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

Methods

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.