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Material Selection in Cushions

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Cushion Materials diagram

Although cushions may look similar in design on the outside, the materials used on the inside can have a significant impact on their performance. Understanding how materials perform will improve the clinician's ability to select a cushion based on their client's goals. This article describes materials, their benefits, and the clinical considerations when selecting the most appropriate wheelchair cushion.

Foam

Foam

Foam is a material which is formed by trapping pockets of gas. It can be a variety of densities, firmnesses, and weights. Based on the uniformity of the cells, they will be more or less rigid.

Closed cell foam has a uniform cell structure which creates rigidity. Open cell foam is softer due to a less uniform cell structure.

The weight of a foam is dependent upon mass, not upon firmness or density. Therefore rigid foam may also be lightweight, whereas softer foam may be heavier.

Memory foam (viscoelastic) has increased viscosity and density which allows it to slowly return to its shape after compression.

Clinical Benefits

  • Inexpensive and lightweight
  • Low maintenance
  • Absorbs impact loads well
  • Firm foam can provide a level base of support
  • Soft foam can be used for comfort
  • Viscoelastic memory foam has vibration dampening properties
  • Various densities can be combined for pressure relief, moving load from high risk bony prominences to lower risk areas

Additional Considerations

  • Compresses over time
  • Difficult to clean and absorbs moisture
  • May create tension on the tissue depending on the softness of the foam
  • May create shear forces on the tissue depending on the foam design
  • May retain heat in hot climates and get harder in cold climates
  • Memory foam may hold more heat than standard foam and it becomes more compressed over time, with less return
  • May increase pressure on bony prominences over time

Elastomer

Elastomer

Elastomer is a man-made polymer with varying degrees of viscoelasticity.

In wheelchair cushions, an elastomer is often a matrix-like polymer structure.

Immersion is dependent upon the softness of the elastomer. It has a greater durability than foam over time.

Clinical Benefits

  • Easy to clean
  • Allows some compression
  • Immersion is dependent upon the rigidity of the elastomer
  • Can be lightweight depending on design (e.g. honeycomb structure)
  • Design may allow airflow which may improve microclimate
  • Has shear reduction qualities due to higher viscoelasticity properties

Additional Considerations

  • Limited displacement
  • Less immersive than lower density foam or fluids
  • More difficult to modify in the field in response to specific postural needs

Gel

Gel

Gel is a solid jelly-like material that can have properties ranging from soft and weak to hard and tough.

Although it is mostly liquid, it behaves like a solid. Therefore it has some viscosity, but does not allow for complete immersion.

Gel maintains its shape even when opened or cut.

Clinical Benefits

  • Some movement and viscosity
  • Often easy to cut, modify, and put in small places
  • Can offer good friction/shear control
  • Easy to clean

Additional Considerations

  • Heavy
  • Temperature-sensitive
  • Holds heat but can feel cold to the touch initially
  • Limited displacement, less immersion than liquid, air, or most foams used in cushions

JAY® Flow Fluid

JAY Flow Fluid

JAY Flow Fluid is a non-Newtonian fluid. A non-Newtonian fluid remains in a semi-solid or highly viscous state.

In a non-Newtonian fluid, viscosity changes when under force to either more liquid or more solid.

Although a non-Newtonian fluid can take a shape without a container, it will flow with pressure. Therefore it needs to be contained when used in a cushion.

Clinical Benefits

  • Good pressure distribution
  • Conforms to the body
  • Weighs less than gel
  • Moves with the body
  • Can help control shear
  • Does not respond to load with counterforce of foam, thereby extremely good for pressure care
  • Provides good immersion while creating a large area of surface contact to distribute

Additional Considerations

  • Heavier than foam or air
  • Needs some maintenance
  • May require redistribution following use due to migration with pressure

Air

Air

Air is a Newtonian fluid. A Newtonian fluid maintains its viscosity independent of stress. A Newtonian fluid only changes its viscosity in response to temperature.

If you provide a Newtonian fluid with stress, it will remain the same thickness (viscosity), but if you heat it up or cool it down it can become more liquid or more solid.

Newtonian fluids such as air or water cannot maintain a shape without a container. However, if the container is opened, it will spill and be released, thereby negating its efficacy.

Clinical Benefits

  • Lightweight
  • Moves with the body
  • Easy to clean
  • Pressure distribution
  • Can be used as entire cushion or within a foam base for stability
  • Provides good immersion while creating a large area of surface contact to distribute

Additional Considerations

  • Reduced stability, particularly in an air-only cushion
  • Risk of puncture
  • Frequent maintenance to ensure proper inflation
  • Sensitive to pressure changes, such as during air travel

References

  1. Polyurethane Foam Association. (1991, rev 2016). Flexible polyurethane foam: A primer. In Touch, 1(1), 1-7. Retrieved from: pfa.org/wp-content/uploads/2019/02/IntouchV1.1a.pdf
  2. Sherman, S. (2011, November). Skin Protection and Cushion Materials. Retrieved from: sunrisemedical.ca/education-in-motion/clinical-corner/november-2011/skin-protection-and-cushion-materials
  3. Watanabe, L. (2017, September). Immersion, envelopment and off-loading. Mobility Management. Retrieved from: mobilitymgmt.com/Articles/2017/09/01/Seating-Strategies.aspx

Published: 2020-04-13

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