Microtiter plates are either made from: 

Polystyrene 
Polystyrene (especially the medical quality, as we are using) is fully transparent, very pure, and so "leachables" are much less of a problem than with all other materials. Therefore, polystyrene is the material of choice when working with cultures that are sensitive to leachables (e.g. animal cells such as CHO and HEK cells).

However, polystyrene has two physical disadvantages:

• It starts melting at approximately 80 degrees Celsius, and can therefore not be autoclaved. Gamma-irradiation by the producer is the state-of-the-art method of sterilisation (also for Enzyscreen). Since most laboratories do not have a gamma-irradiation facility, polystyrene products are sold as "disposable" and "single-use". Though re-use and sterilisation is possible to some extent (by soaking in ethanol, and subsequent drying in a laminar flow cabinet), this property severely limits the re-use, and so is co-responsible for the large amounts of plastic laboratory waste produced worlwide.
• It is a relatively hard and brittle material, which puts limits to the shapes that can be produced by injection molding (the method of choice for production of MTPs). The main limitation is the "draft angle" which - for example - makes it technically difficult to produce 96-square deepwell MTPs. As a consequence, these are only available in polypropylene at present (only 96-half-deepwell MTPs CR1496c can be produced in polystyrene)

Polypropylene
Polypropylene is milky white, relatively soft, autoclavable, and so re-usable. It is a very suitable material for injection molding, and is therefore the material of choice for many "difficult" shapes, such as the 96-square deepwell MTP.

Also polypropylene has two distinct disadvantages:

• Even when marketed as "ultraclean" or "virgin", it generally contains relatively large amounts of chemicals, especially weakeners or "plasticizers" (e.g. phtalates), whiteners, heavy metal (e.g. cobalt).  These contaminants (also known as "leachables") may leach from the plastic into the growth medium, and affect the growth characteristics, and - in the worst case - totally inhibit growth. In practice, the inoculum size appears to be an important factor; leachables are most harmful in projects where the inoculum only contains a small number of cells.
  For this reason, Enzyscreen "detoxifies" the polypropylene MTPs that it sells, by a procedure involving boiling in alkaline and acid solutions. We have been doing that for 20 years now, and feedback from our customers learned us that growth (both for microbes and mammalian cells) in these plates is more reliable and reproducible than in untreated polypropylene microtiter plates.
• Since the material is relatively soft, the surface of the top of the walls between the wells is  irregular. This may result in cross-contamination between cultures in adjacent wells. Therefore, we "flatten" the top-surface of the polypropylene microplates that we sell, by melting the upper 0.2 mm of the walls using a hot flat glass plate.

Both the detoxification process (involving multiple manual steps), and the flattening procedure are very labour-intensive, and therefore the price has to be relatively high (all work is done in the Netherlands)