text: In the unswollen state, hydrogels can be modelled as highly crosslinked chemical gels, in which the system can be described as one continuous polymer network. In this case:
  where G is the shear modulus, k is the Boltzmann constant, T is temperature, N is the number of polymer chains per unit volume, ρ is the density, R is the ideal gas constant, and  is the (number) average molecular weight between two adjacent cross-linking points.  can be calculated from the swell ratio, Q, which is relatively easy to test and measure.
  For the swollen state, a perfect gel network can be modeled as:
  In a simple uniaxial extension or compression test, the true stress, , and engineering stress, , can be calculated as:
  where  is the stretch.