Printing Electronics
Printing electronics is a recent development whereby metal nanoparticles are mixed with organic solvents to form an ink that can be deposited in thin layers on various surfaces with inkjet printers or other printing techniques. After printing the nano ink gets reduced by heating or by applying intense light making it highly conductive. The printing process allows for a quick prototyping and the thin layers are suited for the production of ultra-thin flexible electronics.Due to their flexibility and softness they can be applied directly to the skin in which case they are called electronic tattoos.
The production of such flexible low cost devices for medical sensors which can be attached directly to the skin measuring vital signs could improve health care around the world by offering a less invasive alternative to state of the art medical devices. The most exciting domain where such electronic tattoos can make an impact is in healthcare. Measuring brain activity in electroencephalography (EEG) studies over prolonged periods of time or recording the heart rate without restricting the patients daily routines are just two examples where ultra thin electronics could make a difference.
Silver has the highest conductivity and does not oxidize making it a popular choice for flexible electronics. Gold on the other hand is very expensive but has the advantage of not reacting with biological systems. The conductivity of Aluminum is only 60% of that of Cu making it less attractive for the use in printed electronics.
Beside the advantage of being easily processed into metallic inks and printed into any 2D shape the high surface energy of metal NP leads to a much lower melting point compared to metallic particles with micrometer scales. When NPs are heated to around 200°C they melt and fuse together to form a solid surface (Sunho Jeong et al., 2011). N. C. Raut and K. Al-Shamery 2018 claim that printing with NPs on flexible substrates is challenging due to gas formation caused by the boiling solvents leading to porous structures weakening the mechanical stability. Sung-Jun Joo et al., 2015 on the other hand showed that a mixture of 5 wt% nano wires (NWs) and 95 wt% spherical NPs improved bending resilience.
Depending on the size nano-Cu inks can be either melted together (sintered) to one solid metallic circuit by heating the ink in an oven to 200°C for 30 min or by applying intense pulsed light (IPL) generated from a xenon lamp. Due to the short time interval (milliseconds) of the light pulses, the nanoparticles can be sintered at very high temperatures without damaging the subjacent substrate layer.
For Cu-based nano inks, better conductivity is achieved when elemental Cu NPs are used. Copper oxide (CuO) NPs are formed when the NP are produced under aerobic conditions. CuO NPs have lower conductivity therefore a protective coating layer applied to the Cu NPs is often used to prevent oxidation prior to sintering.
Increasing the cartridge temperature up to 70°C has been shown to reduce the surface tension and viscosity promoting jetting (Sooman Lim et al., 2013). Chemical sintering methods are also possible and reduce the barriers of using inkjet printing for fast prototype design (Yoshihiro Kawahara. et al., 2013).
Electrohydrodynamic printing is the state of the art printing technology for very thin and delicate electronics, allowing the printing in nm scales (Khalid Rahman et al., 2012).
The use of commercial inkjet printers for printing NP based ink allows for a very low hurdle of entry into the area of do-it-yourself (DIY) electronics (Yoshihiro Kawahara. et al., 2013).
For the production of metal ink the nanoparticles are suspended either in water (Sunho Jeong et al., 2011) or a mixture of organic solvents.
Dispersing 24 wt% metal nanoparticles in the form of powder in an organic solvent mixture of proportional 70 wt% ethyleneglycol (stabilizer), 20 wt% 2-methoxyethanol (stabilizer) and 10 wt% methanol was shown to work well for inkjet printing.
In the literature nanoparticles for printing are always produced chemically and under the addition of some kind of dispersing agent like poly(N-vinylpyrrolidone) (PVP). PVP is a polar molecule with a long hydrophilic tail which helps the NP when bound to it to disperse in the solution and prevent agglomeration.
When starting the experiments with naked NP they need to be coated with PVP or similar coating first so they do not sing to the ground and disperse equally well in the solution.
The nanoparticles used should also have different sizes to create smoother packing which is less susceptible to cracking at the particle fusion sites (Ummartyotin, S., et al., 2012).
The kind of ink and the chosen printing technique determines number of steps, energy consumption and resolution of the print (Raut, N. C., & Al-Shamery, K. 2018).
A plasma printer, suitable for printing bacteria carrying elevated amounts of metal was developed by the National Aeronautics and Space Administration (NASA) in 2017 (Rothschild, L. J., et al., 2017)
Since the size of the NPs in the ink can get larger than the size of the color particles in standard ink for example when multiple NP cluster together, the nozzle can become clogged. Brother printers are known to have a larger nozzle opening making them better suitable for printing metal NP inks. Filtering steps before printing e.g. by using a 5-μm nylon mesh can also help to prevent nozzle clocking (Park, B. K., et al., 2007). The built-in printer cleaning operation can also be used to clear clogging if run a few times (Yoshihiro Kawahara et al., 2013).
In general, the resistance of the circuit towards bending is proportional to the thickness of the material to the third power. (Liu, Y., Pharr, M., & Salvatore, G. A. 2017).
