Development of a Benchtop Model for Evaluating the Compatibility of Wound Dressing Materials with Negative Pressure Wound Therapy Systems

Summary

This study presents a benchtop model designed to evaluate the compatibility of wound dressing materials with negative pressure wound therapy systems by assessing pressure and fluid collection over 72 h under continuous and intermittent pressure settings.

Abstract

Negative pressure wound therapy (NPWT) systems facilitate wound healing by applying sub-atmospheric pressure to the wound bed, which promotes granulation tissue formation and reduces inflammation. Wound dressings can be used with these systems to enhance healing; however, the effects of dressings on NPWT device performance are challenging to assess. The purpose of this study was to develop a benchtop flesh analog model for testing the compatibility of wound dressing materials with NPWT devices. In this study, a chitosan-based advanced wound care device was evaluated for its effects on NPWT performance under maximum and minimum therapy pressures. The goal was to use the model to compare pressure readings and fluid collection for samples with and without the chitosan wound care device. The benchtop model was constructed using a plastic box connected to multiple pressure gauges. A circular defect was created on a piece of pork belly, used as the flesh analog, and inserted into the box. The defect was filled with standard NPWT foam or foam combined with the wound dressing. Simulated body fluid containing bovine serum was added to the box, which was then tested at either maximum (-200 mmHg) or minimum (-25 mmHg) pressures for 72 h. Pressure and fluid collection were recorded every 12 h. The NPWT system successfully maintained pressure over the 72 h test period, both with and without the test dressings. The addition of the wound dressings did not impact fluid collection. The test box proved effective as a benchtop model, as it could be sealed and maintained vacuum conditions over the 72 h testing period. This model successfully demonstrated its utility in evaluating the compatibility of wound dressing materials with NPWT systems.

Introduction

Different therapeutic approaches exist to aid in the management and healing process of wounds. Such therapeutic approaches include advanced wound dressings, growth factors, hyperbaric oxygen therapy, skin substitutes, and negative pressure wound therapy (NPWT)¹. NPWT refers to wound dressing systems that continuously or intermittently apply sub-atmospheric pressure to the system, which provides negative pressure to the surface of the wound. NPWT has become a popular treatment modality for the management of acute or chronic wounds². The NPWT system consists of an open cell foam, adhesive wound dressing, a fluid collection system, and a suction pump³. The suction pump, or vacuum, is used to maintain a steady pressure on the wound, which helps increase blood flow and reduce the risk of infection⁴. NPWT promotes granulation tissue formation by removing fluid from the wound and reducing swelling¹. Clinically, the amount of suction pressure used for wounds ranges from -20 mmHg to -200 mmHg, but the most relevant pressure tested is -125 mmHg⁵.

Ex vivo experiments of NPWT are a challenge due to a lack of adequate benchtop models for testing. Current methods for testing NPWT systems include finite element analysis (FEA) computer simulations, which have been used to test how NPWT affects incision sites⁶. Other models include benchtop agar-based wound models, which can be used to test fluid uptake⁷. In vivo, porcine models also have been used to examine wound healing⁸. These models have advantages such as being easy to simulate on a computer for predicting how a wound should heal in theory, as well as testing fluid being pulled through a model material. In vivo testing is definitive for determining whether the system works in live subjects⁸. These models all have disadvantages as well. A computer simulation may not accurately represent how a wound would heal in real life. An agar-based model may show good fluid collection being pulled through the wound but may not represent how fluid would be pulled through tissue and muscle⁷. In vivo models are expensive and require significant resources to complete a study. Also, it can be difficult to keep animals semi-immobile, so there may be challenges with them pulling at the system, which may have confounding results.

A benchtop model is needed for NPWT so that new materials can be tested for use with the system using actual tissue. The new model should be able to reflect how fluid collection is affected by tissue and muscle. The new model should also be able to provide pressure readings inside the wound bed to determine whether the wound was receiving as much pressure as the vacuum pump was supplying. New materials/devices may also be tested, such as additional wound dressings, different types of foam, and different adhesive dressings on top of the wound.

Certain wounds require additional wound dressings to aid in the healing process by reducing the risk of infection. Another reason additional wound dressing materials may be required is to prevent tissue ingrowth between the surface of the wound bed and the open-cell foam. This additional dressing reduces the risk of the wound bed adhering to the open-cell foam, which helps reduce damage and pain when stopping the NPWT system⁹. These additional dressings can be placed around the open-cell foam to act as a barrier membrane between the wound bed and the foam. Certain materials have been used as an interface between the wound bed and foam, such as paraffin or Vaseline-embedded gauze. Paraffin has shown positive potential as a wound dressing by not affecting the transfer of pressure from the system to the ound⁹. However, Vaseline-embedded gauze was reported to inhibit fluid collection and thus was not considered to be an appropriate additional material⁹.

