Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
Multi-modal Flow Visualization and Optimization of
Pneumatic Blood Pump for Sorbent Hemodialysis System
Fangjun Shu1, Robert Parks2, John Maholtz3, Steven Ash?4, and James F.
Antaki1
1
Department of Biomedical Engineering, Carnegie Mellon University, Pittsburgh, PA
2
3
Daedalus Excel, Pittsburgh, PA
RenalSolutions, Inc., Warrendale, PA
4
Proposed to submit to Artificial Organs
?????
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
Abstract
RenalSolutions Allient hemodialysis system utilizes a two-chambered pneumatic pump to
avoid limitations associated with peristaltic pumping systems. Single-needle access is
enabled by counterpulsing the two pump chambers, thereby obviating compliance
chambers or blood reservoirs. Each chamber provides 25 cc per pulse of approximately 6
seconds, corresponding to a peak Reynold’s number of approximately 10,000.
A multi-modal series of flow visualization studies (tracer particle, dye washout and dye
washoff) were conducted on a sequence of pump designs with varying port locations and
diaphragms to improve the geometry with respect to risk of thrombogenesis. Experiments
were conducted in a simplified flow loop using occluders to simulate flow resistance
induced by tubing and dialyzer. Tracer visualization revealed flow pattern and
qualitatively indicated turbulence intensity. Dye washout exposed dwell volume and flow
stagnation. And Dye washoff resulted directly linked to surface washing.
The visualization resulted showed that compared to centered inlet, a tangential inlet
introduced a single vortex, and kept the flow lamina. It also provided better surface
washing on the pump inner surface. But a tangential outlet didn’t present much benefit as
expected. On the contrary, it created a sharp bend to the flow when transiting from filling
flow to injection flow.
Key Words: Thrombogenicity-Surface wash-Dye washout-Dye washoff-Blood pump.
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
Introduction
The Allient® Sorbent Hemodialysis System is intended for convenient home therapy for
acute and chronic renal failure. (See Figure 1a.) One of its unique features is the use of a
dual-chambered pneumatic pump to withdraw and infuse blood from/to the patient. This
configuration enables single-catheter vascular access without the need for a blood
reservoir or compliance chamber.
(Maholtz: 1) is there any previously published description or data on this system or
pump? 2) can you provide any clinical or pre-clinical performance data that we could use
in our introduction?)
This study was undertaken to develop a suitable blood pump for this system. The design
objectives were to supply the required flow and pressure (0.5-1.0lpm at 100mmHg,
Maholtz: please confirm), while avoiding deleterious fluid dynamics that could promote
thrombosis and hemolysis. A baseline design of the pump, illustrated in See Figure 1a., is
pneumatically driven and comprised of a blood chamber separated from an air chamber
by a flexible, elastic diaphragm. The geometric features of the pump under investigation
included position and orientation of the blood ports as well as the contour of the housing
and diaphragm. Due to the coupling between these design features, various alternatives
were considered in combination. It is understood that blood trauma is multifactorial, and
affected by the fluid dynamics as well as surface chemistry and blood coagulability. This
study focused specifically on the fluid dynamic factors, and presumed that best practices
will be utilized for material selection and anticoagulation.
The flow field inside a diaphragm-type blood pump is generally confounded by several
factors. The curved surfaces of virtually every component of the blood path prevents
visualization of the full region of interest. Additionally, the continuously flexing
diaphragm will cyclically block the optical path, and limit the focal depth. Therefore,
three flow visualization techniques were applied in this study to perform a comprehensive
assessment of the relevant flow characteristics. The candidate designs were evaluated on
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
the basis of optimizing surface washing while minimizing flow disturbance and
stagnation.
Materials and methods
Dialyzer system and experimental setup
The blood flow path within the dialysis system depicted in Figure 1a includes the blood
pump as well as vascular access catheters, a dialyzer, and interconnecting tubing.
The pump is driven by a pneumatic control unit (Pulsar XXX, RenalSolutions, Inc.)
providing both positive pressure and vacuum in a cyclical fashion at 10 beats per minute.
Pneumatically actuated tubing occluders upstream and downstream of the pump, are
synchronized with the drive pulse to provide unidirectional flow.
