Vanderbilt University – Senior Design 2010-2011
Determining Properties of
Wound Dressings for
Negative Pressure Wound
Therapy
Lora Aboulmouna, Ryan Frye, Lisa Lewicki
Mentor: Josh Smith, Vice President of Pioneer Technology
Advisor: Dr. Jack Fisher, Associate Clinical Professor of VU
Table of Contents
1. Abstract ..................................................................................................................................................... 3
2. Introduction ............................................................................................................................................... 4
3. Methodology ............................................................................................................................................. 5
4. Results ....................................................................................................................................................... 7
5. Discussion ............................................................................................................................................... 11
6. Conclusions ............................................................................................................................................. 12
7. Future Directions .................................................................................................................................... 12
8. Acknowledgements ................................................................................................................................. 12
9. References ............................................................................................................................................... 13
10. Appendix ............................................................................................................................................... 14
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1. Abstract
Negative pressure wound therapy (NPWT) is a form of wound healing therapy that involves the
application of a vacuum pressure to an open wound filled with a wound dressing and covered
with a film dressing (Malmsjo, 2008). Currently, surgical gauze and a foam patented by Kinetic
Concepts Inc. (KCI), are used as dressings for wounds undergoing NPWT. A new dressing,
Sorbact, is a hydrophobic, wide-meshed material that has shown promising results when used
clinically in NPWT, especially in fistula, or tube-like, wounds. These three materials were
compared in terms of fluid flow properties as well as density, wick time, and the amount of
compression experienced under negative pressure. The resistance to fluid flow of each material
was determined by inserting material inside a fistula model tube, with a vacuum pump attached,
and then measuring the flow rates over a range of clinical NPWT pressures (50 to 120 mmHg).
Gauze produced the highest resistance, but the slowest wick time. Sorbact and KCI had similar
resistances, but with Sorbact exhibiting a faster wick time. This suggests that Sorbact will
perform similarly to KCI foam in a fistula environment, whereas gauze is non-ideal for this
application.
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2. Introduction
Negative Pressure Wound Therapy (NPWT) is a form of wound healing therapy that involves the
application of a vacuum pressure to an open wound filled with a wound dressing and covered
with a film dressing (Malmsjo 2008). This vacuum, or negative pressure, is applied in order to
stretch the wound in two manners, through micro and macro deformation. Macro deformation
involves the inward mechanical stretching of the wound bed in order to reduce the wound size,
whereas micro deformation works on a cellular level. The benefits of NPWT are numerous and
include the promotion of a moist wound healing environment, reduced bacterial colony counts,
increased granulation tissue formation, removal of edema, stimulated cell-mediated immune
response, mechanical deformation of the wound edge tissue, decreased permeability of blood
vessels, and stimulation of angiogenesis and blood flow to the wound margins (Borgquist 2009).
NPWT is specifically designed for use on larger, chronic wounds, such as diabetic ulcers or
MRSA infections. It is not intended for use on most surgical wounds or injuries, unless other
methods have previously failed to promote healing in the wound.
NPWT, also known as Vacuum System Closure (VAC) Therapy, has only been commercialized
in the last twenty years, although the basic concept has been around for centuries. Kinetic
Concepts Inc. (KCI) located in San Antonio, Texas successfully commercialized NPWT through
the production of their reticulated foam dressing.
The two wound dressings currently used in NPWT are KCI’s reticulated foam dressing and
standard surgical gauze. The more common of these two is the KCI foam. The foam is
constructed from polyurethane, and offers a reticulated hydrophobic structure that allows for
both macro-strain and micro-strain. Gauze, on the other, is comprised of cotton, which is
hydrophilic (Ljungh et al, 2006). While gauze is easily obtainable and affordable for clinicians,
its high water retention can be a problem for NPWT. When the gauze becomes saturated, the
internal pores of the material become occlude and prevent the delivery of fluid and pressure
through the dressing interface, and therefore preventing micro-strain. This is not only a problem
for proper wound exudate removal, but also for reduction of infection as well as frequency of
dressing changes.
The choice of wound filler influences the biological effects of the therapy. In several clinical
studies, both foam and gauze stimulated growth of granulation tissue in the wound bed.
