Background
Although in vivo tissues have a three-dimensional structure, studies of the structure, function and pathology of human tissues often rely on in vitro two-dimensional (2D) models and animal models. However, since the structure of in vitro monolayer models is very different from the cell microenvironment in vivo, cell behavior and function, such as cell-cell and cell-matrix interactions, are greatly affected; in addition, species differences often make animal models unable to reproduce human characteristics. Therefore, three-dimensional (3D) culture of tumor cells has been considered as an alternative method, which is simple and practical and has the advantage of simulating the cell microenvironment in vivo.
3D Cell Culture Matrix
3D cell culture matrix simulates one or more properties of the extracellular matrix (ECM) and tumor microenvironment, and is usually composed of a porous structure with a diameter of less than 300nm, which can provide sufficient space for cell growth. Cancer cells can form 3D aggregates or spheres within the matrix. According to the main components, 3D cell culture matrix can be divided into two categories: matrix based on natural materials and matrix based on synthetic materials. Matrices based on natural materials can provide a biological environment, but the mechanical properties of the materials are usually poor, and the differences between batches cannot be completely eliminated. Natural materials are usually used to form hydrogel composites. Synthetic materials are generally polymers, such as polyethylene glycol (PEG), polylactic acid (PLA), poly (lactide-co-glycolide) (PLGA/PLG), etc., which are biodegradable and renewable. Among these materials, thermogel synthetic copolymer hydrogels with sol-gel phase transition show lower critical solution temperature (LCST) behavior, which is meaningful for 3D cell culture matrix. When the sol-gel transition temperature of the smart hydrogel is between 5°C and 37°C, the matrix has advantages in further separating materials and cell aggregation.
3D Culture Medium of PLGA-PEG-PLGA
Hydrogels have always been widely concerned in tissue engineering and 3D cell culture due to their flexible matrix, high water content and responsive network structure. Among them, PEG hydrogels can prevent nonspecific protein adsorption, have good biocompatibility, and are approved by the US FDA for use in humans, making them excellent biomaterial candidates. Thermosensitive hydrogels based on PLGA-PEG-PLGA triblock copolymers have been used to deliver proteins and water-insoluble drugs. The appropriate LCST and good biocompatibility of PLGA-PEG-PLGA triblock copolymers make them a good choice for in vitro cell culture matrix.
Figure 1. Sol–gel transition of avastin/PLGA-PEG-PLGA hydrogel. (Xie B, et al.; 2015)
PLGA-PEG-PLGA triblock copolymer is a thermosensitive polymer. Below the LCST, it is hydrophilic and soluble in water. When heated above the LCST, it collapses and precipitates. However, the sol-gel phase transition temperature is affected by many factors, especially the concentration of the triblock copolymer in water. The PLGA-PEG-PLGA triblock copolymer was characterized by UV spectrophotometry, and its LCST was 26.2°C. The sol-gel phase transition temperature of the PLGA-PEG-PLGA triblock copolymer solution with a mass fraction of 30% was further determined by the test tube flip method. The study found that the sol-gel phase transition occurred only when the mass fraction of the PLGA-PEG-PLGA triblock copolymer was increased to 30%. When heated to 37°C, the triblock copolymer still remained in a gel state, indicating that the thermosensitive hydrogel can be used as a cell culture matrix at the culture temperature. Some researchers jointly developed a thermogel hydrogel based on the PLGA-PEG-PLGA triblock copolymer and used it as a cell culture matrix. Ovarian cancer cells (HO8910) were cultured in a hydrogel with a porous structure, and the results of cell morphology characterization proved that this smart hydrogel provided an appropriate microenvironment for the proliferation of ovarian cancer cells.
Conclusion
Different from the complex requirements of tissue and organ culture equipment in bioengineering, the PLGA-PEG-PLGA triblock copolymer matrix is simple, efficient, and low-cost, which meets the requirements of 3D culture of tumor cells. Due to the good hydrophilicity of the PEG segment, the interface of the PLGA-PEG-PLGA triblock copolymer hydrogel is also hydrophilic, and cells are not easy to adhere to the hydrogel. Based on this anti-cell adhesion interface property, cells cannot form cell-matrix interactions, but form cell-cell interactions and aggregates. Therefore, the PLGA-PEG-PLGA triblock copolymer hydrogel is a good 3D cell culture medium matrial.
Reference
- Xie B, et al.; An injectable thermosensitive polymeric hydrogel for sustained release of Avastin® to treat posterior segment disease. Int J Pharm. 2015, 490(1-2):375-83.