For the flow visualization studies, the fluid loop was simplified by replacing the dialyzer
and cannula with an equivalent fluid resistance provided by an occluder valve. Since the
two pump chambers shown in Figure 1b are symmetric, and do not communicate with
one another, only one was studied for the purpose of these experiments. The resistance
was adjusted to achieve a minimum inlet pressure of approximately -40 mmHg during
filling, and a maximum outlet pressure during ejection of approximately 125 mmHg –
corresponding to actual clinical operation of the system. A solution of glycerin and water
(1:2 volume, approximately 3.7 cP, and 1.08 g/ml) was used as blood analog.
A parametric study of alternative pump caps and diaphragms was conducted. (See
Table 1.) These designs were all generated by solid-modeling software (SolidWorks 2006,
SoldWorks, Inc.) and rapidly prototyped by stereolithography (3D Systems, Valencia,
CA) in optically clear polyurethane (Somos XYZ?, Sandoz? -- Rob, please confirm). The
baseline pump design (A0) features a flat diaphragm and co-axial inlet and outlet ports,
centered within the body of the cap. The baseline flat diaphragm is nominally centered
between upper and lower housings, and is inflated into the blood chamber during ejection
and vacuumed into the air chamber during filling. The prototype domed diaphragms are
made of silicone rubber and are flexed rather than stretched during operation.
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
Consequently their time-varying contour is not as uniform (congruent) as the inflated
diaphragms. However the advantage, and motivation, for this design was to reduce elastic
stresses and risk of pull-out. Caps 1, 2, 3 and 4 are modified variations of the baseline cap,
intended to improve the hemodynamics. Cap 1 incorporates two parallel ribs axially
aligned with the inlet and outlet ports, intended to guide the flow and limit the contact of
the diaphragm with the housing. Cap 2 features a tangential inlet port, intended to
establish a primary vortex upon filling. Caps 3 and 4 have both tangential inlets and
outlets. The ports of Cap 3 are diametrically opposed, while the ports of Cap 4 are
aligned on the same side of the housing. These design options resulted in ten potential
combinations that were studied, however for practical purposes only four representative
combinations are presented here.
Flow visualization techniques
Three flow visualization methods were used in this investigation. Tracer particle
visualization was used to investigate the pathlines of the flow field inside the pump
housing throughout the pump cycle. A dye washout (aka dye dilution) technique was used
to assess the overall residence, hence clearance of blood through the pump chamber. Dye
washoff (aka dye erosion) was performed to validate the surface washing caused by
surface shear, not easily visualized by the previous two modalities.
Tracer particle visualization was achieved using neutrally buoyant spherical fluorescent
polystyrene particles (Duke Science Corp. Palo Alto, CA, USA) with a mean diameter of
33 m to mark the flow field. A monochromatic light sheet (appx. 1mm thick) generated
by a solid state diode-pumped laser (Millennia™ II, Spectra-Physics Lasers, Inc., 532nm),
operated at 0.6 Watts, was aimed through the pump housing to illuminate the area of
interest. A high-resolution digital camera (D70, Nikon Corp., Japan) was used to record
the flow pattern. The exposure time was between 10-50ms, adjusted to optimize the
image quality. A long pass optical filter (550 nm cut-on wavelength, Edmunds Optics,
Barrington, NJ) was used to eliminate the boundary glare thus improve the contrast.
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
A dye washout technique was used to reveal regions of stagnation or recirculation that
may potentially lead to blood deposition. An aqueous solution of fluorescein disodium
salt (C20H10O5Na2) with a concentration of 5 ppm was used as fluorescent dye. Due to
Stokes shift phenomenon, the dye absorbs irradiated light at 532 nm and emits a broad
band longer wavelength. The same laser sheet was used to illuminate the area of interest
as the tracer particles visualization method. A 2 ml bolus of dye was injected into the
inlet tubing of the pump during the beginning of the its filling phase. The same long pass
filter was used to eliminate background laser illumination and boundary glare. A standard
DVD camcorder (VDR-D100, Panasonic) was used to record the flow pattern for further
analysis.
We employed a paint dot flow visualization technique, modified from the method
described by Merzkirch (Merzkirch 1987) and described by our group previously (Wu
1999) to evaluate the efficacy of surface washing. Highly viscous droplets of pigmented
paint (Alizarin Crimson, Winsor & Newton, London, England) were applied on the
interior of pump caps using a quill-type spotter. The diameter of the droplets was
approximately 1 mm. After drying overnight, the pump was gently assembled, connected
into the flow loop, operated for 3.5 minutes, and disassembled immediately thereafter. A
high resolution digital camera (D70, Nikon Corp., Japan) was used to photograph the
residual droplets within the cap.
