Smooth operators: Lubricous additives & medical device manufacturing

Josh Robertson, Compounding Solutions outlines the place that lubricious additives hold in medical device manufacture.

Lubricity in medical device technology is nothing new has been a need since the early days of the industry in everything from simple guide catheters to complex articulating neural catheters, drug delivery systems and more.  The need for lubricity is a function of design based on final performance in either an extruded tube or an injection moulded part. Surface friction between extruded tubes moving inside of one another can quickly derail any type of fine control in a catheter design and/or simply lock up other moving parts for instance a trigger or slide in a handle set. 

Many catheter designs are excellent candidates for PTFE liners to provide lubricity for devices travelling within. Very few companies provide such materials as a great deal of technical expertise is required and pricing reflects the complexity.  The ram extrusion process, etching process and handling/shipping processes for exceptionally thin walled extrusions (typically .002”-.0005”) makes the cost justifiable. The technology is very well proven, effective and available.  A PTFE liner is reflowed with braid or coils and an exterior thermoplastic jacket such as Pebax materials from Arkema. That liner inside the tubing provides the gold standard for all other material comparisons.

PTFE powders and oils can also be compounded into many thermoplastics to provide lubricity.  The powders can be effective, however, at Compounding Solutions we’ve learnt that this method generally requires a ‘break-in’ phase.  The polymer surface does have PTFE on it however the individual granules of powder must be effectively smeared across the surface and that requires polymer essentially being removed to flatten and press out the powder more evenly on the surface. 

Oils can help bridge that gap but it is not preferable. While only a bit of surface needs be sluffed off to impart lubricity, very few devices have the ability to undergo such preparation. In general, the more friction that occurs (break in) the better the powder loaded polymer performs. This type of technology can be effective in some re-useable devices made in injection moulding applications however, catheter-based technologies are not well suited for compounded powders. 

Feedback from our customers has shown that the powders create bonding issues for the devices in everything from tipping operations to reflow processes. The PTFE powder on the surface makes bonding the soft tip of a catheter to the shaft a more difficult operation for heat bonding and at times adhesives. Conversely the same powder can help with the use of a tipping die because it helps with release from the die after the heat forming process.

Another avenue is hydrophilic coating, which is generally applied to the exterior of a device but not exclusively. This is an exceptional provider of lubricity. Based on our testing and reviews nothing we have seen in the medical device industry can deliver the same effects.  Adhesion of the coatings can be an issue however, expertise and effort has yielded good results for many companies. The providers of such materials and coatings provide excellent service and final results. For exterior lubricous needs this is the gold standard.

Costs are always a concern. The desire to reduce the costs involved in medical devices in general is not an unreasonable request. The lowest cost material is not necessarily the pathway to a low cost device.  What most companies do not review is that a small increase in raw material costs for extrusion can greatly reduce the overall device cost.  That can be achieved via dramatic decreases in scrap rates based on highly effective dispersion leading to reduced inclusions and smooth process profiles.

Dispersion is the ability of any given compounder to spread the ingredients in a compound evenly and achieve homogeneity. Pellet to pellet consistency is our hallmark. Such success is a result of switching from an existing vendor to Compounding Solutions materials as our dispersion techniques are the result of proprietary screw designs and machine operations.

Technology improvements involved in medical interventional devices may be viewed as incremental and may not be clearly obvious unless viewed within the lens of the final cost benefit analysis for any given device. Using the example of PTFE liners in catheter based therapies is the first concept we reviewed while considering alternatives to well-known materials in the marketplace. 

The ability to extrude a very thin wall and reflow it into any given construction has provided a pathway to achieve an extremely lubricous inner surface and establish a baseline for performance standards. Compounding Solutions has developed Mobilize not as a replacement to PTFE but as an opportunity to achieve lubricous surfaces without the additional cost and the myriad of issues possible in laminate constructions. The opportunity to use a range of bonding techniques is open with Mobilize as we have yet to see any issues arise in heat or adhesive based bonding.

Mobilize is an additive that is compounded into a range of plastics commonly used in catheter constructions such as PEBA, nylon, polyester and PE.  The additive provides benefits with a minimal cost increase to the overall extruded material costs. Despite this minor increase in raw material cost the impact on the final device is an overall cost reduction because PTFE can be removed.  Within the injection moulding realm, the same benefits can be seen in disposable/single use devices due to Mobilize imparting reduction in frictional forces immediately.

Knowing that new materials can be viewed as a pariah at worst or novelty to some at best we forged ahead with testing to help our customers understand Mobilize is a reasonable alternative to existing market options. Aging data was compiled and tested under ASTM conditions (Figure 1).

This kind of test simulates two things. Accelerated shelf life as would be expected during long-term storage in a warehouse, and the impact of high heat environments during shipping, storage, etc. Our experiment is representative of approximately one year of storage in a 72F warehouse, and three months of storage in a 115F shipping container. Injection moulded tensile and flex specimens were heat-aged at 45C in our oven for three months and tested at days 0, 30, 60 and 90. All test parts were made from the same lot of respective material. 

Conclusion

A range of additives/modifiers are available and each has an appropriate playing field. For the majority of cost-sensitive single-use disposables, additives such as Mobilize are an ideal balance between performance and cost. As we can see from the collected mechanical data on Pebax with and without Mobilize it has a minimal mechanical performance change relative to the base material without any additives. Pellethane and other TPU materials do see a greater degree of change. However, based on the application it could actually enhance the performance. Long term usage and storage of the additive in both polymers is stable and minimal change occurs between the Mobilize loaded material and the natural material.  The addition of heat and UV stabilisers could benefit this longer term performance as well. 

A major observation is that tactile tests of TPU materials loaded with Mobilize are a dramatic change from the base resin.  The tacky feel and clumping of tubing or other parts is essentially non-existent which provides a side benefit as a manufacturing aid.

The gold standards of the industry will remain as such and Mobilize provides another option the market can bring to bear for lubricous needs.