Chitosan-based wound dressings may be a good additional dressing to add during NPWT due to their antimicrobial effects and biocompatibility^10,11. Chitosan is an N-deacetylated derivative of chitin, which is a natural polysaccharide found in fungi and arthropods^12,13. Chitosan has exhibited inherent antibacterial properties in a broad spectrum of gram-negative and gram-positive bacteria¹⁴. Therefore, chitosan membranes have become popular in the treatment of wounds because they can be easily produced, have a long shelf life, and show innate antimicrobial effects¹⁰. These membranes also show good biocompatibility biodegradation, and are non-toxic¹⁰.

In this study, Foundation DRS, a chitosan and glycosaminoglycan advanced wound care device, was examined to determine its biocompatibility with NPWT. Foundation DRS is a biodegradable dermal regeneration scaffold manufactured for ideal handling characteristics and porosity to promote cellular invasion and neo-angiogenesis in wounds. This device is advantageous for healing in a range of different injuries and uses. It was created for intended use in a wide range of wounds, such as pressure ulcers, diabetic foot ulcers, first-degree burns, trauma wounds, dehisced wounds, and surgical wounds^10,11. Foundation DRS is a good option for use in NPWT due to its manufacturing process, which prevents the device from turning into a hydrogel when it is wet. This device maintains an open pore structure when wetted, which should allow fluid to flow during the application of NPWT^12,13.

The objective of this study was to develop a benchtop flesh analog model that could be used to test the compatibility of wound dressing materials with NPWT devices. Clinically, pressures range from -80 mmHg to -125 mmHg for most NPWT applications⁴. To simulate worst-case clinical use conditions, a higher and lower pressure setting were used (-25 mmHg and -200 mmHg). Another objective of this study was to determine if the addition of the chitosan wound care device interfered with the pressure readings and fluid collection of the NPWT. Disruptions in fluid collection or losses of pressure during NPWT could lead to poor wound healing and clinical outcomes. The fluid collection should be similar to the test groups with and without the chitosan wound care device. Pressure readings should also be similar across the test groups over 72 h. In clinical settings, the wound dressing is changed every 48-72 h, so each sample was tested for 72 h in this study³. During testing, the pressure readings should be observed to ensure there is not a drop in pressure.

Protocol

The details of the reagents and the equipment used in this study are listed in the Table of Materials.

1. Creation of the test box

1. Obtain a 3.2-cup plastic container.
2. Create a 2-inch diameter hole in the center of the container lid. Also, make two 3/8 holes in two corners of the container lid approximately 1/2 inch from the edge seal. Use a hole saw to create the holes.
   NOTE: A schematic showing the overall testing setup using a commercial NPWT machine connected to a lab-built benchtop flesh analog box is shown in Figure 1. This schematic outlines how the box is used for experiments. The box created for this experiment is shown in Figure 2.
3. On the first of the 3/8 holes, connect a pressure gauge directly to the hole.
   NOTE: This gauge was used to monitor for pressure drops outside of the test tissue, which would indicate leaks in the tissue.
4. On the second 3/8 hole, feed a small flexible IV tube with an outer diameter of less than 3/8 through the hole to a length of 7 inches on the inner side of the lid. Then, fit the pressure tube to a low-pressure gauge outside the container.
   NOTE: The pressure tube was placed in the wound bed during testing.

2. Flesh analog preparation

1. Use commercially available salted pork belly, known hereto as tissue, to simulate the muscle and fat tissue for NPWT testing.
2. Create a circular wound defect in the surface of the tissue using a #21 blade scalpel that is approximately 1.5 inches wide by 0.75 inches deep. Then, fenestrate the tissue through the fat on each side with a #21 blade scalpel.
3. After the wound defect is created, wipe down the tissue to remove excess fat from the skin, and then soak overnight in deionized water to remove excess salt.

