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Printed Electronics World
Posted on October 11, 2007 by  & 

Digital Fabrication / Non Impact Printing Conference - Alaska Day 5

20 Sept. A plenary session "Engineering Challenges in Biofabrication" covered an inkjet application aiming to fabricate biological structures. As such the "bioinks" need to be water based so as to assemble materials in layers without killing living cells in the bioink or the layers below. There is also a need for absorbable materials for the scaffold.
There was some discussion of the relative attributes of thermal and piezo print heads. The author expressed some preference for thermal units which work well for biological compounds of high molecular weight (>100,000) and have cost and flexibility benefits for nozzle sizes >50 microns. This community seems to take consumer desktop inkjet cartridges and adapt them for this purpose. Despite some pointed questioning from members of the audience with a commercial interest in piezo heads for this application he stuck to his preference.
This is an excellent example where non-impact printing is ideal as it reduces the possibility of interlayer contamination and eases the problems of sterility. It was shown that after printing the cells seem to act in a normal fashion. It is believed that the shear stress is too short (about 10 microseconds) for the cells to get stressed.
So where are the applications? It looks like early applications will be in drug screening and cosmetic testing with organ building to come in the future. Is this important for Printed Electronics? Not yet, but watch for the future. As we get more into printing sensor systems biological membranes will become a key component in some assemblies.
One interesting aspect to this application was the very high value treated paper substrate used in this application. There are obviously options for high value added substrates (glass, plastic, paper) in this market.
I then shared my time between the Image Permanence and Printing Systems Engineering and Optimisation sessions in the Non Impact Printing (NIP) conference and the New and Novel Direct Write Methods session at Digital Fabrication. Although this session has no direct relevance to Printed Electronics at this time, the models being developed are going to be applicable to applications prone to environmental degradation processes, particularly printed displays. There is an International Standards group working on these models and as Principal UK Expert on this I am interested in following this up.
In the Printing Systems Engineering and Optimisation session a paper from Chemnitz University of Technology examined the transition of industrial printing from conventional colour printing into Printed Electronics. The work indicates that, although the capabilities of these printed devices will be well below that currently available from silicon based fabrication technologies, the types of presses in use are capable of four orders of magnitude greater productivity with a commensurate decrease in cost per unit. The authors believe that from conventional printing technologies flexo, offset and gravure are suitable for printed electronics and will be joined by inkjet and toner based digital systems. Gravure printing systems have the advantage of a simple inking system that will probably ease adaptation to the new fluids required. However, the high set up time may limit application to long print runs or high quality applications. By way of contrast setting up an offset press is comparatively easy but the fluid formulation is likely to be more difficult.
This session also contained a very interesting paper from Hewlett Packard on heat transfer issues in an inkjet print head. While our first thoughts may be that this is only an issue with thermal inkjet, the point was made that external heating sources such as UV curing units and driers may be equally important. Temperature changes can influence the rheology of inks and even prompt the nucleation of bubbles that can block the ink channels. With increasing levels of miniaturisation and nozzle densities this is set to become a more important topic and HP deserve credit for the level of work they have done on this.
Although these two papers were in the NIP conference they would be equally at home in both conferences. Once again it shows the close fit between the Non Impact Printing and Digital Fabrication conferences.
I spent most of the afternoon in the New and Novel Direct Write Methods strand of the Digital Fabrication conference. We were treated to a broad spectrum of methods and applications. This served to further underline the fact that piezo actuated inkjet is not the only game in town!
A presenter from Waseda University showed an electrostatic inkjet system for printing electronic conductors. Although this is a single nozzle system it is capable of jetting highly viscous liquids that are effectively pastes so can cope with very high metal loading inks yet still produce fine lines. Still early days for this technology but one to watch for - those difficult to print "fluids".
A presenter from Atotech showed an interesting method of applying a resin coat to a metal foil. This is an element of the production process of multilayer PCBs for devices such as mobile phones. Typically these layers are solvent coated and require long oven lines to remove the solvent. The aim of this work was to investigate the use of electrostatic technology to powder coat the resin layer, thus avoiding the use of solvents. Again, the work appears to be in early stages of development but it will be interesting to see if this printing technique can be modified to produce a patterned coating, just as in conventional electrostatic printing.
Another presentation from the Chemnitz University of Technology dealt with the construction of micro membranes patterned using inkjet technology. Rather than use inkjet to build the membrane material the fluid drops are used to repel the membrane construction to provide the location and size of the pores. In addition to biological applications this technology could well find application in the fabrication of printed electronic sensor elements.
Finally, a presentation from Hitachi explored the use of electrophotography for printing conductive features of around 10 micron width. Although once again at the experimental stage this paper shows the potential of electrophotography as a digital fabrication technique that could well be more productive than inkjet in some applications.
The conference day finished with the interactive poster session. Here delegates display a poster describing their work and then stand by the side of it to discuss it with passers by. This is a really good forum, particularly for work in progress.
I have already described a lot of this work as it was previewed within the conference sessions. However, two papers stood out... The first was on coating papers for use as substrates for printed electronics where the authors were using technology originally developed in the photo industry. I presented a paper at the Digital Fabrication conference in 2006 on this topic so it was good to see the theme continuing. The work showed that polyethylene coated paper was as good a substrate for PEDOT printing as polyester base.
The second paper was previewed in the Print and Image Quality section of the Non Impact Printing (NIP) conference and covered the optical properties of printed images, in particular attributed related to sharpness. As such, one would not expect this to be of particular relevance to printed electronics. However, a closer look at the technical content of this paper revealed the potential relevance to the investigation of image quality in printed displays. It was very apparent that there are a number of technologies under development that will be of use in Printed Electronics in the future. Watch this space!
Alan Hodgson
Independent consultant
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