There are a couple of things going on here that are contributing to the failures under accelerated aging but that are not seen after product sits on the shelf and undergoes real-life aging. Cyanoacrylates are susceptible to a chemical reaction called hydrolysis. When placed in a hot and humid environment, the chemical polymer in the cyanoacrylates tends to break down. EtO sterilization uses steam or humid water vapor as the carrier to bring the EtO into the chamber, so we typically view this as a moisture-rich environment. A slight decrease in bond strength with EtO is expected.

It is also important to realize that cyanoacrylates are very brittle adhesives. If you have ever stuck your fingers together accidentally with a cyanoacrylate and then moved them together in a rubbing motion, the adhesive will crack and eventually enable you to separate your fingers. As you put the parts into a 50°C oven, you correctly lower the humidity, but there is still a concern, since the temperature remains fairly high, which helps drive chemical reactions. The length of time involved is fairly long, and with even a low amount of humidity, additional hydrolysis reactions may be occurring.

Another issue relates to the coefficient of thermal expansion (CTE) differences between the stainless steel, the polycarbonate, and the adhesive. Materials expand at different rates at a given temperature. The metal expands at one rate, the polycarbonate expands at another, and the adhesive at yet another. This puts additional stress on the bond line, a phenomenon that does not occur when product undergoes aging in real life. This interaction may be enough to crack or break enough of the cyanoacrylate bonds to cause your bond-strength test to fail.

Between the two substrates, the cyanoacrylate is most similar to the polycarbonate. Therefore, the weak bond will most likely be to the stainless steel, which matches your observations. In addition, adhesives such as cyanoacrylates can penetrate slightly into the surface of the polycarbonate and become chemically embedded or intertwined with the polymer. In such cases, the only bonds available on the stainless steel are weaker van der Waals forces and mechanical interlocks into the microscopic nooks and crannies on the metal surface. These bonds are typically the first to break.

Other adhesive options such as urethanes, acrylates, light-curable acrylated urethanes, heat-curable acrylated urethanes, or epoxies can have a lower modulus value, which will be able to absorb the stresses of the CTE differences. Even rubber-toughened cyanoacrylates may be an option. These adhesives are able to withstand shock or impact better than cyanoacrylates because they are not as brittle. This makes them better suited to survive much higher temperatures and stresses.

There are a number of options to look at when trying to answer this question. Many adhesives are available that will do the job. The range of products available includes epoxies, structural adhesives (acrylated urethanes), threadlockers, and other adhesives that all offer excellent adhesion to metal.

Typically, the first choice that comes to mind when we need to mate two threaded parts is anaerobic threadlockers, which are available with different degrees of strength and torque resistance. There are permanent threadlocker grades available on the market from a number of suppliers. Epoxies offer brute strength as well. Structural adhesives such as acrylic or acrylated urethanes include the Dymax 846-GEL families or Loctite 300-series structural adhesives, which also offer exceptional strength.

The questions that need to be answered to inform your product choice include: What types of metal and plastic are being adhered together? There is a difference between bonding stainless steel or cold-rolled steel to anodized aluminum or galvanized zinc. The same is true for plastics. What type of environment will the part be exposed to? Can we use an activator to cure the material, or can we use heat or other curing methods? What is the bond gap between the surfaces? Does the material have to be medical grade? Does it require impact strength?

There are a number of options and trade-offs to consider before we can identify the best system for your application. Contacting a local representative or distributor may be the best way to determine the most suitable adhesive.

A few options are available to you. Choosing a permanent adhesive such as an epoxy or an acrylated urethane for the base will allow the device to stay bonded, and using a temporary adhesive or removable mask to temporarily bond the top of the device may be an option. Peelable or water-soluble masks will allow you to remove the material cleanly and efficiently. Water-soluble masks will dissolve in either water or solvents (alcohols or chloroform). The use of a mild and gentle solvent such as water or alcohol will have less of an impact on the permanent adhesive. A good idea is to use the shortest time necessary to remove the “top” adhesive, thereby limiting the exposure of the bottom adhesive.

