C - DTU Nanotech - Danmarks Tekniske Universitet

5 Freezing Different Samples using
Peltier Cooling
This chapter describes the feasibility to use Peltier elements for freezing different samples.
In a first test series the most suitable design of a freezing chamber for Peltier cooling was
figured out by freezing phosphate buffered saline (PBS). As main material PMMA was
used for the freezing chamber because of the possibility to make a rapid prototyping by
using the CO2–laser. In subsequent experiments we determined the viability of E. coli
after freezing the cells using Peltier cooling. The last tests engaged the freezing of different
volumes and the temperature during the freezing of PBS–buffer. PBS–buffer was used as a
standard, because our cell culture was diluted in PBS. While PBS–buffer was the standard,
in addition the freezing tests with different volumes were made with Luria–Bertani–medium
(LB–medium), because in a bio–microreactor the extracellular metabolites of the E. coli cells
will be normally dissolved in LB–medium or a variation of the LB–medium.
5.1 Materials and Methods
5.1.1 Fabrication of four different freezing chambers
The freezing chambers had a round vessel with a diameter of 8 mm and working volumes
from 75 µl to 100 µl. By fabricating the freezing chambers, the focus was on the bottom of
the vessel, since the Peltier element was beneath the vessel and thus a good negative heat
transfer through the bottom of the vessel was needed. The main part of all freezing chamber
vessels consisted of PMMA. We tested four different designs: the PMMA plate in design I
and II was only machined with the CO2–laser (shown in Fig. 5.1). For design III and IV we
also needed the CO2–laser, but in III, the bottom of the vessel consisted of an aluminium foil
which was glued to a laser machined PMMA plate with vacuum grease, and for fabricating
the bottom in IV, a PMMA foil was thermally bonded to a laser machined PMMA plate
(see Fig. 5.1).
Additionally two different top covers were fabricated for the freezing chambers.
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