D2 Research and Development



Overview of Research and Development Activities

State of the Science on Spinal Cory Injury

Research Projects: Effect of Local Cooling | Effects of Weight Shifting | Handrim Technology

Development Projects: Inflammation Modeling | Low Shear, Cool Cushion | Propulsion Training Tools


D2 Development of a low-shear, cool cushion

Task Leader(s): Patricia Karg, MS; David Brienza, PhD

Co-Investigator: Erik Porach, BSME

Other participants: Andrew Malkiewicz, BSE (bioengineering graduate student); Jon Akins, MS (bioengineering doctoral student); Robert Hoover, Sunrise Medical (industry consultant)


Project Overview

Pressure ulcer risk factors influenced by seat cushions are pressure, heat, moisture, and shear stress. Seat cushion technology designed to manage these factors has evolved over the past 50 years from pillows to the fluid-filled, gel and foam devices available on the market today. The materials and components of current wheelchair cushions include foam (elastic and viscoelastic), gel, viscous fluid, elastomer, pad or cells/bladders. The components may be used alone in a product or in combination. The cushions are most often marketed as providing "pressure relief" or "pressure reduction" through immersion or pressure redistribution using precontouring. Shear reduction is usually addressed through cushion covering materials with two-way stretch and/or a segmented/cube construction that reduces horizontal stiffness. Although products incorporate features into their designs that theoretically reduce shear forces, the impact of these features on tissue distortion has not been studied. This project aims to advance wheelchair cushion design by defining design specifications and developing and evaluating design concepts for reduction of shear strain in sub-dermal soft tissues.

There are four strategies currently used to manage temperature. The first, more common method is the use of a cushion cover that permits air-exchange and is offered on a variety of cushion types. The second uses a system of air cells interwoven throughout the body of the cushion to provide continuous access to air and facilitate ventilation. Another adds an electrical fan to increase airflow through the network of air cells or channels. The fourth approach achieves temperature management through the incorporation of a special Phase Change Material (PCM) in the cushion. Microcapsules contain the PCM, which has a melting point just below normal body surface temperature (34ºC). During the phase change, the heat is absorbed and cools the surface up to 10ºF. It will absorb heat for approximately 2-4 hours and resets in 5-8 hours. During the reset time the material solidifies and releases the stored heat. Presently, few products have temperature control features and those that do employ passive (i.e., open loop) control. Thus, this project also aims to advance wheelchair cushion technology by developing design specifications and incorporating design features that measure and control temperature at the body-seat interface.


Project Objective(s)

Advance the state-of-the-art in seat cushion design through:

  • Development of design specifications for shear strain reduction of buttock soft tissue
  • Development of design specifications for temperature control at the body-cushion interface
  • Development of design concepts for a cushion containing shear reduction and temperature control features
  • Evaluation of cushion design concepts

Support and participate in the development of national and international standards for measurement of cushion characteristics relevant to shear reduction and temperature control

RTE-110 equipment



There is convincing evidence that temperature control and shear reduction are important for preventing pressure ulcers. However, research and development is needed to translate this information into design specifications for use by industry. The first step in the development process will be the specification of design requirements for the reduction of shear strain in buttock soft tissue. Magnetic resonance imaging techniques will be used to quantify distortion of buttock soft tissue of seated subjects under various loading conditions and surfaces. The tissue distortion will be correlated to interface pressure distributions and shear force measurements.

In Year 2 we will begin developing design specifications for temperature control based upon the results from Project R1. Project R1 will provide us with how temperature affects tissue response to loading and methods for selecting appropriate cooling parameters based upon degree of neurological impairment. These results will be translated into specifications for required cooling capacity, spatial resolution and location of cooling, range of temperature settings, sound level for the cooling unit, etc.

The goal of this development effort is to enhance wheelchair cushion devices by improving how the device responds to stimuli in its local environment and the interaction between the body and cushion. This can be accomplished through integration of sensors, active materials and systems. The cushion design will take into account immersion and pressure redistribution features, but will focus on optimizing temperature control and shear strain reduction according to the design specifications determined in the first phase of the project.


Expected Findings and Deliverables

This project will result in population-specific design specifications followed by the development and evaluation of a novel seat cushion incorporating active cooling and low shear features. The project will develop population-specific design specifications for cooling cushions, innovative concepts for reducing shear, and scientific knowledge on the nature of shear in the seated posture.


Project Updates


We previously reported our goal to advance the cushion design by introducing temperature control. We began our work this reporting period by determining our plans for the Phase II prototype cushion based on the results of the previous research. After the layout for the next step of the cushion was decided, the materials for the cushion were researched and ordered to prepare for construction of the Phase II device. Once the cushion was constructed, we began with the next portion of the assembly which was the hardware needed for data acquisition and temperature control. We evaluated the prototype to determine if it would provide the cooling that is required, we began testing under various conditions to fine tune the hardware settings. The next step was to complete the data acquisition (DAQ) system to monitor the prototype assembly during testing.

