1. Droplet culture method
The droplet culture method is a three-dimensional cell culture technique proposed by Harrion et al. that utilizes a culture solution containing cell droplets to form backbone-free three-dimensional cell spheres through surface tension.
Compared with other three-dimensional culture methods, this method is easy to operate, promotes tight polymerization between cells, and enhances cell-to-cell and cell-to-matrix interactions, thereby increasing the differentiation potential of cells.
However, there are some problems with the droplet culture method, such as the difficulty in controlling the culture environment and the fact that the cell droplets are easily contaminated and not easily suspended.
In addition, cellular polymers may cause necrosis in the center due to insufficient oxygen and nutrients during culture. This problem can be avoided by controlling the number of cells inoculated.
To overcome these limitations, droplet culture methods can be used in conjunction with microchip technologies, such as microfluidic droplet chips and superhydrophobic chips, which utilize a pumping device to continually renew the culture medium, replenish depleted oxygen and nutrients, and support the long-term culture of cells.
2. Microfluidic chips
Microfluidic chips are devices based on microfluidic control technology that contain functional components such as micron-sized culture chambers, channels, and micropumps that enable precise preparation, reaction, separation, and detection of cells, tissues, and chemical reagents.
Such chips typically consist of micrometer-sized channels of two incompatible fluids, the aqueous phase and the oil phase, which form multicellular spheres in the aqueous phase when the aqueous-phase fluid containing the cells flows in the oil phase.
Microfluidic chips have the following advantages:
① Capable of precisely controlling the flow of cells through micron-sized channels by adjusting dynamic parameters to simulate specific environments to meet the needs of multicellular spheroid culture;
② The ability to precisely control the volume and shear stress of multicellular spheroids reduces cell damage and lowers reagent consumption;
③Precise regulation of the microenvironment of cells in space and time, can adjust the concentration of substances, solution temperature, pH value, etc., more realistically simulate the complex environment of stem cells in vivo, widely used in the study of disease mechanisms and drug screening and other fields.
3. Magnetic suspension culture method
Magnetic levitation culture method is a three-dimensional culture technique that utilizes magnetic force to bind cells to magnetic particles, thus maintaining the cells in suspension.
By adjusting the magnetic field, the shape of the cell clusters can be precisely controlled to promote contact and aggregation between cells to form cell spheres.
This culture method offers several advantages: low cost, ease of handling, biocompatibility, and viability of cultured cells with less necrosis in the core of the spheroplast.
Compared to traditional three-dimensional culture techniques, magnetic levitation enables more rapid formation of cell spheroids that are tightly connected and autonomously secrete endogenous ECM, eliminating the need for artificial substrates or special culture media, and the spheroids can be cultured for long periods of time.
In addition, the magnetic levitation culture method allows the shape of the cell spheres to be changed by adjusting the strength of the magnetic field, and the structure of the cell spheres remains intact even when the magnetic field is removed.
Compared to non-magnetic 3D cultures, magnetic cultures are more compact, contain more lipid droplets and extracellular vesicles, and have greater differentiation potential.
At the same time, cells cultured by this method showed significantly higher levels of multilineage differentiation capacity, surface markers, expression of extracellular matrix, and osteogenic and angiogenic proteins than those cultured in two-dimensional or other three-dimensional aggregates.
4. Rotary Cell Culture System (RCCS)
RCCS is a microcarrier technology that uses a simulated microgravity environment to rotate cells, tissues, and culture fluids in a free-fall-like state, thereby generating three-dimensional cell spheroids that can be both wall-adherent and suspended, which in turn can be used for large-scale cell expansion.
This system promotes cell proliferation and induces differentiation through a microgravity environment and low shear stress.
Compared with static culture methods, RCCS-cultured cells are subjected to less external mechanical force, less mechanical damage, higher cell freedom, and more likely to form three-dimensional spheres.
In the RCCS reactor, the cells are in free-fall and rotation and mixing are not limited by gravity in a single direction, thus allowing cells to grow in all directions.
The microgravity environment also affects the polymerization and depolymerization of microtubules inside the cell, reducing internal pressure and leading to changes at the molecular level that allow the cell to maintain good morphology.
In addition, the cells were subjected to shear forces caused by blood flow during the culture process, which enhanced the formation of stress fibers, slowed down the proliferation rate of the cells and slowed down the aging process, thus enabling the cells to maintain better activity in subsequent experiments.
5. Centrifugal pelletization
Centrifugal sphere-forming culture method enhances intercellular adhesion by centrifugation, and cells are precipitated and resuspended in culture medium, then dispensed into 96-well culture plates with cell-repelling surfaces to form three-dimensional cell spheroids.
The method is able to mimic cell development in vivo and enhance cell-to-cell and cell-to-ECM interactions.
The separation process of cell spheroids was rapid and efficient due to the use of non-protein water-soluble substrates. Compared with monolayer culture, the three-dimensional culture model of rat DPSCs obtained by centrifugal sphere-forming culture method showed little change in cell proliferation ability, but its alkaline phosphatase (ALP) activity was significantly increased.
ALP promotes dentin mineralization by regulating collagen synthesis and therefore may play an important role in the treatment of caries and endodontics.
6. Liquid mulching
The liquid-covered method reduces the adsorption of cells to the surface of the culture plate by using low-viscosity non-adherent culture plates, thereby inducing cells to attract each other and aggregate to form cell spheroids.
Low-viscosity surfaces are usually made of materials such as agar, agarose gel, or polymethylmethacrylate, which effectively inhibit cell attachment.
Liquid covering method is easy to operate, low cost, does not require special equipment, and the cells are subjected to less shear stress, easy access to the cell sphere, suitable for long-term large-scale culture of cell spheres.
However, there are some drawbacks to this approach:
①Liquid-covered method is static culture in which the substances in the medium remain static and metabolites accumulate with the culture time, which may adversely affect the normal physiological activities of the cells;
② On non-adherent culture plates with low adsorption surfaces, the growth of cell spheroids is easily disturbed, resulting in less reproducible cultures.
7. Manual scaffolding method
Conventional in vitro cell culture usually lacks extracellular matrix (ECM) components, which may be detrimental to cell survival and proliferation, and especially have a greater impact on stem cell culture.
The artificial scaffold method improves the proliferative activity of cells by mimicking the natural ECM.
The method inoculates cells on a scaffold consisting of an artificial matrix, and the cells gradually migrate to the interior of the scaffold after attachment to the scaffold and form three-dimensional cell spheres in the pores of the scaffold.
Hydrogels are commonly used biomaterials that mimic ECM and have a three-dimensional mesh structure and excellent biocompatibility, which can provide support for cell adhesion, growth and migration.
By modulating the physical properties of the hydrogel or introducing different functional groups according to specific research needs, microenvironments can be created that are suitable for specific cell growth and differentiation.
Natural hydrogels (e.g., collagen) not only provide support for cells, but also provide a good space for the exchange of substances between cells and have advantages such as degradability and low toxicity.
Studies have shown that type I collagen, in particular, is effective in promoting the proliferation and mineralization of dental pulp stem cells.
ZH_CN 
EN