\frac{dx_{1}}{dt} = \left(1 \cdot k_{18} \cdot k_{1} \cdot x_{2} \cdot x_{1} \cdot \left(1 - x_{1} / k_{2}\right) + -1 \cdot k_{18} \cdot k_{5} \cdot x_{1} + -1 \cdot k_{18} \cdot k_{3} \cdot x_{3} \cdot x_{1}\right) / k_{18}\\
\frac{dx_{2}}{dt} = \left(-1 \cdot k_{18} \cdot k_{10} \cdot x_{2} + -1 \cdot k_{18} \cdot k_{9} \cdot x_{1} \cdot x_{2} + -1 \cdot k_{18} \cdot \left(k_{13} + k_{7} \cdot \left(k_{8} + x_{3}\right)\right) \cdot x_{2} + 1 \cdot k_{18} \cdot k_{10} \cdot x_{4}\right) / k_{18}\\
\frac{dx_{3}}{dt} = \left(1 \cdot k_{18} \cdot k_{7} \cdot \left(k_{8} + x_{3}\right) \cdot x_{2} + 1 \cdot k_{18} \cdot k_{4} \cdot x_{1} \cdot x_{3} + -1 \cdot k_{18} \cdot k_{6} \cdot x_{3} + -1 \cdot k_{18} \cdot k_{14} \cdot x_{1} \cdot x_{3}\right) / k_{18}\\
\frac{dx_{4}}{dt} = \left(1 \cdot k_{18} \cdot k_{10} \cdot x_{2} + 1 \cdot k_{18} \cdot \operatorname{piecewise}(k_{12} \cdot \sin\left(6 \cdot \pi \cdot t\right), k_{11} < k_{12} \cdot \sin\left(6 \cdot \pi \cdot t\right), k_{11}) + -1 \cdot k_{18} \cdot k_{13} \cdot x_{4} + -1 \cdot k_{18} \cdot k_{10} \cdot x_{4}\right) / k_{18}