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BACKGROUND AND SUMMARY
The present invention relates to a system for selectively applying either
heat or cold to a local area, and particularly when it is desired that the
temperature of the applicator be held substantially constant. The present
invention is useful in many areas, principally in the medical field. One
of the advantages of the present system is that, depending upon the
applicator pad which is attached to the system, the applicator may be used
as a conventional external applicator, or it may be used during surgery to
apply to incisions or open wounds, or it may be used to rapidly cool or
re-heat an organ during transplant, etc.
The present invention is preferably used with an applicator pad of the type
disclosed in co-owned U.S. patent application Ser. No. 381,733, of Francis
C. Moore and Leon R. Perkinson, for "Disposable, Sterile Temperature
Control Applicator Pad for Medical Application", filed July 23, 1973.
Heat applicators have long been used in the medical field, and they
frequently take the form of a padded or blanket type of applicator
provided with internal resistive wiring for generating heat. Probably the
most commonly used type of cold applicator in the medical field is a
flexible plastic package containing two chemicals which, when mixed absorb
heat. The chemicals are packaged on either side of a rupturable membrane
so that the application of pressure to the exterior of the package
ruptures the membrane and causes the fluids to mix and producing the
heat-absorbing reaction. Another type of cold applicator uses a
compressor, refrigerant, condensing and evaporator coils, and a permanent
or non-disposable applicator. Such units are heavy and cumbersome.
The present invention uses a heat transferring liquid stored in a
reservoir, such as a standpipe, which is pumped by a peristaltic pump
through a heat exchanger containing thermoelectric diodes. As is known, if
a current is forced in one direction (anode to cathode) through a
thermoelectric diode, the diode will produce heat through ohmic losses. If
a voltage of opposite polarity is applied to the junctions of the diode,
then the diode will absorb heat from its surrounding area. Control
circuitry, under control of an operator, determines which polarity of
electrical potential is applied to the thermoelectric diodes in the heat
exchanger, and in either case the applied voltage is a rectified sine
wave. The control circuitry also determines the firing angle at which the
applied voltage is coupled to the thermoelectric diodes, and this firing
angle is adjustable. Hence, the operator, in addition to determining
whether the system will supply heat to the transfer liquid or absorb heat
from the transfer liquid, may adjust the temperature of the transfer
liquid in either the hot or cold region. A temperature-sensitive
transducer is associated with the passageway from the transfer liquid
reservoir and included in the electronic control circuitry in such a way
as to regulate the temperature of the transfer liquid once a setting has
been made by the operator.
Another feature of the invention is that a second subsystem of reservoir,
heat exchanger and pump may be used, connecting the shaft of the second
peristaltic pump in tandem with the shaft of the first so that they are
driven by a single motor. In this system, the second fluid system may be
used, for example, to flush a transplant organ with saline solution at a
controlled temperature, either hot or cold or successively cold and hot.
Thus, great flexibility in usage is provided. In the case of one heat
transferring liquid system, it is preferred that the total liquid in the
system, including reservoir, heat exchanger and tubing be less than about
two quarts, and the liquid may be water. This permits a very rapid
response in the applicator pad to changes in temperature setting. Further,
the present system provides greater conduction of heat to or from the
application surface because it permits the use of a wet applicator pad.
Such pad, as disclosed in the above-identified application further permits
the application of medicaments or sterilizing agents to the application
area.
Other features and advantages of the present will be apparent to persons
skilled in the art from the following detailed description of a preferred
embodiment accompanied by the attached drawing.
THE DRAWING
FIG. 1 is a perspective view, partly broken away, of a system constructed
according to the present invention;
FIG. 2 is a diagrammatic showing of the elements of the heat transferring
liquid loop, showing two pumps in tandem;
FIG. 3 is a circuit schematic diagram of the controller for the present
invention; and
FIG. 4 is an idealized waveform of the voltage applied to the
thermoelectric diodes, illustrating adjustment of the duty cycle.
