Engineered materials for organoid systems

Abstract

Organoids are 3D cell culture systems that mimic some of the structural and functional characteristics of an organ. Organoid cultures provide the opportunity to study organ-level biology in models that mimic human physiology more closely than 2D cell culture systems or non-primate animal models. Many organoid cultures rely on decellularized extracellular matrices as scaffolds, which are often poorly chemically defined and allow only limited tunability and reproducibility. By contrast, the biochemical and biophysical properties of engineered matrices can be tuned and optimized to support the development and maturation of organoid cultures. In this Review, we highlight how key cell-matrix interactions guiding stem-cell decisions can inform the design of biomaterials for the reproducible generation and control of organoid cultures. We survey natural, synthetic and protein-engineered hydrogels for their applicability to different organoid systems and discuss biochemical and mechanical material properties relevant for organoid formation. Finally, dynamic and cell-responsive material systems are investigated for their future use in organoid research.

Fig. 1 |. Timeline of milestones for biomaterials, organoids and stem cells.

PEG, polyethylene glycol.

Fig. 2 |. Organoid cell sources and types.

a | The initial cells used for organoid culture can be human adult stem cells, pluripotent stem cells or primary tissues collected via biopsy. Each cell type has associated advantages and disadvantages as a source for organoids. b | Although each type of organoid develops with unique morphological and physiological features, a general pattern of development is followed. Different studies may desire organoids in different stages of development or fabrication platforms. The proliferation and early differentiation of single cells and cell clusters leads to the generation of early-stage organoids, sometimes referred to as spheroids. Late-stage organoids are structures that resemble native tissue morphology, contain correctly oriented and specialized cells, and display organ-like processes. Organoid cultures are often placed within a 3D matrix at the single-cell or spheroid phase. The physiological relevance and complexity of organoids can be improved by various biofabrication techniques.

Fig. 3 |. Cell-matrix interactions.

a | The matrix microenvironment influences cellular behaviour by cell-matrix interactions established by cell-adhesive ligands bound to cell-surface receptors, by the mechanical properties of the matrix (for example, stiffness) and by the degradability of the matrix. b | The 3D geometry of the matrix can be described by the relative pore or mesh size compared with the cell size. Matrix geometry defines the nature of cell-matrix contacts and how a cell senses other matrix properties, such as cell-adhesive ligands or mechanical properties. Fibrous matrices, for example, natural extracellular matrix and biomaterials such as collagen, have a pore size on approximately the scale of cells. Highly crosslinked hydrogels, such as polyethylene glycol hydrogels, have mesh sizes much smaller than the scale of a cell, which can inhibit cell growth and migration in the absence of degradability. Macroporous materials have large pores that can be on the scale of a cell or larger. c | Viscoelastic materials, for example, native extracellular matrix, display stress-relaxation behaviour. Upon application of stress to the matrix, the molecules rearrange to dissipate the stress over time. By contrast, elastic materials cannot dissipate stress. Plotting normalized stress versus time of a stress-relaxing material and an elastic material under constant strain shows that the stress in a stress-relaxing material decreases over time, whereas it remains constant in an elastic material. d | In materials with stress-stiffening behaviour, an applied stress (σ) that is greater than the characteristic critical stress point (σ_c) leads to stiffening of the material, owing to local molecular stretching. The modulus of a non-stiffening material remains the same regardless of the applied stress, whereas the modulus of a stress-stiffening material increases once σ > σ_c.

Fig. 4 |. Dynamic organoid niches.

Engineered materials may enable the control of lineage commitment of stem and progenitor cells in time and space. a | The release profile of matrix-immobilized growth factors can be designed to sequentially deliver signals through growth factor-material affinity or proteolytic release, providing the appropriate biochemical cues for maturing organoids. b | Materials can be patterned with growth factors to spatially control lineage commitment and, thus, the development and maturation of organoids. c | Materials can be patterned with cell-type-specific adhesive ligands to guide cell self-sorting of early cell clusters and organoids. d | The mechanical properties of a material impact cellular migration. e | Material degradation can be designed to control cellular migration and morphogenesis of the developing organoid. f | Biofabrication techniques provide opportunities to produce spatially controlled, engineered matrices for organoid culture. Bioassembly could be used to spontaneously form microtissues with zonal organization of region-specific organoids. g | Bioprinting enables the rapid fabrication of complex tissue architectures, for example, vascularized constructs. h | Organoids can also be incorporated into organ-on-a-chip platforms. These platforms can provide powerful models of clinically relevant multi-organ interactions^–.