Evidence of strain hardening in DNA gels

Strain hardening observed in many biological gels is nature's defense against the external forces to protect the tissue integrity. Here, we show that double-stranded (ds) DNA gels also stiffen as they are strained. Chemical DNA gels were prepared by solution cross-linking of ds-DNA (about 2000 base pairs long) using the cross-linker ethylene glycol diglycidyl ether (EGDE), while physical DNA gels were prepared by the heating - cooling cycles. Stress relaxation experiments show that strain hardening in both chemical and physical gels starts to appear at 40% deformation, the extent of which increases when the amplitude of the deformation is increased up to the yield strain amplitude. The degree of strain hardening greatly depends on the contour length L_c of DNA network strands as well as on the time scale of the measurements; the gel exhibits strong strain hardening at short time scales and soften at long time scales. The maximum degree of hardening appears if the contour length of the network chains approaches 100 nm, which is the Kuhn length of ds-DNA, DNA gels exhibit universal scaled stiffening behavior that can be reproduced by a wormlike chain model taking into account the entropic elasticity of DNA strands. The results of our experiments also show that chemical DNA gels exhibit liquidlike response at strain amplitudes above 1000%, but reversibly, if the force is removed, the solution turns back to the gel state. The partial recovery of the initial microstructure of gels suggests stress-induced denaturation of ds-DNA network strands.