Low-cost and customizable inkjet printing for microelectrodes fabrication
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Microelectrodes for detection of chemicals present several advantages over conventional sized electrodes. However, rapid and low-cost fabrication of microelectrodes is challenging due to high complexity of patterning equipment. We present the development of a low-cost, customizable inkjet printer for printing nanomaterials including carbon nanotubes for the fabrication of microelectrodes. The achieved spatial resolution of the inkjet printer is less than 20 µm, which is comparable to advanced commercially available inkjet printers, with the advantage of being low-cost and easily replicated.
KeywordsInkjet printing Carbon nanotubes Microelectrodes
microfluidic impact printing
multi-walled carbon nanotube
sodium dodecyl sulfate
Inkjet printing has seen tremendous development in the deposition of nanomaterials for the fabrication of microelectrodes. It has been considered as an alternative for printing nanomaterials in the applications of flexible electronics and point of care sensors. The superior properties of inkjet printing such as solution processing and maskless printing enable the technology to be used in a variety of applications. A common issue in the development of inkjet-printed sensors is the high cost of inkjet printing research equipment.
Classically used for printing text and images, inkjet printing has recently found applications in many branches. Its advantages include the ability to print solutions of various materials, the extremely low waste and fine control of deposition parameters such as droplet location and number of printed droplets. Inkjet printing has been used to print thin film transistors on plastic substrates , fabricate stretchable FET transistors , tactile sensors , two dimensional force sensors , biosensors , chemiresistive sensors  and a variety of other applications . While many of the above-mentioned studies used professional inkjet printers, some employed office inkjet printers which are widely available at low cost. Office inkjet printers offer the advantage of printing large area patterns with high speed, at the expense of lower resolution and lack of control of the quantity of material deposited.
Several approaches have been employed in the development of low-cost ink deposition processes. In microfluidic impact printing (MIP) [8, 9] a pin which is driven by an electromagnetic actuator strikes an elastic membrane. The ink contained in the microfluidic channel below the membrane ejects in the form of a small droplet. In electrohydrodynamic jet printing  a high voltage is applied between the print head and the substrate, which ejects ink from the nozzle onto the moving substrate to form a pattern. Printed lines with widths as small as 700 nm can be achieved. These approaches provide an alternative to standard printing procedures, however they are not easily replicated since the cartridge and nozzle have to be microfabricated in both cases.
Here, we present a customized inkjet printer with comparable characteristics to advanced commercially available inkjet printers, while being affordable by utilizing off-the-shelf components. Furthermore, it is capable of printing several nanomaterial solutions including carbon nanotubes, graphene oxide, and silver nanoparticles. We demonstrate the capabilities of the printer by patterning carbon nanotubes on polymer substrates. This inkjet printer can easily be replicated and provides a foundation for development of flexible and disposable sensors.
Development of the inkjet printer stage
Ink solution preparation
Ink development is an important step for successful inkjet printing. For the nanomaterials utilized in the custom inkjet printer, a surfactant was added to provide a stable dispersion. The optimal formulation for inkjet printing on PET film has been previously developed in our group . For the ink preparation, 10 mg/ml of multi-walled carbon nanotube (MWCNT) (Cheap Tubes Inc., Brattleboro, VT, USA) and 7 mg/ml of sodium dodecyl sulfate (SDS) (Alfa Aesar, Ward Hill, MA, USA) were added to 5 ml of deionized water in a vial, sonicated for 30 min at 80 W (Fisher Scientific FS20D), transferred to centrifuge tubes and centrifuged for 5 min at 12,000 rpm. The supernatant was recovered and directly injected into a cartridge. The cartridge was opened and thoroughly cleaned before addition of the ink.
For characterization of the inkjet printer, PET sheets (Inkpress ITF851150) were cut into 70 mm × 50 mm and used as the substrate. In order for the substrate to maintain flatness during printing, it was attached to a glass slide which was previously coated with poly(dimethyl siloxane) (PDMS). All measurements of dimensions were performed under a stereomicroscope (Leica MZ16), while sheet resistances were measured with a custom four-point probe setup. The current low-cost inkjet printer does not contain a heating stage, so the experiments were performed at room temperature. Alternatively, a heating stage would allow improvement of the evaporation rate of the droplets.
Results and discussion
Performance of the custom printer
Comparison of inkjet printing performance
Minimum line width
25 μΩ∙m at 8 prints
Canon BJC 4550
40 kΩ/sq at 90 prints
30 kΩ/sq at 1 print
Microfab JetDrive III
0.7 kΩ/sq at 14 prints
PEDOT/PSS, polyurethane prepolymer
0.61 kΩ/sq at 5 prints
Inkjet printing of humidity sensor
We have developed a cost effective and customizable inkjet printer offering a resolution of 18 µm (1412 dpi) with the components available off-the-shelf, which is comparable to the advanced inkjet printers available for research. The inkjet printer was employed to print patterns of carbon nanotubes on PET film to demonstrate the capability of printing microelectrodes, which can be used in several applications such as electrochemical and humidity sensors. A relative humidity sensor was fabricated by printing carbon nanotube onto PET film and subsequently transferring to PDMS. The sensor showed 2.6% change in resistance when the humidity changed between 60 and 88% (from ambient to a high humidity environment). In addition, the flexibility of the sensor allows the integration in wearable applications for humidity monitoring.
THC and JWC designed the research. THC performed the experiments and drafted the manuscript. JWC supervised the research and drafting the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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