DETAILED DESCRIPTION
Referring then to FIG. 1, reference numeral 10 generally designates a
console which houses the heat exchanger, pump, control circuitry and so on
for the present invention. The console 10 is portable, including an upper
handle 11. The front portion of the console 10 includes a faceplate panel
12 on which are mounted the operator controls and signal lights. A window
13 in the faceplate panel 12 permits the operator to view the amount of
heat transferring liquid which is present in the reservoir. Preferably,
the rerservoir takes the form of a standpipe, designated 14 in FIG. 2, the
heat-transferring liquid being denoted 15.
Returning to FIG. 1, a peristaltic pump 16 is mounted to the lower portion
of the faceplate panel 12, and the lower portion of that panel is
partially broken away to show a drive motor 17 and a fluid manifold 18, to
be discussed presently.
On the faceplate panel 12, there are an ON/OFF switch 20, a hot/cold switch
21, a green indicator light 22 for "COLD" a red indicator lamp 23 for
"HOT", and a manually adjustable temperature dialing switch 24.
The heat-transferring liquid is communicated from the applicator 18 through
a first conduit 30 to an applicator pad generally designated P; and the
liquid is returned to the manifold 18 by means of a second conduit 31. As
indicated above, the pad P may be constructed according to the teachings
of the above-identified patent application Ser. No. 381,733 of Francis C.
Moore and Leon R. Perkinson, for "Disposable, Sterile Temperature Control
Applicator Pad for Medical Application", filed July 23, 1973. Alternative
shapes may be used, depending upon application; and briefly, the pad P
includes two sheets of thin plastic material laminated together to form a
serpentine conduit through which the heat transferring liquid is
communicated, and an outer layer of absorbent material which may be wet in
order to improve heat transfer qualities. Further, the pad P may be an
applicator which has a special shape, depending upon the application area.
It is considered an important feature of the invention that the applicator
pad P is disposable and may take on various forms as this greatly expands
the utility of the system. By using a variable volume reservoir, such as a
standpipe 14 for the heat-transferring liquid, the amount of liquid in the
applicator pad P does not affect system operation. Further, by placement
of the temperature-sensitive device as will be disclosed below, regulation
of the temperature of the heat-transferring liquid is practically
independent of load so that the design and volume of the applicator pad P
does not seriously affect system operation.
Turning now to FIG. 2, fluid 15 from the reservoir 14 is coupled through a
bored conduit 33 and an exterior conduit 34 to the heat exchanger and
control circuitry functionally indicated by the block 36. As mentioned,
the height of the heat-transferring liquid 15 in the standpipe 14 may be
observed through the window 13 on the faceplate panel of the console 10.
A temperature-sensitive resistor or "thermistor" 37 is located in the
conduit 33 at a location adjacent the outlet of the reservoir 14 and, more
importantly, sensing the temperature of the heat-transferring liquid prior
to forcing the liquid through the heat exchanger or through the applicator
pad (which represents the "load"). With this arrangement, control of the
temperature of the heat-transferring liquid has been facilitated and made
substantially independent of variations in load (within design range) such
as may be caused by differences in contact area, differences in applicator
pad heat-transfer characteristics or volume, and so on. The resistance of
thermistor 37 is an inverse function of temperature. The control circuitry
will be explained in more detail below.
After exiting from the heat exchanger, the heat transferring liquid flows
back through the manifold 18 and a flexible tube 40 which forms a part of
the peristaltic pump 16. Pumps of this type are well known in the medical
field, comprising a plurality of freely rotatable orbital rollers 45
mounted between end plates, one of which is shown at 46. The end plates
are driven in rotation by a shaft 47. A continuous fluid-tight tube, such
as that shown at 40, is arranged in a race through the pump which is
spaced from the outer ends of the rollers 45 a distance sufficient only to
permit collapse of the tube 40, so that the shaft 47 turns, the orbital
rollers 45 are moved to first close the flexible tube 40, then force a
quantum of liquid trapped in it along the direction of movement of the
orbital rollers to an outlet which, in this case, is coupled back to the
manifold 18 at 48. The heat-transferring liquid is thence communicated
through an internal bore 49 to the flexible conduit 30 and the applicator
pad P. The liquid returning from the pad P through the conduit 31 is
coupled to the standpipe 14 by means of a bore 50 which, again, is in the
manifold 18. A conduit 52 communicates the fluid in the discharge bore 50
with a pressure relief valve 53 in the manifold 18. The input to the
pressure relief valve 53 is coupled to the conduit 49 which feeds the
applicator pad P. Thus, should the pad P or the flexible tubes 30, 31
become occluded, the resulting increase in pressure appearing in the
conduit 49 will actuate the pressure relief valve 53, and the fluid will
be transmitted directly to the reservoir or standpipe 14. This will
prevent bursting of any portions of the system near the subject on whom
the pad P is applied until the obstruction is removed or obviated.
