[ Eq(Derivative(x1, t), (-1)*k1*k2*(x1 - x2)/Piecewise((500, Le(t, k12)), (400, True))),
  Eq(Derivative(x2, t), (k1*k2*(x1 - x2) - k1*k2*(x2 - x3) - k3*x2*Piecewise((33/100, Le(t, k12)), (33/100, True)) - k4*x2*Piecewise((33/100, Le(t, k12)), (33/100, True)))/Piecewise((33/100, Le(t, k12)), (33/100, True))),
  Eq(Derivative(x3, t), (k1*k2*(x2 - x3) - k7*x3*Piecewise((380, Le(t, k12)), (280, True)) - k11*x3*Piecewise((380, Le(t, k12)), (280, True)))/Piecewise((380, Le(t, k12)), (280, True))),
  Eq(Derivative(x4, t), (-1)*k9*k2*(x4 - x5)/Piecewise((500, Le(t, k12)), (400, True))),
  Eq(Derivative(x5, t), (k9*k2*(x4 - x5) - k9*k2*(x5 - x6) + k3*x2*Piecewise((33/100, Le(t, k12)), (33/100, True)))/Piecewise((33/100, Le(t, k12)), (33/100, True))),
  Eq(Derivative(x6, t), (k9*k2*(x5 - x6) + k7*x3*Piecewise((380, Le(t, k12)), (280, True)))/Piecewise((380, Le(t, k12)), (280, True))),
  Eq(Derivative(x7, t), (-1)*k10*k2*(x7 - x8)/Piecewise((500, Le(t, k12)), (400, True))),
  Eq(Derivative(x8, t), (k10*k2*(x7 - x8) - k10*k2*(x8 - x9) + k4*x2*Piecewise((33/100, Le(t, k12)), (33/100, True)))/Piecewise((33/100, Le(t, k12)), (33/100, True))),
  Eq(Derivative(x9, t), (k10*k2*(x8 - x9) + k11*x3*Piecewise((380, Le(t, k12)), (280, True)))/Piecewise((380, Le(t, k12)), (280, True))) ]