Hemodialysis and cardiopulmonary bypass are two medical procedures in which blood is extracted from the body, treated, and pumped back into the body. Hemodialysis is used to cleanse toxins from the blood of a patient in kidney failure, and uses a typical blood flow rate of 400 milliliters per minute (ml/min). Cardiopulmonary bypass is used to oxygenate the blood of a patient undergoing open heart surgery, and uses a typical blood flow rate of 4 liters per minute (1/min).
A peristaltic pump, often called a roller pump, is a fluid pump in which an enclosed flow channel is compressed by a roller or rollers, or by a series of compression blocks or fingers, to propel a fluid along the channel from a channel entrance to a channel exit, in rough analogy with the peristaltic pumping action of biological structures such as intestines. Advantageously, the fluid being pumped contacts only the interior surfaces of the flow channel, and complex components such as valves or pistons, which would be subject to leakage or sliding wear, are avoided.
Both hemodialysis (HD) and cardiopulmonary bypass (CPB) employ peristaltic pumps to pump blood. These pumps use compressible flow channels comprising soft, round tubing having a central lumen, typically made of polyvinyl chloride (PVC) softened by plasticizers such as phthalates. The tubing is compressed, and its lumen is partially or fully occluded by passing rollers, or blocks as mentioned above, to push blood through the flow channel.
Six problems arise from the use of this soft, round tubing. One problem is spalling of particles from the tubing into the blood flow as the lateral edges of the soft tubing undergo wear during compression due to stretch and shear of the tubing material [1]. A second problem is spalling of particles from the tubing into the blood flow as the interior faces of the soft tubing undergo contact caused by roller compression, which contact can be a grinding contact when soft tubing is used. A third problem is leaching of plasticizers into the blood flow from tubing walls and spalled particles. A fourth problem is hemolysis (blood cell destruction) due to crushing of blood cells between interior faces of tubing walls. A fifth problem is hemolysis due to grinding of blood cells between interior faces of tubing walls. A sixth problem is hemolysis due to excessive fluid shear stress (for example in excess of 150-560 Pascals [2],[3]) caused by high velocity gradients in the blood near roller compression regions.
The problems of spalling due to crushing contact, spalling due to grinding contact, leaching of plasticizers, hemolysis due to crushing contact, hemolysis due to grinding contact, and hemolysis due to excessive fluid shear stress can be reduced by the pump operator (called a “perfusionist” in CBP practice) adjusting the pump roller force during setup to provide a tubing lumen which is not fully occluded (“under-occluded”) during operation. For example, a tube having a circular lumen 12.7 mm in diameter in its uncompressed condition may be set up to have a gap of 1 millimeter (mm) between interior wall faces during compression by a roller. But this force adjustment is different for each tube due to manufacturing tolerances, and for a given roller force setting the lumen gap decreases during pump operation as the tubing wears.
Flow channels for peristaltic pumps having a non-round cross-sectional shape, which herein will be called the Davis-Butterfield shape, or DB shape, after the inventors, can have performance advantages over a round tube or hose, including low spallation, low mechanical stress, long channel life, and high-pressure capability. Also, as they allow for the use of stiff materials rather than soft materials, the need for plasticizers is reduced or eliminated. However, even in channels having the DB shape, pump roller pressure can cause contact of the interior faces of the flow channel, and can result in one or more of spalling from the contacting faces, leaching of plasticizers, and, in hypothetical cases where such channels might be used for pumping blood, hemolysis due to crushing of blood cells between the contacting faces, and hemolysis due to fluid shear stress. The roller force can be adjusted to leave a small residual lumen (under-occlusion) in a DB-shaped channel, the force being for example 5% less than the force required to completely occlude the lumen. However, due to manufacturing tolerances of the channel shape, that force level can't be accurately predicted and must be experimentally determined during use for each channel. No prior art using the DB channel shape for pumping blood is known.
