Abstract
Air cell cushions are widely used in wheelchair seating and medical support surfaces to reduce interface pressure and prevent pressure injuries. Their effectiveness depends on maintaining proper internal air pressure, which can fluctuate with environmental changes such as temperature and atmospheric pressure. These fluctuations pose a risk in clinical and transport settings where cushions may be exposed to extreme conditions. This study examines the behavior of air cell cushions made from silicone, polyurethane, and TPU-coated fabric under different environmental conditions. Using a proportional–integral–derivative (PID) control algorithm, we regulated the internal air-cell pressure to remain constant under changing temperature (15–35 °C) and atmospheric pressure (101–75 kPa) conditions. Results indicate that the cushion material significantly influences pressure stability, with TPU-coated fabric being the most sensitive to environmental changes. Applying the closed-loop control algorithm, it is possible to maintain the set pressure within ±0.05 kPa regardless of the cushion material, loading conditions, and environmental variations.
Introduction
Pressure injuries (PIs) pose a serious health risk for individuals with limited or no mobility. PIs can lead to complications such as surgery, amputation, infection, and sometimes death. 1 As the main contributing factors of PIs include prolonged pressure and shear on soft tissues, clinical guidelines recommend frequent repositioning and the use of support surfaces like wheelchair cushions and mattresses that help evenly distribute pressure. 2 Among the three main types of commercial cushions ‒ gel, foam, and air ‒ air cell cushions have demonstrated greater effectiveness in reducing PI risk.3,4 Air cell cushions are typically made from soft, flexible materials and comprise an array of interconnected air cells. Air movement among the cells redistributes pressure evenly, and inflation or deflation of air cells can be performed to achieve the desired interface pressure. The redistribution and lowering of air cell pressure enable immersion of a human body into the cushion, distributing pressure across the body’s surface area to reduce high pressure points. 5 However, air cell cushions come with several inherent disadvantages. Setting and maintaining optimal internal air pressure is crucial for a user to prevent PI formation. Air cushions are prone to leaking, which can lead to deflation and bottoming out. Conversely, if the cushion becomes overinflated due to misuse, the interface pressure can become too high. 5 To ensure optimal performance, users often need to visit seating clinics where professionals use pressure mapping tools to calibrate the cushion. Regardless, users can still be at risk of a failing cushion if there is no indication of a leak, or under- or over-inflation.
Several factors can contribute to dangerous levels of unintentional under- and over-inflation, particularly extreme changes in environmental pressure and temperature. Air contracts in cold temperatures and under high barometric pressure, while it expands in warm temperatures and at lower barometric pressures. 5 Wheelchair users encounter sudden environmental changes in both everyday life and unique scenarios, making it essential to take proper precautions when using air cell cushions. Temperature fluctuations can occur in common situations such as moving from an air-conditioned building into hot outdoor weather, entering cold storage rooms, or being near heat sources like furnaces. These shifts can affect the internal pressure of air cushions, potentially compromising their performance.
Travel adds another layer of complexity. Leaving an air cushion in a car or airplane cargo hold can expose it to extreme temperatures, which will change the internal pressure from its initial setting. Additionally, activities involving significant altitude changes such as air travel or visiting mountain destinations can cause rapid air expansion or contraction within the cushion. 6 In these scenarios, users must plan to monitor and adjust inflation levels to maintain safe interface pressure. Wheelchair users adopt a variety of methods to maintain low interface pressure during airplane travel, including air cushions. However, a 2019 survey of 695 wheelchair users found that 26% experienced skin pressure issues during flights. 7 Without an appropriate pressure-sensing system, air cushion users have no reliable way to verify whether their cushion is maintaining safe pressure levels.
