U.S. patent number 3,677,444 [Application Number 05/017,169] was granted by the patent office on 1972-07-18 for micropump.
This patent grant is currently assigned to Hans H. Estin, Leonard W. Cronkkite, Jr. & William W. Wolbach, Trustees. Invention is credited to Edward W. Merrill.
United States Patent |
3,677,444 |
Merrill |
July 18, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
MICROPUMP
Abstract
A pump for delivering liquid at a relatively constant rate from
a chamber having a compliant membrane wall which exerts a
substantially constant resistive force when expanded under a
pressure within the range of between zero and atmospheric and
within the mechanical limits imposed on it. The outside surface of
the membrane is located in a second chamber wherein the pressure is
increased to atmospheric by diffusing gas through an elastomeric
diffusion membrane. The pump is especially adapted for delivering
microquantities of liquid at a constant rate for long periods.
Inventors: |
Merrill; Edward W. (Cambridge,
MA) |
Assignee: |
Hans H. Estin, Leonard W.
Cronkkite, Jr. & William W. Wolbach, Trustees (Boston,
MA)
|
Family
ID: |
21781107 |
Appl.
No.: |
05/017,169 |
Filed: |
March 6, 1970 |
Current U.S.
Class: |
222/135; 222/215;
604/132; 604/407; 222/541.6; 141/61; 222/386.5; 604/405 |
Current CPC
Class: |
A61M
5/152 (20130101); F04B 43/06 (20130101); F16N
13/00 (20130101); F04F 3/00 (20130101); F04B
43/0072 (20130101) |
Current International
Class: |
A61M
5/145 (20060101); A61M 5/152 (20060101); F16N
13/00 (20060101); F04B 43/06 (20060101); F04B
43/00 (20060101); F04F 3/00 (20060101); B67d
005/40 () |
Field of
Search: |
;222/479,207,205,386.5,541,209,353,437,457,206,215,135,1
;128/DIG.5,2,276,214F ;141/8,59,61,215 ;417/148,437 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Reeves; Robert B.
Assistant Examiner: Bartuska; Francis J.
Claims
I claim:
1. A pump for delivering a liquid at a relatively constant rate
comprising a first chamber and a second chamber for retaining
liquid each defined by a separate rigid wall and a separate
elastomeric membrane, means for filling the first chamber and
second chamber with liquid, an orifice in each of said first
chamber and said second chamber for removing liquid from said first
chamber and second chamber, each of said elastomeric membranes
exerting a substantially constant resistive force when expanded
under a pressure within the range of between zero and atmospheric
and within the mechanical limits imposed upon said elastomeric
membranes, and a diffusion membrane located in a third chamber
defined by said elastomeric membranes to permit gas diffusion from
the atmosphere into the third chamber.
2. A pump for delivering a liquid at a relatively constant rate
comprising an elastomeric tube, said tube exerting a substantially
constant resistive force when expanded under a pressure within the
range of between zero and atmospheric and within the mechanical
limits imposed upon it, means for filling the tube with a liquid,
an orifice for removing liquid from said tube, said tube located in
a second chamber and a diffusion membrane located in said second
chamber to permit gas diffusion from the atmosphere into the second
chamber.
3. The pump of claim 2 wherein the tube is surrounded by a porous
retaining means to impose the maximum limit of expansion.
Description
This invention relates to a method and apparatus for delivering
liquids at a relatively constant rate, particularly at a very low
rate.
Presently, there exists a wide variety of systems requiring the
delivery of minute quantities of liquid at a relatively constant
rate and over relatively long periods such as in the administration
of pharmacological drugs to humans and to experimental animals.
Other exemplary systems involve delivering lubricants to machines,
or delivering bactericidal agents to running water supplies for
purification. Liquid delivery to these systems is effected at a
rate of the order of a few cubic millimeters over a period ranging
between an hour and several days or more.
Presently available pumps for delivering microquantities of liquids
involve cumbersome expensive machinery combining complicated
electromechanical devices to assure delivery of fluid with
specified accuracy and rate over a long period. A typical device
utilizes an infusion pump having as its principle of operation the
progressive mechanical insertion of a hypodermic syringe barrell
into a syringe. This device, as well as others now available, is
impractical for use in remote places, or on moving systems such as
moving experimental animals due to the cumbersome auxiliary control
apparatus.
