Making Fast & Easy Connections with Swelling Fluids

Elizabeth Norwood, SR Chemist, Microcare LLC | Nov 21, 2022

Many medical devices employ new and more sophisticated components in their designs. This includes the tubing and hoses used in venous access devices, drainage catheters, dialysis machines, infusion pumps, enteral feeding devices, and others that transfer fluid, medications, gases, guide wires, cameras, or other materials.

Some of these more sophisticated devices feature state-of-the-art multi-lumen tubing where multiple channels run inside a single tube, simplifying the devices’ operation. In addition, some devices use tubes with a smaller diameter and thinner-walled tubes, making them lighter and more flexible for easier handling, storage, and use.

However, the drawback to these updated tubing designs is that it often makes manufacturing the devices more challenging. Especially connecting the soft or thin-walled tubes to rigid plastic or barbed fittings, which can be labor-intensive and slow. This is where swelling agents play a key role.

Assembly Troubles

For many medical device fabricators, the preferred tubing materials are silicone elastomers which have been used for decades. Medical-grade silicone tubing is bio-compatible, durable, flexible, and versatile. It also resists bacterial growth and is easy to sterilize. But, joining complex silicone tubing onto parts made of harder materials can be difficult. First, while silicone is flexible, it generally does not expand or stretch without assistance. Second, silicone has a high coefficient of friction, or a tacky surface, that makes sliding a silicone tube onto a fitting tough.

Manually pushing or twisting tubes into place is hard for assemblers, sometimes causing carpal tunnel, wrist problems, and other workplace-related injuries. Also, forcibly pressing small silicone tubing onto a barbed fitting sometimes results in stress-cracked tubes and scrapped materials.

Fortunately, there are fluids to make connecting fittings to tubes quicker and easier. There are three common fluid types: silicone oil, isopropyl alcohol, or swelling fluids.

Silicone Oil

Silicone oil makes tube assembly easier, but it is messy. The oil stays often migrates throughout the production facility, coating other equipment, and surfaces. It captures surrounding dirt in the process, making facility hygiene a constant challenge.

IPA (Isopropyl Alcohol)

Ultra-pure IPA (Isopropyl Alcohol) is readily available and is relatively inexpensive. But IPA dries slowly, which increases assembly cycle time. Also, IPA allows thin-wall tubing with less structural rigidity to collapse or fold over during assembly operations. This makes it harder to press or slide the tubing onto a fitting.

Swelling Fluids

Using swelling fluids to temporarily swell tube ends to increase their size helps make fitting tubes onto barbed connectors simpler and faster. Two of the most common swelling fluids are hexane or engineered silicone-based swelling fluids.

Hexane works well for swelling but must be used with care. It is extremely aggressive. It might remove ink markings or surface coatings, damage plastic components, or permanently change the physical properties of the tubing. Hexane smells strong, so proper ventilation or PPE (Personal Protective Equipment) is essential. It is also classified as a hazardous air pollutant (HAP) and may contribute to air quality issues.

Silicone-based Swelling Fluid

A better option is engineered silicone swelling fluid. One end of a silicone tube soaks in the swelling fluid causing the tubing wall to uniformly expand. Soaking time depends on how much tube expansion is required. For instance, a tube that needs 1–2% expansion for assembly, typically soaks for less than a minute.

The swelling fluid does not change the physical properties of the tubing. After the tube fits into the connector, the swelling fluid evaporates quickly and completely. The tubing recovers to its original size, color, shape, durometer, compression, and strength. It forms a tight, leak-proof, secure grip over the fitting, no matter how complex the geometry. It does not bond or weld the tubing and fitting together, making it easier to separate them later if required.

Good Compatibility

The swelling fluid's active ingredient is methyl siloxane, a member of the silicone family. It is not aggressive, so it has excellent materials capability and is safe to use on most elastomers and polymers including polycarbonate or polyurethane, and has excellent compatibility with metal components.

There are other swelling fluids available for many tubing materials including neoprene, EDPM, polyethylene, polypropylene, polyimide, and other molded thermoelastomer tubing and hoses.

Process Validation

Since silicone swelling fluids don't require heat, glue, or other adhesives that can affect the integrity of the tube, fittings, or device it is easier to qualify and validate in the assembly process. In addition, swelling fluids do not produce residue that could negatively impact clean room integrity or adversely affect the qualification of the manufacturing process.

Meet Environmental Standards

Silicone swelling fluids meet safety, quality, and regulatory standards, whether it's medically specific or environmental. They have a low GWP (Global Warming Potential) and zero ODP (Ozone Depleting Potential). They are not classified as Hazardous Air Pollutants (HAPs) and are not subject to NESHAP regulation.

To Wrap Up

It is critical that today's medical devices perform flawlessly with leak-free connections between the medical device and the tubing. Swelling fluids provide an effective and efficient way to join silicone, polyurethane, or other thermoelastic tubing to fittings and molded parts. It is especially useful for swelling tubing with thin, soft walls or those with larger diameters.

Using a swelling fluid helps simplify medical device assembly, by increasing throughput and boosting overall productivity in the safest and most sustainable way. Many swelling fluids are an easy-to-handle and reliable alternative to messy silicone or aggressive hexane. When choosing a swelling fluid, it is best to consult with a company that specializes in medical lubricating and coating technology. They have the experience and expertise to help choose the best-swelling fluid for each individual application.

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