development that would allow optical elements to be reduced in size is if aspherical lenses are used in optical systems. With an asphere, you can achieve a similar quality image as produced by a doublet lens, in addition to only requiring one element, not two or more.’ But there are drawbacks to using aspherical
lenses, such as high tool and manufacturing costs. From a manufacturer’s perspective, the production of aspherical lenses is worthwhile only if the demand is great enough to push up volume. ‘This approach would become a more acceptable option when producing disposable endoscopes, as after several uses they would be replaced,’ explained Gretschel. ‘This would increase demand and therefore validate production of these endoscopes with aspherical lenses.’
Single use endoscopes
The smaller the
are already in development; however, they are not as common in medicine as endoscopes that are repeatedly used. They act as valuable alternative, as they dramatically reduce the chances of a patient acquiring an infection through having an endoscopic procedure.
In addition to manufacturing constraints,
there are also physical limitations that prevent the optics within endoscopes being miniaturised. The angular resolution of an image is diffraction- limited, which puts a restriction on the size to which an endoscope can be reduced. Beyer commented: ‘0.5mm in diameter is a reasonable estimate as to how small endoscopes can go, but with reduced image quality. To compensate for the physical diffraction limits, new ways of imaging have to be found’. This physical diffraction limit must be
overcome before optical systems can be reduced further in size. Research teams have been searching for a suitable alternative material that will allow them to ‘break’ this diffraction limit. ‘The principle has recently been demonstrated using metal-based materials – predominantly gold – but these were too inefficient and too costly to mass-produce,’ according to an announcement in January from Swinburne University of Technology, Australia.
Xiaouri Zheng, a PhD student at the Swinburne institute, attempted to produce a lens using graphene oxide. This ultra-light, transparent material is 200 times stronger than steel, while being incredibly flexible and acting as a superb conductor. The high functionality and durability
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lenses used in an optical system, the less the quality. In order to compensate this, very complex aspherical designs or software “tricks” are needed
of this material make it the perfect candidate for acting as a material that can break the diffraction limit. Using a 3D printer, a team at the Swinburne institute, led by associate professor Baohua Jia, was able to produce a strong and flexible lens through the use of a graphene oxide solution. Using a laser to reduce the graphene oxide, the team created three concentric rings on the surface of the lens, which created high-quality focusing properties. The resulting lens produced by the team was incredibly flat – 300 times thinner than a sheet of paper – and was able to view objects of 200nm in length, at visible and near-infrared wavelengths. Graphene-based lenses could be exactly what endoscope manufacturers are looking for in order to reduce the size of their optical systems. With this research being only published in September 2015, these graphene-based lenses are still in the early stages of development. The research team is currently working towards integrating the lens with a fibre to create
a smaller, safer and more sophisticated endoscope for non-invasive surgery. With endoscopic optical systems already being
targeted for miniaturisation via aspherical and graphene-based lensing, medical professionals could one day be using a new set of endoscopes that are able to capture high resolution images of objects only nanometres in size.
A needle in the haystack Certain areas of medicine require a delicate approach to seeing inside the body that cannot be offered by conventional endoscopes. Whilst operating on brain tumours, surgeons must identify the borders of cancerous tissue so that they can remove the tumour without damaging the healthy tissue surrounding it. Professor Marloes Groot at the Vrije Universiteit Amsterdam is using micro-optics to develop an endoscope that is able to achieve this.
The endoscope will take the form of a handheld device that will deliver scattering photons into the brain tissue of a patient. It will be an extension of a table-top system that can scan an excised piece of brain tissue using second and third-harmonic generation, a physical phenomenon where single photons are generated via multiple interacting photons. These single photons are produced at a shorter wavelength, which allows them to scatter through the brain tissue. Scattered photons contain information
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