3. Loading of the test chamber

1. Fill the bottom of the test chamber with open-cell foam that is 1.5 inches thick. Then, place the tissue on top of the foam.
   NOTE: Manually center the tissue sample so the wound defect created is directly under the hole in the top of the lid.
2. For the experimental groups, add the chitosan wound care device inside the wound defect so that the bottom and sides of the defect are covered. Then, fill the rest of the defect with the open cell foam.
3. Insert the pressure tube connected to the pressure gauge on the testing chamber into the open cell foam that is used to fill the defect. Ensure that this tube is placed approximately halfway down from the surface of the wound defect.
4. Cover the tissue with the adhesive wound dressing. Then, create a small cut on the adhesive dressing, directly on top of the middle of the open cell foam, filling the wound defect.
5. Thread the vacuum nozzle through the lid of the testing chamber and place it on top of the adhesive dressing, where the small cut was made. After placing the vacuum nozzle, close the lid of the testing chamber to press the adhesive wound dressing and vacuum nozzle down, which helps create a seal.
6. Connect the 500 mL fluid collection cannister to the vacuum pump and then connect the vacuum nozzle to the fluid collection cannister.

4. Creation of the simulated body fluid

1. Create a simulated body fluid according to Marques et al.¹⁵.
2. Make the simulated body fluid by combining 8.035 g of NaCl, 0.355 g of NaHCO₃, 0.225 g of KCl, 0.231 g of K₂HPO₄3H₂O, 0.311 g of Cl₂Mg6H₂O, 0.292 g of CaCl, 0.072 g of NaSO₄^2-, 6.118 g of (HOCH₂)₃CNH₂, and 39 mL of 1 M HCl in 960 mL of deionized water to bring the total solution to 1 L.
   NOTE: The composition of the simulated body fluid is shown in Table 1.
3. Then, combine the simulated body fluid with bovine serum in a 3:1 ratio. Supplement the final solution with 5% of 10x antibiotics/antimycotics for microbial control. Stir the solution after adding the bovine serum and antibiotics/antimycotics, and then store it in a refrigerator.
   NOTE: The final solution will be referred to as the complete solution. This solution should not be kept sterile and should be made fresh before each sample is tested.

5. Test conditions

1. Adjust the settings on the vacuum pump for the samples depending on the test condition.
   NOTE: The test groups are: Group 1 Control (n = 3): Foam alone with continuous suction at -200 mmHg; Group 2 Control (n = 3): Foam alone with intermittent suction from 0 to -200 mmHg; Group 3 (n = 3): Chitosan Wound Care Device under Foam with continuous suction at -200 mmHg; Group 4 (n = 3): Chitosan Wound Care Device under Foam with intermittent suction from 0 to -200 mmHg; Group 5 Control (n = 3): Foam alone with continuous suction at -25 mmHg; Group 6 Control (n = 3): Foam alone with intermittent suction from 0 to -25 mmHg; Group 7 (n = 3): Chitosan Wound Care Device under Foam with continuous suction at -25 mmHg; Group 8 (n = 3): Chitosan Wound Care Device under Foam with intermittent suction from 0 to -25 mmHg.
2. For maximum pressure test groups, set the pressure at -200 mmHg. For minimum pressure testing groups, set the pressure at -25 mmHg. Then, place the vacuum pump settings on intermittent or continuous pressure. Run all samples for 72 h.
   NOTE: The continuous setting applied pressure continuously for 72 h. The intermittent setting applied pressure at a 5/2 ratio (5 min of pressure, followed by 2 min with no pressure) for 72 h. The maximum and minimum values were chosen based on the pressure range that clinical NPWT systems can use. A 72 h cycle was chosen based on the length of time NPWT is typically used clinically before performing a bandage change³.
3. During testing, record the pressure on the pressure gauge and the amount of fluid in the fluid collection canister every 12 h for 72 h.
4. If the amount of body fluid analog drops below 75% of the top of the testing chamber, as observed visually, remove the secondary pressure gauge and add a complete solution to the chamber.
   NOTE: The preparation of samples and testing setup can be seen in Figure 3.
5. After 72 h, turn the vacuum pump off and disconnect the fluid collection canister from the vacuum nozzle. Remove the fluid collection canister from the vacuum pump.
6. Remove the tissue from the testing chamber and pull the adhesive wound dressing off. Then, take out the open cell foam and observe whether the chitosan wound care device was still intact. It is considered intact if it can be removed without breaking, ripping, or tearing; however, minor tears or thinning are acceptable if the membrane can be removed completely.