Beyond personal mobility, air cell technology is also being explored in medical evacuation scenarios because PIs are a major concern when transporting immobilized patients.8,9 In such a scenario, patients may remain immobilized for extended periods on support surfaces, where sustained pressure can lead to vascular occlusion and soft tissue damage.10,11 As evident from military reports, PI prevalence rates were 4.9–9.3% during the Iraq War, underscoring the need for reliable pressure-relieving solutions.12,13 Air cell cushions can be used for preventing pressure injuries in these scenarios.14,15 These devices must be designed to withstand the full range of environmental conditions encountered during emergency transport, including rapid changes in temperature and altitude.
Some companies have developed technology to combat the issue of air pressure fluctuations in wheelchair cushions. The Smart Check® by ROHO© is a sensor that can be attached to a single-valve cushion, where it stores the clinical pressure setting for the individual user and can alert them when the cushion pressure is too far out of range. 16 A similar technology that can work with multi-valve cushions is the LUCI + Air™, a sensing device that can monitor air pressure and sitting behavior with an app. 17 In both cases, the user would have to manually adjust the pressure of their cushion and use the device to verify it is back to the optimal level. To remove the burden of manual adjustment, Kalogon developed a device called the Sentinel that not only senses incorrect air pressure but also automatically adjusts it to the user’s correct clinical setting. 18 The device is compatible with single-valve cushions, particularly with Star Cushions. While technologies are being developed to improve air pressure regulation for wheelchair users, there is value in further understanding the behaviors of these air cushions in variable environmental conditions. There is little data on the characterization of air cell behavior for support surface applications, where cushions are made from a variety of materials, exposed to varying environments, and operate under loads. Understanding the behavior of these air cells in typical operating conditions will contribute to the improvement of these devices and help to develop pneumatic hardware and new control schemes for further applications such as air mattresses in aeromedical emergency scenarios.
This manuscript explores the air pressure behavior of air cell cushions in varying environmental conditions. The cushion materials ‒ polyurethane, silicone, and TPU-coated nylon fabric ‒ were utilized from our parallel work that investigated air cell designs for a spine board overlay. 8 Environmental chambers were built to test the cushions in a range of temperatures and atmospheric pressures, with and without load. Additionally, two control algorithms were developed and tested for maintaining a set air pressure regardless of environmental temperature and pressure changes. The resulting data shows how the internal pressure of air cells responds to varying environmental conditions for different cushion materials, as well as compares the air cell pressure behavior with and without the control algorithms.
Materials and methods
The cushions and pneumatic hardware used in this work are based on our proprietary air cell cushion technology, which typically consist of an array of air cells with multi-valve pneumatic control for individually monitoring and controlling each air cell in the array.8,19,20 For this study, the air cell cushion configurations and pneumatic control hardware—including cell layout, valve count, and sensor placement—were modified to meet the experimental design requirements. These modifications enabled evaluation of environmental effects on the cushion and implementation of closed-loop control to mitigate those effects. The overall test system details (cushion and control unit) are described in the next sections.
System description
Each air cell cushion consists of an array of air cells that are interconnected (single-valve). Three cushions made of different materials were tested: polyurethane rubber, silicone, and TPU-lined nylon fabric. The cushion connects to a pneumatic control unit, where a microcontroller controls the valves and pumps to inflate/deflate the cushion according to feedback from pressure and vacuum sensors. The user interface can be used to set the internal pressure of the air cell array to any desired pressure.
Air cell cushion fabrication
Figure 1 shows the three cushion types that were fabricated using (a) polyurethane rubber (PMC-724), (b) silicone rubber (XIAMETER RTV-4334-T4), and (c) thermoplastic coated nylon fabric. Dimensions of each air cell are 9.0 × 2.6 × 2.5 inches (L × W × H). All cells were fabricated individually, then they were attached onto a thin fabric base to maintain their configuration and connected pneumatically using tubes and connectors. The 4 × 2 array air cell cushions (size: 19 × 12 × 2.5 inches) shown in Figure 1 were used for temperature testing, while air cell cushions with a 3 × 1 array (size: 9.0 × 8.2 × 2.5 inches) were used for environmental pressure testing. Different arrays were used due to the size constraints of each testing chamber. Air cell cushions made of (a) polyurethane rubber, (b) silicone, (c) TPU-coated nylon fabric.
