Mechanical behaviour of flexible 3D printed gyroid structures as a tuneable replacement for soft padding foam

Various areas of healthcare utilise custom foam cushioning to treat or mitigate conditions like pressure ulcers, or to provide personalised support structures for patients with specific clinical needs. Polyurethane foams are often used; however, such materials require significant time and expertise to combine different foam types into a device that provides sufficient structural support in some areas, with soft pressure distribution in others. In this paper, flexible 3D printed gyroid based metamaterials are investigated as a tuneable replacement for polyurethane foams. The impact of changing key gyroid structural characteristics on the material's mechanical response is examined. Samples with six different unit cell geometries for each of two flexible TPU 3D printing filaments (NinjaFlex and Flexion X60) were produced using fused filament fabrication, tested, and compared to three types of conventional polyurethane rehabilitation foam. Compression tests were conducted focussing on compressive stress-strain response, strain rate effect, print layer effect, and cyclic fatigue behaviour. In all tests it was observed that gyroid samples of both filament types were able to produce compressive responses comparable to the foams. Solid volume fraction was determined as the critical gyroid geometric parameter that influenced compressive response, and solid volume fractions capable of reproducing the specific response of each of the three rehab foams were determined. It is shown that 3D printed gyroid materials are a viable replacement for soft polyurethane foams, and the direct control of material response possible with simple geometric changes means such metamaterials may lead to improved optimisation of rehabilitation cushions.