The Fabrication And 3D Prototyping Techniques Of Heterogeneous Micro/Nanofiber Nonwoven Materials

Nonwoven materials are important soft and porous textiles which are directly fabricated from fibers or extruded filaments using nonwoven techniques.Owing to the unique pore structure,high porosity,high permeability,tunable mechanical properties and ease of functionality,nonwoven materials have been widely used in various applications such as tissue engineering,personal hygiene,medical care,filtration,car,aerospace,military industry,etc.Heterogeneous micro/nanofiber nonwoven materials are promising in solving key problems in the area of regenerative medicine,etc.However,the fabrication and 3D prototyping techniques are still under development.Electrospinning is a promising technique capable of producing ultrafine fibers.Attributing to their unique properties including high specific area,high porosity,and interconnected pores of their assembled nonwovens,electrospun fibers hold great promise for various applications such as filtration,tissue engineering,etc.However,electrospun fibers are usually solid and have a smooth surface,thus it is of great importance to achieve routine fabrication of heterogeneous ultrafine fibers,which will contribute to improved application performance.In this study,the secondary structures of electrospun grooved fibers including groove number,groove depth,interior structure and diameter of grooved fibers can be tuned in a controllable manner,by systematically exploring the solvent ratio,solution concentration and relative humidity using PS/(THF/DMF)solutions.Results indicated that the surface of fibers can be tailored from no groove to single groove or multiple grooves,while the interior structure of grooved fibers can be tuned from solid to porous.Importantly,grooved fibers in diameter of 326 nm were fabricated which is the finest reported until now.Moreover,grooved beads on a ring fibers were fabricated through electrospinning,the surface of beads can be adjusted from porous,macro-porous to wrinkled,while the fiber between beads can be tailored from single groove to multiple grooves.In addition,the effect of solvent system including single and binary solvent systems on the secondary structure of grooved fibers was investigated.Solvents selected can be classified as low boiling point solvent(LBPS): dichloromethane(DCM),acetone(ACE),tetrahydrofuran(THF),high boiling point solvent(HBPS): N,N-dimethylformamide(DMF),cyclohexanone(CYCo),and non-solvent(NS): 1-butanol(Bu OH).It was found that single solvent systems produced non-grooved fibers,LBPS/DMF solvent systems generated fibers with different grooved textures,while LBPS/CYCo led to fibers with double grooved texture.Grooved fibers can also be fabricated from LBPS/LBPS,NS/LBPS,and NS/HBPS systems under specific conditions.The results indicated that grooved fibers could be fabricated from binary solvent systems as long as the DER(the difference of evaporation rate)is high enough.The formation of grooved texture should be attributed to three main mechanisms,namely voids based elongation,wrinkles based elongation and collapsed jet based elongation.To improve the controllability over the secondary structure of electrospun fibers,in situ mixing electrospinning(IME)as a convenient and reliable method was developed to directly fabricate macro-porous ultrafine fibers that allows for the simultaneous electrospinning of solution immediately after mixing.By systematical investigation of various solvent systems and mixing solvents,it was proposed that the formation mechanism of macro-pores(> 50 ?m)should be attributed to incomplete mixing coupled with nonsolvent-induced phase separation.The results indicated that macro-porous fibers owned much higher specific surface area,larger pore size and pore volume than conventional electrospun porous fibers.The diameter of the macro-porous fibers can be tuned from 1.80 ± 0.40 ?m to 6.75 ± 0.48 ?m by adjusting the feeding rate of CYH and the concentration of PS solution.The macro-porous fibers showed excellent performance of oil absorption of 95.68 ± 7.48 g g-1,57.98 ± 4.19 g g-1,and 34.82 ± 2.44 g g-1 for silicon oil,motor oil,and peanut oil,respectively.Moreover,the development of IME has greatly expanded the solution scope for electrospinning from stable solution system to unstable or sub-stable solution system,thus providing intriguing opportunities for the investigation and fabrication of heterogeneous fibers by in situ mixing of various immiscible solvents/solutions.However,it is still a vital challenge to use electrospun fiber nonwovens for 3D cell culture and tissue regeneration.Therefore,3D bioprinting technique was employed to fabricate cell-laden heterogeneous nonwoven materials.Cell-laden bioinks made of gelatin methacryloyl(Gel MA)physical gels(GPGs)were developed to direct bioprint highly porous and soft Gel MA cell-laden constructs at relatively low concentrations of the bioinks(down to 3%,compressive modulus: 1.8 k Pa).Attributed to their shear-thinning and selfhealing properties,the GPG bioinks could retain the shape and form integral structures after deposition,allowing for subsequent UV crosslinking for permanent stabilization.The structural fidelity was demonstrated by bioprinting various 3D structures including a rhombus,a diagonal square,a thin-walled tube,and a cone-shaped tube,which are typically challenging to fabricate using conventional bioinks under extrusion modes.It was also showed that the bioprinted constructs not only permitted cell survival but also enhanced cell proliferation as well as spreading at lower concentrations of the GPG bioinks.We believe that GPG bioprinting will provide many opportunities in convenient fabrication of 3D cell-laden constructs for applications in tissue engineering,regenerative medicine,and pharmaceutical screening.To achieve the controllable fabrication of complex and multi-component tissue constructs that recapitulate the complex physiological microenvironment of natural 3D tissues and organs,a multi-material extrusion bioprinting platform was developed which is capable of continuously and/or simultaneously depositing up to seven types of bioinks with fast and smooth switching among different reservoirs for rapid fabrication of complex(tissue)constructs.The concept was demonstrated by mounting a single printhead consisting of a bundle of 7 equal-sized capillaries,each connected to a unique bioink reservoir that could be individually actuated by digitally controlled pneumatic pressure.The ejection process,when synergized with the movement of the motorized stage,allows for rapid deposition of two-dimensional(2D)patterns and 3D architectures composed of multiple desired bioinks in a spatially defined manner,at a speed an order of magnitude faster than most existing nozzle-based bioprinting modalities.The capability of our rapid continuous multi-material bioprinter was further demonstrated by generating miniaturized cell-laden constructs containing several types of cells,fabricating gradient structures,and prototype multi-component bioelectronics.The proposed technology is likely to advance the field of extrusion bioprinting by offering strong capacity for engineering highly complex functional biomaterials,tissues,and devices in the future.