However, the granulation tissue formation from the foam dressing was consistently “thick but
fragile” whereas that of the gauze was “thinner and denser” (Borgquist, 2009). In addition,
removal of the KCI foam required more force than removal of the gauze. This not only results in
a greater disruption of the granulation tissues, but it also results in heightened pain for the
patient. Studies have also shown that different pressure ranges affect biological processes on the
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wound edge, including blood flow and macro deformation. A pressure of -80 mmHg was found
to provide maximal effects of macro deformation in NPWT (Borgquist et al, 2007).
A third dressing for NPWT, however, has recently entered the spotlight and may improve upon
both KCI foam and gauze. Pioneer Technology of Nashville, TN has obtained exclusive rights to
a proprietary wound dressing known as Sorbact. Sorbact is wide-meshed and is acetate gauze
treated with a fatty acid ester DACC (dialkyl carbamoyl chloride), making it hydrophobic. It is
hypothesized that the fluid dynamics of Sorbact, under NPWT, will closely resemble that of the
foam dressings available by KCI.
In order to compare Sorbact to its competitors, a number of factors need to be considered. First,
it must be understood how the materials behave when placed in a negative pressure environment.
Secondly, each material’s resistance to flow must be measured in order to determine whether or
not the material occludes the flow of exudate from the wound after it becomes saturated. Finally,
the natural wick time, or the time that it takes the material to dry via evaporation, should be
examined for each material to better understand the material’s affinities for water.
The primary objective of this project was to create a model that allows for a comparison of these
fluid dynamic properties of all three wound dressing interfaces. Sorbact is expected to have the
least resistance of the three materials, closely followed by KCI foam. Gauze is expected to have
a much larger resistance than both KCI foam and Sorbact.
3. Methodology
Material densities of each of the three materials were first determined. This was done through a
fluid displacement model, where each material was individually placed in a graduated cylinder
filled with water, air bubbles were eliminated from each of the materials, and water displacement
was measured. Mass and volume were used to determine the density of each of the materials.
The densities were then used to equate the three materials by volume, for use in the final
experimental setup.
Compression of each of the materials in a negative pressure environment was then measured. In
order to test this, a model wound bed was created out of Dragon Skin, a high performance
platinum cure silicone rubber (Pea, 2011). An equated amount of each material was placed inside
of the wound bed. A vacuum and a sealant drape were attached to the system. A pressure of
180mmHg, which falls into the clinical pressure range, was applied and material compression
was observed.
Intrinsic wick time was then investigated by saturating each material in water and then looking at
the rate at which each material dried. To do this, Sorbact, KCI foam, and gauze were first
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weighed at 0.31g, 0.13g, and 0.30g respectively. These weights were chosen because they are
equated, by volume. The materials were each saturated for 15 minutes, shaken loosely, and then
weighed directly afterward. The materials were weighed every 10 minutes until a fully dried
state was reached. (Note: Measurements of gauze were stopped before it reached a fully dried
state because it would have taken many additional hours to reach its final wick time.)
Flow
rates
were
measured using an inline flow meter at a
pressure of 26mmHg
for
each
of
the
materials, as well as a
“no-material”
setup,
serving as a control.
This setup is shown in
Figure 1. It consisted of
a fluid reservoir, which
attached to a tube
containing the material
(or an empty tube in the
case of the control),
Figure 1: Experiment design setup where Q is flow rate, h is water
which attached to a
height, and P is the pressure created by the weight of the water.
flow meter, which then
attached to a vacuum pump. The fluid reservoir contained water and was maintained at 32°C.
There was a constant stream of water pouring into the water reservoir, making it overflow, to
ensure that the height of the water was consistent for all trials. The tube containing the material
had a ½ inch inner diameter, to emulate a fistula-style model. The material was placed in the tube
at the same location for each trial. The flow meter had a range of 0-40mL/min. The vacuum
pump applied a selected pressure to the system. Since the vacuum pump could only move in
increments of approximately 20mmHg, a pressure valve was attached to allow for more discrete
pressure selection. Additionally, a pressure gauge was added so that the new pressure being
applied to the system could be accurately measured. Ten trials were conducted using the in-line
flow meter at a pressure of 26mmHg. Each trial consisted of measuring the flow rate when gauze
was inserted into the tube, when KCI foam was inserted, when Sorbact was inserted, and when
the tube was left empty. Each of these individual experiments was run for five minutes after the
flow initially stabilized, and then a flow reading was taken via the flow meter every 30 seconds.
