For precise blood pressure monitoring in hospitals and surgeries, the accurate handling of medical devices is paramount. Human error or device failure can result in inaccurate readings, which compromise the safety of patients, and it is critical, therefore, that device components, such as connectors, not only ensure a leak-free connection but are also simple and easy to use.
For blood pressure monitoring, connections have traditionally been formed using a standard metal pneumatic connector — a comparatively expensive option that requires a concerted physical effort to screw in or unscrew. Attempts to mimic the performance of metal connectors in plastic have had limited success. When subject to vibration or interference, existing designs unscrew too easily and do not provide a leak-free fit, which compromises the accuracy of blood pressure readings.
Advances in material science have yielded a new line of connectors that overcomes these challenges. Developed through close consultation with medical device manufacturers and professionals, the new ScrewThread connector retains the familiar shape of its metal equivalent but with more features and increased usability.
The ScrewThread connector came about through Colder’s discussions with medical device manufacturers, who were experiencing difficulties with the metal and plastic versions of the fitting. Colder was challenged to design a new solution that would work with industry standard parts and compensate for any deficiencies. In addition, medical staff could not be required to learn how to use a completely new technology. The fitting itself would need to perform for several years, or up to 10,000 cycles of use.
Colder began by holding discussions with medical professionals to understand the challenges they faced with the existing products. This brought to light two key requirements for a new solution. First, the fitting had to be simple and comfortable to use, so that staff were not distracted from their patients by trying to force two parts together. Second, staff needed to be confident that a secure, leak-free connection had been made.
Colder’s design team understood how to meet the needs of both the device manufacturers and the medical professionals themselves. With the original metal connector and its first plastic iteration, both parts of the fitting — the rotating outer-sleeve and the inner barbed stem — were made from the same materials. This limited the scope of what could be achieved. Colder instead chose to treat them individually.
Improving the Stem
The first challenge was to ensure that the inner fitting would perform to the required specifications. Colder designed a part that would be leak-free, and even able to overcome the poor quality of some existing industry standard parts.
The engineering team overcame this by experimenting with the O-ring on the nylon stem. When inserted into the ID of the mating part, the O-ring would compress to eliminate leaks. By using a higher- performance ring, Colder ensured a leak-free fit even if the mating part was inferior. On the other end of the inner fitting — a barbed end designed to grip the interior of the tube — Colder created a parting line free barb. This eliminated the protruding ‘ridge’ that could allow air to escape around the side when under pressure. Using proprietary molding techniques, the inner fitting has three sharp barbs designed specifically to securely hold the stiffer hoses common to blood pressure monitors.
Redesigning the Sleeve
The outer sleeve of the fitting spins freely and the user grips and rotates it to screw the fitting on to the mating part. Staff had previously complained that the metal fitting was too sharp, and the plastic version too rough, to use.
In order to make the outer sleeve easy and comfortable to grip, Colder used twoshot molding to combine a rigid polypropylene inner core with a tactile soft-touch outer layer made of thermoplastic elastomer. Colder’s molding expertise overcame the traditional challenge of the inner core distorting under the heat and pressure of the second mold by increasing the wall thickness of the polypropylene core and fully supporting it on a steel pin throughout the molding process. As a result, the core remained intact and was not at risk of collapsing.
Medical staff had also explained the difficulty of securing the existing metal and plastic fittings — it was hard to screw the metal version onto the plastic mating part, and the plastic versions spun loosely right up the point where they abruptly stuck at the end.
To confirm that a secure connection had been made, Colder customized threads on the sleeve so that the last turn was slightly shallower than the rest, creating interference with the threads of the mating part. Now the fitting threaded on easily, but tightened noticeably at the end for a clear fit that would not come loose. Colder also replicated the 2½ turns required to fully secure the metal fitting so that users would not be distracted by an unfamiliar product.
Functionality and Aesthetics
As the fitting was created for medical applications, the team chose a clean white design that medical staff confirmed looked and felt ‘cleaner’ under strong hospital lighting. This also made it easier to see dirt accumulating so staff could clean or replace the fitting. The soft-touch grip was molded in deep blue, which contrasted with the white and made the connector easy to see.
Colder also tested all materials to the same standards for biocompatibility that medical device manufacturers themselves would employ. The fitting meets all requirements for cytotoxicity, sensitization, and intracutaneous injection, making it suitable not only for blood pressure monitoring but also a wide range of other medical applications.