Infusion pumps: Plastic plain bearings help with high-precision dosing

With the Perfusor, B. Braun Melsungen AG has achieved what is considered one of the highest goals in marketing: a brand name has established itself as a generic term for an entire product group. In this case for the syringe or infusion pumps invented by B. Braun. These medical devices are widely used in every hospital today. They dose the medication contained in syringes into the patient's bloodstream with high precision and, if necessary, over long periods of time.

Up to 24 such perfusors can be used per patient in an intensive care unit. They supply the patient with painkillers, adrenaline, nutrient solutions, beta blockers or blood, for example. They can be grouped together and attached to the patient's bed and can be networked via a communication module and an online suite. As they remain at the bedside during transport, they are powered by rechargeable batteries.

A smooth movement must be realised during infusion. For B. Braun Melsungen AG, this required a particularly good coefficient of friction of the components in order to realise a smooth movement in the smallest possible installation space. The solution also had to be easy to install.

With a specially manufactured clip bearing made of the material iglidur® J, a simple solution is guaranteed despite the tight installation space. The clip bearings are also maintenance and lubrication-free, which is absolutely necessary in this application to avoid contamination. Their low coefficient of friction ensures that the infusion pump functions smoothly despite the very short stroke.

Perfusers are an important product group for B. Braun, but only one of many. The company, which is based in Melsungen near Kassel and has been family-owned for 180 years, is active in no fewer than eighteen product and application areas of medical technology. These include pumps, dialysis machines, disposables such as syringes and "Braunülen" (another product name as a generic term, this time for venous catheters) as well as the surgical instruments of the subsidiary Aesculap.
 
In the 2017 financial year, B. Braun AG, which employs around 62,000 people worldwide, achieved sales of around €6.8 billion. This corresponds to an increase of 5% and characterises the growth strategy: sales rose by € 2.7 billion in just eight years. Investments totalled just under € 970 million and were financed entirely from the company's 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 changes in 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 the inserted syringe, for example, and monitors all essential processes itself. The control system is redundant; the two processors communicate with each other and report immediately if they detect deviating values." And only when the device has recognised the syringe is it locked and the drive started.

The operator guidance is designed with usability in mind: Many syringe types and active ingredients are stored in the integrated database. The operating theatre staff enter the delivery rate, which depends on the patient's weight, for example, and can rely on this value being precisely adhered to. Predefined infusion profiles are also possible. Dr Dirk Aljets: "With some active substances, a bolus, i.e. a certain level, must be reached quickly first, then only a subsequent dose is administered."

With a delivery rate of 0.01 to 999.9 millilitres per hour (bolus rates even up to 1800 ml/h) - typical values are around 5 ml/h - the (linear) drive of the perfusor must fulfil extreme requirements in terms of accuracy, and this with a very slow movement sequence with which the syringe contents are delivered into the supply line.
 
The Braun engineers have therefore invested a great deal of expertise in the design of the drive. Dr Dirk Aljets: "We use a stepper motor that acts on a spindle with a special thread developed by us." This ensures the desired accuracy and reliability for the minimal linear movements. This accuracy can even be expressed in figures: "Our perfusors work with a maximum deviation of less than +/- 1 %."
 
When designing the drive, B. Braun takes great care in the selection of bearings and has been using plastic bearings from the igus® construction kit since 1992. A new bearing was developed for the new Compact series. Dr Dirk Aljets: "With the previous model, the shaft guide was quite complex - with two guide rods and one igus® bearing with special tolerances and one as a floating bearing. The new drive is guided by a round rod with a slide. This is simpler, saves installation space and also costs - and is just as precise."

The sliding movement of the carriage on the round rod is performed via bearing bushes made of the high-performance polymer iglidur® J. The fact that these bushes are lubrication-free is not decisive for B. Braun, because the spindle has to be lubricated. Much more important is a smooth glide at extremely low speed of the linear movement. This requirement is met with the iglidur® J bushings. In addition, the push rod is attached to the sheet metal plate via an iglidur® clip bearing with special contour. For this application, igus® and B. Braun have jointly developed a specification that guarantees a reliable bearing and makes it impossible for the push rod to tilt.

Over the years, B. Braun has developed a continuous collaboration with igus®, which has resulted in various iglidur® plastic bearings being used in medical devices - in the Compact series, for example, on the unlocking device for the syringe. Other examples are collar bearings in standard and special dimensions, for example with recesses for the feed-through of cables. Dr Dirk Aljets: "Sometimes we don't even need high-performance plastic for such applications; a PE component would do just as well. But we are impressed by the quality and delivery capability of igus®. That's why we like to use these bearings, even if it's not technically necessary." And that is why they are not only used in the perfusors, but also in the dialysis machines from B. Braun.