Bottom-up tissue engineering

Abstract

Recapitulating the elegant structures formed during development is an extreme synthetic and biological challenge. Great progress has been made in developing materials to support transplanted cells, yet the complexity of tissues is far beyond that found in even the most advanced scaffolds. Self-assembly is a motif used in development and a route for the production of complex materials. Self-assembly of peptides, proteins and other molecules at the nanoscale is promising, but in addition, intriguing ideas are emerging for self-assembly of micron-scale structures. In this brief review, very recent advances in the assembly of micron-scale cell aggregates and microgels will be described and discussed.

Figure 1

Modular approach to the production of scaffolds. Nano- or microscale modules consisting of cells, cell aggregates, particles or gels are allowed to assemble. The assembly process may be guided by specific interactions or may be random. Following crosslinking in some manner, a three-dimensional construct is available for in vitro expansion of cells or for in vivo transplantation.

Figure 2

(A) Complementary oligonucleotides were covalently coupled to the surfaces of different cells by click chemistry. (B–E) Two non-adherent cell types were mixed, and did not aggregate if their surfaces were modified with: (B) no oligonucleotides, (C) non-complementary oligonulceotides. However, specific aggregation was observed if the cell surfaces were modified with complementary oligonucleotides (D&E) . (F) Aggregation of DAPI stained cells (blue), with the central cell modified with fluorescein-conjugated oligonucleotides (green). (G) 3D reconstruction of an aggregate of Texas Red-labeled (red) and fluorescein-labeled cells (green). From Gartner ZJ, Bertozzi CR. “Programmed assembly of 3-dimensional microtissues with defined cellular connectivity”, Proceedings of the National Academies of Science, USA, 106:4606–10. Copyright 2009 to the authors, permission for reuse not required.

Figure 3

(A) Assembly of hexagonal PEG microgels floating on a perfluorinated solvent. (B) Fluorescence image of liver cells (red) in some of the hexagonal microgels and fibroblasts (green) in the rest. (C) Assembly of lock-and-key microgels. (D) Fluorescence image of liver cells (red) in the lock microgels and fibroblasts (green) in the key microgels. From: Zamanian B, Masaeli M, Nichol JW, Khabiry M, Hancock MJ, Bae H, et al. “Interface-directed self-assembly of cell-laden microgels”, Small, 2010, 6:937–44. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. (E–H) Micromasonry , using surface tension of a polymerizable solution of PEG to assemble microgels, followed by photopolymerization. (E&F) Tube formed around a PDMS cylinder. (G&H) Sphere formed in a non-aqueous solvent. From: Fernandez JG, Khademhosseini A. “Micro-Masonry: Construction of 3D Structures by Microscale Self-Assembly”, Advanced Materials. 2010;22:2538–41. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. (I&J) Photomasks were used to sequentially polymerize PEG (left) and gelatin (right) around a cell cluster (center). Qi H, Du Y, Wang L, Kaji H, Bae H, Khademhosseini A. “Patterned differentiation of individual embryoid bodies in spatially organized 3D hybrid microgels”, Adv Mater. 2010;22:5276–81. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. (K) Concentric rings formed by sequential photopolymerizations using different masks. (L) The rings stack to form tubes in mineral oil. Du Y, Ghodousi M, Qi H, Haas N, Xiao W, Khademhosseini A. “Sequential assembly of cell-laden hydrogel constructs to engineer vascular-like microchannels”, Biotechnol Bioeng. 2011; In press. Copyright John Wiley & Sons. Reproduced with permission.

Figure 4

(A) Millimeter-sized gels decorated with α- or β-cyclodextrins, or n-butyl or tert-butyl groups aggregate upon shaking, but only with gels containing their appropriate binding partners. Reprinted by permission from Macmillan Publishers Ltd.: Nature Chemistry, 3:34–37. Copyright 2011. (B) PEG microspheres were formed with identical chemistries and similar size, but with different buoyancies. Upon centrifugation microspheres separated out into distinct layers based on the buoyancy differences to produce graded structures. From: Roam JL, Xu H, Nguyen PK, Elbert DL., “The Formation of Protein Concentration Gradients Mediated by Density Differences of Poly(ethylene glycol) Microspheres”, Biomaterials. 2010;8642–8650. Copyright Elsevier, used with permission.