A simple two-step method was introduced to improve the hydrogel mechanical strength by forming an interpenetrating network (IPN). For this purpose, we synthesized polyacrylate/polyacrylate (PAC/PAC), polyacrylate/polyacrylamide (PAC/PAM), polyacrylamide/polyacrylamide (PAM/PAM) and polyacrylamide/poly(vinyl alcohol) (PAM/PVA) IPN hydrogels. The PAC/PAC IPN and PAC/PAM IPN hydrogels showed compressive strength of 70 and 160 kPa, respectively. For the PAM/PAM IPN and PAM/PVA IPN hydrogels, they exhibited excellent tensile strength of 1.2 and 2.8 MPa, and elongations at break of 1750% and 3300%, respectively. A strain relaxation was also observed in the case of PAM series IPN hydrogels. From FTIR, TGA and SEM measurements, we confirmed that physical entanglement, hydrogen bonds and chemical crosslinking played major roles in improving hydrogel strength and toughening. The two-step technique contributes to the understanding of ideal networks, provides a universal strategy for designing high mechanical strength hydrogels, and opening up the biomedical application of hydrogels. (C) 2009 Elsevier Inc. All rights reserved.