The flow rates of each material, as well as the no-material scenario, were individually averaged
together for each trial.
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Since higher pressures needed to be tested in order to observe the materials in a clinical pressure
range, a new flow meter was ordered. This flow meter had a range of 0-200mL/min, and
therefore a smaller scale. Since the flow meter had a smaller scale and the differences between
fluid flows for each of the materials are inherently small, the flow rates measured for each of the
materials were all approximately equal. Because a flow meter containing a large range is a
necessary component when applying higher pressure to the system and the differences in the
flow rates for the materials are so small, a flow meter could not be used to measure flow in
clinical pressure ranges. To address this issue, a manual flow measuring technique was utilized.
A manual volumetric flow measurement technique was used to measure flow when applying
pressures of 50 mmHg, 79 mmHg, and 105 mmHg. The setup from the in-line flow meter
experiment was used in order to keep the amount of resistance in the system constant. Ten trials
were run at each of the pressures. Each trial consisted of taking a flow measurement when the
tube contained KCI foam, Sorbact, and gauze, and also for an empty-tube scenario. Each
individual experiment was run for one minute. The water that was pulled through the system in
that time, via the vacuum pump, was collected and measured in a graduated cylinder. The
amount of water displaced through the system and the amount of time it took for this to occur
allowed for the volumetric flow rates to be calculated.
Material resistances were then calculated using the following relationships:
𝑔ℎ1 +
𝑃 𝑣1
𝑃𝑎𝑡𝑚 𝑣2
+ = 𝑔 ℎ2 +
+
𝜌 2
𝜌
2
𝑃𝑠𝑦𝑠𝑡𝑒𝑚 = 𝑃 − 𝑃𝑣𝑎𝑐
𝑅=
𝑃𝑠𝑦𝑠𝑡𝑒𝑚
𝑄
where ρ = density of water (1 g/cm3), R is the system resistance, Q is the flow rate, h2 is the top
of the water reservoir, h1 is the bottom of the reservoir where the tube attachment is located, and
h is total reservoir height (h2 – h1 = h).
4. Results
In order to examine equal quantities of the
three materials used, the density of each
material was calculated, as shown in Table
1. Since the materials were completely
submerged in water, variations in the
material porosities were also factored in.
Table 1: Densities of each material calculated
from fluid displacement model.
Material
Density (g/ml)
Gauze
1.48
KCI Foam
0.62
Sorbact
1.36
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Gauze was found to be the densest material and KCI foam was found to be the least dense
material. In later experiments, the densities were used to equate the materials via equal volumes.
Below, Figure 2 shows the compression that each of the wound dressing materials underwent in
a mimicked wound environment. KCI foam was show to compress the least and gauze was
shown to compress the most. (Note: KCI foam utilizes a different vacuum tube attachment
because a tube cannot be inserted within the material.)
Figure 2: From left to right, Sorbact (density 1.36 g/mL), gauze (1.48 g/mL), and KCI (0.62
g/mL) compressed 16 mm, 19 mm, and 13 mm respectively, under negative pressure.
Flow Rate (mL/min)
Using the design set-up
38.5
shown in Figure 1, flow
38
rates were recorded in 30
37.5
second increments for a total
37
period of five minutes, for
36.5
each of the three materials.
36
This
information
was
35.5
plotted, as shown in Figure
35
3, to determine if averaging
34.5
the flow rates over the five34
minute time period would
0
2
4
6
provide a relatively accurate
Time (min)
overall flow rate. Since the
flow
rates
remained
relatively constant over the Figure 3: Flow rates over a 5 minute range for Sorbact
given time range, it was (blue), KCI foam (red), and gauze (green).
decided that taking an
average of the flow readings over the five-minute time period would be adequate in determining
the flow rate through the materials.
