Review: in vitro microvessel models

Abstract

A wide range of perfusable microvessel models have been developed, exploiting advances in microfabrication, microfluidics, biomaterials, stem cell technology, and tissue engineering. These models vary in complexity and physiological relevance, but provide a diverse tool kit for the study of vascular phenomena and methods to vascularize artificial organs. Here we review the state-of-the-art in perfusable microvessel models, summarizing the different fabrication methods and highlighting advantages and limitations.

Fig. 1

Schematic illustration of the fabrication methods and features of in vitro microvessel models. Models can be categorized as organ-on-a-chip platforms, that have moderate complexity and are high throughput, and organogenesis-based models that are characterized by high complexity and low throughput. Organogenesis refers to the self-organization of stem cells and/or organ-specific progenitor cells into tissue that resembles a specific organ. Platforms exploiting self-organization are generally challenging due to difficulties in differentiating and culturing stem cells, and time consuming since endothelium formation takes 1–2 weeks. Organ-on-a-chip platforms are devices that use microfabrication and microfluidics technologies to recapitulate specific aspects of organ structure and function. In general, organ-on-a-chip platforms are relatively easy to fabricate and endothelial layers can be formed in 2–4 days. The fabrication methods include: microfluidics, templating, 3D printing, and self-organization.

Fig. 2

Schematic illustration of membrane-based and ECM-containing microfluidic devices. (a) A membrane device with endothelial cells cultured on a porous membrane sandwiched between two orthogonal polydimethylsiloxane (PDMS) channels. Electrodes for transendothelial electrical resistance (TEER) measurements can be embedded in the top and bottom channels. These platforms are similar to transwell devices with the addition of shear flow. (b) ECM device with ECM separating two parallel channels. Endothelial cells are seeded onto the vertical sidewall of one of the channels. In addition, other cell types can be co-cultured in the ECM.

Fig. 3

Microvessel fabrication with cylindrical template. (a) A template rod inserted into a PDMS mold defines the location of the vessel. (b) A solution of the ECM, often collagen type I or fibrin, containing cells is introduced around the cylindrical template within the PDMS housing. (c) After gelation/cross-linking, the template rod is removed. (d) The platform is connected to a flow loop and endothelial cells are seeded into the cylindrical channel. (e) Adhesion and spreading of the endothelial cells on the internal surface of the ECM form the vessel lumen.

Fig. 4

2D microvessel array fabrication by lithographic patterning. Standard lithographic patterning is used to create a 2D array of rectangular channels in a matrix material. Following seeding with endothelial cells, the microvessels have rounded corners and display the versatility of co-culture with multiple cell types. RBC - red blood cells, WBC – white blood cells, EC – endothelial cells, and other relevant cells within the extracellular matrix (ECM).

Fig. 5

Direct bioprinting of ECM and ECs in a dissolvable matrix. (a) Gelatin containing HUVECs printed as a cylinder embedded in a collagen ECM. (b) Following printing and gelation, the gelatin is dissolved by heating to 37 °C. During this step, the device is rotated to enhance adhesion of the HUVECs along the internal walls of the cylinder. (c) Proliferation and spreading of endothelial cells results in the formation of a vessel lumen, and the microvessel is connected to a flow loop for perfusion.

Fig. 6

Schematic illustration of microvessel models formed by 3D template printing. (a) A printed 3D network of carbohydrate glass filaments is embedded in a hydrogel matrix. Other cell types, such as fibroblasts or smooth muscle cells can be embedded in the matrix. (b) The template is dissolved to form a perfusable network of cylindrical channels in the ECM. (c) Endothelial cells in suspension are introduced into the network of channels and allowed to adhere and spread to form the endothelium.

Fig. 7

Schematic illustration of microvessel models formed by self-organization. (a) Guided capillary self-organization and (b) guided capillary angiogenesis.

Fig. 8

Schematic illustration of the steps in guided capillary self-organization of microvessels. (a) Cells are seeded into an ECM and introduced into the PDMS housing. (b) Interstitial flow drives self-organization. (c) Cells organize into a network of perfusable capillaries/microvessels.

Fig. 9

Schematic illustration of the steps in guided capillary angiogenesis. (a) Endothelial cells are seeded into one of the microfluidic channels (Ch2), forming a monolayer on the side-wall of the ECM. Endothelial cells, fibroblasts, and other cell types can also be seeded into the ECM. (b) Chemical and/or pressure gradients between Ch2 and Ch3 promote formation and growth of angiogenic sprouts from the source channel (Ch2) towards the sink channel (Ch3). With the addition of endothelial cells in the ECM, both angiogenesis and self-organization contribute to the formation of a microvessel network. (c) A perfused microvessel network is formed between the source and sink channels.