Fracture mechanics studies of adhesion in biological systems

A fracture mechanics based methodology for quantifying adhesive interactions between soft solids, or between a soft solid and a rigid substrate, is reviewed. An emphasis is placed on the application of these techniques to the characterization of adhesive interactions in biological systems. Results from experiments involving the adhesion of gelatin hydrogels to hydrophilic and hydrophobic substrates are described as an illustration of the application of these methods. In these experiments a hemispherical gelatin cap is brought into contact with a flat surface. Separation of the two materials is described in terms of crack propagation along the gelatin/substrate interface. Simultaneous measurements of the applied load, the resulting displacement, and the contact area between the two materials enable us to determine the elastic modulus of the cap, in addition to the crack driving force, or energy release rate. The adhesive behavior of the interface is quantified by the relationship between the energy release rate and the crack velocity. Analogies are made to information obtained from contact angle measurements, and from measurements made with the Israelachvili surface forces apparatus.