This link will direct you to some removable masking materials and guide you through the selection process. Temporary pressure-sensitive adhesives may also give you the option to remove the adhesive layer, if needed.

Unfortunately, not that I am aware of. Technically, most light-curable adhesives are acrylated urethanes or epoxy-based systems that would not survive permanent implantation. There are other hybrid light-curable technologies, but as far as I know, none have been released technically for long-term implantation. In addition, the legal liability is too high for most applications. Perhaps something from the dental-cement industry might be a suitable option.

It is very common to employ an adhesive to make a nice, smooth tapered transition between cuff edges and a tube or marker bands, or to achieve transitions in which there is a jump from one tube size to another. A low-viscosity adhesive in the 200- to 600-cP range, which cures rapidly to a smooth tack-free finish, is ideal. Light-curable materials such as Dymax’s 1120-M-UR or 204-CTH-F are often used in these types of applications. The adhesive can be applied in either a vertical or horizontal position, or even at a slight upward angle to achieve the taper. If a horizontal or tilted orientation is required, rotating the shaft during application and curing it with a spot system for a typical duration of 1 to 8 seconds allows the material to cure in the proper profile without causing it to slump or run. Typically, such transitions have a maximum height of 2 to 5 mil or less. Lower-viscosity adhesives have a sharper taper, while higher-viscosity ones have a shallower taper. Selecting the right needle dispense tip helps users to control the adhesive quantity and position it on the catheter shaft.

Common solvents for these systems are often toluene or xylene. Sonication baths help to loosen the material and remove it to expose underlying layers, but it still might take two to three days for thin coatings and longer for thick coatings. Scoring the surface helps give the solvent areas to work on and through to penetrate deeper into this thick coating. For an environmentally cleaner solution, Dynaloy Dynasolve CU-6, 217, and 220 from Dynaloy LLC (Indianopolis) have been effective at removing different coatings. Dynasolve 217 and 220 are listed as being able to dissolve cured silicone. In any case, I would suggest doing a test on a separate wire without silicone around it to see if it affects the wire jacketing and insulation material.

To determine the proper cure time of any light-curing adhesive when exposed to light from any source, there are a couple of different approaches that can help. The best tool is a radiometer, which tells you how much intensity you have at the bond line. The PC-5 is an older-model flood lamp with an intensity of 50 to 150 mW/cm² over a 5 x 5-in. area. The different approaches depend on how you use the adhesive. If you use it between two substrates in a bond-line thickness of 0.002 to 0.006 in., measuring the fixture time should be sufficient. For Loctite TDS, fixture time at this intensity should be <5 seconds. If you are potting a deeper section, then the depth of cure is important, and you can reach a depth of 2 mm in approx 12 seconds. The Loctite TDS plots the depth of cure at an intensity of 50 mW/cm². If the adhesive bond line has some squeeze-out or has a surface exposed to air, a tack-free surface cure may be important. Tack-free time is the point at which the adhesive is sufficiently cured that you will not get smearing or residue transfer onto a gloved finger.

With any of the three described situations, measuring this yourself is the best way to figure out the proper cure time, whether you are looking at fixture time, depth of cure, or tack-free time. Set the bond line at the lowest intensity you can use—say, 50 mW/cm². Do this by increasing the distance away from the lamp until the radiometer measures 50 mW/cm². You will want to manufacture your parts at a higher intensity to start and manufacture within a window of intensity and time to control your process. Under constant intensity, cure the adhesive several times. You will see the tensile strength, burst pressure, tack-free time, depth of cure, durometer, or other data point climb to a maximum value and then plateau. Once you have identified the start of the plateau, figure in a safety margin, and you have the foundation for your process. You can also record the same data points after setting the time constant and varying the intensity. You will want to define your process by knowing the minimum and maximum intensity and the minimum and maximum time needed to cure the adhesive.

Two options come to mind when looking for a medical-grade light-curable adhesive for polystyrene: 1201-M-SC and 1120-M-UR from Dymax. These materials adhere to polystyrene and are ISO10993/USP Class VI tested.

