5. Conclusions
Culturing cells in three dimensions will soon be the preferred way to investigate cell–cell interactions, growth into tissue, mechanisms of stem cell differentiation, and improved drug efficacy, to name a few areas. Various materials are available to enable 3D cell culture among which is the polysaccharide alginate. Immobilizing cells in alginate hydrogels is a mild process that occurs under physiological conditions. In addition, cells can be retrieved from alginate hydrogels by a simple de‑gelling process that does not require disaggregation of multi-cellular structures. Alginate can be modified by the attachment of peptides that mimic extracellular matrix proteins, such as RGD, thereby allowing immobilized cells to seemingly interact with the alginate hydrogel. We have shown here that some cells actually require the presence of RGD in order to proliferate and form 3D structures.
Encapsulating cells in alginate hydrogel droplets was first described in the 1980s and various formulations are still under investigation for constructing artificial organs for, for example, treatment of Type I diabetes. Recently, two commercial alginate-based 3D cell culture systems have made their appearance. Cells are immobilized in an alginate foam-like scaffold and can then proceed to grow in three dimensions. Publications describing these 3D cell culture systems have begun to appear demonstrating their utility in several areas. Especially important for the use of alginate in 3D cell culture is the ability to change the physical characteristics of the hydrogel by changing the amount or type of gelling ions and/or alginate. One can now tailor-make an environment to which cells can adapt or differentiate.
In fields as diverse as tissue engineering and drug discovery, alginate-based 3D cell culture systems show a significant advantage over classical 2D culture techniques. In addition, automation of 3D culture techniques, especially for high throughput screening will greatly increase the use of culturing cells in this manner. Although most in vitro cell-based assays were originally designed using 2D cell cultures, it will be important to validate assays using a 3D culture system. New or adjusted detection chemistries may need to be developed in order to optimize the 3D cell model. This should not be detrimental to the use of 3D culture systems but rather an opportunity to improve and customize assay systems for multi-cellular structures.
The future promises ingenuity in adapting 3D culture systems into the fields of regenerative medicine. Supporting and improving cardiac function after infarct, correcting osteoarthritic cartilage degradation, and providing artificial skin for in vitro safety studies are among the fields 3D culture can bring new products and ideas. Finally, the adaptation of 3D bioprinting using an alginate-based bio-ink shows great promise. Patient-specific printed constructs can soon be made using alginate and a patient’s own cells. The availability of a commercial ultrapure, low endotoxin sodium alginate as well as peptide‑coupled alginate allows discrete cell signaling to be applied during 3D cell growth.