With the DAQ system in place, the completed the assembly allowed for testing of the entire system. A temperature mat was placed on top of the cushion to confirm that the target temperature was being reached. Testing revealed the need to adjust the hardware settings to reach the desired temperature at the surface of the cushion. Once the target temperature was achieved, we wanted to retest the modified cushion and make sure that the modifications did not compromise the pressure distribution benefits of the original cushion. We used a pressure mat and a standard cushion indenter to compare the modified cushion to an unmodified one, and after comparing the pressure test data, we felt the modifications made an unacceptable change to the pressure relieving functionality of the base cushion. We then began to discuss potential changes to the cooling design to remedy the increased pressure.

University of Pittsburgh Office of Technology Management is planning the submission of a provisional patent application. We have also submitted a NIDRR FIP to further develop the cushion. To begin transfer of the technology we approached an industry partner to collaborate on an NIH SBIR to commercialize the cushion. A non-disclosure agreement was put in place and the industry partner agreed to work with us. An SBIR submission is planned for August 5, 2012. Discussions with the industry partner have already begun by including future changes to deal with the pressure issues created by the current design. Design work will continue to improve the pressure distribution while maintaining cooling, as well as to develop the SBIR proposal.


Work continued on advancing the temperature control cushion design. A working prototype was created to allow bench testing for evaluation and further development of the design. Proof of concept experiments were performed on a cushion providing a partial cooling area. We continued collaboration with a local plastic manufacturing company (PPM), which provided our laboratory with several materials that they felt would suite our needs. Laboratory testing was performed and material selected for further use in the cushion design. Experiments were then performed to determine temperature control parameters.

The next step was to conduct a series of experiments in order to evaluate this cooling design's ability to deliver local cooling with respect to: reliability of interface cooling to the target temperature, the cushion's ability to distribute pressure, and to evaluate any heat and water vapor transmission accumulating at the interface. One-hour seated tests with the current design and experimentally derived control parameters showed no significant difference in interface temperature from our target temperature (25°C) after fifteen minutes of continuous cooling over 14 trials. Pressure distribution was evaluated using the FSA pressure measurement system and the Peak Pressure Index (PPI), the average of the highest recorded pressure values within a 9-10 cm2 area. The PPI of the prototype was compared to a ROHO high profile cushion. This comparison showed no significant difference between the cushions, suggesting our design does not compromise pressure distribution. Finally, we tested the cushion with a thermodynamic rigid cushion loading indenter to evaluate heat and water vapor transfer characteristics using an international draft standard protocol. We compared the cooled region of the prototype with its own contralateral, uncooled side. Results indicated a steady state temperature difference of 3-4°C on the cooled side of the indenter. Humidity measurements were lower on cooled side as well, particularly during the pressure relief lift towards the end of each trial.

These evaluations identified that our design is a valid approach to cooling the seated surface to therapeutic temperatures. They also identified where design improvements were needed. The next step in this design project is to produce a full-scale cushion with a customizable cooling area and control circuits for evaluation with human subjects. Future iterations of this design would standalone from any computer controller or power source, with the electronic components housed beneath the wheelchair seat.

Project investigators continued to participate on national and international standards committees for wheelchair seating and other support surfaces, including beds. Work continued to create a draft standard for the evaluation of heat and water vapor transfer characteristics of these surfaces.



Since the prototyping of the shear reduction cushions, we have now been working towards identifying and evaluating design specifications and concepts for a temperature control cushion design. We began with a literature search of technologies currently employed to manage temperature, both acutely and systemically. This search illustrated several support surfaces with passive temperature controls, personal cooling equipment embedded into articles of clothing, phase change materials, rapid hypothermic induction technology, and air-fluidized support surfaces. Good examples of cooling strategies were also identified in the electronics field including fans, improved heat sink designs, liquid cooling circuits, utilization of heat pipes, and application of thermally conductive pastes and polymers. Moreover, published studies demonstrating the protective effects of lower skin temperatures elucidated a roughly 10°C drop in temperature is most effective for maintaining healthy tissue integrity.

Based on our literature search we decided to implement a temperature controlled fluid circuit and imbed this system into the seat cushion interface. Our initial prototype consisted of a standard foam cushion with channels tooled into the surface to hold the circuit tubing. The tubing was in denser concentration about the IT’s and sacrum to target high pressure and temperature areas of the buttocks. The water was propelled and cooled through the system by a commercially available microprocessor cooling kit. Flow rate and input/output water temperature monitors were included in the design and 25°C was the set point for the circulating water temp. This design proved highly inadequate due to the insulating properties of standard foam and the limited contact between the skin and cooling element (cold water tubing). Skin was not able to be cooled through the tubing thickness, foam insulation, and cover layer. Moreover, the cool circulating water had an excessive buildup of condensation at the cushion interface. Moisture accumulation at the skin surface can cause maceration and accelerate skin breakdown.