Results
Tracer particle visualization
The flow pattern illustrated by the streaks of tracer particles in the baseline pump are
shown in Figure 2. Eight phases of the pump cycle are shown, with the respective point in
the volume-time graphs indicated in the inset of each figure. At the beginning of filling
(Figure 2a), pneumatic vacuum draws fluid into the pump chamber through the inlet and
was observed to form a counter-rotating vortex pair. The vortex was initially confined to
a region adjacent to the inlet, and migrated downstream towards the center of the pump
(Figure 2b). During the remainder of the filling phase, the vortex grew to occupy the
entire chamber. This resulted in a sharp deviation at the distal wall causing retrograde
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
flow along the side walls of the chamber. Two regions of disturbed flow were apparent
near the outlet and inlet, characterized by a sharp deflection of the particle pathlines. The
vortex pair also caused the retrograde flow along the side walls to collide with the inflow
jet. These collisions produced small scale turbulent structures (Figure 2c and d), hence
dissipating turbulent kinetic energy (TKE) generated at the collision region. The loss of
kinetic energy resulted in lower flow velocity at the end of filling. The small turbulent
structures disappeared gradually at the beginning of ejection. (See Figure 2e and f.)
Therefore there were fewer small scale structures observed throughout ejection. Near the
end of ejection, the flow inside the pump was almost laminar (Figure 2g and h).
Pump B0 (baseline with domed diaphragm), A1 and B1 (ribbed housings) showed similar
results, thus not presented. The effect of the ribs and domed diaphragm was overwhelmed
by the flow disturbance caused by the colliding flow streams.
To eliminate the disturbed regions induced by collisions in the baseline pump, a
tangential inlet was considered. The results corresponding to pump A2 are presented in
Figure 5. At the beginning of filling, the flow has a relatively high velocity – indicated by
the length of the streaks. This was caused by the thin gap between the pump housing and
the diaphragm (Figure 3a). After a very short period of chaotic flow (Figure 3b), the fluid
in the pump was observed to rotate clockwise. No small scale turbulent structures were
observed. Consequently, the accumulated kinetic energy of filling maintained the rotation
throughout the beginning of ejection (Figure 3e and f). Another beneficial property of the
tangential inlet of pump A2 was found to be the smooth transition of the flow velocity
from filling to ejection. (See Figure 3e and f.) Due to minimal residual volume at the end
of ejection, the vortex was interrupted at the end of ejection. (See Figure 3h.)
Pump A3 and A4 have identical tangential inlets to A2, and therefore the filling phase is
virtually identical. Contrasted with the co-axial outlet of A1 and A2, the outlets of A3
and A4 are tangential, although oriented 180 degrees from one another. The ejection flow
fields of A3 and A4 were quite different from A2; however very similar to one another.
Therefore only results for pump A3 is presented. (See Figure 4.) Unlike A2, where the
flow converged symmetrically through the outlet collector, the tangential outlet caused an
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
asymmetric fluid trajectory (from upper right quadrant towards the outlet in the figure.)
Because the main vortex was arrested suddenly at the end of filling, the ejected volume
was initiated from rest; hence there was no benefit observed by locating the outlet
tangentially. Overall, the flow patterns throughout ejection in A3 and A4 were more
turbulent than A2.
at various phases of the cycle. Images are presented as in Figure 2.
Figure 5 presents the flow patterns for pump B2, featuring the domed diaphragm. The
trajectory of the domed diaphragm was less symmetrical than the flat diaphragm, and
more influenced by the flow. However, no apparent difference in the flow patterns was
observed as compared to the baseline (flat) diaphragm (A) e.g., pump A2.
Dye washout
Figure 6 and Figure 7 provide representative images of dye washout results for pumps A0
(baseline) and A2 (tangential inlet), respectively. The baseline pump exhibited nearly
instantaneous mixing of fluorescent dye after injection – evidenced by the uniform
brightness of the illuminated region. (See Figure 6f & g.) This demonstrates a high level
of turbulent mixing as found in the particle tracer results. At the end of ejection, all of the
dye was expelled from the pump, with the exception of the inlet region in which there
remained a residual volume. This residual volume mixed with the incoming fluid on the
next cycle and was almost imperceptible at the end of the second cycle.
For tangential inlet pumps (A2-B4), there was less mixing observed during filling. This
was evidenced by the non-uniformity of the dye concentration at the end of filling and
even beginning of ejection. Bright streaks were observed throughout the cycle, indicating
incomplete mixing, and hence a low level of turbulence intensity. This also agreed with
the particle tracer results.
