2.4.2. Rheological properties of carrageenan-based bioink
The rheological properties of various concentrations of carrageenan and carrageenan composite hydrogel were measured using a rheometer (HR-2, TA Instruments, USA) with a 100-mm round plate at a constant temperature of 37°C. All rheological experiments were performed in triplicate. To assess the rheological properties for optimising the printing parameters, the shear rate sweep test and frequency sweep test were performed. For various shear-thinning materials including carrageenan-based hydrogels, the power-law model has been widely used to analyse the viscoelastic properties. The power-law index (n) and zero viscosity (η0) were calculated from the shear rate-viscosity curve of the shear rate sweep test in this study. The printing parameters such as the pressure and printing speed were calculated from the power-law index equation, as described below:
η=η 0 γ n−1
where, η is the shear viscosity, γ is the shear rate, and η0 is the viscosity at the reference shear rate called the zero viscosity (Morrison, 2001; Malkin and Isayev, 2017). The viscosity (η) of the composite hydrogels was assessed using the shear rate sweep test under the shear rate range of 0.1–500 s-1. The frequency sweep test was performed in the angular frequency of 0.1–500 rad/s at a strain in the linear viscoelastic region, where the test was performed without destroying the structure of the sample to assess the storage modulus (G’) and the loss modulus (G”).