!
The material is packaged in several basic forms: as a concentrate (Stabilant
22), as a diluted material (22A, 22E, or 22L), and in
a partially filled bottle of concentrate (Stabilant 22S) allowing
the end-user to add the solvent of his choice.
Stabilant 22 is the actual block-polymer, which we refer to as the concentrate,
and can be used directly where lubrication is an important consideration.
Many customers find it more convenient to use Stabilant 22A which
is the isopropyl alcohol diluted form of the material (cut 4:1 by volume).
The viscosity of Stabilant 22A, and 22E, are much lower,
allowing them to be used, for example, on socketed IC's without removing
the IC's from their sockets.
Where other solvents are preferred, Stabilant 22S can be supplied.
Here, the concentrate occupies about one-fifth the bottle's volume simplifying
the housekeeping problems in adding special diluants to the concentrate.
Stabilant 22 is quite soluble in alcohol's, perchloroethylene, or the Freons(TM).
The Stabilants do not affect elastomers save for some slight
swelling on some materials, the diluants employed are much more likely
to cause problems. Almost no plastic is affected by the concentrate, although
some plastics are affected by long exposure to isopropanol. We do not recommend
the use of the Stabilants on low-cost deposited-carbon-film type potentiometers.
Where are the Stabilants used?
Briefly, the Stabilants can be used on virtually all low-voltage
connectors or switches wherever a mechanical contact has to carry an electrical
current. They are at their most valuable when used on micro-power circuits
such as CMOS logic. The only employment that is contra-indicated is their
use in a "Shotgun Application" approach where inter-contact voltages of
greater than 100 volts are encountered. This does not prevent them from
being applied to individual pins, but they should not be allowed to spill
across the inter-conductive surfaces at voltages of greater than 100 volts.
Too high a field strength could cause the "switch to conductive state"
effect to occur. For this same reason some caution should be used where
a connector assembly (in the 100 volt plus applied voltage class) is so
designed that there are narrow cracks running between contacts; narrow
cracks which could fill with Stabilant through capillary action.
Because the switching speed is very slow, in the typical order of several
seconds there is no discernible harmonic distortion introduced by the Stabilants
except at frequencies lower than several seconds per cycle. Indeed the
Stabilants are used in recording equipment to reduce distortion and improve signal
to-noise ratios by eliminating thin film rectification artifacts from mechanical
They may be applied to socketed IC's and transistors, edge card connectors,
rack and panel connectors, 'D' type connectors, co- axial and tri-axial
connectors such as BNC'S, slide switches, rotary switches, key switches
(a major use in the computer industry is in servicing keyboards) to name
but a few.
Bio-medical electronic equipment on which Stabilant has been used:
Patient monitoring equipment such as ECG and respiration monitors,
Defibrillator Infusion Pumps, EEG equipment, Scanners, Ultra-sound equipment,
Recording equipment, and computers. (to cover but a few)
Our customers report that in many cases, defective equipment could
be returned to service once its connectors were treated with Stabilant 22, and from that point on, its reliability as expressed in mean time
between failures, was generally much greater. They have also commented
on their practice of treating all the connections on a piece of out-of-service
equipment before resorting to more orthodox troubleshooting. As noted,
this very quick and easy treatment is all that is needed in many cases
and has cut the servicing load substantially.
In new installations of complex computer-controlled networks of monitoring
equipment, combining existing plant with new plant, the use of Stabilants
has often reduced the initial trouble shooting period from several weeks,
to under a day!