Fraunhofer IPMS’ micro-scanning mirrors are manufactured on silicon wafers; as many as 1,000 individual mirrors can be produced on a six-inch wafer
manner to large desktop laboratory machines. Dr Jonathon Liu, who chaired a session titled ‘Miniature instruments for endoscopic microscopy’ at the MOEMS-MEMS conference at Photonics West in February, said that the use of confocal microscopy in endoscopic devices was a theme that came up: ‘There were a few talks on the use of MEMS mirrors for scanning a laser beam in a confocal microscope. That’s the most common use of MEMS mirrors in endoscopy – to scan the illumination light into the tissue to create an image.’
A confocal microscope is designed such that out-of-focus light is eliminated to increase resolution and contrast, and to image objects clearly in low background light. ‘In order to do that, these technologies [confocal microscopy] typically only allow you to image a single pixel within a tissue at a certain time,’ said Dr Liu, assistant professor of the Biomedical Engineering department at Stony Brook University in New York, USA. ‘So, in order to create an image, you need to scan that spot through the tissue – so that you reconstruct the image point by point.’
and, in future, could replace existing lengthy laboratory-based procedures.
Diagnosing cancer usually involves a biopsy followed by tissue analysis, which is time- consuming. Histopathology, which is currently the gold standard for diagnosis, is carried out in a laboratory by a specialist pathologist, and involves cutting, staining and examining tissue samples under a microscope. However, not all lesions turn out to be
cancerous, and unneeded biopsies are not only painful and stressful for the patient, but are expensive for healthcare authorities. Advances in MEMS technology over recent years has allowed the development of tiny microscopic devices that perform in a similar
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A team of scientists at the Fraunhofer Institute for Photonic Microsystems (IPMS) in Germany has developed a MEMS microscope head that can operate as a confocal system. The microscope uses a MEMS scanning mirror measuring less than a millimetre; the device itself has a diameter of just 6mm and is designed to fit on the tip of an endoscope. ‘The advantage [with the MEMS mirror] is that you can use a confocal system. Conventional scanners are way too big to be incorporated into the tip of an endoscope,’ said Dr Michael Scholles, head of business development and strategy at Fraunhofer IPMS. ‘We used a MEMS mirror with a diameter of a fraction of a millimetre so it then fits into the tip [of the endoscope].’ The endo-microscope can be part of a larger system for in vivo analysis of cells and other microscopic biological structures in real time, allowing doctors to diagnose cancer more rapidly.
The microscope head uses light from an
external laser transmitted via an optical fibre. The MEMS scanning mirror fitted in endoscope deflects the laser beam and illuminates different points of the tissue as it oscillates. The light reflected by the tissue is
detected by a sensor, and a two-dimensional image is reconstructed by combining the position of the mirror and image sensor signals.
The micro-mirror is able to oscillate in
more than just one plane so that the laser beam can be reflected in any direction. This is achieved by a gimbal suspension which allows the scanning mirror to be deflected on two different and mutually independent levels, making it flexible and able to replace complex constructions usually employed to deflect laser beams.
A difficult part of
the development was to produce a suitable micro-assembly for the endoscope head
The MEMS mirror was designed and manufactured at Fraunhofer IPMS on six-inch silicon wafers by photolithography. As many as 1,000 chips can be produced on a single six-inch wafer, which will be advantageous when the micro-endoscope goes into mass production, according to Scholles: ‘One advantage of producing MEMS devices on a wafer level is that if you go to higher quantities they become inexpensive – with one fabrication step you get a lot of individual chips.’ However, MEMS technology is being used in more and more new devices,
and higher volumes are not always required in the development stage. In this situation, the process is more costly. ‘If you have a specialised application where you only need a handful of devices then you still have to pay for the whole lot [of MEMS components],’ explained Scholles. ‘So it’s a trade-off between the possibilities of higher-volume fabrication based on MEMS processes and the demand from the customer.’ A difficult part of the development was to
produce a suitable micro-assembly for the endoscope head. ‘The technical challenge was the need to come up with the mounting technology for incorporating the MEMS mirror plus fixing the glass optical fibre,’ Scholles said. ‘Here we faced the challenge of making the complete system suitable for installation in the endoscope, and we managed to do it.’ Scholles went on to say that the integration of the MEMS component helped the researchers in other projects that are using a similar setup, such as a biometric device currently under development at Fraunhofer IPMS that scans the retina of the eye. ‘A lot of knowledge that we gained in the endoscope design has been carried over to the retina
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