-How Spraybase® could help fill in the gaps in engineered tissues.-
One of the most interesting and potentially life-saving areas of endeavour that could be supported by the electrohydrodynamic technique is tissue engineering.
The prospect of making viable human body parts to order would have no end of uses, not least not least as material for vital transplants, or as replica tissues for more rigorous medical testing. Recent attempts to reproduce some tissues, such as blood vessels and windpipe linings, have proven the principle [^2], but have also made the daunting complexity of tissues more apparent. Overcoming these challenges will require both highly consistent and highly versatile electrohydrodynamic apparatus.
Tissues are more than just a cell culture: they are a cluster of several strains of cooperating cells, bound by vital structural components. These specialised cells share the same genetic code, but they will have inherited different ways of reading it from their stem-cell ancestors, who will have been induced to differentiate, grow, and/or migrate by chemical signals at the embryonic stage.
These populations of cells grow, divide, and die in response to chemical signals they send each other, and that they receive from elsewhere in the body. They also build and maintain a web of material called the extracellular matrix, which holds them in place. This ECM is more than a strengthener: the body’s chemical vocabulary is relatively limited, so a cell that is out of place will not interpret its signals out of context: indeed, in some cases, the ECM's topography can force the cells into shapes which cause differentiation independently of chemicals[^4].
In tissue engineering, the challenge is to ignite this complex system outside the context of the embryo and womb. Tissues are considered too intricate to build cell-by-cell, so the challenge is to encourage the cells to differentiate and position themselves, by using appropriate stem cells, and subjecting them to embryonic conditions. Cells need to be seeded into a structure called a tissue scaffold which will approximate the ECM, holding the cells in place, and in shape, so that they can be exposed to appropriate gradations of chemical signals.
For a tissue of any complexity, or depth, or with aligned cells, effective gradients of signalling molecules will have to be produced by peptides released from within the scaffold, over the week-long timescales the cells will require to develop their own ECM and viable tissue. Neither fully motile nor immobilised growth factors can recreate these conditions, but a steady release from encapsulated drugs might.
Electrospinning has received a great deal of attention for that reason: the technique can readily lay down nanofibrous mats which not only have similar diameters and chemical compositions to ECM, but can mimic its nanotopography. Coaxial nanofibers has also been shown to release functional signalling peptides which can induce cellular differentiation over the timescales required.
However, the design quality of nanofibrous tissue scaffolds needs to be improved, and this will probably require more consistent and versatile machines to produce them. Often fibers are laid too close for cells to proliferate into thick tissues. This cellular perfusion problem has only been convincingly solved by the concurrent deposition of cell-soluble sacrificial fibers (of poly-ethylene oxide)[^1] in among the scaffold fibers. It has also been found that drug-eluting fibers make poor scaffold material because they tend to degrade unevenly, and rather too fast, whereas incorporating localised peptide-delivering nanoparticles, which need not serve a structural function, can improve the acuity of the signalling gradients.
Although the concept of electrohydrodynamics brings us much closer to tissue engineering, sophisticated tissues cannot be feasibly researched and developed without the use of more versatile, and more stable electrospinning and electrospraying apparatus. The Spraybase® system is continually improved on with a view to addressing challenges such as these.