The ultimate goal of tissue engineering is to assemble different types of cells into one functional tissue/ organ. Tissue assembly from cellular elements could be achieved by few different “bottom-up” approaches: bioprinting, self-assembly alone, dielectrophoretic patterning, micromolding, magnetic levitation, using specific devices (for example Bio3P) and robots. In the last few days we learned about two more new methods for tissue assembly.
The first method was invented by researchers from University of Toronto and published in Science Advances. The authors made bio-scaffold, called Tissue-Velcro, which allows scalable assembly of 2D cellular structures into 3D multicellular functional tissue:
… by adding T-shaped hooks onto the scaffold mesh, we created a Tissue-Velcro system allowing multiple cell types to be cultured individually and then assembled together vertically or horizontally to instantly establish a 3D mosaic coculture system that could be disassembled on demand.
They tested Tissue-Velcro on cardiac tissue assembly. A quote from Extreme Tech:
The beauty of this arrangement is that tissues of arbitrary thickness, and therefore potential pumping power, can be made to order. The team has already made modular hybrid tissues up to three layers thick that include fibroblasts and endothelial cells in addition to the cardiomyocytes. The polymer base should last several months as it is slowly degraded and replaced by a natural secreted extracellular material.
Ability to disassemble these 3D tissues on demand and re-assemble them again without loss of functionality of individual layers, makes this approach really modular.
The second method, called DNA-programmed assembly of cells (DPAC), was described by researchers from University of California San Francisco and Berkley in Nature Methods. The idea of using DNA strands pairing for biological structures assembly is not new. DNA-directed assembly and nucleobase pairing were used before for self-assembly of hydrogels.
DPAC was used for the first time for multiple types of cells to assemble them into microtissues and organoid-like structures. DPAC is a multi-step process, which includes (1) printing of DNA spots on a glass slide, (2) labeling of cells with lipid-modiified (to attach it to cell membrane) complement DNA, (3) first assembly of DNA pattern and complementary cells, washing, (4) layer-by-layer or different patterns assembly, (5) embedding cell patterns and layers into extracellular matrix gel with DNase, (6) DNase cleave DNA, gel is peeled or/and added for support, (7) cells condense into 3D microtissues. You can watch a process on this video. A quote from ArsTechnica:
It’s important to note, however, that the authors only test this with cell types that tend to naturally form coherent tissues if put in the right environment. Not all cell types are quite that cooperative.
And there are trade-offs. 3D printing probably allows finer control and construction of more complicated structures. But the cells are squirted out of the print heads at random, meaning that their density is going to vary quite a bit. That may limit their ability to interact and organize. Here, while the structures probably aren’t as complicated, the cells necessarily start out in contact with each other, making it a bit easier for them to self-organize.
The authors discussed the future development of DPAC and its limitations (references removed by me):
There remain numerous opportunities for improving DPAC. For example, delivery of structured chemical, physical and hemodynamic signals to assembled microtissues, as well as the potential to perfuse embedded vasculature, could be achieved by merging DPAC with microfluidic technologies such as those used in organs-on-a-chip. Merging DPAC with 3D printing could provide a means to control the spatial heterogeneity of ECM in addition to the spatial heterogeneity of cells. Combined with DPAC, folding, stacking or rolling techniques could generate thicker microtissues. Finally, the incorporation of stem cells or even whole tissue fragments as building blocks could enable the study of organoid development and disease processes in higher throughput and in a more reproducible 3D setting. However, DPAC is fundamentally limited to cells or tissue components that can survive dissociation and that can be labeled by DNA.
I’m really amazed by a progress in tissue assembly. There are variety of methods with pros and cons, but everyone can choose the right one for particular application. Most of these methods look beautiful in vitro, but for therapeutic applications they should be tested in animal models. Some of them may not “pass” in vivo tests, but sill be applied for drug R&D and disease modeling. Finally, combining of these methods together sounds very interesting. I’ll continue watch this field and keep you updated.