import numpy as np

def sigmoid(x, b, c):
    y = 1 / (1 + c*np.exp(-b*x))
    return y

x_data = [4, 6, 8, 10]
y_data = [0.86, 0.73, 0.53, 0.3]

popt, pcov = curve_fit(sigmoid, x_data, y_data,(28.14,-0.25))