Thus, there is a need for peristaltic pumps having flow channels which reduce or prevent one of spalling due to contact, spalling due to grinding, leaching of plasticizers, hemolysis due to crushing, hemolysis due to grinding, and hemolysis due to fluid shear stress. Further, there is a need for peristaltic pumps having flow channels which do not require the operator to adjust the pump for under-occluding pump operation.
The present invention includes a flow channel suitable for use with a peristaltic pump, the flow channel comprising: an upper wall having a bowed upward shape; a lower wall having one of a bowed downward shape and a flat shape; and one or more spacers between the upper wall and the lower wall disposed between lateral edges of the upper and lower walls, each spacer having a height. The upper wall, lower wall, and the one or more spacers define a lumen, wherein, when the upper wall is compressed toward the lower wall by compressing members, the one or more spacers limit vertical movement of the compressing members such that the lumen is maintained in an under-occluded condition.
In one aspect, the bowing of one of the upper and lower walls has a recurved shape. In another aspect, one of the upper and lower walls has a uniform thickness. In yet another aspect, the lumen in its under-occluded condition has a lumen width and a lumen height, the lumen width being wider than the width of an under-occluded lumen of an area-equivalent circular tube exhibiting the same under-occluded lumen height.
The present invention includes a method of fabricating a flow channel suitable for use with a peristaltic pump; the method comprising: forming an upper wall having a bowed upward shape; forming a lower wall having a bowed downward shape; and joining the upper wall and lower wall to create a lumen of the flow channel; wherein one of forming the upper wall and forming the lower wall comprises forming one or more spacers protruding therefrom, such that after joining the upper wall and lower wall, the one or more spacers serve as lateral bounds for the lumen; and wherein, when the upper wall is compressed toward the lower wall by compressing members, the one or more spacers limit vertical movement of the compressing members such that the lumen is maintained in an under-occluded condition.
Embodiments described herein include a flow channel suitable for use with a peristaltic pump, the channel having spacer features at the lateral edges of the flow channel which provide under-occlusion to reduce or prevent one of spalling, leaching of plasticizers, and hemolysis. The flow channel may have upper and lower walls having a shape similar to that of walls in a Davis-Butterfield flow channel, or to that of walls in a flow channel having another advantageous shape. In another aspect, the invention comprises a flow channel having a compressed lumen width larger than the compressed lumen width of an area-equivalent circular tube, thereby providing reduced hemolysis due to fluid shear stress. In another aspect, the invention comprises a planar flow channel plate incorporating the above flow channel. In yet another aspect, a disposable kit for a peristaltic pump comprises the above flow channel and one or more additional elements; wherein the flow channel and the one or more additional elements are integrated to form a single assembly.
In
Descriptive language in this disclosure and in associated claims refers to flow channels in the orientations shown in
In
A major disadvantage of roller pumps using soft tubing is that the under-occlusion setting must be done manually, resulting in a large variability depending on the operator. Because of production tolerances in wall thickness, the occlusion setting of a roller pump needs to be controlled before each procedure in order to avoid excessive blood damage and to ensure correct blood flow.
Another disadvantage of soft round tubing is excessive shear (change in velocity versus position), which leads to excessive shear stress on blood cells, which leads to hemolysis. Computational fluid dynamic (CFD) simulations1 show that shear stress occurring in soft, round tubing used in peristaltic pumps can produce hemolysis even when the tubing is under-occluded, having for example an under-occlusion gap 204 of 1 mm between interior walls of the tube. The hemolysis problem arises at regions near the roller-compressed portions of the tubing as the fluid in the tube squirts rapidly ahead of the compression roller, producing damaging peak shear stress of 994 Pa or higher in relation to a cell damage threshold which has been variously estimated to be 150-560 Pa. 1J. W. Mulholland, J. C. Shelton, and X. Y. Luo, “Blood flow and damage by the roller pumps during cardiopulmonary bypass,” J. Fluids Struct., vol. 20, no. 1, pp. 129-140, January 2005.
