Direct-write Equipment and Jetting Optimization Process

The deposition technique chosen for this research is a modified inkjet printing technique designed by MicroFab Technologies Drop-On-Demand Test Stand inkjet system (MicroFab Technologies, inc., Plano TX). This system consists of a pneumatic

controller, a MicroFab JetDrive III external waveform generator with heat source, JetServer software with waveform amplifier, horizontal- and vertical-plane optics system and a piezoelectric nozzle tip with a 50-micron orifice diameter [48]. The schematic of the MicroFab test stand is pictured in Figure 3.1.

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Figure 3.1 (a) Depiction of MicroFab JetLab 4 inkjet printing machine (b) nozzle printing apparatus and motion panel

Direct-write inkjet printing can be accomplished using one of two different inkjet technologies, Continuous (CIJ) and Drop-on-Demand (DOD) inkjet printing. More

suitable for this research is the DOD (or direct-write) technology. This method requires electromechanical pulses (or an applied voltage) over a piezoelectric material that causes the material to deform. The deformation of this material causes an increased pressure within the nozzle after which a single droplet is ejected [48].

After the droplet is ejected, there is an obvious reduction of fluid volume within the solution-containing reservoir, thus the solution must be replaced. As the piezoelectric crystal returns back to its resting state, a negative pressure forces the replacement of the ejected fluid [48]. A list of the input parameters required to characterize the above described process is in Table 3.1.

Table 3.1 List of piezoelectric Direct-write parameters

Piezoelectric Direct-Write Process Parameters uniformly shaped droplets for the deposition of thin film coatings onto prepared titanium substrates. The parameters, which influence this optimization process, include: pulse waveform, print head design, and jetting fluid properties (Figure 3.2). A description of the print head design has been discussed previously. It is designed to include a nozzle orifice as low as 10 micrometers in diameter where it is expected that the variations of the droplet size (diameter) distribution from the orifice diameter may be neglected.

Figure 3.2 Direct-write process parameter relationship

The pulse waveform requires its own set of parameters that can be optimized for the manipulation of drop size and speed, jetting sustainability, drop placement and drop uniformity. The waveform parameters include the frequency, positive/negative jetting voltage, rise time, dwell time, fall time, and final rise time denoted by f, +V/-V, TR, TD, TF, and TFR, respectively. Where the jetting voltage is the optimal voltage (+V) applied causing deformation of the piezoelectric material. –V is usually set at 0, setting the voltage back to 0 and returning the material back to its initial resting state [81].

The rise and fall times with respect to the characterization of the pulse waveform are the time for the driving signal to reach the optimal amplitude (voltage) and the time for which it takes to decrease the voltage back to its original state. The dwell time that the optimal voltage is applied to the piezo material and the frequency refers to the number of drops jetted over a given time span. Figures 3.3 (a) and (b) below depicts the resulting waveform for piezoelectric direct-write processes and the MicroFab JetServer interface used to vary these parameters respectively.

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Figure 3.3 (a) Depiction of JetLab interface (b) pulse waveform formation Much research has been conducted on the manipulation of the waveform shape and parameters to determine their effects on the jetting process. Such a parameter that contributes greatly to the success of the jetting process is jetting stability. The jetting stability is greatly affected by the retraction of the meniscus (fluid surface at the nozzle

buildup at the nozzle tip negatively affects the success of the jetting process. In literature this is referred to as puddling and is directly related to the frequency of droplet accumulation.

A final parameter having a significant effect on the jetting process is the jetting fluid chosen and its associated fluid properties [82]. The fluid properties significantly affects the jettability of a substance, which is the fluid’s ability to maintain a jet stream of droplets for an extended period of time (e.g. two to three hours). Jettable fluids can be broken down into two categories: Newtonian and Non-Newtonian fluids [82]. This research deals with polymeric solutions, which display Non-Newtonian properties, that is, fluids with high viscosity, density, and surface tension values.

The manipulation of fluid properties can be obtained by varying fluid concentration (in this case, polymer concentration) given a solid polymer percentage (weight, %) and solvent (volume, mL) to obtain a weight/volume solution. More specifically, the concentration of a given polymer such as poly(lactic-co-glycolic acid) (PLGA) used in our experiments, can be altered by adjusting its ratio of lactic and glycolic acid to obtain a higher concentration of solution thus resulting in a higher viscosity. The effects of the fluid properties directly relate to tail break-off of the jetting solution. Also, a more viscous solution will have a longer tail (break-off period), which will slow the drop speed.

Residual vibrations may occur even after a single drop has been ejected and could influence the nature in which the resulting drops are ejected. Robustness against disturbance deals with the optimal parameter settings and system abilities in dealing with

such disturbances as dust and air bubbles that could stop the jetting process. Lastly, aging of the piezo material could have a profound effect on the optimal achievable jetting process parameters. There are many other operational issues that could limit the optimality of the direct-write printing process, however, these issues were controlled to the best extent possible.