1. Introduction
The world around us, including the human body, is constructed in three dimensions. Since the 1940s, cells have been cultured, often attached to glass or plastic surfaces, essentially in two dimensions. Today, there is a need for more realistic and controllable culture systems that support cell growth, organization and differentiation essentially as found in tissues and organs. Growing cells in 3D adds a variety of aspects more physiologically significant than would be possible in 2D. A few of these are: cell culture and tumor formation of malignant cells; more relevant drug development and testing; in vitro culture of multi-cellular tissue for later implantation.
Despite the major differences compared to the naturally occurring 3D cell environments found in tissue, most cell culture studies in vitro are performed using cells cultured as monolayers (2D) on hard plastic or glass surfaces because of the ease, convenience and high cell viability associated with this culture method. However, forcing cells to adapt to an artificial flat and a rigid surface can alter cell metabolism and change or reduce functionality, thereby providing results that may not be similar to expected behavior in vivo [1,2]. A powerful and reliable tool for evaluation of cell behavior is gene expression data. Significant changes comparing cells cultured in 2D compared to 3D can be found associated with key biological processes such as immune system activation, defense response, cell adhesion and tissue development [3,4]. There is no doubt that 3D systems are biologically more relevant and 3D cell culture is therefore expected to also provide cellular responses that will be of higher biological relevance.
The significance and potential of in vitro cell culture studies are great considering the need for more cost efficient development of new drugs, time efficient treatment of cancer patients, and an understanding of developmental biology and mechanisms of stem cell differentiation. One example relates to drug development where, currently, only 12% of drugs that enter clinical trials are eventually approved for use in humans [5]. Most drugs fail due to efficacy, which likely could have been revealed at an earlier time point with more reliable cell culture models. Consequently, appropriate cell models would also reduce the need for animal trials, especially for toxicity assays [6]. Reducing the number of animal trials would also be in alignment with the principles of the 3Rs [7] (Replacement, Reduction, Refinement) which are considered an ethical framework for conducting scientific experiments using animals humanely. To better predict the clinical outcome of medical treatments such as chemotherapy, the selection of drugs can be optimized based on the response from isolated cancer cells from the patient.
There are several formats and materials available that enable 3D cell culture. We will focus on the “physical” differently shaped hydrogel formats like beads, moldable gels, injectable gels and macroporous structures. However, other technologies such as hanging drop, low-binding plastic, pyramid plates, etc., are also available for culturing cells in 3D. Some macroporous scaffolds such as meshes, fibrous patches or foams, enable cell seeding throughout the thickness of the matrix and cells may be spatially organized. Such systems are, however, considered semi-3D or 2.5D [1,8] as the initial cell–matrix interaction will be more similar to what is found in 2D with cells spreading on the surface of fibers or pore walls. This is especially true for polystyrene-based 3D cell culture materials.
Nearly all cells that make up tissue reside in an extracellular matrix (ECM). The ECM consists of a complex three-dimensional (3D) fibrous meshwork of collagen and elastic fibers embedded in a highly hydrated gel-like material of glycosaminoglycans, proteoglycans and glycoproteins [1]. All together they provide complex biochemical and physical signals to the cells. A wide range of biomaterials have demonstrated applicability as matrices providing a biologically more relevant environment for cells mimicking several characteristics of the ECM such as physical, mechanical and biological properties. 3D cell culture can be defined as when cells are embedded in a scaffold or matrix and signals from the scaffold and surrounding cells can be received from all directions [1,8]. Cell to cell communication can occur in three dimensions as well. This requires that cells are first suspended in a hydrogel precursor solution and next entrapped by a gel initiation reaction forming covalently or non-covalently linked molecules [9,10]. Polymer hydrogels are considered well suited for 3D cell culture as they have similarities to natural extracellular matrix. Examples of synthetic materials with the capability of forming hydrogels are polyethylene glycol (PEG), poly(hydroxyethyl methacrylate) (polyHEMA), polyvinyl alcohol (PVA) and polycaprolactone (PCL). Natural polymers (and proteins) able to form hydrogels are alginate, chitosan, hyaluronan, dextran, collagen and fibrin where alginate hyaluronan (as a product of bacterial fermentation) and dextran represent non-animal derived materials.
Despite the homogeneous nature of synthetic polymers, their use as cell-entrapping materials has to some extent been avoided due to harsh polymerization conditions [1]. However, some initiator systems for photopolymerization of, for example, PEG-diacrylates are suitable for cell based hydrogel formation considering cytotoxicity, crosslinking efficiency and crosslinking kinetics [11]. Components of animal tissue are naturally recognized by cells due to the presence of cell binding ligands [12] and have been considered as good materials for scaffolds. However, these materials are less attractive because of a reduced degree of experimental control due to batch-to-batch variations as a result of their inherent diversity in material composition. Animal-derived materials may also have limited availability, and for use in the clinic, there are potential risks of immunogenicity and pathogen transmission; hence, obtaining regulatory approval for such applications may be challenging [8]. Natural hydrogels of non‑animal origin are of great interest because of their outstanding biocompatibility and mild gelation conditions, although limited control of gelation kinetics, inherent variations in material composition, and limited control over mechanical properties have been reported [1].
Alginate hydrogels have demonstrated high applicability as a structure for cell immobilization. Different soft and elastic hydrogels with typically 98%–99% aqueous media can be formulated at physiological conditions with preservation of cell viability and function. Since alginate microbeads were used for the first time in humans as an artificial pancreas in the 1980s [13], the polymer has been used with different cell types both in vivo and in vitro. Alginate is recognized for properties and characteristics such as its ability to make hydrogels at physiological conditions, gentle dissolution of gels for cell retrieval, transparency for microscopic evaluation, gel pore network that allows diffusion of nutrients and waste materials in addition to its non-animal origin. Culture of cells in alginate beads is well known [14], and a standard guide describing cell encapsulation in alginate is available from ASTM International [15]. Well-characterized alginates with high purity should be used to prepare hydrogels with consistent mechanical properties for cell encapsulation.
In this review, we will give an introduction to physicochemical and biological properties of alginates and the interaction of alginate hydrogels with cells. In addition, we will focus on 3D cell culture techniques and present aspects of immobilization of cells in alginate beads and new alginate‑based 3D cell culture kits commercially available for use with standard cell culture well plates.