PD-Dental's flexible plastic cannula design to replace metal
Franck Didierjean, sales and operations manager, and Thierry Maire, technical supervisor for Branson Ultrasonics at Emerson share how Produits Dentaire SA (PD-Dental), an endodontic specialty company, has designed a flexible plastic cannula that is designed to outperform and replace metal irrigation cannulas used in root canal surgery.
PD-Dental reasoned that a flexible plastic cannula with side-facing irrigation vents near the tip would be safer and more effective, since it would slide more easily through the complex curves of root canals while enabling more complete irrigation and disinfection.
After preliminary designs showed promise, the next challenge was to produce the cannula on a commercial scale. Following considerable effort to develop a workable injection moulding process, the manufacturer settled on a cannula comprising two injection-moulded components, both made of sterilisable, medical-grade polypropylene.
The first component was the hollow, conical cannula stalk, which incorporated dual side-facing irrigation ports, while the second was a cannula base. One end of the base was a luer taper for connecting the cannula to a hypodermic irrigation syringe, while the other end hermetically sealed to the cannula structure using an ultrasonic plastic weld. PD-Dental planned to name the new, flexible irrigation cannula product IrriFlex.
Ordinarily, making an ultrasonic weld between two polypropylene parts is not difficult. However, the unusual geometry and lightweight structure of the cannula/base components posed special welding challenges. So special, in fact, that a number of ultrasonic welding companies refused to develop the application, saying the weld was impossible to make on a production scale.
Emerson believed it could adapt Branson ultrasonic welding technology to meet the challenge. Over a period of weeks, the Emerson technical team for Branson ultrasonic welding in Geneva worked with the customer to develop a solution. It was an iterative process, requiring numerous changes and adaptations to the custom-built tooling that would ultimately make the welds.
As noted above, the geometry and structure of the components posed two problems:
First, the Emerson team found that even small variations in injection moulding quality could affect the height and profile of the energy director — a fine rim of plastic on the cannula base onto which the stalk would be welded. The team determined that an ideal energy director should be 250 microns in height, with a plus-minus tolerance of 50 microns to ensure a strong weld and a hermetic seal. The team then collaborated with the injection moulding vendor to manage this critical quality specification in production components.
Solving the second problem, inherent in the cannula’s long, narrow shape, fell solely to Emerson. Every ultrasonic weld requires that the upper component be contacted by a sonotrode, which transmits the ultrasonic energy through the component to complete the plastic weld with the base component. In this case, the long, narrow tip of the cannula stalk would be held in a hollow cavity of the horn. However, when a typical horn design was used to weld the cannula stalk to the base, the high-frequency oscillation required to weld it to the base created an unwanted side effect, called diaphragming. Similar to the ripple effect of a pebble hitting water, diaphragming means that the acoustic energy required to weld the bottom of the cannula to the base was causing the top — the fragile stalk — to flex rapidly inside the top of the horn. So, although the initial sonotrode design made successful welds, diaphragming caused the fragile, cannula stalks above to deform and crack, resulting in a scrap part.
To enable good welds without cannula breakage, Emerson created an ultrasonic welding horn that incorporated a simple system of acoustic “dampeners.” The dampeners would cushion and isolate the fragile cannula stalk within the sonotrode without affecting the weld energy directed to the weld zone at the cannula base. When proof-of-concept welds showed the concept could work, Emerson continued to improve the sonotrode design. Ultimately, a “perfected” sonotrode, augmented with a vacuum port to enable automatic part loading/unloading of the welder, delivered the commercial-scale weld quality and consistency required for the finished parts. The design and tooling process took less than six weeks.
The new sonotrode was paired with the Branson 2000XC Micro ultrasonic welder, a welding platform that was “well suited” to PD-Dental’s plans for fully automated welding of the very small IrriFlex cannula assemblies. The Branson 2000XC Micro can support medical product production by collecting and storing weld production records that comply with the U.S. Food and Drug Administration’s 21 Code of Federal Regulations (CFR) Part 11 requirements for electronic records and signatures.
To date, two Branson 2000XC Micro welders have supported the production of more than 6 million IrriFlex cannula assemblies. The design of the flexible cannula, which includes dual side-facing outlets near the tip, ensures that irrigation fluids are dispensed evenly throughout complex root structures.