Preparation And Applications Of Functional New Materials Inspired By Biological Additive Manufacturing Process

Organisms in nature face many challenges,such as environmental changes,food access,predator threats.As a result,organisms respond to these challenges by developing highly intelligent and sophisticated biological structures over billions of years of natural evolution.The structure and function of biomaterials provide great inspiration for the design of materials with new structures and functions.Therefore,“bioinspired materials”has been a research hotspot in the past few decades.In fact,biomaterials are determined by specific structure-forming processes,and in most cases,biological processes take place under environmental conditions.Hence,learning these biological processes can not only provide insight into the nature of specific functions of biomaterials,but also have great enlightenment for the development of new material processing and preparation technologies.Natural materials often undergo a growth process from small to large,which is a natural additive manufacturing process.The formation of these biomaterials involves many physiological processes from nano to macro scale.By studying the structure and formation process of biomaterials,we understand them from point to facet and from outside to inside.The microscopic repeating blocks of enamel are columnar structures and remain highly consistent across species.Inspired by the formation process of tooth enamel,an artificial enamel thin film（TiO₂-CS/GO）_nwith controllable thickness was obtained by hydrothermal growth of titanium dioxide（TiO₂）nanorods and deposition of polymer matrix chitosan（CS）on fluorine-doped tin dioxide（FTO）conductive glass layer by layer.The hardness（1.56±0.05 GPa）and Young’s modulus（81.0±2.7 GPa）of composite（TiO₂-CS/GO）₄are comparable to that of natural enamel.In addition,（TiO₂-CS/GO）₄also has great viscoelasticity,and the corresponding loss modulus is0.76±0.12 GPa.The excellent mechanical properties of（TiO₂-CS/GO）_nbiomimetic films can be attributed to three aspects:（i）the use of TiO₂nanorods with good chemical stability and mechanical strength as columnar inorganic layer can give the composites high mechanical properties;（ii）natural polymer CS uniformly fills the gap of TiO₂array,which can endow the composite with viscoelasticity;（iii）graphene oxide（GO）,which contains a large number of hydroxyl and carboxyl groups,as well as suspended bonds and defects,can provide multiple bonding positions for CS and TiO₂nanorods and ensure tighter interlayer bonding.Oysters can place unsolidified secretions on top of other oyster shells and form oyster reefs by trapping sand,bacteria or diatoms.Inspired by the formation process of oyster reefs,a Ca-PAA-ACP mineral hydrogel was synthesized by using high molecular weight polyacrylic acid（PAA）for chelation with calcium ions(Ca²⁺)followed by physical crosslinking with modified amorphous calcium phosphate（ACP）nanoparticles.The Ca-PAA-ACP mineral hydrogel exhibits versatility,including plasticity,3D printability,self-healing（85%recovery within 1 min）,stretchability（over 1500%elongation）,and ionic conductivity.The ionic skin prepared with this mineral hydrogel can accurately sense the repeated bending motion of the index finger and identify the bending degree of the index finger,with a response time of about 33 ms.In addition,the Ca-PAA-ACP ionic skin can also recognize external temperature stimuli and work stably in the temperature range from room temperature to 75°C.The human skin is one of the largest organs in the body,covering the surface of the body and being able to sense all kinds of stimuli with great precision.Inspired by the multifunctions of human skin,carbon nanotubes（CNTs）were combined with chelates of PAA,sodium alginate（SA）,and Ca²⁺to prepare a bioinspired hydrogel with multiple conductivity.The hydrogel possesses favorable stretchability,self-healing ability,and 3D printability.The hydrogel was printed on a stretchable insulating tape and connected with wires to fabricate a skin-like strain sensor.The sensor responds stably to external stimuli（such as finger bending,knee bending,and breathing）simultaneously with signals of relative resistance changes and relative capacitance changes,and has excellent sensitivity(6.29 in resistive mode and 1.25k Pa^–1in capacitive mode).Furthermore,when the strain sensor is encapsulated between two layers of polyethylene terephthalate（PET）film,it can be used as a flexible writing keyboard,accurately recognizing the letters written on it.Mussel adhesive protein has high strength,toughness,water resistance and strong adhesion to substrate.It also has good biocompatibility and degradability,showing bioadhesive with great advantages and potential.Inspired by mussel secretions,Ca-PAA-CNF-MXene hydrogel was designed by incorporating transition metal carbon/nitride（MXene）nanosheets into a three-dimensional chelate network formed by PAA and cellulose nanofibers（CNF）with Ca²⁺under alkaline conditions.Due to the abundant carboxyl groups on the PAA chain,many carboxyl groups and hydroxyl groups on the CNF chain,and various functional groups on the MXene surface,the Ca-PAA-CNF-MXene composite hydrogels can be formed by physical and chemical cross-linking,thus exhibiting excellent stretchability,plasticity,adhesion,3D printability,self-healing,biocompatibility and electrical conductivity.Furthermore,when the printed hydrogel was encapsulated in stretchable insulating tapes,a multifunctional sensor can be fabricated.This multifunctional sensor can be used simultaneously as a strain sensor,vocal sensor,signature sensor and Morse code transmitter.In addition,due to the moderate integration of water molecules,polymer matrix and MXene in the porous structure of the hydrogel,the hydrogel also produces the function of electromagnetic interference shielding.This paper focused on the preparation of functional new materials by multi-scale additive manufacturing technology inspired by biological processes.Firstly,the structure,function and formation process of natural biomaterials are characterized and analyzed to seek and design suitable bioinspired models;Then,the materials are synthesized from nanoscale to macroscale;Finally,additive manufacturing technology is used to prepare and apply the synthetic materials according to the bioinspired models,and the mechanical,thermal and electromagnetic properties of these materials,as well as the synergy and correlation between structure and property are studied.