It has been proposed to pump liquids from a chamber by permitting
expansion therein of a compressed hollow circular tube made from an
elastomeric material. Expansion is effected by diffusion of air
through the portion of the tube wall, which is pervious, extending
from the chamber into the atmosphere. The expanded tube displaced
liquid in the chamber which then is delivered to any desired
location. Although tube expansion is slow and extends over
relatively long periods, this device is unsatisfactory. The rate of
tube expansion varies substantially from linearity thereby making
the rate of liquid delivery non-linear. This is undesirable
especially when the liquid delivered is a drug. Furthermore, the
total amount of liquid delivered and the variance from linearity is
dependent upon the degree of original tube compression, thereby
rendering it difficult to control delivery of the liquid. This is
especially undesirable when a drug is the liquid being delivered
since it is desirable and sometimes essential that drugs be
delivered to animals in precise amounts and at a substantially
constant rate.
In accordance with this invention, liquid is delivered at a
predetermined and essentially constant rate from a chamber defined
by walls including an elastically compliant wall, by application of
gas pressure increasing from zero to about atmospheric to the
outside surface of the elastically compliant wall. The chamber is
filled by replacing the gas therein with the liquid so that the
compliant wall is expanded. The gas pressure is then applied to the
outside surface of the expanded wall by diffusing gas through an
elastomeric wall into a chamber originally under vacuum and
surrounding the expanded wall to cause it to contract at a
substantially constant rate. The compliant wall is formed so that
it exerts a substantially constant resistive force over a
relatively wide sub-atmospheric pressure range.
By employing a compliant wall which exerts a substantially constant
resistive force when expanded, a slight pressure change on its
outside surface effects a large change in the chamber volume
defined by the wall. In addition, the rate of volume change as a
function of change in outside pressure is substantially constant.
By regulating the outside gas pressure to increase at very slow
rates, liquid delivery can be effected for long periods at a
substantially constant rate. Any structure and composition for the
compliant wall can be employed so long as it exerts a substantially
constant resistive force when expanded within a wide pressure range
between zero and atmospheric pressure. The rate of gas pressure
increase on the expanded wall is regulated by diffusing gas through
a porous barrier open to the atmosphere and to a chamber under
vacuum adjacent the compliant wall. The diffusion ate and
consequent rate of pressure increase is regulated by the structure
and composition of the diffusion barrier.
The pump provided by this invention is portable, does not require
additional apparatus to regulate liquid delivery and can deliver
liquid at a relatively constant rate even at low flow rates for a
long period.
Reference is made to the attached drawings for further
understanding of this invention.
FIG. 1 illustrates the relationship between internal pressure and
volume in an elastomeric tube stretched and filled to form an
aneurism.
FIG. 2 is a cross-sectional view of an embodiment wherein the
inflated tube filled with a liquid is retained within a semi-rigid
container.
FIG. 3 is a cross-sectional view of an elastomeric bladder used to
force liquid through a delivery system.
FIG. 4 is a cross-sectional view of a pump employing the bladder of
FIG. 3 with its interior being under partial vacuum.
FIG. 5 is a cross-sectional view of the pump of FIG. 3 with the
interior of the bladder at atmospheric pressure.
Referring to FIG. 1, a tubular elastomeric material having the
desired properties suitable for the device is stretched and then
filled with varying amounts of gas and the pressure measured. The
tube is made from cross-linked poly dimethylsiloxane and is
available under the trademark "Silastic." It is 10cm .times. 0.183
inch .times. 0.132 inch and is then stretched and held to a length
of 20cm. The stretched tube is filled with gas under pressure to
determine the pressure-volume relationship in the tube. As shown by
FIG. 1 after aneurism is initiated, the pressure in the tube is
relatively constant over a wide gas volume range. Over a four fold
range of volume the absolute pressure difference changed by about
10 percent, from 550 to 450 millimeters of mercury. Consequently,
as the aneurism collapses, the absolute pressure in the gas space
inside the tube gradually but slightly increases toward atmospheric
by a rather small percentage. Thus, the driving potential for gas
diffusion through the elastomeric wall namely, the difference
between exterior pressure and internal pressure remains nearly
constant. These data show that the delivery rate of fluid from the
aneurism under the force exerted by the expanded elastomer is
insensitive to pressure on the delivered fluid different from
atmospheric by at least .+-. 20 mm Hg. Beyond an aneurism volume of
50 cm.sup.3 the entire tube had become converted into a single
aneurism extending over the entire length, and further injection of
gas caused rapid increase in pressure.
