Preparation And Properties Of Stimuli-responsive Hydrogels With High Mechanical Strength Based On Ionic Crosslinking

Stimuli-responsive hydrogels with variable physical or chemical properties,controllable volume or shape changes in response to external stimuli such as temperature,pH,ionic strength,light,electric field,or magnetic field have widespread applications in biomedical field such as tissue engineering,drug delivery and wound dressing.Their practical applications,however,are often limited by their weak mechanical properties.In this paper,we focused on two stimuli-responsive hydrogels,namely,Poly?N-isopropylacrylamide?based thermal-responsive hydrogels and chitosan based pH-responsive hydrogels with good mechanical properties by using ionic crosslinking,combined with doping nanoparticles.Firstly,we prepared stimuli-responsive hydrogels with high strength,high elastic modulus and high toughness by using inorganic nanoparticles and cations or anions as crosslinkers.Secondly,the mechanical properties and inner structure of thses hydrogels could be easily controlled by adjusting the feed ratios or the concentration or types of the ions.Poly?N-isopropylacrylamide?based hydrogels are the most extensively studied thermal-responsive hydrogels.In this paper,we prepared a physical mono-crosslinked P?N-isopropylacrylamide-co-AANa?hydrogel using nanoclay as crosslinkers via a simple radical copolymerization,and then immersed this hydrogel into Fe³⁺aqueous solutions with different concentrations for 3h,finally,the dual-crosslinked thermal-responsive hydrogels with good tensile and compressive strength,high elastic modulus,and high toughness were obtained by a soaking process in deionized water for 24h.The mechanical properties of the dual crosslinked hydrogels could be adjusted by changing the concentration of nanoclay,sodium acrylate or Fe³⁺,or by changing the types of multivalent metal cations such as Ca²⁺and Al³⁺.Under optimal conditions,the fracture tensile strength,elastic modulus,toughness?represented by the area of tensile stress-strain curves?and crosslinking density could reach up to 1.1MPa,1.4MPa,1.3MJ/m³,and 202mol/m³,respectively.This thermo-responsive hydrogels with good mechanical properties have potencial applications in tissue engineering and drug delivery.Chitosan is a natural polyelectrolyte derived from deacetylated chitin and is often used in preparing pH-responsive hydrogels.In this paper,we firstly prepared vinyl-functionalized silica nanoparticles,and used as crosslinkers to fabricate a semi-IPN chitosan/Polyacrylamide hydrogels,and then immersed this semi-IPN hydrogel into potassium carbonate aqueous solutions to obtain the double network chitosan/Polyacrylamide hydrogel based on electrostatic interactions between cations and anions,and the formation of microcrystalline of chitosan chains in alkaline solutions.The fracture tensile strength,elastic modulus and toughness could reach up to 3.85MPa,1.57MPa and 5.41MJ/m³,respectively.Due to the viscoelasticity of polymers,obvious hysteresis and internal friction could be observed during cyclic tensile tests.And the dissipated proportion of the double network hydrogel could reach up to 93%.More importantly,the mechanical properties could be easily tuned by changing the concentration of chitosan or potassium carbonate.Finally,by using different anions such as citrate,succinate,SO₄^2-,HPO₄^2-,H₂PO₄^-or S₂O₃^2-,double network hydrogels with different mechanical properties were obtained.The X-ray diffraction patterns and the water content results indicated that the difference in mechanical properties originated from charge of anions and basicity of the aqueous solutions of anions.