Study On A New Type Tissue Skin Scaffold Made From Grass-d Erived Nanocomposite And Its 3D Bioprinting Process

3D printing technology facilitates rapid manufacture of customized models and accurate control of their internal microstructures.Thus,it provides a new method for the preparation of tissue engineered skin scaffolds.This thesis takes bio-material for human skin repair and skin scaffold structures as its research subjects,and aims to use cellulose nanocrystals(CNC)extracted from humulus japonicus stems(HJS)and gelatin(GEL)as raw materials to produce a CNC/GEL composite material suitable for 3D printing.Its printability as skin scaffold material and its preparation process are systematically studied for the purpose of obtaining skin scaffolds with desirable mechanical properties and porous structure.It is hoped that the results of this research will provide theoretical basis for the application of pure bio-materials and 3D printing technology in dermatology clinical field.(1)CNC is extracted from HJS through sulfuric acid hydrolysis and is transformed into a nanomaterial additive with high aspect ratio through a high-temperature and high-pressure treatment process.It is then compounded with GEL to obtain a gel composite suitable for 3D printing.The micromorphology,mechanics performance and bio-compatibility of this new gel composite are measured to prove and demonstrate the feasibility of using it as material for tissue engineered skin scaffold.The results show that HT140-HJS-CNC can be prepared by purification,sulfuric acid hydrolysis,and a high-temperature high-pressure treatment process.It has an average aspect ratio of about 63,an average diameter of about 6.84nm,and an average length of about 433.66nm.The addition of CNC into GEL significantly improves its elastic modulus and fracture toughness.Adding 10%CNC to 5%gelatin can increase the elastic modulus by 7.9 times and the fracture toughness by 6.73 times.Moreover,the composite gel prepared has no obvious cytotoxicity and inhibitory effect on cell proliferation and culture,making it an excellent bio-material.(2)The related process parameters of the hydrogel were measured,and the effects of temperature and material ratio on its viscosity of the composite gel were studied.The measurement results provide a feasible basis for the processing method of CNC/GEL composite gel to prepare tissue engineered skin scaffolds by pneumatic extrusion and cooling molding.Adopting finite element simulation of the hydrogel extrusion process,the velocity and shear force distribution of the hydrogel flowing in the barrel and the nozzle are studied.Results show that the viscosity range of the material suitable for the selected 3D printer is 4 Pa.s to 40 Pa.s.The process parameters that should be used during printing,including printing speed,air pressure,temperature,etc.are determined.Skin scaffolds were printed according to the selected process parameters,and a 3D scaffold with an aperture of about 500 um was obtained.The mechanical properties were significantly improved compared to pure gelatin scaffolds.(3)Based on systematic analysis of the effect of 3D printing typing algorithm on the porosity parameters and mechanical properties of the scaffold,this thesis puts forward a new hexagonal typing algorithm.In addition,scaffold models with different filling structures are established by CAD software,the porosity and modulus of elasticity were measured.3D-printed scaffolds were made,and the mechanical properties of the scaffolds in different directions were compared.Study results show that the proposed hexagonal algorithm can reduce the overlap length during the printing of the scaffold.At the same overlap length,the porosity of the hexagonal algorithm is increased by about 20%compared with the orthogonal algorithm.In terms of mechanical properties,the hexagonal scaffold's compression performance in the vertical direction is about 20%higher than orthogonal algorithms.At the same time,it has better ductility in the horizontal direction.Therefore,the hexagonal algorithm is very suitable for the scaffold forming of biological tissues.(4)In order to realize automatic parting and printing of the hexagonal algorithm,an auxiliary software which can realize the automatic parting of a given model and output a 3D printing process file is developed using the VC++platform.Study results show that this auxiliary software can automatically type 3D models with given contour size,plan the movement trajectory of the printer nozzle when filling the scaffold,and estimate the coordinates of each starting point,inflection point and end point.3D printing process files that can be read and recognized by the printer control software are automatically generated according to the compilation logic of the CNC system.(5)Based on comprehensive analysis of processing equipment needed for the 3D printing process,currently available 3D bioprinting equipment is structurally optimized.The 3D printing nozzle as well as the mechanical performance testing device used for testing the scaffold are optimized.Two sets of differential bridges were used to realize the output of stress and strain,and the equipment was planned for the control system and the software of the host computer.Study results indicate that the optimized design of the printing nozzle enables fast cylinder replacement and significantly shortens the time needed for switching between the print mode and refilling mode.In addition,the stress-strain gauge designed based on a parallel cantilever mechanism helps to solve the problem created by weak mechanical properties of biomass gel materials.