Figure 8.

Measurement of the oxygen concentration in a well-stirred reactor to determine the oxygen diffusion coefficient within the hydrogel. (A): The oxygen concentration $c$ (mg L⁻¹) of the liquid phase in the well-stirred reactor was measured by an oxygen sensor as a function of time. N₂-bubbling for 10 min allowed for degassing of the oxygen dissolved in the liquid phase (DPBS) until $c=0.18$ mg L⁻¹ (“Degassing”). The well-stirred reactor was closed for 40 min until oxygen concentration stabilized at $c_1=0.90$ mg L⁻¹ (“Equilibrium”). The liquid phase was quickly renewed with oxygen-saturated DPBS until the maximal oxygen concentration $c_2=4.02$ mg L⁻¹. Then, the dissolved oxygen diffused from the liquid phase to the hydrogel for 50 min until the final oxygen concentration $c_{\infty }=3.25$ mg L⁻¹ (“Diffusion”). (B): The diffusion coefficient $D$ was deduced from the linear regression of the quantity $-\ln \left(\mathrm{c}\left(\mathrm{t}\right)-c_{\infty }\right)$ against the time $t$ between $t=840$ s and $t=1500$ s. The origin of time $t$ is indicated in (A) and corresponds to the beginning of the stepwise concentration variation from $c_1$ to $c_2$.