Hydrogels have the remarkable ability to swell in water and aqueous solvents. During the process of swelling, surface instability can occur. This instability depends on the thickness of the hydrogel layers and the surface tension. A higher surface tension stabilizes the flat surface of the hydrogel, which is the outer-most layer. The swelling ratio of the flat layer can be calculated using the following equation derived from the Flory–Huggins theory of free surface energy in hydrogels:
Where λ is the swelling ratio, μ is the chemical potential, p is pressure, k is the Boltzmann constant, and χ and N are unitless hydrogel constants. 
As swelling increases, mechanical properties generally suffer.