For the polyurethane rubber and silicone cushions, an injection molding process was used for fabrication. 21 Molds were designed in Solidworks CAD software, 3D printed using stereolithography, and then post-processed for liquid polymer molding. Each air cell was fabricated by assembling a mold set, mixing the liquid polymer, and injecting it into the mold using a piston. The molded polymer was then cured in an environmental oven at 70°C for 1 h.
For making a fabric cell (Figure 1(c)), the fabric was cut using a template for consistency, and an iron was used to seal the fabric into a three-dimensional rectangular cell with the same dimensions as the rubber and silicone cells. To attain air flow into the fabric cell, a threaded tube fitting was attached by pushing the fitting head through a hole in the fabric cell with washers and a gasket and tightening a screw over the fitting.
Control unit
Figure 2 shows the architecture of pneumatic and electronic components used to control the air cells’ pressure to accommodate environmental changes. All the components are controlled using a microcontroller. Two pumps are used: one for pressurizing and the other for vacuuming the air cells. Air flow control during inflation and deflation is managed by two proportional valves, while two solenoid valves alternate the airflow between pressure and vacuum modes. The air cell cushion is equipped with both a pressure sensor and a vacuum sensor to enable accurate monitoring across a full range of operating conditions. The pressure sensor (Honeywell ABPMANN060KGSA3) measures gauge pressures from 0 to 60 kPa and is used during inflation to reach a set initial internal pressure of 1.5 kPa. However, this setpoint was selected for the specific experimental conditions and can be adjusted as needed depending on the application. The pressure sensor is used during inflation to reach a target internal gauge pressure of 1.5 kPa and can reliably measure pressures down to 0 kPa. However, when the cushion is exposed to a vacuum environment, such as inside a vacuum chamber, the internal pressure can drop below 0 kPa (i.e., negative gauge pressure), beyond the functional range of the pressure sensor. In such cases, a vacuum sensor (NXP MPXV4115VC6U) is employed, which can measure gauge pressures from 0 to −115 kPa. It can continue to provide accurate readings of sub-atmospheric pressures, ensuring continuous feedback for pressure control. In addition to the sensors on the cushion, a separate vacuum sensor is placed inside the environmental chamber to monitor the external ambient pressure, which ranges from atmospheric pressure (101 kPa) down to 75 kPa during vacuum tests. This sensor provides real-time data on the surrounding environment, allowing the control system to appropriately compensate for external pressure fluctuations that may affect cushion behavior. It is important to note that this sensor configuration is specific to our experimental setup. In real-world applications, the system would operate under normal atmospheric conditions, not within a vacuum chamber. In such cases, the environmental sensor would simply monitor atmospheric pressure or temperature as needed, rather than chamber-induced fluctuations. Hardware architecture to maintain air cell pressure with respect to environmental changes.
Figure 3 presents a closed-loop control system used to regulate the internal pressure of air cell cushions. The system compares the desired reference pressure (p
r
) with the actual internal pressure (p) measured by a pressure sensor attached to the air cell cushion. The resulting error signal (e) is processed by a PID controller, which generates a PWM signal to drive a proportional valve. This valve modulates airflow into the air cell cushion, adjusting its internal pressure accordingly. The pressure sensor continuously monitors the actuator’s pressure and feeds it back into the loop, enabling real-time correction. This control architecture ensures that the cushion maintains a stable internal pressure despite environmental fluctuations, such as changes in temperature or atmospheric pressure. Control of internal pressure of air cell cushion.
To maintain internal pressure during temperature changes, the pressure sensor (p) attached to the cushion is used for feedback, as temperature primarily affects the internal pressure through thermal expansion or contraction. However, during environmental pressure fluctuations—such as those occurring inside a pressurized chamber—the system uses the difference between the air cell pressure sensor (p) and an additional pressure sensor that measures the environmental pressure (p
v
). This differential pressure (
The final PID gains used in this study were P = 10, I = 2, D = 1, which provided stable pressure regulation within ±0.05 kPa across all tested environmental conditions.
