Static in Medical Device Manufacturing
by Matt Fyffe
In 600 B.C., the philosopher and mathematician Thales of Miletus reported that after rubbing a piece of amber on the fur of a cat the amber attracted and held feathers. This was the first account of static electricity (literally, electricity at rest).
What Thales observed was what we now know as triboelectric charging, where certain materials become electrically charged after contact with a different material through friction. While generating a controlled static charge has positive applications in some manufacturing scenarios – allowing the temporary adhesion between two or more surfaces of opposite polarity, for example – in many operations across a multitude of industries, uncontrolled static electricity causes serious production problems. These range from downtime due to machinery jams because of product contamination to product loss in industries such as electronics, where even a low static voltage can destroy sensitive components. People can be damaged, too, with employees suffering electric shocks. Where flammable materials are used, there also is the real possibility of fires and explosions.
Uncontrolled static attraction is a particular problem for plastics industries, and consequently, for medical device manufacturers. Even in the most stringent cleanrooms, static charge attracts particulates from people, processes and equipment, so it is important to take appropriate measures to ensure static is kept to a minimum, if not completely eliminated.
Static can cause damage
The primary problems resulting from electrostatic charges are electrostatic attraction, material misbehavior and operator shocks.
Electrostatic Attraction (ESA). Not only are airborne contaminating particles attracted to charged surfaces; charged airborne particles also can be attracted to surfaces free of any charge. This problem affects most plastic-based industries in one form or another, but static in medical device manufacture is the most common cause of rejected products. The problem affects a range of devices, including catheters, syringes, replacement joints, pacemakers and stents.
Material Misbehavior. Uncontrolled ESA gives rise to problems besides product contamination. It can disrupt automated processes by misrouting, repelling or causing parts to stick to each other or to equipment. This imposes significant cost penalties because it forces manufacturers to run their machines at much slower speeds than might otherwise be necessary.
Operator Shocks. Operator shocks typically are the result of an accumulated charge, or battery effect, occurring during the collection of parts in a bin or assembly area. While they can be painful, in most cases the effects are nonlife-threatening and short-lived. However, there also are cost implications in the recoil reaction associated with the initial shock, after which there can be a moment of disorientation, bringing with it subsequent hazards such as collision with other operators or machinery.
The same fundamental principle governs every technique for neutralizing static: Where a material has a positive surface charge, electrons must be added to the surface to rebalance the charge; where the surface charge is negative, the excess electrons must be removed.
The two basic techniques for rebalancing charge are conductivity and replacement. The former involves making an insulator conductive and then grounding it. Ways of achieving this include humidification and applying anti-static chemicals (either as coatings or adding them to plastics during manufacture). Carbon similarly can be added during manufacture to make plastic conductive.
When it comes to tackling static during the production process, the replacement technique using active air ionization may be more practical. Active air ionization employs high-voltage ac or pulsed dc to produce ionized air to neutralize surface charges. The voltage is fed to an array of titanium emitter pins mounted on an ionizing bar. This creates a high-energy ion cloud made up of a high number of positive and negative ions, which are attracted to particles or surfaces carrying an opposite charge, thus rapidly neutralizing the surface.
Active air ionization in medical device manufacturing
As awareness of the problems uncontrolled static can cause grows, more medical device manufacturers are installing static-neutralization equipment. For example, one manufacturer specializing in the development and manufacture of mold tools, plastic injection-molded components and the assembly of complex devices for the pharmaceutical, drug-delivery, medical and healthcare industries uses static-control equipment for the assembly of injection-molded drug-delivery devices and throughout the injection-mold process, right up to hand assembly.
During assembly of the plastic components, ionizing blowers and nozzles neutralize the parts and remove the excess plastic-flash and statically attracted airborne contaminates. This is carried out within a Class 7 cleanroom. During the operation of the bench-mounted ionizing nozzle, the removed particulate is directed toward a tack-mat area, where it is captured to avoid future recontamination of the product. Once clean, the drug-delivery device is manually inspected under an illuminated magnifying glass for cleanness.