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Electrospraying, also well known as electrohydrodynamic jetting, is a phenomenon that is completely determined by an electric field. The protocol jets media passed through a conducting needle that is held at a higher potential with respect to a grounded electrode. The charged media exiting the needle enters an external electric field that is set up by the application of a potential difference between the needle and some ground electrode. The parameters governing this jet protocol are essentially the applied voltage to the needle with respect to the ground electrode, the flow rate of the media and its consistency to the needle and the media properties of electrical conductivity, viscosity, surface tension, density and relative permittivity. Establishing a balance in these three parameters for a given system set-up is of utmost importance for the jetting process to take place in stable conditions. Unstable conditions are not attractive as they have been reportedly found to generate a polydistribution of droplets and residues. The stability and continuity of the jet has been reported to form a near monodistributions of droplets and residues in the micro- and nanometers. In comparison to ink-jet printing, this protocol has some unique advantages, which makes this process more attractive. These feats are the ability to process much higher viscosity flow of either a single- or multiphase medium while exploring large-bore needles from which droplets of residue in the micro- to nanometers are generated with ease. This protocol has, in the past decade or so, been extensively exploited in the processing of structural and functional materials for bio-related applications (surface patterning, drug delivery and medical screening). Furthermore, very recently, this technique was combined with a specially tailored living siloxane sol, which gave rise to the formation of nanodroplets that were assembled to directly form either continuous threads to 3D structures with complexities previously hitherto unachieved with any other stand-alone jet-assisted methodology. With respect to its applicability to biology, in 1989 Fenn et al. combined mass spectrometry with electrosprays to pioneer a bio-analytical approach that has far-reaching consequences in molecular recognition to cancer therapy. This discovery was recognized with the Chemistry Nobel prize in 2002. Electrosprays have been explored in several areas of research, but it was in 2005/6 that these electrified jets were uncovered for their ability to directly handle living cells in an uncompromising manner. Although cells were jetted via this technique and found to be viable, the jetting protocol was taking place in an unstable condition that is not attractive, while jetting in the single-needle configuration. This unstable jetting condition gave rise to the polydisperse distribution of cell-bearing droplets owing to the cell media properties, namely the high electrical conductivity and the low viscosity, two properties that need to be low and high, respectively, to achieve jet stability. Hence, this was later resolved by the modification of the jetting needle to one that was coaxial or concentric. Here, the outer needle held a high viscous and low conducting medium, which stabilized the jetting process. This polymer would not only be useful in stabilizing the jetting process but would enable the encapsulation of cells within droplets to assist as a binding agent if the necessity is to stack these cell-bearing droplets on one another, giving rise to the creation of a biological microenviroment.
This branch of electrosprays are now referred to as ‘bio-electrosprays’ (Figure 3) and have so far demonstrated some significant landmarks. These jets have been shown to handle large volumes of cells from which cell-bearing droplets an order of magnitude smaller than those generated in ink-jets have been formed. Furthermore, by way of flow cytometry, the post-treated cell’s viability was assessed in comparison with several controls, elucidating a cell viability of more than 70%. Although this figure is smaller than viable cell populations reported in IJP studies, it should be noted that those viable cell populations in ink-jet studies have not identified cells undergoing programmed cellular death (apoptotic cells) or cellular debris. A closely related technique to electrosprays is electrospinning, unlike the former, the jet does not undergo break-up but is drawn into a uniaxial continuous thread, which, as in electrosprays, have been found to form threads with diameters ranging from the micro- to the nanometer. Therefore, the coaxial bio-electrospray set-up underwent an extension to its portfolio where the flowing polymer in the outer needle viscosity was modulated to more than 12000 mPas, which promoted the direct formation of continuous cell-bearing fibres. Increasing the collection time or the substrate exposure to the threading process (cell electrospinning [CE], Figure 4) gave rise to the formation of cell-bearing scaffolds (living scaffolds). Several cell types both immortalised to primary which also included stem cells have been treated either by way of BES or CE and found to have no significant effects from a molecular level upwards.
Figure 4. Cell electrospinning heterogeneous cell-bearing living threads,
scaffolds and membranes
Figure 3. BES in the stable cone-jetting mode
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