Supplemental Online Appendix: Initial Clinical Experience Using a Novel Ultraportable Negative Pressure Wound Therapy Device

SUPPLEMENTAL ONLINE APPENDIX:

SNaP™ SYSTEM vs. TRADITIONAL ELECTRONICALLY POWERED PUMPS   The delivery of NPWT differs between the SNaP™ System and traditional electrically-powered pumps. Traditional pumps utilize a positive displacement mechanism: either a piston or a diaphragm cyclically expands a cavity into which gas is drawn, sealed off, and exhausted. Removal of gas molecules from the system reduces the density of the remaining gas and the mass flow rate is eventually cancelled by leakage and backstreaming. In wounds with exudate, fluid buildup reduces the effective enclosure volume available for gas, increasing the pressure. This is counteracted by running the pump continuously at a given volume flow rate.   The SNaP™ System, in contrast, reduces gas density by maintaining equilibrium between constant force springs and generates force from decreased pressure on the opposite side. If exudate buildup at the wound increases pressure, the opposing tension force further expands the system, resulting in reduced gas density and pressure within the system. Potential energy stored in the constant force springs provides the only energy required in the SNaP™ System. Although the ultimate negative pressure delivered to the wound bed should be nearly identical, because differences do exist, this study included the experiment below to evaluate the comparability of pressure, pressure gradient, temperature, and foam deformation patterns between the SNaP™ System and a traditional electrically powered NPWT system. Although the SNaP™ Wound Care System utilizes gauze, the experiments were performed with medical grade foam for comparison purposes only. BENCHTOP COMPARISON METHODS Pressure and Pressure Gradient through Foam Testing   All comparisons were performed between the SNaP™ System and a Medela® Vario 18 Powered Pump. Polyurethane foam (4 x 4 x 1 cm, ~500 micron pore size) and a thin-film polyurethane sealant layer were placed over a polycarbonate test surface to simulate a wound treatment scenario. Although the SNaP™ System is a gauze based system, foam was utilized in these tests for experimental comparison testing to the most widely used NPWT systems. A port served as a conduit for delivery of negative pressure and allowed for the pressure above the foam to be measured. The underside of the test surface was equipped so pressure level beneath the foam could be measured, enabling calculation of the pressure gradient from the top to the bottom of the foam. The SNaP™ System and the powered pump were activated at nominal -125mmHg. Pressure data at the top and bottom of the foam was recorded at a rate of one sample per second for 4000 samples. Data was measured with a calibrated data logging manometer (Sper Scientific Model 840080) and recorded with Handheld Meter Data Logging Software (Sper Scientific). Wound Bed Temperature Change Testing    The SNaP™ System utilizes forced expansion of a closed volume of gas to produce negative pressure. Testing was done to evaluate if the expansion was isothermal and if all of the work was transferred to reducing system pressure rather than changing system temperature. Similarly, this test evaluated if the electrically-powered pump was also isothermal. To simulate a wound treatment scenario, a wound sealing kit consisting of polyurethane foam (4 x 4 x 1 cm, 500 micron pore size) and a thin-film polyurethane sealant layer with an attachment port was placed over a polycarbonate test surface. A standard J-type thermocouple (Sper Scientific) was placed underneath the foam and connected to a digital thermometer (Sper Scientific Model 80005). Either the SNaP™ System or the powered pump was connected to the wound sealing kit and activated to -125 mmHg setting. Temperature readings were taken immediately before and 5 seconds post activation. All tests were performed at ambient temperature of 22.9˚C. Foam Deformation Testing   A hallmark of NPWT is retraction of wound edges and the concurrent contraction of the foam under negative pressure. Testing to evaluate the volume of foam deformation by both the SNaP™ and the electrically-powered pump was undertaken. Polyurethane foam (uncompressed: 6.6 x 6.6 x 1.0 cm, 500 micron pore size) was placed under a thin-film polyurethane sealant layer with an attachment port. Measurements of foam height h and major and minor diameters D₁ and D₂ were taken before and after sequential activations of the SNaP™ System or the electrically-powered pump. Foam volume was estimated by approximating the foam piece as a right elliptical prism where   Foam dimensions were measured both with calipers (Mitutoyo) as well as optically through digital camera images. BENCHTOP TESTING RESULTS Pressure and Pressure Gradient through Foam was Equivalent    The mean pressure delivered by the SNaP™ System was -130.5±1.4 mmHg and by the electrically-powered pump was -129.2 mmHg ± 0.3 mmHg (Appendix Figure 1). The mean pressure differential between the top and bottom of the foam for the electrically-powered pump was 0.8mmHg, or 0.64% of the total pressure applied. The mean pressure difference between the top and the bottom of the foam for the SNaP™ System was 0.4mmHg, or 0.32% of the total pressure applied. In both cases, the pressure differential was very small compared to the total pressure applied. Therefore, the SNaP™ System and the electrically-powered pump produce nearly identical effects in terms of measured pressure as well as pressure differential through a foam contact layer. Wound Bed Temperature Change Did Not Occur with Activation of Either System   Neither the SNaP™ nor the powered pump demonstrated any appreciable change in temperature at the wound bed upon activation of the device. Thus, both mechanisms were able to deliver negative pressure with the same lack of effect on wound temperature (Appendix Figure 2). Volume of Foam Deformation with Activation was Equivalent   The mean percentage volume difference and activation pressure for the SNaP™ System were 29% and -123mmHg, respectively. The mean percentage volume difference and activation pressure for the powered pump was 28% and -121mmHg, respectively. In addition to volume approximations, the visual foam deformation produced by pump and the SNaP™ System was indistinguishable (Appendix Figure 3). In both scenarios, the foam assumed a hard, raisin-like texture. SUMMARY   Because the mechanism for delivering NPWT is slightly different between the SNaP™ System and traditional electrically-powered pumps, we were interested in evaluating whether the delivered negative pressure had the same effect at the level of the wound bed. Benchtop testing was performed which demonstrated very similar results between the two systems in terms of pressure at the level of the foam interface layer and wound bed, temperature at the wound bed, and deformational characteristics of the foam interface layer upon application of negative pressure.   *Spiracur and SNaP are trademarks of Spiracur Inc. All rights reserved. Online Appendix Figure 1: Pressure delivery profile of the electrically-powered pump (left) and SNaP™ System (right) over time demonstrating near identical negative pressure delivery. Blue data points represent measurements taken at the top of the foam. Red data points represent measurements taken at the bottom of the foam at the wound base. Online Appendix Figure 2: Table shows temperature measurement trial results with no change in temperature with delivery of negative pressure for both the SNaP™ System and the electrically-powered pump. Online Appendix Figure 3: The images (top) shows typical case where the foam is unloaded (left), negative pressure is applied by the SNaP™ System (center) and negative pressure applied by the electrically-powered pump (right). Table (bottom) shows volume measurement trial results. Note near identical results between the SNaP™ System and electrically-powered pump. Original Article:https://www.woundsresearch.com/content/initial-clinical-experience-using-novel-ultraportable-negative-pressure-wound-therapy-device