6. Statistical analysis

1. Use the pressure values that were recorded every 12 h during the testing period from the three test specimens per test condition for statistical analysis. For statistical analysis, the final fluid collection value from the three test specimens was used per test condition.
   NOTE: For all statistical analyses, the level of significance was set at α = 0.05.
2. Calculate the mean and standard deviations(n = 3/group) at each time point. Before performing the statistical analysis, perform normality testing for each group using the Shapiro-Wilk test (e.g., continuous suction at -200 mmHg, continuous suction at -25 mmHg, intermittent suction at -200 mmHg, and intermittent suction at -25 mmHg) to determine if ANOVA or Kruskal-Wallis test is appropriate.
3. Analyze data for experimental and control groups subjected to the same pressure test conditions (e.g., continuous suction at -200 mmHg; continuous suction at -25 mmHg; intermittent suction at -200 mmHg or intermittent suction at -25 mmHg) using a two-way ANOVA or Kruskal Wallis test using membrane type and time as main factors.
4. If statistical differences were identified, perform post-hoc analyses. Use Tukey's HSD post-hoc test after the ANOVA or the Dunn post-hoc test after the Kruskal-Wallis test to determine which groups are different.
5. Using the final fluid collection values for each sample in the control and experimental groups, perform a two-tailed t-test assuming unequal variances.
   NOTE: Pressure was analyzed at each time point to ensure there was no significant drop in pressure across the duration of the testing period. While fluid collection was examined at each time, it was only analyzed at the final time point. This is because each tissue had different fat and muscle profiles, resulting in different fluid collection rates, making overall fluid collection more useful for analysis than fluid collection by time points.

Results

The goal of the study was to develop a benchtop model for NPWT that uses a tissue analog and to use the model to investigate the compatibility of wound dressing materials with a negative pressure wound therapy machine. The model was used to study if the NPWT machine was able to maintain pressure over time with the addition of a wound care device. The model was also used to determine if the pressure generated and fluid collected by the NPWT machine in the presence of a wound care device were different as compared to the a...

Discussion

There are a few benchtop models for NPWT, but they have significant limitations. Loveluck et al. developed an FEA computer model to determine how NPWT affected sutured incision sites but did not account for additional wound dressing materials⁶. Rycerz et al. developed agar-based models to evaluate instillation solution distribution to wounds during NPWT⁷. While the agar provided a medium for assessing the distribution of water-soluble materials/dyes in the different models,...

Materials

Name	Company	Catalog Number	Comments
100x antibiotics/mycotics	Gibco	15240062	This is the 100X antibiotics/antimycotics used in the simulated body fluid
3 M KCI ACTIV.A.C Therapy System	KCI Mdical Products	VFTR006619	This is the vacuum pump used in the study.
3 M KCI InfoV.A.C Canister w/Gel 500 mL	eSutures.com	M8275063	These are the fluid collection canisters used in the study
3 M KCI V.A.C GranuFoam Medium Dressing Kit, SensaT.R.A.C	eSutures.com	M8275052	These are the wound dressing packs with the vacuum nozzle including the open cell foam.
Bovine Serum	Gibco	16170086	This was used to mix with the simulated body fluid and the antibiotics/antimycotics
Calcium Chloride	Fisher Scientific	C614-500	This was used to create the simulated body fluid
Excel/Powerpoint	Microsoft Office	N/A	This was used to run the statistics and create the schematic for Figure 1
Foundation DRS Solo	BioNova Medical	N/A	This is the advanced chitosan wound care device used in the study.
Hydrochloric Acid	Fisher Scientific	SA54-1	This was used to create the simulated body fluid
Magensium Chloride	Fisher Scientific	M33-500	This was used to create the simulated body fluid
Phosphate buffered saline	Thermo Scientific	J62036.K3	This was used to dilute the 100x antibiotic/antimycotic to 10x
Potassium Chloride	SIGMA	P-3911	This was used to create the simulated body fluid
Potassium Phosphate Dibasic	Fisher BioReagents	BP363-500	This was used to create the simulated body fluid
PRM Vacuum Gauge 0 to -10 in Hg	PRM Filtration	PGCNBTY630652J10HG	Two pressure gauges are needed for the testing chamber.
Salted Pork Belly	Hormel Food Corporations	UPC: 0003760037988	Salted pork belly can be bought from Kroger. It cannot be sliced. It is best to pick samples that have less fat, and more muscle.
Sodium Bicarbonate	SIGMA	S5761-500G	This was used to create the simulated body fluid
Sodium Chloride	Fisher Scientific	S640-500	This was used to create the simulated body fluid
Sodium Sulfate	Fisher Scientific	BP166-100	This was used to create the simulated body fluid
Tris(hydroxymethyl) aminomethane	Fisher Scientific	BP152-500	This was used to create the simulated body fluid
Tupperware Brands Corp, Kissimmee , FL	Tupperware	N/A	This is the box used as the testing chamber.