For a circular tube having a lumen diameter D, which is equal to the lumen's width in its uncompressed state, the length of the lumen's inner perimeter is πD, which is approximately equal to 3.14 D, and the lateral width of the lumen in its compressed state is approximately half that inner perimeter i.e. πD/2 or approximately 1.57 D.
It does not seem to have been appreciated until the present invention that the magnitude of shear stress is related to the distribution of flow over the compressed lumen width (πD/2 in a circular channel) and that if the compressed width can be increased sufficiently in an “area-equivalent” wider flow channel, then the flow can be distributed over this larger width, and the magnitude of shear stress in an under-occluded lumen can be decreased to a non-damaging, non-hemolytic level.
The term “area-equivalent” is used herein to refer to two different flow channels having in their uncompressed states equal cross-sectional lumen areas. Other factors being equal, two channels which are area-equivalent require equal pump roller speeds to produce equal flows. The present invention as described herein can increase the width of the compressed under-occluded lumen by a factor of more than two, and as much as three, compared to the width of the compressed under-occluded lumen of an area-equivalent circular tube.
The word “round” used herein to describe flow channel shapes connotes flow channels having lumens which, when viewed from inside the lumen, have a shape which is concave everywhere, such as a circle, oval, or ellipse. Flow channel shapes having points, cusps, tips, or regions which are convex when seen from within the lumen are non-round.
If non-round flow channels having channel walls similar to those in
The use of a recurved shape for the flow channel walls has a key advantage over its use in the prior-art Davis-Butterfield channels. A recurved channel wall shape of the present invention is defined, for example with reference to upper wall 501, as a shape that, proceeding from a starting point, for example at the left, towards the end point, in this example at the right, starts off with a zero slope at the leftward extent of the lumen, then in a first section curves smoothly upward until it reaches an inflection point at a point of maximum positive slope, then in a second section curves smoothly downward until it reaches zero slope at a lateral midpoint between the two lumen lateral edges, then in a third section curves smoothly downward until it reaches another inflection point at a point of maximum negative slope, then in a fourth section curves smoothly upward again until it reaches zero slope at the rightward extent of the lumen. The recurved shape serves to distribute mechanical bending stress, experienced during channel compression or distension, over the width of the channel instead of merely concentrating stress at channel edges. Ideally for the purposes of stress distribution, each of the four sections is of equal width, but less-than-ideal section widths will function within the spirit and scope of the present invention.
The shape description above is expressed in terms of shape change from left to right. Of course, the shape could equally well have been defined with a starting point at the rightward extent of the lumen and an end point at the leftward extent.
In the Davis-Butterfield flow channel shape the upper channel wall meets the lower channel wall at two cusps or notches or points having acute angles which are estimated, from examining the figures in U.S. Pat. No. 9,683,562, to be less than 5 degrees.
It is well known that acute angles such as those shown in
In contrast to the burst pressure advantage gained using a recurved wall shape, adapting a simply-curved wall shape as shown in
Intermediate wall shapes between simply-curved and recurved are possible. For example, an upper wall can begin as would a recurved wall shape on the left, starting off with a zero slope at the leftward extent of the lumen, then in a first section curving smoothly upward until it reaches an inflection point at a point of maximum positive slope, then in a second section curving smoothly downward until it reaches zero slope at a lateral midpoint between the two lumen lateral edges, but then departing from the full recurved shape by curving smoothly downward until it reaches the right lateral edge of the lumen. A lower wall shape can, for example, be an upside down left-right mirror image of the upper wall shape described above.
The use of uniform wall thicknesses for walls 501 and 502 is advantageous but is not essential to the invention. Walls having non-uniform thickness can be present, due for example to manufacturing tolerances, or due to a desire to create a flow pattern shifted more toward the channel edges or the channel center.
The width 512 of under-occluded lumen 5051, with a value of 60.1 mm, is more than three times the width 205 of under-occluded lumen 2011 in
For a given under-occluded lumen height in both a non-round channel of the present invention and an area-equivalent circular channel, a “lumen width ratio” or LWR can be defined as the width of the under-occluded lumen in the non-round channel to the under-occluded lumen in the area-equivalent circular channel. For the example above, the LWR is equal to 3.08.
