Integrated Biomaterials Science

Infection and Sterilization 825
features of the material. At room temperature 7 days may be necessary, at
50° with forced ventilation 12 hours, and at 60°, 8 hours. Polyvinylchloride
is the material that requires the longest period of time; nonporous materials,
such as glass or metals, require shorter time intervals.
To improve safety, many attempts were made (Berth and Wolftbrandt,
1992) to obtain nonexplosive mixtures using diluents for EtO, such as
chlorofluorocarbon (Freon 12). Following the EC rules on abolition of this
gas, alternative diluents were investigated, such as carbon dioxide, nitrogen,
and hydrofluorocarbon.
Another chemical sterilizing product is formaldehyde, which is a
colorless gas, highly irritant to eyes and mucosae. It has a well-demonstrated
microbicidal effect, with a wide range of action on Gram + and Gram –
bacteria, spores, mycetes, and viruses. However, its effect may be impaired
by the organic residues present on the surface of the material, which
therefore has to be carefully cleaned before starting the procedure.
Sterilization is carried out in autoclaves, where the gas obtained
through evaporation of an aqueous solution of formaldehyde circulates. The
effectiveness of this process is related to the concentration of formaldehyde,
exposure time, pressure, and temperature. In standard conditions, the
bactericidal power is obtained with a concentration of 1500 mg/l. In a steam
atmosphere and at a temperature of 65°C, concentration drops to 65 mg/l.
Also, exposure time varies with temperature: 2 hours at 80°C, 4 hours at
55°C.
The advantages of this procedure are mainly represented by the
possibility of sterilizing materials that cannot be exposed to high temperatures;
the product is not explosive or inflammable and involves low cost.
The limit to its use is the product toxicity.
28.2.3. Irradiation
Today, irradiation sterilization is a widely employed procedure
(Brinston, 1991). Since the discovery of radioactivity, the feasibility of its use
in this field had been considered, but it came into general use in the late
1950s. The gamma rays used for this purpose are photons of electromagnetic
radiations with an energy range of 1–10 MeV (Henon, 1992). Interactions
between radiations and material involve three types of consequences:
a.
b.
c.
Physical events: Interactions between atoms and radiations produce
unstable molecules (primary products).
Physicochemical events: Primary products react with each other and
produce highly reactive ions and radicals.
Chemical events: There are interactions and reactions among the
unstable reactive groups.