Improving scalpel blade cut quality using laser surface texturing

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Paul Butler-Smith and Tian Long See demonstrate how the surface friction of stainless-steel medical tools can be reduced using pulsed and ultrashort-pulsed laser processing

The development of ultrashort pulsed lasers over the past decade has enabled significant advances in the functional performance of engineered surfaces to be realised for a wide range of applications. This includes areas such as surface friction, tactile behaviour, anti-wetting and icing, light absorption, bacterial formation and more.

The design, generation and evaluation of such surfaces, complemented with advanced laser machining and laser-assisted metal joining, forms a significant part of the laser processing capabilities offered by the Manufacturing Technology Centre (MTC) in Coventry, one of UK’s High-Value Manufacturing Catapults, which work with thousands of companies each year to boost the country’s manufacturing performance.

As part of investigations for the European Horizon 2020 SHARK project, the MTC in collaboration with Fraunhofer IWS and Johnson and Johnson, designed textures and developed laser processes for application on stainless-steel scalpel blades for the reduction of friction and improvement of cut quality. To enable the production of a wide range of groove geometries, both direct laser interference patterning (DLIP) and direct laser writing (DLW) processes were employed on independent laser systems. Schematic diagrams of the two arrangements are shown in figure.1.

Figure 1: a) DLIP and b) DLW configurations showing optical and scanning arrangements and beam paths for the generation of groove textures1

Micro-texturing using the DLIP process was carried out using a TECH-1053 solid-state near-infrared nanosecond pulsed laser by Laser Export. The optical path incorporates a DLIP module (Fraunhofer IWS, DLIP-µFab) which enables direct laser interference patterning by splitting the beam using a diffractive optical element, parallelisation of the beam using a biprism arrangement and convergence using an aspheric converging lens. A line-like interference pattern is created in the overlapping volume (interference volume) of the beams with a pattern period depending on the overlapping angle, the laser wavelength, and the angle between the beams. The arrangement was set up for this study to produce groove textures with pitch separations of 4, 6 and 8μm.

The DLW process was used to produce larger groove textures in combinations of widths (50-500μm), pitch (100-550μm) and depths (5-32μm). The processing was carried out using a five-axis Georg Fischer Machining Solutions LP 400 U laser system, incorporating an Amplitude femtosecond pulsed laser of Gaussian energy distribution and near-infrared wavelength, a dual-axis galvo scanner and beam focusing optics.

Figure 2: The observed improvement in cut quality between an untextured and laser textured scalpel blade

The range of groove texture designs (three DLIP and 14 DLW) were produced on polished 316 stainless-steel coupons for tribology testing and on one side of the selected scalpel blade profile, oriented parallel and perpendicular to its long cutting edge.

The friction tests were carried out using a laboratory tribometer on the produced coupons at an applied load of 1N using a natural polyurethane block of 1cm3 as the counterpart material. The friction results were compared with those of untextured polished plates to provide a reference. The tests revealed significant reductions in contact friction for a number of the texture designs in both the parallel and perpendicular texture orientations. The best performing DLIP-produced textures achieved reductions in the coefficient of friction of up to 32 per cent, while the best DLW-produced textures achieved reductions of up to 17 per cent.

Figure 3: The setup used to test the textured scalpel blades

Using a bespoke test rig, cutting tests were carried out under controlled conditions on each of the DLIP and DLW textured scalpel blades, and on untextured reference blades. Test cuts of 6mm depth and 150mm length were produced in the identical polyurethane used in the tribology tests. A good correlation was found between the friction results and scalpel blade cutting results for textures in the perpendicular orientation, achieving up to 41.5 per cent cutting force reduction in the downward (Fz) and 24 per cent reduction in the transverse (Fx) directions for the DLIP-produced textures, and up to 38 per cent (Fz) and 5 per cent (Fx) reductions for the DLW-produced textures. Improvements in the cut quality were also observed from textures produced by both laser processes, as shown in figure 2 above.

The cutting test arrangement and results for the best performing DLIP and DLW produced textured scalpel blades are shown in figures 3 (above) and 4 (below).

Figure 4: Results showing the percentage reduction in cutting forces compared with untextured blades1

It has been shown that laser texturing using either the DLW or DLIP processes can significantly reduce the surface friction of stainless-steel scalpel blades and enhance the quality of cut. Laser surface texturing can also offer benefits in other medical applications such as hypodermic needles, stents probes and endoscopes, where the reduction in contact friction and patient comfort are important considerations. Surface texturing applied by either of these processes can also offer significant benefits in a vast range of applications where interactions between rigid and compliant materials occur.

Dr Paul Butler-Smith is a senior research engineer at The Manufacturing Technology Centre.

Dr TianLong See is a technology manager at The Manufacturing Technology Centre.

Acknowledgment

The authors gratefully acknowledge the financial contribution from the European Union’s Horizon 2020 Framework Programme for research and innovation under grant agreement No. 768701.

Reference

[1] Butler-Smith, P., et.al. ‘A comparison of the tactile friction and cutting performance of textured scalpel blades modified by Direct Laser Writing and Direct Laser Interference Patterning processes’, Procedia CIRP, 2022 – Elsevier (In-press)

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