4.6. Hydrogel Compression Testing

The stiffness and toughness of the hydrogels were tested using an Instron model 5583 compression tester (Instron, Norwood, MA, USA). The gels were placed on top of the lower platen while the upper plan as lowered on top of the gel during compression at a rate of 1 mm/min, while force and extension were recorded with a 500 N load cell. These values were then used to calculate compressive stress and strain which were then used to calculate compressive modulus, stress and strain at failure and toughness with MATLAB (Mathworks, Natick, MA, USA ). Each load cycle was used to generate a stress–strain plot similar to the sample data shown in Figure 1d. The point of failure was interpreted to be the point at which stress was greatest, and each curve was manually inspected to ensure this assumption was accurate. The compressive modulus was estimated using a zeroth order fit of data between 5 and 15% compressive strain to remain within the linear compression region and avoid effects stemming from the low compression toe-region.