As shown in FIG. 2, rigid housing 1, made of glass, hard plastic,
metal or the like having axial symmetry is provided with necks 3, 4
and 5 at both ends and on one side, these necks not necessarily
having the same inside and outside diameters. A rubber tubing 6,
made for example, from natural gum rubber, silicone rubber, or
other highly elastic polymer suitably vulcanized is extended to
approximately twice its length at rest, drawn through housing 1 and
then everted over the opposite ends of necks 3 and 4 thereby being
held in extension. An internal mesh-like retainer 7 fabricated from
plastic screening or other suitable porous material, previously
inserted into or built into housing 1, circumferentially surrounds
the thus extended tubing 6 supporting it over its extension between
the opposing necks 3 and 4.
When the aneurism as represented by dotted lines 8 is later formed,
by means described below, it is contained by and prevented from
indefinite expansion by the inner surface of retainer 7. Following
the everting of the extended rubber tubing over the necks 3 and 4 a
piece 10 is inserted into the lumen of the tubing 6 to its limit so
that the cap portion 11 compresses the tubing 6 around the neck 4
thereby locking it while the extension 12 occupies volume inside
the tubing 6 that would otherwise later be occupied by the liquid
to be dispensed.
A dispensing nozzle 13 is fitted onto the neck 3 of the housing
thereby locking the extended tube 6 onto the neck 3 and at the same
time forming a needle-like passage 14 for the subsequent dispensing
of the liquid. It is convenient to make the dispensing nozzle with
a tip 16 attached to the main body 13 with a circumferential notch
17 so that when subsequently a vertical thrust is exerted on tip 16
it breaks off at the notch 17 thereby opening the dispensing
orifice 18. The internal geometrical configuration of the collar 20
is similar to that of the cap 11.
A piece of thin-walled silicone rubber or other tubing 21, which
serves as a diffusion membrane, is conveniently carried inside the
housing 1 between the inner surface 22 and the exterior surface of
the retaining cage 7. The diffusion tubing 21 sealed at one end is
connected via its other end to the side neck 5 of the housing 1 by
everting the tubing 21 around the neck 5. A covering piece 24 made
of plastic or metal has a collar 25 designed to compress snugly the
everted neck 26 of diffusive tubing 21 after its positioning on
neck 5. Covering piece 24 has an orifice 27 terminating at a blind
end. At a subsequent time the tab 28 is snapped off by the user
thereby opening orifice 27 into the diffusion tube 21 to admit
atmospheric air. At any convenient location on the body of housing
1 a small orifice 30 is covered by seal 31 carrying a thin layer of
contact sensitive adhesive whereby the seal 31 is maintained over
the orifice 30 until intentionally removed. Cement layers 32 around
the everted tubings 6 and 21 and the respective caps 10, 25 and 20
serve to prevent significant leakage of atmospheric air into the
device during storage. Preferably cement 32 is chosen from the
well-known high vacuum sealants or cements. Liberal quantities may
be used as necessary.
To prepare the pump for service it is convenient to start with the
assembly shown except that orifice 30 is open and dispensing nozzle
13 has not yet been applied. The rest of the assembly is placed in
a vacuum chamber and fully evacuated down to a few millimeters
mercury of absolute pressure. It is often desireable to flush the
whole device with nitrogen, carbon dioxide, or some other inert gas
in order that after vacuumization there will be little or no oxygen
left in the device capable of causing chemical deterioration of the
rubber tubing 6 or the diffusion tubing 21. After evacuation, foil
31 is applied over orifice 30 and dispensing nozzle 13, intact with
the attachment 16, is applied over the everted end of the interior
tubing 6. The interior of housing 1 is now under vacuum and remains
so.