For all the pumps tested, there was a stasis region near the inlet. But the dye was cleared
after two pump cycles.
Dye washoff
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
Dye washoff results of the baseline pump are shown in Figure 8. The original paint dots
are shown in Figure 8a, and Figure 8b shows residual paint after the pump was operated
for 3.5 minutes. The lateral walls and central spine of the pump chamber (regions 1, 2
and 3) demonstrated greater surface washing than the inlet diffuser region (6) and the
regions corresponding to the cores of the two counter-rotating vortices (regions 4 and 5).
(See Figure 2b.) The schematic of the flow structure in this region is shown in Figure 8c.
Results of pump A2 are presented in Figure 9 to illustrate the contrast between the pumps
displaying the greatest and least turbulence in the previous studies. After 3.5 minutes of
operation, almost all the paint dots within pump A2 were washed off. An exception was
small spot located in the inner side of the inlet (mark A in Figure 9b) corresponding to
the likely zone of separation due to the excessive diffusion angle of the inlet.
Discussion
Rational design of blood-wetted devices requires careful consideration of shear induced
trauma (hemolysis) and activation of blood elements (thrombus) (5). Previous clinical
experience with the baseline RSI Allient blood pump revealed minimal hemolysis in this
system, [John: Is there any reference?]thus was not a concern of this study. On the
contrary, blood clots were observed at multiple locations, therefore this study mainly
focused on minimize thrombogenicity.
Stagnation and recirculation zones within the blood flow are directly linked to deposition
of blood elements on biomaterial surfaces.[REF-ask Sam.] Therefore a common objective
is to assure that all surfaces are “washed” to prevent accumulation of platelet agonists and
reduce the residence time in contact with the biomaterial surface. Turbulent or disturbed
flow can also contribute to thrombogenesis by activating platelets.[Snyder 2006or7.] The
three flow visualization techniques used in this study aimed to evaluate these deleterious
flow features. The multi-model approach to flow visualization proved to be beneficial,
both for corroborating each other, and providing different information.
Strong turbulent flow was observed in the baseline design during the filling phase. This
was mainly due to the fluid collisions near the inlet and outlet. After fluid enters the
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
chamber and flows downstream, the stream impinges normally on the outlet port and
turns back to form a pairs of counter rotating vortex. The retrograde flow also collides
with the inflow near the inlet port.
The tangential inlet was introduced to minimize fluid dynamic disturbance during filling.
Fluid entering the pump chamber establishes a single vortex, and rejoins the inlet jet
without disturbing the vortex. Elimination of flow disturbance or turbulence during the
filling phase allows the accumulated kinetic energy to maintain the velocity of the
circumferential motion (like a “fluid flywheel.”)
In additional to tangential inlet, a tangential outlet was introduced with the expectation
that the main vortex initiated during filling would continue throughout ejection.
Therefore there would be minimal change of inertia by aligning the port with the
(anticipated) circumferential velocity field. Other blood pumps have reported such a
behavior (6). However, we found that the vortex in pumps A3 and A4 was damped
abruptly at the beginning of ejection, and therefore the fluid was accelerated from rest.
Hence, the tangential port presented more of a detriment by arresting the vortex, and
causing the fluid volume to exit the chamber asymmetrically. The attenuation of the
vortex in pumps A3 and A4 with tangential outlet compared to A2 with an axial outlet
was counterintuitive. This may be attributed to the relatively spherical shape of the
current device relative to the flat “pancake” shape of the Novacor LVAS pumping sac.
The current results more closely resemble the flow described within the Penn State
device (Deutsch et. al., 2006) which also features a relatively spherical fluid chamber.
Improvements to the diaphragm were considered due to the physical stretching that
occurred in the baseline pump, which had a flat diaphragm. A domed design was
introduced to replace stretching with bending, and to alleviate the tension at the anchor
locations. Although the choice of diaphragm did not present any perceptible
hemodynamic benefits, the domed shape of the diaphragm B is preferable based on its
superior mechanical characteristics.
Dye washout (dilution) studies were conducted to reveal regions of flow stasis which
would prolong the dwell time of blood within the pump. We had anticipated an
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
incremental dilution and clearance of dye over several cycles. However there was no dye
visible in any of the pumps after two cycles. This is understandable due to the very small
residual volume at the end of ejection, and the absence of crevices which would trap
stagnant blood.
