Synthetic tissues

Abstract

While significant advances have been achieved with non-living synthetic cells built from the bottom-up, less progress has been made with the fabrication of synthetic tissues built from such cells. Synthetic tissues comprise patterned three-dimensional (3D) collections of communicating compartments. They can include both biological and synthetic parts and may incorporate features that do more than merely mimic nature. 3D-printed materials based on droplet-interface bilayers are the basis of the most advanced synthetic tissues and are being developed for several applications, including the controlled release of therapeutic agents and the repair of damaged organs. Current goals include the ability to manipulate synthetic tissues by remote signaling and the formation of hybrid structures with fabricated or natural living tissues.

Keywords: 3D printing; artificial; droplets; synthetic biology; synthetic cells; tissues.

Figure 1.. Arrangements of cells in living tissues.

(a) Light micrograph of a section of the skin from the back paw of a mouse. Basement membrane (red arrow); basal epithelial cell (yellow arrow) [43] (Adapted with permission of Rockefeller University Press; permission conveyed through Copyright Clearance Center, Inc. Arrows added). (b) Light micrograph of a longitudinal section of human Achilles tendon. Collagen fiber bundles (yellow arrow); fibroblasts (black arrows) [44] (Adapted under CC BY 4.0. Image cropped, yellow arrow added). (c) Light micrograph of a section of the human retina. The cells are patterned to permit light detection and signal transmission to the brain [45] (Adapted under CC BY 3.0. Image cropped, labels added). Tissues of both high and low cell density are illustrated (‘a’ and ‘b’). Cells at high densities in tissues can be patterned (‘c’).

Figure 2.. Synthetic tissues from assemblies of compartments.

(a) ‘Colonies’ of giant lipid vesicles. Vesicles with negative surface charge were aggregated with poly-l-arginine [4] (Republished with permission from Wiley-VCH, Copyright 2012). Scale bar 30 µm. (b) Lipid vesicles connected by nanotube networks [46] (Republished with permission from the National Academy of Sciences, Copyright 2002). Arrows mark three-way junctions. Scale bar 5 µm. (c) An engineered dimeric α-hemolysin pore designed to act as a gap junction [8] (Labels reproduced). Left: cartoon (not to scale) showing the pore connecting a liposome and a planar lipid bilayer. Right: transmission electron micrograph showing the dimeric pore connecting two liposomes. (d) Diffusion-based communication between cell-mimics comprising DNA-containing hydrogel compartments within porous polymer membranes. A diffusive signaling molecule, here T3 RNA polymerase (T3 RNAP), effects the expression of a reporter gene in the recipient cell-mimic [10] (Republished under CC BY 4.0).

Figure 3.. Synthetic tissues based on droplet-interface bilayers (DIBs).

(a) Formation of a DIB in a lipid-containing oil [12] (Adapted with permission from The Royal Society of Chemistry. Labels added). (b) A 3D-printed patterned network based on DIBs [13] (Republished with permission from AAAS). (c) Droplets connected by DIBs encapsulated in oil sealed within an alginate shell obtained by a microfluidics approach [15] (Adapted under CC BY-NC 4.0. Image cropped). (d) An assembly of aqueous droplets formed by injection into an oil drop encased in agarose [14] (Republished under CC BY 4.0). Scale bar 1 mm. (e) A patterned network in an aqueous environment after 3D printing within a suspended oil drop [13] (Republished with permission from AAAS). Scale bar 400 µm. (f) Spheroidal assembly of a coacervate droplet: a protein-polymer ‘proteinosome’ [21] (Republished with permission from Springer Nature, Copyright 2018). Scale bar 50 µm.

Figure 4.. Functional synthetic tissues.

(a) A conductive pathway in a 3D-printed synthetic tissue [13] (Republished with permission from AAAS. Axis labels reproduced). When electrodes are placed at both ends of the pathway (upper panel), a current flows through the system. Scale bar 500 µm. (b) A folding printed synthetic tissue based on osmotic water flow [13] (Republished with permission from AAAS). Scale bar 200 µm. (c) A three-droplet system, which performs a three-step enzymatic reaction cascade, one step in each droplet. The fluorescent resorufin product appears yellow [25] (Adapted under CC BY 4.0. Reaction schematic and labels reproduced). Scale bar 250 µm. (d) Light-activated expression of the fluorescent mVenus protein in a 3D-printed network [28] (Adapted under CC BY-NC 4.0. Labels reproduced). (e) Signaling systems in synthetic tissues. There is a range of inputs, modes of transmission and outputs (see the text). (f) Hybrid tissues: (i) synthetic tissue and droplets containing living cells printed together and (ii) synthetic tissue embedded in living tissue.