In contrast to soft, round tubing, the flow channels of the present invention benefit from the use of stiff materials having high elastic modulus and high hardness rather than soft materials having low elastic modulus and low hardness. For example, soft round tubing used for peristaltic pumps is recommended to have a Shore A Durometer hardness less than 652, which corresponds to an elastic modulus (Young's modulus) less than 24 megaPascals (MPa). In contrast, rigid polyvinyl chloride (PVC) advantageously employed in the present invention has a Shore D Durometer hardness of 80 and a Young's modulus of 3800 MPa, thereby being 158 times as stiff as soft, round tubing. 2“Material Selection for Peristaltic Pump Tubing|Whitepaper|Grayline LLC.” [Online]. Available: https://www.graylineinc.com/whitepapers/peristaltic-pump-tubing.html. [Accessed: 27 May 2018].
A benefit of using material having a high Young's modulus is the ability to achieve a high lumen width ratio (LWR) for low shear stress and low hemolysis. For the example of channel 500 discussed herein, using rigid PVC, the LWR is 3.08. It can be shown by engineering modeling that as the Young's modulus decreases while area-equivalence with the circular lumen of
Under-occluded lumens are necessarily leaky lumens, potentially allowing backflow through fluid resistance, so the pump roller speed must be adjusted to create the desired forward flow despite backward leakage. For the present invention the under-occluded gap height may be set so that fluid resistance of the under-occluded lumen matches that of an under-occluded lumen of area-equivalent round tubing. It is known that for a lumen having a width much greater than its height, fluid resistance varies as the inverse of the third power of lumen height. Thus, if the under-occluded lumen 2011 from
It will be appreciated that a flow channel of the present invention can be formed by extrusion of a channel having upper and lower walls and spacer features defined by the extrusion process, by lamination, by some combination of extrusion and lamination, or by other means.
Principles of the present invention can be embodied in channels having straight flow paths or curving flow paths.
Channels like channel 702, with further extensions, are suitable for use in a circumferential-roller pump like that shown in
Strain relief means 807 are shown in
The flow channel plate 800 can be formed by laminating together two separate sheets of material 810 and 811 as discussed herein. Similar flow channel plates can be fabricated by other means. For example, flow channel plates can be formed by fine-featured three-dimensional printing means known as micro-stereolithography, using a single printing material or various materials. Other possible means of fabricating flow channel plates include, but are not limited to, stereolithography, three-dimensional printing, injection molding followed by lamination, vacuum forming followed by lamination, lamination around a mandrel, and investment casting.
The material comprising flow channel plate 800 or similar flow channel plates embodying the present invention may be one or more of poly-ether ether ketone (PEEK), polycarbonate, cyclic olefin copolymer (COC), polyvinyl chloride (PVC) with plasticizers, polyvinyl chloride without plasticizers, polymethyl methacrylate (PMMA or Plexiglass®), polyethylene, high density polyethylene, ultra high density polyethylene, polyethylene terephthalate (PET or PETE), polypropylene, Formlabs printing resin, other printing resin, silicon, glass, silicone rubber, polyimide, stainless steel, brass, and bronze. The use of other materials is also possible.
The pump head 900 shown in
In order for channel 1000 to function well, bottom wall 1002 must be able to stretch laterally as the top wall 1001 expands laterally when it is compressed vertically. The lateral stretch of bottom wall 1002 can be accomplished if bottom wall 1002 comprises a material that is less stiff (having a lower Young's modulus) than the material comprising upper wall 1001, or if lower wall 1002 is thinner than upper wall 1001, or some combination.
If lower wall 1002 comprises a material less stiff than that comprising upper wall 1001, lower wall 1002 can also be much thicker than upper wall 1001, the lower wall 1002 for example comprising part of a wider thick substrate atop which spacers 1003 and 1004 and wall 1001 are disposed.
