Fung-elastic material
Fung developed a constitutive equation for preconditioned soft tissues which is 

  
    
      
        W
        =
        
          
            1
            2
          
        
        
          [
          
            q
            +
            c
            
              (
              
                
                  e
                  
                    Q
                  
                
                −
                1
              
              )
            
          
          ]
        
      
    
    {\displaystyle W={\frac {1}{2}}\left[q+c\left(e^{Q}-1\right)\right]}
  with

  
    
      
        q
        =
        
          a
          
            i
            j
            k
            l
          
        
        
          E
          
            i
            j
          
        
        
          E
          
            k
            l
          
        
        
        Q
        =
        
          b
          
            i
            j
            k
            l
          
        
        
          E
          
            i
            j
          
        
        
          E
          
            k
            l
          
        
      
    
    {\displaystyle q=a_{ijkl}E_{ij}E_{kl}\qquad Q=b_{ijkl}E_{ij}E_{kl}}
  quadratic forms of Green-Lagrange strains 
  
    
      
        
          E
          
            i
            j
          
        
      
    
    {\displaystyle E_{ij}}
   and 
  
    
      
        
          a
          
            i
            j
            k
            l
          
        
      
    
    {\displaystyle a_{ijkl}}
  , 
  
    
      
        
          b
          
            i
            j
            k
            l
          
        
      
    
    {\displaystyle b_{ijkl}}
   and 
  
    
      
        c
      
    
    {\displaystyle c}
   material constants.
W
      
    
    {\displaystyle W}
   is the strain energy function per volume unit, which is the mechanical strain energy for a given temperature. Isotropic simplification
The Fung-model, simplified with isotropic hypothesis (same mechanical properties in all directions).