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As mentioned previously, the flow rates measured at each pressure were used to determine the
resistances of each material. Figure 4 shows the calculated material resistances from the flow
rates measured using the in-line flow meter (Figure 4a), as well as from the flow rates measured
using the manual volumetric flow technique (Figure 4b-d). This data was used in a paired t-test
to compare material resistances at varying pressures.
a)
c)
b)
d)
Figure 4: Calculated material resistances of each material at an applied pressure of a)
26mmHg, b) 50 mmHg, c) 79 mmHg, and d) 105 mmHg.
Table 2, on the following page, shows all p-values reported in the statistical analysis. The null
hypothesis, Ho, states that there is no difference between the material resistances at the selected
pressure. The alternate hypothesis, H1, states that at least one of the materials is significantly
different from another material. At 26.16 mmHg, 50.49 mmHg, 79.475 mmHg, and 104.72
mmHg, the null hypothesis for Sorbact versus KCI foam was accepted, with p-values greater
than 0.05. This means that there is not a significant difference between the two materials, at all
pressures tested. At a pressure of 104.72 mmHg, the null hypothesis was accepted for the
resistance values of the tube versus Sorbact and KCI foam, indicating no significant difference.
Gauze was shown to be significantly more resistant than the other two materials and the control.
Finally, the resistances for Sorbact and KCI were not shown to be significantly different from
each other (p> 0.05) at all tested pressures.
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Table 2: P-values taken for each of the pressures taken from Figure 4
Material
Tube vs Sorbact
Tube vs KCI
Tube vs Gauze
Sobract vs KCI
Sorbact vs Gauze
KCI vs Gauze
26.16 mmHg
50.49 mmHg
79.475 mmHg
104.72 mmHg
p-value
Ho
p-value
Ho
p-value
Ho
p-value
Ho
0.000404 reject
0.00021 reject 0.001054 reject
0.1494 accept
8.27E-05 reject 0.001147 reject 0.006165 reject
0.2125 accept
9.98E-05 reject
7.60E-07 reject
3.34E-08 reject
5.28E-10 reject
0.9019 accept
0.6745 accept
0.8857 accept
0.4012 accept
9.41E-05 reject
4.35E-06 reject
1.68E-08 reject
9.10E-08 reject
0.000404 reject
5.69E-06 reject
4.58E-09 reject
2.13E-08 reject
Figure 5 shows four different pressures for each material and the calculated resistances. A linear
relationship is shown between resistance and pressure applied to the system for each of the
materials, as well as for the control. As the pressure increased, the resistance decreased. These
pressures represent pressures in both Sorbact's clinical pressure range, -80 to -120 mmHg, and
KCI foam’s clinical pressure range, -50 to -125 mmHg.
a)
c)
b)
d)
Figure 5: Material resistances as a function of vacuum pressure for a) control (no material),
b) KCI, c) Sorbact and d) gauze. All three materials (and the control) display a linear
relationship over the range of clinical vacuum pressure ranges (50 to 120 mmHg).
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Wick time, or natural drying rate, for Sorbact, KCI foam, and gauze were 0.67 hours, 3.2 hours,
and 5.7 hours, respectively. Figure 6 shows the wick times for the three wound dressing
materials. The rate at which the materials dried showed that Sorbact had the fastest rate of
decrease, 0.588g/hr, and that KCI foam had the slowest drying rate, 0.114g/hr. (See Appendix
for more detailed graphs of wick time.)
Figure 6: Wick time for Sorbact, KCI foam and gauze.
5. Discussion
Upon analyzing the results, observations could be made on the compression, wick time, and
resistance of each of the materials.
Gauze compressed the most during the compression test (19 mm) as well as had the highest
density (1.48 g/mL), the slowest wick time (5.7 hours) and the highest resistance (1.717 mmHgmin/mL at 79 mmHg). Since gauze experienced the greatest compression, it was expected that
the resistance would also be largest for gauze. The calculated resistances from the flow
measurements did, in fact, show that gauze was the most resistant of the materials. The
summarized data above suggests that gauze retains the most fluid and would occlude the flow of
exudate from the wound the most, making it non-ideal for NPWT.
KCI foam compressed the least under the compression test (13 mm) but had the lowest density
(0.62 g/mL). KCI had a faster wick time than gauze (3.2 hours) and a resistance similar to that of
Sorbact (1.567 mmHg-min/mL at 79 mmHg). This suggests that both KCI foam and Sorbact
would perform similarly under NPWT.
