The new millennium has seen an impetus in bridging the physical sciences and the life sciences. Biotechnology and bioengineering, in particular, are at the interface of these two disciplines. Here, one set of emerging methodologies is the use of jets for the direct deposition of cells.
Ink-jet printing1 was the first jet technique for directly handling living cells. This method exploits either piezoelectricity or controlled thermal systems for the generation of droplets. Multidimensional cellular structures have been fabricated by means of this approach [1], [2] and [3], revealing great promise for the fabrication of biologically viable tissues, and one day possibly organs.
Although the technique has paved the way for the community to pursue the fabrication of such biological constructs, the process is limited by the concentration of cells that can be processed and the spatial resolution of the deposited cell-bearing residues. Biological architectures fabricated using ink jets have resolutions of several hundreds of microns for highly concentrated cell suspensions. High cell concentrations could be jetted by employing large-bore needles, but this implies a significant increase in residue size. Furthermore, increasing the cell concentration for a standard ink-jet needle increases cell mortality, the lower viable cell population resulting from the shear stresses imposed on the cells within the needle.
As an alternative, electrospray [4] – a technique completely determined by high-intensity electric fields – has recently been used to jet living cells [5]. The process, now referred to as ‘bioelectrospray’, can employ large-bore jetting needles charged to several thousand volts and has no compromising effects on cell behavior when compared with controls5 and [6]. Furthermore, bioelectrosprays can generate droplets a few tens of microns in size from millimeter-sized needles, and handle cell concentrations an order of magnitude higher than ink-jet printing [7].
Both these methods have the capability of fabricating three-dimensional structures, but the different jetting phenomena lead to different resolutions. In our hands, bioelectrosprays have a resolution dependent primarily on cell size and not on the jetting phenomenon.
The electrospray technique not only generates droplets a few microns in size for deposition, but its promise in a range of healthcare applications has also been shown. In 2002, a share of the Nobel Prize in Chemistry [8] was awarded for the coupling of electrospray ionization with mass spectrometry for the identification of biomolecules. We can expect that bioelectrosprays will not only be explored as a biopatterning technique as ink jets have, but will be developed alongside other established technologies, such as mass spectrometry, where cellular disorders could be identified. We might speculate that unhealthy cells would draw a different charge at the needle from heathly ones, and could be identified by the mass spectrometer. This possibility has tremendous implications in the healthcare industry. Other applications include the development of biochips and biosensors, where cells and tissues are integrated with modern bioanalytical methods.
Although ink jets and bioelectrosprays have successfully demonstrated their ability to jet living cells and organisms, there are two challenging issues for these novel processes. The first is the ability to control the number of cells within a jetted droplet, which has a direct effect on the spatial resolution of the fabricated construct. Secondly, the fate of ink-jetted and bioelectrosprayed cells needs to be thoroughly studied over the short and long term. These studies will not only determine whether the jetting protocols have any effect on cellular DNA and gene expression, but will decide the progression of either method to a clinical setting. It is my opinion that this endeavor will determine the applicability of these two processes for a range of investigations in developmental biology, and regenerative and therapeutic medicine.
If these two direct cell-engineering methodologies can be developed to jet a controlled number of cells without any effect on the post-processed cells, complex three-dimensional architectures could be directly fabricated incorporating viable cells. This would negate the need for sacrificial templates currently being investigated in areas such as tissue engineering.
Further reading
[1] T. Boland et al. Biotechnol. J., 1 (2006), p. 910
[2] N.E. Sanjana, S.B. Fuller, J. Neurosci. Methods, 136 (2004), p. 151
[3] V. Mironov et al. Biotechnol. J., 1 (2006), p. 903
[4] J.B. Fenn et al. Science, 246 (1998), p. 6
[5] S.N. Jayasinghe et al. Small, 2 (2006), p. 216
[6] S.N. Jayasinghe et al. Biotechnol. J., 1 (2006), p. 86
[7] S.N. Jayasinghe et al. Lab Chip, 6 (2006), p. 1086
[8] J.B. Fenn et al. Angew. Chem. Int. Ed., 42 (2003), p. 3871