If the vertical thickness of spacers 1003 and 1004 approaches zero and becomes zero, the channel 1000 becomes a channel having no spacers which can have a fully occluded lumen during pump operation. This arrangement thus comprises a flow channel suitable for use with a peristaltic pump, the flow channel comprising an upper wall having a bowed upward shape, and a lower wall having a flat shape, wherein the upper wall and lower wall define a lumen, and wherein the bottom wall is flat, and the upper wall comprises a material having a first Young's modulus, and the lower wall comprises a material having a second Young's modulus and the second Young's modulus is lower than the first Young's modulus. This arrangement is novel in relation to prior art which used a simply bowed upper channel wall comprising a soft material having a low Young's modulus disposed atop a stiffer and more rigid flat substrate having a higher Young's modulus. Fully occluded lumens have the advantage of no leakage or low leakage in comparison to the under-occluded lumens discussed elsewhere in this description, but fully occluded lumens do not have the advantage of low hemolysis given by under-occluded lumens. When not used for pumping blood or other liquids containing fragile components, a fully occluded channel can be advantageous.
The use of under-occluded flow channels of the present invention in peristaltic pumps relieves the operator or perfusionist of the need to adjust under-occlusion before pump use, produces stable under-occlusion during pump use, and reduces or prevents hemolysis due to crushing or grinding. The use of wide channels of the present invention reduces or prevents hemolysis due to fluid shear stress. The use of channels of the present invention comprising stiff materials such as rigid PVC reduces or prevents wear and spalling of the channel material, and reduces or prevents leaching of plasticizers into blood.
Flow channels of the present invention may be built using combinations of materials rather than a single material. For example, the top or bottom wall may comprise layers of materials having different properties. The spacer regions may comprise a different material than the walls.
Flow channels of the present invention may be built using spacers of different height at the left and right lateral edges of the channel, and the height of one spacer may approach zero, or become zero so that there is a spacer only on one side of the channel. Spacers of different height may be advantageous, for example, in a pump having a channel which follows a planar semicircular path wherein the radially outward region of fluid flow tends to be faster than the radially inward region of fluid flow, resulting in higher fluid shear for the radially outward region of the under-occluded lumen. By making the radially outward spacer thicker than the radially inward spacer, fluid shear across the radial width of the under-occluded lumen can be made more uniform.
A channel of the present invention can be built having interior surfaces exposed to the pumped fluid which are more biocompatible than the rest of the channel, as is known for other blood-contacting devices.
The invention is useful for pumping fluids other than blood, including fluids having fragile components such as large fragile molecules.
Although the invention has been described with respect to particular embodiments thereof, these particular embodiments are merely illustrative, and not restrictive.
It will also be appreciated that one or more of the elements depicted in the drawings/figures can also be implemented in a more separated or integrated manner, or even removed or rendered as inoperable in certain cases, as is useful in accordance with a particular application.
Thus, while particular embodiments have been described herein, latitudes of modification, various changes, and substitutions are intended in the foregoing disclosures, and it will be appreciated that in some instances some features of particular embodiments will be employed without a corresponding use of other features without departing from the scope and spirit as set forth. Therefore, many modifications may be made to adapt a particular situation or material to the essential scope and spirit.
This application claims priority from U.S. Provisional Patent Application Ser. No. 62/726,351, entitled “Under-occluding wide flow channels for peristaltic pumps”, filed on Sep. 3, 2018, which is hereby incorporated by reference as if set forth in full in this application for all purposes.
Number | Name | Date | Kind |
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1765360 | Baumann | Jun 1930 | A |
3508587 | Mauch | Apr 1970 | A |
4275761 | Waldhauser | Jun 1981 | A |
5215450 | Tamari | Jun 1993 | A |
Number | Date | Country | |
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20200072210 A1 | Mar 2020 | US |
Number | Date | Country | |
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62726351 | Sep 2018 | US |