Under pressure

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Tim Gillett and Rob Coppinger find ways to aid stroke recovery with lasers

In 2014 a prototype skull-cutting laser system could be evaluated for medical trials.

Under development by German research institutes, the laser system would be used to release pressure on the brains of stroke victims. When a major stroke occurs, it may cause the brain to swell. The pressure in the cranial cavity increases, blood-flow to the brain diminishes and the brain can suffer further damage.

Until now, surgeons will relieve the pressure by opening the patient’s skull using a mechanical device called a trephine. However, that approach is considered high-risk because the surgeon could inadvertently injure the brain’s meninges, a membrane which covers the brain, which could lead to meningitis and, in the worst-case scenario, death. Instead, the laser would be used to cut through the skull, allowing a section to be lifted without the meninges being touched.

Dr Michael Scholles is head of business development and strategy at the Fraunhofer Institute for Photonic Microsystems IPMS, in Dresden. Along with two other institutes, the Fraunhofer Institute for Laser Technology ILT and Fraunhofer Institute for Integrated Circuits IIS, it is developing a prototype. Scholles said: ‘This is an internal project funded by the Fraunhofer central administration, and they are in the final stages of building a first prototype. Later this year, in the third quarter, the project will end.’ Then the search will begin for medical partners for that evaluation.

The laser system uses a high-energy femtosecond laser; the surgeon can guide the beam and choose where to cut the cranial bone. The surgeon has a hand piece and the laser is fed into it through an articulated mirror arm. The arm consists of two new types of micro-mirrors, developed by the researchers at Fraunhofer IPMS. Scholles added: ‘It uses our MEMS [micro electro mechanical systems] scanners for the deflection of light.’ The first mirror makes the cranial vault incision, directing the laser beam dynamically across the cranial bones. The second adjusts any malpositioning.

The crucial aspect of the new system is that the components are miniaturised, but can tolerate up to 20W of laser output – about 200 times more than conventional micro-mirrors. These can already reach their limits at 100mW, depending on their specific design. In addition, at 5mm by 7mm or 6mm by 8mm, the new MEMS mirrors can guide larger-diameter laser beams. By comparison, conventional micro-mirrors measure from 1mm to 3mm.

For the prototype, the researchers applied highly-reflective electric layers to the mirrors’ silicon substrate. Normally the silicon panel in conventional micro-mirrors is mirrored by an aluminium layer measuring a 100nm thick. The highly-reflective electric layers will, according to the Fraunhofer researchers, reflect 99.9 per cent of the laser light in the visible spectral range, compared to 90 per cent with typical components. Using these highly-reflective electric layers, much less of the high-energy radiation penetrates into the substrate and that means the mirror can tolerate the greater power.

The challenge the researchers had to achieve was to apply the high-power coating to the silicon substrate at just a few micrometers thick, which is commonplace in microsystems technology.

One difficulty was that applying the several different layers, which are required and are a few micrometers thick, causes mechanical stress – and this can deform the coating substrate. The substrate arches, and this diminishes the optical quality of the mirror. To counterbalance this, the same coating is applied on the reverse side of the substrate.

While no medical staff have been involved in the prototype development, Scholles is confident that a user for an evaluation and possible future trial can be found later ths year as the project comes to a close.

‘At the end of the year they will do something or probably prove that it can be used somewhere in medical applications,’ said Scholles.

About the author

Greg Blackman is the editor for Electro Optics, Imaging & Machine Vision Europe, and Laser Systems Europe.

You can contact him at greg.blackman@europascience.com or on +44 (0) 1223 275 472.

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