Medical technology: High demands on tools and processes

Tools for manufacturing medical products should always ensure high precision and reliability, as few other industries are so strictly regulated. With the new European Medical Device Regulation (MDR), high quality requirements have been re-emphasised and a system for identifying and tracing products has been introduced. Repeatability and validated manufacturing processes are therefore extremely important on the production line.

In particular because extensive validation processes are required in order to obtain manufacturing approval, changing manufacturing processes at a later date is no longer possible or extremely difficult. Changes to the machining process and during tool selection must therefore be planned and tested extremely thoroughly.

A distinction is made between the following parts ranges when it comes to the workpieces to be created: Implants, instruments and cutting tools for use during operations. Turning, milling, rolling and drilling are the machining processes involved. Hardly any standard tools are used, but rather a number of individually manufactured special tools for certain medical devices.

Additive manufacturing processes are also being used ever more frequently to produce customised, patient-specific implants, for example.

Surface machining

The assessment of surface quality is heavily dependent on the type of product. Implants surfaces are often not polished, but rather roughened to allow the implant to "grow in" to the body better. By contrast, the surface quality for surgical implements is extremely high following high-gloss polishing. This is important to ensure minimal germ adherence. 

Materials determine the tool selection

When selecting tools, the focus needs to be on the required thermal, chemical and mechanical properties of the materials, the machinability of the surfaces and the ability for them to be sterilised using standard procedures. In addition, extremely high levels of corrosion resistance and biocompatibility are often required.

Titanium alloys, ceramics or fibre-reinforced plastics are frequently used. For cutting tools, these materials usually present a challenge, as the alloying components that would simplify machining, such as sulphur and phosphorous, are either prohibited or only permitted in extremely small ratios for medical technology applications. The surfaces of the tools for medical technology must also be specially treated and coated if necessary. In order that no unwanted substances are deposited on the component, it must also be ensured that this coating is non-hazardous.

The materials that are processed are primarily titanium grade 1 to 5, cobalt-chrome alloys as well as many chrome-nickel steels such as 1.4035, 1.4441, 1.4057 or 1.4571. These materials boast a very high tensile strength of 800 to 900 N/mm, hardnesses from 220 to 250 HB30 and are biocompatible.

Titanium undergoes a very strong chemical reaction with the cutting material of the tool at high temperatures, which can lead to rapid wear. In addition, titanium has poor thermal conductivity, so the heat generated during machining must be dissipated primarily via the tool.

Plastics are subject to a range of requirements: heat resistance for sterilisation-proof plastics, impermeability for X-ray radiation, small thicknesses to save instrument weight, or colourfulness for identifying size or application. This leads to the use of thermosetting and thermoplastics; PEEK, PP, PPSU, UHMWPE or POM are all widely used. Yet they all have one thing in common: poor thermal conductivity. The heat generated during machining must therefore be dissipated with the chips.
 
Carbon fibre-(reinforced) plastic (CFRP), or carbon, consists of carbon fibre and a plastic matrix, usually epoxy resin. Machining is performed by breaking the fibres. This material is heat-sensitive, as the matrix can degenerate. There is also the risk of the CFRP delaminating. 

When it comes to tools for cutting bone material, stainless steels are normally used.

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Biocompatibility - a special case

The question of biocompatibility arises primarily if devices are to be used directly on the patient. Only then and if corresponding classification is required must the device be biocompatible. Biocompatibility tests are a fixed component in the assessment of biological risk. These tests assess the compatibility of a medical product with a biological system and investigate the interactions between the product and the different types of body tissue and cells that will come into contact with it when is used on the patient. Successful testing in accordance with ISO 10993 is standardised proof of compatibility and as such is required for the approval of medical products. The first step in this regard is cytotoxicity testing in accordance with ISO 10993-5. Cytotoxicity tests determine the potential for cell damage in the event of contact with the medical product based on cell cultures. A cytotoxicity test can be performed by the cutting tool manufacturer on its carbides and/or coatings to avoid cross contamination. Theoretical residues on the medical product can be classed as non-hazardous if the test is passed.

Cutting tools in focus

Unlike tools for use on the human body, cutting tools are used for manufacturing medical products and devices and do not come into direct contact with the body. However, the documentation of processes applied in the production of medical products and devices is becoming an increasingly important issue. With new regulations coming into force, the first building block for significantly more transparent production has been laid, and this will likely be required in greater detail in future.

The environment of medical devices is key

If medical products or devices such as tools or instruments are to be used in and on the human body, the environmental conditions must be "medical". Special servicing concepts and inspection strategies exist for these devices. Only specific equipment and processes may be used for cleaning and preparation. These tools and instruments are prepared in controlled environments in accordance with specially prepared plans and workflows. It must then be possible to use these tools in a low-risk environment close to the body following maintenance, cleaning and inspection.