Drop-on-demand inkjet-printing (DOD IJP) is a versatile digital printing technique that allows for direct patterning of a wide range of functional materials such as polymers, polymer composites, as well as different types of nanomaterials. In this technique, pico-liter droplets are generated and precisely positioned on substrates. The possibility of positioning a small volume of material precisely on the substrate as well as contactless patterning has made DOD IJP an interesting technique in the fields such as fabricating optical components (e.g. microlens arrays), electronic devices (e.g. thin film transistors), sensors (e.g. chemical sensors), as well as tissue engineering (i.e. cell patterning).
Working principle and some examples
In LMIS1 we are using a piezo-actuated drop-on-demand inkjet printer developed by Microfab Technology. In this printer, the dispenser consists of an annular piezoelectric element, which is attached to a capillary glass tube. When a pulsed voltage waveform is applied to the piezo-element a sudden change of volume occurs in the capillary glass that generates acoustic pressure waves inside the capillary tube and consequently expels a small volume of liquid from the orifice. For droplets to be generated physical properties of the ink e.g. viscosity and surface tension, have to be carefully tuned and printing parameters (e.g. applied waveform) have to be optimized. The image below shows how a droplet is generated from an ink containing a polymer, using a piezo-actuated nozzle.
|Figure 1: Generation of pico-liter droplets using a piezo-actuated DOD IJP.
Liquid encapsulation in microdevices
Current technology allows us to create amazingly small physical devices. However, the precise deposition and protection of pL volumes of liquid need to be improved as a consequence. The time-window to encapsulate picolitres of water-based ink is less than a minute before significant evaporation takes place. The loss of liquid can disturb the concentrations contained within the ink for correct function. We use DOD IJP to print our desired ink into defined open vessels. We immediately print an oil-based UV-curable encapsulating layer. Thereafter upon UV curing, crosslinking occurs and forms a barrier to protect the underlying ink from evaporation and external influence.
|Figure 2: PDMS samples with wells of 300µm internal lengths are filled with a water-based ink. Before complete evaporation takes place, an immiscible ink is deposited on top to encapsulate the ink underneath before UV treatment.
Printing polymer composites for gas sensing applications
One of the ongoing research activities in our lab regarding IJP is to formulate inks containing polymer nanocomposites (PNCs) for applications in chemical gas sensing using sensor arrays (i.e. electronic nose devices). Since IJP enables us to directly deposit functional materials on a substrate it is a suitable technique for fabricating sensor arrays where each sensor consists of a different material. For this aim we formulate inks containing PNCs, study their printability and investigate their response to different volatile organic compounds (e.g. acetone, and ethanol).
|Figure 3: The image (a) shows an exploded view of the sensor device used for characterization of inkjet-printed polymer composites. The image (b) demonstrate working principle of chemiresistive sensors based on polymer composites. The image (c) shows a representative result indicating the response of a printed polystyrene/carbon black composite to acetone. The inset of the image shows a picture of the printed sensor.