Preparation And Application Of Nanochitin/Zinc Ion Reinforced Polyacrylamide Composite Hydrogels

Hydrogel is a kind of physical or chemical crosslinked polymer material with three-dimensional network structure,which can absorb water and swell but not dissolve.The swelling degree can be adjusted by the composition of the polymer and the density and property of the crosslinked network in the gel matrix.Due to the lack of effective energy dissipation mechanism,the traditional hydrogels have poor mechanical properties,which limit their wide application.With the rapid development of nanotechnology,the mechanical strength of nanocomposites can be improved by using nanoparticles as reinforcing materials and combining them with polymer matrix.Nanocomposite hydrogels are characterized by high swelling rate,good selectivity,hydrophilicity,sensitivity to stimulation,good mechanical properties and so on.The applications of nanocomposite hydrogels in sensors,microfluidics and biotechnology are expanded.Chitin is the second most abundant biopolymer next to cellulose in nature.The nanoscale of chitin is an effective way for its high value utilization.As a kind of natural green nanomaterials,because of the nano size,large surface area,biodegradable,non-toxic and renewable,low density,easy modification,etc,are studied and used as a high-performance environmentally friendly biodegradable nanocomposite enhancing additives,in biomedical materials or tissue engineering has a broad application prospect.Therefore,the preparation of nanocomposite hydrogels with chitin as filler is of great significance to further broaden our understanding of nanochitin-mediated interaction networks in covalent cross-linked hydrogels.Chitin nanometer whiskers（ChNWs）were prepared by alkali-treated partial deacetylation.After chitin powder was purified to remove impurities such as minerals,proteins and pigments,then the partial deacetylation was conducted with 36%Na OH,and acetic protonation and multiple high-pressure homogenization treatments were added to obtain the transparent positively charged ChNWs dispersion solution.The average diameter of individual ChNWs was 30±7 nm and the length was 488±25nm.Through a series of characterization of chitin powder and ChNWs and comparison:the Zeta potential value of ChNWs is+53.4 m V,has a stable dispersion state,while the Zeta potential value of chitin powder is+32.4 m V,cannot be uniformly dispersed.The deacetylation degree of ChNWs and chitin powder was38.6%and 10.6%,respectively.And the crystallinity of ChNWs（74.8%）was lower than that of chitin powder（88.0%）due to the destruction of crystallization area.In addition,through thermogravimetric analysis,chitin powder and ChNWs started to degrade at about 300℃,both of which had good thermal stability.A physical/chemical double crosslinked PAAm/ChNWs/Zn²⁺nanocomposite（NC）hydrogel was prepared by in situ radical polymerization.Pure acrylamide（PAAm）,PAAm-Zn²⁺and PAAm-ChNW-0.5 hydrogels were used as control.According to the SEM,PAAm-ChNW-0.5-Zn²⁺hydrogel has a relatively complex layered porous dense structure,and ChNWs has bridging phenomenon,indicating that the NC hydrogel has a stronger and tougher network structure,and Zn is present and uniformly distributed in the NC hydrogel network.In the NC hydrogel,the hydrogen bond formed by between PAAm and ChNWs,and the coordination bonds formed by Zn²⁺,PAAm chain and ChNWs.They constitute a reversible non-covalent cross-linked network.Moreover,ChNWs is evenly dispersed in the hydrogel without aggregation,which improves the thermal stability of the hydrogel.PAAm-ChNW-0.5-Zn²⁺hydrogel has the maximum storage modulus（G’）and loss modulus（G’’）due to its strong hydrogen bond and metal coordination interaction.With the increase of ChNWs and Zn²⁺concentration,the mechanical properties of NC hydrogel were enhanced,and the optimal tensile and compressive strength are 321.9±8.2 k Pa and 6.95±0.20 MPa,respectively.When the resting time is 30 min,the self-recovery efficiency of hydrogel reaches more than 50%.Theself-recovery efficiency increases with the increase of resting time.The ten successive cycle loading-unloading compression test of the hydrogels nearly restore to its original shape after removal of external force,show that those hydrogels have good resilience,self-recovery and anti-fatigue performance.In addition,through the self-healing experiment of PAAm-ChNW-0.5-Zn²⁺hydrogel,the results showed that the self-healing efficiency of NC hydrogel reached 34.3±1.3%after 96 h of healing,and there was no obvious damage on the incision surface.Potential applications of PAAm/ChNWs/Zn²⁺nanocomposite hydrogels were investigated.The analysis of the swelling property of the hydrogel showed that the PAAm hydrogel with a lower crosslinked density had a larger swelling rate,while the PAAm-ChNW-0.5-Zn²⁺hydrogel with a tighter crosslinked density had the lowest swelling capacity,and its swelling rate decreased from 23.9±1.5 g/g to 11±40.1 g/g.The hydrogel showed good drug delivery performance.The PAAm hydrogel with high swelling performance had the maximum drug load（5908.0±136.4μg/0.3 g）and the cumulative drug release percentage（81.7±1.6%）within 48 hours,while the cumulative drug release percentage of PAAm-ChNW-0.5-Zn²⁺hydrogel was 54.6±1.3%,still reaching the drug release rate of more than 50%.In addition,with the increase of drug concentration,the maximum drug loading of different hydrogels also increased.The cytotoxicity test of the hydrogel showed that the relative cell viability was above 87%,and PAAm-ChNW-0.5-Zn²⁺hydrogel had the best cell viability effect,reaching 96.2±3.7%,indicating that the NC hydrogel had good cell compatibility.The initial and self-healing PAAm-ChNW-0.5-Zn²⁺hydrogels have stable electrical properties.When the hydrogel is used to design a wearable flexible strain sensor,the resistance change ratio of the human when bending fingers,back of hand,elbow and knee can be monitored in real time,indicating that the hydrogel strain sensor has good mechanical properties,bending repeatability and electrical stability.