One simple but effective method to monitor if the curing has gone to completion is incorporated into the 1201-M-SC. This material uses a technology called “See Cure,” where the material starts off with a brilliant blue color and changes to clear as it cures. This visual indicator ensures that complete cure has been achieved in all parts of the bond line. Other methods to determine the state of cure include destructive testing of the components to measure tensile force or a drop of adhesive at the bond-line surface and using this droplet to measure for tack/semicure (a go/no-go measurement observed by the presence or absence of adhesive transfer onto a gloved finger). A more-complex method includes microscope FTIR analysis of the adhesive to identify the presence of the double-bond peak (on the spectrum) before cure and the removal of the double bond peak after cure.

Dosimeters are necessary to accurately measure light exposure, and there are different versions, with different sensors, that measure different parts of the UV and visible light spectrum. While most light-curable adhesives cure with a combination of UV-A, UV-B, UV-C, and visible light, it is often convenient to reference the UV-A light spectrum coming from the light source. UV-A is commonly referred to as 365 nm, but it actually covers a range of approximately 320 to 395 nm. This can be measured using an Accu-Cal 50. If the polystyrene is UV blocking, we would have to rely on the visible-light spectrum of the lamp. The Accu-Cal 50V measures 395 to 465 nm. Both units can give you average intensity (mW/cm²), peak intensity, and total energy (Joules/cm²). Other options are available for special lamps, such as LED lights, which only emit a single wavelength at either 385 nm or 405 nm. The Accu-Cal 50 LED was developed to integrate around the center of the lamp spectrum.

A postbake is not necessary on most adhesives, but there are a few adhesives with a peroxide thermal initiator that can use heat to cure areas not able to see light.

Regarding outgassing of UV-curable adhesives during cure, it is sometimes observed that a small amount of smoke comes up from the adhesive surface during the cure step. This is typical, since the adhesive may emit trace amounts of some of the ingredients (or fractions of the ingredients) contained in the formulation while light is shining on the adhesive and cure is taking place. Sometimes this can be overcome by varying the intensity and duration of cure or by choosing different adhesives and light sources. This does not happen when the adhesive is used between two surfaces. Proper ventilation can help remove this smoke. If the smoke deposits onto a spot or flood lamp, then periodic cleaning of the end of the light guide or lamp housing is necessary to remove the film that may form there, since this thin film can reduce the intensity at the bond line.

Assuming that most surgical tools are made from stainless steel and need to survive repeated autoclaving, I would recommend looking at two-part epoxies as your base chemistry. Options are available from Loctite, 3M, and Epoxy Technology—to name a few. And some formulations have biocompatibility certificates on file. If the surgical tools are disposable or made from plastic and only need to withstand a single autoclave, EtO, or gamma sterilization cycle, a light-curable acrylated urethane like Dymax’s 1120-M-UR would be my first choice. Acrylated urethane light-curing adhesives offer excellent adhesion, are simple to apply as a one-part material, and cure in less than a second.

When light-curable adhesives cure under either UV or visible light, cross-links form between polymer chains. This pulls the chemical chains closer to each other very rapidly. We typically see 1 to 2% linear shrinkage, which could translate into 2 to 5% volumetric shrinkage. This may stress some plastics or optical components. There is a relaxation effect, usually over the next few hours or overnight, where the chains relax slightly as they rotate into an optimum alignment. Polymer chains like to spoon together and snuggle. If they are at odd angles to each other, they still touch, but you want to find that alignment where they are in the same direction and bending the same way. Chemical bonds can stretch and spin around their axis, allowing for this relaxation. It is also good to note that products with a low modulus stretch easier under stress than products with a very high modulus, which do not stretch much at all. Silicones on one extreme can have a modulus as low as 300 psi, whereas epoxies can have a modulus as high as 2,000,000 psi. Many UV-curable adhesives are urethane acrylates, which can have a wide range of modulus values. Your product data sheet should indicate this value.