Preparation And Characterization Of The PAAm Hydrogels Toughed By Chemical And Physical Crosslinking

Hydrogels as hydrophilic gel with relatively high water content are similar to biological tissue with the water content of 70%.Hydrogel with good biocompatibility,flexibility and stimulate responsiveness is applied in biological field and tissue engineering.However,its mechanical properties are relatively poor due to the hydrogel matrix material consist of amount of water,seriously limiting the hydrogel in the field of biomedical applications.Traditional chemical cross-linked hydrogels are composed of permanent,irreversible covalent bonds,which often make the hydrogel brittle,poorly transparent,and cannot self-heal after break.However,the chemical cross-linked network structure is easy to adjust and can fully change the mechanical properties of the final material.Therefore,the traditional chemical cross-linked network structure cannot be ignored,and should be modified to get tough hydrogel.In this paper,we introduce both physical cross-linking and chemical cross-linking into hydrogel network systems to obtain tough,hydrogels with anti-fatigue properties and rapid self-recovery properties.In the first part of the experiment,we introduced the hydrophobic association into the hydrogel network as a physical crosslinking center,N,N'-methylenebisacrylamide as a chemical crosslinker linked to the hydrophilic backbone Poly(acrylamide)(PAAm)as a chemical crosslinking center by free radical polymerization.In this hybrid hydrogel network system,the chemical crosslinking center provides a rigid skeleton for the hydrogel network to support the entire hydrogel network matrix.Hydrophobic association as a physical crosslinking center could effectively dissipate energy with micelle deformation and the reversible dissociation of the LMA chains.Therefore,the hybrid hydrogel has a high compressive strength of 8 MPa and a corresponding compressive strain of 95%.In addition based on continuous cycle compression test,this hydrogel shows time-dependent,rapid self-recovery and anti-fatigue properties.In the second part of the experiment,we expect a robust,extensible hydrogel.We improved the physical crosslinking center and adjusted the density of the chemical crosslinking network,resulting in a high tensile strength,fast self-recovery,anti-fatigue properties and high elongation at break hydrogel(LMA-PBA+MBA gel).In this hybrid hydrogel network system,chemical crosslinking still supports the entire hydrogel network as a rigid skeleton.LMA-PBA hybrid micelles as physical crosslinks by micelle deformation,reversible dissociation of LMA chains,PBA soft particle deformation and reversible dissociation of LMA-PBA dissipate a large amount of energy.The maximum tensile stress of the LMA-PBA+MBA gel can reach 1.4 MPa and the elongation at break is up to 2500%.These hybrid hydrogels toughened by physical and chemical cross-linking with rapid self-healing and anti-fatigue can expand the application of biological fields in the load material,for instance,hydrogel cartilage,tendons,artificial muscle and tissue engineering etc.