Dye washoff (erosion) studies were undertaken to indirectly evaluate the shear stress on
the blood contacting surfaces, hence evaluate “surface washing.” This is a rather effective
and efficient method of evaluation since it does not require elaborate interpretation of
complex flow patterns, and most closely represents the potential of deposition due to
inadequate washing. These results also proved to be very sensitive at distinguishing the
differences between designs.
The presence of residual dye indicators was found to correspond to regions in which low
velocity was observed in the particle tracking studies, such as the center of the vortices.
Likewise, regions in which the dye was completely eroded corresponded to high surface
velocity, such as the periphery of the vortices. The residual dye within the inlet diffuser
also revealed diminished surface shear, caused by boundary layer separation, which was
not readily observable by the other techniques.
Conclusion
Design of pulsatile pumps dates to the origins of artificial heart development (Liotta,
1962). Many of the devices currently in clinical use have evolved over several decades,
benefiting from a great amount of research and incremental improvements.
In this study, we attempted to optimize a pump design in a relatively short time frame.
Geometric modifications to the baseline design were introduced primarily based on
intuition and experience. Due to the immature design of the initial baseline pump, it was
relatively easy to introduce dramatic improvements. However, others have shown that
there are many minor nuances to the design that are important for optimizing flow, and
hence reducing risk of thrombosis. (Deutsch et. al., 2006) Due to the vast selection of
design parameters, it is impractical to explore all possible design variations and
combinations. Numerical optimization may facilitate this process of “shape optimization”
by replacing time-consuming experiments with computational fluid dynamics (CFD)
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
simulation(1-4) Hund?2007?. CFD can also provide a more quantitative and elegant tool
to optimize the pump geometry. However, CFD simulation of pulsatile blood pumps is
particularly challenging due to dynamic interaction between the fluid and the moving,
flexible diaphragm. (Avrahami, 2006 and Medvitz 2007). Therefore flow visualization
experiments, combined with rapid prototyping, offers a pragmatic approach to rapidly
identify potential problems and gain dramatic improvements to the hemodynamics.
QED!
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
Figure Captions
Figure 1. Allient sorbent dialysis system. (a) Dual pneumatic pumps allow single needle access to
patient. (b) Construction of pump comprised of snap-fit housing pair with elastomeric diaphragm
separating pneumatic chamber and blood chamber.
Figure 2. Flow pattern in the baseline pump (A0) at various phases of the cycle. The flow direction is
from bottom (inlet) to the top (outlet.) Figures (a), (b), (c) and (d) (top row) designate the filling phase,
and (e), (f), (g) and (h) (bottom row) the ejection phase. Inset illustrates approximate phase of each
figure.
Figure 3. Flow pattern in the tangential inlet pump (A2) at various phases of the cycle. Images are
presented as in Figure 2.
Figure 4. Flow pattern in the tangential inlet/outlet pump (A3) at various phases of the cycle. Images
are presented as in Figure 2.
Figure 5. Flow pattern in the tangential inlet pump with domed diaphragm (B2) at various phases of
the cycle. Images are presented as in Figure 2. Images appear darker due to black coloring of the
diaphragm.
Figure 6. Dye washout results of the baseline pump (A0) at various time locations of one circle,
fluorescent dye was injected in the beginning of the diastolic procedure. (a), (b), (c), (d), (e) and (f)
diastolic procedure, (g), (h), (i), (j), (k) and (l) systolic procedure
Figure 7. Dye washout results of pump A2 at various time locations of one circle, fluorescent dye was
injected prior to the beginning of filling. Images (a), (b), (c), (d), (e) and (f) represent the filling phase,
and images (g), (h), (i), (j), (k) and (l) the ejection phase.
Figure 8. Dye washoff results of the baseline pump (A0) (a) Dye droplets deposited prior to pumping.
(b) Residual dye droplets after 3.5 minutes of operation. Well washed regions (e.g. 1, 2, and 3) found
interspersed with poorly washed regions (e.g. 5 and 6). (c) Separation zones responsible for poor
surfaced washing.
Figure 9. Dye washoff results of pump A2 (a) Dye droplets deposited prior to pumping. (b) Residual
dye droplets after 3.5 minutes of operation. Residue found in two small regions: near the inlet
diffuser and outlet collector.
Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
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Legend: Yellow = Maholtz; Teal = Shu; Green = Antaki; Magenta = Parks
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