Biomedical Engineering Reference
In-Depth Information
Table 5.2
Properties of the most commonly used infiltration-embedding or embedding resins
Low fluidity
(+) to very
fluid (+++) Solubility
Stability
under
electrons
Wa t er
miscibility
Ease of
cutting
Resin
Polymerization
Epoxy:
(Araldite/
Epon)
+
Alcohol,
acetone,
epoxy 1,2
propane
Non-polar
Very good
+++
Under heat
Epoxy: ERL
(Spurr)
++
Alcohol,
acetone
Non-polar
Good
++
Exactly 333 K
Epoxy: Epofix
++
Non-polar
Good
++
Room
temperature
Acrylic: LR
white
+++
Water,
alcohol,
acetone
Polar
Good
++
At 333 K or
UV at 277 K
Acrylics:
Lowicryl
K4M, K11M
++
Water,
alcohol,
acetone
Polar
Average
+
<273 K UV
Lowicryl:
HM20, HM22
++
Alcohol,
acetone
Non-polar
Average
++
<273 K UV
The resin must be poured (around the sample) in an embedding mold whose
chemical composition does not interact with the components of the embedding
polymer.
The time and method of resin polymerization are specific to the type of
embedding resin used and are provided by the manufacturer.
To improve adherence of the resin to the surface of the material, a preliminary
treatment (primer) composed of 1-2% silane mixed with an ethanol solution may
be used.
For materials presenting a high surface roughness, the use of a procedure similar
to that used for infiltration (successive baths), as well as embedding under vacuum,
could improve adhesion of the polymer to the material surface.
6.5 “Positive-Staining” Contrast Principles
Biological materials and polymers often present poor contrast during TEM obser-
vation. Since image contrast results from variations of electron density within the
thin slice of the material, the principle behind this technique is to make heavy met-
als react with the structures to be observed in order to stop as large a number of
electrons as possible. The addition of this contrast reagent must be selective, i.e., it
must act on certain structures and not on others to be able to differentiate them. This
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