When the micropump consisting of the elements recited above is to
be placed in service to dispense a particular liquid, it is
convenient to place this liquid in a beaker the floor of which may
serve as a means to accomplish fracture of the piece 16. The
micropump is filled by thrusting the entire assembly down against
the floor of the beaker thereby detaching the tip 16 at the notch
17 whereupon the internal vacuum allows the liquid under
atmospheric pressure to enter through the dispensing orifice 18
into the tubing 6 between its inner wall and extension 12. The
tubing 6 is selected so that it will undergo aneurism formation at
the pressure difference corresponding to unit atmosphere of vacuum
of about 700 to 760 millimeters mercury. An aneurism progressively
forms in the tubing 6 taking the shape progressively of 8 and this
aneurism eventually fills the entire retaining cage 7. It will be
understood that since the tip 16 is always immersed under liquid
during filling, only liquid enters to take up the volume formed by
the aneurism. When it is desired to dispense the liquid, usually
within a short period after filling the micropump, first the tab 28
is broken off allowing atmospheric air to enter to interior of
diffusion tube 21. Air will diffuse through it into the space
between housing 1 and retaining cage 7 at a rate proportional to
its area, which is constant and the pressure difference between
ambient air and the instantaneous value inside the housing 1. To
start the pump, tab 31 is pulled clear of orifice 30 to allow
sufficient atmospheric air to enter via that port so that the
aneurism 8 can begin to collapse in response to the elastic strain
of the extended rubber tube 6. When the first drop of liquid
appears at the end of dispensing nozzle 5, the tab 31 is quickly
resealed over orifice 30. From that point, air can enter the
interior of the housing 1 only via diffusion tube 21. As it does
so, the aneurism 8 will gradually collapse expressing liquid from
the orifice 18.
The pressure-volume data shown in FIG. 1 is typical of how the
aneurism can collapse (decrease volume) under a nearly constant
pressure. When nearly a total vacuum is required to render the
tubing 6 in the aneurism form, as it collapses, it can pump against
significant external pressures of the order of at least 50
millimeters of mercury. By the same token it can deliver liquid
into systems substantially below atmospheric pressure, without
affecting the nearly linear operation of the pump. It also will be
understood from this design that as there is no absolute limit on
the total volume of liquid that may be dispensed by this means.
However, it is particularly suitable for dispensing quantities
ranging from a few cubic millimeters to a few cubic centimeters,
and it will further be understood that once the dispensing has been
set in motion by opening the diffusion tube 21 to the atmosphere it
may be stopped by resealing the tube 21.
Another embodiment of this invention is illustrated in FIGS. 3, 4
and 5. The bladder 40 can be formed from vulcanized silicone rubber
stock or any other appropriate elastomeric stock as long as it is
not significantly swollen by the liquid to be dispensed. When the
bladder 40 is formed by compression molding, two identical pieces
are assembled back to back, one piece being the mirror image the
other piece. Alternatively, either half of the composite assembly
taken along the horizontal axis as shown in FIG. 3 may be
fabricated by compression molding and used by itself in a suitable
housing. The bladder 40 comprises conical element 41 with tapering
walls terminating in a bead 42 and defining an angle .theta..sub.o
with respect to the midplane of at least 60.degree. and preferably
75.degree.. The conical element 41 terminates in a base 43 having
circumferential grooves 44 and extensions 45 terminating in a bead
46 that may be mechanically grasped. It will be observed that below
the dashed line representing the midplane, an identical molding is
placed back to back with the above-described molded piece.
When the vulcanizable elastomeric stock has been pressure molded,
the molecules of the molded element constituting the cross linked
elastomeric network are in their relaxed configuration when the
piece as a whole has the shape shown in FIG. 3. When both halves of
the rubber are to be used back to back in the micropump, they are
assembled and clamped between rigid sections 47 and 48 fashioned
out of a suitable non corrosive metal or a hard plastic like
polycarbonate, either by injection molding, machining, or any other
technique leading to reasonable precision of dimensions. The
sections 47 and 48 have a circumferential bead 49 which engages in
the circumferential channel 44 of the bladder 40. The bladder 40 is
stretched radially by the equivalent of a tenter frame that grasps
the bead 46 and extends the pieces until their channel 44 is
enlarged to such a circumference that it will mate with the bead 49
of the outer housing pieces 47 and 48. Permanent clamping is
accomplished by means of several circumferential equally spaced
rivets 50. As a consequence of this radial stretching, the conical
sections 41 become stretched into a very flat cone of shallow angle
and the sections 51 go under very high tension tending to restore
the cone to its unstressed configuration as shown in FIG. 3.