Experimental studies
Experimental studies.
Temperature effect
Figure 4 shows the test setup built for monitoring the effect of temperature change on air cell cushion pressure. A polycarbonate chamber was built with two openings made for heating and cooling units which were operated using a temperature controller (InkBird ITC-308). The chamber used for temperature testing measured 32 × 25 × 25 inches. A DeWALT 1.65 kW electric heater and a 2500 BTU portable air conditioner were employed to achieve controlled temperature variations between 15°C and 35°C. Temperature sensor InkBird ITC-308 provided stability within ±0.5°C throughout the tests. A 4 × 2 array of air cells was placed inside the chamber with a temperature sensor at the top-center of the cushion. The cushion was inflated to 1.5 kPa initial pressure. Internal pressure data of the cushion over time was recorded through the GUI of the cushion controller. Starting at room temperature 23°C, the chamber was cooled down in 2°C increments until reaching 15°C, where temperature was maintained (±0.5°C) for 3 min at each increment. Following the cooling test, once the chamber temperature returned to 23°C, the chamber was heated up in 2°C increments until reaching 35°C, where temperature was maintained (±0.5°C) for 3 min at each increment. To establish a relationship between temperature and the collected pressure data, timestamps were recorded at each instance the system reached a target temperature in the predefined sequence. A range of 15–35°C is typically observed on the ground and in-cabin during flight.
23
When testing with load, a 15 kg buttocks model was placed on top of the inflated cushion. During tests with the PMA, the algorithm would be activated just before adjusting the temperature. Temperature test chamber.
Atmospheric pressure effect
Figure 5 shows the test setup built for monitoring the effect of environmental pressure on air cell pressure. The vacuum chamber had a height of 9 inches and a diameter of 17 inches. It was connected to a vacuum pump (US Vacuum Pumps LLC, Model: DRPC005 000 M4A) to create the required pressure conditions from 75 kPa to 101 kPa. The chamber was modified to have two ports: (1) for controlling internal pressure of a 3 × 1 array of air cells inside the chamber and (2) for controlling the chamber pressure. First, the cushion was placed inside the vacuum chamber and connected to its pneumatic line that flows into the control unit outside of the chamber. Next, the lid was placed on top of the chamber, and the cushion was inflated to 1.5 kPa while the chamber was at atmospheric pressure (101 kPa). Finally, the vacuum was turned on, and the chamber pressure controller was set to six discreet values ‒ 96.9, 92.5, 88.2, 83.8, 79.4, and 75.1 kPa ‒ where average internal pressure of the cushion was recorded at each of these chamber pressures. These pressures cover the range of 101 kPa (atmospheric) to 75 kPa (aircraft cabin), environmental pressures experienced during aircraft flight.
24
When testing with load, a 9 kg weight was placed on top of the inflated cushion (as the buttocks model did not fit inside the chamber). During tests with the PMA, the algorithm would be activated just before applying vacuum. Pressure test chamber.
Results
Temperature test
Figure 6 shows the results from the first and second set of experiments, where three air cell cushions made of different materials were tested in a chamber under cooling and heating conditions to observe the effect on their internal air pressure with and without the pressure-maintaining algorithm. Change in internal pressure of air cells over time as chamber temperature is (a) cooling 23–15°C and (b) heating 23–35°C for different cushion materials: Polyurethane, silicone, and fabric, with and without control from the pressure-maintaining algorithm.
Without Control: Looking at the black lines in Figure 6(a), the cushions’ internal pressures decreased from their initial value as temperature decreased over time for all materials. Conversely, Figure 6(b) shows the internal pressure of the cushions increase as temperature increased over time for all materials. The polyurethane and silicone cushions displayed similar behavior, where internal pressure for both decreased by about 0.2 kPa in cooling conditions and increased by about 0.25 kPa in heating conditions. However, the fabric cushion showed greater sensitivity to temperature, where internal pressure decreased 1.05 kPa in cooling conditions and increased 2.25 kPa in heating conditions. A fluctuating pattern can be seen in all cases. However, the internal pressure fluctuated more drastically for the fabric material compared to the silicone and polyurethane.