As has already been mentioned, if it is desired to adapt the invention to
include a second heating or cooling system which is controlled
independently of the first, a second paristaltic pump 16A may be added,
with its shaft 47A connected in tandem with the previously-mentioned shaft
47 of the pump 16, so that they are driven by a common motor. As best seen
in FIG. 1, the housing for the pump 16 includes a number of bosses 56
which facilitate mounting of the second pump to the front of the console
10, and the shaft 47 may be slotted as at 57 to facilitate driving of the
second shaft.
Referring again to FIG. 2, associated with the second pump 16A is a second
manifold 18A as well as a second heat exchanger and control circuitry,
reservoir, and so on, which are not shown because they may be similar to
those corresponding elements which have already been disclosed. Thus, the
second fluid system may contain a separate fluid and a separate control.
This would be useful, for example, in a transplant operation wherein one
applicator pad may be used to reduce edema on the incision or to surround
the organ being transplanted, while the second fluid system could be used
to flush the organ with a saline solution to preserve it.
Among the advantages of using a peristaltic pump are that electrical
isolation between the applicator and the control system are greatly
reduced, thereby reducing the shock hazard. Further, the fluid is
completely isolated from the pumping mechanism and can therefore be
maintained is a sterile condition if this is desired.
ELECTRICAL CONTROL SYSTEM
Turning now to FIG. 3, reference numeral 70 designates the input lines from
a conventional 120-volt, 60 Hz. source, such as wall outlet. One of the
electrical feed lines is provided with a fuse 71 and the contacts 20A of
the ON/OFF switch are also connected in this line. Preferably, the
ungrounded or "HOT" line is fused and provided with the ON/OFF switch.
Power is fed once the contacts 20A are closed to a fan motor 73 for
circulating room air around the heat exchanger and to the winding of the
motor 17 which drives the pump 16.
Power is also supplied to the HOT/COLD switch 21 which is a four-pole,
double-throw switch having a "HOT" and a "COLD" position. One of the legs
or decks of the switch 21, designated 21A in FIG. 3 is used to couple
power to the filaments of the lamps 22, 23. In the position shown (namely,
the "HOT" position), the red lamp 23 is illuminated, and power is also
coupled to the coil of a relay R.sub.1. The other four decks of the switch
21 are designated respectively 21B, 21C and 21D, and will be discussed
below.
The contacts of the relay R.sub.1 form a double-pole, double-throw switch
generally designated 75 in the left half portion of FIG. 3. When the coil
of the relay R.sub.1 is energized, the contacts 75 are in the position
shown for coupling energy through an inductor L to the thermoelectric
diode bank generally designated by reference numeral 76.
Power is coupled to the contacts 75 from a bridge circuit B1 which, in
turn, is energized by means of a transformer T1. An indicator lamp 78 is
connected across the terminals of the secondary of the transformer T1, and
this lamp is located on the faceplate panel to indicate to the operator
that current is being fed to the thermoelectric diode bank 76, when it is
lit. A series of three thermostats designated respectively 80, 81 and 82
are located between the main power switch 20A and the primary of the
transformer T1. Each of the thermostats 80-82 is a normally-closed
thermostatic switch such that if the ambient temperature exceeds a
predetermined value, the switch opens. The thermostat 80 is located in the
main power supply and will open if the temperature rises above 190.degree.
F. The thermostats 81 and 82 are both located in the heat exchanger, one
opening at 130.degree. F. and the other opening at 190.degree. F., thereby
providing a redundant protection against overheating in the heat
exchanger.
The primary winding of the transformer T1, as mentioned, has one terminal
connected to the "HOT" power line through the thermostats. The other
terminal is coupled to the other power line through a controlled
semi-conductor switch which, in the illustrated embodiment is a triac Q1.
