Biomedical Engineering Reference
In-Depth Information
plates were standardised. Some analytical or diagnostic applications of multiwell
or microtiter plates involve attachment or immobilisation of biological agents or
probes to a surface within the wells, mostly the bottom surface, and the perfor-
mance of one or more reactions in the wells is followed by some sort of quanti-
tative and/or qualitative analytical process. For example, the biological agents or
probes, such as DNA or proteins, are immobilised on the bottom surface of each
well in form of a microarray, i. e. as multiple spots each containing the same or
different probes. The putative target(s) to be analysed are mostly fluorescently,
colorimetrically, or chemiluminescently labelled and brought into contact with the
microarray of probes under highly stringent conditions. Alternatively, the hybri-
dised or bound unlabelled target is detected by a labelled secondary agent. Sub-
sequently, target incubation is determined via an appropriate detection method.
Thus, if different probes are used for each spot a multitude of reactions can be
performed in only one well.
Direct printing into 96 well microplates is a potentially straightforward and
inexpensive way to support a diagnostic platform. Unfortunately the immediate
advantages of an inexpensive polymer substrate, well known to most diagnostic
users, its easy automation and its surface modification properties comes together
with several challenges: with high fluorescent backgrounds for a number of
polymers, electrostatic charging of the surfaces and therefore making spot depo-
sition very difficult as well as long manufacturing times due to the requirement of
always doing a z-movement into the well being the most obvious ones.
For microarray based applications robotic instruments have been developed to
process the plates fully automated. Although the standard structure of the plates
has facilitated such automated processing, at the same time this structure presents
challenges with regard to certain procedures. For example, the deposition of
biological agents by printing them as microarray spots onto the bottom surface of a
well is done by robotic liquid-handling systems, so-called arrayers. These depo-
sitions require the pipetting head or printing pin of the arrayer to move signifi-
cantly up and down (z-axis) as it spots one well of a microtiter plate after the other
with small drops of liquid comprising only about 0.1-4 nl. Depending on the kind
of microarray printed it might be necessary to move the pipetting head over 100
times into one well which makes this process very time consuming. Furthermore,
these movements increase the risk of damaging the pipetting or spotting head.
3.1 Contact Printing Versus Non-Contact Printing
Although it is theoretically possible to print 96 identical arrays into microplates
using contact or pin printing, this technology plays no role in diagnostic manu-
facturing. This is mainly due to proteins being destroyed due to mechanical stress
when contact spotted to the surface and the high CVs in the print process, which
are mostly based on the inefficient transfer of the fluids from the print tips end to
the surface. Fig.
1
shows in detail the transfer of a protein dissolved in spotting