With Control: The light-grey lines in Figure 6 illustrate that the PMA successfully stabilized cushion pressure at 1.5 ± 0.05 kPa across all cushion materials, regardless of cooling or warming conditions.
Figure 7 shows the results of the third experiment. First, the polyurethane cushion was initially inflated to 1.5 kPa, and then a load was added, increasing the internal pressure of the cushion to 4.5 kPa. After applying load, temperature was varied in the chamber and internal pressure of the cushion was recorded over time with and without the pressure-maintaining algorithm. Without the PMA, cushion pressure decreased 0.16 kPa in cooling conditions (Figure 7(a)) and increased 0.24 kPa in warming conditions (Figure 7(b)). With the PMA, internal pressure of the cushion was maintained at 4.5 ± 0.05 kPa regardless of temperature change. The potential application of the PMA for use with medical air cell cushions is further supported by its effectiveness at stabilizing cushion pressure while under load and in varied temperature conditions. Polyurethane cushion with load - change in internal pressure of air cells over time as chamber temperature is (a) cooling 23–15°C and (b) heating 23–35°C.
Pressure test
Figure 8 shows the results from the first and second set of experiments, where three air cell cushions made of different materials were tested in a vacuum chamber to observe the environmental pressure change on their internal air pressure with and without the pressure-maintaining algorithm. Comparison of internal air cell pressure data for different cushion materials with and without a pressure-maintaining algorithm (PMA) under varied environmental pressure conditions.
Without Control: Figure 8(a) shows the relationship between the chamber pressure and internal air cell pressures for different cushions without the PMA. The difference in pressure (
With Control (column 2): To maintain
With Load: After a load was applied, the initial air cell pressure of 1.5 kPa increased to 2.8 kPa. Figure 9(a) shows how Polyurethane cushion with load - comparison of environment and internal air cell pressure data with and without a pressure-maintaining algorithm (PMA).
Discussion
Temperature test
In Figure 6, the fluctuating pattern can be attributed to the nature of the temperature control of the chamber, where temperature was fluctuating ±0.5°C when attempting to maintain the target temperature. The expansion and contraction behavior of the air cells in response to temperature changes is consistent with the van der Waals equation (behavior of real gases), which demonstrates that internal pressure increases with growing temperature. Differences in material properties of the air cells contribute to the sensitivity of their pressure change in varied temperatures. Silicone and polyurethane have a significantly higher heat resistance compared to TPU fabric. 25 The higher density and lower thermal conductivity of rubbers makes it difficult for heat to transfer to the air inside of the air cells, slowing down temperature equalization. In contrast, the air in TPU fabric cells adjusts more quickly to environmental temperatures due to better thermal conductivity. Additionally, silicone and polyurethane are hyperelastic materials which allow for expansion that slow down the rate of increasing pressure, while non-elastic TPU-lined fabric prevents expansion, contributing to higher pressure. The light grey lines in Figure 6 highlight the PMA’s effectiveness and underscores its necessity in compensating for temperature-induced pressure fluctuations. As the data reveals that cushion material influences pressure stability under varying thermal conditions, pressure control like this PMA is essential for ensuring the safety of medical air cell cushion users.
