Medical device fabricators are using new and more sophisticated components to incorporate into their device designs. Patients and healthcare providers want smaller, easier to use devices. These include drainage catheters, IV tubes and bags, dialysis machines and many others.
Some of these complex devices use state-of-the-art tubing designs with smaller inner and outer diameters and thinner wall thickness. Some may also multi-lumen tube construction with multiple channels running inside the tubing to deliver fluids, gases, guide wires or other materials within a single tube. But, attaching these tubes to fittings to devices during manufacturing can be difficult and slow. This is where swelling agents play a key role.
Making hard connections easier
For many medical device manufacturers, silicone elastomers are the preferred tubing material. The remarkable physical properties of silicone — durability, flexibility, low cost, structural versatility, resistance to bacterial growth, low extractables and easy sterilization — make it a good choice for medical grade tubing.
But, mating complex silicone tubing onto parts made of harder materials can be problematic for two reasons. First, while silicone is flexible, it won’t generally expand or stretch without assistance. Secondly, silicone has a high coefficient of friction, or tacky surface, that makes sliding a silicone tube onto a fitting difficult.
Fortunately, there ways to make connecting silicone tubing quicker and easier. There are three common methods of joining silicone tubing to parts: lubricating with alcohol, pre-treating with silicone oil, or swelling with Swellex™ Swelling Fluid.
IPA (Isopropyl Alcohol)
Ultra-pure isopropyl alcohol (IPA) can lubricate silicone tubes for assembly. IPA is easy to source, is relatively inexpensive and it evaporates without residue. But IPA dries slowly, which can increase assembly cycle time. Also, if the connection isn’t completely dry, there may be problems with the device later on. Plus, IPA isn’t the best when assembling thin-wall tubing because the tubing collapses or folds, making it difficult to press or slide the tubing onto a fitting.
Silicone Oils
Lubricating with silicone oil is effective, but messy. The oil stays on a surface indefinitely. Also, silicone oil migrates everywhere int factory, making housekeeping difficult. Plus, the oil captures dirt from the environment and, medical grade silicone oils are expensive. As with IPA, thin wall tubing often collapses or folds with silicone oil, making assembly a challenge.
Swellex™ Swelling Fluid
Swelling makes tubing easily slide over a barbed fitting or other connector. The two most common swelling agents are hexane or an engineered silicone swelling fluid, like the MicroCare Medical Swellex™ Swelling Fluid. Hexane has a strong odor so proper ventilation or PPE (personal protective equipment) is essential. Hexane is also an extremely aggressive solvent so it might remove surface coatings or ink markings, damage plastic components or permanently change the physical properties of the tubing. Although effective at swelling silicone, hexane is a volatile organic compound (VOC) that contributes to poor air quality. A better choice is to use an engineered silicone swelling fluid like Swellex™.
How It Works
One end of a silicone tube is immersed and soaked in the swelling fluid. The fluid uniformly expands the elastomer like a sponge absorbing water. Exposure time determines how much the tube swells. Because tubing dimensions may only need to expand by 1–2% for assembly, the entire swelling process is often be accomplished in less than a minute.
The tubing swells in a highly predictable manner without changing the physical properties of the tubing. This makes assembly faster and easier when connecting thin wall tubing to barbed fittings.
Once the tube is in place, the swelling fluid evaporates quickly and completely from the tube. The tubing recovers to its original size, durometer, compression, color shape and strength. It forms a tight, leak-proof, secure grip over the fitting, no matter how complex the geometry. Since the Swellex™ fluid’s active ingredient is a methyl siloxane — a member of the silicone family — it doesn’t affect the chemical structure of the tubing or the physical properties of the fitting.
In addition, Swellex™ has excellent materials compatibility. It works on thin wall and even softer, larger diameter tubing. It doesn’t cause a long-term change to the mechanical properties with the tubing material. In addition, it won’t weld or bond the tubing onto the fitting, so the tubing can be easily removed later if needed.