Medical Plastics That Reduce Hospital Infections

One out of every 25 patients in U.S. acute care hospitals contract a health care associated infection, according to a survey by the Center for Disease Control (CDC).  One in every four of these are associated with medical devices. In-dwelling devices, such as central venous (CV) catheters, are particularly susceptible to bacteria colonization which can enter the blood stream.  Antimicrobial additives can be melt compounded into plastics to kill harmful bacteria on the surface of device components.

Silver and copper have been used for thousands of years to prevent microbial infections.  These continue to be used to kill modern day microorganisms, such as methicillin-resistant staphylococcus aureus (MRSA) and carbapenem-resistant enterobacteriaceae (CRE).

Metallic silver (Ag) is inert, insoluble in water, and unable to kill bacteria.  However, when combined with certain elements (e.g., chlorine) silver atoms lose an electron and become ionic (Ag+). This form of silver is colorless, water soluble, and highly reactive with other elements.  Ionic silver attacks bacterial cell membranes making them more permeable.  It also interferes with cell metabolism resulting in overproduction of reactive oxygen compounds that are toxic to the cells.

Copper is an important nutrient for cells, including bacteria.  However, in high doses it is known to efficiently kill bacterial cells.  Research into copper’s precise antimicrobial mechanism of action is ongoing and inconclusive.  Yet a number of studies indicate that copper effects the integrity of the bacterial cell wall and inappropriately binds to proteins within the cell causing loss of function.

When used as an additive in medical plastics, these metals are often bound to inert carriers that impart chemical stability and extend the duration of antimicrobial action.  Common organic carriers include zeolites, phosphates, titanium dioxide, montmorillonite and mesoporous silica.

Zeolites are particularly effective carriers for melt blending with medical plastics.  These crystalline aluminosilicates are compositions of aluminum, silicon and oxygen, with uniform cavities and pores.  This geometric construction controls release of the antimicrobial metals.  Importantly, zeolites can withstand the high melt processing temperatures of thermoplastics.