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Ceramic bearings for medical robots

17 April 2024

Although martensitic stainless steels have been favoured for bearings in robot applications, their performance falls short of rising standards to prevent contamination in medical environments. Chris Johnson explains why ceramic high-precision bearings hold the answer

FROM SURGICAL assistance robots that help perform minimally invasive surgeries, to AI-equipped therapeutic robots used in rehabilitation for injuries and paralysis, medical robots have come a long way since simple robotic arm technologies were first introduced in the 1980s. Although surgical robots are becoming cheaper, modern medical applications are also demanding higher precision and speed capabilities from these robots.

Traditionally, in medical instruments, bearings — that optimise the rotation of axes like robot arms by limiting or avoiding friction and jolts — have been made from high purity metals such as martensitic stainless steel. Examples include medical robots that help undertake keyhole surgery where the utmost precision is required. Surgical robot bearings must deliver the highest possible rotational accuracy, while also being small enough to fit into these often compact and portable hospital devices.

However, steel bearings have limitations including contamination. While steel bearings are tough and capable of withstanding highly-corrosive environments, they are susceptible to particulate contamination due to the very high contact pressures against the steel within the bearing while it moves. If left unchecked, this contamination can cause denting and wear.

Of course, contamination in general is unacceptable in health and safety environments. As robots become more prevalent in medical applications, regulations are getting stricter in order to prevent contamination.

Replacing steel

The need to avoid contamination accompanies other design requirements such as high speeds, low noise and non-magnetic properties. For instance, because Magnetic Resonance Imaging (MRI) scanners use a strong magnetic field to generate two- or three-dimensional images of any living subject, steel bearings cannot be used due to their magnetic properties.

All of these factors influence the choice of material for bearings in medical environments – but what material can replace steel? The medical equipment sector is increasingly choosing bearings made exclusively of zirconia (ZrO₂), or other exotic ceramic compounds. According to the Institute of Materials, Minerals and Mining, “ceramic bearings made from ZrO₂ are tough with similar expansion properties to chrome steel and 440 stainless steel, although they are 30% lighter”.

Indeed, full ceramic precision bearings are harder than steel, with superior corrosion and heat resistance, higher dimensional stability and lower density. However, ceramic bearings are expensive. This is a big issue for health institutes that operate within tight budgets, so the decision to invest in them must be well-considered. Fortunately, ceramic bearings hold a number of advantages for medical environments.

One is that the bearings don’t react to chemicals. That means they don’t corrode or weaken when subjected to harsh chemicals used for sterilisation in hospitals. Ceramics don’t require lubrication that would otherwise draw contaminants like dust, water or humidity and this also eliminates the need for maintenance processes like relubrication. They are also water-resistant, so can be washed-down regularly.

High-value applications

So, is investing in ceramic bearings for medical environments worth it? There are still certain applications where the speed and precision of steel are crucial, like in surgical robots for example. However, there are other instances where using the wrong components in such medical or laboratory equipment can contaminate study conditions, or cause the study to cease altogether.

Instead, the extra investment in ceramic bearings can help ensure that medical robots run with effectiveness and longevity — especially as they are “cast more as companions providing emotional support, rather than seen as tools.”