The National Physical Laboratory (NPL) recently completed a consultancy to identify a suitable measurement system for the oscillating movement of a razor blade used with medical biopsies. Out of plane movement of the blade can damage the membranes and biopsies being sliced, so accurate measurement of the blade is critical to the system's use. CEMMNT partner, NPL, has put forward a design solution and verified its effectiveness.

A medical instrument company has designed a new device with an adjustable blade which oscillates at a range of frequencies, used to cut samples from medical biopsies for analysis. The alignment and movement of this blade is critical to the instrument and misalignment can cause the blade to move out of plane, especially at the higher frequencies, damaging the cells to be analysed. The designers required consultancy on a method to measure the error in movement of the edge of the blade to allow for better alignment by the user.

NPL devised an optical system to solve this measurement problem. The idea was to use a light source shining over the blade, which would then be detected by an optical sensor behind - with any deviation in the blade varying the amount of light and intensity measured by the sensor.

NPL built a test rig for this method whereby an oscilloscope was used to measure the output of the light sensor, and a stage micrometer was used to move a blade across the beam of light at controlled, 10 μm steps, to devise a change in the sensor output as a function of the blade position. The point at which the blade is only half exposed to the sensor is the most sensitive to the movement and is therefore the crucial zone.

The limits of the sensor and set up were tested through a series of experiments. It was possible to identify the resolution of the sensor voltage output and the effective change in light to this system when a blade is moving through the beam. This test work demonstrated that the principle will be effective and should be sufficient to measure changes of less than 0.00005 V. and so, based on the results found, should theoretically meet the required accuracy of 0.1 μm.

The four-day consultancy allowed the company to evaluate a method for measuring the error in their instrument's blade movement. This movement is critical to the use of the device and NPL's work has helped provide the company with an understanding of key areas to consider when developing a system to measure and help adjust for this movement. It also provided them with detailed information and data on a suitable system for doing this, its limitations, and where to source the required components. Following on from this research the medical instrument company has now incorporated this system, in principle, into their production design for the blade movement metrology device, leading to a large improvement to their instrument and will hopefully lead to improved outcomes to patients, thanks to the analysis of more accurate biopsy samples.

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