Pressure test
When the chamber is vacuumed, the pressure inside drops exponentially. Although the pressure inside the air cells also decreases, the drop in environmental pressure causes the air cells to expand. The hyperelastic membrane of the polyurethane and silicone air cells expands gradually to accommodate the pressure change. This behavior is demonstrated in Figure 8(a), where the slope of the chamber pressure is steeper than the slopes for internal air cell pressure. However, in the case of the fabric material, the air cells stop expanding after a certain limit due to its non-elasticity. Once the fabric cushion could no longer expand, the
Limitations of this study include the exclusion of interface pressure measurements under varying environmental conditions. While we incorporated the effects of temperature and atmospheric pressure on internal air cell pressure, the influence of these factors on user–cushion interface pressure was not assessed. Additionally, the evaluation was restricted to a single-valve cushion configuration, which may not reflect performance in multi-valve designs. Physiological responses such as tissue perfusion and blood flow were also not measured, limiting insight into the combined impact of cushion mechanics and environmental changes on clinical outcomes. These constraints may affect the generalizability of the findings to real-world scenarios.
Conclusion
Air cell cushions are widely used as support surfaces for prevention of pressure injuries, yet they are highly susceptible to losing their effectiveness in common conditions such as sudden temperature and atmospheric pressure changes due to the usage environment (high altitudes and airplanes). These conditions cause air to expand or contract, affecting the internal pressure of the air cells which are essential to maintain safe levels of interface pressure. In this work, we examined air cells cushions of three different material types (polyurethane, silicone, and TPU-coated fabric) with and without load to better understand the behavior of air cell cushions in changing environments.
This study highlights how environmental changes in temperature and pressure can significantly affect the performance of air cell cushions, potentially compromising their ability to prevent pressure injuries. By testing cushions made from polyurethane, silicone, and TPU-coated fabric, the research demonstrated material-dependent sensitivity to these conditions. A closed-loop pressure-maintaining algorithm was developed and shown to effectively stabilize internal cushion pressure within ±0.05 kPa across varying environments, both with and without load. These findings support the integration of intelligent control systems to enhance the reliability and safety of air cushions in diverse real-world applications. Beyond the immediate results, the experimental methodologies presented in this study offer a replicable framework for future investigations. By detailing how air cell pressure responds to varying environmental conditions, this work provides a foundation that researchers can build upon to explore different air cell cushion designs, materials, and control strategies. This contribution not only advances understanding in the field but also promotes standardized approaches for studying pressure dynamics in applications where ambient pressure and temperature are critical factors.
In future work, we will investigate how varying environmental conditions influence interface pressure in seated individuals or applied loads on widely used commercial air cell cushions. That study will identify ranges of temperature and atmospheric pressure that may elevate the risk of PIs across different cushion material types and loads. We also plan to evaluate the PMA with a multi-valve version of our cushion to confirm its effectiveness and compatibility in this configuration. Future work will include evaluating the combined effect of environmental changes on both internal air cell pressure and physiological responses such as tissue perfusion and blood flow in weight-bearing regions, using clinically relevant loads and advanced monitoring techniques.
Footnotes
Acknowledgments
We would like to acknowledge the Trauma Research and Combat Casualty Care Collaborative for their funding support. We also thank the participants involved in testing and the students who contributed to the fabrication of the air cells.
Author contributions
Inderjeet Singh (PhD) built the control unit for the cushions and was the lead developer for the algorithms which control air cell pressure, as well as the pressure-maintaining algorithms. He helped design the protocol and assisted in data collection and analysis. Dr. Singh contributed significantly to writing this manuscript.
Alexandra Jamieson (BS) designed and lead the manufacturing and assembly of the experimental setups. She also assisted in the air cell design and manufacturing of the cushions. Ms. Jamieson designed the protocol and helped in data collection and analysis, as well as writing a significant portion of this manuscript.
Yixin Gu (PhD) created the pressure-controlled embedded system for the vacuum chamber and assisted in building the control unit for the cushions. Dr. Gu contributed to writing portions of this manuscript related to his work.
Muthu Wijesundara (PhD) leads the Biomedical Technology Division team at the University of Texas at Arlington Research Institute. He oversaw the design, development, assembly, and testing of the cushions and algorithms. Dr. Wijesundara contributed to the writing and reviewed the manuscript for submission.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Trauma Research and Combat Casualty Care Collaborative [TRC4 grant number TRC4-2024—0000000068].
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.