As the bladder is loaded between the frame plates 47 and 48 and
clamped between them, the angle of the conical section with respect
to the plane of symmetry, .theta., is close to .theta.max as shown
in FIGS. 4 and 5 an angle of approximately 12.degree. to
15.degree., thereby making a very flat conical section 41.
A hollow needle 55 clamped between the identical members of the
bladder 40 at any position around the circumference of the assembly
provides means whereby the space between surfaces 56 of opposing
bladder pieces may be evacuated. When this evacuation occurs, the
angle .theta. defined by the conical piece 41 and the plane of
symmetry goes nearly to 0.degree.. The rigid sections 47 and 48 are
provided with ports 58 and 59 through which liquid subsequently to
be dispensed may be drawn into the chamber as the bladder is being
collapsed to the configuration shown in FIG. 4 by application of a
vacuum on needle 55. When the chamber defined between the bladder
surfaces 60 and the housing surfaces 61 is filled with liquid, a
predetermined length of diffusion tubing 62 is attached at the
terminus of hollow needle 55. Diffusion tubing 62 terminates at its
other end by a suitable plug 63. This tubing may be coiled or wound
in any convenient configuration. It serves the same function as the
tube 21 in FIG. 2, namely to permit the gradual ingress of air by
molecular diffusion through its wall to permit relaxation of the
bladder elements. Relaxation occurs by the progressive lifting of
the conical surfaces owing to the tension in the areas 51 in the
rubber molecules. The elastically deformed rubber structure owing
to the limited angle or operation, that is, between zero and
.theta. max of about 15 percent, provides nearly linear operation
for precisely the same reasons as do the aneurism collapse in the
device of FIG. 2. That is to say, as .theta. increases slowly from
zero to about 15.degree., the tension in the rubber is relaxed by
about 10 to 12 percent and thus the pressure difference between the
space 56 inside the bladder and the liquid space 65 decreases only
by that value. Therefore, the net diffusion driving force to cause
diffusion of air through tube 62, namely the external atmospheric
pressure less the inside pressure between the bladder surfaces,
remains almost constant. In consequence, the rate of deflection of
the bladder elements with increasing angle .theta. is nearly
constant as gas progressively fills the space 56. Since the
thickness and modulus of elasticity of the rubber stock as
compression molded according to FIG. 3 is chosen such as to require
nearly one full unit atmosphere of vacuum to elongate the conical
section 41, this device, like the device of FIG. 1, operates
successfully against variable output pressures within plus or minus
15 millimeters of mercury of the ambient atmospheric pressure.
The particular embodiment shown in FIGS. 3, 4 and 5 can be achieved
by using precisely half of the assembly on either side of the plane
of symmetry. For example, with reference to FIG. 3 the section
lying above the midplane of symmetry may be the sole bladder
element. This is drawn out by the tenter frame referred to above
with a flat plate replacing the other half of the gasket. The
principles of operation are exactly the same as explained above
except that for a given device the total liquid output is
approximately half of what it could be.
While employing the designs shown by FIGS. 4 and 5, it is possible
to have different liquids in spaces 65 and 66 or they be the same
liquids and the delivery may be joined by connecting delivery tubes
70 and 71 to a common connection. It will be understood equally
either section 70 or 71 shown in FIGS. 4 and 5 may be filled with
liquid exclusively of the other and the other section may be left
open to the atmosphere, without affecting the operation of this
device. The beads 42 are intended only for reinforcement and may be
omitted entirely. When present, the beads ultimately rest in hole
72.
Furthermore, the break-off tabs may be supplied as in the other
embodiment of this invention described in reference to FIG. 2 and
that the whole assembly may be prepackaged in a variety of forms.
It will also be understood that whereas the invention has been
described with reference to elastomeric vulcanized materials it is
equally possible to use metal or other bellows to achieve the same
objective, the common requirement being that over the range of
their expected deflection their resistive force remains constant
within plus or minus about 10 percent.
* * * * *