Cold cure treats silicone

Those who have worked with silicone rubber will know the many unique advantages of this material. It is highly flexible, transparent and pleasant to the touch. In addition, the material can withstand extreme heat and subzero temperatures, it resists aging and retains its hydrophobic properties. However, it can have drawbacks when considering its use in some medical applications.

These drawbacks arise from the fact that the silicone rubber is created by the cross-linking of polymers where the individual polymers link with each other into a chemically bound network. However, there are some components known as silicone oils or oligomers, that fail to react with the chain. These oils may disrupt further processing and are undesirable in products that are destined for the food and medical industries.

The traditional method used to overcome this issue is called heat curing or “baking”. In this method, silicone rubber is treated in ovens in order to evaporate the unwanted volatiles. Nanon’s innovative alternative – its patented ColdCuring process – goes much further: it takes the amount of soluble residuals to the lowest possible levels in just a fraction of time needed for heat curing.

The secret of the new method is the use of liquid CO2 as an extracting super-solvent in place of heat.

A simple comparison of the two methods shows how far advanced and promising the new cold technique is. The oven technique can take as long as 4 hours of treatment at a temperature of 250°C in order to remove residues down to 0.5%, whereas the 45-minute new cold method treatment extracts oligomers down to the same levels using room temperature. ColdCuring treatment also has the capability to extract residues to the almost non-existent level of 0.005%.

The amount of cyclic siloxanes D4-D20 is reduced to below 0.005% (50 ppm), so the resulting material is significantly purer and cleaner. For example, in the medical device industry, products made of silicone that have been treated using ColdCuring meet much higher safety standards, including that of the European Pharmacopeia Specification, 5th edition. This includes the specification for devices such as silicone tubes, pouches for carrying blood, valves for syringes and baby pacifiers.

An added benefit of the ColdCuring technique is no "slit-healing". Because no heat is applied, any small pre-cut slits and other fine design details will not “heal” over. This enables precision moulding prior to the treatment. Nor is there any re-polymerisation effect.

The ColdCuring process takes place in what could be described as an advanced “washing machine” using pressurised CO2 in its liquid state as a solvent. The liquid CO2 penetrates into the polymer matrix, dissolves, and washes away low molecular weight residues. Extracted oils are collected from the distillation vessel, from which they can be disposed of safely.

Although it is a completely new application, the same technology has been used in the food industry for many years. The equipment has been previously used for cleaning coffee and spices, and is approved for production. The effectiveness of ColdCuring has also been tested on a wide variety of products, including baking forms, kitchen utensils, key pads, baby pacifiers and medical disposables. The technique is particularly noteworthy since it is an environmentally-friendly technology that provide benefits not available through other processes.

Biofilm removal

Bacterial attachment and growth on medical device materials is a major problem in the healthcare sector. It is estimated that nearly half of hospital-acquired (nosocomial) infections are associated with implants or internal medical devices. For example, each year approximately 80,000 catheter-related bloodstream infections occur in US intensive care units alone, at a cost of $296 m to $2.3 bn (€225m - 1.75bn) .1 These infections are associated with somewhere between 2,400 and 20,000 deaths per year.2 In Denmark it has been estimated that infections acquired during hospitalisation cost Dkr1bn (€0.13bn) per year.3

It is widely recognised that new strategies for interrupting the growth of micro-organisms are required in oder to combat the problems associated with bacterial colonisation on medical devices. This includes the need for new chemical treatments that can render surfaces of medical materials resistant to the random deposition of biological matter, i.e., protein adsorption, bacterial attachment and subsequent biofilm formation.

Under the umbrella of Nanon’s patented Softplasma project, the company has been creating coatings using ultra-hydrophilic polymers, such as polyvinylpyrrolidone (PVP), for application on silicone tubes. The objective of the programme is to investigate the potential of various surface modification technologies for use in the medical device field. Specifically, the focus will be on initial interactions between the bacterial membrane and the polymer surface.

The company’s bio-coatings are applied to the substrate using the patented Softplasma technology. This enables bio-coatings to be applied on almost all surfaces – even those such as silicone, which are difficult to coat.

In recent years increasing emphasis has been put on the adverse biological effects of latex and PVC for use in medical devices. To a large extent silicone can replace these materials in modern medical devices.

This creates a demand for innovative coating techniques, and Softplasma technology is a good alternative to wet chemical techniques that often require the use of harsh chemical agents. Furthermore, the Softplasma technology creates a very thin (nano-scale) coating that does not compromise the physical properties of the device – a factor that is highly desirable in medical device manufacture.

Antibacterial impregnation

Another interesting project being undertaken by Nanon and using its patented Cohancement technology is related to the impregnation of silicone materials with anti bacterial agents or drugs. Cohancement offers significant potential for the medical device sector.

The CO2-based impregnation technology enables the addition of active drugs and antiseptics to the upper layer of swellable medical device materials such as silicone. This has the potential to decrease the build-up of biofilms on the surface by actively killing colonising bacteria. Impregnation into the material facilitates a slower release of drugs and therefore prolongs the anti bacterial effect by several weeks. This technology could be used to administer drugs in a site-specific manner to medical device-carrying patients.

Nanon focuses on inert surfaces for the following reasons:

• the immediate or long-term side effects of using active drugs on the surface are eliminated

• production of such devices will be significantly less costly

• (and perhaps most importantly) an inactive anti-biofouling coating directed at preventing bacterial attachment will not involve the use of antibiotics and, thus, will not contribute to the growing problem of developing multi-resistant bacteria.

Nanon focuses primarily on well-established coating materials that have a well documented non-harmful effect when used in the human body. An example of a coating material currently in the development phase is polyvinylpyrrolidone (PVP), which has been used for more than half a decade in drugs and medical devices and is safe and inert in the human body.