Silicone’s excellent biocompatibility makes it a desirable material for medical device makers, though its challenges can be formidable. Medical device design engineers are familiar with the difficulties of connecting silicone tubing to barbed fittings or mating molded silicone parts with complex geometries to hard plastic parts. The consistency of silicone tubing is very flexible, similar to a rubber band. However, unlike a rubber band it isn’t able to expand or stretch without mechanical or chemical assistance. This makes assembly with rigid parts difficult. And the tacky surface of silicone rubber makes it all the more difficult to work with.

Lubricating vs. swelling

While lubricating the silicone with isopropyl alcohol or silicone oil is one option of mating silicone parts to each other or to hard plastic or metal parts. Traditionally, lubrication methods were used to connect silicone tubing with barbed fittings. Isopropyl alcohol (IPA) and silicone oil are two methods commonly used. Both of these methods are familiar to engineers and have their own sets of benefits. IPA is relatively inexpensive and does dry without leaving a residue. However, IPA doesn’t dry quickly. The main drawback to using silicone oils is that they migrate during use and require significant engineering controls to avoid excessive cleanup at assembly workstations, thereby slowing down productions and increasing manufacturing time and costs.

One effective alternative solution is using volatile methyl siloxane-based swelling agents. Once these solvents are absorbed into the silicone rubber, regardless of the geometry, the silicone rubber will expand in size and once it has expanded enough to allow the silicone to easily accept the fitting or rigid part, the solvent evaporates and the silicone rubber returns to its original size. What remains is the silicone rubber component attached to another part with a tight and secure fit.

In addition to being able to design freely, knowing that this solution exists allows designer engineers to specify the use of dissimilar parts with unique geometries, since this technology allows them to be joined. This is common in finished products that are assembled with both molded hard plastic and silicone rubber, like oxygen masks.

Swelling silicone using hexane solvents is another popular method; however, the odorous and highly aggressive hexane solvents are regulated as a volatile organic compound (VOC) by the environmental protection agency (EPA) and they can actually affect the physical properties of the silicone.

However a newer class of swelling agents, including the MicroCare Medical Swellex, use volatile methyl siloxane (VMS) technology and are considered as chemically “like materials” to silicone rubber and therefore will not change silicone elastomer properties such as tear strength, compression, or color. They are more environmentally friendly and higher performing than the traditional hexane solvents. Also, they evaporate completely, leaving no residue, a critical factor in medical given the importance of clean parts.

One of the greatest benefits of using VMS solvents to swell silicone is that it ensures two parts are joined tightly and securely. If the mated parts need to be separated, they could be – and without damage to either part. With excellent materials compatibility, VMS solvents will not chemically soften or change the cosmetic or chemical properties of commonly used mating plastic components such as polycarbonate. Other swelling solvent based swelling agents may attack or soften polycarbonate based plastics, resulting in the plastic surface physically bonding to the silicone, so that replacing either part or trying to detach one from the other is impossible.

‘Swell’ swelled examples

One leading manufacturer of silicone tubing is using VMS technology to attach the silicone tubing that goes from oxygen breathing masks to brass or other metal fittings onto the oxygen tanks in ambulances and hospitals. This is a situation that usually involves mating small-diameter silicone tubes to relatively wide and rigid barbed fittings to maintain an airtight seal. The swelling agent is applied to the silicone tubing and after it has expanded in size, the rigid fittings can be easily fitted into the tube. Once the fluid evaporates, the tubing returns to its original size resulting in a secure, airtight seal to the oxygen mask and tank. By eliminating the extra housekeeping or increased drying time that was necessary with other methods, this company was able to save time and money.

The method of swelling silicone rubber using VMS technology is also providing an effective solution for one leading developer and manufacturer of catheter-based therapeutic products. This company is using VMS solvents as an effective and environmentally acceptable way to swell extremely narrow silicone tubing that is used to deliver fluids intravenously. They join a syringe or metal fittings onto the tubing. For this company, silicone rubber is used to surround a syringe and guideline to help insert the catheters with minimal trauma.

In this situation, the swelling fluid is used to attach very narrow silicone tubing to medical sharps or other metal fittings. The most important consideration is that the connection produces a dependable leak-tight seal to prevent penetration of infectious agents since the device is actually going into the body. Nothing should be able to get in or out of the device unless it is intended. Also unique about this example is that the fitting that is being joined with the silicone is not threaded or barbed and resulting in a secure connection.

Other companies are finding that swelling silicone allows them the option of connecting the same size diameter tubing to different fittings with multiple diameters. Especially true in high speed, high assembly situations, silicone swelling is providing an easier and more efficient method of joining dissimilar parts than what was used in the past.