A resistor 83A and a capacitor 83B form a noise suppressing network
connected between the second terminal of the primary winding of
transformer T1 and ground or common. Thus, the polarity of voltage fed to
the diode bank 76 is controlled by the contacts 75 of relay R1 which, in
turn, is controlled by the manual selection switch 21A. However, the duty
cycle or duration of coupling of power to the contact 75 from the bridge
B1 is determined by control of the triac Q1, as will presently be
discussed. Turning now to the upper left-hand portion of FIG. 3, a
limiting resistor 84 couples power to a second bridge circuit B2. The
output of the bridge B2 is coupled through a limiting resistor 87 to a
Zener diode 88 which establishes a fixed DC potential for the subsequent
circuitry, which potential is established on a line designated 89. The
deck 21B of the switch 21 has a terminal 90 connected to the wiper blade
and first and second fixed terminals 91, 92. Similarly, the deck 21C has a
terminal 93 connected to the wiper arm and first and second fixed
terminals 94 and 95. The deck 21D also has a terminal 96 connected to the
wiper arm and first and second fixed terminals 97 and 98. The contact 91
of deck 21B is connected to the contact 95 of deck 21C. Similarly, contact
92 of deck 21B is connected to contact 94 of deck 21C. Terminals 93 and 96
respectively of decks 21C and 21D are also connected together.
It will be observed from the above that the decks 21B and 21C are connected
to form a double-pole, double-throw switch. The thermistor 37, shown
schematically in FIG. 3 as a resistive element is connected between the
terminal 95 and the anode of a diode 100. The cathode of the diode 100 is
connected to the gate lead 101 of a unijunction transistor 102. A
capacitor 103 is connected between the gate lead 101 and the negative
terminal of bridge B2. A fixed resistor 105 and a variable resistor 106
are connected in series between a positive terminal of the bridge B2 and
the gate lead 101 of the unijunction 102. The junction between the
thermistor 37 and diode 100 is coupled by means of a fixed resistor 109 to
the contact 92 of the deck 21B.
The fixed contact 98 of the deck 21D is connected to the movable contact of
a multi-position switch generally designated by reference numeral 110 and
having five fixed contacts, as illustrated. Resistors 111-115 are
connected respectively between these fixed contacts and the negative
terminal of the bridge B2. Similarly, the fixed contact 97 of deck 21D is
connected to the movable contact of a second multi-position switch
generally designated 116. Fixed resistors 117-21 are connected
respectively between these fixed contacts and the negative terminal of the
bridge circuit B2.
The switches 110, 116 may be a decade switch having two separate decks,
each deck being provided with ten individual contacts. However, only five
contacts are used for each deck in a manner such that only one deck is
connected to a resistor at a given time. That is, the switch 116 may have
the first five positions connected to the resistors 117-21 whereas the
switch 110 may have the second five positions connected to the resistors
111-115. The switches 110, 116 comprise the manually temperature control
switch 24 of FIG. 1. The switch 116 controls the temperature when the
system is operating in the "COLD" mode, and the switch 110 controls the
temperature setting when the system is operating in the "HOT" mode.
It will be observed that the junction between the fixed resistor 109 and
the thermistor 37 comprises a voltage take-off point for the triggering of
the unijunction transistor 102 which is connected to operate as a
monostable circuit, as will be discussed. When the switch 21 is in the
position shown (i.e. the HOT mode), the thermistor 37 is on the lower
voltage side of this take-off junction relative to the gate lead 101 so
that as the temperature of the heat-transferring liquid increases, the
resistance of the thermistor 37 will decrease correspondingly, and thereby
reduce the take-off voltage from the voltage divider network comprising
the resistor 109, thermistor 37, and one of the fixed resistors 111-115.
This will retard the firing angle of triac Q1 and supply less current to
the bank of thermoelectric diodes 76 until a state of equilibrium is
reached.
When the switch 21 is switched to the COLD state (that is, the movable
contacts are connected to the contacts 91, 94 and 97), the thermistor 37
is above the voltage take-off point of the voltage divider network. In
this state, the voltage divider network includes the thermistor 37, the
resistor 109, and one of the fixed resistors 117-121.
