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* These authors contributed equally
Presented here is a mild 3D printing technique driven by alternating viscous-inertial forces to enable the construction of hydrogel microcarriers. Homemade nozzles offer flexibility, allowing easy replacement for different materials and diameters. Cell binding microcarriers with a diameter of 50-500 µm can be obtained and collected for further culturing.
Microcarriers are beads with a diameter of 60-250 µm and a large specific surface area, which are commonly used as carriers for large-scale cell cultures. Microcarrier culture technology has become one of the main techniques in cytological research and is commonly used in the field of large-scale cell expansion. Microcarriers have also been shown to play an increasingly important role in in vitro tissue engineering construction and clinical drug screening. Current methods for preparing microcarriers include microfluidic chips and inkjet printing, which often rely on complex flow channel design, an incompatible two-phase interface, and a fixed nozzle shape. These methods face the challenges of complex nozzle processing, inconvenient nozzle changes, and excessive extrusion forces when applied to multiple bioink. In this study, a 3D printing technique, called alternating viscous-inertial force jetting, was applied to enable the construction of hydrogel microcarriers with a diameter of 100-300 µm. Cells were subsequently seeded on microcarriers to form tissue engineering modules. Compared to existing methods, this method offers a free nozzle tip diameter, flexible nozzle switching, free control of printing parameters, and mild printing conditions for a wide range of bioactive materials.
Microcarriers are beads with a diameter of 60-250 µm and a large specific surface area and are commonly used for large-scale culture of cells1,2. Their outer surface provides abundant growth sites for cells, and the interior provides a support structure for spatial proliferation. The spherical structure also provides convenience in monitoring and controlling parameters, including pH, O2, and concentration of nutrients and metabolites. When used in combination with stirred tank bioreactors, microcarriers can achieve higher cell densities in a relatively small volume compared to conventional cultures, thereby providing a cost-effective way to achieve large-scale cultures3. Microcarrier culture technology has become one of the main techniques in cytological research, and much progress has been made in the field of large-scale expansion of stem cells, hepatocytes, chondrocytes, fibroblasts, and other structures4. They have also been found to be ideal drug delivery vehicles and bottom-up units, therefore taking on an increasingly important role in clinical drug screening and in vitro tissue engineering repair5.
To meet mechanical property requirements in different scenarios, multiple types of hydrogel materials have been developed for use in the construction of microcarriers6,7,8,9,10,11. Alginate and hyaluronic acid (HA) hydrogels are two of the most used microcarrier materials owing to their good biocompatibility and crosslinkability12,13. Alginate can be easily cross-linked by calcium chloride, and its mechanical properties can be modulated by changing the cross-linking time. Tyramine-conjugated HA is cross-linked by the oxidative coupling of tyramine moieties catalyzed by hydrogen peroxide and horseradish peroxidase14. Collagen, due to its unique spiral structure and cross-linked fiber network, is often used as an adjuvant to mix into the microcarriers to further promote cell attachment15,16.
Current methods for preparing microcarriers include microfluidic chips, inkjet printing, and electrospray17,18,19,20,21,22,23. Microfluidic chips have been proven to be fast and efficient in producing uniform-sized microcarriers24. However, this technology relies on a complex flow channel design and fabrication process25. High temperature or excessive extrusion forces during inkjet printing, as well as intense electric fields in the electrospray approach, may adversely affect the properties of the material, especially its biological activity19. Besides, when applied to various biomaterials and diameters, the customized nozzles used in these methods result in limited processing complexity, high cost, and low flexibility.
To provide a convenient method for microcarrier preparation, a 3D printing technique called alternating viscous-inertial forces jetting (AVIFJ) has been applied to construct hydrogel microcarriers. The technique utilizes downward driving forces and static pressure generated during vertical vibration to overcome the surface tension of the nozzle tip and thus form droplets. Instead of severe forces and thermal conditions, small rapid displacements act directly on the nozzle during printing, causing a minor effect on the physicochemical properties of the bioink and presenting great attraction for bioactive materials. Utilizing the AVIFJ method, microcarriers of multiple biomaterials with diameters of 100-300 µm were successfully formed. Besides, the microcarriers were further proven to bind cells well and provide a suitable growth environment for adhered cells.
1. Cell culture
2. Preparation of nozzles
3. Preparation of hydrogel bioink
4. Microdroplets formation based on AVIFJ
5. Microcarriers formation based on AVIFJ
6. Inoculating cells on the surface of microcarriers
7. Analysis of microdroplets/microcarriers formation
Printheads of varied convergence rates and diameters were fabricated to achieve the printing of multiple types of materials. The nozzles obtained with increasing pull strength are shown in Figure 1B. The nozzles were divided into three areas: reservoir (III), contraction (II), and printhead (I). The reservoir was the unprocessed part of the nozzle, in which the liquid provided static pressure and bioink input for printing. The contraction area was the main part for generating downward drivin...
The protocol described here provides instructions for the preparation of multi-types of hydrogel microcarriers and subsequent cell seeding. Compared to microfluidic chip and inkjet printing methods, AVIFJ approach to constructing microcarriers offers greater flexibility and biocompatibility. An independent nozzle enables a wide range of lightweight nozzles, including glass micropipettes, to be used in these printing systems. The highly controllable processing enables parameters including the volume of the reservoir, the ...
The authors have nothing to disclose.
This work was supported by the Beijing Natural Science Foundation (3212007), Tsinghua University Initiative Scientific Research Program (20197050024), Tsinghua University Spring Breeze Fund (20201080760), the National Natural Science Foundation of China (51805294), National Key Research and Development Program of China (2018YFA0703004), and the 111 Project (B17026).
Name | Company | Catalog Number | Comments |
A549 cells | ATCC | CCL-185 | Human non-small cell lung cancer cell line |
Bright field microscope | Olympus | DP70 | |
Confocal microscope | Nikon | TI-FL | |
Fetal bovine serum, FBS | BI | 04-001-1ACS | |
Gelatin | SIGMA | G1890 | |
Glass micropipettes | sutter instrument | b150-110-10 | |
GlutaMAX | GIBCO | 35050-061 | |
H-DMEM | GIBCO | 11960-044 | Dulbecco's modified eagle medium |
Horseradish peroxidase powder | SIGMA | P6782 | |
Hydrophobic agent | 3M | PN7026 | Follow the manufacturer's instructions and use after dilution |
Micro-forge device | narishige | MF-900 | |
Non-essential amino acids, NEAA | GIBCO | 11140-050 | non-essential amino acids |
Penicillin G and streptomycin | GIBCO | 15140-122 | |
Petri dish | SIGMA | P5731-500EA | |
Puller | sutter instrument | P-1000 | |
Sodium alginate | SIGMA | A0682 | |
Trypsin | GIBCO | 25200-056 | |
Type I collagen solution from rat tail | SIGMA | C3867 |
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