Study On Multiple Crosslinked Alginate/Polyacrylamide Hydrogel With High Mechanical Strength

Traditional hydrogels usually has a low mechanical properties.The appearance of double network hydrogels,however,overcome those disadvantages and make their tensile strength and compressive strength are both close to megapascal.In view of the excellent mechanical properties of double network hydrogels,they are often employed in the field of artificial joint cartilage materials.Calcium alginate/polyacrylamide double network hydrogel?Ca-SA/PAM double network hydrogel?is well known especially for its good toughness and mechanical recovery among these double network hydrogels,while there are still problems such as insufficient Young's modulus,compressibility,and difficult in quantitative analysis of wear when employed as a knee joint articular cartilage material.In order to solve these problems,this paper firstly took advantage of the feature that SA can be cross-linked by various metal ions to cross-link SA in sodium alginate/polyacrylamide?SA/PAM hydrogel?to obtain double network gels with different metal ions.In order to further improve the mechanical properties of the double network hydrogels crosslinked by different ions and SA,the carboxylate-containing acrylic acid?AA?was introduced into the PAM network to increase the crosslink sites with the metal ions in the PAM network simultaneously,inter-crosslinking of two types of networks were added to produce multiple chemical/physical cross-linked hydrogels with high strength.Taking the sliding friction of human knee joints as the research background,the sliding friction test of multiple cross-linked hydrogels under two conditions of walking and jogging was simulated.Finally,in vitro biological evaluation of multiple cross-linked hydrogels was performed.Double-network hydrogels?Ca-SA/PAM,Fe-SA/PAM,Al-SA/PAM,Cu-SA/PAM and Mn-SA/PAM?were prepared from sodium alginate?SA?and acrylamide?AM?using Fe³⁺,Al³⁺,Cu²⁺,Mn²⁺,and Ca²⁺as cross-linkers.In order to further improve the mechanical properties of Fe-SA/PAM double network hydrogels,SA,AM,and AA were used as the main materials.SA/P?AM-co-AA?was first prepared by thermal initiation of free radical copolymerization and chemical crosslinking and then physically cross-linked by SA and P?AM-co-AA?in Fe³⁺and P?AM-co-AA?hydrogels to prepare five kinds of chemical/physical cross-linked hydrogels With the molar ratios are 0,5,10,15,and 20mol%?Fe-SA/P?AM-co-20%AA??,respectively.The physicochemical properties of the double network hydrogels were characterized by universal testing machine,Fourier transform infrared spectrometer?FTIR?and atomic absorption spectrophotometer?AAS?.The cross-sectional morphology of the double network hydrogels was observed by scanning electron microscope?SEM?.Double-network hydrogel swelling test showed that the cross-linking degree of the five double network hydrogels Mn-SA/PAM,Ca-SA/PAM,Cu-SA/PAM,Al-SA/PAM and Fe-SA/PAM increased in sequence.The mechanical properties showed that Fe-SA/PAM double network hydrogel was better than the other four hydrogels in tensile strength?0.98MPa?,toughness?5.8×10⁶ J/m³?,Young's modulus?2.7 MPa?and fracture energy?380 J/m²?,showed"hard and tough"properties.However,the Young‘s modulus and compressive strength of Fe-SA/PAM DN hydrogels have not yet reached the level of human knee articular cartilage?such as Young's modulus:9.6 MPa;maximum contact pressure:7.5 MPa?.Swelling performance tests of multiple cross-linked hydrogels indicated the cross-linking degree of the five multiple cross-linked hydrogels?Fe-SA/PAM,Fe-SA/P?AM-co-5%AA?,Fe-SA/P?AM-co-10%AA?,Fe-SA/P?AM-co-15%AA?and Fe-SA/P?AM-co-20%AA??increased sequentially.The mechanical properties showed that the Fe-SA/P?AM-co-10%AA?multiple cross-linked hydrogel had a certain rigidity?Young's modulus of 9.9 MPa?without losing its toughness?3.8×10⁶ J/m³?and still had a good morphology when compressed to 90%and the compressive strength was 16.7 MPa;exhibited efficient energy dissipation during the two loading-unloading processes?There was no significant difference between the relative values of the two hysteresis loops?;After the first loading-unloading,the compression set can be restored for 1 h with good mechanical recovery.Among the five cross-linked hydrogels,the mechanical properties of Fe-SA/P?AM-co-10%AA?multiple cross-linked hydrogels are closest to the mechanical strength of human knee articular cartilage.The friction coefficient and wear amount of Fe-SA/P?AM-co-10%AA?multiple cross-linked hydrogels were studied through a reciprocating sliding friction tester and simulated the behavior of human knee joint according to the data of human body‘s load,speed and step frequency during walking and jogging conditions.The results showed that the intermittent walking had a lower average coefficient of friction?0.082?and a smaller amount of wear?0.64 mg?than the continuous walking.In the three jogging modes,the warm-up intermittent jog had the smallest average coefficient of friction?0.084?and the lowest wear volume?0.90 mg?.Hence,the suitable walking method for Fe-SA/P?AM-co-10%AA?multiple cross-linked hydrogels applied to artificial knee cartilage was intermittent walking and the appropriate jogging method was warm-up intermittent jogging.The biological evaluation of Fe-SA/P?AM-co-10%AA?multiple cross-linked hydrogels co-cultured with MG-63 cells showed that the proliferation rate of MG-63 increased significantly with the increase of co-cultured time,and except for co-cultured for 1 d,the proliferation rate of MG-63 cells was not significantly different from that of the control group.The results of inverted phase contrast microscopy and fluorescence microscopy showed that MG-63 cells could maintain their spreading state within 1,3,and 5 days,and showed a normal"spindle"morphology.It revealed that the Fe-SA/P?AM-co-10%AA?multiple cross-linked hydrogel was essentially non-cytotoxic.The results of this study indicated that the Fe-SA/P?AM-co-10%AA?multi-crosslinked hydrogel not only had a close mechanical properties to the human knee joint cartilage,but also exhibited good tribological performance during the simulation of the human body's knee joint sliding friction.At the same time,it showed good biocompatibility in co-cultured with MG-63cells which provided a scientific basis to further research of the application of Fe-SA/P?AM-co-10%AA?multiple cross-linked hydrogel in biomedicine field.