Plastic Bearings Help With High-Precision Dosing

The infusion pump's linear movement of a few centimeters is very, very slow and very steady, as it administers medication into the patient's bloodstream over several hours. These medical devices have established themselves worldwide under the brand name 'Perfusor'. They are manufactured by B. Braun Melsungen AG. In "Compact Plus" the latest in the Perfusor series, the plain bearings from the motion plastics specialist igus, guide the linear drive of the pump smoothly and with high precision.
 
With Perfusor, B. Braun Melsungen AG has achieved what is considered one of the highest goals in marketing: a brand name that becomes the generic term for an entire product group. Here it stands for the syringe or infusion pumps invented by B. Braun. These medical devices are common in every hospital today. They dose the medication contained in syringes into the patient's bloodstream with great accuracy and, if necessary, over long periods of time.
 
In an intensive care unit, up to 24 such Perfusors can be used per patient. For example, they provide the patient with painkillers, adrenaline, nutrient solutions, beta-blockers or blood. They can be grouped and attached to the bedside and can be networked via a communication module and an online suite. Since they remain on the bed during transport, batteries undertake the power supply.

During the infusion a steady movement must be performed. For B. Braun Melsungen AG, this required a particularly good coefficient of friction for the components in order to achieve smooth movement in the smallest possible installation space. The solution should also be easy to install.

With a specially manufactured iglide® J clip bearing, an easy solution is guaranteed despite the tight installation space. The clip bearings are also maintenance-free and dry-running, which is absolutely necessary in this application to prevent contamination. Its low coefficient of friction ensures that the infusion pump functions smoothly despite very short strokes.

The Perfusors are an important product group for B. Braun, but just one of several. The 180-year-old family-owned company based in Melsungen near Kassel is active in no less than eighteen product and application areas in medical technology. This includes pumps, dialysis machines, disposable items such as syringes and “Braunülen” (another product name used as a generic term, this time for venous catheters) as well as surgical instruments from the subsidiary Aesculap.
 
In 2017, B. Braun AG, which employs around 62,000 people worldwide, generated sales of around € 6.8 billion. This corresponds to an increase of 5% and characterises the growth strategy: in just eight years, sales rose by €2.7 billion. The investments amounted to just under €970 million and were financed entirely by their own funds.

With the "Compact plus", B. Braun launched a newly designed Perfusor series on the market in 2017/18. The new development was necessary due to modified international and national regulations, which have a major influence on the design and functions of medical devices. The new Perfusors are equipped with numerous safety functions. Dr. Dirk Aljets, Lead Engineer Infusion Pumps of the "Compact" series: "The device recognises, for  example, the applied syringe and monitors all essential processes itself. The control system is redundant; the two processors communicate with each other and report immediately if they record different values." And only when the device recognises the syringe, it is locked and the drive is started.
 
The user guidance is designed entirely for "usability". Many syringe types and active ingredients are stored in the integrated database. The operating staff enters, for example, the delivery rate depending on the weight of the patient and can be assured that this value will be maintained accurately. Predefined infusion profiles are also possible. Dr. Dirk Aljets: "For some active ingredients, a bolus must first be given fast, which means a certain level must be reached, then only the additional doses are given."

With a delivery rate of 0.01 to 999.9 milliliters per hour (bolus rates even up to 1800 ml/h) - values around 5 ml/h are typical - the (linear) drive of the Perfusor must meet extreme accuracy requirements, and this with a very slow motion, by which the content of the syringe is conveyed into the supply tube.
 
Braun engineers have therefore invested a great deal of know-how in the design of the drive. Dr. Dirk Aljets: "We use a stepper motor that acts on a lead screw with a special thread that we developed." This ensures the desired accuracy and reliability with minimal linear movements. This accuracy can even be expressed in numbers: "Our Perfusors work with a maximum deviation of less than +/- 1 %."
 
When designing the drive, B. Braun takes great care in the selection of the bearings and has been using polymer bearings from the igus® construction kit since 1992. A new bearing was developed for the new compact series. Dr. Dirk Aljets: "In the predecessor model, the shaft guide was quite elaborate - with two guide rods, each with one bearing with special tolerance and one as a floating bearing. The new drive is guided via a round bar with a carriage. This is easier, saves installation space as well as costs - and is just as precise."

The sliding movement of the carriage on the round bar is carried out using bearings made from the high-performance polymer iglide® J. The fact that these bushings are self-lubricating is not decisive for B. Braun, because the lead screw must be lubricated. Jerk-free motion at extremely low linear motion speeds is much more important. This requirement is met with the iglide® J bushings. In addition, the push rod is attached to the sheet metal plate using an iglide® clip bearing with a special contour. For this application, igus® and B. Braun have jointly developed a specification that ensures reliable mounting and makes the tilting of the push rod impossible.
 
Over the years, B. Braun has been working closely  with igus®, with the result that various iglide® polymer bearings are used in medical devices - for the Compact series, for example, on the release device of the syringe. Further examples are flange bearings in normal and special dimensions, with recesses for the cable feed-through.