Process design and fabrication of 3D printed custom-contoured wheelchair cushions

Custom-contoured wheelchair seating refers to a seating system that consists of a seat, a back support, or both types of cushions that is based on and matches the wheelchair user’s body contours. Custom-contoured seating (CCS) is prescribed to prevent, accommodate, or correct skeletal deformities; manage pressure and prevent pressure injury; provide trunk stability and limit limb contractures; and enhance autonomic nervous system functions such as breathing, swallowing, and digestion through postural management, among other reasons. Foam is the most common material used in custom-contoured wheelchair seating. Although generally effective at pressure relief and postural management, foam acts as an insulator, sometimes increasing the temperature and moisture levels at the user-seat interface. High moisture and temperature levels increase the risk of tissue breakdown, a factor that increases risk of pressure injury development in the wheelchair user with time. Additionally, current manufacturing methods for CCS products have room for improvement. Some are labour-intensive, create waste due to the subtractive nature of the process, or create products that do not allow for acute angles to be captured and supported in their shape due to the nature of the materials used in the CCS product. This thesis aims to resolve the issues with current manufacturing methods of CCS and lower the risk of pressure injury development in wheelchair seating users by developing a method of making custom-contoured seating systems through additive manufacturing. Initial work with clinical and technical seating experts showed a need for a simpler method for preparing digital models of custom-contoured seating for manufacturing. A semi-automated mesh-to-CAD process was created to address this, with user testing showing improvements in the time to complete digital model creation for people with and without CAD skills. The design of an effective 3D-printed custom-contoured wheelchair cushion was then pursued. First, an investigation of two common 3D printing materials—a rigid plastic, acrylonitrile butyl styrene (ABS) and a flexible polymer, thermoplastic polyurethane (TPU)—for use in wheelchair seating were assessed through uniaxial compression tests. The behaviours of the 3D printed samples were compared to a viscoelastic polyurethane foam used in wheelchair seating. The effects of infill pattern and infill percentage on the compressive behaviour of ABS and TPU were also investigated. Findings from the study suggest that infill pattern and density impact the compressive response of a 3D printed material, with low-density gyroid-pattern infill performing most closely to the viscoelastic foam in compression testing. Further comparative testing between TPU and the viscoelastic foam suggested that a 3D printed custom-contoured cushion would insulate users less than traditional foam cushions, a property that is desirable for some CCS users for increased comfort and for decreased risk of pressure injury development. With the temperature and humidity hypothesis proven, full-scale wheelchair cushion testing against ISO standards of performance were designed and conducted on a custom-contoured 3D printed and a custom-contoured viscoelastic foam cushion that matched the standard rigid cushion loading indenter from ISO 16840 for wheelchair seating systems. Results from these tests showed that the 3D printed cushion designed and manufactured here varies in its mechanical performance from foam, showing higher stiffness and stability, agreeing with the compression testing conducted on small 3D printed samples. Discomfort trials and pressure mappings with healthy volunteers were conducted. Feedback from volunteers suggested that the 3D printed cushions were of similar comfort to the foam cushions overall, with some volunteers preferring one cushion over the other but no discernible trend in preference. The pressure readings also showed similar findings. These results are positive for the future of 3D printable custom-contoured wheelchair seating, suggesting that 3D printed cushions would provide similar comfort and pressure redistribution to current foam cushions, with a possible added benefit for some users of a lower temperature and humidity level at the user-seat interface. Finally, a validated finite element model was created to aid in the design of future custom-contoured wheelchair seating. The model simulates a rigid cushion loading indenter sitting onto a custom-contoured wheelchair cushion. The model can be used to improve upon the design of future 3D printed custom-contoured wheelchair cushions by eliminating the costly printing and testing of full-sized cushions and replacing that with less expensive printing and testing of small samples that define the material behaviour of a cushion.