Besides biological and chemical cues, cellular behavior has been found to be affected by mechanical cues such as traction forces, surface topology and in particular mechanical properties of the substrate. In previous studies involving hydrogel substrates, mechanical characterization was performed assuming Poisson’s ratio to be equal to one-half. However, this might not be true in all cases and might alter the calculation of stiffness of hydrogels.
The present study mainly focuses on characterizing the Young's modulus (E), shear modulus (G) and Poisson's ratio (v) of soft hydrogels using a non-intrusive technique. For this purpose, an apparatus referred to as the "four magnet setup", which allows the determination of local gel elastic properties, was developed. Closed form equations involving E, G and v of the hydrogel were derived and finite element analysis was employed to validate the equations. Linear elastic properties of bis-gels and DNA gels were obtained using the apparatus and verified using rheometry and bead experiments. This is the first report in literature in which the mechanical properties consisting of E, G and v were simultaneously obtained for soft hydrogels.
A DNA gel design space involving parameters such as crosslinker concentration, side-chain concentration and lengths of DNA strands was systematically developed and the mechanical properties were evaluated using bead experiments. It was found that stiffness of DNA gels can be modulated over a wide range by modifying the various design parameters.
Addition of DNA crosslinks generates force and alters the mechanical properties, which has implications for cell and tissue culture substrate design. Two techniques have been developed to characterize the force actuating potential of DNA gels. It was found that the force generated was proportional to the elastic modulus of the gel. Also, at higher temperatures the stiffness of the gels decreased and the amount of force generated also decreased. A comparison of the force generated in both methods showed that either method can be successfully employed. The force was found to be in the range of values reported in the literature for axonal growth in spinal cord neurons.Ph.D.Includes bibliographical referencesby Uday Chippad