We have developed a second design that will not be discussed in detail due to proprietary information.  Materials are being investigated and evaluated for use in the prototype. We plan to bench test the performance of these materials implemented in this novel design concept using both a Thermodynamic Rigid Loading Indenter and with skin-mounted temperature and humidity sensors.

David Brienza and Patricia Karg (Project PIs) participate on national (RESNA) and international (ISO) standards committees for wheelchair seating and support surfaces. They attend multiple committee meetings throughout the year and chair working group activities.  This past year, the RERC lab participated in validation testing of a draft test method on heat and water vapor characteristics, resulting in the validation of this method and agreement by the standards committee to pursue publication of this method in a standard for support surfaces.



We have been working towards the objectives of developing design specifications for shear strain reduction, and the development and evaluation of shear-reduction design concepts. Two parallel approaches have been pursued, (1) characterizing interface shear forces using shear force sensors, and (2) characterizing shear strain of sub-dermal buttock soft tissue using a combination of MRI and finite element modeling (FEM). Interface shear force measurements have been collected on 21 commercial wheelchair seat cushions using a methodology developed to quantify interface shear stress and calculate overall and local horizontal stiffness values. The results have been presented as an extended abstract and are being drafted for submission to a peer-reviewed journal. Results from the evaluation of commercial wheelchair seat cushions provided evidence of materials and technologies that may reduce the risk of pressure ulcers. Three prototype cushions were conceptualized and prototyped into a closed-loop control system using similar ideologies. One prototype was further developed and evaluated. The Remodu wheelchair seat cushion is a fully operational bench-top prototype. The design incorporates a closed-loop control system that monitors and modulates cushion properties. A ROHO Quadtro cushion was retrofitted with highflow valves and an automated pneumatic system to allow adjustability of the cushion properties to flow valves and an automated pneumatic system to allow adjustability of the cushion properties to provide the user with improved pressure distribution and stability. A user control interface displays visual feedback of cushion status and allows the user to manually modify cushion properties. Future design modifications will include an improved air supply, reduction in power consumption, and a handheld user control interface. The design was entered into the Student Design Competition for the RESNA 2009 annual conference. An invention disclosure was submitted prior to publication.

Subcutaneous buttock soft tissues were investigated using a finite element model. This model improved upon image collection methodology and validation techniques in the literature. Magnetic resonance (MR) images of one subject were collected in three seated postures and were used to create 3-D models of the buttock. A non-linear 3-D finite element model was developed with anatomical geometries using hyperelastic and viscoelastic constitutive models. Interface pressure, interface shear stress, and soft tissue displacements were used to validate the model. A parametric analysis resulted in a partially validated model that provided subcutaneous stresses and strains for the upright-seated posture. The validated model will be the basis for future studies evaluating the SCI population and commercial and prototype wheelchair seat cushions. This work published in a Masters thesis, was accepted as an extended abstract for the RESNA 2009 annual conference and will be prepared for submission to a peer-reviewed publication later this year.

In April 2008 we began work on developing design specifications and concepts for a wheelchair seat cushion with self-adjusting interface temperature control properties. This effort began with the assessment of current technologies and materials used for controlling skin temperature at a support surface interface. These data have been used to shape the design inputs of a temperature controlled cushion prototype. The materials, cooling method, location of cooling, range of temperature settings, etc continue to be evaluated and improved upon.

The first graduate student on the project recently graduated with a Masters degree. A second graduate student has been recruited and has begun the work on the temperature control development work.



Interface and subcutaneous stresses are currently being investigated to develop design specifications for shear stress reduction in wheelchair seat cushions. Interface stresses include pressure and shear stress and are the external forces applied to the wheelchair user via the cushion. Two recently available shear sensors were used to collect data from commercially available cushions in pilot and full-scale studies. Subcutaneous stresses are stresses located within the soft tissues below the epidermis. Magnetic resonance imaging (MRI) has been used to develop a non-linear 3D finite element model to quantify soft tissue deformation (strain) of the buttocks under seated loads. Using material properties from the literature and the finite element model, the subcutaneous stresses are calculated. Insight into existing commercial cushion characteristics on interface and subcutaneous stresses will establish a foundation for development of design specifications and a prototype cushion.