In summary, the voltage at the anode of diode 100 is a control signal
representative of the difference between the actual temperature of the
heat-transferring liquid (sensed by thermistor 37) and a desired
temperature (as determined by the setting of the temperature control
switches 110 or 116 depending upon the mode of operation).
A resistor 130 is connected between one power terminal of the unijunction
transistor 102 and the lead 89, and a second resistor 131 is connected in
the other terminal circuit of the unijunction transistor 102. The
unijunction transistor 102 and its associated circuitry, particularly
capacitor 103, resistors 105, 106 and the voltage divider circuitry
feeding current to diode 100 is arranged so that the unijunction
transistor 102 generates an output pulse for each cycle of the 60 Hz.
input voltage. This output or trigger pulse is synchronous with the
voltage being applied through transformer T1, bridge B1 and switch 75 to
the bank of thermoelectric diodes 76. The triggering time within a given
cycle will be advanced or retarded by the previously mentioned control
signal depending upon the mode in which the switch 21 is set (hot or
cold), the temperature of the liquid taken from the standpipe, and the
particular setting of whichever of the temperature control switches 110,
116 is connected in circuit.
OPERATION
To summarize the operation of the control circuitry, assuming that the
switch 21 is set in the "HOT" mode, as illustrated, a voltage divider
network will be set up comprising resistor 109 in series with the
thermistor 37 and one of the resistors 111-115, depending upon the setting
of switch 110. The resistors 111-115 are calibrated such that a higher
resistance value produces a higher operating temperature for the heat
transferring liquid since it will advance the firing angle for the triac
Q1.
The resistance of the thermistor 37 is an inverse function of
temperature--that is, as temperature rises the ohmic resistance of the
thermistor 37 decreases. Hence, for the "HOT" setting mentioned, as the
temperature of the liquid flowing from the standpipe reservoir increases,
the value of resistance of the thermistor 37 will decrease, thereby
withdrawing some of the current that would otherwise be used to charge the
capacitor 103 at the beginning of a cycle. Thus, the magnitude of the
control signal will be reduced. This will retard the firing angle of the
unijunction transistor 102. In other words, the charge on the capacitor
103 must build to a fixed point in order to trigger the unijunction
transistor 102. The lower the value of resistance in the lower leg of the
voltage divider network feeding the diode 100, the lesser current will be
available to charge the capacitor 103, and this will retard the firing
angle. Still referring to operation in the "HOT" mode, as the temperature
of the liquid reduces, the value of resistance of the thermistor 37 will
increase, and this will advance the firing angle for the unijunction
transistor 102, as is desired because as the temperature of the
heat-transferring liquid reduces while operating in the "HOT" mode, more
electrical energy must be converted to heat energy by the thermoelectric
diode bank 76 to reach an equilibrium.
When operating in the "COLD" mode, the position of the movable contacts of
the switch 21 are reversed from those illustrated, thereby reversing the
relative positions of the thermistor 37 and fixed resistor 110 in the
voltage divider network feeding the diode 100. Again, the temperature is
determined by the setting of the switch 116, and as the temperature of the
liquid rises, the resistance value of the thermistor 37 will decrease.
However because the resistor is now in the top leg of the voltage divider
network, more current will be available to charge the capacitor 103, and
this will advance the firing angle, as is desired when operating in the
"COLD" mode and the temperature of the heat-transferring liquid rises.
Conversely, when the temperature of the liquid reduces, the resistance
value of thermistor 37 will increase, thereby reducing the current
available to charge the capacitor 103 per cycle and retarding the firing
angle, again achieving the desired result when operating in the "COLD"
mode and the temperature of the liquid reduces.