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Sorbact’s compression amount was in between that of KCI foam and gauze (16 mm). It had the
quickest wick time (0.67 hours) and a resistance similar to KCI foam (1.566 mmHg-min/mL at
79 mmHg). This shows that although Sorbact’s resistance is the same as KCI foam, it retains less
fluid, potentially leading to better fluid absorption from the wound and less build up of exudate.
For the three lower pressures tested (26 mmHg, 50 mmHg and 79 mmHg) a significant
difference was seen between the tube and each of the three materials. At 105 mmHg, due to the
high flow rates, it was hard to acquire a measurable difference; therefore, the small amount of
material present was likely not able to provide enough resistance to the system. Gauze was
shown to have a much larger resistance than both KCI foam and Sorbact. A significant difference
was not found between the resistances of Sorbact and KCI foam.
6. Conclusions
Our data suggests that gauze has a much larger resistance than both KCI foam and Sorbact, as
initially expected. Although we anticipated Sorbact to have the least resistance of the three
materials, closely followed by KCI foam, it was found that Sorbact and KCI foam do not have
significantly different resistances. While KCI foam and Sorbact have similar resistances, Sorbact
has the additional advantage of a decreased wick time. Based on this, it is thought that Sorbact
provides additional benefits for use in the application of negative pressure wound therapy, in a
fistula wound model.
7. Future Directions
Although the design used in this experiment was unable to determine the saturation points of
each material, this information would allow for the examination of the change in resistance over
time. Because of this, it is recommended that a design is created that allows for saturation points
to be tested. Additional improvements to this experiment involve using a wider range of clinical
pressures, better mimicking realistic wound conditions, and also investigating wounds with
varying size and severity.
8. Acknowledgements
We would like to thank Josh Smith, Dr. Jack Fisher, and Pioneer Technology for sponsoring our
research, as well as providing guidance throughout the process. We would also like to give a
special thanks to John Dunbar, Dr. Amanda Lowery, Dr. Robert Galloway, and Dr. Frederick
Haselton for providing us with their expert knowledge. Additionally, we would like to thank Pat
Tellinghuisen, Mark Holmes, and the Vanderbilt Biomedical Engineering department for
providing us with necessary supplies throughout the project.
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9. References
Borgquist O, Ingemansson R, Malmsjö M. “Wound edge microvascular blood flow during
negative pressure wound therapy: examining the effects of pressures from -10 to -175
mmHg.” Plastic Reconstruction Surgery. (2007) In press.
Borgquist, Ola, Lotta Gustafsson, Richard Ingemansson, and Malin Malmsjo. "Tissue Ingrowth
Into Foam but Not Into Gauze During Negative Pressure Wound Therapy." Wounds21.11
(2009): 302-09.
Ljungh, A, N Yanagisawa, and T Wadström. "Using the principle of hydrophobic interaction to
bind and remove wound bacteria." Journal of Wound Care 15.4 (2006): n. pag. Web. 6
Nov 2010.
Malmsjo, Malin, Richard Ingemansson, Robin Martin, and Elizabeth Huddleston. "Negative
pressure Wound Therapy Using Gauze or Open-cell Polyurethane Foam: Similar Early
Effects on Pressure Transduction and Tissue Contraction in an Experimental Porcine
Wound Model." Wound Repair and Regeneration 17 (2009): 200-05.
Pea, David. "Dragon Skin® 10, 20, 30 Silicone Product Information | Smooth-On." Smooth-On,
Inc. - Mold Making & Casting Materials Rubber, Plastic, Lifecasting, and More. Web.
25 Apr. 2011. <http://www.smooth-on.com/Platinum-Cure-Sili/c1115_1129/index.html>.
Smith, Jan, and Peter Robertsson. Method for Dressing a Wound. , 2010. Web. 27 Oct 2010.
<http://www.google.com/patents/about?id=ApHVAAAAEBAJ&dq=sorbact>.
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10. Appendix
Rate of wick time for a) Sorbact, b) KCI foam, and c) gauze
a)
b)
c)
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