Interface Stress Investigation

The objective of this study was to quantify the interface shear stress of commercial wheelchair seat cushions and to determine if relationships exist between the horizontal stiffness of a cushion and interface shear stress. In a pilot study, shear characteristics of eight seat cushions were obtained. Interface pressure and interface shear force were measured at the left ischial tuberosity of an indenter using pressure and shear force sensors (Predia, Molten Corp., Japan & FSA, Vista Medical, Canada). The FSA sensor did not produce repeatable results and was abandoned after two cushions; all data reported was from the Predia sensor. Interface shear stress ranged between 2.3 - 12.8 kPa. The interconnected air-cell cushion resulted with significantly lower interface shear stress (p<.001) and a flat HR70 open-cell polyurethane cushion resulted with significantly higher interface shear stress (p<.013). No correlation was found between the overall horizontal stiffness and interface shear stress; however, a positive correlation was found between local horizontal stiffness and interface shear stress. This study was presented at National Pressure Ulcer Advisory Panel Support Surface Standards meeting April 15-17, 2008.

Due to the success of the pilot study, we purchased a new Predia sensor for data collection in the full-scale study including at least three cushions from each Healthcare Common Procedure Coding System (HCPCS) category. Shear characteristics of 21 wheelchair seat cushions were obtained with the instrumentation integrated into a data acquisition system to collect data at 10 Hz. Interface shear stress ranged between 1.0 - 12.4 kPa. Cushions constructed with a combination of foam and viscous fluid (or gel) and independent air cells resulted in the lowest interface shear stresses, while honeycomb designs resulted in poor interface pressure and shear stress measurements. No correlation was found between overall horizontal stiffness and interface shear stress, a positive correlation was found between local horizontal stiffness and interface shear stress, and a positive correlation was found between the overall horizontal stiffness and horizontal force. Only limited similarities were found within HCPCS categories. This study was presented at the ISO Support Surfaces and Wheelchair Seating standards meetings in Japan on May 27-30, 2008.

Subcutaneous Stress Investigation

This study is investigating the effects of interface pressure and shear stress on subcutaneous tissue using a finite element model. Researchers have used finite element models to investigate subcutaneous tissue response to mechanical loading; however, this study improves image collection methodology and develops a working finite element model to apply to the spinal cord injury (SCI) population. MR images of one subject have been collected in three seated postures and used to create a 3-D solid model of the buttock. Subcutaneous stresses and strains are determined from a non-linear 3-D finite element model. The finite element model will be validated using interface measurements and image geometry.



Akins J, Karg P, Brienza D. Measurement & Analysis of Wheelchair Seat Cushions' Shear Characteristics. Accepted for: Proceedings of the RESNA 31st International Conference on Technology and Disability; 2008 June 28-30; Washington, DC. Arlington, VA: RESNA Press; 2008.

Akins J, Karg P, Brienza D. Subcutaneous Buttock Tissue Stresses and Strains using Finite Element Analysis. In: Proceedings of the RESNA 32nd International Conference on Technology and Disability; 2009 June 25-27; New Orleans, LA. Arlington, VA: RESNA Press; 2009.

Akins J, Karg P, Brienza D (2011). Interface shear and pressure characteristics of wheelchair seat cushions. Journal of Rehabilitation Research and Development, 48(3), 225-34.

Matthew Johnson, Ana Allegretti, PhD, David Brienza, PhD, Patricia Karg, MSE
Department of Rehabilitation Science and Technology, University of Pittsburgh, Pittsburgh, PA
Preliminary Data on Occurrence Rates of Pressure Ulcers in Spinal Cord Injury Patients Across Care Settings
Symposium on Advanced Wound Care 2011

Shilpa Krishnan, MS; Ana Allegretti, PhD; Patricia Karg, MS; Gwendolyn A. Sowa, MD, PhD; and David M. Brienza, PhD
Department of Rehabilitation Science and Technology, Department of Bioengineering, Department of Physical Medicine and Rehabilitation, Department of Orthopedics; University of Pittsburgh, Pittsburgh, PA
Association Between Presence of Pneumonia and Pressure Ulcer Incidence in Individuals with Spinal Cord Injury

Incidence Rate of Pressure Ulcers in Individuals with Spinal Cord Injury in Acute CareBased on ASIA Impairment Scale and Age as Risk Factors
Rachelle Brick1; Shilpa Krishnan, MS1; Ana Allegretti, PhD1; Patricia Karg, MS1; and David M. Brienza, PhD1,2
1Department of Rehabilitation Science and Technology, School of Health and Rehabilitation Sciences; 2Department of Bioengineering, Swanson School of Engineering

Malkiewicz, A (2011). Masters thesis: Development of a wheelchair seat cushion with site-specific temperature control for pressure ulcer prevention. Pittsburgh: University of Pittsburgh.

Return Home

This work is funded by the National Institute on Disability and Rehabilitation Research (NIDRR),
Rehabilitation Engineering Research Center (RERC) on Spinal Cord Injury, Grant #H133E070024
The ideas and opinions expressed herein are those of the authors and not necessarily reflective of the NIDRR.

Contact Webmaster | Accessibility Statement

Last Updated: 07.10.2012 | 16:35

Smart Code