Thus, the output signal of the unijunction transistor 102 is a voltage
level or pulse which reduced to zero at the end of each cusp of the
full-wave rectified input voltage. The firing angle is advanced or
retarded as just discussed, and this signal is coupled to the gate lead of
a Silicon Controlled Rectifier (SCR) 135. The output pulse of the SCR 135
is coupled through a diode 136 and a voltage divider network including
resistors 140 and 141 to the gate lead G of the triac Q1, causing it to
conduct, and thereby completing the circuit for the primary winding of
transformer T1. When the circuit is completed, the sinusoidal input wave
will energize bridge circuit B1 (and hence, the bank 76 of thermoelectric
diodes) in the polarity selected by relay R1. Briefly, a triac is an AC
switch such that when the input or control current reaches a certain
value, the triac conducts causing a short circuit between the terminals
MT1 and MT2. The triac is turned off when the polarity of the power
reverses and the current fed to the gate lead falls below a predetermined
value, such as five microamps.
In summarizing the operation of the control system, reference is made to
FIG. 4 wherein two half-cycles of a full-wave rectified source voltage are
shown. The polarity of this voltage as applied to the bank 76 of
thermoelectric diodes is determined by the position of the contacts 75,
which, in turn, is determined by the position of the deck 21A of the
switch 21. Assuming that the system is operating in the "HOT" mode, and
that the firing angle is the time T1 as indicated in the cycle, the shaded
area indicates the amount of time during each cycle in which power is
supplied to the bank 76 of thermoelectric diodes to generate heat. If the
heat-transferring fluid becomes hotter, the resistance value of the
thermistor 37 will decrease, and this will retard the firing angle to
reduce the shaded area of FIG. 4 and to apply less average power to the
diode bank 76. If, on the other hand, the temperature of the fluid
reduces, the firing angle will be advanced and thereby provided more
average power to the diode bank. When operating in the "COLD" mode, the
operation of the circuitry is reversed because in this mode, as the
temperature of the liquid rises, it is desired to increase the average
power to the diode bank 76 (but in reverse polarity, of course) so as to
withdraw more heat from the liquid.
The thermoelectric diode bank may have as many diodes connected in series
branches with the branches connected in parallel, as are required for the
amount of heating or cooling for which the system is designed.
Thermoelectric diodes of this type are well known in the art, operating
according to the Seebeck/Peltier effect, and they are commercially
available.
To summarize the principal advantages of the invention, it avoids the use
of a bulky, heavy compressor in providing cooling capability for medical
applications. Not only is it lightweight and therefore portable, but the
same source (namely, the thermoelectric diodes) is used both for heating
and cooling the heat-transferring liquid which is transmitted to the
applicator pad.
By using a minimal amount of heat-transferring liquid, (approximately two
quarts or less in the total system), and by employing an applicator pad
having a thin wall between the heat transfer liquid and the applicator
surface, very rapid response times can be achieved for reducing the
temperature of the surface to which the applicator pad is applied. This is
particularly useful in skin grafts or the like, where it is also useful to
achieve a very rapid and substantial temperature change, such as going
from extreme "HOT" to very "COLD" or vice versa. By placing the
temperature-sensitive thermistor adjacent the outlet of the liquid
reservoir and by controlling the temperature of the liquid after it is
removed from the reservoir, very close tolerances can be held on the
temperature of the liquid, and response time is further reduced. This also
facilitates producing large temperature changes with a minimum of
heat-pumping capacity.
The use of a peristaltic pump provides excellent electrical isolation
between the fluid loop and the applicator surface, and the use of the
pressure relief valve in the manifold minimizes accidental spilling of
liquid should the flexible applicator pad or tubing leading thereto become
occluded.
By using a second peristaltic pump and independent fluid system, one of the
fluid loops can be sterile, for use in flushing an organ with saline, and
the temperature of the second loop may be independently variable.
The heat exchanger may take the form of a rectilinear copper fluid
exchanger having a 1/2 inch conduit milled for conducting the fluid. A
first set of four thermoelectric diodes are silver-soldered to the outer
surface of one side of the exchanger, and these are connected in series
electrically. A second set of four thermoelectric diodes are
silver-soldered to the outer surface of the opposite side of the
exchanger, and these are also connected in series electrically. The first
set of diodes is connected in parallel with the second set.
Having thus described in detail a preferred embodiment of the present
invention, persons skilled in the art will be able to modify certain of
the structure which has been disclosed and to substitute equivalent
elements for those illustrated while continuing to practice the principle
of the invention; and it is, therefore, intended that all such
modifications and substitutions be covered as they are embraced within the
spirit and scope of the appended claims.
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