Biomedical Engineering Reference
In-Depth Information
deaths. While the source of the PRCA induction is not clearly settled, it is
generally attributed to a change in the drug formulation that included a new
surfactant, which caused unexpected leaching of an elastomer compound
from the rubber stopper.
Polymers and Additives
The materials used to fabricate single-use processing equipment for biophar-
maceutical manufacturing are usually polymers, such as plastic or elasto-
mers (rubber), rather than the traditional metal or glass. Polymers offer more
versatility because they are lightweight, flexible, and much more durable
than their traditional counterparts. Plastic and rubber are also disposable,
so issues associated with cleaning and validation can be avoided. Additives
can also be incorporated into polymers to give them clarity of glass or to add
color to labels or to code parts.
Unlike metals, where the risk lies mainly in oxidation, polymers are
affected by heat, light, oxygen, and autoclaving and, thus, degrade over
time if not stabilized, and this can adversely affect the mechanical prop-
erties. Polymers are thus stabilized by incorporating chemicals that are
prone to leaching during the manufacturing process and storage of biologi-
cal materials.
When a plastic resin is processed, it is often introduced into an extruder,
where it is melted at high temperatures and its stability is influenced by its
molecular structure, the polymerization process, the presence of residual
catalysts, and the finishing steps used in production. Processing condi-
tions during extrusion (e.g., temperature, shear, and residence time in the
extruder) can dramatically affect polymer degradation. End-use conditions
that expose a polymer to excessive heat or light (such as outdoor applica-
tions or sterilization techniques used in medical practices) can foster pre-
mature failure of polymer products as well, leading to a loss of flexibility
or strength. If left unchecked, the results often can be the total failure of the
plastic component.
Polymer degradation is controlled by the use of additives, which are spe-
cialty chemicals that provide a desired effect on the polymer. The effect can
be stabilization, which allows a polymer to maintain its strength and flex-
ibility or performance improvement, which adds color or some special char-
acteristic such as antistatic or antimicrobial properties. There are typically
three classes of stabilizers:
• Melt processing aids such as phosphites and hindered phenols, anti-
oxidants that protect a polymer during extrusion and molding
