Tensile Mechanical Properties And Viscoelastic Constitutive Model Of Koilin Membrane

Due to the excellent biocompatibility,bioadhesion,and biodegradability,natural polymers attract much attention in tissue engineering research.However,at present,the mechanical strength of natural polymer materials is generally low.Furthermore,the strength of hydrophilic polymer materials would greatly reduce in vivo,and fail to meet the mechanical requirements of tissue engineering scaffolds.Among the methods on solving this problem,designing the structure of scaffolds can only increase their strength to a limited degree.Synthesizing composites always sacrifices the biological properties of natural materials.While adjusting the molecular structure has the potential to intrinsically modify the properties of polymers.But this method is limited by the lack of proper polymer references.Thus,in order to synthesize polymers with both excellent hydrophilic and mechanical properties,returning to nature and learning from natural biological materials,which adapt well to the environment,for references is necessary.In this paper,through bionic ideas,we studied a biological material,koilin membrane.Through preparing tensile specimens along and across wrinkles of koilin membrane,the dependences of tensile properties on directions were tested.By building tensile experimental system under controllable humidity,the tensile mechanical properties of koilin membrane under different moisture contents were obtained.After pre-soaking specimens in hydrochloric solutions with different pH values,the effects of acid solutions on the tensile properties of koilin membrane were measured.Through tensile experiments at different strain rates,quasi-static loading and unloading tensile experiments and stress relaxation experiments,the viscoelasticity of koilin membrane was tested.Then generalized Maxwell viscoelastic constitutive model was used to fit the experimental data and establish the viscoelastic model of koilin membrane.On microstructure,by SEM,the sections of liquid nitrogen frozen specimen and the fracture surfaces of specimen at different moisture contents were observed.And the elementary compositions of specimen sections were tested through EDS.The results show that koilin membrane can absorb much water which weighs 80%-100%of its own dry weight,which means koilin membrane is well hydrophilic.The strength of koilin membrane along wrinkles is stronger than that across wrinkles,which means that the mechanical properties of koilin membrane are direction-dependent.Along wrinkles,the strength at high moisture content is about 4.8 times of that at low moisture content,which means that the koilin membrane can be enhanced by moisture.While the strength of other well-known materials,such as hydrophilic double-network hydrogel,spider silk,and etc.,would decrease with increasing moisture content.Meanwhile,the strength and elongation of koilin membrane would not largely decrease in acid solution whose pH value is 3,which means it can adapt to the acid environment within gizzard.Through viscoelastic tests,a viscoelastic consititutive model of koilin membrane applied for true strain 0.262-0.916 and strain rate 0.001 s^-1-0.1 s^-1was established.On microstructure,koilin membrane consists of koilin rodlets,which form along thickness direction of koilin membrane,and surface koilin,which fills the gaps among koilin rodlets.When moisture content is low,the brittle fracture of surface koilin would occur after small deformation.As moisture content increases,the brittle fracture of surface koilin would transform to ductile fracture,and the flaw which causes the fracture of specimen would transform from the initial cracks on surface to dimples inside the specimen.On elementary compositions,koilin rodlets contain more Ca content than surface koilin.Overall,the mechanical properties of koilin membrane can fit in the acid environment within gizzard and meet the mechanical demands during the movement of gizzard.The strength of koilin membrane increases with increasing moisture content,and this property could provide bionic reference for enhancing the mechanical strength of natural polymers in moist environment.The viscoelastic constitutive model of koilin membrane established in this paper could be used in related mechanical research.On microstructure,surface koilin mainly bears the stretching along in-plane direction of koilin membrane,and koilin rodlets mainly bear the compression in the thickness direction and the friction of food from inner lumen of gizzard.