Electropolishing for Medical Devices: Relatively New ... Fascinatingly Diverse

Dimitri Aslanidis*, Gisele Roebben, Johan Bruninx and

Wilfried Van Moorleghem

@medical technologies n.v. (AMT), Daelemveld 1113 3540 Herk-de-Stad, Belgium

www.amtbe.com

Keywords: electrolyte, electropolishing, nitinol, medical device

Abstract. Surface with high quality is needed for all medical devices. New materials inspire designers; and design drives high surface requirements. Nitinol and other titanium alloys are emerging as key materials in device manufacturing. The standardisation in the medical industry throws heavier emphasis on more standard surfaces processes for better performance and life-extension. Among all surface treatments, electropolishing is the most critical in the final surface appearance. For this purpose a consistent and standard electropolishing process has been developed which is suited for precision surface levelling and brightening.

Introduction In order to guarantee a high degree of performance, long fatigue life, low friction, improved

wetting, bonding properties or just a clean surface, it is necessary for devices to undergo surface treatment process to a give a product free of surface defects, irregularities, appropriate levelling and smoothness. These processes can be based on variety of techniques which often differing surface levelling, cleanness and roughness. However, one technology which can not be disregarded is the electrochemical polishing or common known as electropolishing. It has been present in the industry since the early 1950s. The process is not complicated at all. It is an electrochemical anodic process. The workpiece is immersed in the electrolytic solution (electrolyte or bath), which carries a direct current (under 1-25V), and metal dissolution takes place from the anode and towards the cathode. The anode is our device and the cathode in usually an inert conductive surface. This is a simple process and can be controlled with the following parameters: Potential and current density (amount of applied current) Bath composition (concentration and preparation) Bath temperature Processing time Hydrodynamics (agitation, cell configuration and geometry) The alloy surface

The main objective in most applications is to level out or make the surface more equivalent, i.e., less peaks and valleys.

The advantages of the electropolishing are the following Complex in shape devices can be easily handled and processed The process control is easy and consistent The process can be very easily automatized Different surface qualities can be achieved in very short processing time _____________ *E-mail address: dimitri.aslanidis@amtbe.com Fax +32 13 553514

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Precision levelling can be achieved in the range of 2m It is up to 30x faster than conventional processes Successfully applied to different grades and alloys

The disadvantages are one or combination of the following

It sometimes requires very harmful electrolytes Solution disposal Hydrogen embrittlement

Taking into account the high quality of produced surfaces, the medical device industry is increasingly adopting electropolishing technology, for final surface conditioning. In addition, the devices industry is attempting to build smaller and denser structures by using a variety of materials. Therefore, it is important to offer very flexible electropolishing solutions by using new electrolytic systems. The objective of this study is to show how @medical technologies develop and supply integrated electropolishing solutions for the medical device industry.

Current Results AMT offers a new generation electropolishing treatment by using a new electrolyte and

advanced process design. These have been most effective in improving the surface brightening, increase the life-time of the bath and provide precision surface levelling. The electrolyte used in our processing line is called EP 1 and the chemical composition is proprietary. Our main objective was to offer the following to the industry Flexible electrolyte for processing of different alloys and surfaces Prolongation of the life of process baths Drag-out minimisation (overflowing, transferred chemicals from one process step to other) Minimisation of water usage No wastes

All of them can be accomplished by using Advanced electrolytes like, EP-1, and Appropriate process design and technology

First, the electrolyte is found to be of paramount importance and focus has been on the following achievements development of stable chemistry use substances which allow surface conditioning during polishing ability for a simple analysis control less sensitive to contaminants

Electropolishing is a process which can be easily designed and implemented for a specific component. The main problem starts when a defined process has to become more flexible by using other type of surfaces, different sizes, alloys and require variety of qualities. On the other hand, this process can be easily automatized with very accurate control. It has been recognised by AMT that the design and automatisation when coupled early in the beginning can offer flexibility and control of processing by using: various current densities accurate temperature control of bath and workpiece critical necessary volume of electrolyte controlled agitation

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appropriate pre- and post- treatments of both electrodes (anode/cathode) appropriate cell design

The appearance of typical surfaces is shown in Figures 1 to 4. Figures 1 to 3 show surfaces which are required by the medical, aerospace and tool industries. Figure 4 is not considered adequate and should be avoided.

During design, the basics of electropolishing should not be forgotten: the anodic current density is highest at the peaks and lowest at the valleys. By increasing the current density, metal from the anode dissolves faster and easier at the peaks, producing a levelled, smooth and "free" of valleys surface. Micro and macro asperities can be easily controlled and removed within a range of more than 1m (see Figure 1 and 2). If the current density is not in the appropriate range, instead of polishing, a type of etching is introduced. in addition, high current densities enhance gassing with a result the formation of heterogeneous surface. Usually the potential should vary between 3-25 V depending on the type of surface, the size of the component and the cell configuration. By using an appropriate set of process parameters, the surface characteristics contain (see Figures 1 to 3) Surface without defects, cracks and the formation of high smoothness (Figure 3) Low surface roughness with high brightness or simply reflection (Figures 2 and 3).

Conclusions Nothing is static. We seek to optimise materials, materials processing and surface technologies to

meet today's needs for high quality medical devices. The development of treatments capable to produce bright, smooth and levelled surfaces is closely linked to the development of controlled electropolishing lines. These should be built on appropriately selected electrolyte and well designed processes. Based on our approach, we show that tailor made surfaces can be produced. These can assist better the performance of medical devices and other components in diverse sectors.

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(a) Before

(b) After

FIGURE 1: Typical surface appearance of wire (a) before and (b) after electropolishing.

FIGURE 2: Detail of electropolished surface of a medical instrument. Difficult machined edges can be easily smoothened and formed by using electropolishing methods. Optimum processing conditions can homogeneously remove layers of 1m.

(a) Surface smoothening:

creation of very flat surfaces.

(b) Surface levelling;

high quality surface for medical devices. FIGURE 3: Surface details of electropolished stent. Very smooth, levelled and bright surfaces are the key characteristics of the technology offered by AMT.

FIGURE 4: Typical surface of not optimized stent electropolishing. The presence of valleys and peaks is clear across large areas. Such surfaces are not acceptable for high performance devices.