Functionally graded biomaterials for use as model systems and replacement tissues

Abstract

The heterogeneity of native tissues requires complex materials to provide suitable substitutes for model systems and replacement tissues. Functionally graded materials have the potential to address this challenge by mimicking the gradients in heterogeneous tissues such as porosity, mineralization, and fiber alignment to influence strength, ductility, and cell signaling. Advancements in microfluidics, electrospinning, and 3D printing enable the creation of increasingly complex gradient materials that further our understanding of physiological gradients. The combination of these methods enables rapid prototyping of constructs with high spatial resolution. However, successful translation of these gradients requires both spatial and temporal presentation of cues to model the complexity of native tissues that few materials have demonstrated. This review highlights recent strategies to engineer functionally graded materials for the modeling and repair of heterogeneous tissues, together with a description of how cells interact with various gradients.

Keywords: Gradient materials; composites; light-based gradients; mesenchymal stromal cells; tissue engineering.

Figure 1.. Native functional gradients enable tissues to withstand compressive and torsional forces and facilitate nutrient transport.

(A) Mineralization gradient as ligament inserts into bone at the enthesis. Blue circles depict fibroblasts while white rhombuses depict hydroxyapatite. (B) Gradient of collagen fiber alignment in articular cartilage illustrates parallel collagen fibers in the superficial zone that become perpendicularly aligned toward the subchondral bone. (C) Porosity gradient from cortical to trabecular bone facilitates a transition from high strength and toughness that provides protection from the external environment to a more porous network that is home to stem and progenitor cells and increased cell exchange and nutrient transport.

Figure 2.

(A) UV generation of continuous gradients with photomasks. (i) UV light penetrates a photomask with a gradient in translucency to create a linear gradient on the substrate below. (ii) A photomask is pulled across a substrate to create a spatiotemporal gradient of UV exposure time. (B) Cross-sectional view of gradient patterns. (i) Discrete linear gradient which is often a result of combining individually fabricated materials. (ii) Continuous linear gradient used to interrogate the relationship of continuous stiffness changes on cell adhesion or differentiation. (iii) Radial gradient. An example of a non-linear gradient such as oxygen tension within and surrounding a cell spheroid. (C) ASCs exhibit more spreading and nuclear localization of Lamin A and YAP as substrate stiffness increases from low (2 kPa) to high (40 kPa); scale bar = 50 μm. Panel C reprinted from Reference 13 with permission from the National Academy of Sciences.

Figure 3.. Cellular response to FGMs.

(A) ASCs exhibit increased spreading and proliferation as stiffness of annealed microgels increases from Layer 1 to Layer 5. Image reproduced from Reference 73 with permission from John Wiley and Sons. (B) MDA-MB-231 breast cancer cells exhibit increased migration by day 7 towards the region of interest (white box) on a perlecan domain I gradient compared to uniform perlecan distribution and a day 1 control, scale bar = 1mm. Reprinted from Reference 61 with permission from Elsevier. (C) Biphasic HAp and CS scaffolds (D2) outperformed monophasic CS (C2), HA (B2), and sham (A2) groups in a rabbit osteochondral defect model. Reprinted from Reference 133 with permission from Elsevier.

Figure 4.

(A) Porogens such as salt or sugar can be distributed in a scaffold by size and subsequently leached out to create a porosity gradient. This construct can model the porosity gradient in bone. Porogen is denoted as yellow spheres, while resulting pores are white with imperfect boundaries. (B) Sugar particles leached out of a PLLA scaffold created a gradient in pore size from 300 to 600 μm. Sugar particle gradient can be seen in part a, with the resultant porosity after leaching in part b. Parts c-f are higher magnification. Reprinted from Reference 114 with permission from Elsevier.